Underwater wireless communications can be carried out through acoustic, radio frequency (RF), and optical waves. Compared to its bandwidth limited acoustic and RF counterparts, underwater optical wireless communications (UOWCs) can support higher data rates at low latency levels. However, the severe aquatic channel conditions (e.g., absorption, scattering, turbulence, etc.) pose great challenges for UOWCs and significantly reduce the attainable communication ranges, which necessitates efficient networking and localization solutions. Therefore, we provide a comprehensive survey on the challenges, advances, and prospects of underwater optical wireless networks (UOWNs) from a layer by layer perspective which includes: (1) Physical layer issues including propagation characteristics, channel modeling, and modulation techniques (2) Data link layer problems covering link configurations, link budgets, performance metrics, and multiple access schemes; (3) Network layer topics containing relaying techniques and potential routing algorithms; (4) Transport layer subjects such as connectivity, reliability, flow and congestion control; (5) Application layer goals, and (6) Localization and its impacts on UOWN layers. Finally, we outline the open research challenges and point out the prospective directions for underwater optical wireless communications, networking, and localization studies.

/pdf/underwater-optical-wireless-communications-networking-and-4b2y1mfv1m.pdf

Underwater Optical Wireless Communications, Networking, and Localization: A Survey

Resonant waveguide gratings (RWGs), also known as guided mode resonant (GMR) gratings or waveguide-mode resonant gratings, are dielectric structures where these resonant diffractive elements benefit from lateral leaky guided modes from UV to microwave frequencies in many different configurations. A broad range of optical effects are obtained using RWGs such as waveguide coupling, filtering, focusing, field enhancement and nonlinear effects, magneto-optical Kerr effect, or electromagnetically induced transparency. Thanks to their high degree of optical tunability (wavelength, phase, polarization, intensity) and the variety of fabrication processes and materials available, RWGs have been implemented in a broad scope of applications in research and industry: refractive index and fluorescence biosensors, solar cells and photodetectors, signal processing, polarizers and wave plates, spectrometers, active tunable filters, mirrors for lasers and optical security features. The aim of this review is to discuss the latest developments in the field including numerical modeling, manufacturing, the physics, and applications of RWGs. Scientists and engineers interested in using RWGs for their application will also find links to the standard tools and references in modeling and fabrication according to their needs.

/pdf/recent-advances-in-resonant-waveguide-gratings-17dyntevmn.pdf

Recent Advances in Resonant Waveguide Gratings

http://ieeexplore.ieee.org/iel7/7569814/7569815/07569878.pdf

Electromagnetic Shielding

In this work we experimentally demonstrated an underwater wireless optical communications (UWOC) link over a 2.96 m distance with two 445-nm fiber-pigtailed laser diodes employing Orbital Angular Momentum (OAM) to allow for spatial multiplexing. Using an on-off keying, non-return-to-zero (OOK-NRZ) modulation scheme, a data rate of 3 Gbit/s was achieved in water with an attenuation coefficient of 0.4128 m−1 at an average bit error rate (BER) of 2.073 × 10−4, well beneath the forward error correction (FEC) threshold.

Multi-gigabit/s underwater optical communication link using orbital angular momentum multiplexing

To increase system capacity of underwater optical communications, we employ the spatial domain to simultaneously transmit multiple orthogonal spatial beams, each carrying an independent data channel. In this paper, we show up to a 40-Gbit/s link by multiplexing and transmitting four green orbital angular momentum (OAM) beams through a single aperture. Moreover, we investigate the degrading effects of scattering/turbidity, water current, and thermal gradient-induced turbulence, and we find that thermal gradients cause the most distortions and turbidity causes the most loss. We show systems results using two different data generation techniques, one at 1064 nm for 10-Gbit/s/beam and one at 520 nm for 1-Gbit/s/beam; we use both techniques since present data-modulation technologies are faster for infrared (IR) than for green. For the 40-Gbit/s link, data is modulated in the IR, and OAM imprinting is performed in the green using a specially-designed metasurface phase mask. For the 4-Gbit/s link, a green laser diode is directly modulated. Finally, we show that inter-channel crosstalk induced by thermal gradients can be mitigated using multi-channel equalisation processing.

/pdf/orbital-angular-momentum-based-space-division-multiplexing-1ggd1x2wan.pdf

Orbital Angular Momentum-based Space Division Multiplexing for High-capacity Underwater Optical Communications.

We report on a Tm:fiber master oscillator power amplifier (MOPA) system producing 109 W CW output power, with >15 dB polarization extinction ratio, sub-nm spectral linewidth, and M2 <1.25. The system consists of polarization maintaining (PM) fiber and PM-fiber components including tapered fiber bundle pump combiners, a single-mode to large mode area mode field adapter, and a fiber-coupled isolator. The laser components ultimately determine the system architecture and the limits of laser performance, particularly considering the immature and rapidly developing state of fiber components in the 2 μm wavelength regime.

Integrated Tm:fiber MOPA with polarized output and narrow linewidth with 100 W average power

A narrowband mid-infrared (mid-IR) guided-mode resonance filter (GMRF) is designed and fabricated using a Hafnium Dioxide film/quartz wafer material system. The fabricated GMRF is then integrated into an erbium (Er)-doped Zr-Ba-La-Al-Na (ZBLAN) fluoride glass fiber laser as a wavelength selective feedback element. The laser operated at 2782 nm with a line-width less than 2 nm demonstrates the viability of GMRFs for wavelength selection in the mid-IR.

Guided-Mode Resonance Filters for Wavelength Selection in Mid-Infrared Fiber Lasers

A guided-mode resonance filter is designed and fabricated for externally locking a GaN blue laser diode. The external mirror design is polarization selective which allows the reflectivity to be tuned, in order to optimize the output coupling of the cavity. The resonance demonstrates a line-width <0.5 nm, centered at 445.6 nm with an output power in excess of 0.5 W of CW power without temperature control.

Blue Laser Diode Wavelength Selection With a Variable Reflectivity Resonant Mirror

Space division multiplexing of optical beams has recently been demonstrated for improving the bandwidth of optical communication links. This paper will explore the use of space division multiplexing utilizing blue lasers for potential undersea applications. Experimental results will be shown for optical vortices utilizing a range of charge numbers corresponding to various Orbital Angular Momentum states.

Spatial multiplexing for blue lasers for undersea communications

We designed, fabricated, and tested, polarization selective, graded-reflectivity resonant filters; based on a radial-gradient spatially-distributed, guided-mode resonance device architecture. The demonstrated filters have polarized spectral-resonance responses, distributed across their aperture extent, in the range between 1535nm and 1540nm wavelengths. Spectral sensitivity was observed on device tests, for wavelength changes as low as 0.2nm. Using multiple lithographic exposures and biasing exposure methods, the devices were engineered to have a sub-aperture region, with no hard boundaries or diffraction anomalies.

Aaron J. Pung

Papers

Integrated Tm:fiber MOPA with polarized output and narrow linewidth with 100 W average power

Guided-Mode Resonance Filters for Wavelength Selection in Mid-Infrared Fiber Lasers

Blue Laser Diode Wavelength Selection With a Variable Reflectivity Resonant Mirror

Spatial multiplexing for blue lasers for undersea communications

Polarization selective, graded-reflectivity resonance filter, using a space-varying guided-mode resonance structure.