The source laser was 1540 nm, the sensing laser was 1550 nm.[1] > For the LUS system reported here, the 1550 nm LDV was specifically selected to maximize permi

The source laser was 1540 nm, the sensing laser was 1550 nm.[1] > For the LUS system reported here, the 1550 nm LDV was specifically selected to maximize permissible optical backscatter from the skin while maintaining safety. > Thus, the 1540 nm optical source was selected to maximize the converted acoustic source amplitude while remaining within the safety limits. They wanted lasers close in the range of 1500 nm, but just over that wavelength has better safety limits. I am pretty certain those lasers were just chosen because that was what was commercially available in the narrow range of frequencies they wanted. The source laser is a Q-switching laser[2], which is needed to produce those pulses. It doesn't particularly matter what wavelength they used (~2000 nm also would have worked); the doppler (aka self-mixing[3]) technique they used is very highly tolerant to noise. [1]: https://www.nature.com/articles/s41377-019-0229-8 https://www.nature.com/articles/s41377-019-0229-8 [2]: https://en.wikipedia.org/wiki/Q-switching https://en.wikipedia.org/wiki/Q-switching [3]: https://en.wikipedia.org/wiki/Self-mixing_laser_interferometry https://en.wikipedia.org/wiki/Self-mixing_laser_interferomet...