Generation of short acoustic pulses from an energetic picosecond laser

Abstract A passively mode-locked laser which produces picosecond energetic laser pulses for the generation of ultrasound is described. High energy extraction from the laser was achieved by using a minimum number of optical elements in the optical resonator and by using a half-symmetric resonator configuration. Output energy in the mode-locked train was 160 mJ. A single pulse from this train, up to 8 mJ in 30 ps, was used to generate ultrasound in thin sheet material, such as 250 μm thick aluminium sheet. Ultrasound detection was achieved by either a PVDF transducer, coupled to an oscilloscope without pre-amplification, or by a laser interferometer. A comparison of acoustic waveform recorded by these two devices showed that the PVDF transducer behaved essentially as a velocity sensor.