Generating underwater acoustic signals from a remote, aerial location by use of a high-energy pulsed infrared laser has been demonstrated. The laser beam is directed from the air and focused onto the water surface, where the optical energy was converted into a propagating acoustic wave. Sound pressure levels of 185 dB re microPa (decibel re microPa) were consistently recorded under freshwater laboratory conditions at laser-pulse repetition rates of up to 1000 pulses/s. The nonlinear optoacoustic transmission concept is outlined, and the experimental results from investigation of the time-domain and frequency-domain characteristics of the generated underwater sound are provided. A high repetition rate, high-energy per pulse laser was used in this test under freshwater laboratory conditions. A means of deterministically controlling the spectrum of the underwater acoustic signal was investigated and demonstrated by varying the laser-pulse repetition rate.

A numerical technique for the propagation of sound in the ocean where velocity is a function of both range and depth has been developed. The technique is not restricted to the narrow-angle (parabolic) approximation and it reduces to an exact solution for a homogeneous media. The given field is transformed into a sum of plane waves via an FFT. This transformed field is propagated without approximation through a homogeneous space represented by an average wave number for that space. Updating the average wave number provides for Snell’s law bending in range. The variations from the average velocity are accounted for by summing deviations from the nominal phase to develop a group of direction-sensitive phase correction masks. A weighted group of plane waves centered about each direction are inverse transformed for multiplication by the phase mask with the results summed. The choice of overlapping weights in the transformed space provides an approximate continuous phase correction mask for all directions. The functional relationship of range-step size to frequency, angular spectrum, and velocity profiles are developed. Several results for a 1500-m channel are shown.

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To be able to select appropriate signal architectures for various underwater acoustic communication channels a computer operable model that simulates the complete communications channel has been developed. The model utilizes time varying impulse responses and windowing techniques to determine the response to any given input signal. Statistical descriptors of the complex impulse and cw responses of the underwater acoustic environment of interest are used to develop the model impulse responses and to modify the input signal. This paper briefly outlines the model, and provides a comparison with some at sea experimental differential phase shift key (DPSK) 11. MODEL DEVELOPMENT The general communication geometry of interest is depicted in Figure 1. As indicated, the source and receiver are in SURFACE BOTTOM Figure 1. Shallow Water Communication With Multiple Paths.