This PDF file contains the front matter associated with SPIE Proceedings Volume 8610, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

The Lunar Laser Communication Demonstration will be NASA's first attempt to demonstrate optical communications from a lunar orbiting spacecraft to an Earth-based ground receiver. A low SWAP optical terminal has been built and integrated onto the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft, presently scheduled to launch in 2013. LLCD will demonstrate duplex optical communications between this small space terminal and a multi-aperture photon-counting ground terminal at downlink data rates of up to 622 Mbps and uplink data rates of up to 20 Mbps. The system will also perform two-way time-of-flight measurements with the potential to perform ranging with sub-centimeter accuracy. As of the time of this conference, the Lincoln-built ground terminal has been constructed at a temporary site near Lincoln Lab nnd the two alternate ground terminals – being built by JPL and ESA – are in preparation.

We report a free space laser communication experiment from the satellite laser ranging (SLR) station at NASA Goddard Space Flight Center (GSFC) to the Lunar Reconnaissance Orbiter (LRO) in lunar orbit through the on board one-way Laser Ranging (LR) receiver. Pseudo random data and sample image files were transmitted to LRO using a 4096-ary pulse position modulation (PPM) signal format. Reed-Solomon forward error correction codes were used to achieve error free data transmission at a moderate coding overhead rate. The signal fading due to the atmosphere effect was measured and the coding gain could be estimated.

Tesat is performing inter-satellite links (ISLs) for over 5 years now. Besides the successful demonstration of the suitability of coherent laser communication for high speed data transmission in space, Tesat has also conducted two major satellite to ground link (SGL) campaigns during the last 3 years. A transportable ground station has been developed to measure the impact of atmospheric turbulence to the coherent system. The SGLs have been performed between the Tesat optical ground station and the two LEO satellites TerraSAR-X and NFIRE, both equipped with a Tesat LCT. The capability of the LCTs of measuring the signal intensity on a direct detection sensor and on a coherent sensor simultaneously makes the system unique for investigating the atmospheric distortion impacts. In this paper the main results of the SGL campaigns are presented, including BER performance for the uplink and downlink. Measured scintillation profiles versus elevation angles at different weather conditions are illustrated. Finally preliminary results of an adaptive optics system are presented that has been developed to be used in the transportable adaptive optical ground station (T-AOGS) acting as the counter terminal for the LCT mounted on Alphasat, a geostationary satellite of the European Space Agency (ESA), in autumn 2013.

Minimizing the mass and power burden of a laser transceiver on a spacecraft for interplanetary optical communications links drives requires operation in a “photon starved” regime. The relevant performance metric in the photon starved regime is Photon Information Efficiency (PIE) with units of bits per photon. Measuring this performance at the detector plane of an optical communications receiver, prior art has achieved performance levels around one bit per incident photon using pulse position modulation (PPM). By combining a PPM modulator with greater than 75 dB extinction ratio with a tungsten silicide (WSi) superconducting nanowire detector with greater than 83% detection efficiency we have demonstrated an optical communications link at 13 bits per incident photon.

A mountain-top-to-valley optical link demonstration was performed in Switzerland between Säntis mountain, 2’502m altitude, and Dübendorf airfield, 448m altitude. The link distance at very low elevation angle of 2° was 55km. Main goal was to evaluate an optical communication system for LEO-to-Ground links in realistic atmospheric conditions, though worst case, comprising the impact on data throughput and on pointing acquisition and tracking performance. Three wavelengths were tested simultaneously, a downlink at both, 1550nm and 808nm together with a 1064nm uplink, thus allowing for comparison of atmospheric transmission impact over a wide wavelength range. Alongside, all transmitters were designed to be eye-safe. The mountain top transmitter was installed inside a service building and the 60cm receiver telescope on the airfield was placed in an open stand. The link demonstration forms part of an on-going development activity started at RUAG Space with support from ESA in 2010. This activity is currently in the Engineering Model phase and aims at the Flight Model to be ready in 2016. Goal is to develop an optical downlink terminal that primarily addresses the needs of the emerging market of small satellites, the optical ground terminal and the ground network topology. The overall test approach is presented and explained together with a summary of all activities performed. Test results are presented and the discovered issues are addressed. Furthermore, a general overview is provided on the development activity and its current status.

This paper describes the design of a wide field-of-view single-mode-fiber coupled free space laser communication terminal which has mutual beacon tracking system using a commercially available fast steering mirror. Using the terminals, we tried a low power laser beam transmission experiment over 500-m distance using a 1064 nm wavelength. This paper also reports the experimental results of terminal performance as well as propagation characteristics.

Pointing, acquisition, and tracking (PAT) systems in spaceborne optical communications terminals can exploit inertial sensors and actuators to counter platform vibrations and maintain steady beam pointing. Interferometric fiber optic gyroscopes (IFOGs) can provide sensitive angle rate measurements down to very low (sub-milliHertz) mechanical frequencies, potentially reducing the required beacon power and facilitating acquisition for a spaceborne optical communications terminals. Incoherent broadband light sources are used in IFOGs to alleviate detrimental effects of optical nonlinearities, backscattering, and polarization non-reciprocity. But incoherent broadband sources have excess noise or relative intensity noise (RIN), caused by the beating of different spectral components on the photodetector. Unless RIN noise is suppressed, IFOG performance cannot be improved once the light on the photodetector exceeds one photon per coherence time (~microWatts). We propose a simple method to dramatically suppress the RIN of an incoherent light source and thereby reduce the angle random walk (ARW) of an IFOG using such a source. We demonstrate 20 dB RIN suppression of a broadband EDFA source, which we predict could improve the angle random walk (ARW) of an IFOG using this source by 12 dB.

Heritage pointing, acquisition, and tracking (PAT) systems have relied on optical tracking with a cooperative remote terminal to stabilize the line-of-sight of optical communications links. A hybrid approach, using new interferometric fiberoptic gyro (IFOG) technology to sense and correct local angular disturbances, blended with optical tracking, is shown to yield two significant advantages over traditional all-optical tracking: (1) line-of-sight stabilization over a very wide disturbance frequency range, down to extremely low frequencies (<<1 Hz), without the need for any optical signal power or cooperation from the remote terminal, and (2) a significant reduction in signal power required for the optical tracker. This paper will present fundamental performance analyses of a hybrid IFOG/optical tracking system and will derive simple design rules that the system designer can use to architect an optimal hybrid IFOG/optical PAT system. In addition, flow-down benefits that can simplify PAT system hardware will be discussed.

Improvements to a ground-based 40W 1.55 micron uplink transmitter for the Lunar Laser Communications Demonstration (LLCD) are described. The transmitter utilizes four 10 W spatial-diversity channels to broadcast 19.4 - 38.9 Mbit/s rates using a variable-duty cycle 4-ary pulse position modulation. At the lowest rate, with a 32-to-1 duty cycle, this leads to 320 W peak power per transmitter channel. This paper discusses a simplification of the transmitter that uses super-large-area single mode fiber and polarization control to mitigate high peak power nonlinear impairments.

We demonstrate highly efficient, 1.5um-fiber-amplifier, optimized for athermal and reliable operation. High efficient operation is sustained for a wide range of pulse-position-modulation (16 to 128-ary PPM) formats with pulse widths varying from 8nsec to 0.5nsec. System achieves 6W average and ~1kW peak power with 8nsec pulses and 3Ghz linewidth. Stimulated Brillion scattering is managed by use of LMA fiber in final stage and precise linewidth control while maintaining the required diffraction limited, and (PER>20dB) polarized output. System maintains performance for ambient temperatures 10-50°C.

Orthogonal On-Off Keying is a coded modulation technique, where the input digital signal is mapped into a block of orthogonal codes. The encoded data, which is in orthogonal space, modulates the laser beam by means of On-Off Keying. At the receiver, two photo cells are Cross Coupled to compensate the sunlight and other atmospheric noise. Since the laser beam is highly directional and can only be acquired by one photo cell, the input laser signal can then be received with little noise, and signal processing is made easier. These techniques are especially beneficial in high bandwidth, long distance secure laser communication applications, such as for use in Unmanned Aerial Vehicles.

Previously we have demonstrated that the orbital angular momentum (OAM) of the light beam may be measured by image transformation that maps the azimuthal to linear transverse co-ordinate (Berkhout et al 2010 Phys. Rev. Lett. 105 153601). For each input OAM state the transmitted light is focused to a different transverse position enabling simultaneous measurement over many states. We present a significant improvement to our earlier design, extending the measurement bandwidth to greater than 50 OAM states and showing simultaneous measurement of the radial co-ordinate. We further demonstrate the transformation working in reverse, potentially allowing for the rapid switching of OAM modes.

The LCRD will demonstrate optical communications relay services between a geosynchronous satellite and Earth over an extended period, and thereby gain the knowledge and experience base that will enable NASA to design, procure, and operate cost-effective future optical communications systems and relay networks. LCRD is the next step in NASA eventually providing an optical communications service on the Next Generation Tracking and Data Relay Satellites (TDRS). LCRD will demonstrate some optical communications technologies, concepts of operations, and advanced networking technologies applicable to Deep Space missions. In this paper we describe the integrated dual format (PPM/DPSK) modem testbed development and performance.

NASA’s Laser Communication Relay Demonstration (LCRD) will be NASA’s first long-duration demonstration of laser communications (lasercom) in space, providing geosynchronous-satellite-hosted bidirectional relay services between two Earth ground stations. LCRD will leverage and enhance existing ground stations. Ground Station 1 (GS-1) will leverage the Optical Communications Telescope Laboratory (OCTL) built by JPL, while Ground Station 2 (GS-2) will leverage the Lunar Laser Communications Demonstration (LLCD) Ground Terminal (LLGT) built by MIT Lincoln Laboratory. While each ground system has unique telescopes and integrated optics, many of the backend subsystems (e.g., communications, environmental monitoring, control, user simulators) will be common to both terminals. Here we provide an overview of the architecture of the LCRD ground stations, and the planned enhancements to the existing facilities.

NASA’s Laser Communication Relay Demonstration (LCRD) aims to demonstrate a geosynchronous satellite laser communications (lasercom) relay between two independent ground terminals. We report on the design of two adaptive optics (AO) techniques for LCRD Ground Station #2 (GS-2). GS-2 leverages the ground terminal developed for NASA’s Lunar Laser Communications Demonstration (LLCD). Equipping GS-2’s 40cm diameter receive telescope with AO to mitigate atmospheric turbulence effects will enable the use of single mode, optically preamplified receivers for high data-rate near-Earth relay applications. In this work a direct wavefront sensing AO approach using a Shack-Hartmann sensor and a continuous facesheet micro-electro-mechanical system (MEMS) deformable mirror (DM) was compared with an indirect sensing, hill-climbing or multidither approach using a segmented MEMS DM. Design concepts and recent experimental progress for the two approaches are presented.

The Laser Communication Relay Demonstration will feature a geostationary satellite communicating via optical links to multiple ground stations. The first ground station (GS-1) is the 1m OCTL telescope at Table Mountain in California. The optical link will utilize pulse position modulation (PPM) and differential phase shift keying (DPSK) protocols. The DPSK link necessitates that adaptive optics (AO) be used to relay the incoming beam into the single mode fiber that is the input of the modem. The GS-1 AO system will have two MEMS Deformable mirrors to achieve the needed actuator density and stroke limit. The AO system will sense the aberrations with a Shack-Hartmann wavefront sensor using the light from the communication link’s 1.55 μm laser to close the loop. The system will operate day and night. The system’s software will be based on heritage software from the Palm 3000 AO system, reducing risk and cost. The AO system is being designed to work at r0 greater than 3.3 cm (measured at 500 nm and zenith) and at elevations greater than 20° above the horizon. In our worst case operating conditions we expect to achieve Strehl ratios of over 70% (at 1.55 μm), which should couple 57% of the light into the single mode DPSK fiber. This paper describes the conceptual design of the AO system, predicted performance and discusses some of the trades that were conducted during the design process.

The NASA owned Optical Communication Telescope Laboratory (OCTL) telescope located at Table Mountain, CA is being readied as a backup ground station for the upcoming Lunar Laser Communications Demonstration (LLCD). The backup ground terminal is called the Lunar Laser OCTL Terminal (LLOT). The 1-m diameter telescope will be configured as a mono-static transceiver for transmitting a laser beacon and receiving downlink at a data-rate of 39 Mb/s. Interfaces to an operations center with near-real time exchange of monitored data at OCTL will also be developed. A system level overview of this backup ground station for LLCD will be presented.

The Lunar Laser OCTL Terminal is an auxiliary ground station terminal for the Lunar Laser Communication Demonstration (LLCD). The LLOT optical systems exercise modulation and beam divergence control over six 10-W fiber-based laser transmitters at 1568 nm, which act as beacons for pointing of the space-based terminal. The LLOT design transmits these beams from distinct sub-apertures of the F/76 OCTL telescope at divergences ranging from 110 μrad to 40 μrad. LLOT also uses the same telescope aperture to receive the downlink signal at 1550 nm from the spacecraft terminal. Characteristics and control of the beacon lasers, methods of establishing and maintaining beam alignment, beam zoom system design, co-registration of the transmitted beams and the receive field of view, transmit/receive isolation, and downlink signal manipulation and control are discussed.

The Lunar Laser Communications Demonstration Project undertaken by MIT Lincoln Laboratory and NASA’s Goddard Space Flight Center will demonstrate high-rate laser communications from lunar orbit to the Earth. NASA’s Jet Propulsion Laboratory is developing a backup ground station supporting a data rate of 39 Mbps that is based on a non-real-time software post-processing receiver architecture. This approach entails processing sample-rate-limited data without feedback in the presence high uncertainty in downlink clock characteristics under low signal flux conditions. In this paper we present a receiver concept that addresses these challenges with descriptions of the photodetector assembly, sample acquisition and recording platform, and signal processing approach. End-to-end coded simulation and laboratory data analysis results are presented that validate the receiver conceptual design.

Ground-based, narrow-band, high throughput optical filters are required for optical links from deep space. We report on the development of a tunable filter assembly that operates at telecommunication window of 1550 nm. Low insertion loss of 0.5 dB and bandwidth of 90 pm over a 2000 nm operational range of detectors have been achieved.

As diode pumped alkali lasers (DPAL) are scaled to powers exceeding 1 kW, the effects of atmospheric transmission, including thermal blooming, is explored. The cesium and rubidium lasers operate near 894 and 795 nm, respectively, in the vicinity of atmospheric water vapor absorption lines. The potassium laser line lies in the high rotational limit of the O₂ X-b (0,0) band near 770 nm. We assess the effects of atmospheric transmission on high power propagation using the High Energy Laser End-to End Operational Simulation. HELEEOS uses the scaling laws of the Scaling the High energy laser And Relay Engagements (SHaRE) toolbox which is anchored to the wave optics code WaveTrain and all significant degradation effects, including thermal blooming due to molecular and aerosol absorption, scattering extinction, and optical turbulence, are represented in the model. The HELEEOS model enables the evaluation of uncertainty in low-altitude high energy laser engagements due to all major low altitude atmospheric effects to include physically-based representations of water clouds, fog, light rain, and aerosols. Worldwide seasonal, diurnal, and geographical spatial-temporal variability in key climatological parameters is organized into probability density function databases in HELEEOS using a variety of recently available resources to include the Extreme and Percentile Environmental Reference Tables (ExPERT) for 408 sites worldwide, the Surface Marine Gridded Climatology (SMGC) database which provides coverage over all ocean areas, the Master Database for Optical Turbulence Research in support of the Airborne Laser, and the Global Aerosol Data Set (GADS) used to provide worldwide aerosol densities. The spectral transmission model is anchored to field data from an open-path Tunable Diode Laser Absorption (TDLAS) system composed of narrow band (~300 kHz) diode laser fiber coupled to a 12" Ritchey-Chrétien transmit telescope. The ruggedized system has been field deployed and tested for propagation distances of greater than 1 km. The TDLAS approach achieves a minimum observable absorbance of 0.2%, whereas an FTIR instrument is almost 20 times less sensitive.

Measurements of partially spatially coherent infra-red laser beam intensity fluctuations propagating through a hot-air turbulence emulator are compared with visible laser beam intensity fluctuations in the maritime and IR laser beam intensity fluctuations in the terrestrial environment at the United States Naval Academy. The emulator used in the laboratory for the comparison is capable of generating controlled optical clear air turbulence ranging from weak to strong scintillation. Control of the degree of spatial coherence of the propagating laser beam was accomplished using both infrared and visible spatial light modulators. Specific statistical analysis compares the probability density and temporal autocovariance functions, and fade statistics of the propagating laser beam between the in-laboratory emulation and the maritime field experiment. Additionally, the scintillation index across varying degrees of spatial coherence is compared for both the maritime and terrestrial field experiments as well as the in-laboratory emulation. The possibility of a scintillation index ‘sweet’ spot is explored.

Backscattering enhancement (BSE) effect is due to the fact that both the initial and back-scattered waves propagate through the same inhomogeneities of the refractive index. Mean value of the back-scattered intensity is higher than it would be with the same obstacle but no inhomogeneities. This effect is named backscattering enhancement (BSE) effect. Numerical modeling of lidar that based on BSE effect was carried out in Rutov-Obukhov approximation in our work. The integral equation was considered which bundles up the distribution of turbulence intensity throughout the space between the source and a scatterer. Coefficient BSE was determined as ratio of relation dispersions of radiation intensity fluctuation that scattered straight back and at an angle. BSE coefficient does not depend on the nature of scatterings in cases of aerosol or molecular scatterers. As example variants of turbulence intensity distribution C_n² between sources in form select layer or boundaries of half-space with enhanced turbulence intensity scatterers were considered. Possibility of detection the sort out the regions with enhanced turbulence intensity was showed in the case isotropic turbulence for molecular or aerosol scatterings. Inhomogeneous distribution of turbulence intensity is reliably picked out on dependence of BSE coefficient on distance between source and probing laser beam. The lidar scheme for BSE measurements with space modulation of probing beam is suggested. It allows suppressing systematic errors. Lidar allows measure BSE coefficient along with the routine lidar sensing. The dependence of BSE coefficient on the line along propagation path has considered for finite receiving aperture and finite diameter of probing laser beam. The results of modeling demonstrate that BSE measurements make it possible to remotely sort out the regions with enhanced turbulence intensity at distances determined by the maximum sensing range.

The turbulent effects from the Earth’s atmosphere degrade the performance of any optical system within it. There have been numerous studies in the effects of atmospheric turbulence on an imaging system that is pointed vertically to the sky looking at distant objects and the seeing conditions associated with it. We investigate the calculation of the seeing conditions with an imaging system pointed horizontally in terrestrial and maritime environments. We have acquired video data of different horizontal paths in the infrared wavelengths and performed data analysis that will be the basis of new characterizations and modeling of horizontal path atmospheric turbulence.

In long-range situations, the performance of free-space optical (FSO) communication links is strongly impacted by atmospheric turbulence. In this paper we compare efficiency of turbulence effects mitigation in FSO communication links using spectral (wavelength) and spatial diversity techniques. Numerical analysis of both techniques is performed considering FSO communication setting with single-mode fiber-collimator transceivers. In the case of spectral diversity setting, the fiber-collimators are based on the use of photonic crystal fibers that provide single-mode operation for three distinct wavelengths (532, 1064 and 1550nm). In the spatial diversity communication setting, analysis is performed using multiple-transceiver configurations. Analysis includes both received signal's statistical and temporal spectral characteristics.

In this paper we study the performance of an atmospheric imaging technique we referred to as digital adaptive optics. The technique consists in two major steps: (1) an optical sensor provides simultaneous measurements of the optical field wavefront phase and intensity distributions (complex field) in the system pupil, and (2) a digital processing approach is used to synthesize a compensated image from the complex field measurement. A numerical analysis of the system performance is provided for an anisoplanatic imaging scenario.

The Laser Communications Relay Demonstration (LCRD) will implement an optical communications link between a pair of Earth terminals via an Earth-orbiting satellite relay. Optical turbulence over the communication paths will cause random uctuations, or fading, in the received signal irradiance. In this paper we characterize losses due to fading caused by optical turbulence. We illustrate the performance of a representative relay link, utilizing a channel interleaver and error-correction-code to mitigate fading, and provide a method to quickly determine the link performance.