Design of a remote and integrated Sagnac interferometer that can generate narrowband guided wave through the use of laser and effective optics to detect defects occurred in plates

To inspect the integrity of specimens that are moving or no space available to mount sensors on their surfaces, laser generated guided wave (GW) becomes the only feasible option. However, conventional GW generated by laser-based emission is in broadband frequency range. Hence, unwanted GW modes, along with the emitted desired GW mode, also get produced simultaneously to the inspected specimen. The results generated from reflected GW become chaotic, making the identification of GW signals truly reflected by defects very difficult. Hence, substantial research efforts have been spent on reducing unwanted GW modes by spatially modulating the laser pattern to emit narrowband GW. Previous methods to generate narrowband GW include the use of slit mask, diffractive grating, lenticular array, and interference of laser beam. Among these techniques, each has certain shortcomings. A recent research achievement is the design of the integrated optical Mach-Zehnder system (IOMZ) (J. Chen et al, Opt. Lett., 42, 4255, 2017). It can emit desired narrowband Lamb wave into the aluminum plate by using pulsed laser and the designed optical system. Although the results showed IOMZ’s ability in emitting narrowband GW mode, significant unwanted noise was also generated, making the signal-to-noise ratio (SNR) of the received GW signals rather low. In this study a new optical system to overcome the shortcoming of the above reported devices is proposed. It is called integrated Sagnac interferometer-based optical system (SIOS) which can minimize generation of unwanted noise and substantially improve the SNR of reflected GW signals. A comparison study is also presented to illustrate the effectiveness of SIOS is superior to that of the IOMZ. Experimental results reveal that the proposed SIOS is promising for remote nondestructive testing and evaluation.