Fiber optic probe hydrophone for HIFU field measurements

Abstract

Fiber Optic Probe Hydrophone (FOPH) has been developed in this research for measurements of HIFU fields and shockwaves. The FOPH offers a unique advantage over other PVDF Hydrophones for measuring higher pressure levels and shockwaves because of its frequency independence and the fact that the sensor tip is repairable if damaged by cavitation. The FOPH also offers small detector size which eliminates the need of spatial averaging. It also provides greater immunity to electromagnetic interferences. The proposed system contains a high power pigtailed laser diode module (1.5W@850nm) connected to a 2x2 arm 3dB optical coupler via FC-FC Adaptor. The optical coupler used in the system employs a 100/140µm multimode optical fiber as the sensing element. A silicon photodetector (0.55A/W@850nm)receives the modulated light signal and converts it into voltage output proportional to the applied acoustic pressure, which is further amplified by using a 40 dB amplifier. Three transducers having resonance frequencies of 1.1 MHz, 2.2 MHz and 5.6MHz have been used for calibration of the FOPH by comparing its output with the output of a calibrated0.2 mm PVDF membrane hydrophone. The sensitivity was determined as 3.6±0.1mV/MPa. Sensitivity was found to be linear and frequency independent. Pressure waveforms measured by FOPH showed agreement with those measured by membrane hydrophone for all transducers. The FOPH system was then tested with two ballistic shockwave devices to measure the pressure of shockwaves of their transducer probes. Shockwaves generated by the ballistic sources were recorded. Positive acoustic pressures in the range of 4 MPa to 20 MPa and negative pressures in the range of -3MPa to -15 MPa were measured.