This PDF file contains the front matter associated with SPIE Proceedings Volume 8971, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Gaussian beam propagation through a turbulent layer has been studied using a split-step methodology. A modified von Karman spectrum (MVKS) model is used to describe the random behavior of the turbulent media. Accordingly, the beam is alternately propagated (i) through a thin Fresnel layer, and hence subjected to diffraction; and (ii) across a thin modified von Karman phase screen which is generated using the power spectral density (PSD) of the random phase obtained via the corresponding PSD of the medium refractive index for MVKS turbulence. The random phase screen in the transverse plane is generated from the phase PSD by incorporating (Gaussian) random numbers representing phase noise. In this paper, numerical simulation results are presented using a single phase screen whereby the phase screen is located at an arbitrary position along the propagation path. Specifically, we examine the propagated Gaussian beam in terms of several parameters: turbulence strength, beam waist, propagation distance, and the incremental distance for Fresnel diffraction for the case of extended turbulence. Finally, on-axis temporal statistics (such as the mean and variance) of the amplitude and phase of the propagated field are also derived.

Optical beam spread and beam quality factor in the presence of both quartic phase aberrations and atmospheric turbulence is numerically analyzed. We obtain analytical expressions for both the mean-square beam radius and the beam quality factor using the moment method, and we compare these expressions to the results from Monte Carlo simulations, which allows us to mutually validate the theory and the Monte Carlo simulation codes. We also discuss the reason for the discrepancy between the classical approach for calculating the ensemble-averaged mean-square beam radius in a turbulent atmosphere that is described by Andrews and Phillips and by Fante versus using the moment method.

Optical feeder links will become the extension of the terrestrial fiber communications towards space, increasing data throughput in satellite communications by overcoming the spectrum limitations of classical RF-links. The geostationary telecommunication satellite Alphasat and the satellites forming the EDRS-system will become the next generation for high-speed data-relay services. The ESA satellite ARTEMIS, precursor for geostationary orbit (GEO) optical terminals, is still a privileged experiment platform to characterize the turbulent channel and investigate the challenges of free-space optical communication to GEO. In this framework, two measurement campaigns were conducted with the scope of verifying the benefits of transmitter diversity in the uplink. To evaluate this mitigation technique, intensity measurements were carried out at both ends of the link. The scintillation parameter is calculated and compared to theory and, additionally, the Fried Parameter is estimated by using a focus camera to monitor the turbulence strength.

Scintillation indices, probability distribution functions and signal spectra are measured simultaneously for spatially coherent and partially coherent optical beams propagating through various distances in an open atmosphere. The partially coherent beam is produced by coupling the broadband output of a superluminescent diode to a multimode optical fiber. A simple system to adjust the coherence radius by controlling the numerical aperture at the fiber output is implemented. Substantial reduction of the scintillation index and signal fade probability as compared to a slightly diverging coherent Gaussian beam are observed at all propagation distances studied up to 6.5 km.

Free space optical communication is impaired by atmospheric effects such as weather conditions, attenuation, scattering and turbulence. We investigate the effect of using different intensity modulation schemes on combating atmospheric turbulence. The results show that the intensity modulated beam is more resistant to turbulence with the differential pulse position scheme showing the better performance in comparison to the pulse position scheme, whereas the on-off keying scheme performs the best among the three intensity modulation schemes.

Optical links at 1.55μm are envisaged to cope with the increasing capacity demand from geostationary telecom satellite operators without the need of Radio Frequency (RF) coordination. Due to clouds blockages, site diversity techniques based on a network of Optical Ground Stations (OGS) are necessary to reach the commonly required link availability (e.g. 99.9% over the year). Evaluation of the N Optical Ground Station Network (N-OGSN) availability is based on Clouds Masks (CMs) and depends on the clouds attenuation taken in the optical communication budget link. In particular, low attenuation of high semitransparent clouds (i.e. cirrus) could be incorporated into the budget link at the price of larger or more powerful optical terminals. In this paper, we present a method for the calibration of the attenuation at 1.55 μm of high semitransparent clouds. We perform OGS localization optimization in Europe and we find that the incorporation of thin cirrus attenuation in the budget link reduces by 20% the number of handover (i.e. switches OGS) and the handover rate. It is also shown that the minimum number of station required in Europe to reach 99.9% link availability is 10 to 11. When the zone of research is enlarged the Africa, this number is reduced to 3 to 4.

The next five to ten years will see more and more free-space optical communication systems being put into practical use as technologies and techniques continue to mature, particularly in the area of mobile and satellite-to-ground communications. To meet the increasing demand of these types of systems, it is necessary to gain a deeper understanding of the various atmospheric effects at play in a free-space optical link in an effort to mitigate their impact on operational systems. In that context, the German Aerospace Center (DLR) has conducted a number of field trials between a Dornier 228 aircraft and its ground station in Oberpfaffenhofen, just south of Munich, Germany. These field trials have involved the concurrent measurement of atmospheric turbulence using three different techniques: pupil plane imaging, focus spot imaging and Shack-Hartmann wave-front sensing. To ensure the accurate synchronization of measurements between the three techniques, a concerted effort was made in the selection of computer hardware and the development of image acquisition software. Furthermore, power measurements in up- and downlink have been taken to be further correlated with the 3 primary instruments. It is envisioned that the resulting analysis of these measurements shall contribute to the implementation of new adaptive optics techniques to facilitate various air and space communication links. This paper shall describe the overall experiment design as well as some of the design decisions that led to the final experiment configuration.

The phase noise that originates in the multi-channel master-oscillator power amplifier (MOPA) system of a coherent tiled fiber-array beam director may drastically impact the efficiency of laser beam projection on a remotely located target in the atmosphere. The recently proposed near-field phase locking (NFPL) technique mitigates the MOPA-induced phase noise and gives an opportunity for programmable control of local (on-subaperture) piston and tip/tilt phases of the outgoing fiber-array beams (beamlets). In the present paper, we evaluate the influence of both NFPL and programmable phase control on the beam director performance for different laser beam propagation paths and atmospheric turbulence conditions. Our analysis is based on wave-optics numerical simulation.

JPL has a continuing program to environmentally test suitable fiber-based laser transmitters as reliable sources for optical communications from space. In lieu of the availability of fully space qualified systems, commercial pulsed fiber amplifiers either upgraded to meet the necessary environmental requirements or off-the-shelf have been tested under a variety of conditions. Three amplifiers that support high peak powers at 1550 nm have been subjected to vibration, mechanical shock, and thermal cycling tests as well as lifetime vacuum operation. The test results point to the robustness of the commercial technology and the readiness for full space qualification.

A Yb LMA fiber amplifier based laser transmitter capable of operating with high average power and high energy (~500W, 1mJ) is presented. The prototype, all-fiber, high TRL level laser transmitter is designed to meet all the single aperture requirements of an eight aperture deep space laser beacon system. The high speed FPGA controlled transmitter supports a directly modulated DFB laser and two acousto-optic modulators which are used to implement an open loop pattern dependent -pulse pre-shaping algorithm. Ultra-fast high power diode drivers are used for generating outer nested PPM modulation with Binary PPM (67usec, 33mJ pulses) and for implementing <1usec loss of signal (LOS) protection. Optical performance to be presented will include diffraction limited (M²~1.2) nested PPM optical outputs with >300W average and 9kW peak power with >70% o-o efficiency for the final power stage.

We present a new class of Erbium-doped optical fibers: the Hole-Assisted Carbon-Coated, HACC fibers. Optical fibers with this particular structure have been made by iXFiber on the basis of an appropriate choice of codopants in their core and claddings. By using an additional pre-treatment with deuterium (D2) loading authorized by the HACC structure, we highlight the efficiency of such components and demonstrated that this new type of fiber presents a strongly enhanced radiation resistance compared to the other types of erbium-doped optical fibers studied in litterature. We also built an Erbium-doped Fiber Amplifier (EDFA) with one of these HACC fibers and compared its radiation response to the one of the same fiber composition but without the HACC structure and D₂ loading. We tested the performances of this EDFA under Υ-rays and characterize its gain degradation up to doses of 315 krad. Before irradiation, the amplifier presents a gain of about 31 dB that is comparable to the optical performances of amplifiers based on HACC fibers without the D₂ pre-treatment and the HACC structure. During irradiation, our results demonstrate that the tested amplifier is nearly unaffected by radiations. Its gain slowly decreases with the dose at a slope rate of about -2.2×10^-3 dB/krad. This strong radiation resistance (enhancement of a factor of ×10 compared to the previous or conventional radiation tolerant EDFA) will authorize the use of HACC doped fibers and amplifiers for various applications in space for missions associated both with low or large irradiation doses.

Deep-Space Optical Communications is a key emerging technology that is being pursued for high data-rate communications, which may enable rates up to ten times more than current Ka-band technology. Increasing the frequency of communication, from Ka-band to optical, allows for a higher data rate transfers. However, as the frequency of communication increases, the beam divergence decreases. Less beam divergence requires more accurate and precise pointing to make contact with the receiver. This would require a three-order-of-magnitude improvement from Ka-Band (~ 1 mrad) to optical (~ 1 urad) in the required pointing. Finding an architecture that can provide the necessary pointing capability is driven by many factors, such as allocated signal loss due to pointing, range to Earth, spacecraft disturbance profile, spacecraft base pointing capability, isolation scheme, and detector characteristics. We have developed a suite of tools to 1) flow down a set of pointing requirements (Error Budget Tool), 2) determine a set of architectures capable of meeting the requirements (Pointing Architecture Tool), and 3) assess the performance of possible architecture over the mission trajectory (Systems Engineering Tool). This paper describes the three tools and details their use through the case study of the Asteroid Retrieval Mission. Finally, this paper details which aspects of the pointing, acquisition, and tracking subsystem still require technology infusion, and the future steps needed to implement these pointing architectures.

A preliminary evaluation of under-water data-rates achievable assuming near Poisson channel capacity achieving codes with pulse-position modulation and photon counting shows that to achieve 10’s to 100’s bits per second at a distance of 1 km requires 1-2m diameter collection areas with conventional blue laser transmitters and photomultiplier tubes. At distance of 200 m the data-rates increase to 6 Mb/s. A simple model is presented to show the rapid decrease in irradiance as a function of range in different types of deep-sea water where additive background noise can be neglected.

The multi-rate DPSK format, which enables efficient free-space laser communications over a wide range of data rates, is finding applications in NASA’s Laser Communications Relay Demonstration. We discuss the design and testing of an efficient and robust multi-rate DPSK modem, including aspects of the electrical, mechanical, thermal, and optical design. The modem includes an optically preamplified receiver, an 0.5-W average power transmitter, a LEON3 rad-hard microcontroller that provides the command and telemetry interface and supervisory control, and a Xilinx Virtex-5 radhard reprogrammable FPGA that both supports the high-speed data flow to and from the modem and controls the modem’s analog and digital subsystems. For additional flexibility, the transmitter and receiver can be configured to support operation with multi-rate PPM waveforms.

We describe a flexible high-sensitivity laser communication transceiver design that can significantly benefit performance and cost of NASA's satellite-based Laser Communications Relay Demonstration. Optical communications using differential phase shift keying, widely deployed for use in long-haul fiber-optic networks, is well known for its superior sensitivity and link performance over on-off keying, while maintaining a relatively straightforward design. However, unlike fiber-optic links, free-space applications often require operation over a wide dynamic range of power due to variations in link distance and channel conditions, which can include rapid kHz-class fading when operating through the turbulent atmosphere. Here we discuss the implementation of a robust, near-quantum-limited multi-rate DPSK transceiver, co-located transmitter and receiver subsystems that can operate efficiently over the highly-variable free-space channel. Key performance features will be presented on the master oscillator power amplifier (MOPA) based TX, including a wavelength-stabilized master laser, high-extinction-ratio burst-mode modulator, and 0.5 W single polarization power amplifier, as well as low-noise optically preamplified DSPK receiver and built-in test capabilities.

NASA’s Lunar Laser Communication Demonstration (LLCD) demonstrates optical communication between ground stations on Earth and NASA’s LADEE spacecraft orbiting the Moon. As a contribution to the LLCD experiment, the European Space Agency (ESA) prepares its existing optical ground station to serve as a complementary ground communication terminal. A ground receiver unit was developed for this purpose. It comprises optical detectors and electronics to recover the frames, decode the data and store them in a large data buffer. The overall ground receiver unit design is presented and explained together with ground test activities. Different optical detectors had been evaluated and two detector types were chosen for implementation. A multimode fiber is used to couple the optical signal from the existing telescope into the detectors. The 16ary-Pulse Position Modulation (PPM) signal is then processed in the electronics. Clock recovery and frame synchronization could be shown to work reliably at low power and under severe power fluctuations. For error correction, a powerful 1/2-rate Serially Concatenated PPM (SCPPM) decoder is applied. The user data is decoded at a throughput of 39Mbps with up to 20 turbo iterations. With the developed ground receiver unit an average signal power of only a few hundred picowatts is required to yield a frame error rate smaller than 10^-5. The ground receiver unit was tested for compatibility with NASA’s ground support equipment and installed in ESA’s Optical Ground Station on Tenerife. It was used in actual optical downlinks in October and November 2013. LLCD downlink results are presented and analyzed.

Free-space optical communications terminals frequently rely on optical telescopes to enhance the transmitted and received efficiency of the communication system. We have designed and patented a suite of monolithic optical telescope systems, fabricated from a single piece of transparent material. In small sizes (5 to 15 cm apertures) these designs hold promise for reducing flight terminal mass and volume, reducing risks associated with telescope alignment, and reducing costs of flight optical terminals when produced in volume. This paper presents variations of optical designs and compares their characteristics, and fabrication tolerances. Results of a prototyping effort demonstrate the feasibility of producing these elements using modern fabrication techniques.

NASA is currently developing optical communications to use with its spacecraft—both in earth-orbit and in deep space. This may allow spacecraft to use small, pencil-beam telescopes instead of large, wide-beam microwave antennas, potentially saving weight, reducing transmission power, and increasing communications bandwidth. The Earth side of such communications links will require a network of low cost, ground-based telescopes. The ground support mission mentioned above would benefit from the development of lightweight, low cost, 1 to 2 meter aperture telescopes. The key is the development of low cost, diffraction limited mirrors that cost orders of magnitude less than NASA’s current telescope mirrors, have a drastically reduced manufacturing time, with significant weight reduction (low areal density). Spin-cast epoxy mirrors do not require any grinding, polishing, or figuring and therefore have the potential for low cost, short production time, and light weight. The specially-formulated thin epoxy described here naturally forms a parabolic surface when spun at constant velocity and once it hardens, the mirror surface is ready for use except for a reflective coating. A recently produced 50cm diameter f/2 spin-cast epoxy mirror has been measured to have a 6-8 micron RMS surface figure deviation and approximately 1 nm microroughness. Other advances include the synthesis and co-polymerization of spiro orthocarbonate compounds (SOCs) to reduce chemical shrinkage and the engineering of a stiff mold to hold the curing epoxy as it spins.

Ball Aerospace & Technologies Corp. (BATC) has developed a Risley Beam Pointer (RBP) mechanism capable of agile slewing, accurate pointing and high bandwidth. The RBP is comprised of two wedged prisms that offer a wide Field of Regard (FOR) and may be manufactured and operated with diffraction limited optical quality. The tightly packaged mechanism is capable of steering a 4 inch beam over a 60° half angle cone with better than 60 μrad precision. Absolute accuracy of the beam steering is better than 1 mrad. The conformal nature of the RBP makes it an ideal mechanism for use on low altitude aircraft and unmanned aerial vehicles. Unique aspects of the opto-mechanical design include i) thermal compliance to maintain bearing preload and optical figure over a wide temperature range; and ii) packaging of a remote infrared sensor that periodically reports the temperature of both prisms for accurate determination of the index of refraction. The pointing control system operates each prism independently and employs an inner rate loop nested within an outer position loop. Mathematics for the transformation between line-of-sight coordinates and prism rotation are hosted on a 200 MHz microcontroller with just 516 KB of RAM.

Three novel multiple-beam forming methods were proposed in this paper for liquid crystal optical phased array. These three methods called sub-aperture method, array division multiplexing method, iterative Fourier transform pattern approximation method could form multiple beams simultaneously for multi-target tracking in lidar and uninterrupted communication among multiple satellites. Principles of these methods were discussed in this paper. Simulations and experiment results were given to verify the feasibility of these methods.

A terrestrial free-space optical communications network facility, named IN-orbit and Networked Optical ground stations experimental Verification Advanced testbed (INNOVA) is introduced. Many demonstrations have been conducted to verify the usability of sophisticated optical communications equipment in orbit. However, the influence of terrestrial weather conditions remains as an issue to be solved. One potential solution is site diversity, where several ground stations are used. In such systems, implementing direct high-speed optical communications links for transmission of data from satellites to terrestrial sites requires that links can be established even in the presence of clouds and rain. NICT is developing a terrestrial free-space optical communications network called INNOVA for future airborne and satellitebased optical communications projects. Several ground stations and environmental monitoring stations around Japan are being used to explore the site diversity concept. This paper describes the terrestrial free-space optical communications network facility, the monitoring stations around Japan for free-space laser communications, and potential research at NICT.

From mid-October through mid-November 2013, NASA’s Lunar Laser Communication Demonstration (LLCD) successfully demonstrated for the first time duplex laser communications between a satellite in lunar orbit, the Lunar Atmosphere and Dust Environment Explorer (LADEE), and ground stations on the Earth. It constituted the longest-range laser communication link ever built and demonstrated the highest communication data rates ever achieved to or from the Moon. The system included the development of a novel space terminal, a novel ground terminal, two major upgrades of existing ground terminals, and a capable and flexible ground operations infrastructure. This presentation will give an overview of the system architecture and the several terminals, basic operations of both the link and the whole system, and some typical results.

The growing data-rate demand on satellite communication systems has led to the increased interest in optical space communication solutions for uplinks and downlinks between satellites and ground stations. As one example for applications that benefit from higher data-rates offered by optical links, RUAG Space studied an uplink scenario from an Unmanned Aerial Vehicle (UAV) to a Geostationary Orbit (GEO), under the European Space Agency project formally known as “Optical Communications Transceiver for Atmospheric Links” (OCTAL). Particularly suitable for optical links through turbulent atmospheres are robust Pulse Position Modulation (PPM) schemes. Communication electronics using a Multi-Pulse PPM (MPPM) scheme have been developed, increasing the data-rate compared to traditional PPM at a constant peak-to-average ratio while allowing a widely configurable data-rate range. The communication system was tested together with a newly developed receiver and transmitter at a wavelength of 1055nm in a field test campaign on the Canary Islands, where the transmitter telescope was located on La Palma while the receiver was installed within the ESA Optical Ground Station on Tenerife. The nearly horizontal link between the two islands with a link distance of 142km allowed validation of relevant system performances under stringent atmospheric conditions. A data-rate of more than 360Mbps could be demonstrated using MPPM, while nearly 220Mbps could be achieved with traditional PPM, well exceeding the targeted data-rate of the studied UAV-to-GEO scenario. Following an introduction on the applied MPPM schemes, the architecture of the test setup is described, different modulation schemes are compared and the test results of this Inter-Island Test Campaign performed in October 2012 are presented.

The Optical Payload for Lasercomm Science (OPALS) system developed by the Jet Propulsion Laboratory, California Institute of Technology, will be used for optical telecommunications link experiments from the International Space Station (ISS) to a ground telescope located at Table Mountain, CA. The launch of the flight terminal is scheduled for late February 2014 with an initially planned 90-day operations period following deployment on the exterior of the ISS. The simple, low-cost OPALS system will downlink a pre-encoded video file at 50 Mb/s on a 1550 nm laser carrier using on-off key (OOK) modulation and Reed-Solomon forward error correction. A continuous wave (cw) 976 nm multibeam laser beacon transmitted from the ground to the ISS will initiate link acquisition and tracking by the flight subsystem. Link analysis along with pre-flight results of the end-to-end free-space testing of the OPALS link are presented.

The Lunar Lasercom Ground Terminal (LLGT) is the primary ground terminal for NASA’s Lunar Laser Communication Demonstration (LLCD), which demonstrated for the first time high-rate duplex laser communication between Earth and satellite in orbit around the Moon. The LLGT employed a novel architecture featuring an array of telescopes and employed several novel technologies including a custom PM multimode fiber and high-performance cryogenic photon-counting detector arrays. An overview of the LLGT is presented along with selected results from the recently concluded LLCD.

The paper describes the operations of ESA’s Optical Ground Station (OGS) during the Lunar Laser Communications Demonstration (LLCD) experiment, performed in October and November 2013 with NASA’s Lunar Atmospheric and Dust Environmental Explorer (LADEE) spacecraft. First the transmitter and receiver designs at the OGS telescope are described, which are geometrically separated to prevent cross-talk. Problems encountered and the lesson learned will be explained. As it turned the chosen arrangement was not sufficiently stable in terms of alignment and the paper will describe the solution found. A new industrial contract has been placed for improvement of the design of two solutions will be presented, which will both be tested in a follow-up laser communication campaign, scheduled for end March 2014.

The Optical Communications Telescope Laboratory (OCTL) located on Table Mountain near Wrightwood, CA served as an alternate ground terminal to the Lunar Laser Communications Demonstration (LLCD), the first free-space laser communication demonstration from lunar distances. The Lunar Lasercom OCTL Terminal (LLOT) Project utilized the existing 1m diameter OCTL telescope by retrofitting: (i) a multi-beam 1568 nm laser beacon transmitter; (ii) a tungsten silicide (WSi) superconducting nanowire single photon detector (SNSPD) receiver for 1550 nm downlink; (iii) a telescope control system with the functionality required for laser communication operations; and (iv) a secure network connection to the Lunar Lasercom Operations Center (LLOC) located at the Lincoln Laboratory, Massachusetts Institute of Technology (LL-MIT). The laser beacon transmitted from Table Mountain was acquired by the Lunar Lasercom Space Terminal (LLST) on-board the Lunar Atmospheric Dust Environment Explorer (LADEE) spacecraft and a 1550 nm downlink at 39 and 78 Mb/s was returned to LLOT. Link operations were coordinated by LLOC. During October and November of 2013, twenty successful links were accomplished under diverse conditions. In this paper, a brief system level description of LLOT along with the concept of operations and selected results are presented.

We have designed and experimentally demonstrated a radiation-hardened modem suitable for NASA’s Laser Communications Relay Demonstration. The modem supports free-space DPSK communication over a wide range of channel rates, from 72 Mb/s up to 2.88 Gb/s. The modem transmitter electronics generate a bursty DPSK waveform, such that only one optical modulator is required. The receiver clock recovery is capable of operating over all channel rates at average optical signal levels below -70 dBm. The modem incorporates a radiation-hardened Xilinx Virtex 5 FPGA and a radiation-hardened Aeroflex UT699 CPU. The design leverages unique capabilities of each device, such as the FPGA’s multi-gigabit transceivers. The modem scrubs itself against radiation events, but does not require pervasive triple-mode redundant logic. The modem electronics include automatic stabilization functions for its optical components, and software to control its initialization and operation. The design allows the modem to be put into a low-power standby mode.

Recently, we demonstrated a multi-rate DPSK modem with high-sensitivity over a wide dynamic range, which can significantly benefit performance and cost of NASA’s Laser Communication Relay Demonstration. This increased flexibility, combined with the need to verify robust operation under challenging free-space environmental conditions, results in a large number of operational states which must be accurately and thoroughly tested. To support this, we developed test and diagnostic capabilities that can be easily reconfigured to assess modem performance across a wide range of data rates and operational modes. These capabilities include internal self-test modes in which test waveforms can be directed from the transmitter into the receiver to determine modem communications performance. We used these self-test capabilities to demonstrate robust performance in realistic environments during thermal-vacuum, shock/vibration, and EMI/EMC testing.