Development of a 50mhz optical fibre hydrophone for the characterisation of medical ultrasound fields

Abstract

A wideband fibre optic hydrophone system based on a polymer film Fabry-Perot sensing interferometer has been developed for the measurement of medical ultrasound fields. The sensor transduction mechanism is based upon the interfer- ometric detection of acoustically-induced changes in the optical thickness of the polymer film. The advantage of this concept is that it offers the prospect of providing sufficiently small element sizes to avoid spatial averaging at frequencies in the tens of MHz range. The sensor is interrogated using a fibre coupled tunable laser operating in the range 1520-1600nm. The acoustic performance of the sensor was characterised in terms of its sensitivity and frequency response by comparison with a calibrated PVDF membrane hydrophone. The noise-equivalent- pressure was approximately 3kPa over a 25MHz measurement bandwidth with a frequency response that extended to 50MHz. The concept lends itself to a wide range of applications, from the laboratory characterisation of medical ultrasound sources, to in- vivo measurement of therapeutic and diagnostic ultrasound. I. INTRODUCTION Modern medical ultrasound equipment increasingly uses higher frequencies to improve spatial resolution. Characteri- sation of the output of such equipment requires a hydrophone with broadband frequency response and an acoustically small element size to minimize spatial averaging. The hydrophone must also achieve these two characteristics without sacrific- ing sensitivity. The devices presently used for this type of characterisation are piezoelectric PVDF needle and membrane hydrophones. Whilst developments in fabrication techniques have allowed the production of needle hydrophones with small active areas (∼40µm), the reduction in sensitivity as active element size decreases is still a limitation with this type of sensor. Several alternatives to the needle hydrophone based on fibre-optic sensing methods have been suggested in the past. These techniques employ reflectometry (1), interferometry (2), diffractivity (3), polarimetry (4) or the inclusion of Bragg reflectors (5) within the fibres to detect the ultrasound. In this paper we present an extrinsic optical fibre sensor based on a polymer film Fabry-Perot Interferometer (FPI). This is a development of previous work in which it was shown that this type of sensor can provide a bandwidth of 20MHz and a wideband noise equivalent pressure of 10kPa (6). The concept has been advanced by making two key developments. Firstly a new range of sensors with bandwidths in excess of 50MHz has been designed. Secondly, in order to develop a relatively low cost system with the necessary robustness for practical field use, the operating wavelength region of the sensors has been shifted to the 1540-1610nm range. This enables the rapidly tuneable, stable and inexpensive fibre-coupled C-L band lasers developed for optical telecommunications applications to be used as the interrogating source.