Optical Underwater Communication: The Potential of Using Converted Green LEDs in Coastal Waters

Abstract

Compared to the open ocean where blue lightemitting diodes (LEDs) perform well for data communications, in coastal and harbor environments optical transmission becomes worse and the color of lowest attenuation shifts to green. Another problem concerns the "green-yellow gap" of LEDs, as the quantum efficiency of current commercially available green LEDs is poor. Since energy consumption is an important factor, particularly for battery-powered systems, using blue LEDs is a tradeoff. Recently, phosphor-converted green LEDs, which are pumped by highly efficient blue LEDs, have been introduced to the market, and this type of LED promises better performance. This paper compares the use of blue, green, and converted green LEDs for applications in optical underwater communication systems in coastal waters. Theoretical aspects of the overall LED-water-detector channel are presented. A method for determining individual system coefficients is developed, and the impact on the communication system performance is explained. Practical approaches are introduced, complemented by measurements in the Baltic Sea. Index Terms-Coastal waters, converted light-emitting diode (LED), green-yellow gap, optical underwater (u/w) communication.

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“…The amplitude of the optical signal received imposes limitations on transmission distance and is determined by a number of impacting factors, such as absorption and scatterance, coupled with the effects of beam spreading, multi-path interference and physical obstruction [16,17]. Issues are further exacerbated by the additive effects of background noise, often from solar irradiance [2,3,18,19]. However, there is the potential to enable systems to communicate at larger distances, as described by Wen et al where distances in excess of 500 m have been determined to be viable through simulation [20].…”

“…However, there is the potential to enable systems to communicate at larger distances, as described by Wen et al where distances in excess of 500 m have been determined to be viable through simulation [20]. The most appropriate wavelengths for underwater optical communications have been explored in detail by various studies [2,3,18,19,21,22], most of which refer to the visible blue and green ranges of the optical spectrum and also indicate that the attenuation coefficients, with respect to signal wavelength, vary depending on the optical properties of the water, as described by Sticklus et al and Mobley [16,19].…”

“…The signal amplitude received is governed by a number of factors, e.g., the inverse square rule with respect to distance, the angle of incidence and the effects of forwards and backwards scatterance; when deployed, attenuation due to turbidity of the water introduces elements of high non-linearity. These effects were described in detail by Sticklus et al [19]. Figure 1 shows a generalised model of optical transmittance reproduced from Mobley [16]; this model formalises the power losses from the incidence power Φ i (λ) due to the effects of spectral absorbance Φ a (λ) and scatterance Φ s (λ) with respect to transmittance power Φ t (λ).…”

“…A green light source was chosen over a blue one for two reasons. Firstly, in salt water, the scattering coefficient in the green spectral range of the visible electromagnetic spectrum is lower than that of the blue [17,19]. Secondly, this study was conducted in preparation for another, which features a camera with a bayer array pixel configuration [23].…”

“…It should also be noted that in the front end of the receiver there is the potential for electrical noise to be picked up as well as optical noise in the photodiode. Raw data received using no filter On inspection of the signal's rise and fall events, there are first-order effects present due to the internal capacitance of both the transmitter and receiver circuits, which was expected and also experienced by Baiden et al, Wen et al and Sticklus et al [19,20,22]. The following task was to design a means of extracting the transmitted data from the signal.…”