Embedded Micromachined Fibre Optic Fabry-Perot Pressure Sensors in Aerodynamics Applications

Small size, high bandwidth pressure sensors are required for instrumentation of probes and test models in aerodynamic studies of complex unsteady flows. Optical fibre pressure sensors promise potential advantages of small size and low cost in comparison with their electrical counterparts. We describe miniature Fabry-Perot cavity pressure sensors constructed by micromachining techniques in a turbine test application. The sensor bodies are 500 µm square, 300 µm deep -with a ~2 µm thick copper diaphragm electroplated on one face. The sensor cavity is formed between the diaphragm and the cleaved end of a singlemode fibre sealed to the sensor by epoxy. Each sensor is addressed interferometrically in reflection by 3 wavelengths simultaneously, giving an unambiguous phase determination; a pressure sensitivity of ~1.8 rad bar^-1 was measured, with a typical range of vacuum to 600 kPa. Five sensors were embedded in the trailing edge of a nozzle guide vane installed upstream of a rotor in a full-scale turbine stage transient test facility. Pressure signals in the trailing edge flow show marked structure at the 10 kHz blade passing frequency. To our knowledge, this is the first report of sensors located at the trailing edge of a normal-sized turbine blade.