Compact Laser Driver for Ultrasonic Arbitrary Position and Width Pulse Sequences Generation

Laser ultrasound offers several benefits in comparison to immersion or air-coupled ultrasound: it directly generates stress on the sample surface; it has a wide bandwidth and a small acoustic source can be produced. Conventionally, high-power pulsed lasers are required therefore equipment is bulky and expensive. Here we propose to use mid-power laser diodes, which are small and low cost. To solve the low signal amplitude problem with lower power lasers, the use of spread spectrum signal, arbitrary position, and width binary pulse sets is proposed. To date, a laser driver for this task has not been available. This article describes the development of such a compact driver, where the essential electronics occupy an area of $10\,\,\times20$ mm. The whole system with a fixture for kinematic mount is comparable in size to a conventional piezoelectric ultrasonic transducer. The driver can supply pulse sets of up to 40 A. Individual pulse duration can vary between 20 and 1000 ns (0.5–25-MHz ultrasound range) and the total pulse train duration is limited by the laser type used. Experiments show that up to 10- $\mu \text{s}$ long pulse sets can be used at 10-A current, without laser degradation. Current waveforms, beam profile, and optical response signals were measured for three rectified topologies. It was concluded that the GaN-based constant current switch topology has the best performance, but a power MOSFET in a source current feedback topology can also be used to generate pulses down to 20 ns. Photoacoustic response signals from chirp and phase shift keying modulation have been demonstrated.