375-nm ultraviolet-laser based non-line-of-sight underwater optical communication

For circumventing the alignment requirement of line-of-sight (LOS) underwater wireless optical communication (UWOC), we demonstrated a non-line-of-sight (NLOS) UWOC link adequately enhanced using ultraviolet (UV) 375-nm laser. Path loss was chosen as a figure-of-merit for link performance in this investigation, which considers the effects of geometries, water turbidity, and transmission wavelength. The experiments suggest that path loss decreases with smaller azimuth angles, higher water turbidity, and shorter wavelength due in part to enhanced scattering utilizing 375-nm radiation. We highlighted that it is feasible to extend the current findings for long distance NLOS UWOC link in turbid water, such as harbor water. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement OCIS codes: (260.7190) Ultraviolet; (010.4458) Oceanic scattering; (010.7340) Water; (140.3460) Lasers. References and links 1. Z. Zeng, S. Fu, H. Zhang, Y. Dong, and J. 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Introduction
Underwater wireless communication (UWC) is of great interest to the military, industry, and the scientific communities due to various applications ranging from tactical surveillance, pipeline and environmental monitoring to oceanographic data survey, marine archaeology, and search or research missions [1,2].Yet the radio frequency (RF) communication system widely utilized for the terrestrial communication can hardly be deployed underwater due to high seawater attenuation [3].Acoustic communication is the commercially viable solution so far, due to the long propagation distance over kilometers in the water [4,5], but it is limited in data-rate by the low channel bandwidth underwater and the high latency [6,7].
Given these inherent drawbacks in RF and acoustic channels, underwater wireless optical communications (UWOC) turns out to be an appropriate solution for real-time high-data-rate communication at giga-bits-per-second over 20 meters [8][9][10].Moreover, in the industry, the underwater transmission distance has been extended over 150 m with a data rate of 12.5 Mbps, as demonstrated by Sonardyne's BlueComm 200 system.Furthermore, the advancements of low-cost and energy-efficient light sources such as LEDs [11] and diode lasers [12] enable the construction of compact miniaturized optical transceivers for data, image, and video transmission [13].
The aforementioned works mainly consider line-of-sight (LOS) configuration, which inherently imposes strict requirements on positioning, acquisition, and tracking (PAT) [14,15].In underwater conditions, the transmitted photons are scattered and absorbed by water molecules, dissolved ions, organic matters, and suspended particulates or planktonic organisms.Underwater LOS communication link may also be obstructed by hills and rocks.Both of these scenarios will lead to scintillation, deep-fading, or complete loss of signals.To this end, non-line-of-sight (NLOS) communication can be implemented to mitigate the abovementioned issues, which can be implemented either through light reflection from the water surface [16] or light scattering [17] from the molecules in the water.A diffusedreflection NLOS communication can also be implemented; for example in [18], which reported a 500 kbps, 6.6 m link.
Path loss can be adopted as a figure-of-merit to study NLOS communication channel as lower path loss indicates higher communication data rate and received signal-to-noise ratio (SNR) performances.It is affected not only by the channel geometries but also the water turbidity, transmission power, and wavelength utilized in the data transmission channel.It is noted that in the literature merely reports on simulation works, for instance those based on Monte Carlo method [19] and Henyey-Greenstein (HG) phase function [3], including impulse response [17,20], bit-error rate (BER) performance predictions [17,21] as well as the effect of channel geometries on path loss [22].Experimental data is much needed to guide the experimental design of eventual long-distance NLOS transmission.
In this paper, we demonstrate the merits of NLOS UWOC link based on 375-nm laser in which link performance can be enhanced predominantly through multiple scattering.For a comprehensive evaluation, we experimentally measured the effect of geometries, water turbidity, and wavelengths on path-loss.On the significance of the present work, we noted that previous wavelength used in UWOC are mainly in the "transparent window" (400~600 nm) for light aquatic-attenuation [23][24][25].However, to achieve both low light aquatic attenuation and high scattering for achieving a good NLOS UWOC link, we use the enhanced scattering property of 375 nm.To the best of our knowledge, this is the first experimental investigation of NLOS UWOC based on 375-nm diode laser.

Experiment details
The experimental setup is depicted in Fig. 1.The transmitter consisted of an ultraviolet diode laser (Thorlabs L375P70MLD) with a maximum output power of 70 mW and a peak wavelength of 375 nm.For cooling and heat-sinking, the laser is mounted on Thorlabs TCLDM9, which is connected to Thorlabs ITC4001 laser-diode and thermoelectric cooler (TEC) controller.Meanwhile, to demonstrate the significant advantage of 375-nm, the path loss data was compared to a 405-nm diode-pumped solid-state (DPSS) laser (Changchun New Industries MDL-III-405-500mW) with a maximum output power of 500 mW and a peak wavelength of 405 nm, whose wavelength is in the "transparent window".A series of planoconvex lenses are used for collimating the laser beam.
The laser beam propagates through the transparent glass water tank, which has a transmittance of ~94% at the wavelengths under investigation.x T Φ in Fig. 1 is the beam divergence angle, which changes in various water conditions due to different scattering effects.x T ϕ is the transmitter azimuth angle that can be changed using a rotation stage.
Besides, the transmitter and receiver are placed in a coplanar configuration.
A water tank with dimensions of length 45 cm × width 30 cm × height 35 cm is utilized.Except for the incident glass wall (45cm × 35 cm), the other inner walls of the water tank are covered using black cloth to diminish the reflection effect from the glass wall.Besides, a beam dump (Newport PL15), which is a device designed to absorb the energy of photons or other particles within an energetic beam, is installed near the back wall inside the glass tank to totally eliminate the effect of reflection.The transmission power is fixed at 50 mW during all the measurements except for the path loss measurements as a function of transmission power.Furthermore, to ensure that the scattered light is purely coming from water, a large enough blackboard was installed between the transmitter and receiver to block off any scattered light from the air.All the measurements were in a darkroom at a temperature of ~20 °C.
At the receiver end, a power meter (Newport 2936-C) and a photodetector (Newport 818-UV/DB) with wavelength response in the range of 200-1000 nm, a clear aperture of 10.3 mm, and calibration uncertainty without attenuator of ± 1% @ 350 -949 nm is utilized.x T Φ in Fig. 1 is the field-of-view (FOV) of the detector, which is 100° in our experiment.x T ϕ is the receiver azimuth angle that can be changed using a rotation stage.In natural waters, the particle size range of interest is limited in the first approximation to roughly 0.01 to 1000 μm.This range is of interest to those researchers who are concerned with the optical properties of seawater and its influence on propagation of light in the sea [26].In this investigation, the water turbidity is precisely controlled by adding commercial antacid (Maalox), which has suspension particles Al(OH) 3 and Mg(OH) 2 with the particle size in part of the above interval range, resulting in both Mie scattering (forward scattering) and Rayleigh scattering.In laboratory experiments, Maalox solution is known to present excellent scattering characteristics similar to real ocean particles [27,28].Laux et al. experimentally validated the use of Maalox as scattering agent by measuring and comparing its volume scattering function (VSF) against two different measurements in a 3 m laboratory water tank in [27] which can be used as a baseline for determining the amount of Maalox concentration needed to emulate different ocean water types [13,[29][30][31].Volume (V in μL) of Maalox was calculated and added to the 23 liters of tap water in an orderly fashion based on [27] to produce four ocean waters, namely clear sea water, coastal water, harbor I water, and harbor II water.Before adding Maalox solution to simulate different water types, the tap water was  To demon communicatio haracterized by on coefficient he added volum ure was suffici surements.
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Fig. 1 .
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