/' Meas. Sci. Technol. 8 (1997) 1050-1054. Printed in the UK PII: SO957-0233(97)80678-9 In-fibre Bragg gratings for ultrasonic medical applications N E Fishert, J Surowiect,.D J Webbt, D A Jacksont, L R GavrilovS, J W Hand$, L Zhangs and I Bennions t Applied Optics Group, Physics Laboratory, The University, Canterbury CT2 7NR, U K $ Radiological Sciences Unit, Hammersmith Hospital, Du Cane Road, London W12 OHS, UK g Photonics Research Group, Department of Electronic Engineering, Aston University, Birmingham 8 4 7ET, UK Received 8 January 1997, in final form 7 April 1997 Abstract. We investigate the feasibility of using in-fibre Bragg gratings to measure ultrasonic fields for medical applications. Two signal processing schemes for interrogating the gratings are described. Preliminary results for each scheme (one a homodyne approach, the other a heterodyne one) give noise-limited pressure resolutions of 2.2 x and 1.8 x atm respectively, each within a 1 Hz bandwidth. The second scheme, however, gives a more stable response. 1. Introduction Fibre Bragg gratings (FBG) are currently attracting considerable research interest in the optical sensing community, particularly in the field of advanced composite materials or 'smart structures' in which the fibres are embedded within the materials in order to provide real- time evaluation of the load, strain, temperature, vibration and so on. FBGs can be holographically written into Ge- doped fibres by side exposure to W interference patterns [l, 21. This results in a periodic modulation of the refractive index in the fibre core which can reflect light of wavelength A satisfying the Bragg condition A = 2nD, where n is the mode refractive index and D is the grating period. Since the grating period and hence the reflected wavelength are dependent on the temperature or strain of the fibre, by monitoring the shift in the reflected wavelength, these measurands may thus be recovered. In addition, the inherently wavelength-encoded output of these devices means that the sensed information is not dependent on light levels (a distinct advantage over other sensing schemes) and also facilitates the use of wavelength-division multiplexing by assigning each sensor to a different portion of the available source spectrum. There is a need for the assessment of the safety of ultrasound for medical applications [3-51 due to the trend towards increasing output powers from diagnostic ultrasound equipment [5] and the widening use of high- intensity ultrasound in a range of therapeutic applications (including ultrasound surgery, hyperthermia and lithotripsy [6-91). Often the assessment of such fields is based upon theoretical models of some complexity, as the acoustic fields present in the body may arise from non-ideal sources and their propagation through heterogeneous tissues in the body is likely to be influenced by many factors. Hence, direct determination of such fields in vivo is of importance. Conventional detection is most commonly achieved using piezoelectric devices. For example, transducers made from piezoceramics such as lead zirconate titanate (PZT) and the piezoelectric polymer polyvinylide difluoride (PVDF) can offer high sensitivities (with, in the latter case, an improved acoustic impedance match to water). Indeed, PVDF hydrophones have found wide use in biomedical applications (although they are in general not ideal for in vivo applications [lo]). However, common to transducers fabricated from both types of material are a susceptibility to electromagnetic interference and signal distortion, and a reduced sensitivity that is due to the electrical-loading effects of the transducer leads. Several approaches for detection utilizing intrinsic optical fibres based either on interferometric or on polarimetric techniques have been reported which avoid these 'electrical' problems [I 1-14]. In addition, the fact that the fibre's diameter is small (125 Fm for single-mode optical fibres) means that the system is particularly well suited for minimally invasive procedures. However, by utilizing in-fibre FBGs for detection there is also the added advantage of being able to multiplex them (along the same fibre) in order to detect the ultrasonic field at several points simultaneously. In this paper, two interferometric schemes for monitoring high-frequency (megahertz) ultrasonic waves using FBGs are investigated. In the first, a straightforward homodyne approach is taken in which the FBG is illuminated with a tunable narrowband light source; in the second, a heterodyne technique based on an unbalanced interferometric wavelength discriminator is described. 0957-0233/97/101050+05$19.50 Q 1997 IOP Publishing Ltd