Patent Application: US-46995304-A

Abstract:
a novel method based on sequential writing for fabricating of advanced fiber bragg gratings is disclosed . as opposed to already presented sequential methods , this scheme uses a continuous wave uv laser source and allows for a very precise control and repetitively of the formation of the gratings . furthermore , one can use high average irradiances without destroying the fiber , resulting in a dramatical shortening of fabricating times for complex gratings .

Description:
further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter . however , it should be understood that the detailed description and specific examples , while indicating preferred embodiments of the invention , are given by way of illustration only , since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description . the setup according to an embodiment of the invention is illustrated in fig1 a . a fiber 1 to be exposed is placed in a fiber holder 2 mounted on an airbearing born carriage 3 , which is translated by a feedback - controlled linear drive . the position of the translator stage relative the uv interference pattern is measured with a heterodyne interference detection system 4 utilizing a he — ne laser as light source . the resulting spatial resolution is approximately 0 . 6 nm , available over the translation length of about half a meter . a light source 5 , e . g . a frequency - doubled argon - ion laser emits light , such as 100 - mw radiation of wavelength 244 nm , into a beam splitter , such as a half transparent mirror or prism , or a phase - mask , where it is divided into two coherent beams . thereafter , the beams are launched into a double sagnac interferometer 6 , which generates the interference pattern forming the grating . one or several cylindrical lenses 61 focus the two interfering beams into a line focus that coincides with the core of the fiber 1 . longitudinally , the focus could extend over about 100 microns , which roughly corresponds to 200 fringes for a bragg wavelength of 1550 nm resonance wavelength . since the fiber is moving , and preferably at a constant speed , the pattern has to follow the fiber movement during exposure . to accomplish this , two mirrors are mounted on piezo crystals 62 in the uv interferometer , one in each beam path . a translation of the mirrors introduces a phase shift between the interfering beams , which shifts the fringe pattern under the envelope defined by the focus . the pattern thus moves with the fiber , continuously exposing the fiber . since the piezo elements cannot move infinitely long , they are fed with a sawtooth signal so that the fringes are perfectly following the fiber over some distance , typically one fringe . at the end of the voltage ramp in each period , the fringes jump back to the original position . during this short moment , the fiber is evenly exposed giving a slight increase in dc index change . each new period of the sawtooth signal results in a new “ subexposure ” of the fiber , corresponding to a new subgrating in earlier sequencial methods . the properties of the piezo crystals limit the maximum writing speed , as it is essential that the extent of the ringing after the jump is much less than the period time of the signal . otherwise , the modulation depth of the generated grating may degrade . the maximum writing speed that could be obtained in our setup was of the order of centimeters per second . in practice , the handling of the fiber before and after the exposure represents the actual time limiting factor in the grating fabrication process . but with the above described setup a maximum writing speed of at least the order of centimeters per second can be obtained . the entire grating structure is determined by the positions of the jumps , i . e . sign reversions of the sawtooth signal as described in detail below . these positions are calculated in advance by a calculation unit 7 , such as a conventional personal computer , and fed into a control unit 8 that eventually performs the sign reversions when the desired positions are reached . the control unit is preferably electronically implemented in hardware . a step motor 63 controlling the angle between the interfering beams could be used to change the period of the interference fringes to match the desired local pitch of the grating . the angle change is preferably performed symmetrically for both beams so that the center fringe does not move its position . in a preferred setup , the resolution for this pitch variation is about 1 . 4 pm in resonant wavelength . in order to prevent unwanted exposure outside the actual grating , the system also comprises a controllable shutter 9 in the uv beam path that is only open within the grating region during the writing . an example of the optical path in the projection system 6 used in an apparatus according to the invention is illustrated in fig1 b . the beam b from the laser 5 is directed to a beam - splitter 601 , such as a semitransparent mirror , where it is separated into two beams b 1 , b 2 . thereafter the beams are led in different paths in the double sagnac interferometer by means of reflecting mirrors 601 - 609 or the like . further , movable mirrors 621 , 622 are introduced in the optical path for controlled introduction of a phase - difference between the beams . the displaceable mirrors may be mounted on piezo - electric elements , as is discussed above . thereafter , the beams b 1 , b 2 are directed to the lenses 610 - 612 to be focused on the fiber 1 . the reflecting elements 604 , 605 are preferably displaceable by means of the step motor 63 , as previously described , for controlling of the angle between the interfering beams . preferably , the mirrors 604 , 605 are mounted on a holder plate or the like , and consequently movable in a coordinated fashion . for simple unchirped and unapodized gratings , all exposed fringes are in phase and the sawtooth jumps will appear at the same integral number of grating pitches throughout the writing process . adding a ( positive ) phase shift , e . g . for dfb structures , merely corresponds to delaying the lump somewhat , as is illustrated in fig2 . the interference pattern will then follow the fiber a little bit longer than otherwise at the position of the phase shift and make a correspondingly larger jump back . if the jump is delayed by δφ / 2 the resulting phase shift in the grating will be δφ . an important issue is the ability to apodize gratings in order to suppress unwanted side lobes in the reflection spectrum . according to the invention this is realised by so - called dithering . the visibility of the grating is changed by alternating the phase offset of the subexposures between two different values , thus in fact superposing two uniform gratings with the same properties , but phase - shifted relative to each other . choosing phase offsets ± δφ will give a total index variation of the form where x is the position along the fiber , k = 2 π / λ and λ is the grating pitch . as can be seen , the phase term directly determines the strength of the index variation . this is the same principle as was used in ref . 6 , but in the present setup , where moving fringes are used , the uv dose given in each of the two phase shifted gratings is easily and precisely controlled . if the dose varies from subgrating to subgrating , which is the case when a pulsed uv light source is used , noise will be introduced in both the phase and the amplitude of the grating structure . [ 0049 ] fig3 a shows the sawtooth waveform for a phase offset ± π / 3 . as can be seen , it alternates between two phase - shifted versions of a uniform sawtooth wave . these waves result in two superposed gratings which add according to the above equation , giving a fringe visibility of 50 %. the sign reversions at positions marked ‘ a ’ in fig3 a sweep the interference pattern back to expose the next part of the grating whereas the reversions at positions ‘ b ’ cause the pattern to jump to the other phase offset . by always letting the sawtooth signal follow each of the two phase shifted waveforms the same number of periods before switching to the other , an even distribution of the uv dose is ensured . the grey line indicates a waveform that would result in a maximum visibility ( non - apodized ) grating with the same phase as the one described above . since the piezo crystals have a finite response time , it is important to choose the switch positions between the two waveforms carefully . if e . g . the positions marked ‘ c ’ in fig3 a are used , the resulting sawtooth signal takes the form as shown in fig3 b . as before , the reversions at the ‘ a ’ positions correspond to sweeping back for the next subexposure and at the ‘ b ’ positions , the pattern jumps to the other phase offset . in this case , though , the sign reversions turn out to be closer spaced the smaller the phase dither δφ is . hence , the piezo crystals could impossibly respond to the signal as the visibility approaches 100 %. choosing the switch positions according to fig3 a instead results in reversions at an approximately constant rate , regardless of current visibility . by continuously changing δφ for every new pair of subexposures , any apodization may be created and further adding single phase shifts as described at the beginning of this section gives any phase profiles . chirping a grating is equivalent to continuously changing the distance between the fringes . a constant chirp , i . e . a linear increase / decrease in grating period and fringe distance , is the same as a quadratic increase / decrease in phase as compared to a non - chirped grating . if the interference pattern is focused to contain only one single fringe , it would be possible to create any chirp throughout the grating . on the other hand , the fabrication time would increase since every overlapping fringe in the apodization scheme described above must be exposed individually . furthermore , the demand of precision of the fiber alignment increases the tighter the uv beam is focused . if the beam spot contains several fringes and the period of the interference fringes is fixed throughout the writing process , it is still possible to apply smaller chirps by merely adjusting the phase offset as long as the phase shift remains small over a length corresponding to one subexposure . larger chirps will give an apodization effect due to overlapping of out - of - phase fringes since the actual interference fringe period differs too much from the desired grating pitch . in order to apply larger chirps , the interference fringe period must be continuously changed during the exposure to always match the desired local grating pitch . in our setup , this is done by changing the angle between the two interfering beams with aid of the step motor described above . with this technique , it is possible to create arbitrary chirps throughout the grating . it is imperative , that the angle adjustment is symmetric with respect to the fiber normal in the plane of incidence , otherwise the overall position of the interference pattern also changes , thus introducing phase errors in the grating structure . in order to test the equipment , several gratings were written . the interference pattern has in each case been focused to a longitudinal size of approximately 100 microns . it has proven important to use the same tv power and writing speed at each exposure and to keep the time between multiple writing passes approximately constant . if these requirements are not met , different temperatures due to different amounts of remaining heat in the fiber result in a misalignment of the fringes from pass to pass , thus causing degradation of the grating rather than reinforcement . the effect is especially prominent when writing chirped gratings . [ 0056 ] fig4 shows the reflection spectrum of a 1 cm uniform unapodized grating that was exposed once at 13 mw with a speed of 5 mm / s . using a highly sensitive hydrogen loaded fiber resulted in a grating with approximately 20 % reflectivity and a modulation depth of roughly 2 · 10 − 5 . as can be seen , the sinc - shaped spectrum is in perfect agreement with the simulated ideal response of such a grating . the resemblance with the simulation is still very good far away from the main peak , as shown in fig5 . the nearly perfect symmetry indicates that the number of phase errors is very low . next , a sinc - apodized 4 cm long grating including 7 sidelobes on each side of the main peak was written in the same type of fiber as above . in order to suppress spectral sidebands , the grating was additionally apodized with a super - gaussian profile . the resulting reflection spectrum is shown in fig6 . the grating was exposed 5 times at 100 mw , which gave a reflectivity of approximately 20 %. the in - band noise is 10 % and the sideband rejection at least 20 db , as is illustrated in fig7 . finally , a 2 cm long and 10 nm chirped super - gaussian apodized grating was written in a redfern gf - 5 fiber . the grating was exposed 10 times at 2 mm / s with 100 mw beam power . use was made of the step motor to continuously optimize the local period . as can be seen from fig8 and 9 , the filter response is very good with a sidelobe rejection of approximately 22 db and a reflectivity close to 100 %. note also the spectral symmetry that , again , indicates a very low amount of phase errors . the invention provides a novel technique for fabrication of high quality fiber bragg gratings . by continuously moving the interference fringes of cw uv light with the moving fiber during the writing process , maximum amount of the light power is used for the actual exposure . as a result , the fabrication time for all kinds of customized gratings is greatly reduced as compared to earlier methods . the invention allows for a very precise control of grating formation , which is demonstrated by a number of realised gratings showing a very low amount of phase errors . all steps of the grating exposure are software - controlled so that any kind of phase and apodization profile can be realised by means of simple programmatic changes . the flexibility and speed of this technique make it a powerful tool for fabrication of arbitrary - shaped fiber bragg gratings . the invention has now been described by way of embodiments . however , many alternatives are possible . for example , different types of beam splitters are feasible , other types of means for introducing phase differences between the beams could be used instead of piezo elements , other types of means for translating the fiber could be used etc . such alternatives are known from the prior art . it should be appreciated by someone skilled in the art that such alternatives are part of the invention , such as it is defined by the appended claims . 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