WO1994019713A1

WO1994019713A1 - Optical waveguide grating

Info

Publication number: WO1994019713A1
Application number: PCT/GB1994/000330
Authority: WO
Inventors: David Neil Payne; Philip Stjohn Russell; Laurence Reekie; Jean-Luc Archambault
Original assignee: University Of Southampton
Priority date: 1993-02-19
Filing date: 1994-02-18
Publication date: 1994-09-01
Also published as: GB2275347A; GB9303374D0

Abstract

A method of fabricating an optical waveguide grating comprises the step of thermally damaging selected regions of the waveguide by exposure of those regions to transverse optical illumination.

Description

OPTICAL WAVEGUIDE GRATING

This invention relates to optical waveguide gratin s.

Optical waveguide gratings, such as fibre Bragg gratings, are used as in-line fibre reflectors and tuning elements for fibre lasers, as optical filters (e.g. for erbium (Er^3*) doped fibre amplifiers) and as sensors for measuring strain, temperature and pressure. Optical waveguide gratings may also be fabricated on planar waveguides.

Traditionally, two methods have been used to make such gratings, namely physical etching of a fibre polished to expose the fibre core, and photorefractive holographic exposure of the core to ultraviolet (UV) or blue light.

The first method, involving physical etching of the fibre core, can provide a permanent grating with a large refractive index modulation. However, it requires a considerable investment of time and effort on the part of the fabricator, and such gratings can take several days to produce with no guarantee of success due to the number of critical steps in the process. Furthermore, it is necessary to strip the protective polymer coating from the fibre and polish away the cladding to expose the core before etching the grating. This locally reduces the strength of the fibre exactly at the point where strength may be required, for example if the fibre is to be used as a strain sensor.

The second method, photorefractive holographic exposure, is described in EP-191063-A, and involves directing two transverse, interfering beams of UV laser light onto the fibre. The two beams interfere to generate a periodic fringe pattern of illumination; at local illumination maxima a photorefractive process takes place in the fibre core to generate regions of slightly modified refractive index. This technique has the advantage of only requiring a short production time, but suffers from several disadvantages: a) a stable interferometer free from drift, vibration and air currents is generally required to expose the fibre to generate a grating; t») the gratings can be erased by exposing to temperatures of around

400°C; c) the gratings can be erased by subsequent re-exposure to UV or blue illumination; and d) the protective fibre coating must be removed before fabricating the grating.

This invention provides a method of fabricating an optical waveguide grating, the method comprising the step of thermally damaging selected regions of the waveguide by exposure of those regions to transverse optical illumination.

Using a method according to the invention, it is possible, using a purely optical technique, to fabricate fibre gratings on a production-line basis which have the advantages of etched gratings, and which are significantly more robust than the photorefractive gratings described above.

The method uses exposure to transverse optical illumination to thermally damage (i.e. physically modify and visibly damage) the material of the waveguide, (e.g. the glass in the core, in the cladding, or at the core/cladding interface of an optical fibre or in or adjacent to the guiding region of a planar waveguide) , thus giving rise to a modulation in the effective refractive index experienced by light guided through the waveguide. In general, the presence of thermal damage may be verified by examination of the waveguide under a microscope.

In one application, the modulation of the refractive index causes light propagating along the waveguide which is incident upon and resonant with the grating to be reflected back along the waveguide. The optical bandwidth over which this effect takes place is dependent on the length of the grating and the strength of the index modulation.

Unlike photorefractive gratings which are formed by modifying the refractive index of the fibre core, gratings fabricated by a method according to the invention may be made robust at temperatures well in excess of 400°C. In addition, in contrast to the photorefractive gratings described above, the thermally damaged regions of the gratings (in particular, any thermally damaged regions of the cladding of an optical fibre) are not significantly perturbed by low levels of light in the green to UV range which can slightly alter the refractive index of the core (thereby erasing or weakening a photorefractive grating). Gratings fabricated in accordance with the invention are therefore suitable for use as, for example, sensors in hostile environments or as feedback elements in fibre lasers operating at the blue/green end of the visible spectrum.

Preferably the transverse optical illumination comprises one or more light pulses. In this case, the method may be used to fabricate gratings using only a single laser pulse with a duration of e.g. a few tens of nanoseconds, thus dispensing with the need for a stable, isolated and enclosed environment. It therefore becomes practical to write such gratings during the fibre drawing process, before the fibre is coated, thus maintaining the fibre strength, since there is no longer a need to strip the coating for subsequent grating fabrication. This is especially significant in the case of quasi-distributed fibre sensors which require many gratings to be written along the length of a single optical fibre.

In a preferred embodiment the transverse optical illumination comprises a plurality of interference fringes generated by interference between two coherent beams from a single light source. This technique generates a pattern of optical illumination having a precise and repeatable periodicity.

In order that the wave fronts of each of the two coherent beams are not reversed with respect to one another, it is preferred that the numbers of reflections undergone by each of the two coherent beams differ by a multiple of two (i.e. 0, 2, . . . ) .

Although the grating may be formed by a single exposure to the optical illumination, in one preferred embodiment the method comprises the step of sequentially thermally damaging a plurality of regions of the waveguide by exposure to a respective plurality of light pulses.

Although other light sources may be used, it is preferred that the optical illumination comprises light generated by an excimer laser.

The choice of light source depends on the absorption characteristics of the material of the absorbing region of the waveguide. Preferably a krypton-fluoride (KrF) laser is employed.

It is preferred that the optical illumination has a wavelength of less than 00 nm.

In one preferred embodiment, the regions form a grating which is periodic along a transmission direction of the optical waveguide. In another preferred embodiment the regions form a grating having a varying pitch along a transmission direction of the optical waveguide. The invention is applicable to various types of waveguide, such as an optical fibre waveguide or a planar waveguide.

In the case of an optical fibre waveguide, it is preferred that the cladding of the optical fibre waveguide is substantially transparent to the optical illumination and the fibre core is doped with an absorbing dopant to absorb the optical illumination. A number of dopants are suitable for raising the absorption of the waveguide material, with the choice of dopant depending on the wavelength of the optical illumination. In preferred embodiments the fibre core is doped with oxides of germanium (which absorb the light generated by a KrF laser) and, optionally, one or more of the transition metals, boron and the rare earths.

Alternatively, in order to promote thermal damage in the fibre cladding (i.e. the generation of a grating in the cladding) it is also preferred that at least a part of the cladding of the optical fibre waveguide is doped with an absorbing dopant to absorb the optical illumination.

In a preferred embodiment, the cladding has a layered structure, with the grating being formed on an inner layer (e.g. a layer adjacent to the fibre core. In other words, it is preferred that the cladding of the optical fibre waveguide comprises an outer cladding layer substantially transparent to the optical illumination, and an inner cladding layer doped with an absorbing dopant to absorb the optical illumination. As an alternative, in order to promote thermal damage towards the centre of the fibre core, in one embodiment the centre of the core is more heavily doped with the absorbing dopant than the radial periphery of the core.

Preferably the absorption by the core of the optical illumination increases with the temperature of the core.

Preferably, the thermal damage is induced by the transverse optical illumination being arranged to provide at least a threshold energy density or at least a threshold transient temperature within the waveguide. In particular, it is preferred that the illumination provides an energy density incident on the waveguide of at least 0.5 Jem^"2 (Joules per square centimetre). Alternatively, or in addition, it is preferred that the transverse optical illumination is arranged to cause a transient heating of at least a part of the waveguide to a temperature of at least 1000 degrees Celsius.

Viewed from a second aspect this invention provides a method of fabricating an optical fibre, the method comprising the steps of: drawing the optical fibre from a heated preform; fabricating a grating in the core of a portion of the drawn optical fibre using a method as defined above; and coating the optical fibre with a protective coating.

Viewed from a third aspect this invention provides an optical waveguide grating in which selected regions of the waveguide are thermally damaged by exposure of those regions to transverse optical illumination.

An optical waveguide grating according to the invention is particularly suitable for use in a laser; an optical amplifier; an optical sensor or a wavelength dependent optical tap. Viewed from a fourth aspect this invention provides apparatus for fabricating an optical waveguide grating, the apparatus comprising means for thermally damaging selected regions of the waveguide by exposure of those regions to transverse optical illumination.

The various preferred features of the invention identified in the present application relating to methods of fabrication are equally applicable to other aspects of the invention (e.g. a grating, an apparatus for fabricating a grating, or a method of fabricating an optical fibre) and vice versa.

The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which:

Figure 1 is a schematic diagram of an interferometer for use in the fabrication of an optical waveguide grating;

Figure 2 is an expanded view of the incidence of two beams on an optical fibre;

Figure 3 is a graph showing the peak-to-peak effective index modulation of the gratings against the illumination pulse energy;

Figure 4 is a graph illustrating the optical performance of the gratings; Figure 5 is a graph showing normalized reflected optical power at the Bragg wavelength against time, for various temperatures;

Figure 6 is a schematic diagram of a planar waveguide grating; Figure 7 is a schematic diagram of a laser;

Figure 8 is a schematic diagram of an optical fibre laser;

Figure 9 is a schematic diagram of an optical fibre sensor;

Figure 10 is a schematic diagram of an optical amplifier; Figures 11a and lib show the output spectrum at the two outputs of the apparatus of Figure 10;

Figure 12 is a schematic diagram of a wavelength selective optical tap; and

Figure 13 is a schematic diagram of a grating array. Figure 1 is a schematic diagram of an interferometer for use in the fabrication of an optical waveguide grating.

The interferometer receives a pulsed beam of ultraviolet (UV) light from an excimer laser such as the Lambda Physik ENG-150 krypton- fluoride (KrF) excimer laser (not shown). In the present embodiment, the laser generates 0.1 Joule, 20 nanosecond (ns) pulses at a wavelength (λ) of 248 nanometres (nm) and with a coherence length of 25mm. The pulse energy is monitored by a pyroelectric energy meter (not shown) which receives a portion of the beam tapped off by a fused silica optical flat (not shown) . In response to the pulse energy detected by the pyroelectric energy meter, the pulse energy of the excimer beam 10 is controlled using a half-wave plate and polarizer arrangement (not shown) having a variable transmission loss.

The excimer beam 10 is passed through two cylindrical lenses 20, 30 before being supplied to a beam splitter 40. The beam splitter 40 splits the excimer beam 10 into two substantially equal beams 50 and 60. The beam 50, representing the portion of the beam reflected by the beam splitter 40, is then reflected from three mirrors 65. 70 and 80 before impinging on an optical fibre 100. Similarly, the beam 60, which represents the portion of the excimer beam 10 transmitted by the beam splitter 40, is reflected by two mirrors 110, 120 before impinging on the optical fibre 100. The beam 50 and the beam 60 are transversely incident on the optical fibre 100 at angles of incidence of ± θ.

In the present embodiment, the pulse energy was controlled to provide an energy density incident on the optical fibre 100 of at least about 0.5 Jem^"2 (0.5 Joules per square centimetre). This gave rise to transient heating of absorbing parts of the fibre to temperatures in excess of about 1000°C (degrees Celsius). The use of these parameters led to thermal damage of the fibre in the present embodiment, rather than mere photorefractive alteration of the fibre material.

In place of the cylindrical lenses 20, 30 which act cr the beam 10, a cylindrical lens 90 may be used to focus the beams 50, 60 onto the optical fibre 100.

The excimer beam 10 has a coherence length of 2 mm. Accordingly, in order to achieve interference between the two beams 50 and 60 split from the excimer beam 10 by the beam splitter 40, the optical path lengths traversed by each of the two beams 50 and 60 between the front surface of the beam splitter 40 and the optical fibre 100 should be substantially identical. A further feature of the apparatus of Figure

1 is that, in contrast to conventional interferometers, the numbers of reflections undergone by the two beams 50, 60 differ by a multiple of

2 (0, 2, 4 ...), which means that the wave fronts of each of the two beams 50 and 60 are not reversed with respect to one another. This ca improve the visibility of interference fringes between the two beams when the spatial coherence across the wave front is poor (as is the case with many excimer lasers).

Figure 2 is an expanded view of the incidence of the two beams 50 and 60 on the optical fibre 100. The two beams are incident on the fibre at complementary angles of ± θ. The beams are refracted as they enter the fibre cladding 130 and are focused by the cylindrical lens 90 (not shown on Figure 2) onto the fibre core 140.

The fibre core 140 contains a dopant which has a high absorption at the UV wavelength being used to write the grating (in this case, 248nm) , such that a substantial proportion of the incident light is rapidly absorbed. An interference pattern is set up in the core of the fibre which periodically modifies the state of the core glass by physically damaging the core or the core/cladding interface. This creates a modulation in the refractive index of the fibre 100. (In alternative embodiments, the damage could be restricted to the fibre cladding or to a region or layer of the fibre cladding) .

For use as a wavelength dependent reflector, the grating is designed such that: λ_D

( 1 )

²ⁿeff

where Λ=the grating pitch, λ_B=the Bragg (resonant) wavelength of the grating and n_eff=the effective refractive index of the core mode.

The Bragg wavelength can be altered by changing the pitch of the interference fringes generated by the two beams. This can be achieved either by changing the angle of the interfering UV beams or by tuning the writing laser to a different wavelength within the absorption band of the glass of the absorbing region of the fibre.

The fringe pattern in the present embodiment has a pitch of a few hundred nanometres, so that the resulting grating is resonant with visible or near infrared light.

In the present case, a silica fibre doped with approximately 15 mol% Ge0₂ (germania) was used. Germania typically gives rise to an absorption in silica around λ=24θnm of:

«₍₂₄o™,₎ -200+lOOx [ Ge0₂] _{moj% (2})

where α is measured in dB/cm. This means that for a fibre containing 15 mol# Ge0₂, α=1700dB/cm, or 0.17dB/μm.

Absorption of the UV light by the doped silica causes a local rise in temperature, which will eventually (around T=1000°C) cause a further increase in the absorption, leading to a more rapid temperature rise, and eventually to local disruption or thermal damaging of the material at temperatures well in excess of 2000°C. These transient writing temperatures are measurable using the so-called coherent anti-

Stokes Raman spectroscopy technique.

The pulse of light used in this fabrication method is sufficiently short that the energy is absorbed within the thermal time constant of the fibre core (or local absorbing region) , allowing the temperature to rise above that required to cause damage.

Assuming that there is good fringe visibility in the interference pattern, then significant damage occurs only at the peaks of the intensity profile. The proximity of this periodically damaged region to the core gives rise to a large effective index change. As an example, a peak-to-peak effective index modulation of Δn * 6xl0^"3 was obtained from a single 40mJ pulse when two beams from a KrF excimer laser operating at λ = 24δnm were focused to an area 15mm x 300μm. This is sufficient to give a grating efficiency (reflectivity) of greater than 99%. i-e. well in excess of the reflectivity due to the photorefractive effect alone (typically 1-2% for a single pulse). In fact, the maximum effective refractive index change which has previously been reported due to the photorefractive effect alone is 1.8xl0^"3.

Several types of fibre are suitable for use in the present embodiment, such as: a) A fibre containing 1 mol% Ge0₂ in silica; b) A fibre containing 17 mol% Ge0₂ in silica; c) A fibre containing 9 mol% Ge0₂ and 7 mol% B₂0₃ in silica; d) A fibre doped with a transition metal such as copper in silica; e) A fibre doped with a rare earth in silica.

The common factor in these fibres (and other suitable fibres employing other dopants and/or other base materials) is the presence of a material with a strong absorption at a given writing wavelength (though not necessarily in the UV region of the spectrum), in order that sufficient absorption can take place to cause thermal damage of the type described above. Furthermore, the cladding region of the fibre (or the region between the heat absorbing region and the outside of the fibre) should be substantially transparent to the writing laser.

For at least some of the above fibre compositions (e.g. (a) to (c)), the absorption of UV light by the fibre increases with temperature. This causes a non-linear 'thermal runaway' effect in which heating of localised regions of the fibre increases the amount of heat absorbed by those regions from the incident illumination.

A number of gratings have been prepared experimentally using the process described above. For each grating, the peak-to-peak refractive index modulation was estimated from the gratings reflection spectrum using the results of standard coupled-mode theory. The results of this test are shown in Figure 3. which is a graph of peak-to-peak effective index modulation against the laser pulse energy in millijoules (mJ) . I is apparent from Figure 3 that a sharp threshold occurs at a pulse energy of about 30mJ (for the particular core composition and focusing arrangement used in this test). Below the threshold, the index modulation grows substantially linearly with pulse energy, whereas above the threshold the index modulation appears to saturate, being then caused by thermal damage of the fibre material rather than photorefractive alteration. For gratings generated above the threshold, the peak-to-peak effective index modulation can be very high, for example as high as 0.006 which is comparable to the core- cladding index difference of 0.02.

The threshold energy required to create the gratings by thermally damaging the waveguide material may be dependent on the composition of the waveguide material used. However, the presence of thermal damage (rather than photorefractive alteration) in the resultant gratings can be verified by examination of the grating under a microscope.

The discussions below concern those gratings generated above the threshold pulse energy. Figure 4 is a graph illustrating the optical performance of the gratings. The transmission spectrum shows at least 26 decibels (dB) extinction at the Bragg (resonant) wavelength, which means that only 0.2 percent of the light is transmitted. Also, the calibrated reflection spectrum confirms that the gratings are nearly 100 percent reflecting. The irregularities in the spectra have been attributed to grating non-uniformity resulting from non-uniformities in the profile of the excimer beam 10, which non-uniformities are strongly enhanced by the highly non-linear response of the heat absorbing glass.

The gratings pass wavelengths longer than the Bragg wavelength, whereas shorter wavelengths are strongly coupled into the fibre cladding 130. This effect is similar to that observed for etched or relief optical fibre gratings.

In another test, when all the wavelength lines of an argon-ion laser were launched into the core 140, most of the light could be seen diffracting out of one side of the fibre, each laser line being well separated in angle. This indicates that the index modulation of a fibre-core grating is not uniform across the core, permitting the grating to operate as an effective wavelength selective tap in e.g. a wavelength division multiplexed telecommunications system. Several gratings were placed in a furnace and were tested for thermal stability at temperatures ranging from 700°C to 1000°C for up to twenty hour periods. The results of this test are illustrated in Figure 5. which is a graph showing the normalized reflected .optical power at the Bragg wavelength against time in hours. Below 800°C and for periods of about twenty hours, no significant changes were observed in the grating reflectivities. At 900°C- the grating under test decayed quite slowly. At 1000°C, most of the grating had disappeared after four hours. By contrast, gratings generated using the prior art photorefractive exposure described above may be erased within a few seconds at 450°C.

An examination of the gratings under a microscope showed a periodic visible damage track at the core-cladding interface of the fibre. The damage track is localised on one side of the core, suggesting that most of the UV light is absorbed or scattered there, perhaps never reaching the other side of the core.

In the present embodiment, the damage tends to occur at the core/cladding interface. This can lead to a radially asymmetric grating being formed which then acts like a "blazed", or angled periodic device. It is possible to avoid this in an alternative method by only doping the centre of the core, or at least by doping the centre of the core more heavily than the radial periphery of the core, and by using other less-absorbing dopants to raise the refractive index of the remainder of the core (e.g. to maintain a uniform refractive index across the core) . This will cause the grating to be formed at the centre of the core, giving rise to a symmetrical structure which will interact with the peak of the guided mode in the core. In other embodiments, the cladding could be doped or otherwise arranged to absorb light from the writing laser, so that a grating is formed in the cladding of the fibre. A cladding structure of two or more layers could be employed, so that an outer layer of the cladding is substantially transparent to the illumination provided by the grating writing laser, and an inner layer (e.g. a cylindrical layer adjacent to the fibre core) is doped or otherwise arranged to absorb the writing laser radiation.

Wherever the grating is formed (core, cladding, core/cladding interface etc) , as an alternative to holographic writing using interfering beams of light, individual grating elements can be written by directing point-focused pulses of light onto the fibre. This modified technique allows gratings having a pitch which varies along the length of the grating to be fabricated. In a further alternative embodiment, short (e.g. 5 mm) lengths of grating can be fabricated from individual laser pulses. This can alleviate any problems caused by the spatial inhomcgeneities across the wave front of light generated by excimer lasers.

Figure 6 is a schematic diagram of a planar waveguide grating comprising a substrate 200 in which a guiding region 210 is formed. The guiding region could cover the whole upper surface of the substrate 200, and could (depending on the application) be covered by a thin film coating. A grating 220 is fabricated on the guiding region 210 using the method described above.

Figure 7 is a schematic diagram of a laser comprising a pumped laser medium 300 (e.g. a semiconductor diode) linked by optical fibres to a reflector 310 and a fibre grating 320 fabricated by the above method. The grating 320 acts as a wavelength selective reflector.

Figure 8 is a schematic diagram of an optical fibre laser, in which a rare earth doped optical fibre 400 acts as the lasing medium and also as a waveguide on which two gratings 4l0, 420 are fabricated as described above. The laser receives optical pump energy through the grating 410 (with a reflectivity of approximately 100% at the lasing wavelength) , and supplies output radiation through the grating 420 (with a reflectivity of less than 100% at the lasing wavelength) .

Figure 9 is a schematic diagram of an optical fibre sensor, in which an array of gratings 10, 520, 530 are mounted in or on a substrate 540. The gratings are arranged to be exposed to environmental conditions such as temperature, pressure, strain etc. The wavelength reflected by each grating depends on the environmental conditions. As an alternative, the gratings need not be supported by a substrate. Figure 10 is a schematic diagram of an optical amplifier, in which a pumped amplifying medium 600 (an Er³⁺ doped optical fibre) is connected via an optical circulator 610 to a fibre grating 620 of the type described above. Light received at a port 6ll of the circulator 610 is output at a port 612, and light received at the port 612 is output at a port 613- Figures 11a and lib show the output spectrum at outputs 630 and 640 of the apparatus of Figure 10 respectively, in terms of the light transmission T, wavelength λ and the signal wavelength λ_s. Such a configuration can be used to filter the amplified spontaneous emission from the amplifier and thus reduce its noise figure.

Figure 12 is a schematic diagram of a wavelength selective optical tap, in which a group of wavelength channels S(λ) is received by an optical fibre 700 on which a grating 710 has been fabricated as described above. The angle θ through which each channel exits the grating is wavelength dependent, and is mapped by a lens 720 into a position x(λ) on a detector array 730. The wavelength resolution depends on the grating length, and can be as low as 0.1 Angstrom (0.01 nm) .

Figure 13 is a schematic diagram of a grating array 800 in which light received on an input optical fibre 810 is passed through an array of fibre gratings of the type described above. Separate output channels at various wavelengths λ₁ ... λ_n (corresponding to the gratings used) are generated.

The gratings may be fabricated during drawings of an optical fibre from a heated preform. In this case, a short length of the newly-drawn fibre may be exposed to transverse illumination generated by the writing laser. If a rapid pulse of illumination is used, the exposure can take place while the fibre is moving (as part of the drawing process). In order to improve the spatial stability of the moving fibre (so that the illumination can be focused onto the fibre successfully) , the exposure can be made as the fibre is about to enter a coating cup through a narrow guiding aperture (i.e. immediately before the fibre is coated with a protective coating) . Conventionally, the guiding aperture is about 0.3mm in diameter; this constraint on lateral vibration of the fibre is adequate if the writing laser is focused to a spot diameter of about 1mm. In summary, the embodiment described above provides the following advantages over conventional grating fabrication techniques: a) The gratings are substantially unaffected by temperatures of up to approximately 800°C. b) The gratings are substantially unaffected by exposure to low levels of light in the green to UV range. c) The gratings can be made using a single pulse from a high-power UV laser. d) It is possible to fabricate the gratings in real time during the drawing of the fibre from a preform. e) An effective index change is produced in the core which is significantly greater than the change which can be obtained using conventional optical writing techniques, such as those described in EP-A-191063.

Claims

1. A method of fabricating an optical waveguide grating, the method comprising the step of thermally damaging selected regions of the waveguide by exposure of those regions to transverse optical illumination.

2. A method according to claim 1, in which the transverse optical illumination comprises one or more light pulses.

3. A method according to claim 1 or claim 2, in which the transverse optical illumination comprises a plurality of interference fringes generated by interference between two coherent beams from a single light source.

4. A method according to claim 3. in which the numbers of reflections undergone by each of the two coherent beams differ by a multiple of two.

5- A method according to any one of the preceding claims, comprising the step of sequentially thermally damaging a plurality of regions of the waveguide by exposure to a respective plurality of light pulses.

6. A method according to any one of the preceding claims, in which the optical illumination has a wavelength of less than 500 nm.

7. A method according to claim 6, in which the optical illumination comprises light generated by an excimer laser.

8. A method according to claim 7. in which the excimer laser is a krypton-fluoride laser.

9- A method according to any one of the preceding claims, in which the regions form a grating which is periodic along a transmission direction of the optical waveguide.

10. A method according to any one of claims 1 to 8, in which the regions form a grating having a varying pitch along a transmission direction of the optical waveguide.

11. A method according to any one of the preceding claims, in which the waveguide is an optical fibre waveguide.

12. A method according to claim 11, in which the cladding of the optical fibre waveguide is substantially transparent to the optical illumination and the fibre core is doped with an absorbing dopant to absorb the optical illumination.

13- A method according to claim 11, in which at least a part of the cladding of the optical fibre waveguide is doped with an absorbing dopant to absorb the optical illumination.

14. A method according to claim 13, in which the cladding of the optical fibre waveguide comprises an outer cladding layer substantially transparent to the optical illumination, and an inner cladding layer doped with an absorbing dopant to absorb the optical illumination.

15. A method according to claim 11 or claim 12, in which the fibre core is doped with oxides of germanium.

16. A method according to claim 11, claim 12 or claim 15, in which the fibre core is doped with a transition metal, boron, or a rare earth.

17. A method according to claim 12 and any one of claim 15 and claim l6, in which the centre of the core is more heavily doped with the absorbing dopant than the radial periphery of the core.

18. A method according to any one of the preceding claims, in which the absorption of optical illumination by the waveguide increases with temperature.

19. A method according to any one of the preceding claims, in which the transverse optical illumination is arranged to provide an energy density incident on the waveguide of at least 0.5 Jem^"2 (Joules per square centimetre) .

20. A method according to any one of the preceding claims, in which the transverse optical illumination is arranged to cause a transient heating of at least a part of the waveguide to a temperature of at least 1000 degrees Celsius.

21. A method according to any one of claims 1 to 10, in which the waveguide is a planar waveguide.

22. A method of fabricating an optical fibre, the method comprising the steps of: drawing the optical fibre from a heated preform; fabricating a grating in the core of a portion of the drawn optical fibre using a method according to any one of claims 11 to 20; and coating the optical fibre with a protective coating.

23. An optical waveguide grating in which selected regions of the waveguide are thermally damaged by exposure of those regions to transverse optical illumination.

24. A grating according to claim 23, in which the transverse optical illumination comprises one or more light pulses.

25. A grating according to claim 23 or claim 24, in which the transverse optical illumination comprises a plurality of interference fringes generated by interference between two coherent beams from a single light source.

26. A grating according to claim 25, in which the numbers of reflections undergone by each of the two coherent beams differ by a multiple of two.

27. A grating according to any one of claims 23 to 26, in which a plurality of regions of the waveguide are sequentially thermally damaged by exposure to a respective plurality of light pulses.

28. A grating according to any one of claims 23 to 27, in which the optical illumination has a wavelength of less than 500 nm.

29. A grating according to claim 28, in which the optical illumination comprises light generated by an excimer laser.

30. A grating according to claim 29, in which the excimer laser is a krypton-fluoride laser.

31. A grating according to any one of claims 23 to 30, in which the regions form a grating which is periodic along a transmission direction of the optical waveguide.

32. A grating according to any one of claims 23 to 31, in which the regions form a grating having a varying pitch along a transmission direction of the optical waveguide.

33- A grating according to any one of claims 23 to 32, in which the waveguide is an optical fibre waveguide.

34. A grating according to claim 33, in which the cladding of the optical fibre waveguide is substantially transparent to the optical illumination and the fibre core is doped with an absorbing dopant to absorb the optical illumination.

35- A grating according to claim 33, in which at least a part of the cladding of the optical fibre waveguide is doped with an absorbing dopant to absorb the optical illumination.

36. A grating according to claim 35, in which the cladding of the optical fibre waveguide comprises an outer cladding layer substantially transparent to the optical illumination, and an inner cladding layer doped with an absorbing dopant to absorb the optical illumination.

37- A grating according to claim 33 or 3 , in which the fibre core is doped with oxides of germanium.

38. A grating according to claim 33, claim 3 or claim 37, in which the fibre core is doped with a transition metal, boron, or a rare earth.

39- A grating according to claim 34 and any one of claim 37 and claim 38, in which the centre of the core is more heavily doped with the absorbing dopant than the radial periphery of the core.

40. A grating according to any one of claims 23 to 39, in which the absorption of optical illumination by the waveguide increases with temperature.

4l. A grating according to any one of claims 23 to 40, in which the transverse optical illumination is arranged to provide an energy density incident on the waveguide of at least 0.5 Jem^"2 (Joules per square centimetre) .

42. A grating according to any one of claims 23 to 4l, in which the transverse optical illumination is arranged to cause a transient heating of at least a part of the waveguide to a temperature of at least 1000 degrees Celsius.

43. A grating according to any one of claims 23 to 32, in which the waveguide is a planar waveguide.

44. A laser comprising a grating according to any one of claims 23 to

43.

45. An optical amplifier comprising a grating according to any one of claims 23 to 43.

46. An optical sensor comprising a grating according to any one of claims 23 to 3.

47. A wavelength dependent optical tap comprising a grating according to any one of claims 23 to 43-

48. Apparatus for fabricating an optical waveguide grating, the apparatus comprising means for thermally damaging selected regions of the waveguide by exposure of those regions to transverse optical illumination.