A fiber optic sensor suitable for non-contact detection of ultrasonic waves has been developed in this study. This sensor is based on the fiber optic Sagnac interferometer, which has a path-matched configuration and does not require active stabilization. It is shown that there exists the specific birefringence producing the quadrature point of operation for the maximum sensitivity of the fiber optic Sagnac interferometer for an arbitrary incident polarization state. The quadrature phase bias between two interfering laser beams in the Sagnac loop is achieved by the control of birefringence using a fiber polarization controller. The birefringence can be controlled in low birefringent single mode fiber due to bending; then the fiber may have a fast and a slow axis. As a result, an arbitrary optical phase delay of the light can be obtained between clockwise and counter clockwise beams in the Sagnac loop. A stable quadrature phase bias is confirmed by observing the interferometer output corresponding to the 2π peak-to-peak input signal produced by a fiber-wrapped PZT ring phase modulator. A fiber focuser is attached to the end of the probing optical fiber to focus the laser beam onto the specimen surface and to collect the reflected light back into the optical fiber probe. Harmonic ultrasonic oscillations produced by a PZT disk and ultrasonic pulses produced by conventional ultrasonic PZT transducers are successfully detected; and the performance of the interferometer is investigated by a power spectrum analysis of the output signal. The interference intensity is affected by the frequency of ultrasonic waves and the time delay of Sagnac loop. An additional signal processing is not needed for the detection of ultrasonic waves using the Sagnac interferometer. The configured fiber optic Sagnac interferometer is very effective for the non-contact detection of small displacement with high frequency such as ultrasonic waves used in conventional non-destructive testing, and can be applied for any other ultrasonic measurements. Based on the validation of the fiber optic Sagnac interferometer, the non-contact detection of laser-generated ultrasound is performed. Longitudinal and Rayleigh waves are generated in a steel specimen by a high powered Q-switched Nd:YAG laser pulse, and are detected by the non-contact optical technique using the fiber optic Sagnac interferometer. Surface acoustic waves are generated by line array illumination of a sample surface with a Q-switched Nd:YAG laser pulse. Multiple slits and a cylindrical lens are used to generate the tone-burst like surface waves. The adjustment of the center frequency of surface wave is performed by the change of distance between the sample and the lens; and the narrowband surface acoustic waves are obtained. Surface acoustic waves generated by laser line array illumination and the optical detection of ultrasound by fiber optic Sagnac interferometer can be applied to non-destructive testing and evaluation in a non-contact manner. Symmetric and antisymmetric Lamb modes are also excited in low-frequency-thickness regime by illuminating a thin plate with an array of Q-switched Nd:YAG laser-generated line sources. The propagation of laser-generated Lamb waves is detected by measuring the out-of-plane displacements in a non-contact manner using the fiber optic Sagnac interferometer. Laser line array illumination using multiple slits and a cylindrical lens is effective for the wavelength control of surface waves and Lamb waves. This scheme is very simple to generate the spatially narrow-banded surface waves and Lamb waves by the wavelength control. The laser generation and non-contact detection of narrowband Lamb waves at the selected operating points with relatively small rate of change in group velocity are presented.
