WO1999045414A1

WO1999045414A1 - Optical interferometer and method for writing phase structures

Info

Publication number: WO1999045414A1
Application number: PCT/AU1999/000133
Authority: WO
Inventors: Gideon William Yoffe; Benedict Smith; John William Arkwright
Original assignee: The University Of Sydney; Siemens Ltd.
Priority date: 1998-03-04
Filing date: 1999-03-04
Publication date: 1999-09-10

Abstract

A method of forming a grating structure (127) in a photosensitive waveguide (128) is disclosed comprising projecting a single beam of light (121) onto a phase mask (122) so as to create two coherent beams of light (123, 124); projecting the two coherent beams (123, 124) around an optical circuit (125, 126) so that they interfere at a first location; translating the phase mask (122) substantially perpendicular to the single beam of light (121); whilst simultaneously translating the photosensitive waveguide (128) substantially perpendicular to the single beam (121) so as to substantially minimize the variation in position of the intensity maxima of each fringe of the interference pattern in the reference frame of the waveguide (128) thus simultaneously writing a grating (127) along the portion of the waveguide within the interference pattern.

Description

- 1 -

OPTICAL INTERFEROMETER AND METHOD FOR WRITING PHASE STRUCTURES

Background of the Invention

The present invention is directed to the writing of accurate low noise grating structures in waveguides such as optical fibres and in particular, discloses an interferometer system for writing fibre Bragg gratings. Background of the Invention

A fibre Bragg grating is a length of optical fibre that has a periodic variation in the refractive index of the glass in the core along its length. This periodic variation is known to be generally formed by holographic exposure from the side by two beams of ultra-violet light that intersect in the core region. The interference pattern so formed is transferred into a permanent modulation of the refractive index. Light propagating along the fibre interacts with this fringe pattern, undergoing partial reflections all the way along it. If the wavelength of the light is such that a whole number of half-wavelengths correspond to the spatial period of the fringe pattern, strong reflectivity can be obtained.

There are two standard techniques for achieving the holographic exposure. A free-space interferometer can be set up, using a beam-splitter to divide the ultra-violet light beam into two parts and mirrors to direct the two beams onto the fibre. Another technique involves placing the fibre directly behind a phase mask. Ultra-violet light that is incident on the phase mask is split into diffracted orders. Two of these orders interfere on the fibre tc generate a periodic pattern. The phase mask technique is easier to use, because stable interferometers are not required, but it is relatively inflexible because the period of the grating (and hence the wavelength that is reflected) is determined largely by the phase mask.

One particular advantageous form of Bragg grating writing is disclosed in PCT Patent Specification No.

PCT/AU96/00782 by Franςois Ouellette et. al. entitled "Ring - 2 -

Interferometer Configuration for Writing Gratings" the contents of which is specifically incorporated herein by cross reference. The method disclosed in the aforementioned PCT specification utilises a Sagnac type loop so as to minimise vibrations occurring in the grating writing process. Fig. 1 illustrates the basics of the aforementioned process wherein an input coherent UV beam 1 is diffracted into two first order beams 2 and 3 by a diffraction grating or phase mask 4. The two diffractive beams 2, 3 are reflected around a series of mirror elements 6, 7 so that each of the beams follow substantially counter-propagating paths such that they again meet and interfere at the point 8 where an interference pattern results and where a photosensitive fibre 9 is placed so as to form a modulated refractive index within the fibre 9. The angle of overlap of mirrors 6, 7 can be controlled so as to alter the grating wavelengths and therefore provide for specific wavelength reflections and/or chirping in accordance with requirements. The aforementioned PCT specification also discloses traversing the beam along the surface of the phase mask 4 so as to form an extended grating structure.

However, when writing extended grating structures, the process of the aformentioned PCT patent application has a number of significant drawbacks. This is perhaps readily evident from the schematic illustration in Fig. 2a and Fig. 2b which illustrates the ray paths followed by an incident beam 10 which is incident on a extended grating 11 which in turn is diffracted into diffracted beams 12, 13. The diffracted beams are reflected by mirrors 14, 15 so as to form a corresponding interference pattern on the fibre at a particular location shown at the point 17 in Fig. 2a and the point 18 in Fig. 2b. Unfortunately, the two arrangements result in substantially different paths for the rays as they traverse the ring and, as the input UV beam 10 traverses the phase mask 11, the corresponding - 3 - beams traverse across the mirror surfaces 14, 15. The traversal across the mirrored surfaces can result in a substantially higher noise factor being introduced which is generally undesirable and calls for extremely flat surfaced mirrors. Further, air current etc. can interfere with the pathway of the beams resulting in non-symmetrical disturbances to the beams which in turn induced further noise. Summary of the Invention It is an object of the present invention to provide for an improved form of grating writing systems which overcomes or at least alleviates some of the disadvantages of the aforementioned arrangements.

In accordance with a first aspect of the present invention, there is provided a method of forming a grating structure in a light sensitive photosensitive waveguide comprising projecting a single beam of light onto a phase mask so as to create two coherent beams of light; projecting the two coherent beams around an optical circuit so that they interfere at a first location; translating the phase mask substantially perpendicular to the single beam of light; whilst simultaneously translating the photosensitive waveguide substantially perpendicular to the single beam so as to substantially minimize the variation in position of the intensity maxima of each fringe of the interference pattern in the reference frame of the waveguide thus simultaneously writing a grating along the portion of the waveguide within the interference pattern. Preferably, the phase mask is mounted on a first translatable stage and the photosensitive waveguide is mounted on a second translatable stage and one of the translatable stages (a top translatable stage) is mounted on the other one of the translatable stages (a bottom translatable stage) . Otherwise, the phase mask can be mounted on a first translatable stage and the photosensitive waveguide is mounted on a second - 4 - translatable stage able to be independently moved relative to the first translatable stage.

The optical circuit ideally comprises a Sagnac type loop with two reflective mirrors although other arrangements are possible. Further, the photosensitive waveguide is preferably translated relative to the phase mask such that the ratio of the speed of movement of the waveguide to that of the phase mask is substantially equal to the ratio of the period of the grating being written to the period of a grating written by direct contact printing through the same phase mask using the same diffracted orders .

In one embodiment, the top translatable stage includes an input signal for driving a transducer element for positioning the top translatable stage and the input signal is driven in a sawtooth manner when writing the grating.

Further, in an alternative arrangement, the translation of the photosensitve waveguide is varied so as to write a chirped grating in the photosensitve waveguide. In accordance with a further aspect of the present invention, there is provided a method of forming an extended grating structure in a photosensitive waveguide comprising projecting a single beam of light onto a phase mask so as to create two coherent beams of light; projecting the two coherent beams around a series of optical reflection elements so that they interfere at a first location; and translating at least one of the optical reflection elements in a first direction substantially perpendicular to the path of the single beam so as to cause a consequential translation of the interference pattern along the grating so as to cause the extended grating structure to be written in the photosensitive waveguide.

The optical reflection elements can be translated in the first direction and when the grating structure is written at a different wavelength than the wavelength of the phase mask, the photosensitive waveguide or phase mask - 5 - can be simultaneously translated in the first direction (with a positive or negative velocity) so as to maintain a stable spatial position of maxima in the interference pattern in the reference frame of the photosensitive waveguide .

Ideally, the two coherent beams form an optical circuit comprising substantially a Sagnac loop and the number of optical reflective elements is two.

In accordance with a first aspect of the present invention, there is provided a method of forming an extended grating structure in a photosensitive waveguide comprising the steps of: projecting a single beam of light onto a phase mask so as to create two coherent beams of light; projecting the two coherent beams around a series of optical reflection elements so that they interfere at a first location; translating at least one of the optical reflection elements in a first direction substantially perpendicular to the path of the single beam so as to cause a consequential translation of the interference pattern along the grating so as to cause the extended grating structure to be written in the photosensitive waveguide; subsequently, translating the phase mask substantially perpendicular to the single beam of light; and simultaneously translating the photosensitive waveguide substantially perpendicular to the single beam so as to substantially minimize the variation in position of the intensity maxima of each fringe of the interference pattern in the reference frame of the waveguide thereby simultaneously writing a grating along the portion of the grating within the interference pattern. Brief Description of the Drawings

Notwithstanding any other forms which may fall within the scope of the present invention, preferred forms of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: - 6 -

Fig. 1 is an illustration of the prior art grating range system;

Fig. 2(a) and Fig. 2(b) illustrates schematically a problem associated with the prior art grating writing systems; and

Figs. 3 and 4 illustrate the operation of a first embodiment;

Fig. 5 illustrates the operation of an alternative embodiment of the present invention; Fig. 6 illustrates schematically the arrangement of a further embodiment;

Fig. 7 illustrates the arrangement of a further alternative embodiment; and

Fig. 8 to Fig. 10 illustrates the method of operation of a combined embodiment.

Description of Preferred and Other Embodiments

In a first embodiment an interferometric design is proposed that creates the correct phase relationship between scanned beams once they have been recombined on the waveguide. Instead of moving the beam along the phase mask, one or both of the mirrors in the arrangement are moved in a direction substantially parallel to the phase mask. The movement of the mirrors results in an interfering beam incident on the fibre with the phase relationship of the beam being such as to write a continuously varying index modulation that maps out a grating in the fibre frame of reference.

Turning initially to Fig. 3, there is illustrated an initial example arrangement. This arrangement is very similar to that aforementioned and includes an incident UV beam 20 which is split into two coherent beams 21, 22 which are reflected around mirrors 24, 25 so that they interfere at the plane in which a fibre is placed 26.

In a preferred embodiment, a shortened phase mask 20 is provided and, instead of scanning the beam along the phase mask 27, the mirror 25 is translated generally in the - 7 - sweeping across the surface of the fibre 26. The interference pattern having spatially stable maxima and minima. The situation at a later time is as indicated in Fig. 4, where the interference pattern has moved to the point 30 having traversed between the two locations. In this way, an extended Bragg grating structure is written on the fibre 26. Of course, the method of Fig. 3 and 4 has the unfortunate disadvantage that the beam is translated along the surface of the mirrors 24, 25 as interference pattern 30 moves along the fibre 26. However, the UV beam 20 remains stable and the portion of the phase mask 27 utilized is substantially unchanged.

As the spot traverses a distance of λ_f/2 (where λ_f is the Bragg wavelength in the plane of the waveguide ie. the desired period of the induced index variation) , the relative phase between the counter-propagating beams will change by one period of the writing wavelength, hence causing the interference pattern formed at the point of intersection to move between adjacent maxima as the mirror is scanned. The Sagnac nature of the interferometer ensures that the fringes formed within the overlapping beams move in such a way during the scanning process that the evolving fringe pattern remains stationary in the waveguide frame of reference. In a modification, the angles of the mirrors making up the Sagnac interferometer can be varied so as to change the period of the fringe pattern formed on the optical waveguide. In this way, gratings of varying period or gratings with continuously chirped period can be written. The embodiment of Fig. 3 and Fig. 4 allows long gratings to be written without relying on the uniformity of an etched phase mask or diffraction grating.

This design also allows direct monitoring of the writing beam since one of the counter-propagating beams will remain stationary on mirror 24 during the scanning process and allows a monitor 32 (Fig. 3) to be placed - 8 - behind the mirror where partially silvered mirrors are used. A disadvantage of this method is that it requires long optically flat mirrors to be used in the interferometer . Turning now to Fig. 5, there is illustrated the method of an alternative embodiment. In this embodiment, both of the mirrors 24, 25 are translated generally in the direction 34. This is achieved by applying a relative motion between the UV beam, fibre and phase mask combination 20, 26, 27 and the mirrors 24, 25.

The net result of this motion moves both of the mirrors 24, 25 in unison in the direction 35 parallel to the waveguide. An advantage of this arrangement is that both mirrors of the Sagnac loop are locked together during the translation, hence providing a more stable interferometer. This arrangement will also allow gratings approximately twice the length of those written using the scanning ring interferometer set out in the aforementioned PCT specification. It should be noted that where it is desired to write an arbitrary wavelength grating, it will be necessary to calculate the differential velocity between the phase mask and fibre. In this case, it will be necessary to translate the fibre so as to maintain a stable grating pattern in the reference frame of the fibre. If the waveguide is positioned in the same vertical plane as the phase mask the required differential velocity will be zero and the written Bragg wavelength will be equal to that produced when the waveguide is exposed in direct contact with the phase mask. It can be seen that, through utilization of a translation of the mirrors, an extended grating structure can be formed utilizing a single portion of the phase mask so as to write an extended structure.

In a further alternative embodiment, a stable static ring interferometer is provided that uses a phase mask as a beam splitter. In this embodiment the input beam is - 9 - maintained in a stable position and the diffracted interfering ultra violet beams follow the same paths and do not move. Instead, the fibre and phase mask are scanned through the region where the beams intersect . Turning initially to Fig. 6, there is illustrated schematically the preferred embodiment 120. In the preferred embodiment, UV light from a laser 121 is diffracted by a phase mask 122 so that two of the orders 123, 124 are reflected by mirrors 125, 126 so as to form an interference pattern and subsequent grating 127. A fibre 128 is positioned with a slight vertical displacement from the incoming UV beam so that the original diffracted orders 123, 124 do not strike the fibre (similar to Fig. 1) . The beams travel at a slight angle to the plane of the interferometer so that the beams that return from the mirrors hit the fibre at the point where they interfere. The interference pattern is created at the intersection point 127 resulting in a subsequent grating being written into the photosensitive core of the fibre. In the static state, the period of grating 127 can be varied by adjusting the angle of one or both of the mirrors 125, 126. This action will also alter the position of the intersection of the two beams and the phase mask position should be adjusted so that the beams overlap the fibre. In the preferred embodiment, the phase mask 122 and fibre 128 are each mounted on a bottom translation stage 130 which can be translated in the direction 131 as indicated. The phase mask is further mounted on a second translation stage 132 which provides for independent motion of the phase mask 122 relative to the fibre 128.

Considering first the case where it is desired to write a grating wavelength equivalent to the phase mask wavelength, the arrangement 120 is set up such that the returning beams intersect in the plane of the phase mask and the fibre is placed immediately adjacent to both slightly vertically displaced from the phase mask. In this -10- case, the fibre 128 and the phase mask 122 can be translated together at the same velocity through the incident laser beam 121 in the direction of 131. The phase of the interference pattern will remain static relative to the fibre and a grating is obtained having a length which is approximately equal to the distance travelled by the fibre and phase mask.

If the interferometer is set up to write gratings with a different period or wavelength with the fibre 128 separated from the plane of the phase mask 122, it is necessary to move the fibre and the phase mask at different speeds relative to the ultra-violet beam and the mirrors in order to accommodate the different rate of evolution of the interference fringes. In this case the fibre and the phase mask can be mounted on separate translation stages 130, 131. The required ratio of the speed of movement of the fibre 128 to that of the phase mask 122 will be equal to the ratio of the period of the grating being written to the period of a grating written by direct contact printing through the same phase mask using the same diffracted orders. In many cases the difference in speeds is small and can be achieved most simply by mounting the translation stage that carries the phase mask on the stage that carries the fibre, or vice-versa. For fine control of the wavelength, it is desirable to use for the differential motion a small translation stage with very precise electrically-controlled motion, using, for example, a piezoelectric transducer. In a further refinement, if this stage cannot accommodate the whole of the required differential movement, a "sawtooth" motion could be adopted so that it moves in the required direction at the required speed for a certain distance and then moves back towards the other end of this range of travel before repeating the desired motion. Ideally this return movement, or "flyback", should be as instantaneous as possible and predetermined so that the fringes written -11- after flying back are coherent with those written beforehand. So long as the motion is correct during the writing part of the cycle, a grating will be written at the desired wavelength. The differential distance travelled between flyback events can be anything between zero and the maximum travel of the stage.

It is also possible in a further refinement to introduce a "chirp", or other spatial variation in the period of the grating, by varying the difference in the speeds of movement of the phase mask and the fibre as the two are scanned through the interferometer. For small variations in wavelength no repositioning of the interferometer mirrors is required, but a broad chirp preferably is achieved by real-time control of the mirror angle as well as control of the differential velocity described above.

The utilisation of a stable ring interferometer arrangement has a number of significant advantages. For example, a more compact ring can be formed and no traversal of the mirror surfaces is required. Further, it is possible to utilises partial reflecting mirrors and to place detectors behind the mirrors so as to accurately monitor the position and intensity of the UV beams being projected around the ring. Further, as each counterpropergating beams follow substantially the same paths, they will have a higher resilience to noise vibrations and fluctuations in air currents etc, in addition to being operated with light sources having a low coherence length. In the further arrangement, the embodiments as disclosed with reference to Fig. 3 to Fig. 5 are combined with the arrangement of Fig. 6 to provide for a combined arrangement which allows for the writing of even longer gratings. The combined arrangement is illustrated in Fig. 7 and involves the utilization of a second separate translation platform 140. The operation of the combined -12- system can then be as illustrated in Figs. 8 - 10 wherein

Fig. 8 illustrates a similar arrangement to Fig. 5 with the mirrors 125, 126 being translated generally in the direction 141 so as to write a grating structure 142 in the fibre 128 via phase mask 122. Subsequently, as shown in Fig. 9, the translation of the mirrors 125, 126 reaches a predetermined point wherein the method of Fig. 6 is utilized to translate the phase mask 122 and fibre 128 generally in the direction 145. The arrangement extending the grating structure so as to, at a later stage, be in the position as illustrated in Fig. 10. In this matter, an extended grating structure can be written in a fibre.

Obviously other intergrated combinations of the arrangments can be implemented. It would further be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

Claims

-13-We Claim:

1. A method of forming an extended grating structure in a photosensitive waveguide comprising: projecting a single beam of light onto a phase mask so as to create two coherent beams of light; projecting said two coherent beams around a series of optical reflection elements so that they interfere at a first location; translating at least one of said optical reflection elements in a first direction substantially perpendicular to the path of said single beam so as to cause a consequential translation of said interference pattern along said grating so as to cause said extended grating structure to be written in said photosensitive waveguide.

2. A method as claimed in claim 1 wherein all of said optical reflection elements are translated in said first direction.

3. A method as claimed in any previous claim wherein said grating structure is written at a different wavelength than the wavelength of said phase mask and said photosensitive waveguide is simultaneously translated in said first direction (with a positive or negative velocity) so as to maintain a stable spatial position of maxima in said interference pattern in the reference frame of said photosensitive waveguide.

4. A method as claimed in claim 1 wherein said translation results in a corresponding path length variation in said two coherent beams.

5. A method as claimed in any previous claim wherein said two coherent beams form an optical circuit comprising substantially a Sagnac loop.

6. A method as claimed in any previous claim wherein the number of optical reflective elements is two.

7. An extended grating structure when formed in accordance with the method of any one of claims 1 to 6. - 14 -

8. A method of forming a grating structure in a photosensitive waveguide comprising: projecting a single beam of light onto a phase mask so as to create two coherent beams of light; projecting said two coherent beams around an optical circuit so that they interfere at a first location; translating said phase mask substantially perpendicular to said single beam of light; and simultaneously translating said photosensitive waveguide substantially perpendicular to said single beam so as to substantially minimize the variation in position of the intensity maxima of each fringe of said interference pattern in the reference frame of the waveguide thereby simultaneously writing a grating along the portion of said grating within said interference pattern.

9. A method as claimed in claim 8 wherein said phase mask is mounted on a first translatable stage and said photosensitive waveguide is mounted on a second translatable stage and one of said translatable stages (a top translatable stage) is mounted on the other one of said translatable stages (a bottom translatable stage) .

10. A method as claimed in claim 8 wherein said phase mask is mounted on a first translatable stage and said photosensitive waveguide is mounted on a second translatable stage able to be independently moved relative to said first translatable stage.

11. A method as claimed in any previous claim 8 to claim 10 wherein said optical circuit comprises a Sagnac loop.

12. A method as claimed in any previous claim 8 to claim 11 wherein said optical circuit comprises two reflective mirrors.

13. A method as claimed in any previous claim 8 to claim 11 wherein said photosensitive waveguide is translated at a speed of movement having a ratio to the speed of movement of the phase mask equal to a ratio of the -15- period of the grating being written to the frequency of the phase mask.

14. A method as claimed in claim 9 wherein said top translatable stage includes an input signal for driving a transducer element for positioning said top translatable stage and said input signal is driven in a sawtooth manner when writing said grating.

15. A method as claimed in any previous claim 8 to claim 14 wherein said translation of said photosensitve waveguide is varied so as to write a chirped grating in said photosensitve waveguide.

16. a grating when produced in accordance with the steps of any of claims 8 to claim 15.

17. A method of forming an extended grating structure in a photosensitive waveguide comprising the steps of: projecting a single beam of light onto a phase mask so as to create two coherent beams of light; projecting said two coherent beams around a series of optical reflection elements so that they interfere at a first location; translating at least one of said optical reflection elements in a first direction substantially perpendicular to the path of said single beam so as to cause a consequential translation of said interference pattern along said grating so as to cause said extended grating structure to be written in said photosensitive waveguide; subsequently, translating said phase mask substantially perpendicular to said single beam of light; and simultaneously translating said photosensitive waveguide substantially perpendicular to said single beam so as to substantially minimize the variation in position of the intensity maxima of each fringe of said interference pattern in the reference frame of the waveguide thereby simultaneously writing a grating along the portion of said grating within said interference pattern.