WO2001075496A1 - Piqure en phase de reseaux de diffraction a longues fibres - Google Patents

Piqure en phase de reseaux de diffraction a longues fibres Download PDF

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Publication number
WO2001075496A1
WO2001075496A1 PCT/GB2001/001347 GB0101347W WO0175496A1 WO 2001075496 A1 WO2001075496 A1 WO 2001075496A1 GB 0101347 W GB0101347 W GB 0101347W WO 0175496 A1 WO0175496 A1 WO 0175496A1
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Prior art keywords
grating
waveguide
grating section
section
writing
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PCT/GB2001/001347
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English (en)
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Michael Kevan Durkin
Mikhail Nickolaos Zervas
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University Of Southampton
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Priority to AU42595/01A priority Critical patent/AU4259501A/en
Publication of WO2001075496A1 publication Critical patent/WO2001075496A1/fr

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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
    • G02B6/02123—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating
    • G02B6/02133—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating using beam interference
    • G02B6/02138—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating using beam interference based on illuminating a phase mask
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
    • G02B6/02123—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating
    • G02B6/02152—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating involving moving the fibre or a manufacturing element, stretching of the fibre

Definitions

  • This invention relates to writing gratings in photosensitive material, for example writing optical waveguide gratings. More especially, the invention relates to the fabrication of long optical fibre Bragg gratings, such as those that can be used for dispersion compensation, to stitching of grating sections in multi-section gratings, and to aligning an apparatus with a grating.
  • Bragg gratings fabricated by illuminating an ultra-violet (UN) interference pattern onto an optical fibre are of interest, especially for telecommunications.
  • chirped fibre Bragg gratings for dispersion compensation.
  • these devices should have a bandwidth covering the useful gain window of an optical amplifier (typically > 30nm).
  • the usefulness of dispersion compensating devices is determined (i) by the total length of dispersive fibre which may be compensated for and (ii) by the useable bandwidth.
  • the dispersion-bandwidth product of a chirped fibre Bragg grating is determined by its length (total group delay).
  • a grating that can compensate for 80 km of non-dispersion- shifted fibre (17 ps/nm/km) over a bandwidth of 30 nm (typical of 32 ITU channels at 100GHz spacing) should be around 3.75 m long.
  • One main kind of technique for fabricating long fibre gratings uses a phase mask in proximity with a fibre to make a series of adjacent grating sections, each having the length of the phase mask (i.e. typically 10-15 cm), thereby to build up a length of 1 - 2 metres from, for example, 10 - 20 individual sections.
  • the quality of the device is critically dependent on phase coherence between the individual sections.
  • the interfaces between adjacent grating sections are referred to as 'stitching' regions. If grating sections are joined in a non-phase sensitive manner then significant artefacts are observed in both the group delay and reflectivity characteristics.
  • Errors introduced into the grating structure by stitching are cumulative, rather than random, in that they increase with the number of stitching regions, or stitches, in the total grating. Cumulative phase errors are particularly damaging for device performance.
  • Another feature of this phase mask technique is that any flaws in the phase mask are reproduced in each grating section. This is a real issue because of the imperfect optical quality of contemporary phase masks.
  • Several approaches have been adopted to ensure phase continuity between adjacent sections in a multi-section grating structure fabricated using the above general phase mask proximity technique.
  • Kayshap et al disclosed a first approach, referred to as "UN-trimming", for providing phase continuity between adjacent grating sections in an article in the Proceedings of the 22nd European Conference on Optical Communication - ECOC '96 Oslo"- ThB.3.2, pp 5.7 - 5.10, entitled “Eight wavelength x lOGb/s simultaneous dispersion compensation over lOOkm single-mode fibre using a single 10 nanometre bandwidth, 1.3 metre long, super- step-chirped fibre Bragg grating with a continuous delay of 13.5 nanoseconds".
  • the initially imperfect phase relation between the grating sections is then adjusted by illuminating each of the regions between the grating sections with UN light in order to change the optical path length, and hence the phase relation, between the adjacent sections by a photo-induced refractive index modulation in the optical fibre. This is referred to as "UN trimming". While simple to implement, the ultimate quality of this technique is questionable with anomalies clearly visible in reflection spectrum and group delay profiles.
  • EP-A-0843186 and EP-A-0878721 disclose two variations of a second, more recently developed approach.
  • This technique exploits the known fluorescence- bleaching effect associated with a photo-induced refractive index modulation to determine the relative phase between an existing grating section and the interference pattern of a phase mask.
  • the level of either transmission (EP-A-0843186) or fluorescence (EP- A-0878721) from the recently written grating section is determined by the relative positioning of the refractive index modulation and the intensity profile of the UN probe beam.
  • the in-phase condition between the UN interference pattern and the recently written grating section is met by adjusting their relative positions in order to maximise transmission (EP-A-0843186) or minimise fluorescence (EP-A-0878721). Once this condition is satisfied the next grating section is written by increasing the UN power and scanning the phase mask.
  • EP-A-0843186 and EP-A-0878721 has some limitations.
  • the alignment of the UN interference pattern and the grating requires either the phase mask or the waveguide to be mounted on a mechanical translation stage.
  • the instabilities associated with using non-static optical mounts can lead to instabilities and noise in the fabrication process that will compromise the overall grating quality.
  • the approach is not attractive for chirped gratings, since a unique phase mask is required for each section of the grating. This is costly and inflexible, bearing in mind that approximately ten phase masks are required per metre length of grating.
  • US 5,822,479 discloses another kind of technique which might have potential for fabricating long fibre gratings.
  • This technique uses an interferometer to generate a fringe pattern directly onto a substrate (or waveguide) and moves the substrate transverse to the fringe pattern while pulsing the fringe pattern.
  • the substrate is thus exposed repeatedly as the substrate traverses the fringe pattern, with the pulsing of the fringe pattern being carefully controlled by a feedback loop so that the traversing movement of the substrate is co-ordinated with the pulsing of the fringe pattern.
  • the substrate is thus moved continuously through the fringe pattern while a secondary beam continuously illuminates the grating.
  • the phase of the diffracted secondary beam is monitored and used to control the triggering of the writing beam, ensuring that the fringe pattern is present only when it is in phase with the grating being written.
  • WO 98/08120 discloses another fabrication technique which, like that of US 5,822,479, is based on multiple exposure of a translating structure, in this case an optical fibre.
  • a grating produced by this method comprises millions of overlapping beam-sized gratings from individual UN exposures on a translating fibre. The exposures are separated by one (or several) grating periods (measured interferometrically) and no scanning of a phase mask is required. This technique has proven to be especially successful in suppressing any stitching effects.
  • the practical length-limitation of gratings fabricated by this technique is the maximum travel of the mechanical stage used to translate the fibre (typically this may be 1-2 m).
  • a method of fabricating an optical waveguide grating comprising: (a) arranging a first length of a photosensitive waveguide on a linear translation device;
  • the determined phase relation may be used to calculate an offset value which is added to a plurality of stored beam modulation positions that are used to control fabrication of the second grating section.
  • the offset value may be determined from a phase offset between turning points of the photo-signal at a plurality of measurement positions, each measurement position being defined by a relative position of the waveguide and the probe beam.
  • the probe beam induces fluorescence in the waveguide, the phase relation between the intensity pattern of the probe beam and the first grating section being determined by monitoring modulation of the fluorescence as the waveguide is translated relative to the probe beam.
  • the fluorescence is advantageously measured by sampling at each of a series of positions along the first grating section, the positions being separated by a sampling period slightly different from a period of the periodic intensity pattern used to write the first grating section. In this way the amount of data that needs to be collected and manipulated can be substantially reduced in comparison to a simple scheme in which the fluorescence is measured at intervals much less than the modulation period.
  • the sampled fluorescence signal is preferably measured until at least one minimum of the fluorescence is identified.
  • the second grating section also has an end region that overlaps that of the first grating section, the end region of the second grating section being written with peaks of exposure intensity that gradually increase in amplitude so that the amplitudes of the peaks of the refractive index in the photosensitive waveguide are substantially equal after writing the second grating section.
  • an apparatus for fabricating an optical waveguide grating comprising multiple grating sections, the apparatus comprising: a translation stage operable to provide linearised motion in a translation direction and having a mounting for securing a waveguide extending in the translation direction; an inscription beam generator operable to generate an inscription beam having a periodic intensity pattern extending in the translation direction and incident on the waveguide; a probe beam generator operable to generate a probe beam having a periodic intensity pattern extending in the translation direction and incident on the waveguide; a photodetector operable to generate a photo-signal from light emitted from the waveguide responsive to excitation by the probe beam; a beam modulator arranged to modulate the inscription beam; and a control device having an output connected to the beam modulator for modulating the inscription beam according to a set of beam modulation positions defining a grating structure and an input connected to receive the photo-signal from the photodetector, the control device having a first operational mode for
  • the photodetector is operable to measure a fluorescence signal from the waveguide and output the photo-signal responsive thereto.
  • the probe beam it is convenient for the probe beam to be an attenuated form of the inscription beam, in which case an attenuator can be used for switching between the inscription beam and the probe beam.
  • a method of fabricating an optical waveguide grating comprising multiple sections, the grating having a structure defined by a set of beam modulation positions, the method comprising: securing a first length of a photosensitive waveguide on a linear translation device; writing a first grating section defined by a first subset of the beam modulation positions into the first length of the waveguide using a light intensity pattern; securing a second length of the waveguide on the linear translation device while keeping a part of the first grating section thereon; determining a phase offset between the part of first grating section kept on the translation device and a probe beam; and writing a second grating section as an extension of the first grating section using a light intensity pattern, the second grating section being defined by a second subset of the beam modulation positions continuing on from the first subset, wherein the beam modulation positions used for writing the second grating section are adjusted according to the phase offset so that the second grating section extends
  • a first grating section is fabricated in a photosensitive waveguide with an inscription beam, basically according to the method of WO 98/08120.
  • the inscription beam has a periodic intensity pattern that may be generated either interferometrically, or by using a diffractive optical element, such as a pi-phase mask.
  • the waveguide is translated by some means such that a portion of the first grating section remains in the range of the linear translation device. That portion of the waveguide which is in the range of translation will therefore partially comprise the first grating section.
  • a beam of probe radiation with a spatially-periodic intensity pattern is used to illuminate some portion of the first grating section that falls with in the range of the linear translation device.
  • the probe radiation must have a wavelength that induces fluorescence in the waveguide.
  • the intensity pattern of the probe beam is conveniently generated by using the same laser and other optical components used to write the grating sections.
  • the power is however strongly attenuated in order that any photo-induced refractive index change caused by the probe beam is minimal.
  • the probe beam may be intensity-modulated or continuous-wave, for example.
  • the fluorescence detected at the output of the waveguide is monitored and used to determine the phase relation between the probe interference pattern and the primary grating structure at the points of measurement.
  • a continuous measurement of fluorescence yields a modulating signal with one oscillation observed when the grating moves by one period.
  • the fluorescence varies according to the relative alignment of the intensity fringes, with the periodic bleaching of fluorescence being associated with the regions of strongest photo-induced refractive index change.
  • a period of the first grating section may be used as the known amount for incrementing the measurement positions.
  • the relative motion between the probe interference pattern and the first grating section is continuous and monotonic, and is measured interferometrically,
  • the value of the offset of the measurement position is simply added to the stored beam modulation positions that are then used to fabricate a second grating section, which is thereby accurately "stitched" to the first grating section.
  • ⁇ the value of the offset of the measurement position
  • the laser beam power is increased from the reduced probe level to regenerate the inscription beam and inscription of the second grating section is commenced using the method of WO 98/08120. That is, as for the first grating section, the second grating section is formed by many exposures of light, typically UN light, nominally separated by a single grating period. The positions of the exposures are pre-calculated and triggered based on interferometric position measurements. These steps can then be repeated to fabricate third and further grating sections as desired. The offset is re-determined for each stitching region after the fibre has been translated forward, prior to writing the next grating section.
  • the invention makes it possible to fabricate long gratings by adjoining grating sections in a phase-error-free manner, whereby: 1) the total length of the grating can be longer than the maximum travel of the linear translation stage;
  • the length of the grating sections between stitching regions can be much longer than a phase mask and is only limited by the physical length (or maximum travel) of the translation stage used to displace the fibre; 3) the number of stitching regions per unit length of grating is kept lower than with any of the above-referenced known techniques of fabricating multi-section waveguide gratings;
  • the system is not reliant on scanning a phase mask to inscribe the grating; 5) the system is not reliant on using a different phase mask for each grating section;
  • the technique used for stitching the grating section requires no additional mechanical movement, so that long-term stability can be maintained during the fabrication process; and 7) the fabrication process can proceed with a continuous, monotonic, relative motion between the waveguide and the interference pattern, both during the stitching process and the grating section fabrication process, there being no need for a break or interruption in the motion between the scanning of the stitching region with the probe beam and the subsequent writing of the next grating section.
  • grating sections are stitched in a phase-error-free manner in the specific embodiment.
  • No physical dithering of optical elements such as a phase mask
  • the phase condition is determined by interferometrically-triggered measurement of UN-induced fluorescence while the interference pattern and the existing grating section are in continuous, monotonic, relative motion.
  • This technique is both inherently stable (as optical elements may be mounted on rigid, stationary mounts) and highly accurate (as the number of stitching regions per unit length is kept low).
  • Figure 1 shows a fibre grating fabrication apparatus according to an embodiment of the invention
  • Figure 2 shows a part of the apparatus of Figure 1 in more detail
  • Figure 3 A is a schematic diagram illustrating fabrication of a first grating section
  • Figure 3B is a schematic diagram illustrating preparation for fabrication of a second grating section after fabrication of the first grating section
  • Figure 3C is a schematic diagram illustrating fabrication of a second grating section
  • Figure 4A illustrates in close up the fabrication step of Figure 3 A
  • Figure 4B illustrates in close up the fabrication step of Figure 3B
  • Figure 4C illustrates in close up the fabrication step of Figure 3C
  • Figure 5 is a graph showing light intensity of the writing inscription beam as a function of position along the grating section during the fabrication steps of Figure 3 A and Figure 3 C;
  • Figure 6 is an idealised graph of fluorescence intensity as a function of position of the translation stage during the fabrication step of Figure 3B;
  • Figure 7 is a graph of measured data equivalent to that of Figure 6, showing detected fluorescence intensity in arbitrary units as a function of position of the translation stage;
  • Figure 8 shows sampling of detected probe fluorescence along an existing grating section which is used to achieve alignment of between an existing grating section and a probe beam
  • Figure 9 shows the stitching region between grating sections in terms of the refractive index modulation against position
  • Figure 10 shows yield, y, required per grating section for a total yield of 50% as a function of total grating length, L, in metres;
  • Figure 11 shows yield, y, required per grating section for a total yield of 10% as a function of total grating length, L, in metres.
  • FIG. 1 shows a grating fabrication apparatus according to an embodiment of the invention.
  • the apparatus is a development of that of WO 98/08120.
  • a laser 2 supplies a beam 7 to a phase mask 14 to expose a photosensitive waveguide in the form of an optical fibre 18.
  • the laser used is a continuous wave (CW) laser producing a beam having a power of up to 100 mW at a lasing wavelength of 244 nm, i.e. in the ultra-violet (UN) region.
  • CW continuous wave
  • AOM acousto-optic modulator
  • the laser beam is in a polarised state as indicated by arrows 5.
  • the beam 7 is deflected through 90 degrees by a mirror (Ml) 8, through a focusing lens (LI) 10, a further lens (L2) 12 and the phase mask 14, thereby to image a periodic intensity pattern onto a section of the optical fibre 18.
  • the phase mask 14 is positioned remote from the optical fibre 18, rather than in contact.
  • a piezoelectric positioning device (PZT) 16 is provided for adjusting the position of the lens 12 to ensure good alignment between the beam 7 and the optical fibre 18.
  • the position adjustment may be in the form of a dither (i.e. periodic spatial oscillation) having a frequency selected to be small in comparison to the rate at which fringes traverse the exposure region (which is typically in the order of kHz). A value of 20 Hz is typical for the dither frequency.
  • the optical fibre 18 is securely held on a bar (B) 34 in first and second N- grooves (Nl & N2) 30 and 32.
  • a mirror (M2) 28 which defines a measurement arm 42 of an interferometer 44 that is used to provide absolute position measurements of the bar 34 which is movably mounted on a linear translation stage 26.
  • Translation mounts (Tl & T2) 56 and 58 mount the bar 34 to the translation stage 26.
  • the translation stage used provided a travel of about 105 cm (42 inches).
  • the interferometer 44 used was a double-pass He- ⁇ e interferometer.
  • a position feed-back connection 46 provides a feed-back signal from the interferometer 44 to the linear translation stage 26 to ensure absolute positioning accuracy.
  • a further connection 48 connects an output of the interferometer 44 to a decision logic unit 52.
  • the decision logic unit 52 receives a further input from a connection 54 which links the decision logic unit 52 to an output of a control computer (PC) 60.
  • the control computer 60 stores a set of pre-calculated beam modulation positions which define the structure of the grating to be fabricated.
  • the set of beam modulation positions may define an aperiodic structure (e.g. a chirped grating) or a periodic structure (e.g. a grating of a single period).
  • the connection 54 relays a signal from the control computer 60 that conveys calculated beam modulation positions to the decision logic 52.
  • the decision logic 52 controls the AOM 6 through a connection 50 and based on the inputs from connections 48 and 54. Namely, the state of the AOM 6 is switched by the decision logic 52 when the measured position received from the interferometer 48 corresponds to the modulation position received from the control computer 60.
  • One end of the fibre 18 is connected to some general diagnostics 25 comprising an optical spectrum analyser (OSA) 20, a 50:50 beam splitter 22 and a broadband optical source 24 which are connected as shown in Figure 1.
  • the other end 36 of the fibre 18 is connected to a photo-detector 38 for measuring fluorescence induced in the fibre 18 by the light beam 7.
  • the detector 38 measures fluorescence from an emission at 400 nm.
  • the detector 38 has an output connected via connection 39 to a tracking circuit for conveying a fluorescence signal to the tracking circuit 40. Responsive to the fluorescence signal, the tracking circuit 40 outputs a dither control signal through a connection 41 to the PZT 16 that provides the above-described dithering.
  • the apparatus is further provided with an additional control connection 68 which is used to supply the fluorescence signal from the detector 38 to the control computer 60.
  • Figure 2 shows internal structure of the control computer 60.
  • the set of pre- calculated beam modulation positions defining the structure of the grating to be fabricated are stored in a storage device 62.
  • the fluorescence signal is supplied by the control connection 68 to an offset calculation unit 66 of the control computer 60.
  • the offset calculation unit 66 is operable to calculate an offset value from the fluorescence signal. _ The significance of the offset value and its calculation from the fluorescence signal are described in detail further below.
  • the calculated offset value is supplied to an offset compensation unit 64 of the control computer 60 which is operable to add the determined offset value to the beam modulation positions, so that the beam modulation positions supplied to the decision logic 52 are compensated to take account of the offset value.
  • the apparatus is further provided with components 70 and 72.
  • These are motorised spools for advancing the position of the fibre 18 on the bar 34 in an automated fashion, thereby allowing the fibre 18 to be moved along after a grating has been written on a section of the fibre 18.
  • the fibre 18 could be advanced by providing a second linear translation stage (not shown) and exchanging the fibre from one stage to the other using standard robotic automation techniques.
  • the fibre 18 can be advanced on the bar 34 manually, as is more convenient for lower production volumes, in which case the spools 70 and 72 could be dispensed with.
  • Figures 3 A to 3C show highly schematically and in chronological order different stages in fabrication of a grating using the apparatus of Figure 1.
  • Figures 4A to 4C show the same steps as Figures 3 A to 3C respectively, showing the fibre and phase mask in more detail.
  • Figures 3A and 4A shows a first section of an optical fibre 18 arranged on the translation stage 26.
  • the fibre is labelled with an axis, 1
  • the translation stage is labelled with an axis, z.
  • the first section of fibre extends from lo to li and is arranged on the translation stage between zo and z ⁇ . Motion of the translation stage in the z direction is then used to cause relative motion between the phase mask 14 and the fibre 18. (In alternative embodiments, the phase mask 14 could be moved instead of the translation stage).
  • a grating structure is thereby written into the first section over the distance W shown in the drawings with an inscription beam (UNwi in Figure 4 A) supplied by the laser and modulated by the AOM 6.
  • the inscription beam has a generally square-wave intensity profile, as illustrated in Figure 5.
  • the written grating section is indicated schematically in Figure 3A by short dashes at right angles to the fibre.
  • the periodic intensity pattern of the inscription beam may be generated either interferometrically, or by using a diffractive optical element, such as a pi-phase mask, as assumed in the present embodiment. It will be understood that the first grating section is thus fabricated basically according to the method described in WO 98/08120.
  • Figures 3B and 4B show the apparatus after the fibre has been moved forward by an amount (li - lo) - ⁇ .
  • the laser beam Prior to moving the fibre forward, the laser beam is turned off or at least strongly attenuated, to avoid accidental writing of the photosensitive fibre.
  • an overlap region P of the first grating section extending between zo and (zo + ⁇ ) and is still accessible to exposure through the phase mask at one end of the travel of the linear translation stage 26.
  • the first grating section is thus translated such that a portion of its length remains in the range of the linear translation device.
  • a second section of the fibre defined by values lj and 1 2 , is arranged to extend between (zo + ⁇ ) and Zj on the translation stage.
  • the overlap region P of the first grating section is then interrogated by a probe beam (UNp in Figure 4B) which has a spatially-periodic intensity pattern. Additionally, the probe beam has a wavelength that induces fluorescence in the waveguide.
  • the intensity pattern of the probe beam is conveniently generated by using the same laser and other optical components that are used to write the grating sections.
  • the power of the probe beam is strongly attenuated, relative to that of the inscription beam, in order that any photo-induced refractive index change caused by the probe beam is minimal. The attenuation can be effected automatically or manually using the AOM 6, polarisers (not shown), or neutral density filters placed in the beam path (not shown).
  • Linear translation of the fibre with respect to this probe beam is then commenced with the overlap region P being illuminated by the probe beam to generate the fluorescence necessary to interrogate the existing grating structure.
  • Highly-accurate linear interferometers are available commercially which have sub-nanometre resolution (e.g. ZYGO ZMI-1000) so that scanning over the overlap region can be controlled with a similar precision.
  • the fluorescence detected at the output of the fibre by the detector 38 is supplied as a signal to the control computer 60 through signal line 68.
  • Figure 6 shows an idealised intensity profile of the fluorescence as a function of the translation. Generally, the fluorescence intensity has a periodic modulation, as described further below.
  • Figure 7 corresponds to Figure 6, but shows a measured fluorescence signal in arbitrary intensity units.
  • the fluorescence signal is monitored by the offset calculation unit 66 of the control computer 60 ( Figure 2) which uses the modulation periodicity in the signal to determine the phase relation between the probe interference pattern and the primary grating structure at the points of measurement.
  • the modulation of the fluorescence signal is caused by the peaks in the probe beam intensity pattern coming into and out of registry with the peaks of the grating structure written into the fibre. Since the fibre is translating continuously through the probe interference pattern a continuous measurement of fluorescence yields the modulating signal shown in Figures 6 and 7 with one oscillation observed when the grating moves by one period.
  • the fluorescence varies according to the relative alignment of the intensity fringes with the periodic bleaching of fluorescence associated with the regions of strongest photo-induced refractive index change.
  • the relative position of the interference fringes and the refractive index modulation of the existing grating section By adjusting the positions at which readings are taken by a known amount, it is possible to determine the relative position of the interference fringes and the refractive index modulation of the existing grating section. For example, a period of the first grating section may be used as the known amount for incrementing the measurement positions.
  • the relative motion between the probe interference pattern and the existing grating is continuous and monotonic, and is measured interferometrically.
  • ⁇ The value of the offset of the measurement position, referred to as ⁇ in the following, is thus deduced from the phase relationship of the fluorescence by the offset computation unit 66.
  • the formalism used for calculating the offset value from the fluorescence signal is detailed further below.
  • Figures 3C and 4C show the apparatus again, with the fibre still in the same position as shown in Figures 3B and 4B, but subsequent to writing a second grating section between positions lj and 1 2 in the fibre.
  • the laser beam is returned to its inscription beam mode, i.e. its power is increased.
  • the offset value ⁇ is added to the stored beam modulation positions output from the storage 62 prior to output by the control computer 60 along the connection 54.
  • the beam modulation positions adjusted by the offset value are then used to fabricate the second grating section, which is thereby accurately "stitched" to the first grating section.
  • the second grating section is in phase with the first grating section.
  • the second grating section is formed by many exposures of light nominally separated by a single grating period.
  • the positions of the exposures are pre-calculated and triggered by the decision logic 52 based on interferometric position measurements by the interferometer 44 and the beam modulation positions supplied by the control computer 60.
  • the important difference from the method of WO 98/08120 is that the beam modulation positions used for writing the second grating section are adjusted by the offset value, determined by the above-described probe beam interrogation process carried out prior to writing the second grating section.
  • ⁇ o is both the period of the grating and the separation of the exposures; n is an integer; ⁇ is the phase offset between the intensity peaks of the probe interference pattern at the measurement positions n/ ⁇ o and the troughs of the induced refractive index structure (corresponding to the regions of least fluorescence- bleaching). Note that / may be dependent on some other function with a dominant periodicity of ⁇ o.
  • Equation (1) may thus be generalised as:
  • ⁇ is the offset of the measurement position. It is thus possible to determine the position offset required for the measurements whereby the level of fluorescence gives a desired phase relation between the existing grating and the interference pattern at the measurement positions.
  • the in-phase condition is thus where /is minimised:
  • the above described fluorescence monitoring method envisages aligning and stitching a new grating section with an existing one by recording continuously the periodic fluorescence pattern generated by the weak, i.e. low power, probe beam.
  • the length resolution e.g. step size in the case of a stepped scan, must then be much smaller than the period of the refractive index modulation, i.e. the grating period, in order to track the evolution of the refractive index modulation.
  • the period of the refractive index modulation i.e. the grating period
  • the fluctuations in the fluorescence level have a direct inverse correspondence to the refractive index modulation that defines the grating.
  • the sampled fluorescence level is the curve traced out by the fluorescence levels at the sample points (hollow circles).
  • the sampled fluorescence also varies smoothly and periodically, but with a much lower frequency than that of the actual fluorescence level (or grating period). The reason for this behaviour is now described. As mentioned above, in this advanced method, the steps are sampled with a period ⁇ sam pi e that is slightly different to the grating period ⁇ o.
  • the sampled fluorescence intensity, at the m sampling point z m is given by:
  • the m .th sampling point is given by:
  • n 0,1,2,3. (7)
  • Equation (8) gives the number of steps that are required before the next minimum is reached.
  • the probe-beam and the existing grating section are in perfect alignment, i.e. in phase.
  • Writing of a further section of grating is then proceeded with, using a second set of predetermined stored data.
  • the positioning stage on which the fibre is held thus now has an absolute frame of reference with which to control the writing of the next grating section.
  • sampling period is not critical, but will typically be within 5% or
  • FIG. 9 shows induced refractive index change as a function of position along the fibre in a stitching region between two grating section.
  • the refractive index profile of the existing grating section is shown with the solid lines.
  • An apodisation has been applied at the end of the existing grating section in the form of a gradual attenuation of the refractive index maxima, from a knee point. The knee point to the end of the grating section defining an overlap region for stitching the next grating section.
  • the existing grating section is then probed with a low level of illumination (horizontal dashed line in the figure) during which the above-described fluorescence-based sampling method is applied.
  • a low level of illumination horizontal dashed line in the figure
  • scanning towards the end of the existing grating section continues until onset of the overlap region.
  • the sampling step is then returned to the initial value ⁇ o (or multiples thereof) and the probe-beam power is increased gradually across the overlap region to the initial writing power.
  • the power increase advantageously takes place gradually over the overlap (stitching) region in order to avoid background-index variations and the resulting unwanted phase shifts.
  • the technique ensures that the total intensity dose applied over the overlap region during the two separate writing processes is substantially the same as in the interior of the individual grating sections where only a single writing process is applied.
  • the absolute amplitudes of the peaks and troughs of the refractive index profile are not different from those away from the stitching regions, as is evident from Figure 9.
  • conventional techniques cause overdosing of light in the overlap region so that even if there are no anomalies in the phase of the refractive index profile in the overlap region, there are still anomalies in the magnitudes of the refractive index profile.
  • the attenuation will be provided by controlling the intensity of the writing beam when a technique based on that of WO 98/08120 is used. If other writing techniques are used in which a whole grating section is made in a single exposure (e.g. proximity phase mask techniques), then the attenuation may be provided with suitable profiling at the edge of the exposure area, for example with neutral density filtering.
  • apodised stitching technique exemplified by Figure 9 may also find general application to many processes of writing gratings in a photosensitive material where multiple sections need to be stitched together and is not limited to gratings written with the stroboscopic process of WO 98/08120.
  • Figures 10 and 11 are used to illustrate the advantages of the present fabrication technique in which a long multi-section grating with long individual grating sections is provided. These advantages are to be contrasted with other techniques of fabrication of long multi-section gratings with shorter individual grating sections.
  • a fundamentally important advantage of the present fabrication technique is the reliability with which long fibre gratings can be fabricated.
  • short grating sections e.g. grating sections limited in length by the dimensions of a phase mask (5- 10cm)
  • it is necessary to stitch many sections to form a grating of any substantial length e.g. more than lm.
  • Y t is the yield of the long stitched device
  • Y ⁇ g is the yield of the individual grating sections
  • Y sMc h is the certainty of obtaining good alignment between the gratings sections
  • L ⁇ is the overall length of the stitched grating
  • L ⁇ g is the length of the grating sections.
  • Figure 10 shows yield, y, required per grating section for a total yield of 50% as a function of total grating length, L, in metres.
  • Figure 11 is a corresponding figure for a total yield of 10%.
  • the required yield per grating section is consistently high, above 90% for both the 5cm and 10cm cases in order to achieve 50% total yield (for the 10% total yield case these values are still above 90% for all but the lm total length).
  • the yields required for the different length gratings starts to converge, for gratings up to 4-5m in length, a very much lower yield is required for gratings with a length ⁇ lm.
  • the very high yield required for 10 cm sections means that it is unrealistic to produce, reliably and consistently, long gratings of several metres in length by stitching phase-mask length grating sections.
  • the above-described technique does not use a physical dithering of either the interference pattern (i.e. the phase mask) or the waveguide to achieve in-phase stitching between adjacent grating sections.
  • the technique presented herein there is a continuous, monotonic, relative motion between the interference pattern and the existing grating, measured interferometrically, and readings of the fluorescence level are made at well-known positions (for example, it may be desirable to take measurements separated by the known period of the existing grating section).
  • the technique is not based on UN beam scanning phase masks (hence grating quality is not limited by phase mask quality); 2) the technique is capable of fabricating gratings of lengths far in excess of the inscription beam size, or the dimensions of a phase mask, if one is employed;
  • the technique has a high accuracy for fabricating grating sections, based on linear interferometer
  • the relative phase of the grating sections can be adjusted highly accurately to fabricate chirped and/or apodised structures.

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Investigating, Analyzing Materials By Fluorescence Or Luminescence (AREA)
  • Optical Integrated Circuits (AREA)

Abstract

L'invention concerne un procédé et un appareil pour créer un réseau de diffraction de guide d'onde optique par l'inscription d'une succession de sections de réseau de diffraction dans un guide d'onde photosensible. Selon l'invention, une première section de réseau de diffraction est inscrite par l'exposition répétée d'un faisceau d'inscription doté d'un diagramme d'intensité périodique sur la première longueur de guide d'onde et par le déplacement du faisceau d'inscription par rapport au guide d'onde entre les expositions successives. La première section du réseau de diffraction étant inscrite, le guide d'onde est déplacé sur le dispositif de translation de telle sorte qu'une deuxième longueur de guide d'onde est disposée pour l'exposition, une portion de la première section de réseau de diffraction restant dans la portée du dispositif de translation linéaire. Un faisceau sonde doté d'un diagramme d'intensité périodique est utilisé pour éclairer la portion de la première section du réseau de diffraction. Le rapport de phase entre le diagramme d'intensité du faisceau sonde et la première section du réseau de diffraction est alors déterminé par la translation du guide d'onde relativement au faisceau sonde et par la mesure de la fluorescence émise par le guide d'onde excité par le faisceau sonde. Selon l'invention, on procède ensuite à l'inscription de la deuxième section du réseau de diffraction.
PCT/GB2001/001347 2000-04-05 2001-03-27 Piqure en phase de reseaux de diffraction a longues fibres WO2001075496A1 (fr)

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AU42595/01A AU4259501A (en) 2000-04-05 2001-03-27 In-phase stitching of long fiber gratings

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EP00302868.5 2000-04-05
US19630900P 2000-04-12 2000-04-12
US60/196,309 2000-04-12

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CN103777270A (zh) * 2014-02-11 2014-05-07 武汉理工大学 自动静态连续制备光纤光栅阵列的装置与方法
US9644966B2 (en) 2014-09-11 2017-05-09 Honeywell International Inc. Integrated optic circuit with waveguides stitched at supplementary angles for reducing coherent backscatter
CN108474902A (zh) * 2016-03-25 2018-08-31 株式会社藤仓 光纤光栅的制造装置及制造方法
CN109491035A (zh) * 2018-11-16 2019-03-19 成都科信达实业有限公司 一种大口径拼接光栅精密调整架
CN117687135A (zh) * 2024-02-04 2024-03-12 安徽中科光栅科技有限公司 一种虚实光栅对准方法
CN117687136A (zh) * 2024-02-04 2024-03-12 安徽中科光栅科技有限公司 一种拼接光栅对准精度检测方法

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Cited By (12)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN103777270A (zh) * 2014-02-11 2014-05-07 武汉理工大学 自动静态连续制备光纤光栅阵列的装置与方法
US9644966B2 (en) 2014-09-11 2017-05-09 Honeywell International Inc. Integrated optic circuit with waveguides stitched at supplementary angles for reducing coherent backscatter
CN108474902A (zh) * 2016-03-25 2018-08-31 株式会社藤仓 光纤光栅的制造装置及制造方法
EP3435125A4 (fr) * 2016-03-25 2019-11-06 Fujikura Ltd. Dispositif et procédé de fabrication de fibre à réseau de bragg
CN108474902B (zh) * 2016-03-25 2020-11-03 株式会社藤仓 光纤光栅的制造装置及制造方法
US10976487B2 (en) 2016-03-25 2021-04-13 Fujikura Ltd. Manufacturing device and manufacturing method of optical fiber grating
CN109491035A (zh) * 2018-11-16 2019-03-19 成都科信达实业有限公司 一种大口径拼接光栅精密调整架
CN109491035B (zh) * 2018-11-16 2024-02-02 成都科信达实业有限公司 一种大口径拼接光栅精密调整架
CN117687135A (zh) * 2024-02-04 2024-03-12 安徽中科光栅科技有限公司 一种虚实光栅对准方法
CN117687136A (zh) * 2024-02-04 2024-03-12 安徽中科光栅科技有限公司 一种拼接光栅对准精度检测方法
CN117687135B (zh) * 2024-02-04 2024-04-16 安徽中科光栅科技有限公司 一种虚实光栅对准方法
CN117687136B (zh) * 2024-02-04 2024-04-16 安徽中科光栅科技有限公司 一种拼接光栅对准精度检测方法

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