Objective  With the increase of marine rights protection, marine resource development, and underwater exploration activities, wireless optical communication has become one of the key technologies for non-contact underwater information transmission due to its advantages of high speed, easy mobility, and good concealment. The absorption, scattering, and turbulence characteristics of visible light in seawater seriously restrict the underwater transmission distance of visible light communication systems. According to the transmission characteristics of visible light in seawater, blue-green light with the range of 450-550 nm is suitable for deep-sea wireless optical communication, which is less affected by seawater. In order to improve the capacity of wireless optical communication, high-order modulation methods such as Quadrature Amplitude Modulation (QAM) are often used in wireless optical communication systems. However, QAM modulation technology requires a linear light source and channel, which limits the emission power of the light source and reduces the underwater transmission distance for underwater wireless optical communication systems. In addition, it is difficult to quickly modulate a high-power laser light sources, so the high-power laser light cannot achieve high-speed optical carrier information transmission. Therefore, the wireless optical communication system with high-speed and high-power is one of the key technologies for long-distance underwater wireless optical communication. In order to obtain high-power signal light, Free Space Optical (FSO) communication usually uses optical amplifiers to directly amplify the signal light by combining the indirect modulation technology to output high-power signal light. However, it is difficult to directly amplify the blue-green light signal to achieve high-power underwater laser output in the UOWC system due to the lack of optical amplifiers in the blue-green light.

  Methods  A high-power and high-speed underwater optical wireless communication (UOWC) system (Fig.1) with wavelength conversion construction (Fig.3) is designed, which adopts the second harmonic theory of nonlinear optics, as well as the 1 064 nm high-speed external modulation and the amplification technology. After analyzing the optical system and system parameters, the wavelength conversion efficiency model is deduced, and the relationship between the angle range and the conversion efficiency at different temperatures is obtained. Finally, the communication system is set up (Fig.9), and experimental analysis is carried out.

  Results and Discussions  The effect of temperature on conversion efficiency and beam quality, as well as the performances of the square wave and sine wave are tested in this paper. The results show that the optimal matching can be achieved at an angle of around 1.57° (Fig.5), no matter how the wavelength and temperature change, the conversion efficiency can be maximized. At the same wavelength, the angle adjustable range corresponding to the wave vector mismatch would be increased with the increasing temperature, but the conversion efficiency will be decreased (Fig.6). Furthermore, the beam's uniform intensity distribution quality of doubling frequency mode is significantly better than that of the modulated fundamental frequency mode at room temperature (Fig.8). In addition, the width of the rising edge and the falling edge of the square wave signal would be compressed (Fig.10), whose relative variation of the rising edge tends to 0.27, and the relative variation of the falling edge tends to 0.06, but the compression phenomenon will converge with the increase of frequency (Fig.11). The sinusoidal signal has no noticeable distortion with the increase of frequency (Fig.12).

  Conclusions  The designed system improves the beam quality compared to the modulated fundamental frequency light mode, and compresses the rising and falling edge widths of the square wave signal, which will converge with the pulse width decreases, but does not distort the sine signal with increasing frequency. So the system has no effect on the high-speed digital signal and high-frequency analog signal. It is verified that the designed system can achieve 1 W high-power 532 nm green light wireless output at room temperature, the analog signal frequency is higher than 1 GHz, and the digital signal rate is faster than 500 Mbps. The research results can provide theoretical and technical support for future long-distance high-speed underwater wireless optical communication.