Fundamental Research On Novel All-fiber Photoacoustic Source For Underwater Communication

This dissertation works for all-fiber underwater communication system, and a novel all-fiber photoacoustic source used in underwater communication is researched fundamentally. Based on the characteristics of all-fiber systems and photoacoustic sources, after the mechanism of laser generated acoustic waves in liquids is investigated theoretically and experimentally, an all-fiber photoacoustic source is explored for short distance, hidden, high speed underwater communication. An all-fiber system of laser generation of acoustic waves is constructed, and the optical acoustic characteristics of the system are analyzed in theory, simulation and experiment.The main results obtained in this dissertation are as follows:(1)The theory of photoacoustic effect through thermoelastic mechanism is analyzed in detail. Under the situation of generation of planar photoacoustic waves, the effects of the time domain waveform of pulse lasers on the characteristics of photoacoustic pulses are deduced and simulated, and the optical-to-acoustic conversion efficiency and the influencing factors are analyzed; Under the situation of generation of spherical photoacoustic waves, the frequency characteristics, acoustic energy and optical-to-acoustic conversion efficiency of the photoacoustic pulses are analyzed theoretically, and the measures to enhance the conversion efficiency are presented. The theoretical results show that thermoelastic mechanism is not valuable in high power engineering practice.(2)A vaporization model of liquid medium is constructed. The threshold energy density of the pulse lasers to generate vaporization and its influencing factor are analyzed. The feasibility to reach the threshold value through focusing and transmission by optical fibers is discussed. The sound source level of photoacoustic waves, which can describe the launching performance of the photoacoustic source better than the pressure at the excitation point in the investigation of all-fiber photoacoustic sources, is analyzed.(3)An experimental system to produce high power pulse lasers is constructed using a DFB semiconductor laser, a LiNiO3 electro-optic intensity modulator, and erbium doped optical fiber amplifiers. To obtain high peak-to-peak power pulse lasers used in the technology of laser generation of acoustic waves and load communication information effectively, an erbium doped optical fiber power amplifying system is designed and made, and the method to control the operating point and the modulation depth of the modulator is investigated. The power gain and the waveform distortion of the pulse lasers are studied, and the experimental results agree with those observed from theoretic simulation. The relevant theory and the technique program are verified. (4)Photoacoustic pulses with the source spectrum level of 110dB and central wave length between 10kHz and 20kHz are obtained based on the all-fiber system of laser generation of acoustic waves in liquids, and the program to obtain sound sources used in short distance, high speed underwater communication through photoacoustic effect using fiber laser is verified. The effects of parameters of pulse lasers on the characteristics of photoacoustic pulses are investigated experimentally, and the results obtained provide foundation for the further development of the technology.(5) To detect photoacoustic pulses of very low intensity and lower frequency, a Helmholtz resonant acoustic cell is designed and made, and the acoustic amplifying performance in water is analyzed and simulated. The feasibility to use the resonator in the system of laser generation of acoustic waves is verified experimentally.(6)The breakdown of water is achieved using an ytterbium doped optical fiber laser. The all-fiber experimental system of laser generation of acoustic waves through optical breakdown mechanism is studied. Photoacoustic pulses with the source spectrum level of 122.4dB and frequency of nearly 22kHz and its 2-4 order frequency multiplication are obtained, and the strength of sound source approaches the demand of short distance, hidden, high speed underwater communication.