Non-visible light underwater optical communications

摘要

One look at the absorption curve for light in water suggests that the best possible window for underwater optical transmission is at the blue-green end of the visible light spectrum, centered at roughly 450 nm for clear, deep water, and shifted more towards green for coastal water [1]. While this holds true for signal transmission in the optical communications channel, it is also true for the solar spectrum, which presents as in-band white noise near the ocean surface. The greatest signal loss and interference is in the 0 to 40 meter depth range, but solar irradiance is still present to a depth of 300 meters and in some cases to a depth of500 meters. Because solar radiation shares the same spectral band as the communications signal, solar light is very difficult to reject without the loss of transmitted communications signal. For wide-angle transmitters and receivers, dielectric, thin-film (TF) filters are not effective due to the wavelength dependence on angle of incidence, thus making narrow-angle band pass filters for underwater use very difficult to design. Absorptive filters can be used to reject large portions of the visible spectrum, but the problem of in-band noise still remains since absorptive filters do not exhibit the sharp cutoff of interference filters. Sources in the range of 350 to 400 nm present a viable means to work at, or just beyond, the edge of the visible spectrum where a majority of sunlight can be rejected. Until recently, efficient, inexpensive, near ultraviolet (NUV) sources have not been readily available; but the demand for new, high power NUV light-emitting diodes (LEDs) for commercial applications has made their use in underwater applications possible. These wavelengths are not visible to the human eye thus making them good candidates for non-visible communications. The Woods Hole Oceanographic Institution's (WHOI's) Optical Communications Group has performed many field and lab experiments with short wavelengths for the purpose of wavelength division multiplexing (WDM), enhanced daylight performance, and non-visible underwater communications. Initially 405 nm LEDs were used for WDM in shallow water and then for long-range transmission tests in deep water at the Axial Seamount of the Juan de Fuca Ridge (JDR). Additional tests were performed using 405 nm and 385 nm near the surface at JDR to assess ambient daylight rejection. Recently, work has been done in very shallow water in Cape Cod Bay to assess range with high ambient light in very turbid water.