9.14-Mbps 64-PPM UWOC system based on a directly modulated MOPA with pre-pulse shaping and a high-sensitivity PMT with analog demodulation

A pulsed fiber master oscillator power amplifier (MOPA), which is combined with second harmonic generation (SHG) and modulated by directly changing the current of the low-power seed laser, is designed in this paper to overcome the 'green gap' of semiconductor lasers and the difficulty of obtaining high-power and wide-bandwidth driving circuits. To decrease the guard slot and increase the data rate of a high-order pulse position modulation (PPM) system, pre-pulse shaping (PPS) is utilized to decrease the fluctuation of pulse power, which is caused by the gain dynamics of multi-order amplification of the MOPA, from 55.6% to 27.5% for 25-ns pulses and from 22.4% to 16.7% for 10-ns pulses, respectively. Moreover, an analog PPM demodulation method is proposed to mitigate the nonlinear effect caused by space charge limitations at dynodes of a photomultiplier tube (PMT) and increase the robustness of the system. In an optical darkroom, a 99-m 64-PPM UWOC transmission, of which the measured link loss is around 13.16 attenuation length (AL), is realized in a water tank with a data rate of 9.14 Mbps. The average received optical power ranges from -60.87 to -52.51 dBm, corresponding to a bit error rate (BER) range of 1.93 × 10^-4 to 2.3 × 10^-3. To further prove the reliability of the proposed system, we implement a 65-m UWOC experiment with the same data rate at a BER of 3.42 × 10^-4 in a 50-m standard swimming pool. The maximum link loss is measured to be 15 AL.