Abstract
To enable high-speed underwater wireless optical communication (UWOC) in tap-water and seawater environments over long distances, a 450-nm blue GaN laser diode (LD) directly modulated by pre-leveled 16-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) data was employed to implement its maximal transmission capacity of up to 10 Gbps. The proposed UWOC in tap water provided a maximal allowable communication bit rate increase from 5.2 to 12.4 Gbps with the corresponding underwater transmission distance significantly reduced from 10.2 to 1.7 m, exhibiting a bit rate/distance decaying slope of −0.847 Gbps/m. When conducting the same type of UWOC in seawater, light scattering induced by impurities attenuated the blue laser power, thereby degrading the transmission with a slightly higher decay ratio of 0.941 Gbps/m. The blue LD based UWOC enables a 16-QAM OFDM bit rate of up to 7.2 Gbps for transmission in seawater more than 6.8 m.
Highlights
To overcome the insufficient data bandwidth of the UWAC system, Moore et al proposed a preliminary simulation to demonstrate that a microwave with tens of kilowatts power can transmit over tens of kilometers on the surface of seawater[8]
From the frequency response of the 450-nm blue laser diode (LD) shown in Fig. 2(c), it can be observed that the relaxation oscillation frequency was continuously up-shifted by increasing the bias from 65 to
By directly pre-leveling the 16-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) encoded data, the blue LD-based high-speed underwater optical communication (UWOC) system was successfully demonstrated in tap-water and seawater environments at 5.6–12.4 Gbps over 10.2–1.7 meters, and 4–7.2 Gbps over 10.2–6.8 meters, respectively
Summary
To overcome the insufficient data bandwidth of the UWAC system, Moore et al proposed a preliminary simulation to demonstrate that a microwave with tens of kilowatts power can transmit over tens of kilometers on the surface of seawater[8]. (LEDs) with lower attenuation were proposed for implementation in the UWOC system to provide a transmission data rate of up to several tens of Mbps[13,14,15,16,17] This is because blue waves exhibit the lowest attenuation coefficient among all visible wavelengths in seawater[18,19]. Previous studies have demonstrated that both the transmitted data rate and the distance of blue LD-based UWOC have potential for being upgraded through improving the bandwidths of direct modulation and detection. The OFDM subcarrier amplitude pre-emphasized data format was employed to directly modulate the blue LD at 450 nm and optimize its transmission performance to more than 10.2 m in both tap-water and seawater environments. With the reflector-folded underwater path scheme, three underwater transmitting distances of 3.4, 6.8, and 10.2 m were selected to characterize the maximal allowable transmission data rates before and after OFDM subcarrier amplitude pre-leveling
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