CA2264879C - Fabricating optical waveguide gratings - Google Patents

Abstract

A method of fabricating an optical waveguide grating having a plurality of grating lines of refractive index variation comprises the steps of: (i) repeatedly exposing a spatially periodic writing light pattern onto a photosensitive optical waveguide; and (ii) moving the writing light pattern and/or the waveguide between successive exposures of t he writing light pattern, so that each of at least a majority of the grating lines is generated by at least two exposures to different respective regions of the writing light pattern.

Description

1015202530WO 98/08120CA 02264879 1999-02-23PCT/GB97/020991FABRICATING OPTICAL WAVEGUIDE GRATINGSThis invention relates to fabricating optical waveguide gratings.Dispersion compensation is an attractive technique allowing the upgrade of theexisting installed standard fibre network to operation at 1.5/LII] where it exhibits adispersion of ~ (about) 17ps/nm.km which would otherwise prohibit high capacity(eg. l0Gbit/s) data transmission.Chirped fibre gratings are currently the most attractive technique for fibredispersion compensation [1]. This is because they are generally low loss, compact,polarisation insensitive devices which do not tend to suffer from optical non-linearitywhich is the case with the main competing technology, dispersion compensating fibre.For present practical applications chirped gratings must exhibit both highdispersion, ~ 1700ps/nm, sufficient to compensate the dispersion of around 100kmof standard fibre at a wavelength of 1.55pm, and a bandwidth of around 5nm. Thisimplies a need for a chirped grating of length lm.Fibre gratings are generally created by exposing the core of an optical fibreto a periodic UV intensity pattern [2]. This is typically established using either aninterferometer or a phase mask [3]. To date, phase masks are the preferred approachowing to the stability of the interference pattern that they produce. The length of thegrating can be increased by placing the fibre behind the phase mask and scanning theUV beam along it. Techniques for post chirping a linear grating after fabricationinclude applying either a strain [1] or temperature gradient [4] to it. However thesetechniques are limited due to the length of the initial grating (~ 10cm with availablephase masks) and the length over which a linear temperature or strain gradient canbe applied. Alternatively more complex step chirped phase masks can be employed[5]. However, all of these techniques are currently limited to a grating length ofabout 10cm.In addition to chirping the grating, it is also sometimes desirable to be able toapodise (window) the gratings to reduce multiple reflections within them and toimprove the linearity of the time delay characteristics. A powerful technique hasbeen developed which allows chirped and apodised gratings to be written directly ina fibre, referred to as "the moving fibre/phase mask scanning beam technique" [6].10202530CA 02264879 1999-02-23we‘ w an 1»'4 _: 1 i ’ 7 ‘3 *7win 10-)2i DruckexemplarThis technique is based on inducing phase shifts between the phase mask and the fibreas the phase mask and fibre are scanned with the UV beam. Apodisation is achievedby dithering the relative phase between the two at the edges of the grating. Like allthe previous techniques the one draw back with this technique is that it is againlimited to gratings the length of available phase masks, ~10cm at present. AThis problem has been overcome in one approach by Kashyap et al usingseveral 10cm step—chirped phase masks [5]. These are scanned in series to obtain alonger grating. The phase "glitch" or discontinuity between the sections issubsequently UV "trimmed" to minimise its impact. However this is a timeconsuming and costly process. In addition the effect of the UV trimming will varywith grating ageing.A technique for potentially writing longer gratings has been reported byStubbe et a1 [7]. In this case a fibre is mounted on .an air-bearing stage andcontinuously moved behind a stationary grating writing interferometer. The positionof the fibre is continuously monitored with a linear interferometer. The UV laser ispulsed to write groups of grating lines with period defined by the writinginterferometer. A long grating can be written by writing several groups of gratinglines in a linearly adjacent series, with controlled phase between the sections. Thephase shift between each grou'._._t of grating lines is controlled via the linearinterferometer and a computer which sets the time the laser pulses. A short pulse,~ lOns, is required such that the position of the writing lines is effectively stationaryand accurately controlled with respect to fibre motion. Having said this, however,jitter in the pulse timing and in the linear interferometer position will give detrimentalrandom phase errors in the grating. Chirped gratings can potentially be fabricated bycontinuously introducing phase shifts between adjacent groups along the grating.Obviously the maximum translation speed is limited by the number of grating lineswritten with one laser pulse and the maximum repetition rate of the pulsed laser. Itis also proposed in this paper that apodisation is achieved by multiple writing scansof the grating.This invention provides a method of fabricating an optical waveguide gratinghaving a plurality of grating lines of refractive index variation, the method comprisingthe steps of:(i) repeatedly exposing a spatially periodic writing light pattern onto aphotosensitive optical waveguide; and1015202530CA 02264879 1999-02-23,‘3(ii) moving the writing light pattern and/or the waveguide between successiveexposures or groups of exposures of the writing light pattern, characterised in thatthe successive exposures or groups of exposures overlap so that each of atleast a majority of the grating lines is generated by at least two exposures to differentrespective regions of the writing light pattern.Embodiments of the invention provide a number of advantages over previoustechniques:1. The realisation that the laser does not have to be pulsed but just has to be onfor a particular duty cycle - preferably less than 50% of the period. This allows anexternally modulated CW (continuous wave) laser to be used.2. With this technique the grating lines are re-written by several successiveexposures of the writing light beam at every grating period (or integral number ofgrating periods). Thus the footprint defined by the writing light beam is significantlyoverlapped with the previous lines. Significant averaging of the writing process isachieved thus improving the effective accuracy and resolution of the system,compared to that of [7] where a group of lines is written in a single exposure, and thefibre is then advanced to a fresh portion where a further group of lines is written ina single exposure. ‘3. Effectively controlling the grating writing process on a line-by-line basisallows accurate apodisation to be achieved. This may be performed in embodimentsof the invention by dithering the grating writing interferometer position in the fibreto wash out or attenuate the grating strength whilst keeping the average index changeconstant.4. The technique offers the further advantage that the CW laser may be extremelystable, whereas pulsed lasers (e.g. those used in [7]) may suffer from pulse-to-pulseinstability which is not averaged. In addition the high peak powers of the pulsedlaser may cause non-linear grating writing effects.5. Arbitrary phase profiles and in particular a linear chirp can be built up byinducing phase shifts electronically along the grating as it grows. In a similar mannerto the "Moving fibre/phase mask" technique [6] the maximum wavelength is inverselyproportional to the beam diameter. This can be further improved in particularembodiments of the invention by incorporating a short, linearly chirped phase mask.Thus as the fibre is scanned the'UV beam may be also slowly scanned across thephase mask, an additional small phase shift is induced, whilst most significantly we1015202530CA 02264879 1999-02-23. 1) 3.‘,,.. — ~ 54-s4have access to writing lines of a different period allowing larger chirps to be built up.This invention also provides apparatus for fabricating an optical fibre gratinghaving a plurality of grating lines of refractive index variation, the apparatuscomprising:a writing light beam source for repeatedly exposing a spatially periodic writinglight pattern onto a photosensitive optical waveguide; andmeans for moving the writing light pattern and/or the waveguide betweensuccessive exposures or groups of exposures of the writing light pattern, characterisedin thatthe successive exposures or groups of exposures overlap so that each of atleast a majority of the grating lines is generated by at least two exposures to differentrespective regions of the writing light pattern.The various sub-features defined here are equally applicable to each aspect ofthe present invention.The invention will now be described by way of example with reference to theaccompanying drawings, throughout which like parts are referred to by likereferences, and in which:Figure 1 is a schematic diagram of a fibre grating fabrication apparatus;Figures 2a to 2c are schematic diagrams showing a grating fabrication processby repeated exposures;Figures 3a and 3b are schematic timing diagrams showing the modulation ofa UV beam; andFigures, 4a and 4b are schematic graphs characterising a 20cm gratingproduced by the apparatus of Figure 1.Figure 1 is a schematic diagram of a fibre grating fabrication apparatus. Anoptical fibre (e. g. a single mode photorefractive fibre) 10 is mounted on a crossedroller bearing translation stage 20 (such as a Newport PMLW160001) which allowsfor a continuous scan over 40cm. The fibre 10 is positioned behind a short (~ 5mm)phase mask 30 (e. g a mask available from either QPS or Lasiris).1015202530WO 98/08120CA 02264879 1999-02-23PCT/GB97/020995The fibre is continuously and steadily linearly translated or scanned in asubstantially longitudinal fibre direction during the grating exposure process.Ultraviolet (UV) light at a wavelength of 244nm from a Coherent FRED laser40 is directed to the fibre/phase mask via an acoustic—optic modulator 50 (e.g. aGooch & Housego, M110-4(BR)) operating on the first order.The relative position of the fibre to the interference pattern of the phase maskis continuously monitored with a Zygo, ZMIIOOO differential interferometer 55. Theinterferometer continuously outputs a 32-bit number (a position value) which givesthe relative position with a ~1.24nm resolution. This output position value iscompared by a controller 70 with switching position data output from a fast computer60 (e. g. an HP Vectra series 4 5/166 with National Instruments AT-DIO—32F) inorder that the controller can determine whether the UV beam should be on or off atthat position. Whether the UV beam is in fact on or off at any time is dependent onthe state of a modulation control signal generated by the controller 70 and used tocontrol the acousto—optic modulator 50.So, as each position value is output by the interferometer, the controller 70compares that position value with the switching position data currently output by thecomputer 60. If, for illustration, the interferometer is arranged so that the positionvalues numerically increase as the fibre scan proceeds, then the controller 70 detects ‘when the position value becomes greater than or equal to the current switchingposition data received from the computer 60. When that condition is satisfied, thecontroller 70 toggles the state of the modulation control signal, i.e. from "off" to"on" or vice—versa. At the same time, the controller 70 sends a signal back to thecomputer 60 requesting the next switching position data corresponding to the nextswitching position.If the fibre was scanned with the UV beam continuously directed onto thefibre, no grating would be written since the grating lines would be washed out by themovement.However if the UV beam is strobed or modulated (under control of theswitching position data generated by the computer 60) with a time period matchingor close to:10152025WO 98108120CA 02264879 1999-02-23PCTIGB97/02099phase mask projected fringe pitchfibre translation speedthen a long grating would grow.This expression is based on a time period of a temporally regular modulationof the UV beam, and so assumes that the fibre is translated at a constant velocity bythe translation stage. However, more generally, the switching on and off of the UVbeam is in fact related to the longitudinal position of the fibre, so that in order togenerate a grating the UV beam should be turned on and off as the fibre is translatedto align the interference pattern arising from successive exposures through the phasemask.Figures 2a to 2c are schematic diagrams showing a grating fabrication processby repeated exposures of the fibre to the UV beam.In Figure 2a, the UV beam from the acousto—optic modulator 50 passesthrough the phase mask 30 to impinge on the fibre 10. During the exposure process,the fibre 10 is being longitudinally translated by the translation stage 20 in a directionfrom right to left on the drawing. Figure 2a illustrates (very schematically) arefractive index change induced in the fibre by a first exposure through the phasemask.Figures 2a to 2c illustrate a feature of the normal operation of a phase maskof this type, namely that the pitch of the lines or fringes of the interference patternprojected onto the fibre (which gives rise to the lines of the grating) is half that of(i.e. twice as close as that of) the lines physically present (e.g. etched) in the phasemask. In this example, the phase mask has a "physical" pitch of 1am, and the linesprojected onto the fibre have a pitch of O.5;tm.The UV beam is modulated by the acousto~optic modulator in a periodicfashion synchronised with the translation of the fibre. In this Way, successiveexposures, such as the two subsequent exposures shown in Figures 2b and 2c,generate periodic refractive index changes aligned with and overlapping the firstexposure of Figure 2a. Thus, the refractive index change providing each individualgrating "element" or fringe is actually generated or built up by the cumulative effectsof multiple exposures through different parts of the phase mask as the fibre moves1015202530WO 98/08120CA 02264879 1999-02-23PCT/GB97/020997along behind the phase mask. This means (a) that the optical power needed togenerate the grating can be distributed between potentially a large number ofexposures, so each exposure can be of a relatively low power (which in turn meansthat the output power of the laser 40 can be relatively low); and (b) the grating canbe apodised by varying the relative positions of successive exposures (this will bedescribed below with reference to Figure 3b). .Although each of the successive exposures of the fibre to UV light through thephase mask 30 could be a very short pulse (to "freeze" the motion of the fibre as theexposure is made), this has not proved necessary and in fact the present embodimentuses an exposure duty cycle in a range from below 10% to about 50%, although awider range of duty cycles is possible. An example of a simple regular exposure dutycycle is shown schematically in Figure 3a, which in fact illustrates the state of themodulation control signal switching between an "on" state (in which light is passedby the acousto-optic modulator) and an "off" state (in which light is substantiallyblocked by the acousto-optic modulator). The period, T, of the modulationcorresponds to the time taken for the fibre 10 to be translated by one (or an integralnumber) spatial period of the interference pattern generated by the phase mask 30.As the duty cycle for the UV exposure increases, the grating contrastdecreases (because of motion of the fibre during the exposure) but the writingefficiency increases (because more optical energy is delivered to the fibre perexposure). Thus, selection of the duty cycle to be used is a balance between thesetwo requirements.Assuming linear growth, the index modulation, ng(z) in an ideal grating canbe described as a raised cosine profile:ng(z) oc 1+sin(27rz/A)where z is the position down the fibre and A the grating period. With the newtechnique we obtain:ng(z) oc (AAONI A) [1 + {sin(1rAA0N/A)/(7rAAON/A)}sin(27r(z + AAON/2)/A)]1015202530WO 98/08120CA 02264879 1999-02-23PCTlGB97l020998where AAON/A is the fraction of the period that the beam is on (i.e. the dutycycle).For small values of AAON/A a near 100% grating contrast is obtained howeverthe efficiency of the grating writing is reduced to ~AA0N/A because most of the UVbeam is prevented from reaching the fibre.The maximum grating strength is obtained for AAON/A=0.5 however the ratioof dc to ac index change is worse. For AAON/A > 0.5 the grating begins to be reducedwhilst the dc index change continues to build.Experimentally, a good value for AAON/A has been found to be ~0.3-0.4.Thus, with embodiments of this technique, exposure of the grating lines orelements is repeated every grating period. Thus the footprint defined by the UVbeam, which might for example for at 500nm diameter beam, ohm, consists of¢>,,,,,,,,,/A( ~ 1000) lines, is significantly overlapped with the previously exposed lines.Significant averaging of the writing process given by (¢>,,c,m/A)”’ is therefore achieved,thus improving the effective accuracy and resolution of the system.The computer in this embodiment actually generates the switching positionsinternally as "real" numbers (obviously subject to the limitation of the number of bitsused), but then converts them for output to the controller into the same unit systemas that output by the Zygo interferometer, namely multiples of a "Zygo unit" of1.24nm. This internal conversion by the computer makes the -comparison of theactual position and the required switching position much easier and therefore quickerfor the controller. A random digitisation routine is employed in the computer 60 toavoid digitisation errors during the conversion from real numbers to Zygo units. Thisinvolves adding a random amount in the range of i0.5 Zygo units to the real numberposition data before that number is quantised into Zygo units. Thus an effectiveresolution can be obtained of:1.24nm/(q5,,m,/A)"2 z0.03nm.The technique offers the further advantage that the CW laser is extremelystable whereas pulsed lasers (as required in the technique proposed by Stubbe et al[7]) may suffer from pulse-to—pulse instability which, in the Stubbe et al technique,1015202530W0 98/08120CA 02264879 1999-02-23PCT/GB97/020999is not averaged over multiple exposures. In addition the high peak powers of apulsed laser may cause non-linear grating writing effects, which are avoided oralleviated by using longer and repeated exposures in the present technique.A refinement of the above technique, for producing apodised gratings, willnow be described with reference to Figure 3b.Using the techniques described above, effectively controlling the gratingwriting process on a line-by-line basis allows accurate apodisation to be achieved.Apodisation is achieved by effectively dithering the grating writinginterferometer position in the fibre to wash out or attenuate the grating strength.However, if the overall duty cycle of the exposure is kept the same, and just thetiming of each exposure dithered, the average index change along the grating is keptconstant.To completely wash out the grating subsequent on periods of the UV laser areshifted in phase (position) by ivr/2(_-l;A/4). To achieve a reduced attenuation theamplitude or amount of dither is reduced. Figure 3b illustrates an applied dither ofabout in/3 from the original (undithered) exposure times.This technique of apodising is better with an exposure duty cycle of less than50%, to allow a timing margin for 100% apodisation.One example of the use of this technique is to generate a grating with acontrast increasing at one end of the grating according to a raised cosine envelope,and decreasing at the other end of the grating in accordance with a similar raisedcosine envelope, and remaining substantially constant along the central section of thegrating. This apodisation can be achieved particularly easily with the presenttechnique, as the central section requires no phase shift between successive exposures,and the two raised cosine envelopes require a phase shift that varies linearly withlongitudinal position of the fibre.The required phase shifts can be calculated straightforwardly by the computer60, under the control of a simple computer program relating required phase shift tolinear position of the fibre (effectively communicated back to the computer 60 by thecontroller 70, whenever the controller 70 requests a next switching position datavalue).Other apodisation schemes are also possible. Compared with previous1015202530WO 98/08120CA 02264879 1999-02-23PCT/GB97/0209910methods of dithering [6] this technique is not limited by the dynamics of a mechanicalstage used for dithering, but instead simply adjusts the switching time of a non-mechanical modulator element 50. It can also achieve substantially instantaneousphase shifts.Furthermore, arbitrary phase profiles and in particular a linear chirp can bebuilt up by the computer 60 inducing phase shifts along the grating as it is fabricated.In a similar manner to the "Moving fibre/phase mask" technique [6] the maximumwavelength is inversely proportional to the beam diameter. However, with thepresent technique an improvement can be obtained (with respect to the technique of[6]) by incorporating a short, linearly chirped phase mask. Thus as the fibre isscanned the UV beam is also slowly scanned (by another PZT translation stage, notshown) across the phase mask. This scanning of the position of the UV beam in itselfinduces a small chirp, in accordance with the techniques described in reference [6],but more significantly the translated beam accesses writing lines of a different periodallowing larger chirps to be built up. This has been tested using a 19mm diameter,~20nm chirped phase mask (sourced from Lasiris) with its central period around1070mn. This allows ~30nm chirped gratings centred around a central wavelengthof 1550nm to be fabricated. _Figures 4a and 4b are schematic graphs showing the characterisation of a20cm linearly chirped grating written at a fibre translation speed of 200p.m/s with thebasic technique described earlier, i.e. with a fixed mask. At this fibre translationspeed, for a projected fringe pitch of O.5;.tm the writing light beam is switched at aswitching rate of 400Hz. In other words, the fibre advances by one projected fringebetween exposures. (It is noted that the limitation on fibre translation speed in theseprototype experiments is the calculation speed of the computer 60 used in theexperiments, and that given a faster computer such as a Pentium or subsequentgeneration PC, much higher translation speeds of, say, 10mm per second or morewould be possible).In particular, therefore, Figure 4a is a graph of reflectivity against wavelength,and Figure 4b is a graph of time delay against wavelength. The wavelength(horizontal) axes of the two graphs have the same scale, which for clarity of thediagram is recited under Figure 4b only.10W0 98/08120CA 02264879 1999-02-23PCT/GB97/0209911A ~4nm bandwidth and dispersion of ~500ps/nm are observed.Such results have not been reported by any other method. Gratings up to 40cmand writing speeds up to 1mm/s have been demonstrated. Lengths in excess of 1mand writing speeds up to 10mm/s are feasible.In the above description, the fibre has been translated with respect to the phasemask, and in the later description the UV beam is translated with respect to the phasemask. However, it will be clear that the important thing is relative motion, and so thechoice of which component (if any) remains "fixed" and which is translated isrelatively arbitrary. Having said this, however, the arrangement described above hasbeen tested experimentally and has been found to be advantageously convenient toimplement. It will also be apparent that in other embodiments each "exposure" couldin fact involve a group of two or more exposures, with the position of the fibre withrespect to the writing light beam being constant or substantially constant for exposureswithin a group, but different from group to group.CA 02264879 1999-02-23WO 98/08120 PCT/GB97/0209912PUBLICATION REFERENCES1. D. Garthe et al, Proc. ECOC, vol. 4, (post-deadline papers), pp. 11-14(1994).2. G. Meltz et a1, Opt. Lett., 14(15), pp. 823-825, 1989.3. KO. Hill et al, Appl. Phys. Lett., 62(10), pp. 1035-1037, 1993.4. RI. Laming et al, Proc.ECOC’95, Brussels, Vol 2, Paper We.B.1.7, pp585-8, 17-21 September 1995.5. R. Kashyap et al, Electronics Letters, Vol 32 (15), pp. 1394-6, 1996.6. M.J. Cole et al, Electronics Letters, Vol 31 (17), pp 1488-9, 1995.7. R. Stubbe et al, postdeadline paper 1, Proc. Photosensitivity and QuadraticNonlinearity in Glass Waveguides, Portland, Oregon, September 9-11, 1995.

Claims (14)

1. A method of fabricating an optical waveguide grating having a plurality of grating lines of refractive index variation, the method comprising the steps of:
(i) repeatedly exposing a spatially periodic writing light pattern onto a photosensitive optical waveguide; and (ii) moving the writing light pattern and/or the waveguide between successive exposures or groups of exposures of the writing light pattern, wherein the successive exposures or groups of exposures overlap so that each of at least a majority of the grating lines is generated by at least two exposures to different respective regions of the writing light pattern.

2. A method according to claim 1, in which step (i) comprises moving the writing light pattern and/or the waveguide between exposures by a distance, in a substantially longitudinal waveguide direction, substantially equal to an integral number of spatial periods of the writing light pattern.

3. A method according to claim 2, in which step (i) comprises moving the writing light pattern and/or the waveguide between exposures by a distance, in a substantially longitudinal waveguide direction, substantially equal to one spatial period of the writing light pattern.

4. A method according to any one of claims 1 to 3, in which step (ii) comprises:
detecting the relative position of the writing light pattern and the waveguide;
comparing the detected relative position to predetermined switching positions related to the spatial period of the writing light pattern; and controlling exposure of the writing light pattern in response to that comparison.

5. A method according to any one of claims 1 to 4, in which:
the writing light pattern is generated from one or more source light beams;
and exposure of the writing light pattern is controlled by directing the one or more source light beams through one or more optical modulators.

6. A method according to claim 5, in which the writing light pattern is generated by directing the source light beam through a phase mask.

7. A method according to claim 5 or claim 6, in which the one or more source light beams are substantially continuously generated (CW) light beams.

8. A method according to any one of claims 1 to 7, in which step (i) comprises moving the writing light pattern and/or the waveguide at a substantially uniform relative velocity.

9. A method according to claim 8, in which step (i) comprises substantially periodically exposing the writing light beam onto the waveguide, the exposures having a substantially constant temporal duty cycle.

10. A method according to claim 9, in which step (i) comprises varying the time at which each exposure of the writing light beam is made to vary the spatial alignment along the waveguide of successive exposures, thereby varying the contrast of grating lines generated by those exposures.

11. A method according to any one of claims 1 to 10, comprising varying the spatial period of the writing light beam during fabrication of the grating.

12. A method according to claim 6 and claim 11, comprising directing the source light beam onto different regions of a chirped phase mask in order to vary the spatial period of the writing light beam during fabrication of the grating.

13. A method according to any one of claims 1 to 12, in which the waveguide is an optical fibre.

14. An apparatus for fabricating an optical fibre grating having a plurality of grating lines of refractive index variation, the apparatus comprising:
a writing light beam source for repeatedly exposing a spatially periodic writing light pattern onto a photosensitive optical waveguide; and means for moving the writing light pattern and/or the waveguide between successive exposures or groups of exposures of the writing light pattern, wherein the apparatus is arranged so that the successive exposures or groups of exposures overlap so that each of at least a majority of the grating lines is generated by at least two exposures to different respective regions of the writing light pattern.