Underwater wireless optical communication (UWOC) is an useful way to transmit date in ocean with short distance, because it has high speed. In application, the first challenge for this technology is establishing optical link underwater. There are two schemes to deal with this issue. The first one is enlarging the angle of divergence of the transmitter, and the field of view of receiver, which reduces the requirement to align the UWOC transmitter and receiver. However, the energy loss is increased in the transmission link. The other method needs a pointing, acquisition, and tracking (PAT) system to establish optical link. The angle of divergence of the transmitter is quite small in this way, which is helpful to reduce energy loss in the transmission link. However, in order to make the optical link stable, the receiver should measure the angle of incident of communication light all the time to provide feedback parameters to PAT system. In this work, a method is presented and demonstrated based on four-quadrant detector (QD) for such issue. It shows that the tested error for pitch angle and azimuth angle is less than 1°, as the communication system could still work with 50Kbps communication rate and 2m distance. It has demonstrated the feasibility of the method.
In this thesis, we designed and experimentally demonstrated a high-power high-speed underwater optical wireless communication (UOWC) system with wavelength conversion construction. External modulation based on 1064nm laser is used for high-speed information communication, as well as the optical amplifier is used to obtain enough optical power of 1064nm laser. After that, according to the quasi-phase-matching (QPM) conditions, the PPLN optical structure is designed to improve the wavelength conversion efficiency for achieving higher 532nm laser output power in 24.5℃~40°C. Compared to the 532nm LD modulation system, this system can output 1.4W 532nm laser power in 100Mbps. This system experiments in single link 100m water tap of the attenuation coefficient 0.73dB/m equivalent to the clear ocean, and the measured bit error rate (BER) is 6.2×10-6 in 100Mbps pseudo-random binary sequence (PRBS) data without the forward error correction (FEC). Based on receiver sensitivity and the seawater channel optical transmission model, the transmission performance was predicted to be 340m@100Mbps and 100m@2Gbps in the attenuation coefficient equivalent to pure seawater.
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