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

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 col...