The blue waveband is the window for underwater wireless optical communication, and high-performance blue lasers are the ideal light source for underwater wireless optical communication. Pulsed blue lasers have high peak power than that of continuous-wave laser, experience less attenuation during underwater transmission and have better communication performance. As an important technique for obtaining pulsed lasers, Q-switching has been widely used in solid-state lasers. However, for semiconductor lasers, it is difficult to obtain large pulse energy or high peak power due to the nanosecond short lifetime of the carriers. But in the other hand, Q-switched semiconductor lasers can produce pulse trains with higher repetition rates because of their shorter carrier lifetime, and combined with their flexible and designable emitting wavelengths, their application range can also be expanded.

In the gain chip of a semiconductor disk laser (SDL), there exists the nonlinear Kerr effect in the semiconductor multiple quantum wells of the active region, where the refractive index in the region is proportional to the light intensity, i.e. n=n₂I, where n is the refractive index of the material, I is the incident light intensity, and n₂ is the Kerr coefficient. For the semiconductor multiple quantum wells materials, the above n₂ is negative. The nonlinear Kerr effect in the active region leads to an equivalent lens depending on the intensity of light, causing the pulsed laser to experience a higher refractive index, while the continuous-wave laser suffers to a lower refractive index. When there is an aperture in the resonant cavity or a so-called soft aperture composed of pump spot on the gain chip, the equivalent lens mentioned above will cause the pulsed laser to experience less loss in the resonant cavity, start the pulse operation of the laser, and maintain stable pulse train output. If the effect of the Kerr equivalent lens is weak, SDL will operate in a Q-switching state, producing Q-switching pulse train with a pulse width on the order of nanosecond, which is called self Q-switching. If the Kerr equivalent lens has a stronger effect, SDL may operate in a mode-locked state, generating shorter picosecond pulses. This article utilizes the above-mentioned Kerr equivalent lens to achieve stable self Q-switching in the SDL. Then, while maintaining the Q-switched operation of the laser, a self Q-switched frequency-doubled blue laser is obtained by inserting a LiB₃O₅ (LBO) nonlinear crystal into the resonant cavity. Finally, pulsed blue laser is used as the light source for an underwater wireless optical communication, and the communication performance of the pulsed blue laser and the continuous-wave blue laser is compared.

In a Z-type resonant cavity composed of the bottom distributed Bragg reflector (DBR) in the gain chip, the high-reflectivity mirror M1, the frequency-doubling output mirror M2, and the planar high-reflectivity mirror M3, when the absorbed pump power is 29.4 W, the maximum output power of the 982 nm fundamental laser is 4.22 W. After the nonlinear crystal LBO is placed in the resonant cavity and the absorbed pump power is 28 W, the maximum average output power of the self Q-switched blue laser is 702 mW, with a pulse width of 8 ns and a period of 16.1 ns, corresponding to a pulse repetition rate of 61.9 MHz. The repetition rate of the output pulses of the self Q-switched blue SDL increases with the increase of the absorbed pump power, but the width of the pulse decreases with the increase of the absorbed pump power. In the underwater wireless optical communication system constructed using the above-mentioned self Q-switched pulse blue laser as the light source, the bit error rate of the pulse optical communication is at least one order of magnitude lower than that of the continuous-wave optical communication under the same water type, data rate, and link length. The reason is that the self Q-switched laser pulses can be regarded as a high-frequency modulated light waves, which have smaller scattering attenuation than that of the continuous-wave laser. Therefore, the pulsed laser power obtained by the receiver will be significantly greater than that of the continuous-wave laser, thereby increasing the signal intensity, improving the signal-to-noise ratio, and reducing the bit error rate.

Based on the nonlinear Kerr effect of the semiconductor medium in the active region of the SDL gain chip, stable self Q-switching of the SDL with an emission wavelength of 982 nm is achieved in a Z-type resonant cavity. After placing an LBO crystal at the smallest waist of the beam inside the cavity, 491 nm pulsed blue laser output is obtained. When the absorbed pump power is 28 W, the maximum average output power of the Q-switched blue laser pulse is 702 mW, the pulse width is 8 ns, and the repetition rate is 61.9 MHz. In the underwater wireless optical communication system constructed using the above-mentioned self Q-switched pulsed blue laser, under the same conditions (input optical power, Maalox solution mass concentration, communication link length, and data rate), the communication performance of the self Q-switched blue SDL is significantly better than that of the continuous-wave blue SDL. When the data rate is 10 Mbit/s and the link length is 18 m, the bit error rate of the self Q-switched blue SDL communication is reduced by about one order of magnitude compared to that of the continuous-wave blue SDL communication.