Interference-based amplification for CW laser-induced photoacoustic signals

Thanks to their low cost, compactness and suitability of use in industrial environments, CW lasers represent a viable alternative to more traditional pulsed lasers for non-contact inspection of structures, both under static and working conditions. However, the lower energy density typically reached by CW lasers requires adequate attention to obtain ultrasonic signals with sufficient amplitude. CW lasers can be modulated by TTL sequences with different duty cycles to constructively use the excitation of a double ultrasonic wave, occurring when the laser is both turned on and off. An appropriate choice in terms of number and spacing of the TTL pulses allows for the optimization of the ultrasonic response from the excitation standpoint: an increase in the amplitude of the ultrasound is enabled, and its frequency band modified. Such a solution allows to better adapt the ultrasonic wave to the receiver band, maximizing the global efficiency. In this work, the influence of the employed modulation sequence on the ultrasonic signal is analysed; by the TTL optimization proposed, application of the technique to the exemplary case of a rail represents a first implementation of CW lasers to the inspection of an industrial component.