An Integrated Optical Pressure Sensor in the GaAs/AlGaAs Ternary System

[摘要] The principal concerns of this thesis are the design, fabrication and demonstration of an optical pressure sensor in the aluminium gallium arsenide (AlGaAs) ternary epitaxial layer system. The device is based on a ridge optical waveguide traversing a section of GaAs wafer which has been wet-chemically etched, in a well-defined area, from the wafer base, through the substrate (approximately 400um thick) to within approximately 10um of the top surface of the wafer. A deflection of the membrane from equilibrium by a pressure differential across its thin dimension results in a stress-induced change in the refractive index of the membrane material. A guided wave on the top surface of the membrane experiences a change of phase as it passes through the stress field produced by the membrane deformation. A mathematical theory of photoelasticity is developed to model the deflection and stress-induced refractive index changes of a membrane of material in the zinc-blende cubic crystal class (43m), of which GaAs is a member. The theoretical model was firstly tested by measuring the deflection of membranes under pressure by observing them in a microscope equipped with a Tolansky interferometer. Using the experimental apparatus described in this thesis, membrane deflection was achieved by evacuating one side of the membranes. This resulted in the pressure being limited to one atmosphere across a membrane (or limited by the evacuating capability of the vacuum pump). Two types of device are investigated -the first uses only straight waveguides and is here referred to as a birefringent pressure sensor while the second incorporates a Mach-Zehnder waveguide structure. (1) The birefringent pressure sensing device was fabricated and tested. It consisted of a number of straight waveguides crossing a membrane and was placed in an external Mach-Zehnder interferometer in order to measure the phase-shift of light in the waveguide when a uniform pressure was applied to one surface of the membrane. The theoretical model indicates that the change in phase for TE and TM polarisations is different i. e there is stress-induced birefringence. Thus linearly polarised light launched into a straight waveguide (equally exciting the TE and TM polarisations) experiences a change of polarisation with pressure. The polarisation of the light output by the device could be monitored without placing the device in an external interferometer -hence the name 'birefringent pressure sensor'. This sensor was studied using both the techniques described above. (2) The design considerations for a waveguide Mach-Zehnder interferometric pressure sensor are based on both the photoelasticity model developed in this thesis and the results obtained from the birefringent pressure sensor. The device parameters were chosen to allow at least 2pi phase-shift (i. e one cycle) within the one atmosphere pressure limit in this thesis. The arm separation of the Mach-Zehnder structure is 300um and the total device length is about 17mm. The Y-junctions are symmetric and are formed by two intersecting S-bends of radius of curvature 40mm. The arm separation and S-bend radii are limited by the losses of the device. Theoretical calculations indicate that considerably smaller radii for the S-bends would be acceptable with only a small reduction in optical transmission. In order to estimate the losses, the transmission of the Mach-Zehnder waveguides was compared to those for straight waveguides of the same overall length. The Y-junction structures forming the Mach-Zehnder waveguide configurations were also fabricated individually in order to assess the losses relative to straight guides. The results have been compared to theoretical models of the losses in S-bends and Y-junctions. The fabrication methods for waveguides and membranes are described. Straight waveguides, Y-junction and Mach-Zehnder waveguide structures were fabricated by both Reactive Ion Etching and wet-chemical etching and a comparison of the propagation characteristics is given. Dry etching was not used for membrane fabrication mainly due to the extensive etching depths required to form membrane structures (several hundred microns). In addition wet-chemical etching was more readily available and better developed for selective etching which allowed greater control over the thickness of the membranes.

[发布机构] University:University of Glasgow

[关键词] Materials science, Electrical engineering