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Single-frequency pulsed laser oscillator and system for laser-ultrasonics

We present a new pulsed laser oscillator and system for the optical detection of ultrasound in materials. A single-frequency laser oscillator based on a pulse pumped Nd:YAG rod inside a ring cavity is proposed. The laser delivers single-frequency pulses of 35 W power. Pulse to pulse stability of the laser is obtained with a classical Pound–Drever–Hall method. Power of about 1 kW can be obtained when the second rod of a dual-rod pumping chamber is used as an amplifier. The performance of the system is then investigated with a GaAs photorefractive crystal-based two-wave mixing phase demodulator. In particular, the intensity noise of the laser can be made small enough to allow the detection limit to be set by the shot-noise of the laser. The coherence length of the laser is about 20 m, which makes it a versatile laser-ultrasonic inspection system operated with a two-wave mixing-based phase demodulator. A complete compact and affordable system is obtained when the second rod of the pumping chamber is used to operate a short pulse laser for the generation of ultrasound. Tests of this laser-ultrasonic system on metallic samples are presented.