WO1998037442A1 - A method of providing in-situ chirped gratings in waveguides and waveguides made by that method - Google Patents

Abstract

A new method for the production of chirped gratings in a photosensitive optical waveguiding medium (11) is described which utilises a bulk optic prism (20) and superimposed divergent or convergent actinic radiation beams (12, 13). A chirped interference pattern is generated on or in the vicinity of one face (23) of a prism (11) by impinging a single or plurality of divergent or convergent actinic radiation beam(s) to a second face (22) of the prism. The chirped grating is formed in the photosensitive optical waveguide to be processed by exposure to the interference pattern, thus forming a chirped grating in the waveguide. In one preferred embodiment, the waveguide is a photosensitive optical fibre (11), having formed in said waveguide an in-fibre chirped Bragg grating by the method disclosed. Characteristics of gratings produced by the method disclosed are dependent upon the angle of incidence of the writing beam, beam divergence or convergence angles, geometry of the prism, the distance between the beam focus and prism surface and the refractive index modulation depth. Transmission and reflection characteristics of the gratings may be tailored by control over these same parameters. The method disclosed possesses advantages over prior art, such as the ability to introduce high chirp magnitudes and relatively long chirped gratings lengths, at a broad range of wavelengths, while maintaining low sensitivity to mechanical vibrations.

Description

A METHOD OF PROVIDING IN-SITU CHIRPED GRATINGS IN WAVEGUIDES AND WAVEGUIDES MADE BY THAT METHOD

FIELD OF THE INVENTION

This invention relates to a method of preparing in-situ chirped gratings in optical waveguides and to waveguides made by that method.

ART BACKGROUND

Optical devices are commonly used in industry and science and include laser cavities, waveguides, lenses, filters and other optical elements and their combinations . Such optical devices are used in a variety of instruments and installations.

Photonics technology has revolutionised the communications and sensor fields. This is mainly due to the rapid development of optical and opto-electronic devices . A wide variety of glass materials, material-dopants and waveguide structures are available and this provisional specification relates to a method of preparing in-situ chirped gratings in optical waveguides and to waveguides made by that method. This technique has excellent potential to greatly reduce the cost and complexity of waveguide systems by replacing conventional optical signal filtering, detection and processing components by in-situ, miniaturised, rapid response, and in-line reflection or transmission gratings in optical waveguides, particularly those utilising optical fibre waveguides. In addition, it would greatly simplify and increase the reliability of manufacturing in-fibre chirped Bragg gratings .

The high expectations of optical fibres as information carriers in communication systems have been justified by their performance over the past two decades . Due to their high bandwidth, low attenuation and mechanical properties, each fibre is capable of replacing up to thousands of copper wires in telecommunication systems. With these characteristics it is no surprise that optical fibres have become the most affordable and efficient medium available in the field of telecommunications. Yet, optical fibres can be more than mere signal carriers.

Silica-based optical fibres have been demonstrated to have photosensitive or photo-refractive properties when doped with germanium and other materials. The photo-induced changes in defect structures and/or material damage in germanosilicate glasses are thought to be responsible for this phenomenon. In such a model, an actinic radiation beam (defined as optical radiation which modifies, chemically or structurally, an optical media by exposure to the said optical radiation) , usually in the ultraviolet (UV) , incident on the germanosilicate glass optical fibre breaks the bonds in the defect structures which results in a slight change in the index of refraction of the glass. If the UV beam is in the form of an interference pattern a highly-ordered, three-dimensional reflection or transmission Bragg grating can be photo-etched or written permanently into the core of the fibre. At high UV intensities, regular structural damage caused at intensity maxima in the interference pattern can have a similar effect.

The advent of in-situ reflection or transmission gratings in optical fibres has created exciting opportunities in a wide variety of applications, with tremendous commercial potential. Some examples are: novel and practical fibre optic sensors, passive wavelength filters in wavelength multiplexed telecommunications (ie., fibre-to-the-home) and sensor systems (ie., spectroscopy and chemical analysis), frequency selective elements in active fibre devices

(instrumentation) , as well as in-fibre devices for signal detection and processing, optical pulse compression, dispersion compensation, flattening the gain spectrum of optical amplifiers and holography.

Methods devised in the prior art for the formation of uniform in-fibre gratings include: forming an external grating structure on the surface of an optical fibre such that it will interact with the evanescent field of the fibre; forming an in-situ fibre grating by setting up a standing wave in a photosensitive optical fibre using two interfering actinic rays from a single frequency laser; forming an in-situ fibre grating by transversely illuminating the side of a photosensitive optical fibre to an interference pattern formed by two overlapping, collimated, suitably angled actinic rays from a single coherent UV laser; forming an in-situ fibre grating by transversely illuminating the side of a photosensitive optical fibre in a point-by-point writing procedure by impinging a single collimated, actinic UV light beam through a slit mask; forming an in-situ fibre grating by transversely illuminating the side of a photosensitive optical fibre to an interference pattern of actinic light formed by impinging a single collimated, UV light beam on a uniform- period phase grating mask; and forming an in-situ fibre grating by transversely illuminating the side of a photosensitive optical fibre to an interference pattern of actinic light formed on one face of a right-angle prism interferometer by impinging a single collimated, UV light beam on another face of the prism from a single coherent UV laser.

When a broad-spectrum source of radiation is injected into the core of an optical fibre with an in-situ reflection or transmission grating a very sharp optical peak at the Bragg wavelength is observed in the reflective-mode, thus the grating operates as a narrow bandpass filter. Reflections as high as 100?-. are possible. In the transmissive-mode, all of the original light is observed except for that in the region of the Bragg wavelength, thus the grating operates as an in-line wavelength rejection (bandstop) filter. The grating pitch (individual line spacings) determines the Bragg wavelength according to the following relationship:

l_β = 2nΛ, (1)

where λ_β is the Bragg wavelength, n is the effective refractive index of the fibre core and Λ is the pitch or line spacing of the grating.

In sensing applications, any perturbations on the optical fibre in the region of the in-situ grating, such as strain or temperature, will alter the grating pitch and thus shift the Bragg wavelength. The correlation is exact and linear, to within the useful range of the measured perturbation. In spectral monitoring applications, the reflected optical signal may correspond to a particular wavelength of interest and thus may be monitored. This has the advantage of replacing a bulk spectrometer, wavemeter, or wavelength demultiplexer with an inexpensive, in-line grating. In communications or signal conditioning applications, where optical signal noise and dispersion effects need to be kept to a minimum, transmissive-mode gratings (acting as bandstop filters) and reflective-mode gratings (acting as bandpass filters) can be used to restore the signal to its original, or otherwise desired, state.

A significant advantage of fibre gratings is that several gratings, written at different Bragg wavelengths, can be incorporated into a single optical fibre. This enables several bandpass filters to monitor several wavelengths simultaneously. Similarly, several Bragg sensors can be incorporated into a single fibre for quasi-distributed sensing applications.

Furthermore, overlayed (superimposed) or closely-spaced gratings can be useful for monitoring several wavelengths simultaneously over a short fibre length in spectral filtering applications. In sensor applications, two overlayed gratings can be used for temperature-compensated strain monitoring.

Much of the early research into in-fibre gratings concentrated on the production of an equally or regularly spaced grating pitch (uniform grating) , with a corresponding very-narrow optical bandwidth. Typical optical bandwidths for uniform gratings are in the 0.1 to 0.5 nm range.

In recent years, the fabrication of chirped in-fibre gratings has been attracting significant research and commercial interest due to the growing need for very broad bandwidth gratings in a number of application areas.

Chirped gratings, which have a continuously varying pitch spacing, can be useful in optical signal conditioning and processing applications. This is a significant capability because signal processing could be performed in the optical fibre itself rather than electronically, thus reducing system cost, size, and complexity.

Both linearly and non-linearly chirped gratings have found numerous applications in the areas of telecommunications and fibre optic sensors. In some applications, such as fibre amplifiers and low coherence fibre Fabry-Perot interferometry, chirped gratings have been employed as reflectors for the formation of broad bandwidth optical cavities. In other applications, chirped gratings have been utilised for flattening the gain spectrum of optical amplifiers and for optical pulse compression and dispersion compensation in high-speed, long-haul telecommunications systems. To date, chirped grating bandwidths in the 1 to 44 nm range have been demonstrated in the prior art.

Presently, there are two general approaches to introducing a chirp; amplitude chirping, where the periodicity of the grating is held constant while varying the refractive index along the grating, and wavelength chirping, which introduces a variation of the grating periodicity. The latter approach has found a predominant number of applications as it offers broader grating bandwidths and ease of production.

Methods devised in the prior art for the formation of in- fibre chirped gratings include: forming a series of in-fibre uniform gratings at varying Bragg wavelengths by transversely illuminating the side of a photosensitive optical fibre to an interference pattern formed by two overlapping, collimated, suitably angled actinic rays from a single coherent UV laser and translating the fibre or interference pattern while varying the intersection angle of the actinic rays; forming an in-fibre chirped grating by transversely illuminating the side of a photosensitive optical fibre, which has a core having a non-uniform refractive index profile along the length of the fibre, to an interference pattern formed by two overlapping, collimated, suitably angled actinic rays from a single coherent UV laser; forming an in-fibre chirped grating by transversely illuminating the side of a photosensitive optical fibre to a continuously translating (along the fibre axis) interference pattern formed by two overlapping actinic rays while varying the actinic wavelength using a suitable wavelength-tunable laser; forming an in-fibre chirped grating by transversely illuminating the side of a photosensitive optical fibre to an interference pattern formed by two overlapping actinic rays at different intersecting angles and/or dissimilar curvatures or wavefronts from a single coherent UV laser; forming an in-fibre chirped grating by transversely illuminating the side of a photosensitive optical fibre to an interference pattern of actinic light formed by impinging a single collimated, UV light beam on a non-uniform-period phase grating mask; forming an in-fibre chirped grating by transversely illuminating the side of a photosensitive optical fibre to an interference pattern of actinic light formed by impinging a single collimated, UV light beam from a single coherent UV laser on a prism interferometric arrangement using a prism with a curved face [19] or by non-uniform, collimated UV light beams from a single coherent UV laser on a. prism interferometric arrangement; forming an in-fibre chirped grating by transversely illuminating the side of a curved or tilted photosensitive optical fibre to a uniform interference pattern of actinic light formed from a single coherent UV laser; forming an in-fibre chirped grating by transversely illuminating the side of a photosensitive optical fibre to an aperiodic interference pattern formed by two overlapping, collimated, multiple wavelength, suitably angled actinic rays from a multiple wavelength laser; forming an in-fibre chirped grating by transversely illuminating the side of a tapered photosensitive optical fibre to an interference pattern formed by two overlapping, collimated, suitably angled actinic rays from a single coherent UV laser; forming an in-fibre chirped grating by transversely illuminating the side of a photosensitive optical fibre to an interference pattern formed by two overlapping, collimated, suitably angled actinic rays from a single coherent UV laser, before or during subjecting the fibre to strain or temperature gradients during the writing process; and forming an in-fibre chirped grating by transversely illuminating the side of a photosensitive optical fibre to a continuously translating (along the fibre axis) interference pattern formed by impinging a single collimated, UV light beam on a translating uniform- period phase grating mask, also known as velocity modulated exposure using a moving fibre/phase mask scanning beam technique. Some arrangements also translate the optical fibre .

To date, reliability problems, optical bandwidth limitations, overall grating length limitations and cost considerations have limited the usefulness of most of the techniques detailed above. Furthermore, the prior art methods for writing of very broad bandwidth gratings can often suffer from such disadvantages as the requirement of very good spatial coherence properties of the writing beam and/or superior mechanical stability.

The object of the present invention is to provide a method and waveguides formed by that method which possesses advantages over prior art, such as the ability to introduce high chirp magnitudes and relatively long chirped grating lengths, at a broad range of wavelengths, along with the same low sensitivity to mechanical vibrations as the conventional prism interferometer configuration.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a method of producing an optical waveguide having in-situ chirped gratings, including the steps of: providing at least first and second convergent or divergent beams of actinic radiation; superimposing the first and second convergent or divergent beams to generate a chirped interference pattern; and exposing the optical waveguide to the interference pattern to form the in-situ chirped grating in the waveguide.

Preferably the first and second beams are separated from at least one beam of actinic radiation.

Preferably the beam of actinic radiation is separated by directing the beam to a block of refractive material having a first face, a second face and a reflective face so that the beam passes into the first face, the beam being refracted so that a first portion of the beam propagates through the block of refractive material to the second face thereby forming the first convergent or divergent beam and a second portion of the beam propagates to the reflective face thereby forming the second convergent or divergent beam, the second beam being reflected from the reflective face to superimpose on the first beam at the second face to create the interference pattern.

Preferably the reflective face comprises a third face of the block of reflective material.

Preferably the block of reflective material comprises a right-angled prism with the first face being the hypotenuse face of the right-angle prism and the second and third faces being the remaining faces of the right-angle prism.

Preferably the optical waveguide is exposed to the interference pattern by locating the optical waveguide adjacent to the second face.

Preferably the first and second convergent or divergent beams have dissimilar wavefronts.

In another embodiment of the invention, the first and second beams can be produced by a single wavefront-dividing mirror interferometric technique .

The invention may also be said to reside in an optical waveguide made according to the method described above.

According to the preferred embodiment of the present invention, a coherent actinic radiation beam from a suitable optical source, preferably a UV laser, is directed and conditioned by a series of mirrors or optical elements, and focussed at a point so as to provide a path of diverging or converging actinic radiation. The diverging or converging actinic radiation is subsequently appropriately directed, at an appropriate angle, to a face of a block of refractive material, preferably, but not limited to, the hypotenuse face of a right-angle prism, such that a first portion of the beam propagates through the prism to a second face and a second portion is reflected at a third face of the block towards the second face. In this way, two divergent or convergent actinic beams, preferably, but without limitation, with dissimilar wavefronts, are superimposed at the second face of the prism, thus generating a chirped interference pattern in the region of overlap. The interference pattern is projected from the second face of the prism, enabling a suitable photosensitive or photo-refractive optical waveguide or fibre to be positioned in the field of the interference pattern. Preferably, the optical waveguide or fibre is positioned against the second face of the prism. An in-situ chirped grating is produced by transversely illuminating the side of the optical waveguide or fibre to the chirped interference pattern of actinic radiation for a suitable or desirable period of time. Production of the in-fibre chirped grating may be monitored during the photo- writing process by appropriate optical arrangements and equipment. Characteristics of chirped gratings produced by the method disclosed are dependent upon the angle of incidence of the writing beam, beam divergence or convergence angles, geometry of the prism, the distance between the beam focus and prism surface and the refractive index modulation depth (the strength of the refractive index change) . Transmission and reflection characteristics of the chirped gratings may be tailored by control over these same parameters, before or during the production process.

The main features of the preferred embodiment of the present invention, without imposing any limitations, involve: optical waveguide or fibre compositions, geometries and/or arrangements designed to optimise photosensitivity or photo-refractivity of the optical waveguides or fibres; mechanical, physical and optical arrangements designed to optimise the production of chirped gratings in said optical waveguides or fibres; a method for the production of in-fibre chirped gratings utilising an optical prism and superimposed divergent or convergent actinic radiation beams, preferably, but without limitation, with dissimilar wavefronts, to form the chirped interference pattern used in the photo-writing process; and a method for the production of in-fibre chirped gratings with the ability to introduce high chirp magnitudes and relatively long chirped grating lengths, at a broad range of wavelengths, while maintaining low sensitivity to mechanical vibrations.

The preferred embodiment of the present invention may be said to reside in a method of preparing or producing in- situ chirped gratings in optical waveguides comprising, but not limited to, the steps of: providing an optical waveguide or fibre (single or multi moded) formed from a waveguide material designed to optimise photosensitivity or photo-refractivity of the optical waveguide or fibre; utilising an appropriately directed single diverging or converging actinic radiation beam, at an appropriate angle, to a face of a suitable block of refractive material, preferably, but not limited to, the hypotenuse face of a right-angle prism, such that a first portion of the beam propagates through the prism to a second face and a second portion is reflected at a third face of the block, which may or may not be mirrored, towards the second face, thus generating a chirped interference pattern in the region of overlap by superimposing divergent or convergent actinic radiation beams, preferably, but without limitation, with dissimilar wavefronts; forming a permanent or temporary chirped grating in the photosensitive or photo-refractive optical waveguide or fibre to be processed by exposure to the interference pattern, thus forming a chirped grating in the waveguide or fibre; otherwise treating or processing the waveguide or fibre during the photo-writing process to optimise or otherwise modify the properties or characteristics of the chirped grating being produced; further treating or processing the waveguide or fibre with the in-situ chirped grating to optimise or otherwise modify the properties or characteristics of the chirped grating; preferably, protecting the optical waveguide or fibre with manufactured chirped grating by encapsulating or coating the desired region of the optical waveguide or fibre in a suitable device or material (ie. heatshrink fusion splice protector, acrylate, enamel, epoxy, polyimide, etc.); and passively or actively employing the optical waveguide or fibre with in-situ chirped grating, or individual or plurality of optical waveguides or fibres with a plurality of in-situ chirped gratings, in an optical waveguide or fibre system.

In another embodiment of the invention, a single wavefront- dividing mirror interferometric technique is used to create a chirped interference pattern with superimposed divergent or convergent actinic radiation beams, preferably, but without limitation, with dissimilar wavefronts.

The present invention is effective on any optical waveguiding material possessing photosensitive or photo- refractive properties and provides the opportunity to fabricate in-fibre chirped gratings.

Preferably the optical waveguide is an optical fibre, either single or multi moded. Preferably the optical waveguide is a doped or specially doped optical fibre, either single or multi moded. Preferably the optical waveguide is further processed prior to photo-processing to optimise photosensitivity or photo-refractivity of the waveguide. Preferably the optical waveguide is further processed following photo-processing to optimise the quality, mechanical stability or longevity of the in-fibre chirped grating. Preferably the optical waveguide is a silica-based optical fibre which can be fusion spliced to the end of a light guiding fibre and allows the incorporation of a number of chirped grating elements along a single fibre length. In another embodiment, a plurality of chirped gratings may be produced by the present invention in an optical waveguide or fibre simultaneously or sequentially, with the same or different optical properties or characteristics.

Preferably the waveguide comprises at least one optical fibre and/or at least one optical fibre device. In some embodiments of the invention the waveguide may merely comprise an optical fibre without any additional elements. However, the optical fibre can include passive or active elements along its length. Furthermore, the optical fibre can include sensing elements along its length and those sensing elements can comprise devices which will respond to a change in the desired parameter in the environment of application and influence the properties and characteristics of the electromagnetic radiation propagating in the waveguide to thereby provide an indication of the change in the parameter.

Preferably any suitable CW or pulsed single-frequency or multiple wavelength actinic radiation source or plurality of sources may be employed. In a preferred embodiment, without limitation, a CW or pulsed coherent UV laser is utilised to supply the actinic radiation.

Preferably the block of refractive material is a right- angle, right triangular or rectangular prism, or any other n-sided prism of suitable geometry. In some embodiments, a plurality of prisms may be employed. Also, mirrors or reflective films may be employed on some of the prism surface (s) .

Preferably the optical waveguide or fibre is positioned firmly against the face of the prism projecting the chirped interference pattern. However, in other embodiments, the optical waveguide or fibre may be translated or located a distance from the prism face. In other embodiments the optical waveguide or fibre may be rotated or curved relative to the prism face. In addition, other optical elements, such as lenses, may be utilised between the prism and the optical waveguide or fibre. Furthermore, additional optical elements, such as lenses, phase masks, amplitude masks, etc., may be employed in conjunction with the present invention to obtain other grating structures.

Preferably, but without limitation, according to the present invention, superimposed divergent or convergent actinic radiation beams with dissimilar wavefronts are used to create a chirped interference pattern on a face of a bulk optic prism. However, in another embodiment, superimposed divergent or convergent actinic radiation beams with similar wavefronts may be used to create a chirped interference pattern.

Preferably, but without limitation, according to the present invention, a chirped interference pattern is generated on one face of a bulk optic prism by impinging a single divergent or convergent actinic radiation beam to a second face of the prism. However, in another embodiment, a more complex chirped interference pattern may be generated on one face of a bulk optic prism by impinging a plurality of divergent or convergent actinic radiation beams to a second face, of the prism.

Preferably, but without limitation, a single or plurality of in-fibre chirped gratings produced by the present invention are used, either passively or actively or both, for optical filtering, dispersion compensation, pulse shaping (broadening or compression) , multiplexing, amplifier gain flattening, sensing or creating optical resonators in optical systems . Sensing elements based on the present invention may be in the form of a single-point sensor, quasi-distributed sensors, distributed sensors and/or multiplexed-demultiplexed sensors.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the present invention will be further illustrated, by way of example, with reference to the following drawings in which:

Figure 1 is a view showing embodiments of the present invention illustrating apparatus suitable for the production of in-fibre chirped gratings;

Figure 2 is a view showing an embodiment of the present invention illustrating a schematic diagram of a single divergent actinic radiation beam impinging and propagating through a right-angle prism;

Figure 3 is a view showing an alternate embodiment of figure 2; Figure 4 is a graph illustrating the theoretical chirping profiles for two different wavelength regions according to the present invention;

Figure 5 is a graph illustrating the theoretical grating length as a function of the divergence angle for 670 nm region chirped gratings according to the present invention;

Figure 6 is a graph illustrating the theoretical behaviour of the chirping parameter as a function of distance between the focus point of the actinic beam and the prism for two different wavelength regions according to the present invention;.

Figure 7 is a graph illustrating the theoretical chirped grating reflected spectral response for two different wavelength regions according to the present invention;

Figure 8 is a graph illustrating the experimentally derived reflected spectral response for an 850 nm region in-fibre chirped grating;

Figure 9 is a view showing an embodiment of the present invention illustrating a schematic diagram of a single convergent actinic radiation beam .Impinging and propagating through a right-angle prism;

Figure 10 is a view showing an embodiment of the present invention illustrating a schematic diagram of two divergent actinic radiation beams with similar wavefronts propagating through a suitable prism to interfere and produce a unique interference pattern, and thus grating, profile;

Figure 11 shows a fibre optic sensor system formed by the method of the preferred embodiments of the present invention;

Figure 12 shows a spectroscopic system employing optical filter device (s) in the form of a fibre spectrometer made according to the preferred embodiments of the present invention;

Figure 13 shows an embodiment of the present invention applied to a long-distance optical fibre telecommunications line; and

Figure 14 shows a wavelength multiplexed fibre- to-the-home system made according to the preferred embodiments on the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to figure 1, a chirped grating 14 is produced in a photosensitive optical fibre 11 by the following method. A single 245 nm UV, coherent actinic radiation beam 2 from a coherent UV source such as a MOPO or exci er laser 1, is. directed and conditioned by a series of apertures 4, beam directing prisms 5, a beam expanding lens 6 and cylindrical lens 7, and focussed at a point by cylindrical lens 9 so as to provide a single diverging actinic radiation beam 10. The diverging actinic radiation beam 10 is appropriately directed, at an appropriate angle, to the hypotenuse face 22 of a right-angle prism 20, such that a first portion of the beam 12 propagates through the prism 20 to a second face 23 and a second portion 13 is reflected at a third face 24 of the prism 20 towards the second face 23. In this way, two divergent or convergent beams 12 and 13 with dissimilar wavefronts are superimposed at the second face 23 of the prism 20, thus generating a chirped interference pattern in the region of overlap 26. The interference pattern is projected 28 from the second face 23 of the prism 20, enabling the interference pattern to be viewed by a suitable screen 29 located an appropriate distance from the prism 20. The photosensitive optical fibre 11 is positioned firmly against or in the viscinity of the second face 23 of the prism 20, thus transversely illuminating the side of the optical fibre 11 to the chirped interference pattern of actinic radiation for a suitable or desirable period of time and forming an in-situ chirped grating 14.

If desired, a probe beam of light at the relevant range of wavelengths can be simultaneously launched into the optical fibre 11 during the photo-writing process. By this method, production of the in-fibre chirped grating 14 may be monitored for optimisation or modification during the photo-writing process by launching light from a broadband source 16 (ie., LED or SLD) into one end of the optical fibre 11 and monitoring the spectral changes of the original light at the output end of the fibre 11 with the use of an optical spectrum analyser 18.

With reference to figure 2, a schematic of the optical beam propagation through a right-angle prism for the case of a divergent beam is shown, in accordance with the method of figure 1.

With reference to figure 3, an alternative schematic of the optical beam propagation through a right-angle prism for the case of a divergent beam is shown, in accordance with the method of figure 1.

For the purpose of demonstration only, an analysis of the beam propagation for the case of figures 2 and 3 follows:

Consider a divergent beam 10 falling onto the hypotenuse face 22 of a right-angle prism 20. Let the half-divergence angle be α and the angle of incidence be i (beam axial ray) . If the axial ray is refracted into the vertex of the prism, then the angles of incidence for the marginal rays are (i+α) and (i-α) and, accordingly, their refraction angles are sin^'1 [sin (i+oz) /n] and sin^'1 [sin (i-a) /n], where n = 1.51 is the refractive index of quartz at λσv = 245 nm. As it follows from the prism geometry of figure 2, two divergent beams are produced 12 and 13, one of which is formed by rays having angles of incidence between i and (i+α), the other formed by rays having angles of incidence between (i-α) and i. The divergent beams overlap within the prism 26, on its surface 23, and in the air 28.

In this case, it can be shown that the two emerging beams 12 and 13 have different angles of divergence and hence, the wavefronts have dissimilar curvatures.

This requires consideration of the angles δ, θi and θ₂ formed by the central and marginal rays and the normal to the prism surface 23. From simple geometry these angles have the following values :

δ = π/4 - sin^'l[sin(i)/n] (2) θ = π/4 - sin^'1 [sin(i - a)/n] ( 3 ) θ₂ = π/4 - sin^~l [sin(i + a)/n] (4 )

and therefore the angles of divergence for the two outgoing wavefronts are determined by the relationships :

/?, = -?, - δ = sin^~l [sin(i)/n]- sin^~i [sin(i - a)/n], ( 5 ) β₂ - δ- θ_τ = sin^~l [sin(i + a)/n)- sin^~ [sin(i)/n] ( 6 )

giving rise to the different angles βx and β₂ for the case when a ≠ 0. Hence the field curvatures for the interfering wavefronts are dissimilar which produces a non-uniform interference fringe spacing along the prism surface 23.

Now considering the beams 12 and 13 overlapping at the prism surface 23 in more detail. The grating length is limited by the common overlap region 26 of the two beams given by L = min(BC,BD) . The relative positions of points C and D will depend upon the combination of parameters i, d, and α. For the geometry depicted in figure 2 when BD < BC, the interference fringe spacing at point D is determined by the angles θi and x formed by rays LD and ZD. Z is the point on the hypotenuse face AS determined by the ray of the incident beam which is deflected in the direction of point D. The local angle of incidence (i+θo), (where 0< Oo < α ) for the ray FZ is calculated as a numeric solution of a trigonometric equation of the form BC(i+αo) = BD(i-α) which is obtained by taking into account the following relationships for the beam propagation:

, _Λ asin[π/4 + sin^'l (sin(i)/n] (7)

AO = cosfsin (sin(i)/n)J dsin(a) (8)

O = cos(i + a) dsin( ) (9)

ON cos(i - a)

(AO - OM) cos[sin^'1 (sin(i X a) /n)] d°⁾

AC = sin[π/4 + sin^'1 (sin(i + a) /n)]

BC = a - AC ⁽ID

(a /2 - AO - ON)sin(3π/4 - θ_l ) (12)

LS = sin(θ, )

BD = (a - LS)tan(θ, ) ⁽13⁾

where a = AB is the prism dimension. It follows that the angle rτ*o is found by numerically solving the equation:

(AO - OM) cosfsin ' (sin(i + oz₀ ) /n)] a _: ^ — ; — ; = (a - LS ) tan(θ. ) sin[π/4 + sin^' (sin i(/ii' +-L a/V₀) 1 / /nn) 1]7 ( 14 ) The angle x is calculated using the following formula:

x ^■=■ π/4 — sin^~ [sin(i + a₀)/n] ( 15 )

For the geometry depicted in figure 3, when BD > BC, the interference fringe spacing at point C is determined by the angles θ₂ and y formed by rays MC and KC. K is the point on the prism face BS determined by the ray which is reflected by face BS in the direction of point C. In this case the angle y is calculated using the formula:

y = π/4 - sin^'] [sin(i - oz₀ )/n] (16)

where oo is the splution of the trigonometric equation of the form BD(i-O-o) = BC(i+α) which is obtained by using the geometric relationships (2-4) and (7-13) and substituting (i-α)by (i-α₀) •

The local angles of intersection between the two interfering wavefronts 12 and 13 and the dependency of these angles on the position along the prism surface 23 can now be calculated. It follows that the local chirped grating periodicity for the two cases of beam overlap has the form:

Λ = "UV n[sin(θ_] ) + sin(x)] ( 17 ) and

"UV

Λ = - n[sin(θ₂ ) + sin(y)] ( 18 )

respectively. Therefore the chirped Bragg wavelength is a function of position along the grating given by:

λ_B (L) - 2n_β£fA (L) , (19) where n_aff is the effective refractive index of the fibre core. Numerical analysis of the chirping profile function A(z) shows, to a very good approximation, it has the form:

Λ(z) = A_uaj_c-bZ² (20)

where A_n^_x is the fringe spacing at the prism vertex. Therefore, by varying parameters i, d, and α, the Bragg wavelength, grating length, and parameter b may be controlled.

Figure 4 illustrates some plots of the theoretical chirping profile λ_B (L) as a function of distance d and angle i for wavelengths in the region of 670 nm and 1550 nm. The chirping profile parameter determines the rate at which the grating periodicity varies along its length and is strongly dependent upon the angle of incidence of the writing beam (hence Bragg wavelength) , the split beam divergence angles and the distance between the beam focus and prism surface.

By carefully selecting these parameters the chirping profile may be controlled to tailor-design the grating spectral response most suitable for a particular application. For example, in dispersion compensation and pulse compression in communications systems, an approximately linearly chirped in-fibre grating can be designed by choosing low values of b; -0.2-0.4 nm/mm² in the 900 nm regime, -0.6-0.8 nm/mm² in the 1300 nm regime and -1.4-1.6 nm/mm² in the 1550 nm regime (for a beam diameter -5 mm, focal length -40 mm and the distance between the beam focus and prism surface optimised for the particular set of parameters) . Linearisation of the grating response is also achievable by masking the fibre near the prism vertex in order to eliminate the nonlinear region of the chirping profile function. Some control of the linear chirping rate is also possible. Figure 5 illustrates a plot of the theoretical grating length L_G as a function of α, for various values of d, for the 670 nm wavelength region.

Figure 6 illustrates a plot of the theoretical dependency of the chirping function parameter b on distance d for gratings with λ_nax = 670 nm and 980 nm.

Figure 7 illustrates the calculated spectral responses for chirped grating reflectivities in the wavelength regions of 670 nm and 1550 nm.

Referring to figure 8, there is illustrated the transmission spectrum of an in-fibre chirped grating produced by the method, of the present invention illustrated in figure 1.

With reference to figure 9, a schematic of the optical beam propagation through a right-angle prism for the case of a convergent beam is shown, in accordance with the method of figure 1. The calculated chirping profiles are similar to those obtained with a diverging beam although the converging beam configuration greatly restricts the grating length and range of the chirping parameter b. This effectively limits the grating full-width bandwidth to approximately 10-20 nm in the 1300-1550 nm regime. However, the ability to control the chirping parameter by varying the distance between the beam focus and prism surface is retained, with the focus situated on the opposite side of the prism. The range of useful values of d for which the grating length and chirping profile (hence bandwidth) can be controlled will be primarily limited by the prism size and beam divergence. Theoretical calculations of the chirping function for the converging beam configuration differ marginally from those for the diverging beam case. Figure 10 illustrates several diagrams related to the present invention, illustrating (a) a schematic diagram of two divergent actinic radiation beams with similar wavefronts 64 and 66 propagating through a suitable block of refractive material 63 to interfere and produce a unique interference pattern, and thus grating 69, profile 68, as well as a (b) plot and (c) diagram of the theoretical chirped grating profile from this embodiment . For the purpose of demonstration only, an analysis of the beam propagation for the case of figure 10 follows:

Consider a holographic interferometer producing the interference of two divergent beams 64 and 66 with equal divergence angles α, as shown in Figure 10(a).

Assume the focus, of each beam is equidistant from the fibre axis, i.e. b, and the central rays are inclined at an angle δ with respect to the normal axis . Triangles AOFi and B0F₂ are therefore mirror mages and the grating length is defined by the common beam overlap region AB 68. Point O is the centre of the interference pattern (i.e., AO=OB) and maximum beam overlap occurs when points Aχ=A and Bι=B which is controlled by parameter Jb. Let the angle between rays F₂0 and F₂A be Orj. This can be found by numerically solving the trigonometric equation obtained by applying the sine theorem to triangles FiAO and F₂AO:

sin a 1

5172 ₀ cos(δ - a) cos(δ + ₀) (21)

Consider point C on the grating axis in order to calculate the local fringe spacing as a function of position along the grating A (x) . Angles OF_xC and OF₂C are denoted as α_* and β_x, respectively. These can be found by numerically solving the trigonometric equations obtained by applying the sine theorem to triangles FiAC and F₂OC:

sin(a - _x ) cos(δ - a)cos( _x - δ) (22) x _ b sin(a₀ - β_x ) cos(δ + β_x )cos(δ + ₀ ) (23)

Angles α_* and β_x can be determined for any value of x along the grating and the chirping profile function can be calculated with the following function:

λ UV

A(x) = sin(δ - _x ) + sin(δ + β_x )

( 24 )

The local Bragg wavelength is determined by the relationship:

^λBra ^X) = ²ⁿeff (^X)

(25)

Figure 10(b) shows an example of a chirping profile for a grating, approximately 14 mm long, in the 900 nm region. The curve-fitted function well approximates a parabola of the form λ_BrΛgg (x) =a+bx+cx² , with a=923.285, b=-1.372 and c=0.099. The parameters used in the calculation are: λuv=245 nm, b=100 mm, α=0.06 rad. and δ=0.4 rad.

Figure 10(c) shows an example of a symmetric chirped in-fibre Bragg grating 69 with a refractive index modulation profile similar to that depicted in Figure 10(b). This device has many potential applications, i.e., positive and negative dispersion compensation in communications systems and sensing using broadband Fabry-Perot interferometry, although the application range and performance of this device is in no way limited to the above these examples .

Figure 11 illustrates a fibre optic sensor system employing chirped gratings 14a and 14b as broad bandwidth reflectors for the formation of a broad bandwidth optical resonant cavity. The gratings have the same or similar chirping profiles. The output of the sensor is monitored by choice of various possible optical signal processing schemes 30 and detector(s) 32. Broad bandwidth light from a superluminescent diode (SLD) , tuneable laser diode (LD) or other suitable optical source 34 is launched into optical fibre 11 via optical fibre 36 and coupler 38. Reflected light from the gratings 14a and 14b is coupled into fibre 39 from coupler 38 and then to possible optical signal processing schemes 30 and detector (s) 32.

Figure 12 illustrates a fibre optic chemical sensor based on evanescent-wave spectroscopy. Broad bandwidth light from sources 44 and 46 is launched into D-shaped or tapered sensing optical fibre 60 via optical fibres 47 and 48, coupler 50, optical fibre 52, coupler 54, optical fibre 56 and fibre splice 58. Reflected light from the mirror or chirped gratings 62 at the end of fibre 60 is coupled into fibre 53 from coupler 54 and then to passive or active chirped grating bandpass filter 40 and detector 42. The detector may also include a processor for performing Fourier transform analysis on the reflected wavelengths.

In the intrinsic sensing configuration illustrated in figure 11, the fibre buffer is partially or totally removed from the desired sensing location(s) and the fibre 60 is brought into contact with the sensing medium. The medium absorbs particular wavelengths corresponding to the chemical species present. These absorption peaks can be monitored in real-time and their intensities related to the concentration of analytes. Fibre 60 selection and the degree of cladding removal are important to the sensitivity of the technique, and in some cases are dependent on the sensing medium. Chirped grating(s) 62 corresponding to wavelengths of interest may be incorporated into the sensing fibre 60 to reflect the desired wavelengths or may be part of the instrumentation 40 which detects and monitors the optical information.

Figure 13 illustrates a simple long-distance, optical fibre telecommunications line 85 with incorporated dispersion compensating chirped grating systems 70 for signal shaping and conditioning. As the optical signal from a suitable source 80 such as a laser or the like travels long distances along the fibre 11 the signal broadens by a dispersion effect. Optical signal shaping and conditioning chirped grating systems 70 can be incorporated at the required distances or intervals along the fibre line 85 to restore the signal to the original or otherwise desired state. The chirped grating systems 70 can be the sole devices incorporated into the fibre 11 or they can be part of a fibre optic amplifier system if the optical power losses in the overall system are high.

Figure 14 illustrates a wavelength multiplexed fibre-to- the-home system utilising in-situ chirped gratings 90. The number of optical channels is increased by using optical waves at different carrier wavelengths. Thus, at the customer receiving end, wavelength (ie. channel) selectors or filters are required to separate the various components. These selectors may be formed by passive or active chirped gratings 90.

EXAMPLES OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention have been tested illustrated by the following examples. The in-fibre chirped gratings were constructed in order to demonstrate the feasibility of producing fibre optic chirped gratings as described herein. Not all of the results obtained to date are detailed in the following examples .

Example 1: Production and Operation of In-Fibre Chirped Grating at λniax = 1476 nm

An in-fibre chirped grating was produced in standard single-mode communications fibre, hydrogen loaded for - 5 days at - 100 atm. pressure. The writing conditions are summarised as: α ~ 0.062 radians d = 20 mm L = 2.5 mm

Hence the chirping profile coefficient is determined to be Jb = 1.12 nm/mm²

The transmission spectrum of the chirped in-fibre grating was acquired with an optical spectrum analyser, and the following characteristics were determined: λ_πuut = 1476 nm Δλ = 2 nm (FWHM) Δλ = 7 nm (Full-width)

R - 70%

Example 2: Production and Operation of In-Fibre Chirped Grating at λ_nax = 854.5 nm

An in-fibre chirped grating was produced in standard single-mode communications fibre, hydrogen loaded for ~ 3 days at - 100 atm. pressure. The writing conditions are summarised as : α ~ 0.124 radians d ~ 30 mm L ~ 4 mm Hence the chirping profile coefficient is determined to be b ~ 0.38 nm/mm²

The transmission spectrum of the chirped in-fibre grating, which is shown in figure 8, was acquired with an optical spectrum analyser, and the following characteristics were determined:

Δλ = 2.4 nm (FWHM)

Δλ = 6 nm (Full-width) R = 70%

Since modifications within the spirit and scope of the invention may readily be effected by persons skilled within the art, it is to be understood that this invention is not limited to the particular embodiments described by way of example hereinabove.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A method of producing an optical waveguide having in-situ chirped gratings, including the steps of: providing at least first and second convergent or divergent beams of actinic radiation; superimposing the first and second convergent or divergent beams to generate a chirped interference pattern; and exposing the optical waveguide to the interference pattern to form the in-situ chirped grating in the waveguide.

2. The method of claim 1, wherein the first and second beams are separated from at least one beam of actinic radiation.

3. The method of claim 2 , wherein the at least one beam of actinic radiation is separated by directing the beam to a block of refractive material having a first face, a second face and a reflective face so that the beam passes into the first face, the beam being refracted so that a first portion of the beam propagates through the block of refractive material to the second face thereby forming the first convergent or divergent beam and a second portion of the beam propagates to the reflective face thereby forming the second convergent or divergent beam, the second beam being reflected from the reflective face to superimpose on the first beam at the second face to create the interference pattern.

4. The method of claim 3, wherein the reflective face comprises a third face of the block of reflective material .

5. The method of claim 3, wherein the block of reflective material comprises a right-angled prism with the first face being the hypotenuse face of the right-angle prism and the second and third faces being the remaining faces of the right-angle prism.

6. The method of claim 3, wherein the optical waveguide is exposed to the interference pattern by locating the optical waveguide adjacent the second face.

7. The method of claim 1, wherein the first and second convergent or divergent beams have dissimilar wavefronts .

8. The method of claim 1, wherein the first and second beams are produced by a single wavefront-dividing mirror interferometric technique.

9. A method of preparing or producing in-situ chirped gratings in optical waveguides including the steps of: providing an optical waveguide or fibre formed from a waveguide material designed to optimise photosensitivity or photo-refractivity of the optical waveguide or fibre; utilising an appropriately directed single diverging or converging actinic radiation beam to a face of a block of refractive material such that a first portion of the beam propagates through the prism to a second face and a second portion is reflected at a third face of the block towards the second face, thus generating a chirped interference pattern in the region of overlap by superimposing divergent or convergent actinic radiation beams; forming a permanent or temporary chirped grating in the photosensitive or photo-refractive optical waveguide or fibre to be processed by exposure to the interference pattern, thus forming a chirped grating in the waveguide or fibre; and otherwise treating or processing the waveguide or fibre during the photo-writing process to optimise or otherwise modify the properties or characteristics of the chirped grating being produced.

10. An optical waveguide made according to the method of claim 1.