Power-Efficient Long Distance Underwater Wireless Optical Communication System Based On Single-Photon Detector

For the satisfaction of the communication needs in the underwater exploration,underwater wireless optical communication(UWOC)emerges as the times require.UWOC has earned a place in the field of underwater communication thanks to its rich bandwidth,high data rate and convenience in usage and maintenance.However,compared with the underwater acoustic communication(UAC),the communication distance of UWOC is severely limited,besides,the alignment requirement between UWOC transmitting and receiving sides in long-reach communication is very strict.The main idea of this thesis is to study the single photon avalanche detectors with high sensitivity and modulation formats suitable for long-distance underwater optical signal transmission,and establish a long-reach,low-optical-power UWOC system based on both.The research content and the results achieved in the thesis are as follows:(1)We first investigated the development of UWOC in recent years,then summarized the prevalent modulation formats and optical detectors used in UWOC.For the modulation format suitable for long-distance underwater signal transmission,we introduced and studied the pulse position modulation(PPM)that has the advantages of high signal-to-noise ratio,high energy efficiency and low error rate.As for the optical detector,we introduced the multi-pixel photon counter(MPPC)that has very high sensitivity and linear response to optical intensity,then mathematically analyzed and deduced its response in the cases of single optical pulse and multiple optical pulses in a period of time.The influence of parameters such as optical signal intensity,dead time and signal width on the MPPC response was also mathematically discussed.Finally,photon equivalent threshold(p.e.threshold)that can filter out MPPC non-signal outputs caused by background light and dark count rate(DCR)was introduced.(2)According to aforementioned analysis and discussion,we built a UWOC system that combined PPM format signal and MPPC.Then we attempted to achieve the communication with extremely low transmitting optical power in a 46-m long underwater channel.In the experiment that based on a blue-light laser,an analog output MPPC and 4-PPM to 64-PPM signals with 5 MHz to 10 HMz slot frequencies,error-free communication was realized with maximum transmitting optical power of 0.172 mW and minimum power of 1.7 ?W.After that,some problems caused by the system,such as amplifier and bias voltage,and their effects on the communication were discussed.(3)In the experiment that based on a blue light-emitting diode(LED),a digital output MPPC with p.e.threshold and 8-PPM to 64-PPM signals with 5 MHz slot frequency,we achieved the communication with bit error rates(BERs)all below the forward error correction(FEC)limit.And in the communication,the maximum and minimum receiving optical power were 19.5 pW and 3.4 pW respectively.We also proved that when p.e.threshold was set slightly lower than the incident signal light intensity,it could effectively filter out non-signal outputs and simplify the signal amplitude calculation method.Finally,a comparison between the models of practical MPPC output and theoretical photon incidence was carried out to prove their similarity under the condition of weak light incidence.