Thermoacoustic generation of narrow-band signals with high repetition rate pulsed lasers

Abstract

The generation of sound by absorption of laser light in water is analyzed for the case of lasers pulsating at high repetition rates. It is shown that pulsating the laser at any arbitrary repetition rate is, in general, not a very effective way to produce a narrow-band signal. In order to produce a signal with a narrow frequency band, one should instead pulsate the laser at an optimum repetition rate that is determined by the optical frequency of the laser and, to a lesser extent, by the laser beam diameter. Expressions for the optimum repetition rate are derived from both a frequency domain analysis and a time domain analysis. It is found that, with the present laser technology, significant gain in signal-to-noise ratio can be achieved by pulsating the laser at its optimum repetition rate, as compared with the more conventional methods for generation of narrow-band signals, such as the method of laser intensity modulation. As a result, it now seems possible to generate continuous thermoacoustic highly collimated sound beams with high repetition pulsed lasers in such a way that these signals are easily detectable several kilometers away from the source. Spatially periodic laser deposition configurations on the water surface are also discussed, and it is shown that further improvement in signal-to-noise ratio is achievable, in principle, for a spatial periodicity tuned to the optimum temporal periodicity of the repetition rate of the pulsed laser.