Remote, Aerial, Trans-Layer, Linear and Non-Linear Downlink Underwater Acoustic Communication

Abstract

Both the linear mechanism for optical to acoustic energy conversion are explored for opto-acoustic communication from an in-air platform to a submerged vessel such as a submarine or unmanned undersea vehicle. This downlink communication can take the form of a bell ringer function for submerged platforms or for the transmission of text and/or data. The linear conversion mechanism, also known as the linear opto-acoustic regime where laser energy is converted to sound at the air-water interface, involves only the heating of the water medium. In this mode of operation, the acoustic pressure is also linearly proportional to the laser power. In contrast, the non-linear conversion mechanism, also known as the non-linear opto-acoustic regime where focused laser energy is converted to sound at the air-water interface, involves a phase change of the water medium through evaporation and vaporization which leads to the production of a plasma. In this mode of operation, the acoustic pressure is non-linearly related to the laser power. The non-linear conversion mechanism provides a more efficient, i.e. higher source level, yet less controllable method for producing underwater acoustic signals as compared to the linear mechanism. A number of conventional signals used in underwater acoustic telemetry applications as well as command and control applications are shown to be capable of being generated experimentally via the linear and nonlinear opto-acoustic regime conversion process. The communication range and data rates that can be achieved in both conversion regimes are addressed. The use of oblique laser beam incidence at the air-water interface to obtain considerable in-air range from the laser source to the in-water receiver is addressed. Also, the impact of oblique incidence on in-water range is examined. Optimum and sub-optimum linear opto-acoustic sound generation techniques for selecting the optical wavelength and signaling frequency for optimizing in-water range are addressed and discussed. Opto-acoustic communication techniques employing M-ary Frequency Shift Keying (FSK) and Multi-frequency Shift Keying (MFSK) are then compared with regard to communication parameters such as bandwidth, data rate, range coverage, and number of lasers employed. In the non-linear conversion regime, a means of deterministically controlling the spectrum of the underwater acoustic signal has been investigated and demonstrated by varying the laser-pulse repetition rate to provide M-ary Frequency Shift Keyed signaling. This physics-based conversion process provides a methodology for providing low probability of intercept signals whose information is embedded in noise-like signals. These laser generated signals can then be used in a frequency hopped spread spectrum technique with the use of the proper receiver structures to take advantage of the frequency diversity and periodicity inherent in this type of signal structure that could also be used to combat frequency selective fading in underwater acoustic channels