ELSEVIER Optics & Laser Teclmology, Vol. 29. No. 6, pp. 333-338, 1997 C 1998 Elsevier Science Ltd PII: SOO30-3992(97)00013-3 Printed in Great Britain. All nghts reserved 003Om3992i97 $17.00 + 0.00 Compact phase-shifted Sagnac interferometer for ultrasound detection P. A. FOMITCHOV, S. KRISHNASWAMY, J. D. ACHENBACH A compact fibre, phase-shifted Sagnac interferometer for ultrasound detection has been developed. The interferometer is a truly path-matched device, and therefore requires no path stabilization or heterodyning. It is a less expensive and more robust alternative to the heterodyne or path-stabilized Michelson interferometer. The device provides high spatial resolution of ultrasonic detection. It has been used in conjunction with conventional piezoelectric transducers (PZT) to detect Rayleigh and Lamb waves and to image a crack in a thin plate, rivet cracks in riveted plates, and for ultrasonic beam profiling. @ 1998 Elsevier Science Ltd. KEYWORDS: laser-based ultrasonics, Sagnac interferometers, non-destructive evaluation Introduction Laser-based ultrasonics (LBU) includes two aspects: (i) the generation of ultrasound by laser illumination, and (ii) the detection of ultrasonic signals by laser interferometric techniques. LBU’s appeal in non- destructive evaluation (NDE) arises from the possibility for non-contact generation and detection, remote placement of equipment using fibre-optics, easy scanning with high spatial resolution, absolute displacement calibration, both broad-band and narrow-band signal generation, wide frequency band measurements, and applicability to curved surfaces. Both laser generation of ultrasound and the subsequent detection of the ultrasonic waves using laser interferometry have therefore been areas of active research over the past decadelm6. In earlier papers, the authors have discussed an LBU system which employs a diffraction grating for illumination of a line-array to generate narrow-band surface waves and Lamb waves4, and a fibre heterodyne dual-probe laser interferometer to measure ultrasonic signals- . 3,5~6 This paper reports the development of a robust, low cost, fibre Sagnac laser inferferometer that is suitable for the detection of ultrasonic signals arising from laser- or PZT-generated sources or from acoustic emissions. The primary advantage of the Sagnac interferometer is that it is exactly path matched and, as such, requires no static path compensation (or heterodyning) for sensor stabilization. Bowers7 first reported the use of a Sagnac-type interferometer for surface acoustic wave detection, and the work reported The authors are at the Center for Quality Engineering and Failure Prevention, Northwestern University, Evanston, IL 60208, USA. Received 12 February 1997. Accepted 1 April 1997. here builds on that effort. A design of such a system using a piezoelectric fibre stretcher as a phase modulator has been previously described by the authors*. The non- linearity and slow rate of phase modulation possible with the fibre stretcher made it impossible to provide true quadrature bias resulting in lowered sensitivity for that system. In this paper, we describe a system with an integrated electro-optic modulator to provide quadrature bias in order to increase the sensitivity of the interferometer so as to detect small amplitude ultrasonic signals. Attention is focused on the development of a compact system that utilizes PZT-generation and interferometric detection to provide high spatial resolution of measurements. Principle of the Sagnac interferometer The fibre Sagnac interferometer is shown schematically in Fig. 1. A linearly polarized laser beam is coupled into a single-mode fibre. This beam is split into two legs by a 2 x 2 coupler and these are, in turn, recoupled by a second coupler. One of the fibre legs contains an electro- optic phase modulator and a time delay fibre loop. The recoupled beams are focused onto a test specimen through a focusing probe. The scattered light from the object is then collected back by the focusing probe, gets split along the two legs, and eventually reaches the photodetector. All the fibres and couplers are polarization-maintaining so that there is minimal polarization scrambling of the beams. We call the central portion between the two couplers the Sagnac loop. There are four paths that the light can travel from the laser to the specimen and back into the photodetector, as shown in Fig. 1(b). Two of these will have returned by the same leg (leg 1 or leg 2) through which they entered (that is, without going through the Sagnac loop). The 333