This PDF file contains the front matter associated with SPIE Proceedings Volume 7923, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

The Lunar Laser Communications Demonstration (LLCD), a project being undertaken by MIT Lincoln Laboratory and NASA's Goddard Space Flight Center, represents NASA's first attempt to demonstrate optical communications from a lunar orbiting spacecraft to an Earth-based ground receiver. The LLCD space terminal will be flown on the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft, presently scheduled to launch in 2013. LLCD will demonstrate downlink optical communications at rates up to 620 Mbps, uplink optical communications at rates up to 20 Mbps, and two-way time-of-flight measurements with the potential to perform ranging with sub-centimeter accuracy. We describe the objectives of the LLCD program and discuss key technologies employed in the space and ground terminals.

Since February 2008 coherent laser communication links are operational in-orbit. Transmitting data at a rate of 5.625 Gbps they verify the capability of laser communication exemplarily in LEO-LEO and Ground-LEO constellations. The LEO-LEO links run with a bit error rate of 10^-11. Acquisition typically is closed within seconds. On the basis of these results laser communication terminals are adapted to LEO-GEO links with a still scalable user data rate of 1.8 Gbps. The terminals will be applied in the European data relay system.

As military sensors and systems become more sophisticated, tactical situations will require reliable, high data rate communications. The current RF communication systems are increasingly competing for the limited amount of RF spectrum and bandwidth. One possible way to augment the current RF communication systems is by the use of free space lasercomm in tactical networks for links in which direct line of sight is possible. Free space lasercomm has been demonstrated over horizontal distances greater than 10 nautical miles and at data rates greater than 1 gigabit/sec. Lasercomm links do not require any RF frequency allocation, nor do they have an RF signature. They are inherently low probability of intercept and detection and they are very difficult to jam due to the very narrow divergence of the communication beams and the very narrow acceptance angle of the receivers.^1-6 The U.S. Naval Research Laboratory has demonstrated the use of free space lasercomm in tactical networks at Trident Spectre 2009 and Empire Challenge 2010. This paper will discuss these lasercomm demonstrations and present packet error rate test data captured at both.

Power-efficient modulation schemes such as PPM are widely adopted in deep-space optical communications. In order to achieve multi-gigabit transmission, the usage of large number of time slots in PPM is needed that imposes stringent requirements on system implementation. In this paper, we propose the use of orbital angular momentum (OAM) based LDPC-coded PPM as a means to satisfy high-bandwidth demands of future interplanetary communications while keeping system cost and power consumption reasonably low. Because OAM eigenstates are orthogonal, an arbitrary number of bits/photon can be transmitted. The main challenge for OAM based deep-space communication represents the link between a spacecraft probe and the Earth station because in the presence of atmospheric turbulence the orthogonality between OAM states is not longer preserved. We show that the proposed OAM based LDPC-coded PPM can operate under strong turbulence regime when used in combination with receiver spatial diversity.

Code/pulse-position-swapping (CPPS) is a communications scheme that substitutes pulse-position-modulation (PPM) symbols with optical-code-division-access (O-CDMA) codes. CPPS retains the multiple bits per symbol communication of M-ary PPM and the asynchronous multiple access of O-CDMA. Additionally, CPPS has the advantages of granular communications, common electrical bandwidth for all users independent of data rates, compatibility with free-space or guided (fiber and waveguide) communication links, and compatibility with intensity modulation/direct detection. The transmitted symbols (codes) of CPPS are translated from a deserialized bit stream that has been divided into words of length log₂M. Thus the receivers associate the detected symbol with the original bit sequence by means of an electronically implemented look-up-table (LUT). This paper describes the architecture and design of a direct translating receiver based on map-coding, which uses optical processing to output the transmitted bit sequence without the need for a LUT. Analyses and computations characterize the receiver concept in terms of bit errors (mistranslations).

The space terminal modem for the Lunar Laser Communications Demonstration (LLCD) provides duplex lasercom capabilities between the Earth and a satellite in lunar orbit with a 0.5-W optical transmitter delivering downlink data rates of 39-620 Mbps and an optically-preamplified direct detection receiver supporting uplink data rates of 10-19 Mbps. The modem consists of four subsystem modules: digital electronics, analog electronics, power conditioning, and electro-optics. This modular approach permits subsystems to be built and tested in parallel and provides design flexibility to address evolving requirements. Other important design considerations for the modem include the utilization of commercial-off- the-shelf (COTS) components to reduce delivery time, cost, minimization of size, weight, and power, and the ability to survive launch conditions and operate over a broad temperature range in lunar orbit.

Generation of chaos from nonlinear optical systems with an optical or electronic feedback has been studied for several years. Such chaotic signals have an important application in providing secure encryption in free-space optical communication systems. Lyapunov exponent is an important parameter for analysis of chaos generated by a nonlinear system. The Lyapunov exponent of a class of a nonlinear optical system showing a nonlinear transfer characteristics of the form sin²(x) is determined and calculated in this paper to understand the dependence of the chaotic response on the system parameters such as bias, feedback gain, input intensity and initial condition exciting the optical system. Analysis of chaos using Lyapunov exponent is consistent with bifurcation analysis and is useful in encrypting data signal.

Pseudo Random Noise (PN) coded laser radar can improve the target detection ability without the demand on high power laser. However, the reflected echoes are generally so weak that they are buried in the thermal noise of the receiver, which raises the problem of choosing an optimal threshold for correctly decoding them since the power of echoes varies from time to time, and the voltage of light generated electrical signal by photo diode (PD) is always positive. In this work, we firstly show the problem we are going to discuss. Then, a novel method basing on Inter Symbol Interference (ISI) is proposed for solving the problem. Next, numerical simulations and experiments are performed to validate the method. Finally, we discuss the obtained results theoretically.

JAXA has made efforts 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 (such as Advanced Land Observing Satellite (ALOS) follow-on missions by JAXA) and manned space flight missions. JAXA's R&D activities for advanced optical communication terminals are introduced. The target of the terminals is to establish the optical data relay link between the LEO user satellite and the GEO data relay satellite up to 2.5 Gbps of data-rate. JAXA has started the development of a Bread Board Model (BBM) of the terminal in order to evaluate the feasibility of the terminal. The terminal is aimed to be small and light-weighted, which is helpful for an onboard capability of the LEO satellite. Furthermore, the modulation of carrier and the acquisition and tracking sequence are selected in order to achieve the interoperability of optical space communication systems. We recently study the feasibility of the acquisition and tracking sensor, the waveguide high power amplifier for a transmitter and the homodyne coherent receiver etc. in the development of BBM.

A conceptual design study titled Deep-space Optical Terminals was recently completed for an optical communication technology demonstration from Mars in the 2018 time frame. We report on engineering trades for the entire system, and for individual subsystems including the flight terminal, the ground receiver and the ground transmitter. A point design is described to meet the requirement for greater than 0.25 Gb/s downlink from the nearest distance to Mars of 0.42 AU with a maximum mass and power allocation of 40 kg and 110 W. Furthermore, the concept design addresses link closure at the farthest Mars range of 2.7 AU. Maximum uplink data-rate of 0.3 Mb/s and ranging with 30 cm precision are also addressed.

KW level fiber based MOPA laser system at 1064nm for uplink deep space communication is developed and characterized. System achieves 11.5 kW peak power (600W average power) at 500 kHz (5% duty cycle) with >70% optical conversion efficiency. Experiments using 16-ary PPM format are presented where without pre-pulse shaping >±60% pulse energy variation is observed. Gain dynamics is identified as main source of pulse to pulse energy fluctuation. Novel, FPGA implementable open loop pulse shaping algorithm is developed and demonstrated. Resulting pulse energy statistics are reported, where <±7% pulse energy variation is achieved for 90% of pulses.

We are presenting some new aspects of detection of absolute time position of picosecond laser pulses. The ground-space optical links for precise and accurate time scale synchronization are running or preparing in frame of several space agencies. We are involved in several projects in position of single photon detector and timing designer. The typical optical link consists of ground segment - picosecond laser pulse transmitter, telescope, single photon avalanche detector, timing electronics, and time reference; and space segment - corner cube retroreflectors, single photon sensitive optical receiver with event timer board, and time reference to be synchronized. To ensure absolute time position we have to calibrate absolute internal delay of several semiconductor photodiodes in picosecond range detecting picosecond laser pulses on several wavelengths. The experimental results demonstrating unobvious dependence on detector type and entire experiment arrangement and its possible theoretical interpretation will be presented.

This paper presents Bread Board Model (BBM) of coherent homodyne receiver with an optical phase locked loop and a frequency compensator of Doppler shifts for inter satellite optical communication link. 2.5Gbps BPSK data has been demodulated with sensitivity of -49.1dBm at bit rate of 1e-6 under initial frequency offset of +/-7 GHz simulated as Doppler shifts due to variation of distance between each satellite.

A hardware prototype of a flight receiver for deep space optical communications has been developed where a single detector array is used for acquisition, tracking, and high-speed data recovery. A counting algorithm accumulates pulses on every pixel in a photon-counting array and extracts signal information encoded with a nested modulation scheme.

The Lunar Laser Communication Demonstration (LLCD) program will demonstrate the first high-bandwidth optical communication payload on a NASA space mission. The inertially stabilized 108 mm aperture telescope will fly on NASA's LADEE spacecraft and is fabricated nearly entirely of beryllium, providing a high stiffness-to-weight ratio. The telescope consists of a two-axis fine positioning stage using inertial sensors and coarse and fine optical tracking. The stabilized telescope uses a two-axis coarse positioning gimbal to provide a large field-of-regard. Inertial stabilization provides local disturbance rejection while allowing modest optical uplink power to provide an absolute pointing reference. The telescope is a three-wavelength design providing separate uplink acquisition and communication wavelengths, and a downlink communication wavelength. Acquisition and coarse tracking of the uplink beacon is via a photodiode quadrant detector, while fine tracking is via nutation tracking and piezoelectric actuation of the receive fiber. Control of the downlink point-ahead angle is via piezoelectric actuation of the transmit fiber. The telescope is thermally stabilized during normal operations. The transmit and receive beams are fiber-coupled to a separate optoelectronic module and the telescope line-of-sight will be stabilized to better than 2.5 microradians during normal operations. Provision for self-test and boresighting during on-orbit operations is provided.

A custom designed and manufactured gimbal with a wide field-of-view and fast response time is developed. This enhanced custom design is a 24 volt system with integrated motor controllers and drivers which offers a full 180o fieldof- view in both azimuth and elevation; this provides a more continuous tracking capability as well as increased velocities of up to 479° per second. The addition of active high-frequency vibration control, to complement the passive vibration isolation system, is also in development. The ultimate goal of this research is to achieve affordable, reliable, and secure air-to-air laser communications between two separate remotely piloted aircraft. As a proof-of-concept, the practical implementation of an air-to-ground laserbased video communications payload system flown by a small Unmanned Aerial Vehicle (UAV) will be demonstrated. A numerical tracking algorithm has been written, tested, and used to aim the airborne laser transmitter at a stationary ground-based receiver with known GPS coordinates; however, further refinement of the tracking capabilities is dependent on an improved gimbal design for precision pointing of the airborne laser transmitter. The current gimbal pointing system is a two-axis, commercial-off-the-shelf component, which is limited in both range and velocity. The current design is capable of 360o of pan and 78o of tilt at a velocity of 60o per second. The control algorithm used for aiming the gimbal is executed on a PC-104 format embedded computer onboard the payload to accurately track a stationary ground-based receiver. This algorithm autonomously calculates a line-of-sight vector in real-time by using the UAV autopilot's Differential Global Positioning System (DGPS) which provides latitude, longitude, and altitude and Inertial Measurement Unit (IMU) which provides the roll, pitch, and yaw data, along with the known Global Positioning System (GPS) location of the ground-based photodiode array receiver.

Since the optical inter-satellite communication has attractive advantages such as high-speed transmission with high confidence, almost no electronic magnetic interference, and low power consumption, it has been activity investigated. However, directivity control of the laser beams requires a bulky and complicated system in satellite mobile communications. A more flexible and high accurate system with small and simple mechanism has been desired. In this study, we propose a new method of optical inter-satellite communication with a dynamically reconfigurable optical directional device in which diffraction gratings are automatically rewritten and reorganized in response of incident conditions by moving satellites. For realizing such a device, we have developed Sn₂P₂S₆ crystals which have a high sensitive photorefractivity and dynamic reconfigurable property. Furthermore, this crystal has hundreds times faster response than conventional photorefractive materials such as BaTiO₃. These features are extremely advantageous to construct a high-speed and flexible communication system with a large tolerance to displacement of moving satellites. To investigate the possibility of the dynamically reconfigurable optical inter-satellite communication system, we experimentally evaluate the temporal and spatial characteristics of Sn₂P₂S₆ crystals for the variation of the beam incident angle. Moreover, the diffraction beam from the crystal has phase conjugate wavefronts of the beam entering from the counter direction. We try to utilize this behavior to suppress the beam spread and to reduce the background light such as sunlight with a spatial filtering technique that has sensitivity in wavefront differences of the signal and background light.

This paper discusses the operational condition for direct single-mode-fiber-coupling FSO terminals under the various adverse weather conditions, such as strong atmospheric turbulences and rain falls. A good correlation between the scintillation index of the intensities of beacon receiving power and the signal fading depth has been observed, which allows us to predict the signal link quality based on the beacon scintillation index provided by the classical scintillation theory and concludes that the scintillation index for the beacon beam should be less than 0.1. This paper also reports the effect of performance enhancements provided by the new adaptive controller for the stable and robust terminal operation.

Laser beams propagating through the atmosphere are affected by optical turbulence, whose static and dynamic properties can be characterized by spatial and temporal fields of the refractive index. The resulting wave front distortions lead to performance degradation in the form of reduced signal power and increased bit-error-rates (BER), even in short links; however, it is impossible to obtain closed-form solutions for instantaneous realizations of these distortions and all the subsequent events. Instead, the statistical properties of the refractive index fluctuations can be studied using one of the well-known spectral models and extended further into the scintillation analysis and analysis of communication performance. From a practical stand point, it would be very advantageous to relate the expected system performance to specific factors responsible for wave front distortions, which are typically linked to some weather variables, such as the air temperature, pressure, wind speed, etc. In this paper, we present the results of a detailed experimental study, where some of these relationships are mathematically justified based on the tests conducted over a period of several months. The measurement data was obtained using a 29-km free-space laser communication link established between two fixed-point terminals and operating at a wavelength of 1550 nm.

System configurations based on single monolithic-apertures that are immune to atmospheric fluctuations are being developed. Main goal is the improvement of the performance achievable in coherent, free-space optical communication systems using atmospheric compensation techniques such as adaptive optics. As an alternative to a single monolithic-aperture coherent receiver with a full-size collecting area, a large effective aperture can be achieved by combining the output signal from an array of smaller receivers. We study the communication performance of field conjugation adaptive arrays applied in synchronous laser communication through the turbulent atmosphere. We assume that a single information-bearing signal is transmitted over the atmospheric fading channel, and that the adaptive array coherent receiver combines multiple dependent replicas to improve detection efficiency. We consider the effects of log-normal amplitude fluctuations and Gaussian phase fluctuations, in addition to local oscillator shot noise. We study the effect of various atmospheric parameters and the number of branches combined at the receiver.

The use of Spatial Light Modulators (SLM), Liquid Crystal Devices for atmospheric turbulence simulation in optical system has increased in the recent years. These devices allow the development of test-beds that can be used to simulate, analyze and improve optical components or systems in a controlled laboratory environment before further implementation on the eld. Most research has been performed at visible wavelengths with the use of a vast array of atmospheric turbulence simulation algorithms. We present preliminary work on an atmospheric simulation test bed which uses an algorithm developed at NRL with a transmissive high denition Liquid Crystal Device SLMs for applications in the short-wavelength infrared, with the main focus of interested at 1550nm. Preliminary results are shown for the application to a high denition re ective Liquid Crystal Device SLM for the same wavelength.

Precision ranging between planets will provide valuable information for scientific studies of the solar system and fundamental physics. Current passive ranging techniques using retro-reflectors are limited to the Earth-Moon distance due to 1/R⁴ losses. We report on a laboratory realization and field implementation of active laser ranging in real-time with two terminals, emulating interplanetary distances. Sub-millimeter accuracy is demonstrated.