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Free-Space Laser Communication Technologies XXII

Listed below are the papers found in this volume. Click the paper title to view an abstract or to order an individual paper.

JPL in collaboration with JAXA and NICT demonstrated a 50Mb/s downlink and 2Mb/s uplink bi-directional link with the LEO OICETS satellite. The experiments were conducted in May and June over a variety of atmospheric conditions. Bit error rates of 10^-1 to less than 10^-6 were measured on the downlink. This paper describes the preparations, precursor experiments, and operations for the link. It also presents the analyzed downlink data results.

Science return and high bandwidth communications are key issues to support the foreseen endeavors on spaceflights to the Moon and beyond. For a given mass, power consumption and volume, laser communications can offer an increase in telemetry bandwidth over classical RF technology allowing for a variety of new options, like more raw scientific data being sent back to Earth where data processing can be performed on ground. Recent European activities in the field of laser communications investigated mission scenarios for deep space and within the Earth's sphere of influence. Various link topologies have been investigated, involving Lissajous orbits at Libration points of the Earth-Sun and the Moon- Earth system, and also Martian orbiters. Different types of lasercom terminal concepts have been investigated, either operating fully autonomously or being attached to dedicated telecom orbiter spacecraft. Enhanced pulse position modulation formats were tested together with tailored FEC and interleaver technology in inter-island test campaigns using ESA's optical ground station on Tenerife. The paper summarizes the findings from all activities, highlights the potential and describes synergy aspects of involved technologies, all in view using lasercom as part of an integrated RF-optical TT&C subsystem to support enhanced science return.

This paper describes optical satellite communication activities based on technology developments, which started in Europe more than 30 years ago and led in 2001 to the world-first optical inter-satellite communication link experiment (SILEX). SILEX proved that optical communication technologies can be reliably mastered in space and in 2006 the Japanese Space Agency (JAXA) joined the optical inter-satellite experiment from their own satellite. Since 2008 the German Space Agency (DLR) is operating an inter-satellite link between the NFIRE and TerraSAR-X satellites based on a second generation of laser communication technology, which will be used for the new European Data Relay Satellite (EDRS) system to be deployed in 2013.

This paper presents the results of the technology demonstration of a short distance free-space optical (FSO) communication link with 2.5 Gbps data rate. Each terminal consists of a gimbaled telescope, an acquisition and coarse tracking CCD and digital control system, a fine tracking system, a fiber-optic interface, transceivers, beacon lasers, and a bit error test set. Two different fine-tracking subsystems were set-up on these two terminals: the first one is based on a micro mechanical steering mirror developed at NICT, Japan; while the second is based on liquid crystal (nonlinear spatial light modulation mechanism), developed at CSA, Canada. Performance of the two systems, their compatibility and the future work are discussed.

This paper discusses a new concept of free-space optical (FSO) communication terminals, characterized as the mutual tracking of near field Gaussian beams and the direct single mode fiber coupling to achieve Tera-bit/seconds (Tbps) class FSO link in short range outdoor environments. It first shows the required optical configurations including beacon transmitters, tracking sensors and fast steering mirrors to construct the mutual tracking system and then discusses their capabilities and performance for this specific near field operation in which conventional tracking system is not enough to assure stable and reliable link performance. It also presents one of the simplest realizations of the new FSO terminals and their test results.

Free space laser communications provides wide bandwidth and high security capabilities to Unmanned Aircraft Systems (UAS) in order to successfully accomplish Intelligence, Surveillance, Target Acquisition, and Reconnaissance (ISTAR) missions. A practical implementation of a laser-based video communications payload flown by a small UAS aircraft is described as a proof-of-concept. The two-axis gimbal pointing control algorithm calculates the line-of-sight vector in real-time by using differential GPS and IMU information gathered from the UAS vehicle's autopilot so that the laser transmitter in the airborne payload can accurately track a ground-based photodiode array receiver with a known GPS location. One of the future goals of the project is to move from UAS-to-ground communications to UAS-to-UAS free space laser communications.

Precision ranging between planetary bodies would provide valuable scientific information, including tests of fundamental physics. Current ranging techniques based on retroreflectors, however, are limited to the Earth- Moon distance due to an inverse fourth power scaling. We present methods for interplanetary distances based on paired one-way ranging, which scales with a more favorable inverse square power. Corrections for clock offset, frequency error, and the Doppler effect are shown. We present the results of tabletop experiments demonstrating sub-millimeter ranging accuracy.

Video imagery was streamed from the ground to an aircraft using a free-space laser communication link. The link operated at 270 Mb/s over slant ranges of 5-9 km in day and night time background conditions. The experiment was designed to demonstrate autonomous link acquisition and served as a first proof-of-concept for a planetary access link between a surface asset and an orbiter at Mars. System parameters monitored during the link demonstration including acquisition and tracking and communication performance are discussed.

JPL has developed a series of software and hardware tools to analyze and record data from a 50Mb/s down and 2 Mb/s up bi-directional optical link with the LUCE terminal onboard the LEO OICETS satellite. This paper presents the data products for this experiment including the system architecture and analysis of the actual data received.

The restarted OICETS-ground laser communications experiments are introduced. The events are sequentially summarized from the launch of OICETS to the end-of-life, where the reopened experiments started from October 2008. In the period, the satellite-ground laser communications campaign with the four optical ground stations of DLR, ESA, JPL and NICT are conducted from April 2009 to September 2009. The open pointing characteristics of OICETS measured in those trials show that the performance remains almost the same as before in 2006. The average rate of the link establishments through the whole period is about 0.6 due to the weather conditions. The viewable periods of OICETS from the four ground stations are analyzed as an example. The result indicates that the satellite could be accessible once an hour from at least one of the four ground stations, which implies a possibility of a LEO satellite-ground quasi-continuous connection.

Today, coherent optical inter-satellite links are operational in LEO-LEO constellations for more than two years. With data transmitted error free at rate of 5.625 Gbps and links established within a few seconds they prove a performance well suited for commercial applications. For the first time coherent LEO-to-ground links have been built up, too. They allow to investigate the atmosphere's impact on optical space-to-ground links for later optimization of optical ground stations.

Successful pointing, acquisition, and tracking (PAT) are crucial for the implementation of laser communication links between ground and aerial vehicles. This technology has advantages over the traditional radio frequency communication, thus justifying the research efforts presented in this paper. The authors have been successful in the development of a high precision, agile, digitally controlled two-degree-of-freedom electromechanical system for positioning of optical instruments, cameras, telescopes, and communication lasers. The centerpiece of this system is a robotic manipulator capable of singularity-free operation throughout the full hemisphere range of yaw/pitch motion. The availability of efficient two-degree-of-freedom positioning facilitated the development of an optical platform stabilization system capable of rejecting resident vibrations with the angular and frequency range consistent with those caused by a ground vehicle moving on a rough terrain. This technology is being utilized for the development of a duplex mobile PAT system demonstrator that would provide valuable feedback for the development of practical laser communication systems intended for fleets of moving ground, and possibly aerial, vehicles. In this paper, a tracking system providing optical connectivity between stationary and mobile ground platforms is described. It utilizes mechanical manipulator to perform optical platform stabilization and initial beam positioning, and optical tracking for maintaining the line-of-sight communication. Particular system components and the challenges of their integration are described. The results of field testing of the resultant system under practical conditions are presented.

Precision laser beam pointing is a key technology in High Energy Laser systems. In this paper, a laboratory High Energy Laser testbed developed at the Naval Postgraduate School is introduced. System identification is performed and a mathematical model is constructed to estimate system performance. New beam pointing control algorithms are designed based on this mathematical model. It is shown in both computer simulation and experiment that the adaptive filter algorithm can improve the pointing performance of the system.

Optical access links can be used for relaying data from the surface of Mars to spacecraft orbiting Mars. In this paper considerations related to the concept of operations, link analysis and low-complexity transceiver design required for future implementation are discussed, along with the description of some prototype transceiver development that has been completed.

Laser beam propagating through the atmosphere is distorted by atmospheric turbulence and platform vibrations, leading to the reduction of received signals, beam pointing error, and bit-error rate (BER) degradation. Mathematical models of atmosphere and platform vibrations were developed simultaneously to simulate the actual laser communication. A realistic wavefront distortion generated by the Kolmogorov spectrum and McGlamery algorithm was applied with a liquid crystal spatial light modulator (SLM). A disturbance signal implemented by a two-dimensional piezoelectric steering mirror is applied to represent the platform jitter. Experimental results demonstrate how signal to noise ratio (SNR), BER and pointing error change with the increase of atmospheric turbulence strength and vibration spectrum bandwidth. This paper presents the modeling and measurement of effects of atmospheric turbulence and platform jitter. The distortion compensation and tracking techniques can be tested based on the system.

A prototype carbon nanotube actuator operated with low voltage is studied to achieve a function to tilt a mirror angle. The process to fabricate the actuator is introduced and the moving characteristics are measured. The response time of this actuator should be improved to be much faster, but the result indicates a potential of the actuator to control the tilt mirror angle.

This paper explores the architecture and design of an optically-implemented 64-ary PPM transmitter and direct-translating receiver that effectively translates incoming electrically-generated bit streams into optical PPM symbols (and vice-versa) at > 1 Gb/s data rates. The PPM transmitter is a cascade of optical switches operating at the frame rate. A corresponding receiver design is more difficult to architect and implement, since increasing data rates lead to correspondingly shorter decision times (slot times and frame times). We describe a solution in the form of a time-to-space mapping arrayed receiver that performs a translating algorithm represented as a code map. The technique for generating the code map is described, and the implementation of the receiver as a planar lightwave circuit is given. The techniques for implementing the transmitter and receiver can be generalized for any case of M-ary PPM.

We provide a summary of the classical information capacity of single-mode free-space optical communication for pure-loss channels. We compare the capacities afforded by structured transmitters and receivers to that of the ultimate communication capacity dictated by the quantum nature of light, and we draw the following conclusions. The ultimate capacity can be achieved with classical coherent states (i.e., ideal laser light), but the capacity-achieving receiver (measurement) is yet to be determined. In photon-starved pure-loss channels, binary phase modulation in combination with the optimal receiver is near-capacity achieving, and more importantly, it is superior to on-off keying with either the optimal receiver (as yet to be determined) or with a photon-counter. Heterodyne detection approaches the ultimate capacity at high mean photon numbers.

Incoherent receivers of Free Space Optical (FSO) signals have no knowledge of instantaneous channel state. Thus, the receiver requires some information about the noise and fading statistics for a maximum likelihood (ML)-based optimal detection. Using pilot-aided symbols, we develop a simple multi slot averaging (MSA) estimation technique to approximate the values of parameters required at the incoherent detector. No channel state information (CSI) is available at the receiver side and this work will not be also trying to estimate it. But the estimation of noise and fading statistics will be practically investigated. We evaluate the bit error rate (BER) performance of FSO links with MSA estimation over both Gaussian and lognormal atmospheric turbulence fading (scintillation) channels. Numerical simulation will be completed to evaluate the estimation error of the MSA estimator. We will see that at signal to noise ratio (SNR)=13dB, the performance loss of the Gaussian estimator improves from 3dB to 0.4dB when we increase the number of pilot symbols from 16 to 64. This paper also presents the hardware design of the estimator using Xilinx system generator.

JAXA has made an effort to build the next generation space data relay network. The inter-orbit optical links are essential segments for such a network in order to fulfill requirements of high resolution earth observation satellite applications and humaned space flight mission. In this paper, JAXA's R&D activities towards realizing advanced optical communication terminals are introduced. The target of the terminals is to establish the optical data relay link between LEO and GEO up to 2.5 Gbps of data-rate. The space data relay network with the terminals will provide seamless and high rate data downlink service for user spacecrafts in LEO.

We propose a solution for pointing and tracking an optical terminal using one or more beacons and a slowly varying background image. The primary application is a deep space optical communication terminal, where multiple source tracking provides robustness against beacon outage. Our solution uses optical orthogonal codes modulated on each beacon to separate the signal from each source for centroiding. This technique allows calculation of the transmit pointing vector from each beacon location as well as from the background image. The latter can be used to track during beacon outages. We present a simple algorithm for performing this separation, and apply it to experimental data from a photon-counting detector illuminated by two beacons and one constant source. Our results show that the photon flux from each source can be accurately estimated even in the low signal, high background regime. We estimate the variance of the signal estimator due to Poisson fluctuations and infer the effect on a centroid estimator for tracking.

There is demand for improved deep-space satellite communications links with increased data rates to accommodate new sensor technologies and increased sensor payloads on spacecraft. It is imperative that new solutions be compact in size, light in weight, be high speed, and highly power efficient. Optical links offer potential improvements in power, size and weight due to a substantially narrower beam and smaller components. Solutions using fiber-laser transmitter master-oscillator power-amplifiers (MOPA) have been investigated previously, but methods for improving the system power efficiency are needed. In this paper we will present methods for improving the wall-plug efficiency of fiber MOPAs for deep-space communications. A high-power, wavelength-stabilized, 1550 nm seed laser with an external modulator is used to reduce the number of amplifier stages. In addition, resonant pumping in the 1430 to 1530 nm band improves pump absorption and, hence, wall-plug efficiency. A first-stage amplifier is used in order to maximize extraction efficiency at high gain. The design targets a wall plug efficiency of 20% with more than 1 kW of peak power per pulse and over 10 W of average power. An amplifier operating at 1532 nm also has the advantage of commercial off-the-shelf components with demonstrated reliability.

A Free Space Optical (FSO) link utilizing mid-IR Interband Cascade lasers has been demonstrated in the 3-5 μm atmospheric transmission window with data rates up to 70 Mb/s and bit-error-rate (BER) less than 10^-8. The performance of the mid-IR FSO link has been compared with the performance of a near-IR link under various fog conditions using an indoor communication testbed. These experiments demonstrated the lower attenuation and scintillation advantages of a mid-IR FSO link through fog than a 1550 nm FSO link.

Deep Space Optical Communications (DSOC)) impose challenging requirements on detector sensitivity and bandwidth [1]. The current state-of-the art of high-repetition rate, high-power lasers recommends using near-infrared (NIR) 1064nm wavelengths for specific DSOC tasks [2]. Large photonic arrays with integrated beam acquisition, tracking and/or communication capabilities, and smart pixel architecture should allow the implementation of more reliable and robust DSOC systems. Integration of smart pixel technology for parallel data read, acquisition and processing is currently available in silicon. Therefore it would be desirable to monolithically integrate the photodetectors with the electronics. However, silicon has a weak absorption at 1064nm. One elegant approach to increase its absorption efficiency is to trap the photons inside the silicon using the cavity resonance effect (resonant cavity enhancement or RCE). We present in this paper the challenges of developing resonant cavity single-photon detector arrays for applications to DSOC. The metrics of the main process parameters to fabricate resonant cavity detectors is analyzed and critical process steps are developed and evaluated. We conclude that such detector arrays are feasible using current state-of-the-art CMOS technology, provided that suitable process control protocols are developed. We report a 10X performance enhancement at NIR wavelengths for the first generation of resonant cavity single-photon detector prototypes, less than 150ps timing performance in photonstarved mode and 20-30ps for multi-photon hits.

Recently, the amount of the data measured by the satellites becomes increase because the recent satellites have been higher performances. Therefore, the faster communication method is demanded. One of the solutions is to use optical communication systems. When widely available fiber optical devices are used in free-space laser communication systems, it is required to couple the laser beam into a single mode fiber. Free-space laser communications systems require the precise tracking control because of very narrow laser beams. It is rather difficult to couple the laser beam into the optical fiber owing to atmospheric turbulence in ground-to-satellite laser propagation paths. We developed a simulation model of the fiber coupling efficiency for ground-to-satellite laser communication links under atmospheric turbulence. And we developed a prototype of the fast steering mirror (FSM) which can operate at high frequencies under the atmospheric turbulence. We experimentally verify the tracking performance of FSM by using ground-to-satellite laser communication links.

The Deep Space Optical Communications Transceiver (DSOCT) was developed as a small demonstrator testbed for evaluating optical components and systems for a deep space optical communications system. The need for a low-scatter optical system derives from the requirement for the transceiver to operate to within 2 degree solar elongation angles. An experiment in which the terminal was set up on Earth and pointed near the Sun demonstrated the terminal's ability to achieve Earth-background limited operation somewhere between 2 and 5 degrees of the edge of the solar disk, depending on the Earth-radiance background assumed as the lower bound for background light and the sky radiance conditions during the experiment. Stray light analysis matches the measured scatter to within a factor of 3, and identifies the system's secondary mirror as the main source of concern.

This paper proposes a space division and time division multiple access (SD/TDMA) technique based on intensity distributions for hybrid line-of-sight (hybrid-LOS) indoor optical wireless communication system. At first, a novel signal discrimination scheme for the spatially multiplexed optical signals is proposed and it is applied to a space division multiple access (SDMA) in hybrid-LOS system. In the proposed scheme, multiple terminals simultaneously transmit their optical signals to access point (AP) using on-off-keying (OOK) modulation, and the spatially multiplexed optical signals are received by a photodetectors array (PD-array), where multiple PDs are disposed to observe the spatial intensity distribution of optical signals. Because the terminals transmit their data using OOK modulation, the spatial intensity distribution observed by the AP equipped with PD-array is subject to the data transmitted from individual terminals, and the AP can identify the terminals transmitting the optical signal by determining the transmitted intensity distribution. Of course, the transmitted intensity distributions are not orthogonal signal and the discriminability of transmitted intensity distributions is much related to the differences of intensity distributions. This implies that the number of terminals that can simultaneously access to the AP will be limited and it is determined by the differences of the candidate intensity distributions. In order to enhance the discriminability of the transmitted intensity distributions, the proposed signal discrimination scheme is further applied to the SD/TDMA. In the SD/TDMA, the discriminability required to enable SDMA is ensured by introducing a scheduling algorithm in which terminals with higher discriminatory of transmitted intensity distributions are allocated to the same time slot. Numerical results show that SD/TDMA using proposed signal discrimination scheme increases the throughput and the number of terminals that can access to the AP.

In hybrid FSO/RF (Free Space Optics/Radio Frequency) systems, the FSO link is the primary link while the RF link is the secondary (backup) link. In current hybrid systems, once the FSO signal to noise ratio (SNR) decreases above a preset threshold, the system switches from FSO to RF to maintain the communication connection. But this scenario does not provide maximum utilization of the available bandwidth in terms of spectrum efficiency. This paper proposes an adaptive FSO bit rate algorithm to maintain communication using the FSO link as long as its bit rate is greater than that can be offered by the RF, thus requiring no switching to the secondary link. The system switches to the secondary link if and only if the FSO bit rate while maintaining the system bit error rate (BER) drops below the bit rate that can be achieved by the RF link. This paper shows the dependence of FSO bit error rate on channel bit rate using analysis and computer simulation. Furthermore, computer simulation is carried out and the results confirm the algorithm's effectiveness on maintaining higher bit rate communication connection using FSO link. Hence, the overall system performance achieves better throughput.

The OCTL to OICETS Optical Link Experiment (OTOOLE) project demonstrated bi-directional optical communications between the JAXA Optical Inter-orbit Communications Engineering Test Satellite (OICETS) spacecraft and the NASA Optical Communications Telescope Laboratory (OCTL) ground station. This paper provides a detailed description of the experiment design for the uplink optical channel, in which 4 beacon lasers and 3 modulated communication lasers were combined and projected through the F/76 OCTL main telescope. The paper also describes the reimaging optical design employed on the acquisition telescope for receiving the OICETS-transmitted signal and the design of the receiver channel. Performance tests and alignment techniques of both systems are described.