AU2819599A - Grating writing method and apparatus - Google Patents

Description

WO99/45417 PCT/AU99/00123 Grating Writing Method and Apparatus The present invention is directed to the creation of grating and/or other structures in photosensitive optical waveguides such as optical fibres or the like. 5 Background of the Invention Optical fibre communication has seen a tremendous growth in the past several years due to increased traffic flow. Wavelength Division Multiplexing (WDM) is one key enabling technology for service providers to improve and 10 increase information flow. WDM has spurred the development of many new optical components. One particularly important device is the fibre Bragg grating which normally takes the form of a periodic modulation in the refractive index along the length of a segment of optical fibre. The periodic 15 modulation is normally created through the utilisation of a photosensitive waveguide. By exposing segments of the fibre utilising a UV beam or the like, it is possible to change its refractive index in an ordered and well known manner. In this way, gratings can be created. One form of 20 popular grating creation method is to utilise the interference pattern from two interfering beams of light so as to crate the grating. With increasing performance demands comes an increasing demand for highly accurate and more complex 25 gratings which require minimising any noise associated with the grating period and strength and the ability to control the grating parameters in a highly precise manner. To realise this, the utilisation of new and improved methods for writing gratings has become increasingly important. 30 For example, in PCT Application No. PCT/AU96/00782 entitled "Ring Interferometer Configuration for Writing Gratings" there is disclosed one form of vibration insensitive grating writing system which utilises a Sagnac type loop to maintain vibrational stability. 35 Although the implementation of such system has been found to give a relatively high grating performance, it is always desirable to provide for improved grating systems that WO99/45417 PCT/AU99/00123 -2 allow more complex and accurate grating designs to be written and so allow for improved grating performance. The improved grating performance being highly prized amongst manufacturers who utilize such devices in WDM transmission 5 systems. Summary of the Invention It is an object of the present invention to provide for systems having improved grating performance. In accordance with a first aspect of the present 10 invention, there is provided a method of forming a grating structure on a photosensitive waveguide, the method including the steps of: moving the photosensitive waveguide at a first substantially constant relative velocity perpendicular to a shuttered beam of light in the 15 photosensitive range and emitting a shuttered beam at the photosensitive waveguide at predetermined times so as to form the grating structure within the waveguide. The shuttered beam of light can form an interference pattern on the photosensitive waveguide and 20 the shuttered beam can be operated so as to strobe the interference pattern such that subsequent strobes are substantially aligned on the waveguide so as to form a grating thereon. The interference pattern can be formed from two 25 coherent beams projected around a Sagnac loop. Alternatively, the shuttered beam can be focussed on the photosensitive waveguide to a point substantially less than the grating period (substantially 500 nanometres). 30 In accordance with a second aspect of the present invention, ther is provided a method of forming a grating structure in a photosensitive waveguide, the method including the steps of: forming an interference pattern from a shuttered light source; moving the grating relative 35 to the interference pattern in a predetermined manner; and controlling the duty cycle or frequency of opening of the shutter so as to form an apodised grating structure. If WO99/45417 PCT/AU99/00123 -3 necessary, the period can be further controlled so as to produce a predetermined chirping in the grating structure and the method can also include modulating the power of the light source so as to maintain substantially constant 5 overall average power. In an alternative arrangement, direct writing can be utilised in conjunction with the shuttered system. Brief Description of the Drawings Notwithstanding any other forms which may fall 10 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: Fig. 1 illustrates a first embodiment of the present invention; 15 Fig. 2 illustrates the formation of a grating in accordance with the first embodiment; Fig. 3 illustrates the fringe forming and smearing as provided in the first embodiment; Fig. 4 illustrates a second embodiment of the 20 present invention; Fig. 5 illustrates a third embodiment of the present invention and; Fig. 67 shows the growth dynamics of the two parameters required to be controlled in an apodised grating 25 including An and average (n). Description of Preferred and Other Embodiments In a first embodiment, the grating writing techniques previously utilised are further developed for the writing of grating structures on photosensitive 30 waveguides or the like which are moving at a relative velocity to the imaging system which is imaging a grating or other device onto the moving photosensitive waveguide. Modern high precision positioning devices are unable to consistently provide the high degrees of accuracy 35 required in writing minute complex grating structures for operation in the optical range, especially around 1500nm. Being unable to accurately control the position of a device WO99/45417 PCT/AU99/00123 - 4 has limited the results produced to date. However, whilst it may be difficult to accurate position in a device, interferometric positioning techniques allow for a somewhat higher accuracy with which a current location of device 5 can be determined. Further, modern positioning devices are somewhat more accurately able to maintain a constant velocity of movement then obtain an accurate absolute stationary position. Therefore, the preferred embodiments of the present utilised these aspects of modern positioning 10 devices to produce gratings on moving fibres which can meet or exceed the gratings produced by other known methods. Turning initially to Fig. 1, there is illustrated a first embodiment which utilises this principle. A fibre 2 upon which a grating needs to be written is mounted on an 15 accurate positioning stage 3 which moves at a velocity V past an imaging area produced by phase mask 4 and mirrors 5 and 6. The fibre 2 being UV sensitive. The arrangement of Fig. 1 uses the low noise Sagnac loop configuration taught by PCT Application No. PCT/AU96/00782 for writing the 20 grating. However it will be readily evident to the person skilled in the art that the present invention is readily extendible to other forms of interferometric grating writing including that disclosed in US Patent 4,725,110, US Patent 4,807,950, and US Patent 5,377,288. 25 Light from a UV laser 8 is reflected by a mirror 9 and passes through a controlled power modulator 10 and shutter 11. The modulator 10 and shutter 11 can be combined into a single unit or crystal modulator such as the N35110-2-UV acousto-optic modulator available from 30 Australasia Acousto-Optic Systems. Otherwise, other suitable forms of shutters can be utilised having a high frequency response function. The light 12 emitted from the shutter 11 is imaged onto the phase plate 4 where it is diffracted so as to produce two first order beams eg. 13, 35 14 which are reflected around a Sagnac loop by mirrors 5, 6 so as to image an interference pattern at point 16. The shutter 11 is operated at periodic intervals determined by WO99/45417 PCT/AU99/00123 -5 the velocity V of fibre so as to imprint a constant interference patter at regular intervals on photosensitive fibre 2. The shutter is strobed in a timed relationship with the velocity such that it is in positional phase with 5 each strobe. The operation of the shutter will now be illustrated schematically with reference to Fig. 2a to Fig. 2d which illustrate the core of the fibre 2 and the corresponding interference pattern 16. In Fig. 2a, the fibre 2 is moving with a velocity V and has previously 10 constructed fringes 18. The shutter is strobed such that the interference pattern 16 is properly aligned with the fringes 18 so as to slowly build up an index modulation eg. 19 with each shuttering of the shutter. Hence, a short time later as shown in Fig. 20, the shutter is again 15 operated with the fibre having moved a distance corresponding to one period in the shutter interval. The process is continued Fig. 2c and Fig. 2d for a subsequent period so as to build up a fibre structure. By accurately timing the shuttering process, a large scale grating 20 structure is built up. Of course, the process will result in a certain degree of "smear" due to the shutter being open for a finite period of time while the grating is moving past. The smear will result, as illustrated in Fig. 3, in the 25 core fibre having a periodic modulation with a central maximum eg. 20, 21 with smearing eg. 23, 24 at each side. The degree of smearing being dependant upon the shutter open period and the velocity V with which the fibre is moving. 30 The location of the central maximum determines the period of an individual fringe and can be controlled by the timing of the shutter. Proper adjustment of the timing prevents unwanted chirp to be induced in an apodised grating structure. Alternatively, an adjustment of the 35 shutter opening time can be used to write a predetermined arbitrarily chirped grating. Fig. 6 shows the growth dynamics of the two WO99/45417 PCT/AU99/00123 -6 parameters required to be controlled in an apodised grating including the An and average n (n). An is determined by he required grating design, and adjustment of the exposure is used to keep the (n) constant. The average (n) 51 will 5 increase with an increased duty cycle, whereas the An will increase to a maximum at point 52 which corresponds to 50 % duty cycle. Apodisation can be achieved by altering the duty cycle so as to change An. Average (n) is kept constant by keeping the area 10 of the curve on a laser power vs. Duty cycle graph constant. Therefore, the curve 53 is utilised and the laser power is halved when going from maximum fringe contrast, 52, to minimum fringe contrast, 54. A number of refinements to the arrangement of 15 Fig. 1 are possible. For example, the current position of the constant velocity moving stage 3 can be interferometrically determined and referred to a computer system for determining the appropriate high frequency control of the shutter 11. Further, when it is desired to 20 write only one frequency, being the frequency of the phase mask, the Sagnac type loop can be dispensed with. For example, such an arrangement is illustrated in Fig. 4 wherein the light from a laser system 25 has its power modulated 26 in accordance with requirements for being past 25 through a shutter 27 with again the shutter 27 and power modulator 26 being optionally combined as aforementioned. The output from the shutter 27 strikes a phase mask 28 wherein it is diffracted forming an interference pattern directly at the back of the phase mask 28. A fibre 29 is 30 positioned close to the phase mask 28 in the interference pattern formed at the back of the phase mask 28 on a stage 30 which is driven at a constant velocity V. The shutter 27 is again operated in a strobed manner such that output occurs whilst overlapping fringes are present. 35 The principle of these embodiments can be further extended to direct writing systems. Such a system could be as illustrated in Fig. 5. Here a light source 40 is WO99/45417 PCT/AU99/00123 -7 provided which again can comprise a UV laser source, however, it may, in certain circumstances, comprise a normal light source. The light passes through a power modulator 41 and shutter 42 which again can be combined 5 before being focussed by focussing optics 43 down to a point. The focussing optics 43 are preferably of a high quality necessary to focus the beam to a very fine point on a fibre 45 which is mounted on stage 46 which undergoes a velocity translation at velocity V. 10 For example, the focussing system can comprise a microscope objective followed by a spatial filter (pinhole) followed by a second microscope objective able to focus the spatial filter output to a small point on the fibre. The stage 46 can again be interferometrically 15 monitored 47 for a current position which is forwarded to a timing unit 48 which in turn controls the shutter 42. Again, by the use of a high speed shutter system 42 it is possible to accurately adjust the emitted light so that accurate fringe patterns are drawn of fibre 45 whilst stage 20 46 undergoes a substantially constant velocity V. It would 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 25 the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

Claims (11)

1. A method of forming a grating structure on a photosensitive waveguide, said method including the steps of: 5 moving said photosensitive waveguide at a first substantially constant relative velocity perpendicular to a shuttered beam of light in said photosensitive range; emitting a shuttered beam at said 10 photosensitive waveguide at predetermined times so as to form said grating structure within said waveguide.

2. A method as claimed in claim 1 wherein said shuttered beam of light forms an interference pattern on said photosensitive waveguide and said shuttered beam is 15 operated so as to strobe said interference pattern such that subsequent strobes are substantially aligned on said waveguide so as to form a grating thereon.

3. A method as claimed in claim 1 wherein said interference pattern is formed from two beams. 20

4. A method as claimed in any previous claim wherein said photosensitive range is in the UV range.

5. A method as claimed in claim 1 wherein said shuttered beam is focussed on said photosensitive waveguide to a point less than substantially 500 nanometers. 25

6. A method as claimed in any previous claim wherein said relative velocity is slowly varied so as to produce a chirp in said grating.

7. A method of forming a grating structure in a photosensitive waveguide, said method including the steps 30 of: forming an interference pattern from a shuttered light source; moving said grating relative to said interference pattern in a predetermined manner; 35 controlling the duty cycle or frequency of opening of said shutter so as to form an apodised grating structure. WO99/45417 PCT/AU99/00123 -9

8. A method as claimed in claim 6 or 7 further comprising modulating the power of said light source so as to maintain substantially constant overall average power.

9. A method as claimed in claim 3 wherein said 5 two beams comprise two coherent beams projected around a Sagnac loop.

10. An apparatus when implementing the method of any of claims 1 to 9.

11. A grating when formed utilizing the method 10 for any of claims 1 to 9.