The characterization of high-frequency ultrasonic fields using a polarimetric optical fiber sensor

A polarimetric ultrasonic sensor consisting of a polarization-maintaining fiber carrying light from a laser is described, and its application to the characterization of high-frequency (1-10 MHz) ultrasonic fields is demonstrated. The principle of the sensor is based on the detection of ultrasonically induced birefringence in the fiber. The advantage of using a polarization-maintaining fiber instead of a single-mode fiber is discussed. The response of the sensor to both continuous and pulsed ultrasound at frequencies ranging from 1 to 10 MHz and at different power levels has been investigated, and a tomographic technique was used to compute transducer beam profiles with high spatial resolution (∼0.1 mm). Profiles measured in this way are compared with those measured using a polyvinylidene fluoride (PVDF) hydrophone, and are shown to have superior spatial resolution. The sensor output is observed to be a linear function of the square root of the total acoustic power, which is measured with a radiation force balance. The advantages of this sensor over available PVDF hydrophones are its ability to withstand very high ultrasonic power, and its excellent spatial resolution for beam profile measurements. It is most accurately employed in situations where the acoustic beam is both narrow and plane wave like as is the case in transducer focal regions. This is just the situation in which conventional "point" hydrophones, with their limited spatial resolution, are least accurate. This sensor should therefore find applications in the characterization of medical ultrasonic transducers.