The deployment of laser communications (lasercom) in space depends upon the availability of key technologies that can support these challenging missions. The development of these technologies is important in broadening the addressable applications that lasercom can support. In addition to surviving long-term missions in a hostile environment, a premium is placed upon new techniques and devices that can reduce the on-board size, weight, and DC power consumption by the lasercom payload, while maintaining high reliability. Specific requirements for these next-generation systems are discussed here, with examples of emerging technologies that appear to be insertion candidates.

The increasing resolution of earth observation sensors will require much higher data rates for the data downlink in future than is feasible with conventional RF-technology. This applies for earth observation satellites as well as for aeronautic observation platforms, such as aircraft or stratospheric high altitude platforms. The most promising solution for this data downlink bottleneck is the application of optical free space transmission technologies. DLR has built diverse atmospheric flight terminals and performed several trials of optical downlinks from space (together with partnering organizations) as well as from atmospheric carriers in recent years. Here we present and compare results of such communication system trials.

The R&D activities and current status in NICT on space laser communications are reported, where it is shown the goal and scenario originating from the satellite-ground laser communication demonstrations with ETS-VI since 1994 for two decades. The experiences obtained in the demonstrations have been inherited to the experiments with OICETS in 2006. The experiments using the satellite are on going in 2008. Among these demonstrations, a laser terminal with combination of key technologies was experimentally produced. For the next space laser communication system, we have started the next version development technologies, on which trial manufactures are currently in progress. They have been (and will be) implemented and tested at the ground station.

A 5.6 Gbps optical communication link has been verified in-orbit. The intersatellite link uses homodyne BPSK (binary phase shift keying) and allows to transmit data with a duplex data rate of 5.6 Gbps and a bit error rate better than 10-9 between two LEO satellites, NFIRE (U.S.) and TerraSAR-X (Germany). We report on the terminal design and the link performance during the measurement campaign. As an outlook we report on the flight units adapted to LEO-to-GEO intersatellite links that TESAT currently builds and on plans to study GEO-to-ground links.

In this paper we present the design concept plus experimental results and evaluation of a newly developed advanced DWDM Radio-on-Free-Space Optical (RoFSO) communication system capable of simultaneous transmission of multiple RF signals. The RoFSO system is evaluated based on the performance metric parameters defined for the various RF signals comprising of different wireless services including terrestrial digital broadcasting signals, cellular 3GPP W-CDMA signals, IEEE 802.11 WLAN based signals etc being transmitted over the RoFSO link. The performance metric parameters being considered include standard optical received power, CNR and BER characteristics, W-CDMA signal transmission metric parameters like Adjacent Channel Leakage Ratio (ACLR) and Error Vector Magnitude (EVM), modulation error ratio (MER) for digital terrestrial television broadcasting signals as well as spectrum mask and EVM for IEEE 802.11 Wireless LAN signal transmission.

This paper shows the design and the performance of a new free-space optical communication terminal including the results of the indoor and outdoor demonstration experiments in a short link distance. To provide flexible and high-speed connectivity to the terrestrial free-space optical communications, a new compact laser communication terminal has been developed at NICT. The terminal has a feature to connect the free-space laser beam directly to single mode fiber by using a special fiber coupler to focus the free-space laser beam and couple it into the single mode fiber, fast and accurate fine tracking system and a small refractive-type telescope with diffraction limited performance. The bandwidth of the fine tracking system is more than 5 kHz using an off-the-shelf miniature Galvano mirror actuator and an analog PID controller.

Real-time monitoring allows new possibilities in applications like disaster management or traffic observation and guidance. The German Aerospace Center is currently developing an aircraft based observation system. Among other sensors a high resolution camera platform together with an optical downlink terminal is an integral part of the system. The optical terminal was tested in the first stage of expansion in November and December 2008. At distances up to 85 km the achieved mean tracking offset with pure CPA tracking was 266 μrad. Initial communication tests have been successfully performed up to a distance of 40 km.

In this paper we discuss recent progress on the implementation of a hardware free-space optical communications test-bed. The test-bed implements an end-to-end communications system comprising a data encoder, modulator, laser-transmitter, telescope, detector, receiver and error-correction-code decoder. Implementation of each of the component systems is discussed, with an emphasis on 'real-world' system performance degradation and limitations. We have demonstrated real-time data rates of 44 Mbps and photon efficiencies of approximately 1.8 bits/photon over a 100m free-space optical link.

Oerlikon Space AG has since 1995 been developing the OPTEL family of optical communications terminals. The optical terminals within the OPTEL family have been designed so as to be able to position Oerlikon Space for future opportunities open to this technology. These opportunities range from commercial optical satellite crosslinks between geostationary (GEO) satellites, deep space optical links between planetary probes and the Earth, as well as optical links between airborne platforms (either between the airborne platforms or between a platform and GEO satellite). The OPTEL terminal for deep space applications has been designed as an integrated RF-optical terminal for telemetry links between the science probe and Earth. The integrated architecture provides increased TM link capacities through the use of an optical link, while spacecraft navigation and telecommand are ensured by the classical RF link. The optical TM link employs pulsed laser communications operating at 1058nm to transmit data using PPM modulation to achieve a robust link to atmospheric degradation at the optical ground station. For deep space links from Lagrange (L1 / L2) data rates of 10 - 20 Mbps can be achieved for the same spacecraft budgets (mass and power) as an RF high gain antenna. Results of an inter-island test campaign to demonstrate the performance of the pulsed laser communications subsystem employing 32-PPM for links through the atmosphere over a distance of 142 km are presented. The transmitter of the communications subsystem is a master oscillator power amplifier (MOPA) employing a 1 W (average power) amplifier and the receiver a Si APD with a measured sensitivity of -70.9 dBm for 32-PPM modulation format at a user data rate of 10 Mbps and a bit error rate (BER) of 10^-6.

We report on a pulsed fiber based master oscillator power amplifier laser at 1550 nm to support moderate data rates with high peak powers in a compact package suitable for interplanetary optical communications. To accommodate pulse position modulation, the polarization maintaining 1 W average power laser transmitter generates pulses from 0.1 to 1 ns with variable duty cycle over a pulse repetition frequency range of 10 to 100 MHz.

Compensation of slowly varying wavefront aberrations of low-cost meter-scale optical communication receiver mirror systems is reported. Our goal is to reduce the surface wavefront error of a low-cost large aperture telescope mirror from multiple waves to about 1-wave or less peak-to-valley (P-V). Both spatial light modulators and monolithic deformable mirrors are applied in our active optical compensation systems. Spatial light modulators with only one wave stroke can compensate for aberrations of over 10-waves P-V by using 2π phase wraps, while deformable mirrors are limited to 10- wave aberrations (P-V), due to stroke limitations of commercially available systems. We demonstrate aberration compensations of a low-cost 0.6-m diameter mirror using a deformable mirror and a 0.3-m diameter telescope using a spatial light modulator. Upon aberration compensation, image quality is recovered close to the original, with the residual distortion limited by the diffraction efficiency. The laser beam spot size is reduced by a factor of four, and the measured received signal level is increased ten-fold.

We have fabricated high-speed Interband Cascade lasers and provided the first experimental evidence that these devices can be directly modulated at a frequency of 3.2 GHz and above. This work has demonstrated suitability of IC lasers as a mid-IR light source for multi-GHz free space optical communications links.

We have validated an autonomous acquisition scheme that is critical for achieving data transfer over proximity links with ranges up to a few thousand kilometers. The sun-illuminated International Space Station (ISS) against a dark sky background during terminator passes over Southern California was used to validate the autonomous acquisition and tracking scheme. A root mean square (rms) accuracy of 83 μrad was achieved.

Mechanical resonators have been extensively used to provide vibration isolation for ground based, airborne, and spaceborne payloads. At low frequency, the effectiveness of these isolation systems is determined mainly by designing a mechanical oscillator with the lowest resonant frequency achievable. The Low Frequency Vibration Isolation Platform (LFVIP) reduces the resonant frequency of the mechanical oscillators into the sub-hertz region to maximize the passive isolation. This mechanical system, which has been expressly designed to isolate spacecraft vibrations from a compact deep space optical communication terminal, is based on the Stewart platform topology. Furthermore, the LFVIP provides tip/tilt functionality for acquisition and tracking of an optical beacon signal. An active control system is used for the DC positioning of the platform and the damping of the resonance of the mechanical oscillator. A summary of the LFVIP system, including analysis design, and preliminary results is presented.

A canonical deep space optical communications transceiver which makes synergistic use of advanced technologies to reduce size, weight, power and cost has been designed and is currently under fabrication and test. This optical transceiver can be used to retire risks associated with deep space optical communications on a planetary pathfinder mission and is complementary to ongoing lunar & access link developments. Advanced technologies being integrated into this transceiver include use of a single photon-sensitive detector array for acquisition, tracking and communications; use of two-photon absorption for transmit beam tracking to vastly improve transmit/receive isolation; and a sub-Hertz break frequency vibration isolation platform is used to mitigate spacecraft vibration jitter. This article will present the design and current test results of the canonical transceiver.

Deep space optical communication transceivers must be very efficient receivers and transmitters of optical communication signals. For deep space missions, communication systems require high performance well beyond the scope of mere power efficiency, demanding maximum performance in relation to the precious and limited mass, volume, and power allocated. This paper describes the opto-mechanical design of a compact, efficient, functional brassboard deep space transceiver that is capable of achieving Mb/s rates at Mars ranges. The special features embodied to enhance the system operability and functionality, and to reduce the mass and volume of the system are detailed. System tests and performance characteristics are described in detail. Finally, lessons learned in the implementation of the brassboard design and suggestions for improvements appropriate for a flight prototype are covered.

In this paper, Multiple-Input Multiple-Output (MIMO) technology is applied to diffuse free-space optical (DFSO) links. We compare the theoretical BER performance of simulated MIMO and Single-Input Single-output (SISO) optical links in an indoor office environment. An iterative site-based simulation tool is used to determine the impulse response of wireless infrared (IR) channels for specified locations within a room. For our purposes, we use a MIMO 4x4 orthogonal space-time block code. Using this scheme a BER calculation is done based on received signal power and the corresponding channel gains. By setting a BER threshold within which the system can operate, we are able to see the coverage area provided by MIMO and SISO DFSO system architecture. We simulate a stationary transmitter while the receiver is moved through 735 different locations in the room, resulting in a BER contour plot of the system for a specified room. Simulation results show that by using 4-element arrays at both ends of the link, along with space-time block coding techniques, allows the effective coverage area to be increased by approximately 4 times. Also, when operating with a BER threshold of 10^-3, the MIMO architecture requires up to 15dB less signal power than the SISO architecture to remain below the threshold. An optical testbed is used to begin hardware validation of our theory, both with and without optical orthogonal frequency division multiplexing (OFDM) techniques. We provide initial measurement results for the proposed optical system.

High-resolution active laser ranging systems for Moon, Mars and beyond are analyzed. Both stand-alone laser-ranging transponders, and laser-communications systems configured to provide millimeter-level ranging data are analyzed. It is shown that a combined dual-function laser-communications and laser-ranging system is feasible.

We describe the performances obtained with the latest developments of voice-coils deformable mirrors for the correction of atmosphere turbulence. Thanks to the electro-magnetic principle of the deformable mirror, very large strokes are obtained (more than 20μm) with a very large bandwidth (1 kHz). We further present the ALPAO Core Engine which is an open and flexible environment allowing fast developments of high performances adaptive optics. We emphasize all the benefits for free space optical communication.

Pseudo-noise (PN) modulation techniques have been used to modulate lasers for time of flight measurements and for profiling atmospheric backscatter. Compared to mono-pulse techniques, PN code modulation allows timing and backscatter measurements to be made with much lower laser peak powers and higher duty cycles. We have developed a new modified version of PN code modulation for laser measurements in which the laser pulse width and position may also be varied within the PN code bit interval. These allow more degrees of freedom for optimizing laser measurements. We have demonstrated these in laboratory measurements using a 974 nm laser diode, return to zero (RZ) PN code modulation at various laser pulse duty cycles, and a photon counting receiver. The results show significant improvements in the receiver signal to noise ratio, sensitivity, and ranging precision for the lower duty cycle RZ PN codes.

Negative Avalanche Feedback photon counting detectors with near-infrared spectral sensitivity offer an alternative to conventional Geiger mode avalanche photodiode or phototube detectors for free space communications links at 1 and 1.55 microns. These devices demonstrate linear mode photon counting without requiring any external reset circuitry and may even be operated at room temperature. We have now characterized the detection efficiency, dark count rate, after-pulsing, and single photon jitter for three variants of this new detector class, as well as operated these uniquely simple to use devices in actual photon starved free space optical communications links.

NASA is developing technology for 64-ary PPM using relatively large PPM time slots (10 ns) and relatively simple electronic-based receiver logic. In this paper we describe photonics-based receiver options for the case of much higher data rates and inherently shorter decision times. The receivers take the form of virtual (array or quadrant) arrays with associated comparison tests. Previously we explored this concept for 4-ary and 16-ary PPM at data rates of up to 10 Gb/s. The lessons learned are applied to the case of 64-ary PPM at 1.25 Gb/s. Various receiver designs are compared, and the optimum design, based on virtual arrays, is evaluated using numerical simulations.

As a part of the EU-FP7 R&D programme, the OMEGA project (hOME Gigabit Access) aims at bridging the gap between wireless terminals and wired backbone network in homes, providing high bit rate connectivity to users. Beside radio frequencies, the wireless links will use Optical Wireless (OW) communications. To guarantee high performance and quality of service in real-time, our system needs techniques to approximate the Bit Error Probability (BEP) with a reasonable training sequence. Traditionally, the BEP is approximated by the Bit Error Rate (BER) measured by counting the number of errors within a given sequence of bits. For small BERs, required sequences are huge and may prevent real-time estimation. In this paper, methods to estimate BER using Probability Density Function (PDF) estimation are presented. Two a posteriori techniques based on Parzen estimator or constrained Gram-Charlier series expansion are adapted and applied to OW communications. Aided by simulations, comparison is done over experimental optical channels. We show that, for different scenarios, such as optical multipath distortion or a well designed Code Division Multiple Access (CDMA) system, this approach outperforms the counting method and yields to better results with a relatively small training sequence.

Free Space Optical (FSO) communications is the only viable solution for creating a three-dimensional global communications grid of inter-connected ground and airborne nodes. The high amount of data exchange between satellites and ground stations demands enormous capacity that can not be provided by strictly regulated, scarce resources of the Radio Frequency (RF) spectrum. Free Space Optical (FSO) communications, on the other hand, has the potential for providing virtually unlimited bandwidth. Furthermore, due to the spatial confinement of laser beams, such links are very secure. In other words, security is guaranteed at the physical layer. However, the promised enormous data rates are only available under clear weather conditions, and atmospheric phenomena such as clouds, fog, and even turbulence can degrade the performance, dramatically. While scattering media such as clouds and aerosols cause pulse broadening in space and time, turbulence presents itself as scintillation and fading. Hence, to exploit the great potential of FSO at its best under all weather conditions, prudent measures must be taken in the design of transmitter and receiver. More specifically, multiple transmitters and receivers can be used to de-correlate the turbulence induced fading and to compensate for pulse attenuation and broadening caused by scattering. In this paper, Multiple-Input Multiple-Output (MIMO) transmitter and receiver designs in FSO communications are investigated and the achievable performance improvements are discussed.