Experimental study of laser‐induced underwater sound

Experiments have been carried out to characterize the sound generated when a $CO2$ “TEA” (pulsed) laser, operated without nitrogen, illuminates a free water surface. Energy densities ranging from below to above the value needed for rapid surface evaporation have been used. Acoustic pulse risetimes, as measured with very broadband detectors, are typically less than 100 ns at shallow depths. The acoustic pulse shape changes during propagation. The exact nature of the change depends upon the laser energy density. Generally a pulse stretches and the trailing edge steepens so that it changes from a nearly sinusoidal shape into an “N” shape, often with the rarefaction having a larger amplitude than the initial compression by the time it has traveled downward 5 cm from the surface. Although nonuniformities (“hot spots”) in the laser intensity make it difficult to determine the exact relation, it appears that the peak pressures are roughly proportional to the energy density (J/cm²) of the laser pulse. An approximate energy density‐to‐pressure conversion constant for pressures measured at a depth in meters is 0.1 (m) (bar)/(J/cm²). The paper will provide an overview of these and other experimental results.