Use of a fibre-optic hydrophone in measuring acoustic parameters of high power hyperthermia transducers

Abstract

A fibre-optic hydrophone consisting of a polarisation-maintaining fibre carrying light from a laser source has been used to measure the acoustic output parameters of a single focused hyperthermia transducer and a six-transducer assembly. Beam profiles of the transducers were measured using the fibre-optic hydrophone and the results compared with those obtained using a PVDF hydrophone. The acoustic power output from the hyperthermia transducer was measured using a radiation force balance. It was observed that the root mean square voltage of the fibre-optic hydrophone output is proportional to the square root of the acoustic power up to more than 80 W. It was also observed that, under continuous-wave operation, the fibre optic hydrophone can stand up to very high power (more than 200 W) without being damaged. As its sensing element is the fibre itself, whose diameter is considerably narrower than the width of the ultrasonic beam, it can provide resolution of about 80 microm in beam profile measurement. The fibre is a line sensor and a computer tomographic technique is used to recover the pressure profile from the hydrophone output voltage. In typical clinical operations, the six-transducer assembly is driven with less than 150 W of electrical power input. In such cases, each individual transducer receives less than 25 W of input power and non-linearity and generation of high frequency harmonics at the focus is not a significant problem.

Detection of high-frequency ultrasound with a polarization-maintaining fiber

“…In comparison with a single-mode fiber, a polarization-maintaining fiber can respond to lower frequencies with better sensitivity and requires no real-time control of the polarization state of the input light. This sensor has already been applied to the characterization of some low-power standard transducers [4] and high-power medical transducers [ 5 ] . The results of the present study can also be applied to the identification of the locations of the local principal axes of a polarization-maintaining fiber.…”

“…where A pa is given by ( 5 ) and A pi is given by (9). As an example, with P44 = -0.072 and n = 1.46 for silica glass, we find from (5) For a fiber with a beat length of 2.4 mm, if we take ( a 2 -a l ) T = -1 x v2 -v1 = 0.02 [12], and vo = v1 = v2 = 0.2, we find from (9) 0 .…”

Piezoelectric and fibre-optic hydrophones

“…To detect the change in polarisation, the light emerging from the distal end of the fi bre is passed through some form of polarisation, sensitive component (De Paula, 1982). This approach has been used to measure the output of hyperthermia transducers (Chan et al , 1989). Another type of intrinsic sensor employs fi bre Bragg grating (FBG) technology.…”

Acousto-Ultrasonic Optical Fiber Sensors: Overview and State-of-the-Art

“…This is the first account of a polarimetric optical fiber acoustic sensor. Further work on polarimetric optical fiber sensors has been undertaken by dePaula et al [64]- [66], and also by Chan et al [67], [70] with Price [68], and Brinch [69]. The work of Chan et al [67] describes a polarimetric optical fiber sensor based on acoustically induced birefringence in a polarization maintaining fiber.…”