We have been developing a silicon-based integrated optic pressure sensor using an intermodal interference between the fundamental TM-like and TE-like modes. The sensor has a micromachined diaphragm with a sensing waveguide as a pressure-sensitive structure. The sensor is theoretically known to have a strong dependence of sensitivity on the sensing waveguide on the diaphragm. According to the theoretical prediction for the sensor based on the elasto-optic effect, the waveguide should be placed along the diaphragm edge to maximize sensitivity. To date, such dependence has not been experimentally examined in detail. In this study, two sensors with 20 or more waveguides placed at 0.1mm intervals on the diaphragm were fabricated to determine the relationship between sensitivity and waveguide position. The diaphragm dimensions were (1) 2.0mmX10mmX35micrometers and (2) 3.0mmX15mmX64micrometers . The ratio between width and length of each diaphragm was 1:5. The maximum sensitivity of 100 mrad/kPa was obtained for the waveguide nearest to the diaphragm edge with a wavelength of 633nm. In addition, the measured sensitivities were very similar for the corresponding waveguide positions in the two sensors since a scaling factor, which is defined as the cube of the either side length divided by the square of the thickness, was set as a constant.
A silicon-based integrated optical pressure sensor using an intermodal interference between the fundamental TM-like and TE-like modes is described. The sensor consists of a micromachiend rectangular diaphragm and a straight polystyrene optical waveguide passing across the diaphragm. The use of the intermodal interference has the potential advantage of shortening the sensor length since it needs neither a Y-branch nor a 3-dB coupler. In the presence of the pressure difference on the diaphragm, the phase difference is yielded between the two guided modes by an anisotropic index change induced by the elasto-optic effect. The phase difference is finally transformed into a change in light intensity by an analyzing polarizer. We fabricated a sensor with a 1.2 mm by 10 mm by 20 micrometers diaphragm over which the waveguides were formed at 50 micrometers intervals. In the experiment, a linearly-polarized He-Ne laser beam at 633 nm was coupled to the two guided modes at equal intensity. The output intensity through a polarizer was sinusoidally changed at a period of 64 kPa, corresponding to a phase sensitivity of 98 mrad/lPa, for the waveguide nearest to the diaphragm edge. The measurement was carried out for other waveguide positions. The largest sensitivity was obtained for the waveguide nearest to the diaphragm edge as theoretically expected.
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