WO2003012535A1 - Optical pulse generator with single frequency drive - Google Patents
Optical pulse generator with single frequency drive Download PDFInfo
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- WO2003012535A1 WO2003012535A1 PCT/US2002/023621 US0223621W WO03012535A1 WO 2003012535 A1 WO2003012535 A1 WO 2003012535A1 US 0223621 W US0223621 W US 0223621W WO 03012535 A1 WO03012535 A1 WO 03012535A1
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- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/21—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour by interference
- G02F1/225—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour by interference in an optical waveguide structure
-
- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/0121—Operation of devices; Circuit arrangements, not otherwise provided for in this subclass
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/501—Structural aspects
- H04B10/503—Laser transmitters
- H04B10/505—Laser transmitters using external modulation
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/501—Structural aspects
- H04B10/503—Laser transmitters
- H04B10/505—Laser transmitters using external modulation
- H04B10/5051—Laser transmitters using external modulation using a series, i.e. cascade, combination of modulators
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/508—Pulse generation, e.g. generation of solitons
-
- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F2201/00—Constructional arrangements not provided for in groups G02F1/00 - G02F7/00
- G02F2201/16—Constructional arrangements not provided for in groups G02F1/00 - G02F7/00 series; tandem
-
- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F2203/00—Function characteristic
- G02F2203/54—Optical pulse train (comb) synthesizer
Definitions
- the present invention relates to optical pulse generation, and in particular, to the generation of very narrow optical pulses for optical time-division multiplexed networks.
- TDM time-division-multiplexing
- OTDM optical time- division-multiplexed
- Narrow optical pulses are pulses that occupy very small intervals of time, or optical pulses that have a steep intensity change produced by a control signal. If the optical pulses are not narrow enough, the pulses would overlap within an optical channel, so that signal integrity in an OTDM network would be lost.
- Several methods have been used in the prior art to generate narrow and predetermined pulse formats.
- Some prior art methods include partially modulating the transfer function of a modulator with a device, such as an electro- absorption modulator, in order to generate fast pulses.
- a device such as an electro- absorption modulator
- Zehnder interferometers are used to generate optical pulses having a very narrow pulse width.
- input signals characterized by different and successively increasing frequencies are applied to each of a series of cascaded interferometers.
- This invention relates to an optical pulse generation system and method, which can be implemented using a relatively small number of commercially available parts to generate very narrow optical pulses with a high extinction ratio, suitable for use in OTDM networks.
- 16 ps pulses are generated at a 10 Gb/sec rate with a 25 dB extinction ratio.
- a principal discovery of the present invention is that ultra-short optical pulses characterized by a relatively narrow pulse width, as well as a relatively high extinction ratio, can be generated by driving a pair of cascaded interferometric modulators at substantially the same drive frequency (for example, a drive frequency within the RF band), and by selecting substantially different bias voltages and drive amplitudes for each modulator.
- a bias voltage for the applied drive signal can be selected relative to the output power-modulation voltage transfer function of the modulator, and a corresponding drive amplitude established, in a way so as to substantially minimize the pulse width of the optical pulses, and in a way so as to substantially maximize the extinction ratio of the optical pulses generated by the optical pulse generation system.
- Both modulators are driven at the same frequency, in contrast to prior art methods in which drive signals characterized by different frequencies and centered around the same bias voltages (typically, at maximum optical transmission) are applied to each modulator.
- An optical pulse generation system in accord with the present invention includes a first optical interferometric modulator and a second optical interferometric modulator.
- the first and second interferometric modulators are Mach-Zehnder modulators.
- the optical output of the first interferometric modulator is coupled to the optical input of the second interferometric modulator.
- Each interferometric modulator includes a modulation input for receiving a modulation voltage drive signal that modulates an optical signal that has been received in the optical input of the modulator.
- Each interferometric modulator is characterized by an optical output power-modulation voltage transfer function.
- the modulator transfer function of each modulator is a "raised cosine" type transfer function.
- the first modulator is characterized by a parameter V ⁇ i representing the voltage required to change the output power from the first modulator from a minimum value to a maximum value
- the second modulator is similarly characterized by a parameter V ⁇ 2 representing the voltage required to change the output power from the second modulator from a minimum value to a maximum value.
- the optical pulse generation system includes means for applying a first modulation voltage drive signal to the modulation input of the first modulator, and means for applying a second modulation voltage drive signal to the modulation input of the second modulator.
- the first drive signal is characterized by a first bias voltage normalized to V ⁇ i, an amplitude Al normalized to V ⁇ i and a frequency FI .
- the first drive signal modulates an input optical signal received by the optical input of the first modulator about the first bias voltage, with the normalized amplitude Al. A first modulated optical signal is thereby generated from the output of the first modulator.
- the second drive signal is characterized by a second bias voltage normalized to V ⁇ 2 , an amplitude A2 normalized to V ⁇ 2 , and a frequency A2.
- the second drive signal modulates the first modulated optical signal, received by the optical input of the second modulator, about the second bias voltage with the normalized amplitude A2.
- the optical output of the second modulator provides a second modulated optical signal in the form of optical pulses.
- the output optical pulses generated by the optical pulse generation system of the present invention are substantially equal to the product of the pulses produced by the first modulator, and the pulses that would nominally be produced by the second modulator if the optical signal received at the input of the second modulator were characterized by an output power constant in time.
- the first and second bias voltages, and the first and second drive amplitudes are chosen so as to substantially minimize the pulse width of the output optical pulses generated by the optical pulse generation system.
- output optical pulses of about 16 ps are generated at 10 Gb/sec with an extinction ratio of about 20 dB.
- the first bias voltage biases the first interferometric modulator substantially at the maximum optical transmission.
- the second bias voltage biases the second interferometric modulator substantially at quadrature, i.e. at the half-power point.
- the corresponding drive amplitude for the first drive signal is about twice V ⁇ i, and the corresponding drive amplitude for the second drive signal is about V ⁇ 2.
- the identical drive frequencies FI and F2 are in the RF band, and are about 10 GHz.
- the first and second bias voltages, and the first and second drive amplitudes are chosen so that the output optical pulses generated by the optical pulse generation system of the present invention are characterized by a relatively high extinction ratio, i.e. a highly suppressed side lobe energy.
- the first bias voltage biases the first interferometric modulator at a maximum optical transmission
- the second bias voltage biases the second interferometric modulator at about 130 degrees relative to the maximum optical transmission.
- the drive amplitude for the first drive signal is about twice V ⁇ i
- the drive amplitude for the second drive signal is about (0.6) * V ⁇ 2 . Both signals have a frequency of about 10 GHz.
- the resulting output pulses have a pulse width of about 16 ps, a repetition rate of 10 Gb/sec, and an extinction ratio of about 25 dB.
- the present invention features a method of generating optical pulses.
- the method includes generating a first modulated optical signal by applying a first modulation drive signal to a first optical interferometric modulator, so as to modulate an input optical signal (for example provided by a CW laser source) received into the modulator.
- the method includes generating a second modulated optical signal, in the form of narrow optical pulses, by applying a second modulation voltage drive signal to a second optical interferometric modulator so as to modulate the first modulated optical signal.
- the first and second drive signals are characterized by substantially different bias voltages and drive amplitudes, and by a substantially identical drive frequency.
- the method includes varying the bias voltages and the drive amplitudes so as to substantially minimize the pulse width of optical pulses in the second modulated optical signal.
- the method includes varying the bias voltages and the drive amplitudes so as to substantially maximize the extinction ratio of the optical pulses in the second modulated optical signal.
- Fig. 1 illustrates an optical pulse generation system constructed in accordance with one embodiment of the present invention.
- Cascaded first and second interferometric modulators are driven with bias and drive conditions chosen so as to generate output optical pulses having a relatively narrow pulse width.
- Fig. 2 illustrates a Mach-Zehnder interferometric modulator, as known in the prior art.
- Fig. 3A illustrates the modulator transfer function for the first interferometric modulator, driven with a drive signal biased at the maximum optical output power, and having a drive amplitude of twice V ⁇ i.
- Fig. 3B illustrates the modulator transfer function for the second interferometric modulator, driven with a drive signal biased at quadrature, and having a drive amplitude of V ⁇ 2 .
- Fig. 4 illustrates the output optical signals from the first and second interferometric modulators, for the bias and drive conditions shown in Fig.s 3A and 3B.
- Fig. 5 illustrates the output optical pulses from an optical pulse generation system having the bias and drive conditions shown in Fig.s 3A and 3B.
- Fig. 6 illustrates on a log scale the output optical pulses from an optical pulse generation system having the bias and drive conditions shown in Fig.s 3A and 3B.
- Fig. 7 illustrates another embodiment of an optical pulse generation system in accordance with one embodiment of the present invention, in which the bias and drive conditions are chosen so as to generate optical pulses having a relatively high extinction ratio, namely about 25 dB, and a pulse width of about 16 ps.
- Fig. 8A illustrates the modulator transfer function and the bias and drive conditions for the first interferometric modulator, in an embodiment of the invention in which the bias voltages and the drive amplitudes are chosen so as to generate optical pulses having a relatively high extinction ratio.
- Fig. 8B illustrates the modulator transfer function and the bias and drive conditions for the second interferometric modulator, in an embodiment of the invention in which the bias voltages and the drive amplitudes are chosen so as to generate optical pulses having a relatively high extinction ratio.
- Fig. 9 illustrates the output optical signals from the first and second interferometric modulators, for the bias and drive conditions shown in Fig.s 8A and 8B.
- Fig. 10 illustrates the extinction ratio of the output pulses when the bias voltages and the drive amplitudes are chosen as illustrated in Fig. 8A and 8B.
- the present invention allows for the generation of pulses having a relatively narrow pulse width, and a relatively low duty cycle, for example 16%.
- a low duty cycle allows for optical time division multiplexing (OTDM) to be accomplished in a relatively simple way.
- OTDM optical time division multiplexing
- an optical pulse must be very narrow. This is because a single clock pulse is split into multiple channels, depending on the ratio by which the data transmission rate of a single channel is to be enhanced through optical multiplexing.
- the optical pulses in all of the channels are optically modulated, in parallel.
- the optical pulse outputs from the multiple channels are then combined together, resulting in an optically multiplexed OTDM signal.
- the original clock pulses must thus be narrow enough so as to avoid overlapping within a single channel.
- optical pulses be narrow enough to fit into a single channel having a bandwidth of at least 40 Gb/sec or higher.
- the present invention provides a relatively simple and low cost system and method for generating narrow optical pulses needed for OTDM systems.
- a pair of cascaded optical interferometric modulators are driven with modulation voltage drive signals.
- Each interferometric modulator is driven at substantially the same frequency, for example at about 10 GHz, although other frequencies are also within the scope of this invention.
- Each modulator is driven about substantially different bias voltages, and with substantially different drive amplitudes.
- the bias voltages and the drive amplitudes of the modulation drive signals for each modulator are varied so as to generate output optical pulses having a desired pulse width, and a desired extinction ratio.
- the bias and drive conditions may be chosen so as to substantially minimize the pulse width and substantially maximize the extinction ratio of the output optical pulses, in a preferred embodiment of the invention.
- Using a single drive frequency for both modulators allows narrow optical pulses to be generated while using a relatively small number of commercially available parts.
- Fig. 1 illustrates one embodiment of an optical pulse generation system 10, constructed in accordance with the present invention.
- the bias and drive conditions for each of a pair of cascaded interferometric modulators are chosen so as to generate output optical pulses having a relatively narrow pulse width, namely about 16 ps.
- the system 10 includes a first optical interferometric modulator 12 and a second optical interferometric modulator 14.
- Optical interferometric modulator 12 includes an optical input 16 for receiving an optical input signal, and an optical output 20 for providing a first optical output signal.
- optical interferometric modulator 14 includes an optical input 18 for receiving an optical input signal and an optical output 22 for providing a second optical output signal.
- the optical output 20 of the first interferometric modulator 12 is coupled to the optical input 18 of the second interferometric modulator 14.
- the optical input signal received by the optical input 16 of the first interferometric modulator is generated by a source (not shown) of optical radiation, preferably a C W (continuous wavelength) laser, and is transmitted through an optical delivery structure, such as an optical fiber 24.
- Each optical interferometric modulator includes a modulation input (26 and 28) for receiving a first modulation voltage drive signal 30 and a second modulation voltage drive signal 32, respectively.
- Each optical interferometric modulator may also include bias means for biasing the first and second drive signals.
- Each optical interferometric modulator is characterized by a modulator transfer function, which defines the optical output power from the modulator as a function of the applied modulation voltage.
- the first interferometric modulator 12 and its modulator transfer function are characterized by a parameter V ⁇ i , representing the voltage required to change the optical output power from the first modulator 12 from a minimum value to a maximum value.
- the second interferometric modulator 14 and its transfer function are characterized by a parameter V ⁇ 2 , representing the voltage required to change the optical output power from the second modulator 14 from a minimum value to a maximum value.
- the first modulation drive signal 30 and the second modulation voltage drive signal 32 have substantially the same frequency.
- the drive frequency is about 10 GHz, although other drive frequencies (for example 20 GHz) are also within the scope of the present invention.
- a single 10 GHz oscillator can thus be used in the illustrated embodiment of the invention, in order to generate both drive signals. From the viewpoint of practical implementation, a single frequency drive thus permits the system 10 to minimize the number of commercially available parts necessary to generate narrow optical pulses, as mentioned earlier. This is in contrast to prior art pulse generators using cascaded Mach-Zehnder interferometers, in which each interferometer is driven at different frequencies, for example harmonically related frequencies.
- the first interferometric modulator 12 is driven with a first modulation drive signal 30 which is applied to the modulation input 26.
- the modulation drive signal 30 is biased at the transfer function maximum, i.e. centered about a bias voltage chosen to correspond to the modulation drive voltage at which the output optical power from the first interferometric modulator 12 has a peak value.
- the RF drive amplitude for the first modulator is chosen to be twice V ⁇ i.
- the second modulator is driven with a second modulation drive signal 32 which is applied to the modulation input 28.
- the modulation drive signal 32 is biased at quadrature, i.e. is centered about a bias voltage corresponding to the modulation drive voltage at which the output optical power from the second interferometric modulator 14 is one half the peak value.
- the RF drive amplitude for the second modulator is chosen to be equal to V ⁇ 2
- the first 12 and second 14 optical interferometric modulators are Mach-Zehnder modulators.
- Fig. 2 illustrates a Mach-Zehnder interferometric modulator 100, known in the prior art.
- an incoming optical signal 102 is split at a Y-junction into two signals, Ei and E 2 .
- Each signal enters a first waveguide branch 104 and a second waveguide branch 106, respectively.
- the signals are recombined into an output waveguide 110, which provides a modulated optical output signal, E 3 .
- the Mach-Zehnder modulator 100 is formed on a lithium niobate substrate.
- lithium niobate Because of its high electro-optic coefficient, lithium niobate provides an efficient means of achieving optical modulation. Because lithium niobate is optically active, the index of refraction of a waveguide region can be altered by applying an electnc field in that region. Typically, a modulation signal 107 is applied to a modulator input electrode 108. The signal 107 causes an electnc field to be applied to one or both of the waveguide branches 104 and 106.
- an electnc field applied to a waveguide branch causes the index of refraction in the waveguide branch to change with the changing amplitude of the modulating signal.
- the change in the index of refraction alters the speed (or phase) of light in the region, resulting in a change in the delay time of the light passing through the region.
- the modulation signal thus enables the optical path length in one or both of the waveguide branches to be controlled, and a phase difference results between the two signals Ei and E 2 , when they are recombined at the output waveguide 110.
- the interference of the two recombined signals results in an intensity modulated output signal E3.
- Fig 3A graphically illustrates the modulator transfer function 200 for the first interferometnc modulator, as well as the bias and drive conditions for the first modulation voltage dnve signal 30.
- the modulator transfer function 200 is a periodic function of d ⁇ ve voltage in the illustrated embodiment
- the transfer function 200 preferably has a "raised cosme" sinusoidal form, although other pe ⁇ odic forms of the transfer function 200 are also within the scope of the present invention.
- the modulator transfer function 200 is symmetncal about a center voltage V ⁇ 0 between a lower dnve voltage Vi_ and an upper dnve voltage V ⁇ + , and is substantially a single period sinusoid, as a function of drive voltage, between V ⁇ _ and V ⁇ + .
- the modulator transfer function 200 has a maximum value at Vio, and a minimum value at Vi. and at V ⁇ + .
- the first dnve signal 30 is a pe ⁇ odic function of time with a frequency FI and a peak-to-peak amplitude Al, normalized to V ⁇ i.
- the first dnve signal 30 is a sinusoidal function of time. In other embodiments, however, the first drive signal 30 may be represented by periodic functions of time other than sinusoidal functions. By way of example, the first drive signal 30 may be a square wave signal.
- the first dnve signal 30 causes time-varying, oppositely directed electric fields to be applied to the two waveguide branches of the first Mach-Zehnder modulator.
- the frequency FI of the first modulation drive signal 30 is in the RF range, about 10 GHz, but other frequencies may be used in other embodiments of the invention.
- Fig. 3B illustrates the modulator transfer function 300 for the second interferometric modulator, in the embodiment of the invention illustrated in Fig. 1, as well as the bias and drive conditions for the second drive signal 32.
- the modulator transfer function 300 is substantially identical to the modulator transfer function 200 (shown in Fig. 3A).
- the modulator transfer function 300 is also symmetrical about a center voltage V 2 o between a lower drive voltage V 2- and an upper drive voltage V 2+ , and is substantially a single period sinusoid, as a function of drive voltage, between V 2 . and V 2+ .
- the modulator transfer function 300 has a maximum value at V 2 o, and a minimum value at V 2- and at V 2+ .
- the second drive signal 32 is a sinusoidal function of time, with a frequency F2 and a peak-to-peak amplitude A2.
- the second drive signal 32 may be represented by periodic functions of time other than sinusoidal functions, including but not limited to square wave functions.
- the second drive signal 32 also causes time-varying, oppositely directed electric fields to be applied to the two waveguide branches of the second Mach-Zehnder modulator.
- V2 V 2 . + V 2 B
- V 2 B is (1/2) * V ⁇ 2
- the second bias voltage V2 biases the second modulator at quadrature, i.e. at the half-power point.
- the corresponding peak-to-peak drive amplitude A2 has a magnitude of V ⁇ 2, or (V 2+ - V 2- ) / 2.
- an input optical signal is generated by an optical source (not shown), preferably a CW (continuous wavelength) laser.
- the modulation driver or other means for applying a drive signal, applies the first drive signal 30 to the modulation input of the first interferometric modulator 12.
- the first drive signal 30 modulates the input optical signal, so that a first modulated optical signal 400 is generated by the first interferometric modulator.
- the first modulated optical signal 400 is composed of RZ (return-to-zero) optical pulses.
- the modulated optical signal 400 from the output of the first modulator consists of a 20 Gb/sec train of optical pulses, having a pulse width of ⁇ 16 ps.
- the modulated optical signal 400 is received into the optical input of the second modulator.
- the second modulation voltage drive signal 32 is applied to the modulation input of the second modulator.
- the second drive signal 32 modulates the first modulated optical signal 400 about the second bias voltage, i.e. at quadrature, with the second normalized drive amplitude A2 that is equal to one half of V ⁇ 2 .
- Fig. 4 illustrates the output optical signals from the first interferometric modulator and the second interferometric modulator, for the bias and drive conditions shown in Fig.s 3 A and 3B.
- the solid curve illustrates the modulated optical signal 400 generated from the first modulator.
- the dashed curve 410 illustrates the pulses that nominally would result, assuming a constant input optical power at the optical input 16, when the second modulator is driven with the second modulation drive signal 32 (shown in Fig. 3B), at a drive amplitude of V ⁇ 2 and a bias voltage at quadrature or half power point.
- a second set of pulses i.e.
- a 10 Gb/sec train of pulses would nominally be generated by the second interferometric modulator, having a pulse width of ⁇ 50 ps.
- the second stage since the optical signal received at the input of the second modulator is not constant, but rather has the time varying modulated output of the first interferometric modulator, the second stage in effect gates the pulses from the first stage to provide, at the optical output 22, a 10 Gb/sec stream of 16 ps pulses.
- Fig. 5 illustrates the output optical pulses 500 generated by the optical pulse generation system 10 constructed in accordance with the present invention.
- the pulses from the second interferometric modulator perform a gating function on the optical signals that are received by the optical input of the second interferometric modulator. The result is that every other pulse, from the first set of pulses generated by the first interferometric modulator, is gated out. A small amount of residual side lobe power 510 is visible, where every other pulse has been gated out, as a result of applying the second drive signal to the second interferometric modulator. The residual side lobe energy is more easily observed on a log scale, shown in Fig. 6.
- the amplitudes of the side lobes 510 are shown in units of dB, on a log scale. It can be seen from Fig. 6 that the side lobes 510 of optical power have amplitudes that are about 20 dB lower, as compared to the amplitudes of the main pulses 500.
- This residual power causes coherent interference, when the pulses are optical time- division multiplexed to 20 Gb/sec rates. It is desirable to minimize leakage power, because such coherent interference reduces the link margin.
- leakage caused by the side lobes there may be additional leakage due to imperfect extinction in the lithium niobate interferometric modulator.
- a seemingly negligible amount of side lobe energy can result, after optical multiplexing, in substantial fluctuations in optical power.
- Side lobe energy of the order of only 1-2 % can create up to 40 % fluctuations in optical power, when the pulse stream illustrated in Fig. 5 is optically multiplexed. It is therefore desirable to lower the side lobe energy, i.e. maximize the extinction ratio of the optical pulses.
- parameters including the pulse width and the extinction ratio are optimized by varying the bias and drive conditions of the first and second drive signals.
- the bias voltages and drive amplitudes for the first and second interferometric modulators can be varied so as the change and/or substantially minimize the pulse width of the output optical pulses, as described in relation to the embodiment illustrated in Figs. 1 and 3-5.
- the bias voltages and the drive amplitudes for the first and second interferometric modulators may be varied so as to achieve a desired extinction ratio for the output optical pulses.
- Fig. 7 illustrates another exemplary embodiment of an optical pulse generation system 10 constructed in accordance with the present invention, in which the bias and drive conditions are chosen so as to generate optical pulses having a relatively high extinction ratio, namely about 25 dB, and having a pulse width of about 16 ps.
- the increase in extinction ratio is achieved by moving the bias point of the second interferometric modulator from the quadrature point (at 90 degrees with respect to the maximum optical transmission) towards the null, i.e. towards the minimum optical transmission, which is located at 180 degrees with respect to the maximum optical transmission.
- the bias point is moved from the quadrature point by about 40 degrees, to a bias point located at about 130 degrees relative to the maximum optical transmission.
- the drive amplitude for the second drive signal is lowered to less that V ⁇ 2 , to about (0.6) * V ⁇ 2 .
- Fig.s 8 A and 8B illustrate the modulator transfer function and the bias and drive conditions for the first and the second interferometric modulators, in an embodiment of the invention in which the bias voltages and the drive amplitudes are chosen so as to generate optical pulses having a relatively high extinction ratio.
- a first drive signal 600 applied to the first modulator and a second drive signal 610 applied to the second modulator.
- the first modulator is biased at maximum, and driven with a 2 * V ⁇ i amplitude, as in the case illustrated in Figs. 3 A and 3B.
- the second modulator is driven very low, with a drive amplitude of about (0.6) * V ⁇ 2 and the bias point chosen near null, namely at about 130 degrees with respect to the maximum optical transmission.
- Fig. 9 illustrates the output optical signals from the first and second interferometric modulators, for the bias and drive conditions shown in Fig.s 8 A and 8B.
- the solid curve 700 illustrates the optical pulses generated from the output of the first modulator when driven with the drive signal 600. These optical pulses are a 20 Gb/sec train of 16 ps optical pulses, as described earlier.
- the dashed curve 710 illustrates the pulses that nominally would result, if a constant input optical power were received into the optical input of the second modulator, and if the second modulator were driven with the second drive signal 610, biased near null at 130 degrees from the maximum, and with a drive amplitude of 0.6 * V ⁇ 2 .
- the maximum pulse height for these pulses would be slightly over 60% of the pulses generated from the output of the first modulator.
- the second modulator effectively gates the pulses from the first modulator, so that the output optical pulses from the optical pulses generation system are a product of the first set of optical pulses from the first modulator, and the second set of pulses generated by the second modulator.
- the output pulse train is illustrated in Fig. 10, on a log scale.
- Fig. 10 illustrates the suppression of side lobe energy when the bias voltages and the drive amplitudes are chosen according to the conditions shown in Figs. 8A and 8B, so as to generate output optical pulses having a relatively high extinction ratio.
- Fig. 10 shows the main output pulses 800 and the side lobes 810, as a function of time.
- the amplitudes of the main pulses 800 and the side lobes 810 are shown in units of dB. It can be seen that the residual power is about 7 dB lower, as compared to Fig.
- the peak amplitude of the main pulses 800 for the embodiment illustrated in Fig. 10 is about 2 dB lower, as compared to the peak amplitude of the main pulses 500 for the embodiment illustrated in Fig. 6. An improvement of about 5 dB is therefore achieved, as compared to the embodiment illustrated in Fig. 6.
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Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP1404037A2 (en) * | 2002-09-26 | 2004-03-31 | Corning Incorporated | Modulator and method for generating variable duty cycle optical pulses |
US20230073002A1 (en) * | 2021-09-06 | 2023-03-09 | Fujitsu Optical Components Limited | Optical device and optical communication apparatus |
Citations (8)
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US5040865A (en) * | 1990-04-20 | 1991-08-20 | Hughes Aircraft Company | Frequency multiplying electro-optic modulator configuration and method |
US5148503A (en) * | 1991-05-29 | 1992-09-15 | Crystal Technology, Inc | Apparatus and method for linearized cascade coupled integrated optical modulator |
US5168534A (en) * | 1991-12-09 | 1992-12-01 | United Technologies Corporation | Cascaded optic modulator arrangement |
US5249243A (en) * | 1992-05-21 | 1993-09-28 | Siemens Components, Inc. | Apparatus and method for cascade coupled integrated optical phase modulator for linearization of signal transfer |
US5278923A (en) * | 1992-09-02 | 1994-01-11 | Harmonic Lightwaves, Inc. | Cascaded optical modulation system with high linearity |
US5321543A (en) * | 1992-10-20 | 1994-06-14 | General Instrument Corporation | Apparatus and method for linearizing an external optical modulator |
US5422966A (en) * | 1994-06-10 | 1995-06-06 | Gopalakrishnan; Ganesh K. | Microwave electro-optic mixer |
US5854862A (en) * | 1996-03-29 | 1998-12-29 | Crystal Technology, Inc. | Linearized optic modulator with segmented electrodes |
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2002
- 2002-07-26 WO PCT/US2002/023621 patent/WO2003012535A1/en not_active Application Discontinuation
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US5040865A (en) * | 1990-04-20 | 1991-08-20 | Hughes Aircraft Company | Frequency multiplying electro-optic modulator configuration and method |
US5148503A (en) * | 1991-05-29 | 1992-09-15 | Crystal Technology, Inc | Apparatus and method for linearized cascade coupled integrated optical modulator |
US5168534A (en) * | 1991-12-09 | 1992-12-01 | United Technologies Corporation | Cascaded optic modulator arrangement |
US5249243A (en) * | 1992-05-21 | 1993-09-28 | Siemens Components, Inc. | Apparatus and method for cascade coupled integrated optical phase modulator for linearization of signal transfer |
US5278923A (en) * | 1992-09-02 | 1994-01-11 | Harmonic Lightwaves, Inc. | Cascaded optical modulation system with high linearity |
US5321543A (en) * | 1992-10-20 | 1994-06-14 | General Instrument Corporation | Apparatus and method for linearizing an external optical modulator |
US5422966A (en) * | 1994-06-10 | 1995-06-06 | Gopalakrishnan; Ganesh K. | Microwave electro-optic mixer |
US5854862A (en) * | 1996-03-29 | 1998-12-29 | Crystal Technology, Inc. | Linearized optic modulator with segmented electrodes |
Cited By (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP1404037A2 (en) * | 2002-09-26 | 2004-03-31 | Corning Incorporated | Modulator and method for generating variable duty cycle optical pulses |
EP1404037A3 (en) * | 2002-09-26 | 2004-06-09 | Corning Incorporated | Modulator and method for generating variable duty cycle optical pulses |
US20230073002A1 (en) * | 2021-09-06 | 2023-03-09 | Fujitsu Optical Components Limited | Optical device and optical communication apparatus |
US11852878B2 (en) * | 2021-09-06 | 2023-12-26 | Fujitsu Optical Components Limited | Optical device and optical communication apparatus |
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