The detection of ultrasound via optical resonators is conventionally performed by tuning a continuous-wave (CW) laser to the linear slope of the resonance and monitoring the intensity modulation at the resonator output. In this work, we develop an alternative CW technique that can significantly reduce the measurement noise by monitoring variations in the phase, rather than intensity, at the resonator output. In our current implementation, which is based on a balanced Mach-Zehnder interferometer for phase detection, we demonstrate a 24-fold increase in the signal-to-noise ratio of the detected ultrasound signal over the conventional, intensity-monitoring approach.
In this work we propose and implement the use of software defined optical interferometry for the development of laser ultrasound non-destructive testing. The interferometer is conceived as an optoelectronic system that can be controlled by software. The system itself consists of five interconnected blocks: an optical system (a heterodyne interferometer), an Electrical-Optical block (a laser + an acousto-optic modulator), an Optical-Electrical block (a balanced photodetector), a software defined hardware (a software defined radio platform), and a programmable controller (a personal computer). In particular, the selected software defined hardware includes a field programmable radio frequency transceiver. On the one hand, it can synthesize the stimulus signals for the acousto-optic modulator which splits the reference and test beams. On the other hand, it can process the photo-detected high-frequency signals accordingly. This software defined radio platform not only simplifies the experimental scheme but also has such a high sensibility that provides a wide dynamic range. In order to show the performance of the system for non-destructive testing, we analyze the signals produced on aluminium plates to detect flaws in weld seams.
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