A Fabry-Perot fibre-optic hydrophone for the measurement of ultrasound induced temperature changes

A wideband fibre optic hydrophone system based on a polymer film Fabry-Perot interferometer has been developed for the measurement of ultrasound fields. The sensor transduction mechanism is based upon the interferometric detection of acoustically-induced changes in the optical thickness of the polymer film. The system is also sensitive to temperature change due to thermal expansion of the polymer film. This permits the sensor to be used to measure temperature changes caused by ultrasound exposure. The advantage of this concept is that it offers the prospect of providing simultaneous measurement of ultrasound fields and induced temperature changes at the same spatial location. Characterisation of the thermal performance of the sensor shows its response to be linear up to 65 degrees C and the resolution nominally 0.25 degrees C. Ultrasonically induced temperature rises of 50 degrees C above ambient were measured when insonating with a HIFU transducer. The response time of the sensor is currently limited to approximately 120 ms due to the tuning speed of the laser.