WO2002099483A1

WO2002099483A1 - Chromatic dispersion and dispersion slope control

Info

Publication number: WO2002099483A1
Application number: PCT/US2002/014767
Authority: WO
Inventors: Kevin W. Bennett; Scott R. Bickham; Daniel A. Nolan; Vitor M. Schneider; David K. Smith; Laura A. Weller-Brophy; James A. West
Original assignee: Corning Incorporated
Priority date: 2001-05-31
Filing date: 2002-05-09
Publication date: 2002-12-12
Also published as: JP2004533014A

Abstract

An apparatus for dynamically controlling chromatic dispersion in an optical signal includes a coupled waveguide structure (704) and a device (706,708) which alters a refractive index of the coupled waveguide structure to effect a change in the chromatic dispersion. In another embodiment, a method for controlling chromatic dispersion includes selectively altering a refractive index profile of coupled waveguide structure to effect a change in the chromatic dispersion in an optical signal.

Description

CHROMATIC DISPERSION AND DISPERSION SLOPE CONTROL

Field of the Invention

The present invention relates in general to optical communication systems, and in particular to a method and apparatus for compensating for chromatic dispersion and dispersion slope in an optical signal.

Background of the Invention

Optical transmission systems, including optical fiber communication systems, have become an attractive alternative for carrying voice and data at high speeds. In optical transmission systems, waveform degradation due to chromatic dispersion (CD) in the optical transmission medium can be problematic, particularly as fransmission speeds continue to increase.

Chromatic dispersion in optical signals results from the fact that in transmission media such as glass optical waveguides, the higher the frequency of the optical signal, the greater is the refractive index. As such, higher frequency components of optical signals will "slow down," and contrastingly, lower frequency signals will "speed-up."

In single mode optical fiber, chromatic dispersion results from the interplay of two underlying effects, material dispersion and waveguide dispersion. Material dispersion results from the non-linear dependence of the refractive index upon wavelength, and the corresponding group velocity of the material, which is illustratively doped silica. Waveguide dispersion results from the wavelength dependent relationships of the group velocity to the core diameter and the difference in the index of refraction between the core and the cladding.

In addition to the above referenced sources of CD, impurities in the waveguide material, mechanical stress and strain, and temperature effects can also affect the index of refraction, further adding to the ill effects of chromatic dispersion.

In digital optical communications, where the optical signal is ideally a square wave, bit spreading due to chromatic dispersion can be particularly problematic. To this end, as the "fast frequencies" in the signal slow down and the "slow frequencies" in the signal speed up as a result of chromatic dispersion, the shape of the waveform can be substantially impacted. The effects of this type of dispersion are a spreading of the original pulse in time, causing it to overflow into the time slot that has already been allotted to another bit. When the overflow becomes excessive, intersymbol interference (ISI) may result. ISI may result in an increase in the bit-error rate to unacceptable levels. As can be appreciated, control of the total chromatic dispersion of transmission paths in an optical communication system is critical to the design and construction of long haul, and high-speed communications systems. To achieve this control, it is necessary to reduce the total dispersion to a point where its contribution to the bit-error rate of the signal is acceptable. In commonly used dense wavelength division multiplexed (DWDM) optical communications systems, there may be 40 wavelength channels or more, with the center wavelength of each channel being separated from its adjacent channels by approximately 0.8nm to approximately 1.Onm. For example, a 40- channel system could have center wavelengths in the range of approximately 1530nm to approximately 1570nm. As can be appreciated, compensating for chromatic dispersion in such an optical system is important.

Summary of the Invention According to an exemplary embodiment of the present invention, an apparatus for dynamically controlling chromatic dispersion in an optical signal includes a coupled waveguide structure and a device, which alters an index of refraction of the coupled waveguide structure, to effect a change in the chromatic dispersion.

According to another exemplary embodiment of the present invention, a method for dynamically controlling chromatic dispersion includes providing a coupled waveguide structure and selectively altering an index of refraction profile of the coupled waveguide structure to effect a change in the chromatic dispersion in an optical signal.

Brief Description of the Drawings The invention is best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion.

Fig. 1 is a graphical representation of the refractive index versus radius of a coupled waveguide structure in accordance with an exemplary embodiment of the present invention. Fig. 2 is a graphical representation of the refractive index versus radius of a coupled waveguide structure in accordance with an exemplary embodiment of the present invention.

Fig. 3 is a graphical representation of the chromatic dispersion versus wavelength in accordance with an exemplary embodiment of the present invention.

Fig. 4 is a graphical representation of the chromatic dispersion versus wavelength in accordance with an exemplary embodiment of the present invention.

Fig. 5 is a schematic block diagram of a dynamic dispersion compensation apparatus in accordance with an exemplary embodiment of the present invention, Fig. 6 is a schematic block diagram of a dynamic dispersion compensation apparatus in accordance with an exemplary embodiment of the present invention.

Fig. 7 is a schematic block diagram of a dynamic dispersion compensation apparatus in accordance with an exemplary embodiment of the present invention.

Fig. 8 is a functional block diagram of a chromatic dispersion compensation module in accordance with an exemplary embodiment of the present invention.

Fig. 9 is a graphical representation of total dispersion variation versus wavelength for an optical fiber at various test temperatures.

Fig. 10 is a graphical representation of the total dispersion variation with temperature for different dispersion compensating fibers coiled having a relatively small radius of curvature.

Fig. 11 is a graphical representation of the total dispersion variation with temperature for different dispersion compensating fibers coiled having a relatively large radius of curvature. Fig. 12 is a functional block diagram of a dispersion compensation module in accordance with another exemplary embodiment of the present invention.

Fig. 13 is a functional block diagram of a chromatic dispersion compensation module in accordance with another exemplary embodiment of the present invention. Fig. 14 is a schematic representation of an optical apparatus in accordance with an exemplary embodiment of the present invention.

Fig. 15 is a graphical representation of the chromatic dispersion versus wavelength for an illustrative dispersion compensating optical fiber used in an optical apparatus in accordance with an exemplary embodiment of the present invention. Fig. 16 is a graphical representation of the slope of the chromatic dispersion versus wavelength for an illustrative dispersion compensating optical fiber used in an optical apparatus of an exemplary embodiment of the present invention.

Fig.17 is graphical representation of the chromatic dispersion versus wavelength for a representative dispersion compensating fiber in accordance with an exemplary embodiment of the present invention.

Fig. 18 is a graphical representation of the nominal dispersion characteristics of three optical fibers in accordance with an exemplary embodiment of the present invention.

Figs. 19(a) and 19(b) are graphical representations of the dispersion versus wavelength of selectively serially coupled optical waveguides fibers in accordance with exemplary embodiments of the present invention. Fig. 20 is a schematic block diagram of an optical communications system including dispersion control modules in accordance with an exemplary embodiment of the present invention.

Fig. 21(a) is graphical representation of the dispersion versus wavelength of three optical waveguides in accordance with an exemplary embodiment of the present invention.

Fig. 21(b) is graphical representation of the dispersion versus wavelength of three optical waveguides in accordance with an exemplary embodiment of the present invention. Figs. 22(a) - 22(c) show the dispersion input versus wavelength for various optical waveguides in accordance with an exemplary embodiment of the present invention.

Detailed Description In the following detailed description, for purposes of explanation and not limitation, exemplary embodiments disclosing specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure, that the present invention may be practiced in other embodiments that depart from the specific details disclosed herein. Moreover, descriptions of well-known devices, methods and materials may be omitted so as to not obscure the description of the present invention. Briefly, as described in conjunction with exemplary embodiments, the present invention relates to a method and apparatus for controlling chromatic dispersion and or dispersion slope in an optical signal. This confrol may be effected dynamically; but may be achieved statically in accordance with an exemplary embodiment. hi accordance with an exemplary embodiment of the present invention, the confrol of the chromatic dispersion and/or dispersion maybe effected dynamically by altering index of refraction profile of a coupled optical waveguide structure. Illustratively, the index of refraction profile is altered by thermally controlling the coupled waveguide structure. In accordance with another exemplary embodiment of the present invention, the index of refraction profile is altered by introducing optical radiation (e.g. via a secondary laser) to the optical waveguide.

The change in the index of refraction profile changes the optical propagation characteristics of supermodes that are excited in the coupled waveguide structure. Control of chromatic dispersion and/or dispersion slope is effected dynamically by controlled alteration of the index of refraction and the dispersion characteristics of supermodes. Further details of supermode propagation and the dispersion characteristics thereof in coupled waveguide structures are described herein. Advantageously, the resultant chromatic dispersion and/or dispersion slope present in an optical signal may be adjusted to zero (i.e., compensated), positive, or negative by an illustrative method and apparatus of the present invention

It is noted, however, that these techniques to alter the index of refraction profile of the optical waveguide in a controlled manner are merely illustrative, and other techniques for altering the index of refraction profile of the optical waveguide in accordance with an exemplary embodiment of the present invention may be used. For example, an external mechanical force applied to the coupled wavguide structure can induce also refractive index changes. Moreover, the pumping of the secondary laser at a Raman resonance where the signal is amplified also can lead to substantial changes in the index of refraction profile in the coupled waveguide structure. Additionally, the use of electrooptic and acoustooptic glasses or crystals in the coupled waveguide structure can lead to control of the index of refraction profile via application of an electric field or an acoustic wave to the wavguide, respectively.

In accordance with an exemplary embodiment of the present invention, the coupled waveguide structure is an optical fiber having a cenfral core and a ring circumferentially thereabout. The core and the ring have indices of refraction which are greater than the index of refraction of the cladding layer between the core and the ring, and of the cladding material that surrounds the exterior of the ring.

The above described coupled waveguide structure is illustrative, and other types of optical waveguides which include coupled waveguide structures capable of supporting supermodes may be used in keeping with the teachings of the present invention. For example, an optical fiber containing more than one ring disposed circumferentially about a core may be used. Cladding material similar to that described in connection with the single ring fiber would be disposed between the successive rings. Additionally, planar coupled waveguides which can support supermodes may be used. To wit, multiple planar waveguides in a substrate may be used, such as those used in integrated optics. Of course, a variety of structures and materials may be used to achieve this end. Turning to Fig. 1, a cross-sectional profile of the refractive index versus radius of an illustrative coupled waveguide structure is shown. The refractive index of the core

101 and the refractive index of the ring 102 are shown as a function of radius for a coupled cylindrical waveguide structure. Moreover, the index of refraction of the cladding material 103 between the core 101 and the ring 102, as well as outside the ring

102 is shown. In the illustrative embodiment shown in Fig. 1, the core and ring are made of the same material suitably doped to effect a desired index profile. Alternatively, the core and cladding may be of different materials. Moreover, the indices of refraction of the cladding layers 103 between the core and ring, and outside the ring may be the same, or different than one another. It is further noted that the rings may be of the same, or different materials, and that the indices of refraction of these rings may be the same or different. In the case of multiple rings each ring can have a different refractive index.

Turning to Fig. 2, a graphical representation of the refractive index versus radius of another coupled waveguide structure is shown. Again, the coupled waveguide structure is illustratively a cylindrical coupled waveguide structure having a core 201, a ring 202, and cladding 203 disposed between the core 201 and ring 202; and outside the ring 202. The core 201 and the ring 202 in this illustrative embodiment are made of different materials. Again, the cladding layers may have the same or different indices or refraction. In coupled waveguide structures such as those shown graphically in Figs. 1 and 2, when light is coupled into the core of the waveguide, it can be coupled to the ring under certain conditions, and in a variety of ways. This type of coupling is generally known as directional coupling. Moreover, the eigenmodes of such a coupled waveguide system are referred to as supermodes. Further details of coupled waveguides and supermodes may be found for example in "Optical Electronics" (3^rd Edition) by Amnon Yariv, pages 437 - 447; and in "Dielectric Laser and Photonic Integrated Circuits", by Coldren and Corzine, pages 282 - 287. The dispersion characteristic of the fundamental and first order harmonic supermodes are used to effect dispersion compensation in accordance with an exemplary embodiment of the present invention.

The coupling between the ring and core causes the dispersion of such coupled optical waveguides to be governed mainly by the waveguide dispersion of the coupled system. Specifically, it can be shown that the group velocity dispersion (chromatic dispersion) may be approximated by:

where GND is the group velocity dispersion of the symmetric and asymmetric

supermodes; v_t and v₂ are the group velocities of the first and second waveguides,

respectively, at the angular frequency ω₀ (that corresponds to a wavelength λ₀); and K is

the coupling constant of the guides.

A few points in connection with equation (1) are at worthy of discussion presently. First, the coupling coefficient K can be shown to be proportional to the

refractive index profile of the coupled waveguide structure. Moreover, when ω = ω₀ , it

can be shown that the optical power is coupled with maximum efficiency between the two waveguides. This is known as the resonance condition of the supermode, and occurs when there is no phase velocity differential between the individual waveguide eigenmode velocities. At resonance, propagated mode experiences a maximum shift in its group velocity, resulting in a relative maximum in the chromatic dispersion. The present invention relates generally to dynamic manipulation of the index of refraction profile to change the chromatic dispersion characteristics as desired. Turning to Fig. 3, an example of a typical graph of the chromatic dispersion versus wavelength for a variety of coupled waveguide structures having different

coupling constants, K , is shown. At a particular wavelength, λ_Q , the chromatic

dispersion reaches a maximum peak 301 for the symmetric eigenmode (fundamental mode), and a minimum peak 302 for the asymmetric eigenmode (first order harmonic). As can be appreciated from a review of Fig. 3, the absolute value of the chromatic dispersion (positive or negative dispersion) is also dependent upon the coupling coefficient K . Moreover, from a review of equation (1), a change in K and the group velocities vi and v₂ will result in a change in the values around the wavelength λ₀. It

follows that a small change in the index of refraction of the core, ring or cladding (or a combination thereof) can lead to significant changes in the chromatic dispersion. Changes in the values the dispersion due to changes in the refractive index profile are also associated with shifts in the resonant wavelength. All these effects that change the dispersion values and shift the resonance are coupled and cannot be disassociated when the profile is changed, hi particular, changes in the index of refraction can result in a shift of the curves along the abscissa (wavelength axis) of the graph shown in Fig. 3.

Finally, it is noted that the dispersion slope (change in dispersion per unit change in wavelength) can be controUably altered by altering the index of refraction profile. As is known, dispersion slope impairments can adversely impact signal transmission. Dispersion slope impairments may result from optical waveguide (e.g. fiber) dispersion slope; dispersion slope from optical components and equipment in the optical transmission system; and from thermal fluctuations, which can alter dispersion slope. The control and correction of dispersion slope becomes increasingly important for high transmission rate systems operating with certain transmission formats. For instance, 40Gbps optical networks using RZ formats can be degrades when the received optical signal has lOOps/nm² or more of chromatic dispersion slope.

Accordingly, one advantage of the present invention as described in connection with exemplary embodiments is the capability to control the dispersion slope of an optical signal. The dispersion slope may be compensated (nullified), or may be adjusted so the net dispersion slope in the optical signal is positive or negative.

Turning to Fig. 4, a graph of experimentally measured chromatic dispersion versus wavelength for an exemplary coupled waveguide structure having a core and ring configuration in accordance with an exemplary embodiment of the present invention is shown for various temperatures. The coupled waveguide used in this exemplary embodiment is not the same as that having dispersion characteristics shown in Fig. 3, but follows the same physical principles described, hi particular, the chromatic dispersion of a core and ring coupled waveguide structure as previously described exhibits a wavelength dependence, which is also temperature dependent. Again, the change in temperature results in a change in the index of refraction profile for the coupled waveguides, which is manifest in a temperature dependence of the chromatic dispersion.

The drift in dispersion with temperature may be increased by using an optical fiber having the core and the ring made of different materials. In this case, depending upon the coefficients of variation of refractive index of the material with temperature, higher drift efficiencies can be achieved. The variation can be made more efficient if the coefficients of the variation of refractive index with temperature have opposite sign. For example, if the variation of index of refraction of the core with temperature is positive and the variation of the index of refraction of the ring with temperature is negative, the effects are more pronounced. Again, it is noted that the cladding material may be made of the same or different material than the core and/or the ring of the coupled waveguide. Moreover, it is noted that if more than one ring (and therefore more than one cladding layer) is incorporated into the coupled waveguide structure, the refractive indices of the subsequent ring(s) are typically not the same.

As referenced previously, altering the temperature of the coupled waveguide structure is one way to alter its dispersion characteristics. Another method involves the use of a secondary optical energy source. Illustratively, a secondary source of optical power may be input to the coupled waveguide structure resulting in nonlinear effects that can lead to a shift in the resonance wavelength. To this end, the refractive index of a material as a function of frequency varies with the intensity of the applied optical source. Specifically:

where n is the refractive index as a function of angular frequency ω and

E| , of the optical source; n₂ is the nonlinear refractive index of the material,

and Ε is the electrical field component of the light propagating in the wavguide. It is noted that although the energy of the optical signal is substantially evenly distributed between the core and the ring waveguides in the coupled waveguide system, in the case of excitation by a secondary optical source, it can be shown that for

wavelengths above the resonant wavelength, λ₀ , most of the power will be concentrated

in the ring. Therefore, depending on the wavelength of the pump and the optical signal traversing the fiber, different parts of the waveguide, namely the core and the ring, may be excited discreetly. This enhances the refractive index of one part of the waveguide while the other, which substantially does not couple radiation of the secondary source, remains unchanged. As can be appreciated, using the illustrative secondary optical source method, all- optical control of chromatic dispersion is feasible if the wavguide is designed to operate close to the resonance wavelength, and proper materials and optical power are chosen. This can be observed again in Fig. 3, where a theorectical fiber has its resonance around 1.56 μm, with high dispersion values. Typical changes in refractive index for achieving

tunable devices are between LrlO^""6 tobdO^-2 , depending on the dispersion values around the resonance.

In addition, the all-optical power control may be more highly enhanced by the use of different materials in the core and in the ring, as described previously. In this case,

materials with different nonlinear refractive indices n₂ can increase the shift in λ_Q with

optical power leading to a higher drift in the dispersion values. Finally, as can be readily appreciated, because chromatic dispersion is given in units of ps/nm-km, the fiber length can be long or short depending on the fiber design, and degree of dispersion compensation desired. In practical deployment, the fiber can be a straight length of relatively short fiber, or a coiled longer length of fiber.

It is further noted that the dynamic alteration of the index of refraction profile of the coupled waveguide optical fiber in accordance with the exemplary embodiments of the present invention may be effected by thermal control, secondary source optical power control, or a combination of both. Also the use of pressure (external force), electric field in electrooptic materials used as dopants in the fiber, and acoustic waves with use of elastooptic coefficients can be used as to dynamically alter the index of refraction profile of a coupled waveguide structure. A combination of techniques potentially offers a wider range of control.

Fig. 5 shows a dynamic dispersion compensation apparatus using all optical control in accordance with an exemplary embodiment of the present invention. A mulitplexer 501 is used to couple the input optical signal 502 with the secondary optical source signal 503. Illustratively, the secondary optical source is laser. Of course, other optical sources may be used for the secondary optical source.

The signals are input to a dispersion compensation module (DCM) 504 which includes coupled waveguide structures. The coupled waveguide structures are illustratively the coupled waveguide optical fiber having the core and ring configuration described above. A demultiplexer 505 is used to split off the output signal 506 which has been compensated for chromatic dispersion in accordance with an exemplary embodiment of the present invention. A bit-error rate analyzer 507 or similar system performance monitor maybe used to effect any changes in the input power of the secondary optical source 503. The changes in the input power are effected illustratively through the use of a laser controller 508.

Typical changes in refractive index for achieving tunable devices are in the range

of approximately 1 10^~δ to approximately lxl0^~2 , depending on the dispersion values around the resonance wavelength. Changes in dispersion values can be in the range of approximately -10⁵ ps/nm-km to approximately 10⁵ ps/nm-km depending on the profile used. The optical power supplied by the secondary optical source signal 503 to achieve the illustrated dispersion compensation is in the range of approximately 0W to approximately 1KW. The wavelength of the secondary optical source signal is illustratively in the range of approximately O.Olμm to approximately lOOμm .

Fig. 6 shows a dynamic dispersion compensation apparatus in accordance with an exemplary embodiment of the present invention using all thermal control. An input optical signal 601 is incident upon a dispersion compensation module (DCM) 602, which includes a coupled waveguide structure such as an optical fiber having a core and ring as described in accordance with the exemplary embodiments above. The temperature of the DCM 602 is controlled by thermal controller 603. A bit-error rate analyzer 604 or similar system performance monitor may be used to analyze the correction of the chromatic dispersion on the output signal 605. Any necessary changes in the temperature based upon the bit-error rate may be effected by the thermal controller in a feedback loop as shown. Typical changes in refractive index for achieving tunable devices are in the range

of approximately l l 0^"6 to approximately 0^-2 , depending on the dispersion values around the resonance wavelength. Changes in dispersion values can be in the range of approximately -lOOOOOps/nm-km to approximately +100000ps/nm-km depending on the profile used. Temperature variations to effect the dispersion compensation in the illustrated range are in the range of approximately -100 °C to approximately +100 °C.

Fig. 7 shows a combination of optical power control and thermal control used to effect dispersion compensation in accordance with an exemplary embodiment of the present invention. An input signal 701 is incident upon a muhtplexer 702 where it is combined with secondary optical signal 703. The combined signal is incident upon a dispersion compensation module (DCM) coupled waveguide of the DCM 704. The temperature of the DCM 704 is controlled by a thermal controller 708. The output of the DCM 704 is input to a demultiplexer 705. The output signal 706 is analyzed by a bit- error rate analyzer 707 to enable changes to be made in the degree of chromatic dispersion compensation by the DCM 704. This may be effected by changing the temperature with thermal controller 708 and/or by changing the input optical power via the laser controller 706.

It is noted that the various elements described in connection with the illustrative embodiment of Fig. 7 are substantially identical to those described in conjunction with the exemplary embodiment of Figs. 5 and 6. To wit, the common elements such as the bit-error rate analyzer 707, the thermal control 708, the secondary optical source 706, and the like are substantially the same as previously described. Likewise, in the combined optical power/thermal control dispersion compensation apparatus, typical changes in

refractive index for achieving tunable devices are approximately l l 0^"6 to approximately

lxl 0^~2 , depending on the dispersion values around the resonance wavelength. Changes in dispersion values maybe in the range of approximately -lOOOOOps/nm-km to approximately +100000ps/nm-km depending on the profile used. Temperature variations are approximately in the range of approximately -100°C to approximately +100°C and optical power from the secondary source is approximately 0W to approximately 1KW at wavelengths for pump and signal in the range of approximately O.Olμm to approximately lOOμm. It is further noted that the above referenced alternative techniques for achieving

CD compensation may be used individually or in combination in apparati similar in architecture to those described in connection with the exemplary embodiments of Figs. 5 - 7. These alternative techniques maybe electro optic or acousto optic in nature, and may be used individually or combined any may be used individually or combined with one or more of the techniques described herein. EXAMPLE I

In accordance with an exemplary embodiment of the present invention as described in the present example, a dispersion compensation module includes a coiled fiber and a temperature source which alters the temperature of the coiled fiber to selectively compensate the chromatic dispersion present in an optical signal traversing the coiled fiber.

As is described in further detail herein, the dispersion compensation apparatus and method in accordance with an exemplary embodiment of the present invention uses an optical fiber which is specially designed for dispersion compensation. Illustratively, the dispersion compensation fiber provides chromatic dispersion compensation values that are approximately equal in value to the chromatic dispersion present in an optical signal but, of course, having opposite sign so the net chromatic dispersion is zero. To wit, if the dispersion in the optical signal is positive, the compensating fiber usefully provides a negative dispersion of equal magnitude to approximately cancel the overall affect of the dispersion in the optical signal. However, it is noted that there may be instances where it is desired to have the net chromatic dispersion be non-zero. Of course, the illustrative embodiments may be adapted to effect this result. As a result of the dispersion compensation method and apparatus in accordance with an exemplary embodiment of the present invention, dispersion compensation may be dynamically controlled in optical communications systems having transmission rates on the order of approximately 10Gbit and greater. Moreover, this dispersion compensation maybe effected over a relatively broad wavelength band, illustratively including wavelengths in the range of approximately 1530nm to approximately 1570nm. Finally, in a manner which will become more clear as the description proceeds, in addition to dispersion compensation, the method and apparatus in accordance with exemplary embodiments of the present invention enable control of the slope of the dispersion curve across the wavelength band of chromatic dispersion compensation. Turning to Fig. 8, a dispersion compensation module 800 in accordance with an exemplary embodiment of the present invention is shown. An input optical fiber 801 is optically coupled to a fiber coil 802. The fiber coil 802 is disposed in a thermally sealed package 803. A controller 804 may be used to effect a change in the temperature of the fiber coil 802. The temperature control of the fiber coil 802 maybe an open loop control scheme without feedback of the system performance (e.g. a temperature is set and maintained at a constant level). Alternatively, the temperature control of the fiber coil 802 maybe a closed-loop control scheme which dynamically adjusts the temperature of the coil based on system performance feedback. For example, the closed-loop control scheme could be based upon the measured bit-error rate BER) of the system. The temperature of the coil could be varied over a certain range (e.g. approximately -40°C to approximately 100°C) to control the bit-error rate. Of course, the BER of the system is but one measure of system performance. Other measures of performance could be used in the illustrative closed-loop control scheme.

It is noted while the above description of control schemes is directed primarily to control of the temperature of the fiber coil in either an open loop or a closed loop scheme, other techniques may be used to effect refractive index changes to dynamically compensate CD in an optical signal in accordance with exemplary embodiments of the present invention. For example, controller 804 may be used to vary the index of refraction of fiber coil 802 by varying the radius of curvature of the coil and/or the tension on the fiber. In one illustrative embodiment where the controller 804 controls the stress forces on the fiber coil 802 and the radius of curvature of the fiber coil 802, the fiber coil 802 may be disposed about a cylindrical (e.g. a mandrel) or other suitably shaped element, which may be controUably rotated via the controller 804. Alternatively, the fiber coil 802 may be statically disposed, and the controller 804 would only change the temperature of the coil.

Finally, as will become more clear as the description of the present exemplary embodiment proceeds, the control of the index of refraction may be achieved using one of the exemplary techniques, or by using a combination thereof. Regardless, this CD compensation may be achieved in a closed-loop control scheme or an open-loop control scheme. The optical fiber, which is wound to comprise the fiber coil 802, is particularly designed for dispersion compensation in accordance with exemplary embodiments of the present invention. As is known, at typical telecommunication and data communication wavelengths of operation, conventional dispersion compensation fibers are specifically designed to operate in a region where waveguide dispersion is greater in magnitude than material dispersion. Typically, this is in the region of approximately 1.55 μm , as the

material dispersion of the silica based optical fiber usually crosses zero dispersion level and turns positive at approximately 1.33μm .

Non-zero dispersion shifted fibers operate with almost zero dispersion values, and are designed by changing the waveguide parameters and shifting the waveguide dispersion and consequently the overall chromatic dispersion, which includes the waveguide and material dispersion, to values near zero at approximately l.55μm . The

result in many cases is a dispersion compensating fiber that has almost zero dispersion values in the operational wavelength, but which usually presents a positive dispersion curve slope.

As the bit rates of optical communications systems exceed lOGbit/sec, it is useful to compensate both for the absolute dispersion value and for the dispersion slope to guarantee low bit-error rate transmission across the chosen wavelength band. This is particularly the case in multi-channel wavelength division multiplexing (WDM) and (DWDM) optical systems.

The optical fiber which is used in the fiber coil 802 in accordance with exemplary embodiments of the present invention is designed to aid in the compensation of both the slope and dispersion at all wavelengths of operation. The optical fibers used in fiber coil 802 typically have a relatively small core diameter and a relatively high refractive index difference between the core and the cladding layer when compared to conventional single mode fibers such as SMF-28.

The above referenced characteristics of the optical fiber (or other optical wavguide) result in the chromatic dispersion characteristics of the fiber being particularly sensitive to index of refraction changes. For example, the chromatic dispersion in the optical fiber of fiber coil 802 is particularly sensitive to changes in the temperature of the fiber. Moreover, as will become more clear as the description proceeds, the radius of curvature of the optical fiber as well as the tension/stress placed upon the optical fiber can also impact the degree of dispersion compensation versus wavelength for particular optical fibers used in accordance with an exemplary embodiment of the present invention.

Turning to Fig. 9, the dispersion versus wavelength is shown for an illustrative optical fiber at various temperatures. As can be seen, when the optical fiber is at a temperature of -5° C, the dispersion curve 901 exhibits a particular dispersion profile versus wavelength. At a temperature of +25° C, the same fiber has a different slope (curve 902), which has a downward profile versus wavelength. At this temperature, the fiber has slightly lower levels of chromatic dispersion for a particular wavelength. Finally, as can be seen, the same optical fiber at a temperature of 55° C (curve 903) exhibits decreasing chromatic dispersion with wavelength, but with less magnitude at each given wavelength than the previous two temperatures. Again, the profiles shown in Fig. 9 are for a typical optical fiber in accordance with an exemplary embodiment of the present invention. As can be appreciated from a review of Fig. 9, the variation of the chromatic dispersion versus wavelength as a result of changes in the temperature can be used to selectively control the degree of chromatic dispersion compensation in an optical signal, hi the illustratively embodiment shown in Fig. 8, the optical fiber in accordance with the exemplary embodiments of the present invention may be disposed in the fiber coil 802 and suitably heated or cooled to change the degree of dispersion over the particular wavelength band, h the dispersion compensation module 800, this would be effected by controlling a temperature source (not shown) with the controller 804.

It is noted that in the exemplary embodiment shown in Fig. 8, as well as other exemplary embodiments of the present example, the optical fiber is disposed in a coiled manner. While this is illustrative of the embodiments of the present example, it is not necessary. To wit, the fiber may be coiled as shown in the exemplary embodiments of Figs. 8, 12 and 13. Alternatively, the fiber maybe completely uncoiled, and just suitably heated/cooled to effect dispersion compensation in an optical signal. Finally, optical waveguides other than optical fibers may be used in keeping with the exemplary embodiments of the present invention. These waveguides may or may not be coiled. The index of refraction of these optical waveguides may be selectively dynamically altered by the techniques as described herein.

As referenced previously, the behavior of the optical waveguide used for chromatic dispersion compensation in accordance with exemplary embodiments of the present invention may be tightly or loosely coiled. To this end, the radius of curvature of the illustrative optical fiber may be relatively small or relatively large, depending upon application. Moreover, it may be useful to introduce stress on the fiber in order to alter the index of refraction, and ultimately the degree of chromatic dispersion. This may be done, for example, by disposing the fiber coil about a mandrel which may be rotated to increase or decrease the tension on the fiber, as is desired. In accordance with the exemplary embodiment shown in Fig. 8, the controller 804 may be used to rotate a servo motor which is connected to a mandrel about which the fiber coil 802 is disposed. It is noted that the radius of curvature and stress on the fiber can be varied independently. For example, a dispersion compensating fiber may be tightly wrapped about a mandrel having a relatively large radius. In this case both the radius of curvature and the stress on the fiber would be relatively large. It is further noted that the dispersion compensation in accordance with an exemplary embodiment of the present invention may be effected through the variety of factors described taken individually or in combination. For example, the use of temperature alteration is merely exemplary, and could be used alone, in combination with other techniques, or foregone in lieu of other techniques used alone or in combination. As described, a variety of techniques for altering the index of refraction to introduce dispersion compensation may be used. Again, these may be effected individually or in combination with one another and/or with temperature variation. Turning to Fig. 10, the change in dispersion versus wavelength is shown for five different dispersion compensating fibers maintained when the temperature is changed from -5°C to +55°C in accordance with an exemplary embodiment of the present invention. The dispersion compensating fibers of the exemplary embodiment shown in Fig. 10 have a relatively low radius of curvature. In this illustrative embodiment, the relatively small radius of curvature is accompanied by a relatively high tension/stress placed on the fiber. For a particular temperature, the different optical fibers shown in Fig. 10 exhibit a different degree of dispersion over wavelength. As can be appreciated, over the particular wavelength band shown, the various fibers in accordance with an exemplary embodiment of the present invention exhibit dispersion variation ranging from approximately 4% to approximately 12%. The degree of dispersion variation depends upon the particular fiber used and its thermal sensitivity in terms of waveguide design. It is also noted that the general trend for the relatively tightly wound fiber coils of the illustrative embodiment of Fig. 10 is a decrease in the dispersion variation with increasing wavelength. Turning to Fig. 11, the total dispersion variation versus wavelength for three different optical fibers having the same relatively large radius of curvature is shown. Illustratively, the tension/stress imposed on the fiber is relatively low. Again, the temperature is changed in the range of -5°C to +55°C for each optical fiber shown graphically in Fig. 11. As can be appreciated from a review of Fig. 11 , the various optical fibers at the particular temperature have significantly different dispersion variations over wavelength. However, as can be appreciated, the total dispersion variation generally increases with increasing wavelengths.

From the above description, it can be appreciated that changes in the temperature, the radius of curvature of the coiled optical fiber, and the tension/stress imposed thereon, alone or in combination enable a wide range of chromatic dispersion compensation over a relatively broad wavelength band. Moreover, it is noted once again that other optical waveguides may be used, and that it is not always necessary to coil the fibers. According to an exemplary embodiment, one or more coiled fibers may be deployed in a dispersion compensation module such as dispersion compensation module 800. For example, it may be useful to have a fiber which has a relatively low radius of curvature in one coil and another fiber which has a relatively high radius of curvature in another coil. Ultimately, a variety of dispersion compensation values and slopes may be realized with such a dispersion compensation module. As before, if desired, the temperature of the optical fibers of the dispersion compensation module 800 may be varied and controlled using controller 804 to effect a desired amount of chromatic dispersion compensation in an optical signal traversing optical fiber 801. Finally, the tension on the individual fiber coils as well as the radius of curvature may be dynamically controlled in a manner described previously.

Turning to Fig. 12, a dispersion compensation module 1200 in accordance with another exemplary embodiment of the present invention is shown. The dispersion compensation module 1200 includes substantially all of the elements of the dispersion compensation module 800 of Fig. 8, with some noteworthy additions. Accordingly, the details of the similar elements and their characteristics will be omitted in the interest of brevity, and only distinctions therebetween will be described in detail.

Dispersion compensation module 1200 in accordance with the exemplary embodiment of the present invention includes an input optical fiber 1201 and a fiber coil 1202. A controller 1205 usefully controls the temperature of the fiber coil and/or tension or stress on the fiber coil as well as its radius of curvature. The optical fiber 1201 used in fiber coil 1202, the controller 1205, and the dynamic control of dispersion compensation are as previously described. In accordance with the exemplary embodiment shown in Fig. 12, mode strippers 1203 are usefully employed. These mode strippers 1203 are disposed in the thermally sealed package 1204. The mode strippers 1203 illustratively include optical fibers or other suitable optical waveguides disposed about a mandrel. The optical fiber/waveguide typically exhibit the same dispersion characteristics as those of optical fiber 1201. The mandrel typically has a radius of curvature which is substantially smaller than the radius of curvature of fiber coil 1202. The mode strippers can be used to induce micro-bending in the optical fiber disposed thereabout. Ultimately, this can improve the dispersion variation, especially in the case where the optical fiber supports multiple modes. It is noted that the mode strippers 1203 may also be dynamically controlled by controller 1205. To this end, the radius of curvature of the fibers used in the mode strippers 1203 may be altered to effect a desired degree of dispersion variation.

Turning to Fig. 13, a dispersion compensation module 1300 in accordance with another exemplary embodiment of the present invention is shown. An input optical fiber 1301 is coupled to a first pin array 1302. The pin array 1302 is coupled to a first mode stripper 1303, which is coupled to fiber coil 1304. Output from the fiber coil 1304 is input to a second mode stripper 1304, which is coupled to a second pin array 1305. All of these elements are disposed in a thermally sealed package 1307. A controller 1306 may be used to control the temperature, the radius of curvature, and the tension on various elements of the dispersion compensation module 1300. As can be appreciated, many of the elements shown in Fig. 13 are substantially the same and carry out substantially the same function as those common elements of the exemplary embodiment shown in Fig. 12. Accordingly, repetition of these details has been omitted, and only distinctions described in detail. First and second pin arrays 1302 and 1305 are particularly useful in introducing micro-bends in the optical fiber. The first pin arrays 1302 arrays are merely illustrative, and other elements could also be used to induce micro-bends in the optical fiber to improve the dispersion variation. It is also noted that the micro bending of the fiber may be dynamically controlled and varied through the use of controller 1306 as well as suitable mechanisms for variation of the micro-bends. The micro-bend can be effected in a variety of ways. Examples include the use of an array of pins of various sizes and spacing therebetween. Moreover, a mandrel that is adapted to expand/contract may be used. Alternatively, a mesh of metallic wire may be used to selectively exert pressure on the fiber. Still other techniques are possible for inducing microbends, as would be readily apparent to one having ordinary skill in the art. EXAMPLE II

The methods and apparati described thus far dynamically vary the refractive index profile of a dispersion compensating waveguide to exact the a desired amount of chromatic dispersion compensation and/or dispersion slope compensation at particular portion of a wavelength band. However, the exemplary embodiments described in the present example enable chromatic dispersion compensation, dispersion slope compensation, or both in a static manner.

In accordance with an exemplary embodiment of the present example, an optical apparatus includes an integer number (n) of serially connected dispersion compensating optical waveguides. Each of the dispersion compensating waveguides is chosen for its particular dispersion characteristic at a particular portion of a wavelength band over which the optical apparatus is chosen to operate. Moreover, because dispersion is normally measured in units of ps/nm-km, an appropriate length of each of the serially coupled dispersion compensating optical waveguides is chosen to exact a desired amount of dispersion compensation at each portion of the wavelength band.

Accordingly, as will become more clear as the description of the present example proceeds, by the appropriate selection of each dispersion compensating optical waveguide for its dispersion characteristic at a particular portion of the wavelength band over which the optical apparatus operates; and the appropriate selection of the length of each waveguide to exact a desired amount of dispersion compensation, a dispersion characteristic may be synthesized for an optical apparatus in accordance with an exemplary embodiment of the present invention. This synthesized dispersion characteristic may be chosen to nullify the chromatic dispersion of an optical signal, the dispersion slope of an optical signal, or both. Additionally, or alternatively, this synthesized dispersion characteristic maybe chosen to yield a net amount of positive or negative chromatic dispersion in an optical signal, as well as a net positive or negative dispersion slope in the optical signal.

Turning to Fig. 14, an optical apparatus 1400 in accordance with an exemplary embodiment of the present invention includes n-dispersion compensating waveguides optically coupled in series via known optical coupling techniques. Illustratively, a first dispersion waveguide 1401 is serially coupled to a second dispersion compensating waveguide 1402, etc., with an (n-l)^th dispersion compensating waveguide (not shown) coupled to an n^th dispersion compensating waveguide 1403 completing the optical apparatus 1400. For purposes of illustration and not limitation, the number of dispersion compensating waveguides (n) is illustratively greater than or equal to (n 2) two and less than twenty (n < 20 ).

An input optical signal 1405 traverses an input optical waveguide 1404 of an optical communications system. The input optical signal 1405 is illustratively a WDM or DWDM signal having a plurality of optical channels; each channel having a prescribed center wavelength, channel bandwidth, and each channel being spaced from its adjacent channels by a prescribed channel spacing.

As mentioned above, various sources of chromatic dispersion contribute to the overall chromatic dispersion present in the input optical signal 1405. Moreover, the chromatic dispersion can vary over wavelength, contributing to dispersion slope, which can have deleterious effects on the signal quality. The resultant chromatic dispersion present in the optical signal from the various sources of chromatic dispersion has a particular dispersion 'shape' or curvature (often referred to as the dispersion characteristic) over a particular wavelength band (e.g. the wavelength band of the multiplexed input optical signal 1405).

For reasons which will become more clear as the present description continues, each of the dispersion compensating optical waveguides of the optical apparatus 1400 is chosen for its chromatic dispersion characteristics and/or its dispersion slope characteristics over a chosen wavelength band. The resultant chromatic dispersion and dispersion slope characteristics of the serial chromatic dispersion compensating waveguides is illustratively nullifies the dispersion and/or dispersion slope present in an optical signal. To wit, the optical apparatus 1400 is designed to have a dispersion curvature over the wavelength band of the input optical signal 1405 which is at every point of the wavelength band equal in magnitude, but opposite in sign to the dispersion curvature that adversely impacts the input optical signal 1405.

Accordingly, in the present illustrative embodiment, the dispersion and/or dispersion slope present in the input optical signal 1405 has been nullified so that the output signal 1407of the output optical fiber 1406 is substantially free of chromatic dispersion and/or dispersion slope. However, as mentioned previously, the optical apparatus 1400 maybe used to introduce chromatic dispersion and/or dispersion slope to the input optical signal 1405 so that the output optical signal 1407 has a finite (positive or negative) net chromatic dispersion, dispersion slope, or both. Moreover, it is noted that over the selected wavelength band, the net chromatic dispersion may be positive over one or more wavelength sub-bands (of the wavelength band), negative over one or more wavelength sub-bands and zero over one or more wavelength sub-bands. Likewise, the dispersion slope dispersion may be positive over one or more wavelength sub-bands, negative over one or more wavelength sub-bands and zero over one or more wavelength sub-bands.

The optical apparatus 1400 may be used in a variety of applications. To this end, the optical apparatus 1400 may be an integral part of the transmission fibers of the optical communication system, and/or the apparatus 1400 maybe a stand-alone module comprising a single optical apparatus or a plurality of such optical apparati connected in series. Moreover, the n-dispersion compensating waveguides are illustratively dispersion compensating optical fibers of the types described below in the present example.

The n-serially coupled dispersion compensating optical waveguides of the optical apparatus 1400 may be chosen from the various types of dispersion compensating optical waveguides described presently. It is noted that these dispersion compensating optical waveguides are intended to be illustrative, and in no way limiting of the present invention. Moreover, it is noted that the n-serially coupled compensating optical waveguides may all be one type of dispersion compensating waveguide; or a combination of various types of dispersion compensating optical waveguides, chosen for their particular chromatic dispersion and/or dispersion slope characteristic over a particular wavelength band.

Examples of dispersion compensating optical wavguide that maybe used for some or all of the n-serially dispersion compensating optical waveguides of the optical apparatus 1400 are selected lengths of known dispersion compensating optical fibers. Moreover, the dispersion compensating optical waveguides may be a plurality of dispersion compensating gratings such as fiber Bragg gratings (FBG), which may be chirped linearly and/or non-linearly, chirped. Of course, in keeping with the present example, each individual dispersion compensating optical waveguide of the optical apparatus 1400 is chosen to contribute a certain portion of the overall curvature of the dispersion of the optical apparatus over a desired wavelength range so that the output optical signal 1407 has a desired magnitude of dispersion (positive, negative or zero), and dispersion slope (also positive, negative or zero), at each point of its wavelength band. In addition to the dispersion compensating waveguides described thus far, coupled waveguides may be used as some or all of the n-serially coupled dispersion compensating optical waveguides in accordance with the presently described exemplary embodiment. These coupled waveguides may be of the type described in detail above. It is noted that the index of refraction profile of this coupled waveguide structure may be dynamically altered by methods described in detail above. This enables the dispersion characteristic of the coupled waveguide structure to be selectively varied. Alternatively, the index of refraction profile may be substantially static, with the selected parameters of equation 1 remaining fixed.

Turning to Fig. 15, an example of a typical graph of the chromatic dispersion versus wavelength for a variety of coupled waveguide structures having different

dispersion reaches a maximum peak 201 for the symmetric eigenmode (fundamental mode), and a minimum peak 202 for the asymmetric eigenmode (first order harmonic).

As can be appreciated from a review of Fig. 2, the absolute value of the chromatic dispersion (positive or negative dispersion) is also dependent upon the coupling coefficient K . Moreover, from a review of equation (1), a change in K and the group velocities vi and v₂ will result in a change in the values of the group velocity dispersion

around the wavelength λ₀. It follows that a small change in the index of refraction of the

core, ring or cladding (or a combination thereof) can lead to significant changes in the chromatic dispersion. Changes in the values the dispersion due to changes in the refractive index profile are also associated with shifts in the resonant wavelength, λ₀. As referenced above, for each of the n-serially coupled optical waveguides of an exemplary embodiment, the choice of parameters such as the index of refraction of the core, ring or cladding may be set for a desired dispersion curve; may be selectively varied to achieve a desired dispersion curve; or a combination of these techniques may be used. All these effects that change the dispersion values and shift the resonance are coupled and cannot be disassociated when the profile is changed. In particular, changes in the index of refraction can result in a shift of the curves along the abscissa (wavelength axis) of the graph shown in Fig. 16, as well as the shape of the dispersion. This enables tuning of the dispersion characteristic and, thereby, the chromatic dispersion and dispersion slope.

By using coupled waveguides for one or more of the n-serially coupled dispersion compensating waveguides of the present invention, selection of a desired chromatic dispersion characteristic with a particular dispersion slope may be achieved. Alternatively, use of coupled waveguide structures for one or more of the n-serial dispersion compensating waveguides of the present invention enables tuning of the chromatic dispersion and/or dispersion slope. This tuning may be used to dynamically change the net dispersion and/or dispersion slope of the output optical signal 1407. Further details of coupled waveguides structures as well as dynamic dispersion and/or dispersion slope compensation using coupled waveguide structures are described in detail above.

As shown in Fig. 15, the dispersion characteristics of the coupled waveguide dispersion compensating waveguides can be approximately flat, linear, concave or convex in shape as is indicated. Moreover, as will become more clear as the present description continues by suitable selection of variables from eqn. (1) (e.g. coupling coefficient K ) for a particular coupled fiber, a particular curvature may be selected over a particular wavelength range. This enables the synthesis of the resultant dispersion characteristic of the n-serially coupled dispersion compensating optical waveguides of the optical apparatus 1400.

Fig. 16 shows examples of the dispersion slope (change in dispersion per unit change in wavelength) over a wavelength band with coupled waveguide structures having various coupling coefficients.

From the above description of the present example, it is appreciated that a variety of dispersion curves are available depending on the selection of the serially coupled dispersion compensating waveguides. The synthesis of a desired dispersion curve to exact a desired amount of dispersion and/or dispersion slope compensation is readily achieved by the selection of certain dispersion compensating waveguides and their respective lengths. An exemplary embodiment incorporating illustrative serially coupled dispersion compensating waveguides is described presently.

Fig. 17 shows examples of dispersion characteristics that maybe achieved using serially connected coupled waveguide structures (e.g., the dispersion compensating fibers described above) in accordance with an exemplary embodiment. To this end, by coupling two or more coupled waveguides in series, a multiformity of dispersion characteristics may be realized over a particular wavelength bands. These dispersion characteristics are illustratively linear of positive slope, linear of negative slope, flat, concave up, concave down, convex up and convex down. (It is noted that high positive dispersion values and characteristics may normally require the excitation and propagation of higher order modes in the coupled waveguide structure.) Moreover, each of these curves may be positive or negative; and it is possible to synthesize a dispersion curve that is positive over a particular portion of the wavelength band, negative over another, and zero over a transition region therebetween. Again, the resultant dispersion characteristic of the n-serially coupled dispersion compensating waveguides depends on the number, the dispersion characteristics and the lengths of the dispersion compensation compensating dispersion compensating waveguides (in this fibers) chosen. h addition, the dispersion characteristic over the wavelength band may be synthesized to have one shape over one portion of the wavelength band, and other shapes over other portions of the wavelength band. Finally, it is noted that the slope of the linear curves, the radii of curvature of the convex/concave curves and the magnitude of the flat curves are merely illustrative. Each may be increased/decreased by the suitable selection of the coupled waveguide structures and the choice of their respective lengths.

Fig. 18 shows the chromatic dispersion versus wavelength for three illustrative dispersion compensating optical fibers (fibers A,B and C) having particular dispersion and dispersion slope characteristics.

As can be appreciated from a review of Fig. 18, fibers A, B and C have different nominal dispersions per Km. As such, the association of these fibers with different lengths would lead to different results.

Fiber A has a negative dispersion characteristic with a negative concave slope over the chosen wavelength band. Fiber B has a positive dispersion characteristic with a slight positive linear slope, and Fiber C has a negative dispersion characteristic with no slope. Again it is noted that the association of these fibers with different lengths would produce completely different results. One can use this association to match dispersion values and dispersion slope values at a wider range of wavelengths. For example, the selective serial connection of 1.5 Km of fiber A, 0.5 Km of fiber B and 1 Km of fiber C leads to the resultant dispersion curves shown in Fig. 19(a). As can be appreciated from a review of Fig. 19(a), the combination of these illustrative lengths of the illustrative dispersion compensating fibers each having different dispersion lengths can lead to a different value of dispersion and dispersion slope at each point along the curve. In the present exemplary embodiment, each selective serial connection of the chosen lengths of fibers A,B and C results in a dispersion characteristic having an overall negative dispersion across this wavelength band, and a negative concave dispersion slope. It is emphasized that the dispersion characteristics shown in Fig. 18 are merely illustrative of the possible combinations that can be realized using the three exemplary optical fibers. For example, Fig 19(b) shows dispersion characteristic realized by the serial coupling of 5 Km of fiber B, and 1 Km of fiber C. In this illustrative embodiment, the dispersion characteristic exhibits positive dispersion and dispersion slope over the illustrative wavelength band.

It is also emphasized that the examples described in connection with Figs. 19(a) and 19(b) are merely illustrative of the invention of the present disclosure. To be sure, other dispersion compensating optical waveguides of other lengths may be chosen to achieve a variety of results. The choice of the particular lengths of particular waveguides having desired dispersion and dispersion slope characteristics, when coupled serially, enables a wide variety of dispersion characteristics over a selected wavelength range. EXAMPLE III

The focus of the disclosure to this point has been on various apparati and methods of dynamic and static control of chromatic dispersion and/or dispersion slope in an optical signal is disclosed. In accordance with exemplary embodiments described in connection with the present example, the principles described thus far are implemented in a control architecture.

In accordance with one exemplary embodiment of the present example, each individual demultiplexed wavelength channel is input to a respective dispersion control module. Each dispersion control module includes at least one coupled waveguide structure which selectively and tunably introduces chromatic dispersion and/or dispersion slope to the optical signal. Advantageously, positive and negative chromatic dispersion and/or dispersion slope can be introduced into the optical signal using a relatively simple construct of one or more coupled waveguide structures.

The chromatic dispersion and dispersion slope introduced to the optical signal are dynamically tuneable and may be variable at a particular wavelength or wavelengths. It is noted that variation of the magnitude of the slope or chromatic dispersion is generally referred to as the variability of the chromatic dispersion adjustment or dispersion slope adjustment at a particular center wavelength. Moreover, each of the dispersion control modules in accordance with exemplary embodiments of the present invention may be adapted to operate over one or more wavelength channels, depending on tolerance requirements and signal conditions.

As will become clearer as the present description proceeds, the chromatic dispersion control module provides a great deal of versatility. For example, the chromatic dispersion control module according to an exemplary embodiment may compensate for CD in the optical signal. To this end, the chromatic dispersion present in the optical signal at the input of the CD control module will be nullified (i.e. compensated) at the output by introducing an equal but opposite amount of CD to the signal. Likewise, any dispersion slope may be nullified by the chromatic dispersion control module.

Alternatively, in accordance with an exemplary embodiment of the present invention it may be useful to introduce corrective, conditioning or altering chromatic dispersion and/or dispersion slope to an optical signal to bring these parameters to some required (non-zero) level. For example, this may be desirable at amplifier locations, and/or at a wavelength add-drop module (WADM) where it may not be desired to nullify chromatic dispersion and/or dispersion slope present in an optical signal. Instead, it may be desirable to have a net positive or negative chromatic dispersion value or dispersion slope value in an optical signal. In addition, it may be desirable to use the CD control module in accordance with an exemplary embodiment of the present invention to effect dispersion equalization, wherein the chromatic dispersion of all channels is adjusted to some desired smoothly varying function. This is analogous to a gain-equalizer at an add/drop module where it is desired to have the optical power in all channels brought to some predetermined level. Equally, it may be advantageous to take all wavelength channels and set them to some optical CD profile.

Turning to Fig, 20, an optical device 2000 in accordance with an exemplary embodiment of the present invention is shown. A multiplexer 2002 optical multiplexes a plurality of individual wavelength channels 2001 at a transmit section. The multiplexed optical signal is input to an optical amplifier 2003, which is illustratively an erbium doped fiber amplifier, well known to one having ordinary skill in the art. A first dispersion control module 2004 maybe included in the amplifier stage. The amplified signal is output from the amplifier 2003, and a second dispersion control module 2005 selectively introduces chromatic dispersion and/or dispersion slope to the output signal from the amplifier 2003.

The optical transmission system may include a wavelength add/drop module 2006. Such a module is useful in adding or dropping optical channels to or from an optical signal. A third dispersion control module 2012 can be connected to the WADM so as to selectively condition the chromatic dispersion as the wavelength channels are added onto the primary optical route. As referenced above, this conditioning is particularly advantageous in WADM applications.

Ultimately, the multiplexed optical signal is incident upon a demultiplexer 2007. The output from the demultiplexer 2007 is a plurality of individual wavelength channels

2008. The n-individual wavelength channels have respective center wavelengths λ_x , λ₂ ,

λ₃ , ..., λ_n , and the bandwidth of each individual wavelength channel generally complies

with a well-known telecommunications standard, such as the International Telecommunications Union (ITU) standard. Each of the plurality of individual wavelength channels 2008 may have an unacceptable degree of chromatic dispersion and/or dispersion slope. As is well-known to one of ordinary skill in the art, sources of this chromatic dispersion may be the optical devices and elements of the optical communication system; and/or ambient and environmental factors to include mechanical strains and stresses, and temperature.

The tolerances for chromatic dispersion and dispersion slope in optical systems become increasingly tighter as the transmission rates increase. For example, the tolerances for dispersion compensation in a 40 Gbit/sec optical communication system is approximately 16 times more stringent than that of a 10 Gbit/sec optical communication system (i.e. the multiplier of the data-rate to a power of two). Accordingly, it may be useful to compensate for chromatic dispersion and/or dispersion slope not only during the transmission of the optical signal, but also after demultiplexing of the individual wavelength channels at the receiver end of an optical communication system.

The demultiplexed wavelength channels are each incident upon a respective one of the plurality of receiver dispersion control modules (r-DCM) 2009 in the illustrative embodiment of Fig. 20. Of course, this is merely illustrative, and is described more fully herein one or more individual wavelength channels may be input to an r-DCM. The receiver dispersion control modules in accordance with the exemplary embodiment of Fig. 20 enable selective chromatic dispersion adjustment and dispersion slope adjustment in a tunable manner and variable using substantially coupled optical waveguide structures. The tunability and variability of the chromatic dispersion adjustment and dispersion slope adjustment is effected in the module by altering the index of refraction of the one or more optical fibers of the dispersion control module 2009. Ultimately, the output from the wavelength channels which have been compensated for chromatic dispersion are input to a receiver 2010. The receiver dispersion control modules 2009 usefully include at least one coupled optical waveguide structure (not shown). The chromatic dispersion exhibited by the coupled waveguide(s), which support supermodes at a resonance frequency, may be changed in magnitude sign and slope by altering the index of refraction of the waveguide. Further details of optical waveguide(s) used for chromatic dispersion adjustment and dispersion slope adjustment, as well as illustrative mechanisms to selectively alter the index of refraction profiles of the coupled waveguides are described above. Accordingly, the receiver dispersion control modules 2009 illustratively incorporate and exploit the optical waveguides and mechanisms of the exemplary embodiments described above. It is also noted that the first and second dispersion control modules, 2004 and

2005, respectively are usefully relatively wide band dispersion control modules. These may be based on the dispersion control methods and apparati of the exemplary embodiments described above. Alternatively, these dispersion control modules may be based on a dispersion control technique above. One such example of the use of the coupled optical waveguides is shown graphically in Fig. 21(a). Fig. 21(a) is a graph of the dispersion versus wavelength for a receiver dispersion control module 2009 in accordance with an exemplary embodiment of the present invention. The receiver dispersion control module illustratively includes three waveguides. Each of these waveguides is a coupled waveguide structure as described above. A first waveguide has a first dispersion curve 2101, and a second waveguide has second dispersion curve 2102. A third waveguide which has a third dispersion curve 2103, is particularly useful in controlling the slope of the dispersion. In the exemplary embodiment shown in Fig. 21(a), the dispersion curves are those of the first harmonic supermodes (also referred to as the symmetric supermode or LP02) of the coupled waveguides. The first harmonic supermodes are usefully excited using a mode converter (not shown). In the present illustrative embodiment, the added chromatic dispersion is positive. Accordingly, the dispersion adjustment provided by this module is positive. As described herein, negative dispersion adjustment may be achieved by exciting the fundamental supermodes (also referred to as the asymmetric supermodes or LP01).

The first dispersion curve 2101 is for a supported optical mode of a first optical

waveguide having a resonance wavelength of ω_0l , which corresponds to a resonant

wavelength of L₀₁ . Likewise, the second dispersion curve 2102 of the second waveguide

has a resonant wavelength of λ₀₂ , and the third dispersion curve 2103 have a third

resonant wavelength λ₀₃ . At the junction of the first and second curves 2104, a

particular amount of chromatic dispersion may be introduced by the receiver dispersion

control module. This junction 2104 is chosen to be at a particular wavelength λ_c , which

corresponds to the center wavelength of the particular wavelength channel which traverses this particular dispersion control module. In addition to introducing a particular amount of chromatic dispersion adjustment, D_a, the slope of the chromatic dispersion adjustment may also be controlled by the third waveguide having dispersion curve 2103.

As can be appreciated, the chromatic dispersion introduced in the present exemplary embodiment is positive, while the slope of the dispersion is negative. Moreover, in the present exemplary embodiment the chromatic dispersion and dispersion slope of the optical signal input to the receiver dispersion control module are adjusted to zero (i.e. compensated) by the introduction of CD and dispersion slope having equal magnitude but opposite sign to that present in the optical signal. This is merely illustrative of the present invention, and as discussed above, according to an exemplary embodiment of the present invention the CD control modules may adjust the CD and dispersion slope to net positive or negative values.

By virtue of the ability to change the index of refraction of the particular coupled waveguides of the dispersion confrol modules of the exemplary embodiment, the resonant frequency/resonant wavelength of the individual waveguides may be changed. To wit, as is explained above in connection with dispersion compensation using coupled waveguides structures, the dispersion curves can be shifted. This enables "tuning" of each of the individual optical waveguides of the dispersion control module. This "tuning" is shown generally with arrows in the positive and negative directions along the wavelength axis in Fig. 21(a). Accordingly, the magnitude of the chromatic dispersion

adjustment at the particular center wavelengths λ_c may be changed as needed through

the change in the index of refraction. Moreover, shifting the third dispersion curve 2103 can result in a change in the magnitude and sign of the dispersion slope adjustment. Additionally, shifting of the curves (e.g. shifting of curve 2102 to curve 2102') can change the wavelength of the junction, and, accordingly enables introduction of

chromatic dispersion and dispersion slope at another center wavelength λ_c' . Again, the

magnitude of the chromatic dispersion adjustment and/or slope adjustment maybe varied at this other center wavelength by altering the index of refraction (shifting of one or more of the dispersion curves).

Moreover, as is known, dispersion slope impairments can adversely impact signal transmission. Dispersion slope impairments may result from optical waveguide (e.g. fiber) dispersion slope; dispersion slope from optical components and equipment in the optical transmission system; and from thermal fluctuations, which can alter dispersion slope. The control and correction of dispersion slope becomes increasingly important for high transmission rate systems operating with certain transmission formats. For instance, 40Gbps optical networks using RZ formats can be degraded when the received optical signal has lOOps/nm² or more of chromatic dispersion slope.

Accordingly, one particularly advantageous aspect of the exemplary embodiments of the present invention is the ability to dynamically and tunably control and adjust the

dispersion slope. Illustratively, the slope adjustment at λ_c in Fig. 21(a) is negative.

However, by "tuning" the third waveguide, the slope adjustment at λ_c may be changed in

both magnitude and sign to achieve a desired result.

Finally, a few points are particularly noteworthy of the receiver dispersion control module in accordance with the exemplary embodiment described in connection with Fig. 21(a).

First, it is noted that the receiver dispersion control module illustratively adjusts

the chromatic dispersion at a single wavelength channel having center wavelength λ_c .

However, the receiver DCM could be designed to adjust chromatic dispersion and/or dispersion slope at more than one wavelength channel. Of course, this could be accomplished by selective tuning (e.g. shifting dispersion curve 2102 to dispersion curve 2102' so the intersection of dispersion curve 2102' and dispersion curve 2101 is at another

center wavelength λ_c' ). This may also be accomplished by having additional

waveguides in the dispersion control module which have dispersion curves which are chosen to intersect at a particular center wavelength(s) of other waveguide channels, and at particular values of dispersion. Moreover, a particular dispersion slope could be added at these wavelength channel center wavelengths as needed.

Second, it is noted that the use of three waveguides to achieve dispersion adjustment and dispersion slope adjustment is merely illustrative. To this end, more or fewer waveguides may be included to introduce a particular amount of chromatic dispersion and/or dispersion slope to an optical signal. For example, as referenced above, it is possible to adjust the chromatic dispersion and dispersion slope by desired amounts at multiple wavelength channel center wavelengths using a plurality of waveguides having a variety of dispersion curves which are chosen to selectively intersect and desired wavelengths. Additionally, it may be possible to use fewer than three waveguides to adjust the dispersion and dispersion slope by desired amounts at a desired wavelength or wavelengths.

Third, it is noted that the dispersion curve 2103, which adds dispersion slope to the optical signal also adds a certain magnitude of chromatic dispersion. This added chromatic dispersion must be accounted for in realizing the final level of dispersion adjustment, D_a.

The exemplary embodiment described in connection with Fig. 21(a) relates particularly to adjusting chromatic dispersion and dispersion slope through the introduction of positive chromatic dispersion and negative dispersion slope. It is noted that the cliromatic dispersion adjustment and the slope adjustment in accordance with exemplary embodiments of the present invention may be positive, negative or zero. An example of negative adjustment of the chromatic dispersion and positive adjustment of dispersion slope is described presently. Consistent with the exemplary embodiment of Fig. 21(a), the exemplary embodiment described in connection with Fig. 21(b) is a receiver DCM which includes three waveguides. These waveguides are coupled waveguides as described above. It is noted however that the supermodes which have negative chromatic dispersion characteristics as shown in Fig. 21(b) are orthogonal to the supermodes to the supermodes which exhibit an equal but opposite (positive) dispersion characteristics. To wit, the presently described supermodes are the fundamental modes of the coupled waveguides of the illustrative receiver DCM. These fundamental modes are the asymmetric (or LP01) supermodes of the coupled waveguides. It is noted that simultaneous excitation of both the symmetric and asymmetric modes of a given waveguide or waveguides of a receiver DCM should be avoided. To this end, the dispersion curves of the fundamental and first harmonic modes are mirror images of one another, and if simultaneously excited would result in no chromatic dispersion adjustment by the receiver DCM. hi practice, the optical mode input of the illustrative receiver DCM may preferentially be coupled to either the symmetric or asymmetric mode of the coupled waveguide using one or more optical couplers (mode couplers) to excite the specific mode required to effect the desired dispersion adjustment and/or slope adjustment.

In the illustrative embodiment shown in Fig. 21(b), a first dispersion curve 2105 of a first waveguide intersects a second dispersion curve 2106 of a second waveguide at a

point 2107 which corresponds to center wavelength λ_c of a particular wavelength

channel. This results in the adjustment of chromatic dispersion compensation by an amount (-)D_a. Moreover, a dispersion slope adjustment is introduced by a third waveguide of the module having a dispersion curve 2103. Illustratively, the dispersion slope adjustment is positive. Of course this is merely illustrative, and this slope could be zero or negative as well.

As noted in connection with the exemplary embodiment of Fig. 21(a), the receiver DCM of the exemplary embodiment of Fig. 21(b) can be dynamically tuned to effect a selected amount of dispersion adjustment and/or slope adjustment at other center wavelengths by shifting the curves (again designated by "tuning" in the Figure). Likewise, the amount of chromatic dispersion adjustment and/or slope adjustment at the

selected wavelength ( λ_c ) can be selectively varied.

Finally, it is noted that the salient points described in connection with the receiver

DCM described in connection with Fig. 21(a) also apply to the present illustrative embodiment. As such, in the interest of brevity, these points are not repeated.

Turning to Figs. 22(a) - 22(c), representative dispersion curves versus wavelength in accordance with an exemplary embodiment of the present invention are shown to more simply describe the tuning ability of the dispersion control modules in accordance with an exemplary embodiment of the present invention. These graphical representations are illustratively portions of the dispersion curves shown in Fig. 21(a) and/or Fig. 21(b) close

to the intersection point at the center wavelength λ_c of the particular wavelength channel

being compensated. As shown in Figs. 22(a) and 22(b), the ability to tune the amount of chromatic dispersion introduced by the receiver DCM at a particular center wavelength is readily effected in accordance with the exemplary embodiments of the present invention by the alteration of the index of refraction profile of the coupled optical waveguides. The tuning, which is shown by arrows in the graphs in Figs. 22(a) and 22(b), ultimately results in the ability to increase or decrease the magnitude of the dispersion at a particular center wavelength as is shown in Fig. 22(c). In the present illustrative embodiment, the dispersion adjustment is positive, and two optical fibers are used to alter the magnitude of the adjustment. Of course, this tuning capability could be used to change the point of intersection of curves resulting in chromatic dispersion adjustment at a different center

wavelength than λ_c' , as described above. Moreover, as described above, the adjustment

of the dispersion slope may be tuned as well.

As noted above, the dispersion control modules in accordance with an exemplary embodiment of the present invention can incorporate one or more optical fibers, and may be applied to a variety of scenarios in an optical communications system. As referenced above, the various standards bodies (e.g., the ITU) prescribe the tolerances for chromatic dispersion in a particular optical communication system. To illustrate the adaptability of the present invention to effect dynamic chromatic dispersion compensation within specified tolerances, three scenarios are described presently.

In a first example, in an add-drop multiplexer (for example wavelength add-drop multiplexer 2006 of Fig. 20) it may in fact be necessary to tailor the individual dispersion confrol modules 2009 for each wavelength channel. To wit, it may be necessary to have a different dispersion control module 2009 for each wavelength channel. In this scenario, each dispersion control module, which is matched to a particular wavelength channel, would have relatively narrow bandwidth (i.e. slightly greater than the bandwidth of the wavelength channel). Over this bandwidth, the dispersion control module would tunably effect the desired magnitude and slope of dispersion adjustment to an optical signal so that the output therefrom and input to the receiver (e.g. receiver 2010) has a desired dispersion value and dispersion slope within prescribed tolerances. As described above, the output of the dispersion control modules in accordance with an exemplary embodiment of the present invention may have zero chromatic dispersion and zero dispersion slope, in which case the chromatic dispersion and slope have been compensated. Alternatively, the output may have positive or negative chromatic dispersion levels and positive or negative dispersion slope, as desired. Each of the dispersion control modules 2009 of the present illustrative embodiment may include one or more optical waveguides described in the above referenced exemplary embodiments drawn to coupled waveguide chromatic dispersion adjustment, and a device which alters the index of refraction in a controlled manner.

In another example, it may be possible that the dispersion control modules 109 could effectively adjust the chromatic dispersion over a plurality of channels. For example, according to an exemplary embodiment, four wavelength channels could be adequately adjusted to within dispersion tolerance by a particular module. At least one optical waveguide structure would be necessary and would include a device to effectively alter the index of refraction profile of the coupled optical waveguides.

According to yet another exemplary embodiment, it is possible to have a dispersion control module 2009 which could effectively tune to any center channel wavelength across a particular band (e.g., all 40 channels of a particular DWDM optical communication system), and introduce a suitable amount of dispersion adjustment to optical signals of each individual wavelength channel to be within prescribed dispersion tolerances. Again, this may be carried out using a dispersion control module having one or more coupled waveguide structures as well as a device to alter the index of refraction profile for the coupled waveguide(s). A dispersion control module that can be used to adjust chromatic dispersion and/or dispersion slope of any optical channel is termed "colorless." Such dispersion control modules are useful in optical networks where wavelength channels are tunably routed. In such cases, it is not known a priori which wavelength channel will be received by which optical receiver.

In some cases, it may be sufficient to have a dispersion control module that can adjust the chromatic dispersion in one of a number of contiguous optical channels. Such a dispersion control module may be termed "banded"; the module may receive any one of a number of contiguous optical channels and be tuned to correct for the dispersion impairment of that channel. It is within the purview of the present invention as described in connection with the exemplary embodiments to have chromatic dispersion control modules that are banded or colorless.

The invention having been described in detail in connection through a discussion of exemplary embodiments, it is clear that modifications of the invention will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure. Such modifications and variations are included in the scope of the appended claims.

Claims

In the Claims:

We Claim:

1. An apparatus for dynamically controlling for chromatic dispersion in an optical signal, comprising: a coupled waveguide structure; and a device which alters an index of refraction of said coupled waveguide structure to effect a change in the chromatic dispersion.

2. An apparatus as recited in claim 1, wherein said coupled waveguide structure further comprises a core circumferentially surrounded by at least one ring.

3. An apparatus as recited in claim 2, wherein a cladding layer is disposed between said core and said at least one ring.

4. An apparatus as recited in claim 2, wherein a cladding layer is disposed about said at least one ring.

5. An apparatus as recited in claim 3, wherein said cladding layer further comprises a plurality of individual layers.

6. An apparatus as recited in claim 4, wherein said cladding layer further comprises a plurality of individual layers.

7. An apparatus as recited in claim 2, wherein said core and said at least one ring have the same index of refraction.

8. An apparatus as recited in claim 2, wherein said core and said at least one ring do not have the same index of refraction.

9. An apparatus as recited in claim 1, wherein said coupled waveguide structure is a planar wavguide structure.

10. An apparatus as recited in claim 1, wherein said device further comprises a source to controUably heat and cool said coupled waveguide structure.

11. An apparatus as recited in claim 1, further comprising a feedback control loop which is adapted to control said device in order to effect a desired change in the chromatic dispersion of the optical signal.

12. An apparatus as recited in claim 11, wherein said feedback control loop further comprises a bit-error rate analyzer which receives a portion of an output from the apparatus.

13. An apparatus as recited in claim 1, wherein said device further comprises a secondary source of optical power which selectively couples to said coupled waveguide structure.

14. An apparatus as recited in claim 13, wherein said feedback control loop further comprises a bit-error rate analyzer which receives a portion of an output from the apparatus.

15. An apparatus as recited in claim 1, wherein said device further comprises a source to controUably heat and cool said coupled waveguide structure and a secondary optical source which selectively couples to said coupled waveguide structure.

16. An apparatus as recited in claim 15, wherein said feedback control loop further comprises a bit-error rate analyzer which receives a portion of an output from the apparatus.

17. An apparatus as recited in claim 1, wherein the apparatus dynamically controls dispersion slope.

18. An apparatus as recited in claim 1, wherein the dynamic controlling of chromatic dispersion results in zero chromatic dispersion in the optical signal.

19. An apparatus as recited in claim 1, wherein the dynamic controlling of chromatic dispersion results in positive chromatic dispersion in the optical signal.

20. An apparatus as recited in claim 1, wherein the dynamic controlling of chromatic dispersion results in negative chromatic dispersion in the optical signal.

21. An apparatus as recited in claim 17, wherein said dynamic control of dispersion slope results in zero dispersion slope in the optical signal.

22. An apparatus as recited in claim 17, wherein said dynamic control of dispersion slope results in positive dispersion slope in the optical signal.

23. An apparatus as recited in claim 17, wherein said dynamic control of dispersion slope results in negative dispersion slope in the optical signal.

24. An apparatus as recited in claim 1, wherein said device is chosen from the group consisting essentially of: a source to controUably heat and cool said coupled waveguide structure; a secondary optical source which selectively couples to said coupled waveguide structure; an electrooptic effect device; and an acoustooptic effect device.

25. An apparatus as recited in claim 24, wherein said feedback control loop further comprises a bit-error rate analyzer which receives a portion of an output from the apparatus.

26. An apparatus as recited in claim 1, wherein a change in chromatic dispersion in the optical signal is in the range of approximately -lOOOOOps/nm-km to approximately +100000ρs/nm-km.

27. A method for dynamically controlling chromatic dispersion in an optical signal, the method comprising: providing a coupled waveguide structure; and selectively altering an index of refraction in said coupled waveguide structure to effect a change in the chromatic dispersion of an optical signal.

28. A method as recited in claim 27, wherein said selective altering further comprises heating and cooling said coupled waveguide structure.

29. A method as recited in claim 27, wherein said selective altering further comprises introducing a secondary optical signal to said coupled waveguide structure.

30. A method as recited in claim 28, wherein said heating and cooling is in a range of approximately -100°C to approximately +100°C.

31. A method as recited in claim 29, wherein said secondary optical signal has a power in the range of approximately 0W to approximately 1KW.

32. A method as recited in claim 29, wherein said secondary optical signal has a wavelength in the range of approximately O.Olμm to approximately lOOμm .

33. A method as recited in claim 27, wherein said selective altering is effected using an electrooptic effect.

34. A method as recited in claim 27, wherein said selective altering is effected using an acoustooptic effect.

35. A method as recited in claim 27, wherein said altering is effected using a technique chosen from the group consisting essentially of: heating and cooling said coupled waveguide structure; introducing a secondary optical signal to said coupled waveguide structure; using an electrooptic effect; and using an acoustooptic effect.

36. A method as recited in claim 27, the method further comprising analyzing a bit-error rate, and compensating for the chromatic dispersion based on said analyzing.

37. A method as recited in claim 27, wherein a feedback confrol loop is used in the method.

38. A method as recited in claim 27, wherein the dynamic controlling of chromatic dispersion results in zero chromatic dispersion in the optical signal.

39. A method as recited in claim 27, wherein the dynamic controlling of chromatic dispersion results in positive chromatic dispersion in the optical signal.

40. A method as recited in claim 27, wherein the dynamic controlling of chromatic dispersion results in negative chromatic dispersion in the optical signal.

41. A method as recited in claim 27, wherein the method further comprises dynamically controlling dispersion slope.

42. A method as recited in claim 41, wherein said dynamical control of dispersion slope results in zero dispersion slope.

43. A method as recited in claim 41, wherein said dynamical control of dispersion slope results in negative dispersion slope.

44. A method as recited in claim 41, wherein said dynamical control of dispersion slope results in positive dispersion slope.

45. A chromatic dispersion compensation apparatus, comprising: an optical wavguide; and a device which dynamically alters an index of refraction of said optical waveguide to adjust the chromatic dispersion compensation to an optical signal traversing said optical waveguide.

46. A chromatic dispersion apparatus as recited in claim 45, wherein said device dynamically alters said index of refraction by controlling a parameter chosen from the group consisting essentially of: a temperature of said waveguide, a radius of curvature of said waveguide and a tension applied to said waveguide.

47. A cliromatic dispersion compensation apparatus as recited in claim 45, wherein said device dynamically alters a temperature of said optical waveguide.

48. A chromatic dispersion compensation apparatus as recited in claim 45, wherein said device dynamically alters a radius of curvature of said optical waveguide.

49. A chromatic dispersion compensation apparatus as recited in claim 45, wherein said device dynamically alters a tension applied to said optical waveguide.

50. A chromatic dispersion compensation apparatus as recited in claim 45, wherein said optical waveguide is an optical fiber.

51. A chromatic dispersion compensation apparatus as recited in claim 50, wherein said optical fiber has a relatively small core diameter.

52. A chromatic dispersion compensation apparatus as recited in claim 50, wherein said optical fiber has a relatively high refractive index difference between a core layer and a cladding layer.

53. A chromatic dispersion compensation apparatus as recited in claim 45, wherein said wavguide is coiled.

54. A chromatic dispersion compensation apparatus as recited in claim 53, wherein said coiled optical waveguide is disposed about a mandrel.

55. A chromatic dispersion compensation apparatus as recited in claim 53, wherein said coiled optical waveguide has a controlled variable tension applied thereto.

56. A chromatic dispersion compensation apparatus as recited in claim 55, wherein said coiled optical waveguide has a variable radius of curvature.

57. A chromatic dispersion compensation apparatus as recited in claim 55, wherein a controller is operatively coupled to said device.

58. A chromatic dispersion compensation apparatus as recited in claim 57, wherein said controller is part of an open-loop control scheme.

59. A chromatic dispersion compensation apparatus as recited in claim 57, wherein said controller is part of a closed-loop confrol scheme.

60. A cliromatic dispersion compensation apparatus as recited in claim 45, further comprising at least one mode stripper.

61. A chromatic dispersion compensation apparatus as recited in claim 45, further comprising a device which selectively introduces micro-bends to an optical fiber.

62. A chromatic dispersion compensation apparatus as recited in claim 45, wherein the apparatus is disposed in a thermally sealed module.

63. A chromatic dispersion compensation apparatus as recited in claim 47, wherein said temperature of said waveguide is in the range of approximately -5°C to approximately +55°C.

64. A chromatic dispersion compensation apparatus as recited in claim 45, wherein the dispersion compensation is in the range of approximately -7 Ops/nm-km to approximately -140ps/nm-km in the range of approximately 1520nm to approximately 157Qnm.

65. A chromatic dispersion compensation apparatus as recited in claim 45, wherein the change in the chromatic dispersion compensation is in the range of approximately 4% to approximately 12% over a wavelength range of approximately 1520nm to approximately 1570nm.

66. A chromatic dispersion compensation apparatus as recited in claim 45, wherein the change in chromatic dispersion is in the range of approximately -1% to approximately 4% over a wavelength range of approximately 1520nm to approximately 1570nm.

67. An optical device, comprising: a chromatic dispersion control module which is dynamically tuneable over a prescribed wavelength range, and which does not include a waveguide grating.

68. An optical device as recited in claim 67, wherein said module further comprises at least one coupled waveguide structure.

69. An optical device as recited in claim 68, wherein said module further includes a device which controUably alters an index of refraction profile of each of said at least one coupled waveguide structures .

70. An optical device as recited in claim 69, wherein said altering of said index of refraction profile is dynamic.

71. An optical device as recited in claim 69, wherein said altering of said index of refraction profile effects said dynamic tuning of said module.

72. An optical device as recited in claim 67, wherein said module dynamically and tunably compensates dispersion slope.

73. An optical devices as recited in claim 67, wherein said module dynamically and tunably adjusts both chromatic dispersion and dispersion slope.

75. An optical device as recited in claim 67, wherein said module includes three coupled waveguides and said module is adapted to variably adjust chromatic dispersion at a particular wavelength channel center wavelength.

76. An optical device as recited in claim 75, wherein said module is adapted to variably adjust dispersion slope at said center wavelength.

77. An optical device as recited in claim 69, wherein said device is chosen from the group consisting essentially of: a temperature controller, and an optical energy source.

78. An optical device as recited in claim 67, wherein said module is adapted to receive a

wavelength channel having a center wavelength λ_c .

19. An optical device as recited in claim 67, wherein said module is adapted to receive a plurality of wavelength channels having respective center wavelengths over at least a portion of said wavelength range.

80. An optical device as recited in claim 79, wherein said module is adapted to a variably adjust chromatic dispersion at each of said center wavelengths.

81. An optical device as recited in claim 79, wherein said module is adapted to variably adjust dispersion slope at each of said center wavelengths.

82. An optical device as recited in claim 67, wherein said wavelength range is approximately 1530nm to approximately 1570mn.

83. An optical device as recited in claim 79, wherein said plurality of wavelength channels is in the range of approximately two channels to approximately 40 channels.

84. An optical device as recited in claim 67, wherein said chromatic dispersion control module is colorless.

85. An optical device as recited in claim 67, wherein at least two of said chromatic dispersion control modules are used together with one or more wavelength division multiplexers/demultiplexers .

86. An optical device as recited in claim 85, wherein said one or more wavelength division multiplexers/demultiplexers direct each of a plurality of wavelength channels to a respective one of the chromatic dispersion control modules.

87. An optical device as recited in claim 85, wherein one of said wavelength division multiplexers/demultiplexers direct bands of wavelength channels to one or more separate chromatic dispersion control modules.

88. An optical device as recited in claim 67, wherein said dispersion control module adjusts a chromatic dispersion present in an optical signal to zero.

89. An optical device as recited in claim 67, wherein said dispersion control module adjusts a dispersion slope present in an optical signal to zero.

90. An optical device as recited in claim 67, wherein said dispersion control module adjusts a chromatic dispersion present in an optical signal to a positive value.

91. An optical device as recited in claim 67, wherein said dispersion control module adjusts a chromatic dispersion present in an optical signal to a negative value.

92. An optical device as recited in claim 67, wherein said dispersion control module adjusts a dispersion slope present in an optical signal to a positive value.

93. An optical device as recited in claim 67, wherein said dispersion control module adjusts a dispersion slope present in an optical signal to a negative value.

94. An optical apparatus which compensates for chromatic dispersion and/or dispersion slope, comprising: a plurality of serially coupled optical waveguides each of which introduces chromatic dispersion and/or dispersion slope to an optical signal that traverses the optical waveguides.

95. An optical apparatus as recited in claim 94, wherein said serially coupled optical waveguides are chosen from the group consisting essentially of: coupled waveguide structures; dispersion compensating optical fiber; and fiber Bragg gratings.

96. An optical apparatus as recited in claim 94, wherein the optical apparatus has a dispersion characteristic over a selected wavelength range that is a resultant of respective dispersion characteristics of said plurality of serially coupled optical waveguides.

97. An optical apparatus as recited in claim 96, wherein said resultant is equal in magnitude but opposite in sign over said selected wavelength range to a dispersion characteristic of an optical communication system to which the optical apparatus is coupled.

98. An optical apparatus as recited in claim 94, wherein each of said plurality of optical waveguides has a dispersion characteristic chosen so as to have, in combination, a desired net chromatic dispersion and/or dispersion slope compensation effect.

99. An optical apparatus as recited in claim 98, wherein, over a selected wavelength range, said net chromatic dispersion and/or dispersion slope compensation effect nullifies chromatic dispersion and/or dispersion slope; adds positive or negative dispersion and or dispersion slope; or a combination thereof.

100. An optical apparatus as recited in claim 94, wherein said plurality is greater than or equal to two and less than twenty.

101. An optical apparatus as recited in claim 94, wherein at least one of said plurality of serially coupled optical waveguides is tunable to a particular dispersion characteristic over a particular wavelength range.

102. An optical apparatus as recited in claim 101, wherein said at least one serially coupled waveguide is a coupled waveguide structure.

103. An optical apparatus as recited in claim 102, wherein said coupled waveguide has a tunable dispersion resonance wavelength and a tunable dispersion slope.

104. An optical apparatus as recited in claim 94, wherein at least one of said plurality of optical waveguides supports a fundamental mode.

105. An optical apparatus as recited in claim 94, wherein at least one of said plurality of optical waveguides supports at least one higher order mode.

106. An optical apparatus as recited in claim 94, wherein at least one of the optical apparati are disposed in an optical communications link.

107. An optical apparatus as recited in claim 98, wherein each of said plurality of optical waveguides has a selected length.