Pulsed Laser Based Underwater Wireless Optical Communication: Smaller Channel Attenuation and Better Communication Performance

Abstract

One of the primary goals in the development of underwater wireless optical communication (UWOC) is to extend the link length. Meanwhile, the key to increasing the link length of UWOC is to decrease the attenuation of the optical signals underwater. This work demonstrates that a decreased underwater attenuation coefficient and improved communication performance can be achieved by using a pulsed laser. The pulsed blue laser used in the experiment is a self-Q-switched semiconductor disk laser, whose Q-switching is initiated by the nonlinear Kerr effect of the multiple quantum wells in the active region of the gain chip. The width of the laser pulse is about a few nanoseconds and the repetition rate of the output pulse train is in the order of tens of megahertz. According to the measurements of the attenuation coefficients at various concentrations of Maalox solution, the pulsed laser has a significantly smaller attenuation coefficient than the continuous-wave laser, and this drop in attenuation coefficient is particularly noticeable in turbid water with a higher concentration of Maalox solution. The attenuation coefficient of the pulsed laser was only 62% of the continuous-wave laser at a Maalox concentration of 2905 mg·m⁻³. Subsequent tests reveal that the lower attenuation coefficient of the pulsed laser is mainly resulted from the reduced scattering it encounters underwater, rather than the ignorable decrease in the attenuation coefficient raised from the water's absorption of the laser. A UWOC system based on the above pulsed laser has also been built and much better communication performance compared to the continuous-waves laser has been obtained. The bit error rate of the UWOC based on the pulsed laser is 1–2 orders of magnitude lower than that of the UWOC with a continuous-wave laser when the data rate is 10 Mb/s and the link length is 18 m.