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• Chinese Optics Letters
• Vol. 17, Issue 10, 100013 (2019)
Mohammed Sait, Xiaobin Sun, Omar Alkhazragi, Nasir Alfaraj, Meiwei Kong, Tien Khee Ng, and Boon S. Ooi*
Author Affiliations
• Photonics Laboratory, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia
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Abstract

Conventional line-of-sight underwater wireless optical communication (UWOC) links suffer from huge signal fading in the presence of oceanic turbulence due to misalignment, which is caused by variations in the refractive index in the water. Non-line-of-sight (NLOS) communication, a novel underwater communication configuration, which has eased the requirements on the alignment, is supposed to enhance the robustness of the UWOC links in the presence of such turbulence. This Letter experimentally and statistically studies the impact of turbulence that arises from temperature gradient variations and the presence of different air bubble populations on NLOS optical channels. The results suggest that temperature gradient-induced turbulence causes negligible signal fading to the NLOS link. Furthermore, the presence of air bubbles with different populations and sizes can enhance the received signal power by seizing the scattering phenomena from an ultraviolet 377 nm laser diode.

When it comes to practical implementation, the increasing demands for high throughput and high data rates in underwater communication channels have led to a plethora of intensive studies that are focused on finding alternative solutions to deficiencies in acoustic and radio frequency wireless communication technologies. Underwater wireless optical communication (UWOC) is a viable approach to providing better data security, higher data transfer bandwidths, lower time latencies, and robust Doppler spread. UWOC opens a wide range of potential applications, ranging from offshore oil rig and pipeline monitoring to submarine communications. However, UWOC channels are optically quite challenging because they suffer from many difficulties, such as undesired absorption, scattering, and turbulence effects, which can induce fading on the propagation path of a signal. These issues limit the propagation distances to around 100 m[1]. When a signal is prematurely absorbed, the photon energy is converted into other forms of energy, such as heat or kinetic energy, which interact with water molecules or other suspended particles. By contrast, scattering is a process during which photons deviate from their original path because of collisions with water molecules or particles. Because the obstructing particles can be smaller in size compared to the wavelength of the transmission radiation, Rayleigh scattering may occur. To the contrary, scattering from particles that are comparable to the incident propagation wavelengths ($λ$) can result in Mie scattering. The addition of absorption and scattering coefficients $γa(λ)$ and $γb(λ)$, respectively, produces the extinction coefficient, which quantifies the total attenuation in the amount of radiation that is passing through a medium[2]: $γt(λ)=γa(λ)+γb(λ).$

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Mohammed Sait, Xiaobin Sun, Omar Alkhazragi, Nasir Alfaraj, Meiwei Kong, Tien Khee Ng, Boon S. Ooi. The effect of turbulence on NLOS underwater wireless optical communication channels [Invited][J]. Chinese Optics Letters, 2019, 17(10): 100013