Optical signal propagation through underwater channels is affected by three main degrading phenomena, namely absorption, scattering, and fading. In this paper, we experimentally study the statistical distribution of intensity fluctuations in underwater wireless optical channels with random temperature and salinity variations as well as the presence of air bubbles. In particular, we define different scenarios to produce random fluctuations on the water refractive index across the propagation path, and then examine the accuracy of various statistical distributions in terms of their goodness of fit to the experimental data. We also obtain the channel coherence time to address the average period of fading temporal variations. The scenarios under consideration cover a wide range of scintillation index from weak to strong turbulence. Moreover, the effects of beamcollimator at the transmitter side and aperture averaging lens at the receiver side are experimentally investigated. We show that the use of a transmitter beam-collimator and/or a receiver aperture averaging lens suits single-lobe distributions such that the generalized Gamma and exponentiated Weibull distributions can excellently match the histograms of the acquired data. Our experimental results further reveal that the channel coherence time is on the order of 10 −3 seconds and larger which implies to the slow fading turbulent channels.

Abstract-In this paper, we experimentally investigate the statistical distribution of intensity fluctuations for underwater wireless optical channels under different channel conditions, namely fresh and salty underwater channels with and without air bubbles. To do so, we first measure the received optical signal with a large number of samples. Based on the normalized acquired data the channel coherence time and the fluctuations probability density function (PDF) are obtained for different channel scenarios. Our experimental results show that salt attenuates the received signal while air bubbles mainly introduce severe intensity fluctuations. Moreover, we observe that log-normal distribution precisely fits the acquired data PDF for scintillation index (σ 2 I ) values less than 0.1, while Gamma-Gamma and K distributions aptly predict the intensity fluctuations for σ 2 I > 1. Since neither of these distributions are capable of predicting the received irradiance for 0.1 < σ 2 I < 1, we propose a combination of an exponential and a log-normal distributions to perfectly describe the acquired data PDF for such regimes of scintillation index.

In this paper, we realize the concept of analog computing using an array of engineered gradient dielectric meta-reflect-array. The proposed configuration consists of individual subwavelength silicon nanobricks in combination with fused silica spacer and silver ground plane realizing a reflection beam with full phase coverage 2π degrees as well as amplitude range 0 to 1. Spectrally overlapping electric and magnetic dipole resonances, such high-index dielectric metasurfaces can locally and independently manipulate the amplitude and phase of the incident electromagnetic wave. This practically feasible structure overcomes substantial limitations imposed by plasmonic metasurfaces such as absorption losses and low polarization conversion efficiency in the visible range. Using such CMOS-compatible and easily integrable platforms promises highly efficient ultrathin planar wave-based computing systems which circumvent the drawbacks of conventional bulky lens-based signal processors. Based on these key properties and general concept of spatial Fourier transformation, we design and realize broadband mathematical operators such as differentiator and integrator in the telecommunication wavelengths.

In this paper, we evaluate the end-to-end bit error rate (BER) of point-to-point underwater wireless optical communication (UWOC) systems with multi-hop transmission. To do so, we analytically derive the BER expression of a single-hop UWOC link as the building block for end-to-end BER evaluation. We also apply photon-counting method to evaluate the system BER in the presence of shot noise. Moreover, we use Gauss-Hermite quadrature formula to obtain the closed-form solutions for the system BER in the case of log-normal underwater fading channels. Our analytical treatment involves all the impairing effects of the underwater optical channel, namely absorption, scattering and fading. Numerical results demonstrate that multihop transmission by alleviating the aforementioned impairing effects of the channel, can significantly improve the system performance and extend the viable end-to-end communication distance. For example, dual-hop transmission in 22.5 m and 45 m coastal water links can provide 17.5 dB and 39 dB performance enhancement at the BER of 10 −6 , respectively.Index Terms-underwater wireless optical communications, BER performance, photon-counting approach, multi-hop transmission, serial relaying. I. INTRODUCTIONNowadays the growing interest to underwater explorations necessitates design of appropriate and efficient underwater communication methods and systems. In comparison to the traditional underwater communication method, namely acoustic communication, the optical counterpart has three interesting advantages: higher bandwidth, lower time latency and higher security. These unique features make underwater wireless optical communication (UWOC) as a powerful alternative for high speed and large data underwater communications. However, presently UWOC systems have the capability to communicate through ranges that are typically less than 100 m, which hampers their widespread usage. This impediment is mainly due to the three degrading effects in UWOC channels, i.e., absorption, scattering and turbulence which cause loss, inter-symbol interference (ISI) and fading on the propagating light wave, respectively.Although, many worthwhile researches have been carried out to investigate the performance of free-space optical (FSO) communication systems over turbulent atmosphere channels [1]- [4], study and design of appropriate UWOC systems yet have received less attention. The primary works in UWOC area have mainly focused on investigating the absorption and scattering effects of underwater optical channels [5], [6]. Meanwhile, UWOC channel impulse response has been mod-eled using Monte Carlo (MC) approach in [7]. Also a cellular UWOC network based on optical code division multiple access (OCDMA) technique has been proposed in [8] while potential applications and challenges of such a network is elaborated in [9]. Furthermore, beneficial application of serial relaying on the performance of OCDMA-based underwater users is investigated in [10].

Leveraging subwavelength resonant nanostructures, plasmonic metasurfaces have recently attracted much attention as a breakthrough concept for engineering optical waves both spatially and spectrally. However, inherent ohmic losses concomitant with low coupling efficiencies pose fundamental impediments over their practical applications. Not only can all-dielectric metasurfaces tackle such substantial drawbacks, but also their CMOS-compatible configurations support both Mie resonances that are invariant to the incident angle. Here, we report on a transmittive metasurface comprising arrayed silicon nanodisks embedded in a homogeneous dielectric medium to manipulate phase and amplitude of incident light locally and almost independently. By taking advantage of the interplay between the electric/magnetic resonances and employing general concepts of spatial Fourier transformation, a highly efficient metadevice is proposed to perform mathematical operations including solution of ordinary differential and integro-differential equations with constant coefficients. Our findings further substantiate dielectric metasurfaces as promising candidates for miniaturized, two-dimensional, and planar optical analog computing systems that are much thinner than their conventional lens-based counterparts.