Optical methods of acoustic detection

This thesis details the experimental investigation of a fibre Bragg gratings (FBG) as to its suitability as a point ultrasonic sensor for medical applications, the FBG being interrogated by a low-coherence psuedo-heterodyne technique. The noise-limited pressure resolution of the FBG sensor was found to be 4.5kPa/√Hz at a frequency of 1.911 MHz. The ability of the FBG sensor to accurately determine the spatial field profile from a focussed ultrasonic transducer was also investigated and compared with results obtained from a commercially available piezoelectric hydrophone. Ultrasonic shielding materials on the bare optical fibre were also experimentally investigated in an attempt to provide a more localised grating response to the ultrasonic field. The ultrasonic response of low-finesse Fabry-Perot cavities based around 50pm thick polymer films was also investigated as a potential alternative to the use of fibre Bragg gratings, the cavity being interrogated by a low-coherence interferometric heterodyne technique. The noise-limited pressure resolution for a low-finesse Fabry-Perot cavity based on a 50pm thick polyethylene teraphthalate film was found to be 72 Pa/√Hz at an ultrasonic frequency of 1.911 MHz and 11 Pa/√Hz at 612 kHz. The ability of this cavity sensor to spatially resolve the ultrasonic field profile was also examined experimentally. Finally, the use of in-fibre Er3+ FBG based lasers as acoustic sensors in the frequency range 200Hz- 20 kHz was examined experimentally using a heterodyne interferometric interrogation method to assess the potential of these devices as highly sensitive acoustic sensors for military applications. The noise limited pressure resolution of the most sensitive fibre laser was found to be 4 x 1O\(^{-3}\) Pa/√Hz over a frequency range of 4-6 kHz. An array of 4 distributed feedback fibre lasers was constructed and two separate methods of demultiplexing the laser array were compared and contrasted. The limiting system noise sources were also measured where possible.