Bridging optical fiber communication and underwater wireless optical communication via third harmonic generation.

In this paper, we propose an optical module, consisting of an Erbium/Ytterbium co-doped fiber amplifier (EYDFA) and a cascaded periodically poled lithium niobate (cascaded-PPLN), to bridge the conventional telecommunication and the emerging underwater wireless optical communication (UWOC). Compared with using two discrete crystals to achieve the third harmonic generation (THG), using a cascaded crystal simplifies the optical system. Under a fundamental power of 5 W at 1550 nm, we have generated an optical power of 6.54 mW at 516 nm, corresponding to a conversion efficiency of 0.1308%. Furthermore, we added a 5-km single-mode fiber (SMF) before the EYDFA, and by adjusting the seed laser power, we successfully maintained the efficiency of the THG process and the output power of the green light. Afterwards, the nonlinearity of the THG process is analyzed, and a simplified nonlinear pre-compensation method has been proposed to tailor the 4-pulse amplitude modulation (PAM4) signals. In such case, the bit error rate (BER) of the modified PAM4 (m-PAM4) can reduce by 69.3% at a data rate of 12 Gbps. Finally, we demonstrate the practicality of our proposed system by achieving a 7-m UWOC transmission in a water tank at a data rate of 13.46 Gbps in an optical dark room. This result demonstrates the feasibility of the hybrid fiber/UWOC system, highlighting its potential for practical implementation.

Underwater laser positioning of targets outside the field of view based on a binocular vision.

An underwater laser positioning scheme based on a binocular camera is introduced. In spite of the scattering, the underwater laser light path can be clearly captured by a camera within an appropriate range depending on the water turbidity. For an emitting laser with a fixed position, the three-dimensional information of the laser source can be calculated from the beam images captured by a binocular camera, even if the laser is out of the camera's field of view (FOV). This method can break through the FOV limitation of traditional camera positioning and perform a 3D spatial positioning for the target even outside the FOV of the camera. We simulate and analyze the scattering light imaging and find that the laser propagation direction can be recognized from the scattering image. The experimental results show that the proposed underwater positioning scheme achieves an average 3D positioning error of 5.53 cm within a range of 5 m when the underwater attenuation is 0.325m -1.

Theoretical analysis and experimental demonstration of gain switching for a PPM based UWOC system with picosecond pulses.

Shortening pulse width can improve the power efficiency and data rate of a pulse position modulation (PPM) based underwater wireless optical communication (UWOC) system at a fixed average optical power, which is more suitable for the energy-limited underwater environment. As a common method to generate short pulses, gain switching has the advantages of a tunable switching frequency and simple structure, facilitating the generation of high-order PPM signals. However, the output characteristics of electrical gain switching seriously affect the demodulation of PPM signals and limit the data rate. To study the performance of gain switching on a PPM communication system, simulation models of the semiconductor laser diode and the driving circuit are built to describe the generation of electrical and optical pulses. The pulse width, pulse peak value, and peak position of optical pulses are analyzed under different symbol durations and PPM orders. Furthermore, a 64-PPM/150-Mbps UWOC system with a 200-ps optical pulse width is demonstrated by using a gain-switched blue GaN-based laser diode in a water tank. The peak average power ratio (PAPR) is 19.5 dB. Via the statistical analysis of experiment results and the output characteristics of electrical gain switching, the main factor limiting the data rate attributes to the time delay fluctuation of gain switching. To the best of our knowledge, this is the first time that gain switching has been experimentally demonstrated and analyzed in a high-order PPM based UWOC system.

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.

Demonstration of a 2 × 2 MIMO-UWOC system with large spot against air bubbles.

In order to reduce turbulence-induced scintillation and deal with alignment problems, a 2×2 multiple-input multiple-output (MIMO) underwater wireless optical communication (UWOC) system is proposed and experimentally demonstrated. With help of the large divergence angle of light beams and large field of view (FOV) of the detectors, the effect of high-density air bubbles is greatly eliminated. Simulation and experimental results confirm that, in most intensity-modulation/direct-detection (IM/DD) MIMO-UWOC systems, the repetition coding (RC) scheme performs better than the space-time block coding (STBC) scheme. In a 50 m swimming pool, the maximum horizontal offset can reach 97.9 cm, which is 421% and 192% higher than that of STBC multiple-input single-output (MISO) and RC-MISO/STBC-MIMO schemes, respectively. With a data rate of 233 Mbps and a transmission distance of 50 m, the large detection range can meet a variety of underwater wireless communication requirements. The experiment indicates that, when the difference in the transmission distance between the two optical signals is higher than 1 m, the bit error rate (BER) of the RC scheme increases sharply, while the BER of the STBC scheme is stable. The MIMO coding scheme needs to be selected according to the actual application environment.

Quasi-omnidirectional transmitter for underwater wireless optical communication systems using a prismatic array of three high-power blue LED modules.

In this study, a quasi-omnidirectional underwater wireless optical communication (UWOC) system is implemented with a prismatic array consisting of three uniformly distributed high-power LED modules as the transmitter. Over a 10-m underwater channel in a 50-m standard swimming pool, a data rate of 22 Mbps is achieved without adopting any digital signal processing algorithm. With zero forcing (ZF) based frequency domain equalization (FDE) and a maximum ratio combining (MRC) algorithm, the maximum net data rates achieved are 69.65 Mbps, 39.8 Mbps and 29.85 Mbps over 10-m, 30-m, and 40-m underwater channels, respectively. In the proposed UWOC system, the receiver could successfully capture optical signals at different directions from the transmitter and the bit error rates (BERs) measured in different directions show small fluctuations. The proposed system could meet the demands of high-speed data transmission among units in a swarm-robot system and last meter user access in an underwater optical cellular network system.

200-m/500-Mbps underwater wireless optical communication system utilizing a sparse nonlinear equalizer with a variable step size generalized orthogonal matching pursuit.

Linear and nonlinear impairments in underwater wireless optical communication (UWOC) systems caused by the limited bandwidth and nonlinearity of devices severely degrade the system performance. In this paper, we propose a sparse Volterra series model-based nonlinear post equalizer with greedy algorithms to mitigate the nonlinear impairments and the inter-symbol interference (ISI) in a UWOC system. A variable step size generalized orthogonal matching pursuit (VSgOMP) algorithm that combines generalized orthogonal matching pursuit (gOMP) and adaptive step size method is proposed and employed to compress the Volterra equalizer with low computational cost. A maximum data rate of 500 Mbps is realized with the received optical power of -32.5 dBm in a 7-m water tank. In a 50-m swimming pool, a data rate of 500 Mbps over 200-m underwater transmission is achieved with a BER lower than the forward error correction (FEC) threshold of 3.8 × 10-3. The number of kernels of the sparse Volterra equalizer is reduced to 70% of that of the traditional Volterra equalizer without significant BER performance degradation. Compared with orthogonal matching pursuit (OMP) scheme and regularized orthogonal match pursuit (ROMP) scheme, the VSgOMP scheme reduces the running time by 68.6% and 29.2%, respectively. To the best of our knowledge, this is the first time that a sparse Volterra equalizer combined with VSgOMP algorithm is employed for the nonlinear equalization in a long-distance high-speed UWOC system.