KEYWORDS: Turbulence, Ocean optics, Atmospheric propagation, Adaptive optics, Global system for mobile communications, Signal to noise ratio, Water, Laser beam propagation, Scintillation, Wave propagation
Reliable communication between aerial and undersea vehicles is a challenging issue because radio frequency signals are attenuated drastically in sea water while acoustic waves are not preferable in terrestrial links. Located in the transmittance windows of both sea water and the atmosphere, blue-green laser based free-space optical communication systems are capable of providing high speed, low latency data links for this very scenario. Apart from the absorbing and scattering attenuations in the air-water channel, another limiting factor impacting efficient laser beam propagation is the turbulence induced intensity fluctuations. Pure attenuation in sea water restricts the laser communication distance to ~100 meters, which will further reduce to ~10 meters in the presence of oceanic turbulence. Meanwhile, atmospheric turbulence can also substantially degrade the beam quality if the aerial vehicle is at high altitude. In this study, we focus our effort on the turbulence effects on beam propagation in the air-water two-stage links, not taking into account media attenuation or water surface distortions. Considering the complexity of the depth dependence of salinity and temperature in sea water and the altitude dependence of air refractive-index structure constant, we use numerical methods to simulate the beam propagation through the two-stage turbulence channel, which is modeled by discrete phase screens generated with parameterized atmospheric and oceanic turbulence power spectrums. On that basis, beam spread, area scintillation and SNR penalty at the receiver end are analyzed for the uplink as well as the downlink transmission.
The ocean surface has considerable impact on air-to-sea (or sea-to-air) imaging, lidar scanning, and optical communication. This surface is rarely smooth, of course, especially in the littoral region (due to a variety of impacts, from wind to ship wakes, etc.). Most current and previous methods for addressing this roughness and its impact on optical propagation are either fully statistical, totally theoretical, or are “mixed methods” based on a combination of statistical models and parametric-based physical models (our preferred approach). To better understand the statistical nature of the sea surface, experiments were performed in a 50 foot long wave tank capable of not only producing large scale, multi-frequency waves, but also wind driven waves over a range of velocities. High speed imaging (i.e., Photron FASTCAM Mini series(R)) of laser beam projection as well as spatial distribution of surface glint, scanned laser velocimetry measurements of the surface, and deflection statistics of the doubled Nd:YAG (532 nm) beam will all be utilized to produce statistical models of sea surface perturbations under various wind loads and larger scale wave forcing. These data, combined with our mixed model, will help us to measure, analyze, and understand the shape of the sea surface and assess its subsequent impact on optical propagation and specifically on aerial to underwater FSO communication links.
Robust and agile reconfigurable free space optical communication (FSO) links over dynamic traffic play a key role in next generation flexible wireless datacenter inter-rack networks, in terms of high throughput, dynamic robustness, cable complexity and energy efficiency. In this paper, we propose and demonstrate an agile reconfigurable 10Gbps FSO system incorporated with intelligent beam acquisition and tracking mechanism based on gimbal-less two-axis MEMS micromirror and retro-reflective film optics. Steering latency and alignment accuracy of reconfigurable FSO links are evaluated over various distances and directions, and get enhanced by intelligent adaptive acquisition and tracking schemes. Optical power loss and bit error rate of reconfigurable 10Gbps FSO links with mobility are reported over a distance of 13 m. Results show that it is feasible to use MEMS retroreflective acquisition and tracking system to achieve agile reconfigurable FSO links for flexible wireless datacenter inter-rack networks.
We explore a novel, free-space optics based approach for building data center interconnects. Data centers (DCs) are a critical piece of today’s networked applications in both private and public sectors. The key factors that have driven this trend are economies of scale, reduced management costs, better utilization of hardware via statistical multiplexing, and the ability to elastically scale applications in response to changing workload patterns. A robust DC network fabric is fundamental to the success of DCs and to ensure that the network does not become a bottleneck for high-performance applications. In this context, DC network design must satisfy several goals: high performance (e.g., high throughput and low latency), low equipment and management cost, robustness to dynamic traffic patterns, incremental expandability to add new servers or racks, and other practical concerns such as cabling complexity, and power and cooling costs. Current DC network architectures do not seem to provide a satisfactory solution, with respect to the above requirements. In particular, traditional static (wired) networks are either overprovisioned or oversubscribed. Recent works have tried to overcome the above limitations by augmenting a static (wired) “core” with some flexible links (RF-wireless or optical). These augmented architectures show promise, but offer only incremental improvement in performance. Specifically, RFwireless based augmented solutions also offer only limited performance improvement, due to inherent interference and range constraints of RF links. This paper explores an alternative design point—a fully flexible and all-wireless DC interrack network based on free-space optical (FSO) links. We call this FireFly as in; Free-space optical Inter-Rack nEtwork with high FLexibilitY. We will present our designs and tests using various configurations that can help the performance and reliability of the FSO links.
Flexible wireless datacenter networks based on free space optical communication (FSO) links are being considered as promising solutions to meet the future datacenter demands of high throughput, robustness to dynamic traffic patterns, cabling complexity and energy efficiency. Robust and precise steerable FSO links over dynamic traffic play a key role in the reconfigurable optical wireless datacenter inter-rack network. In this work, we propose and demonstrate a reconfigurable 10Gbps FSO system incorporated with smart beam acquisition and tracking mechanism based on gimballess two-axis MEMS micro-mirror and retro-reflective film marked aperture. The fast MEMS-based beam acquisition switches laser beam of FSO terminal from one rack to the next for reconfigurable networks, and the precise beam tracking makes FSO device auto-correct the misalignment in real-time. We evaluate the optical power loss and bit error rate performance of steerable FSO links at various directions. Experimental results suggest that the MEMS based beam steerable FSO links hold considerable promise for the future reconfigurable wireless datacenter networks.
KEYWORDS: Receivers, Free space optics, Transmitters, Collimators, Single mode fibers, Free space, Analog electronics, Radio optics, Switches, Modulation
We report details of an experimental demonstration involving a 15 meter pointed indoor optical wireless link in the 1550-nm wavelength range, that is comprised of a uni-directional Cable Television (CATV) signal and a bi-directional link comprised of two 10 Gbps data links. Four port wavelength division mux-demuxes have been used on both ends of the link. The CATV transmission system is connected to port 1 of the mux-demux. CATV signal consists of both analog and digital parts, and its bandwidth is 1 GHz. The laser is directly modulated by the CATV signal, and at the receiver end, the optical signal is demodulated and fed to a TV. Port 2 of the mux-demux is left unused. Ports 3 and 4 are used for the 10 Gbps links. A bit error rate tester has been used to generate the 10 Gbps signals that are converted to optical wavelengths by enhanced Small Form Factor Pluggable (SFP+) modules at both ends of the setup. Collimators are used at both ends to transmit the combined optical signal that is the output of the mux and to receive the optical signal by focusing it onto a single-mode fiber as the input of the demux. We present results on the CATV portion of the setup and the bit-error-rate performance of the two 10 Gbps links. This experiment shows the feasibility of using pointed optical links in datacenters as secondary links to alleviate the loads of highly utilized wired connections and improve the overall throughput performance of datacenters.
This paper proposes a novel indoor positioning algorithm using visible light communications (VLC). The algorithm is implemented by preinstalled light-emitting diode illumination systems. It recovers the VLC channel features from illuminating visible light and estimates receiver locations by analytically solving the Lambertian transmission equation group. According to our research, the algorithm is able to provide positioning resolution higher than 0.5 mm, in a practical indoor environment. The performance significantly exceeds conventional indoor positioning approaches using microwaves.
We propose a wide field-of-view optical receiver design based on a fisheye lens and an off-axis catadioptric structure for free-space optical communications. The design utilizes a novel fisheye lens group to compress a wide field angle into a narrow field angle and produce the appropriately collimated light that can effectively be coupled into the following aperture of a catadioptric telescope. An off-axis catadioptric telescope with aspheric surface mirrors is designed to compress the incident beam spot size, compensate for the high order optical aberrations and eliminate light loss due to an obstruction. The parallel exit rays are reflected on a double-level tracking mechanism by feeding the position signal from a quadrant detector to correct the pointing error and optimize the coupling efficiency into an optical fiber. The final wide field-of-view optical receiver design is presented along with the evaluation of optical performance results and tracking characteristics. The proposed optical receiver not only can provide a 60-deg wide field-of-view to expand the tracking range, but also mitigates optical aberrations to improve the tracking accuracy for free space optical communication systems in a turbulent atmosphere.
As Light-Emitting Diode (LED)'s increasingly displace incandescent lighting over the next few years, general
applications of Visible Light Communication (VLC) technology are expected to include wireless internet access,
vehicle-to-vehicle communications, broadcast from LED signage, and machine-to-machine communications. An
objective in this paper is to reveal the influence of system parameters on the power distribution and communication
quality, in a general plural sources VLC system. It is demonstrated that sources' Half-Power Angles (HPA), receivers'
Field-Of Views (FOV), sources layout and the power distribution among sources are significant impact factors. Based on
our findings, we developed a method to adaptively change working status of each LED respectively according to users'
locations. The program minimizes total power emitted while simultaneously ensuring sufficient light intensity and
communication quality for each user. The paper also compares Orthogonal Frequency-Division Multiplexing (OFDM)
and On-Off Keying (OOK) signals performance in indoor optical wireless communications. The simulation is carried out
for different locations where different impulse response distortions are experienced. OFDM seems a better choice than
prevalent OOK for indoor VLC due to its high resistance to multi-path effect and delay spread. However, the peak-to-average
power limitations of the method must be investigated for lighting LEDs.
The free space optical communication systems should utilize optical antennas with beam tracking mechanisms. However,
the narrow field of view and optical aberration of antennas degrade the tracking performance of the system. In order to
overcome the problems, we investigate the wide field of view optical antenna technology. The optical antenna consists of
fisheye lens, compensating lens and a catadioptric telescope with off-axis aspheric surface mirrors. The structures and
performances of the optical device elements are numerically analyzed so that their designs can make positive
contribution in enlarging the field of view and reducing the optical aberration. The final optical antenna design is
presented, along with the evaluation of optical performance and tracking characteristics. The proposed optical antenna
could not only provide a wide field of view with approximately 60 degree and expand the range for tracking mechanism,
but also mitigate the optical aberration and improve tracking accuracy of free space optical communication systems in
turbulent atmosphere.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.