FIBERIZED SAGNAC INTERFEROMETER FOR ULTRASOUND MEASUREMENT Pavel Fomitchov, Sridhar Krishnaswamy, and Jan D. Achenbach Center for Quality Engineering and Failure Prevention Northwestern University Evanston, IL 60208 INTRODUCTION Laser-based ultrasonics (LBU), i.e. the generation of ultrasound by laser illumination and the measurement of ultrasonic signals by laser interferometric techniques, has many advantages for applications to nondestructive evaluation (NDE). These include non-contact generation and detection, remote placement of equipment using fiber-optics, easy scanning, 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 a laser interferometry are areas of active research [1-6]. In earlier papers, the present 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 waves [4], and a fiberized heterodyne dual-probe laser interferometer to measure signals [3]. This paper reports progress toward the development of a robust low cost fiberized Sagnac laser interferometer suitable for field applications. Bowers first reported in reference [7] the use of a Sagnac-type interferometer for surface acoustic wave detection, and this work builds on that earlier effort. The primary advantage of the Sagnac interferometer is that it is exactly path matched and as such requires no heterodyning or static path compensation for sensor stabilization. The Sagnac interferometer described below is suitable for the measurement of ultrasonic sUlface waves alising from laser- or pzt-generated sources or from acoustic emissions. The laser-based ultrasonics (LBU) system can be used to detect and characterize discrete defects such as fatigue cracks as well as distributed regions of reduced material properties. PRINCIPLE OF THE SAGNAC INTERFEROMETER The fiberized Sagnac interferometer is shown in Figure 1. A linearly polarized laser beam is coupled into a single-mode fiber. This beam is split into two legs by a 2x2 coupler and these are in turn recoupled by a 2xl coupler. One of the fiber legs contains a fiber phase modulator. The recoupled beams are then focused onto a test specimen through a grin lens focusing probe. The scattered light from the object is then collected by the grin lens probe, gets split along the two legs, and eventually reaches the photodetector as shown in the figure. All the fibers and couplers are polarization-maintaining to minimize polarization scrambling of the beams. We will 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. Two of these will have returned by the same leg (leg I or leg 2) through which they entered (i.e. without going through the Sagnac loop). The other two are the one that enters leg 1 and returns by leg 2 (traveling the Sagnac loop clockwise) and the one that enters leg 2 and returns by leg 1 (traveling the Sagnac loop counter-clockwise). Review of Progress in Quantitative Nondestructive Evaluation, Vol. 15 Edited by D.O. Thompson and D,E, Chimenti, Plenum Press, New York, 1996 645