LASER ABLATION OF WAVEGUIDE STRUCTURES

Abstract: A method of processing an optical device incorporating a waveguide, the method comprising the step of utilizing a laser to heat a surface of the device to alter an optical characteristic of the waveguide, wherein the power density of the laser is selected to effect ablation.

Description: LASER ABLATION OF WAVEGUIDE STRUCTURES Field of the Invention The present invention relates broadly to laser ablation (or alternatively sometimes referred to as laser etching, hereinafter referred to as ablation) processing of waveguide structures . Background of the Invention During the construction of optical waveguide devices, it is common that changes associated with a particular construction step occur which may affect the operational characteristics of the devices. For example, optical devices are often constructed utilizing an adaptation of semiconductor fabrication techniques and can commonly include a number of layers constructed on a silicon substrate. As a result of differential thermal expansion coefficients of the materials, various compressive stresses are induced during normal operating conditions. These stresses can have the effect of changing the operational characteristics of any device formed on the substrate. In particular, the compressive stresses can often give rise to anisotropic birefringent properties in optical waveguides which can substantially effect their operation. Interim solutions suggested have included techniques such as employing complex hybrid technologies where bulk polarizing elements are slotted into an optical chip and set to compensate for birefringence, or providing time consuming chemical etching steps for etching strain relieving grooves on either side of a waveguide. Treatment via a UV laser to create damage at the substrate which leads to compensating stresses has also been suggested, however, the utilization of UV laser treatment has significant problems in that the lasers are expensive and can be unreliable in a manufacturing environment. They typically require constant skilled maintenance. Summary of the Invention The present invention provides a method of processing an optical device incorporating a waveguide, the method comprising the step of utilizing a laser to heat a surface of the device, to alter an optical characteristic of the waveguide, and wherein the power density of the laser is selected to effect surface ablation. The laser may comprise a carbon dioxide laser source . The method may be utilized to alter the birefringent properties of the waveguide, e.g. the TM and TE birefringent modes may be substantially aligned. The step method may further comprise the step of may further comprise masking the surface with a thermally conductive material having an aperture defined therein to limit exposure of the device to the laser. The device may comprise a sensor. The method may further comprise the step of depositing a material layer on the surface. Accordingly, the method itself may be utilised to form the device. The device may eg. comprise a semiconductor device or a Si0 2 /Si planar waveguide device . Step of depositing the material layer may comprise depositing the material layer on portions of the surface affected by the ablation. The material layer may be provided as an electrode for electrically contacting the device. The method may further comprise the step of mounting a further component in a groove formed in the surface as a result of the ablation. The further component may comprise a modulator for modulating a characteristic of the device. The method may be conducted at different locations of the device so as to form an optical structure. The optical structure may comprise a grating structure. The optical structure may comprise a polarisation filter. The method may be used to diminish UV induced changes present in the waveguide. The device may comprise an optical fibre. The method may be utilised to mark the device by way of the ablation. The laser may comprise a semiconductor laser operating at a wavelength of more than about 1.8 micro metre. This wavelength range may be preferable where the surface of the device comprises Si0 2 . The method may further comprise the step of providing an absorber material to facilitate the heating of the surface of the device. The invention may alternatively be defined as providing an device incorporating a waveguide, wherein the device has been processed utilising a laser to heat a surface of the device, to alter an optical characteristic of the waveguide, and wherein the power density of the laser is selected to effect surface ablation. 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: Fig. 1 illustrates schematically the operation of the method of the preferred embodiment; Fig. 2 illustrates the ablation of a wafer surface; Fig. 3 illustrates the change and effective index in an experiment utilizing the preferred embodiment; Fig. 4 illustrates a further change in the effective index of an experiment utilizing the preferred embodiment ; Fig. 5 illustrates the initial profile of a Mach- Zehnder (MZ) interferometer prior to application of preferred embodiment showing both the TM and TE modes; Fig. 6 illustrates the spectral response for TE and TM modes of a MZ interferometer after application of the preferred embodiment for the device of Fig. 6; Fig. 7 illustrates an alternative form of processing a wafer; Fig. 8 illustrates the process of ablation around a core of a waveguide; Fig. 9 illustrates the utilization of ablation in changing the device characteristics of a waveguide; Fig. 10 illustrates the utilization of ablation in conjunction with deposition of further layers on a wafer; and Fig. 11 illustrates the construction of a long period lossy grating. Description of Preferred and Other Embodiments In the preferred embodiment, an inexpensive and relatively compact C0 2 laser device is utilized to provide mid infrared laser processing of a waveguide structure so as to obtain both birefringence compensation and tuning of optical components. The processing set up is illustrated schematically in Fig. 1 wherein a waveguide 3 is subjected to ablation utilizing a C0 2 laser 5. An example of the ablation processing is illustrated in Fig. 2 wherein a waveguide 6 having a internal core 7 is processed so as to include ablation channels 8, 9. In a first example, the C0 2 laser was used to enhance the device characteristics of an asymmetric Mach-Zehnder (MZ) interferometer fabricated utilizing hollow-cathode PECVD techniques . The mid-IR radiation was used to thermally relax stresses at the core and substrate as well as affect a change in the refractive index. At sufficiently high temperatures, the core and cladding waveguide glasses can be softened, melted and vaporised. These ablation processes themselves can be used to generate faster relaxation than would otherwise be possible as well as provide a source of polarisation dependent loss for energy stripping within waveguides for functions such as optical attenuators, and for other more classical applications, including the laser ablation of stress-relieving grooves. Most heating was found to occur through non-radiative transfer of absorbed light into vibrational modes of the silica molecule. For given exposures utilized in experiments, the substrate temperature was thought to be approximately the same as that induced at the surface. The laser was operated initially with unfocussed 10W of C power ( - 140W/cm 2 ) . When the laser was later focused, temperatures readily exceeding the melting point of silica were achieved resulting in laser vaporisation and ablation. Since the asymmetric MZ spectral response is characteristic of the interferometer established between the input and output couplers, birefringence compensation, as measured by matched TE and TM spectra (the TM identified to have a higher effective index by writing a weak Bragg grating in a straight waveguide manufactured on the same wafer), can be achieved between the two couplers. From an experimental point of view, if true birefringence reduction in this region is demonstrated, the change in TE and TM notch wavelength due to an increase in effective index must be such that they converge when processing the longer arm, and diverge when processing the shorter arm. The reverse is the case for a decrease in refractive index. Otherwise, spectral compensation of polarisation is possible within a MZ whilst actually worsening the intrinsic polarisation dispersion. When convergence is achieved, then the feasibility of polarisation compensation can be established which is generally applicable to other optical components and straight waveguides as well as the asymmetric MZ device chosen here. In the experiments to determine the parameters of operation, optical spectra were taken, of the MZ device using a broadband erbium-doped fibre amplifier (EDFA) and a spectrum analyser with a resolution of 0.05nm, limiting the birefringence splitting which can be measured to -lxlO "5 . In initial experiments, the longer arm of a MZ device (12um Si0 2 cladding and buffer layers, 4x5 μm Ge0 2 - doped core, . Δn - 0.01) was processed for testing and confirmation of the concept. Measurements were taken at intervals after briefly halting the exposure at fixed times since the fibre coupling was increasingly affected by longer exposures. It was noted that both TE and TM shifted to longer wavelengths indicating an increase in refractive index. The TE effective index eventually increased more rapidly such that the splitting was reduced as shown in Fig. 4 which shows the change in wavelength splitting between TE and TM eigenstate with exposure to unfocussed light. Initially, however, as shown in Fig. 3, an increase in the splitting observed. We believe is related to an initial increase in compressive stress and subsequent compaction of the core glass. The magnitude of reduction is sufficient to allow compensation of birefringence in most planar silica-on-silicon devices where the splitting is much lower than the device chosen here. Further, this value is unsaturated. The power density of the C0 2 laser was then increased to -1.3kW/cm 2 by focussing to a lOOμm spot size such that we exceed the threshold necessary for vaporisation for an exposure of less then 0.2s. Ablation was confirmed under an optical microscope after gently cleaving through one damage region of the surface. By controlling the duration of the exposure it was possible to control the depth to which material is removed. The spectral responses when exposing the longer arm were found to shift to shorter wavelengths indicating a decrease in refractive index. However, the TM state was found to decrease more rapidly resulting in a large drop in the birefringence splitting. The decrease in refractive index and the localised ablation indicates that in this case dilation and stress compensation or relaxation at the core are the main factors responsible for the reduction in birefringence. Fibre coupling is significantly more stable (an important advantage for in-si tu monitoring) and the process is clearly more efficient than thermal annealing of the material. This will affect the effective propagation constants for each polarisation state as well as introduce some polarisation dependent loss. The result is that this can be used to achieve balancing of the optical energy for each eigenstate and hence result in a matched and improved spectral response of the device. Fig. 4 shows the change in effective index as a function of shots fired in the same region and shows the convergence of birefringence when exposing the longer arm. It was noted that subsequent successive shots on the same region did not contribute any further. Indeed a small reversal was observed. Exposing the shorter arm showed spectral divergence, as expected if polarisation compensation has been achieved. The spectral response for a second device utilized in experiments (essentially a polarisation splitter) prior to irradiation is shown in Fig. 5. The poor fringe contrast and the difference between TE and TM responses indicates that the input coupler is polarisation sensitive and that different amounts of light are split for each eigenstate. As a result, the intensity of light in each arm is not equal leading to poor fringe contrast upon recombination at the output coupler, particularly in this case of the TE state. The polarisation sensitivity between couplers is very difficult to eliminate completely in silica-on-silicon systems where strain is not readily removed. Fig. 6 illustrates the end results on the device once the process of irradiation ablation was optimised. An improvement is fringe contrast to 20dB for both TE and TM states was achieved after five shots along the longer arm (power density -lOkW/cm 2 ) . The total increase in loss necessary to balance the polarisation states in this particular device was -0.12 dB for TM and -1.2dB for TE . A number of further modifications and applications of the aforementioned technique are possible. Firstly, the utilisation of the C0 2 laser can be refined as illustrated in Fig. 7 by utilizing a metal plate 20 containing a slot 21 with the metal plate acting as a heat sink so as to extract heat from the laser beam 22 outside specific locations. In this manner, more refined processing 23 of the waveguide can be achieved. Secondly, the ablation of the waveguide can also be extended, as shown in Fig. 8, to the area surrounding the core 25. This can be utilized to effect the operation of the core and the overall device. For example, in Fig. 9, there is illustrated the utilization of ablation to form a refined surface 30 which can be utilized to provide for more accurate sensing by the core 31. Further, the ablation of the surface can be utilized in the construction of complex semi-conductor devices having predetermined operational characteristics. For example, in Fig. 10, there is illustrated the example of deposition of a subsequent layer 33 which can comprise zinc oxide, BiT0 3 or the like so as to provide for a functional semiconductor device . A further example refinement is illustrated in Fig. 11 where a series of ablations 40-42 are written at regular intervals along a core 43 so as to provide for a long period "loss" grating structure. Other applications can include modification of polarization controllers and attenuators etc. The aforementioned laser process has a number of other uses. In particular, it can be used to provide for accelerated aging of components by means of C0 2 thermal heating of optical devices such as UV processed gratings formed on a planar waveguide. The accelerated aging can provide for improved operational characteristics and can include precise localised aging of components. Further, the thermal annealing can be utilized to anneal out the UV processing of portions of a previously processed waveguide. This can be taken to the extent of almost totally annealing out the UV processing effect. Further, the above processing steps are also directly applicable to fibre devices. 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 the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

Claims: We Claim: 1. A method of processing an optical device incorporating a waveguide, the method comprising the step of: - utilizing a laser to heat a surface of the device, to alter an optical characteristic of the waveguide, and wherein the power density of the laser is selected to effect surface ablation. 2. A method as claimed in claim 1 wherein the laser comprises a carbon dioxide laser source. 3. A method as claimed in any one of the preceding claims wherein the method is utilized to alter the birefringent properties of the waveguide. 4. A method as claimed in claim 3 wherein the TM and TE birefringent modes are substantially aligned by the method. 5. A method as claimed in any one of the preceding claims further comprising the step of masking the surface with a thermally conductive material having an aperture defined to minimise exposure of the device to the laser. 6. A method as claimed in any one of the preceding claims wherein the device comprises a sensor. 7. A method as claimed in any one of the preceding claims further comprising the step of: - depositing a material layer on the surface . 8. A method as claimed in claim 7, wherein the step of depositing the material layer comprises depositing the material layer on portions of the surface affected by the ablation. 9. A method as claimed in any one of the preceding claims further comprising the step of: - mounting a further component in a groove formed in the surface as a result of the ablation. 10. A method as claimed in claim 7 or 8 , wherein the material layer is provided as an electrode for electrically contacting the device. 11. A method as claimed in claim 9, wherein the further component comprises a modulator for modulating a characteristic of the device. 12. A method as claimed in any one of the preceding claims wherein the step of utilising the laser to heat the surface is conducted at different locations of the device so as to form an optical structure. 13. A method as claimed in claim 12, wherein the optical structure comprises a grating structure. 14. A method as claimed in claims 12 or 13 , wherein the optical structure comprises a polarisation filter. 15. A method as claimed in any one of the preceding claims wherein the method is used to diminish UV induced changes present in the waveguide. 16. A method as claimed in any one of the preceding claims wherein the device comprises an optical fibre. 17. A method as claimed in any one of the preceding claims wherein the method is utilised to mark the device by way of the ablation. 18. A method as claimed in any one of the preceding claims, wherein the laser comprises a semiconductor laser operating at a wavelength of more than about 1.8 micro metre. 19. A method as claimed in claim 18, wherein the surface of the device comprises Si0 2 . 20. A method as claimed in nay one of the preceding claims, wherein the method further comprises the step of providing an absorber material to facilitate the heating of the surface of the device. 21. A device incorporating a waveguide, wherein the waveguide has been processed utilising a laser to heat a surface of, the device to alter an optical characteristic of the waveguide, wherein the power density of the laser is selected to effect ablation.

Inventors: ASLUND MATTIAS, CANNING JOHN, ASLUND, MATTIAS, CANNING, JOHN
Applicants: ASLUND MATTIAS, CANNING JOHN, UNIV SYDNEY, ASLUND, MATTIAS, CANNING, JOHN, THE UNIVERSITY OF SYDNEY
Application year: 2000
Main classification: G02B6
IPC codes: G02B5, G02B6, H01S3