Optimizing Acoustic Signal Quality for Linear Optoacoustic Communication

In underwater wireless networks, optoacoustic energy conversion using high energy laser pulse is the only known viable option for communication from an airborne unit to a node at large depth, e.g., a submarine or an unmanned underwater vehicle. However, controlling the generated acoustic signal through this process is very complex. Specifically, if the repetition rate of laser pulses is low, the corresponding acoustic signal is very broadband. The higher frequency components of this broadband signal attenuate more if the underwater node is very far from the surface. Hence, a relatively narrowband signal with lower frequency components is desirable for long distance communication. The frequency component of the broadband acoustic signal depends on the incident angle of the laser light and observation angle of the receiver, i.e., the position of the underwater hydrophone. Both of these angles also change continuously for a wavy water surface, which makes it more complex to determine the frequency components of this kind of signal. In this paper, we show that by carefully choosing the relative position of the airborne unit and underwater node, we can generate a narrowband acoustic signal with lower frequency components for both flat and wavy water surfaces. We provide theoretical analysis and simulation results to capture the effect of these angles on the generated acoustic signals. We further provide guidelines for optimum angle setting for improving the quality of the optoacoustic communication link.