We report our first experimental demonstration of an all-fiber true-time-delay (TTD) system integrated into our free-space laser-communication laboratory breadboard. Using far-field interference fringes at the target to monitor both temporal and spatial coherence, we demonstrated optical beam steering over nearly ±40° FOV with our high-bandwidth TTD system (i.e., 2.7-ps optical pulses at 10 GHz repetition frequency). The planar output aperture of the optical beam-steering device has an overall aperture-diameter of ~4 cm, while the TTD architecture utilized piezo fiber-stretcher components for low-cost, efficient programmable time-delay performance.

Stationary high-bandwidth experiments with a portable lasercom (laser communication) system were performed over a wide range of scintillation indices (< 0.1 to 1) at the Department of Energy’s Nevada Test Site in the summer of 2003. Active alignment was performed with a quad-cell tracking detector at the transmitter transponder and a conical-scan tracking beam at the receiver transponder. During good scintillation conditions, 2-km 10-Gb/s and 11-km 2.5-Gb/s capabilities were demonstrated at error-free bit-error rates over continuous intervals on the order of half an hour. The experimental transponder configuration, which had 2.5-cm transmit-side and 8-cm receive-side aperture diameters, is described and test results are presented. Modifications to the stationary beacon-tracking transponder system that support a semi-autonomous (aided-pointing), mobile, lasercom capability are discussed.

Laser based free-space-optical communication (FSOC) links are known to provide covert, secure, jam-proof and very high bandwidth performances. For mobile platforms, precision pointing and tracking schemes are critical for continuous guiding of a modulated laser beam to establish data link maintenance. In this paper, preliminary experiments of an angle-discrimination based smart pointing and tracking scheme suitable for high-speed, closed-loop, FSOC is discussed. A dual-axis, high-speed, galvo-mirror based scanner was utilized for conical scanning at 550 Hz. Greenwood frequency in the presence of moderate atmospheric turbulence over a range of 1 km at 1.5 μm was measured. It is shown that selection of a scan frequency much higher than the Greenwood frequency reduces scintillation effects on scan angle measurements for track loop maintenance. The measured scan angle value of the receiver with respect to transmit beam when fed back to the scanner through an optical transponder would allow pointing error estimation and correction. Based on our initial phenomenology study, it is shown that the scan-angle modulation based pointing and tracking scheme would provide data-link reliability for dynamic platforms traveling on rough terrains.

A space platform for optical communications could benefit from nonmechanical beam steering in which no inertia is used to redirect the laser communications link. This benefit is to come in the form of compact, low-power, light-weight optical phased arrays that provide greater flexibility in their steering capability. Non-mechanical beam steering eliminates the need for massive optomechanical components to steer the field of view of optical systems. A phased array approach also allows for random access beam steering. This paper discusses nonmechanical beam steering based on liquid crystal on silicon optical phased array technology. Limitations of the current technology and improvements are presented.

We describe the status and initial results of a long term campaign to measure the effects of atmospheric turbulence upon a horizontally directed laser beam, whose altitude above the sea surface is 2 metres. The measurements are made with a video rate Shack-Hartmann wavefront sensor. Currently the source-receiver distance is 110 metres and additional sites have been identified to extend this single pass geometry to around 1 km. To date we have sampled over 70 hours of data, from December 2003 to April 2004. The preliminary scintillation power spectra and phase structure functions have been determined for some of the data sets, which we present here.

Design and demonstration of a versatile liquid crystal-based scanner is shown for steering a laser beam in three dimensions. The scanner consists of a unique combination of digital and analog control polarization-based beamforming optics resulting in both continuous and random fashion beam steering. The scanner features a novel device biasing method, large aperture beamforming optics, low electrical power consumption, and ultra-fine as well as wide angle coarse beam steering. Demonstrations include one, two and three dimensional beam steering with a maximum of 40.92° continuous scan, all at 1550 nm. The minimum scanner aperture is 1 cm diameter and uses a combination of ferroelectric and nematic liquid crystals in addition to Rutile crystal birefringent prisms.

A 2.5 Gb/s optical link at 1550 nm over a distance of 1.1 km has been demonstrated between two identical terminals by using standard commercial optoelectronic devices without any optical amplification and with direct optical coupling into the receiver fiber optics pigtail. Each terminal consists of one 200 mm diameter light-weight Ritchey-Chrétien telescope, obtained through electroformed nickel replication technology and used for both reception and transmission. The terminal is a first phase demonstrator towards lean and cost effective design for low and medium distance inter-satellite links at high data rate. Although a tracking system has not yet been implemented, the system can work continuously up to several hours with a cumulative bit error rate lower than 10^-9, even in heavy rain conditions.

In this paper we describe a novel scheme that uses wavelength space division multiplexing to provide high bit rate communications using indoor line of sight (LOS) optical links. In the approach, a dispersive grating, or more complex diffractive optics, is used for passive beam steering. Light from a ceiling mounted hub is steered to a corresponding Mobile Terminal (MT) within a cell in the coverage area, depending on the wavelength that illuminates the dispersive element. The hubs can be optically transparent, passive and theoretically reciprocal. An identical or similar dispersive component can be used in the transceivers at both ends of the hub uplink and downlink. The hubs have the potential to provide data rates of Gbits/s, terminal mobility as well as low complexity. In order to test the concept a one-dimensional (1D) system has been fabricated. A simple 1^st order plane diffraction grating with a pitch of 1.1 μm is used to provide 1D beam steering. A dispersive angle of approximately 10 degrees over a wavelength range of 30nm in the wavelength division multiplexing (WDM) 'C' band (1530nm - 1560nm) is obtained. Simulation shows that the dispersive angle can be further increased up to 90 degrees using a secondary optical system. Transmission of 1Gb/s data has been demonstrated using this initial configuration. In the paper we discuss these results and further refinement to the system.

We propose a novel reconfigurable erbium doped fiber amplifier (EDFA) design for high performance optical pre-amplifier applications. A fiber Bragg grating (FBG) with high reflection at a specific single channel wavelength and two circulator are used in the EDFA design to realize higher gain and optical signal-to-noise ratio (OSNR). This proposed system was compared with conventional co- and counter propagating EDFA normally used as pre-amplifiers. The selected channel amplification was in the C-band. A maximum gain of 38dB with a gain improvement of 10.8dB was achieved at input signal level of - 40dBm at 1550 nm. An OSNR improvement of 12.6dB was achieved as compared to the other two designs. An average noise figure of 5.6dB was obtained and with the appropriate selection of FBG, the design can be configured as a pre-amplifier for any channel within the C-band. The experimental results were compared and agreed with modeling results of the system.

Sensors which collect information for intelligence, surveillance and reconnaissance generate a substantial amount of data. High data rates are required to transmit the information in reasonable time intervals. This paper describes the objectives and progress on the development of laser communication terminals for aircraft, transmitting high speed data over distances greater than 100 kilometers. The terminals are being designed and built under the Air Force Research Laboratory’s EO Sensor Technology & Evaluation Research (ESTER) program. The free space laser communication system consists of three terminals developed around commercial fiber-optic standards. They are designed to be an open system, full duplex, protocol neutral, and operate at 2.5 gigabit per second, capable of expanding to 10 gigabit per second. The terminal’s data rate can be reduced if atmospheric effects cause a significant increase in bit error rate. The free space optics are mounted in a pointing & tracking gimbal which interfaces with an avionics unit that measures aircraft GPS information. Fine pointing and tracking is maintained by using a beacon signal which switches from wide to narrow divergence when signal lock is achieved. The terminals are designed to operate in air-to-ground and air-to-air scenarios. Subsystem laboratory tests are being completed and the terminals are being prototyped. The first set of tests will be conducted between ground sites and the first flight demonstration will involve interfacing with the multi-purpose common data link (MP-CDL) on an aircraft. We provide a description of our terminal design, show our estimated operational performance, describe the results of laboratory testing, and outline the future program activities.

Modulating retro-reflectors (MRR) couple passive optical retro-reflectors with electro-optic modulators to allow free-space optical communication with a laser and pointing/acquisition/tracking system required on only one end of the link. In operation a conventional free space optical communications terminal, the interrogator, is used on one end of the link to illuminate the MRR on the other end of the link with a cw beam. The MRR imposes a modulation on the interrogating beam and passively retro-reflects it back to the interrogator. These types of systems are attractive for a asymmetric communication links for which one end of the link cannot afford the weight, power or expense of a conventional free-space optical communication terminal. Recently, MRR using multiple quantum well (MQW) modulators have been demonstrated using a large area MQW placed in front of the aperture of a corner-cube. For the MQW MRR, the maximum modulation can range into the gigahertz, limited only by the RC time constant of the device. This limitation, however, is a serious one. The optical aperture of an MRR cannot be too small or the amount of light retro-reflected will be insufficient to close the link. For typical corner-cube MQW MRR devices the modulator has a diameter between 0.5-1 cm and maximum modulation rates less than 10 Mbps. In this paper we describe a new kind of MQW MRR that uses a cat’s eye retro-reflector with the MQW in the focal plane of the cat’s eye. This system decouples the size of the modulator from the size of the optical aperture and allows much higher data rates. A 10 Mbps free space link over a range of 1 km is demonstrated. In addition a laboratory of a 70 Mbps MQW focal plane is described.

One unusual aspect of a modulating retroreflector (MRR) system is that the interrogating laser and the receiver are collocated. This fact, plus the lack of optical amplifiers capable of low data rates, makes the use of a coherent receiver attractive. By splitting off a portion of the interrogating beam, a local oscillator (LO) is made available to mix with the returning signal beam. This self-homodyning is accomplished at very low cost, without the need for an additional laser or an optical phase-locked loop. Changes are only required in the interrogator/receiver, so increased ranges could be realized with no changes to MRR transmitters that have already been developed. A similar method could be used to realize a heterodyne system. To cancel the effects of atmospheric turbulence, a phase-diversity receiver configuration was used. A maximum gain of about 17 dB was observed, limited by saturation of the receiver electronics. The use of coherent interrogator/receivers can increase receiver sensitivity, extending the range of existing systems and showing great promise for modulating retroreflector links.

The results of experiments demonstrating the first amplified retro-modulated free-space optical communications link are presented. The amplifier increases the effective area of the retro-modulator by a factor of 318. The first experimental demonstration of a retro-modulator operating at a data rate of 2.5-Gbps is also presented. We will present the details of the experimental system, a simple theoretical model explaining the system performance, and the results of the first amplified retro-modulated link experiments.

An innovative approach to future space telescopes that enables order of magnitude increased science return for astronomical, Earth-observing and planetary science missions is described. Our concept, called Multiple Instrument Distributed Aperture Sensor (MIDAS), provides a large-aperture, wide-field, diffraction-limited telescope at a fraction of the cost, mass and volume of conventional space telescopes. MIDAS integrates many optical interferometry advances as an evolution of over a decade of technology development in distributed aperture optical imaging systems. Nine collector telescopes are integrated into MIDAS as the primary remote sensing science payload, supporting a collection of six back-end science instruments tailored to a specific mission. By interfacing to multiple science instruments, enabling sequential and concurrent functional modes, we expand the potential science return of future space science missions many fold. Passive imaging modes with MIDAS enable remote sensing at diffraction-limited resolution sequentially by each science instrument, as well as in somewhat lower resolution by multiple science instruments acting concurrently on the image, such as in different wavebands. Our MIDAS concept inherently provides nanometer-resolution hyperspectral passive imaging without the need for any moving parts in the science instruments. For Earth-observing and planetary science missions, the MIDAS optical design provides high-resolution imaging at high altitudes for long dwell times, thereby enabling real-time, wide-area remote sensing of dynamic planetary surface characteristics. In its active remote sensing modes, using an integrated solid-state laser source, MIDAS enables surface illumination, active spectroscopy, LIDAR, vibrometery, and optical communications. Our concept is directly scalable to telescope synthetic apertures of 5m, limited by launch vehicle fairing diameter, and above 5m diameter achieved by means of autonomous deployments or manned assembly in space. MIDAS is a proven candidate for space flight missions, enabled by our continued investments in focused technology development areas.

GaN /AlGaN transistors are being developed for a variety of RF electronic and high temperature elctronics applications that will replace GaAs and Silicon devices and circuits for commercial and military applications. AlGaN/ GaN based HEMT device structure shows significant potential to meet these needs. In this paper, we present a GaN/AlGaN based HEMT design with modeling results, that includes AlN buffer layer followed by AlGaN layers on lattice matched semi-insulating SiC substrates. These devices were grown using RF Plasma Assisted MBE Technique. This approach has demonstrated very uniform epitaxial layers. Key to high quality HEMT structures is the ability to grow high quality AlN Buffer layers. Details of the electrical and optical characteristics of the HEMT layers and devices are presented and a short overview of semi-insulating SiC crystal growth is given.

Research and development of laser communication in Israel encompasses many different areas including inter-satellite communication, terrestrial communication and indoor communication. The R&D activity is mainly being carried out in two sectors: academia and industry. Additionally there is some activity in start-up companies. In this paper, I survey several areas of R&D activity to which Israel has contributed. 1) Broadband Laser Inter-satellite Link (BLISL). This is a joint Israeli-German five-year applied research project supported by the DIP foundation. Its objective is to develop technologies and know-how for broadband optical inter-satellite communication terminals for applications on small satellites in LEO/MEO constellations. 2) Urban optical wireless systems. This area has received much attention in academia. The main task is to quantify atmospheric effects on system performance and to apply this information in industry. 3) Short range communication for indoor applications. The main aim here is to develop adaptive techniques 4) Agile laser scanning/steering. This technology is essential for the next generation of optical wireless systems. 5) Atmospheric probing. We discuss a new concept for atmospheric probing known as firefly clustering.

A procedure for the analysis, modeling, and a practical trial of a Free-Space Optics (FSO) system is presented. The procedure has been conducted in the urban area of Rio de Janeiro, in 2002. Firstly, the transmitter and receiver characteristics are considered. Next, three additional parameters are introduced: they are: the atmospheric loss, the geometric loss and the scintillation. In this last parameter, a few ways how scintillation might be expressed in dB and translated into a power balance equation, is presented. Other fixed parameters, dealing with additional losses, are subsequently inserted. The FSO system availability is exhibited, using airports visibility data, leading to a prediction of the systemic availability. Attention is then focused on the Bit Error Rate, BER, which relates with the Recommendation ITU-T G.826. Within this last Recommendation, it is possible to perform a FSO analysis with respect to the climatic variation. The experiment has encompassed some short periods in which this city presents a strong morning fog. It is finally shown that FSO is a competitive and reliable transmission technology, provided proper and correct use.

The effects of clouds on optical down-links (satellite to ground) have been recognized by a variety of authors. Generally the approach taken for dealing with these effects is to seek statistics on the “cloud-free line of sight.” In contrast, our approach has been to develop a model that incorporates the effects of clouds on the performance of an optical communication system. We previously reported on the use of existing cloud data bases together with a radiative transport model to describe the spatio-temporal spreading effects of multiple scatter. In addition to the cloud optical properties, the communication channel model incorporates various geometrical features such as cloud base, detector aperture, field of view (FOV), and transmitter beam size. We illustrate how manipulation of the various model parameters allows optimization of the communication system. As an example, we note that the signal level increases with FOV. However, so too does pulse stretching. Competition between these two effects (greater signal level, lower allowable bandwidth) leads to an optimum FOV. This FOV, which maximizes the error-free data rate, depends in turn on the optical thickness of the cloud. Finally, consideration of the cloud statistics provides an optimum system design for a given geographical location.

In this paper we summarize work done on the crosstalk effect of aerosol backscatter on the performance of an urban optical wireless communication (UOWC) system. The communication link is a segment within a metropolitan area network (MAN), where a WDM transmitter and receiver are housed in one transceiver unit with parallel, or near-parallel, optic axes. The crosstalk at the receiver is caused by light from the transmitted signal of the same transceiver, which has been backscattered by molecules and aerosols in the atmosphere. This is exacerbated in the presence of fog and haze, when both the desired signal from another transceiver is attenuated by scattering and the backscatter-induced crosstalk increases. In our research we derive a bit error rate (BER) model which takes into consideration the dominant noise sources, which include the backscatter-induced crosstalk and the signal mixing with amplified stimulated emission (ASE) from an optical pre-amplifier at the receiver. Our numerical calculations indicate that in a moderate fog the BER may increase by an order of magnitude, due to backscatter.

In this paper, we present an improved concept of “Laser Firefly Clustering” for atmospheric probing, elaborating upon previous published work. The laser firefly cluster is a mobile, flexible and versatile distributed sensing system, whose purpose is to profile the chemical and particulate composition of the atmosphere for pollution monitoring, meteorology, detection of contamination and other aims. The fireflies are deployed in situ at the altitude of interest, and evoke a backscatter response form aerosols and molecules in the immediate vicinity using a coded laser signal. In the improved system a laser transmitter and one imaging receiver telescope are placed at a base station, while sophisticated miniature distributed sensors (fireflies), are deployed in the atmosphere. The fireflies are interrogated by the base station laser, and emit non-coded probing signals in response. The backscatter signal is processed on the firefly and the transduced data is transmitted to the imaging receiver on the ground. These improvements lead to better performance at lower energy cost and expand the scope of application of the innovative concept of laser firefly clustering. A numerical example demonstrates the potential of the novel system.

Atmospheric effects can be devastating in FSO (Free Space Optics). The major ones are scintillation and beam wander. Often acquisition and tracking of the source is extremely difficult as well. Active optics is widely used to combat this problem. Passive optics using a modulated retrofreflector has proved much easier and possibly more effective, but the double pass through the atmosphere effectively doubles the attenuation coefficient, making it less attractive for long distances. We suggest here a system with the best features of both - one-way path with passive compensation. It uses a special, large, low-cost reflective diffuser screen with designer choice of illumination geometry and viewing direction and field of view. Essentially 100% of the incident light is scattered diffusely into that field of view. The limitations this poses on signal bandwidth and the advantages it offers in terms of acquisition, scintillation, beam wander, radiometric efficiency are discussed.

The use of laser beams with partial spatial coherence can reduce spatial intensity variations (scintillation) due to turbulence and therefore improve the performance of atmospheric laser communication links. For optimal performance the partial coherence needs to be managed such that there is a balance between scintillation reduction and beam spread. However, the practical creation of a partially coherent beam can be problematic. We discuss a concept where a simplified modulated phase pattern is imparted to a coherent source beam that, along with the inherent time averaging of the photodetection process, produces a beam with partially coherent characteristics. Results from a laboratory demonstration are presented that illustrate the potential application of this concept for free-space laser communications.

Predictions of scintillation for ground to space collimated Gaussian beams generated from a numerical wave optics simulation are compared with recent weak scintillation theory developed from the Rytov perturbation approach (L.C. Andrews, R.L. Phillips, P.T. Yu, Ap Opt 34, p 7742-7751, 1995; J.D. Shelton, JOSA A 12, p 2172-2181, 1995). Significant discrepancies are revealed for intermediate-sized beams, defined as beams whose initial diameters place the near ground turbulence in the transmitter near field and the remote space target in the transmitter far field. By adding wander tracking to the wave optics simulation, and by developing a separate analytic model of the beam wander scintillation mechanism, we show that the scintillation for intermediate-sized beams is dominated by turbulence-induced beam wander at the target, and that the results from the wave optics simulation are accurate. We conclude that the analytic theory’s treatment of beam wander is incomplete, leading to the output of incorrect predictions for the second moment of irradiance. The error is most severe at the target point on the transmitter’s optical axis.

The U.S. Navy has an interest in the use of laser systems for surface ships. Such systems must operate within a thin near-surface environment called the marine atmospheric surface layer. There exist substantial gradients in temperature and momentum within this layer which make turbulence a strong function of height. We are interested in robust and simple optical turbulence models that can be used to predict turbulence along near-horizontal paths. We discuss several different models that are based upon similarity theory, and we compare the models with field transmission data taken from both over-water and over-land propagation paths.

We evaluate a similarity-based optical turbulence model that estimates diurnal values for Cn2 from easily obtained local terrain and environmental information by comparing it with scintillometer data taken at the Army Research Laboratory’s A_LOT facility in Adelphi, Maryland. The A_LOT facility is characteristic of many planned urban sites for free-space laser communication. One end of the test site is on top of a two-story building, and the other end is a water tower about 70 meters high. This comparison examines the effects of the asymmetric location, such as the non-uniform height above ground and surface roughness length. We found that by emphasizing the terrain type directly in front of the receiver and assuming the height above ground to be the height of the receiver, model results compare favorably with experimental data.

We present an optical turbulence model that has evolved from the PAMELA model. After a preliminary report in SPIE 2003 it became apparent that more data was needed to refine this adaptive model. This led us to take twelve months of over-land data (~100 meters pathlength) at the Chesapeake Bay Detachment of the Naval Research Lab. We present data throughout the year with varying environments with comparison with the model prediction. Our recent modification includes segmenting the windspeed to 3 sections, morning, afternoon, and night for better fitting. This is an attempt to incorporate variable wind speed into the model which is known to contribute significantly to the turbulence in the atmosphere. In addition, we present preliminary results from the over-the-bay data (10 km pathlength).

Methodology is presented using observations from a radar and new measurement system to address several fundamental turbulence issues related to laser beam propagation that impact high energy laser (HEL) and laser communication systems. The successful design and operation of these laser systems require high-fidelity realistic laser beam propagation models coupled with a thorough and comprehensive knowledge of the real turbulent atmosphere. To date, modeling and simulation of laser beam propagation through atmospheric turbulence have relied upon a traditional theoretical basis that assumes the existence of homogeneous, isotropic, stationary, and Kolmogorov turbulence. The approach and methodology is discussed to assess the impact of real atmospheric turbulence on laser beam propagation. Analysis will include effects of non-classical turbulence as well as inner (l_o) and outer scale (L_o) effects. Data will be obtained from a new measurement platform using a free-flying balloon that lifts a ring with a boom upon which are mounted fine wire (1μm diameter) sensors to measure high-speed temperature and velocity fluctuations from which the turbulent quantities can be calculated including the refractive index structure parameter (C²_n) and the eddy dissipation rate (ε). The “ring” is actually 8-sided with a diameter of 30 feet and trails the balloon with several risers. This design eliminates contamination of the balloon wake that plagues conventional systems.

We investigated an edge response of an extended object in a turbulent atmosphere using imagery data acquired with a double-waveband passive imaging system operating in the visible IR wavebands and an actively illuminated optical sensor. We made two findings. We found that the edge response of an extended object is independent of an exposure time, and an atmospheric tilt does not contribute to the image blur of an extended object. In addition, we found that turbulence-induced image blur for an extended object reduces, not increases, with the imager diameter. Therefore, one can reduce the turbulence-induced image blur for an extended object reduces, not increases, with the imager diameter. Therefore, one can reduce the turbulence-induced blur by increasing aperture diameter of an imaging lens. Both findings contradict the predictions of the conventional imaging theory, suggesting that the conventional theory is not applicable to extended anisoplanatic objects. We provided physical interpretation for the results obtained. In addition, we discussed the mitigation techniques that allow us to reduce both turbulence-induced image blur and edge waviness in optical images.

The European Space Agency (ESA) has built an optical ground station (OGS) for commissioning and checkout of its laser communication payloads in orbit. The first such payload is the laser communication terminal (LCT) onboard ARTEMIS, ESA's latest data-relay and telecommunication satellite in geostationary orbit. ARTEMIS is now routinely relaying Earth observation data sent via a similar LCT from the low-earth orbiting satellite SPOT-4 to a ground station in Toulouse. This paper focuses on bidirectional space to ground laser communication experiments, which have been performed between the OGS and ARTEMIS. ESA's interest in laser communication is first briefly explained, then the design of the ground and space terminals is introduced, the pointing, acquisition and tracking strategies are explained and a summary of all laser links performed so far is given. Experimental uplink and downlink results are presented in terms of temporal irradiance behavior and link statistics. The uplink irradiance behavior is investigated with changing number of transmit beams. Finally, ESA's future activities and upgrades planned for the OGS are discussed.

MEMX Corporation in collaboration with Johns Hopkins University Applied Physics Laboratory (JHU/APL) has developed micro-mirror technology applicable to free-space multi-access optical communications terminals. Based on their previously developed micro-electro-mechanical systems (MEMS) optical switches, these new units are being evaluated for applications on spacecraft. These devices must operate within very accurate digitally-controlled pointing and tracking subsystems, which are an essential adjunct to the long-haul optical communication channels that would be operated potentially from geosynchronous earth orbit (GEO) to ground. For such spacecraft applications high-powered laser diodes are likely be the required transmitter. Coupled with their potential operation in a vacuum or at partial atmospheric pressures, MEMS mirror shape stability and fabrication tolerances are of key concern to a system designer. To this end we have measured the performance of preliminary micro-mirror units in terms of angular jitter, focal spot stability, and open and closed-loop response versus laser transmitter power in both ambient air and at low partial pressures. We will describe the fabrication process as well as the experimental test configurations and results in the context of optical beamsteering. We will also discuss the applicability and scalability of this technology to multi-access terminals.

As a step towards the development of a full scale intersatellite laser communication system, the on-ground simulation experiments and performances validation are necessary in the beginning. In this paper, we designed an optical simulator for on-ground communication performances validation of the free-space laser communication system. The simulator basically consists of a Four-transform lens of big aperture and long focal length and following three optical imaging amplifiers. It becomes possible to evaluate communication-performances of optical terminals by distance transform and background noise simulated. System analysis and simulated experiments show that the simulator is feasible and has the great potential for wide applications.

Capturing the very faint optical communications signals expected from the Mars Laser Communication Demonstration (MLCD) experiment to fly aboard the Mars Telecommunications Orbiter (MTO) in 2009 requires a sensitive receiver placed at the focus of a large collecting aperture. For the purpose of demonstrating the potential of deep-space optical communication, it makes sense to employ a large astronomical telescope as a temporary receiver. Because of its large collecting aperture, its reputation as a well-run instrument, and its relative convenience, the 200-inch Hale Telescope on Palomar Mountain is being considered as a demonstration optical 'antenna' for the experiment. However, use of the telescope in this manner presents unique challenges to be overcome, the greatest of which is pointing the telescope and maintaining the communication link to within a few degrees of the Sun. This paper presents our candidate approaches for adapting the Hale telescope to meet the demonstration requirements, modifications to the facilities and infrastructure, the derivation of requirements for baffles and filters to meet the near-Sun pointing objectives, and initial data on the potential of candidate modifications to meet the requirements.

To maximize the cost-effectiveness of the Mars Laser Communication Demonstration (MLCD), the project is pursuing the use of ground-based astronomical telescopes as large-aperture optical receiving antennae. To facilitate communication as the spacecraft approaches solar conjunction, a large membrane filter is being considered to reject approximately 95% of the sun’s power, while efficiently admitting light at the 1060 nm signal wavelength. Through the use of this filter and some additional facility modifications, the problems of thermally-induced telescope aberrations and dangerous focusing of solar power can effectively be mitigated. The use of a membrane filter is expected to be cost competitive, introduce less scattered light, and provide more flexibility in placement and operations than alternative approaches. This paper addresses the initial design of the filter and preparation of test samples to evaluate candidate materials.

In this paper, a subsystem of coupling semiconductor laser diode to the single-mode fiber and producing the diffraction-limited beam is described in detail, and beam wavefront is measured by a double shearing-interferometer experimentally. As a result, the divergence of the collimated beam is only 65.84μrad, which indicates that the transmitted beam achieves the diffraction-limited, and this optical subsystem is coincident very well with our design.

This paper presents the development of an ultra precision optical satellite relative-trajectory simulator. It is commonly known that by combining two prisms of equal apex angle in near contact and by independently rotating them about an axis parallel to the normals of their adjacent faces, a ray can be steered in any direction within a limited cone. Due to the request of high precision, we use the table-looking method rather than the conventional approximately formula to transform the angle between the azimuth and elevation angle of the relative-trajectory and the angle of the servo-motor. To achieve a stable and accurate control of the system, a Proportional-Integral-derivative (PID) controller is used and the controller was optimized use the Genetic Algorithm. Furthermore, we simulated the system with Matlab program under different bandwidth. The results demonstrate that the proposed position control system achieves a good control performance. The simulation indicates that PID controller performed well on the satellite relative-trajectory simulation.

Atmospheric turbulence is known as the major channel impairment for outdoor long-distance free space optical communication. This paper investigates the performance of high-rate fast-speed Reed-Solomon (RS) codes on log-normal fading FSOC channels. The cases where the channel state information is available and unavailable to the decoder are investigated. The impacts of turbulence strength, temporal correlation, interleaver type and size, as well as performance comparison to turbo codes are reported. Through theoretical analysis and extensive computer simulations, it is shown that impressive coding gain can be achieved for RS codes with very little overhead.

A new method is presented to perform the synchronization and detection of the binary data from the FSO signal. Based on the wavelet transformation, a new band-pass filter is developed and implemented. It is shown that the Haar wavelet is an excellent choice for this purpose. The center frequency of this filter is a function of the scale and could be adjusted to adapt to the variation of the channel. The output of the filter is zero-mean and is closely related to the derivative of the binary data. The filter has a linear phase; therefore, its output is used for synchronization and detection of the data. Analysis of the method is presented using Fourier transformation. In addition, simulation experiments are performed and presented using the real data. The results of the experiments indicate that the Haar wavelet transform is a robust and effective tool in dealing with FSO data.

Laguerre-Gaussian (LG) light beams possess discrete values of orbital angular momentum (OAM) of l&barh; per photon, where l is the azimuthal index of the mode. In principle l can take on any integer number, resulting in an unlimited amount of information that can be carried by any part of the beam - even a single photon. We have developed a technology demonstrator that uses OAM to encode information onto a light beam for free-space optical communications. In our demonstrator units both the encoding and decoding of the orbital angular momentum states is achieved using diffractive optical components (holograms). We use 9 different OAM values; one value is used for alignment purposes, the others carry data.

The paper deals with a statistical model of free-space optical (FSO) data link that takes into account the duration of individual fade events. It is well known that a general data transmission using the Internet protocol (IP) is almost insensitive to link interruptions for hundreds of milliseconds while real-time video services will be badly affected. The statistical model is based on the knowledge of the probability density function of random attenuations and the probability density function of durations of the fades. The paper shows the results from our test site obtained from 1999 to 2001.

It is commonly believed that the main obstacles in functioning of adaptive optical systems in the atmosphere under the conditions of strong turbulence are wave front dislocations (optical vortices). It is assumed in this case that wave front singularities connected with vortices disappear at disappearance (annihilation) of vortices, so it makes no problem to correct for the distorting effect of the atmosphere on the propagating laser beam using, for example, flexible mirrors. This paper is devoted to the study of properties of the wave front phase distribution at the stage following annihilation of optical vortices. It is established that the absolute values of the mean curvature and the Gaussian curvature of wave front parts increase greatly in the areas of vortex aftereffect. It is shown that the efficiency of an adaptive optical system reduces due to the increase of the error at the stage of retrieval of such phase distribution in the process of wave front sensing. This effect should be also taken into consideration when developing principles of construction of single-mirror and multiple-mirror adaptive optical systems operating by the phase conjugation method and intended for work under the conditions of strong atmospheric turbulence.

It is required for the laser communication that laser beam transmitted from the optical terminal must be highly parallel. Optical diffraction limit angle is the minimum divergence the beam can obtain while limited by the definite aperture under ideal conditions, here wavefront still has an error of 0.3 wave. This paper introduces a new method for wavefront analysis. In this method a circular aperture diaphragm used to sample the tested wavefront, a focusing lens, a microscope objective and a fiber optic probe are arranged coaxial. Axial intensity profile behind the focusing lens is plotted by registering the positions of the microscope objective on the axis and the readings of the radiometer. The sampled wavefront height is estimated from the distance between two symmetrical positions along the axis where the intensity is zero. The tested wavefront height is calculated from the sampled wavefront height. The theory and the simulation results are given. It can be applied in coarse measurement of any wavelength laser wavefront. Due to simplicity of the method and its low cost, it is a promising method for checking the collimation of a laser beam.

There is often large optics of several hundreds millimeters aperture in the laser communications transceiver which is nearly diffraction-limited laser beam at the same time. A revised Mach-Zehnder double-shearing interferometer is designed with 300 millimeters aperture which can be used for wave front analysis. Compare to the aperture-divided double lateral-shearing interferometer presented before, which is based on Jamin interferometer, this structure is easy to achieve for large aperture measurement without thick Jamin plates. In this paper, the interferometer with six same size plates is explained. One plate of them occupied as reflection is aperture-divided. Three changeable schemes are attained to get different wave front measurement ranges through adjusting angle between half-aperture plates. The interferometer is a white light interferometer. The interferometer is so designed that the equal optical path interference is hold though the shear is changing. It is useful for the short coherent length such as laser diode. The wave front aberration is deduced from the interferogram. The calibration is important especially for the Mach-Zehnder structure. Here two interferometers are adopted. One is Fisuea type used for parallel or angle adjustment, another generates plane wave better than 0.2λ with the laser and the high-quality plates. The method to use these apparatus is also explained in detail.

In this paper, we propose some squamosal ball mirrors for free space optical communication. The system is simulated by ray tracing for analyzing the refracted and scattered optical signal. Considering the effect of Gaussian beam of the incident signal being refracted or scattered by the squamosal ball mirror, based on the ray tracing study, we investigating the beam uniformity for the covering area. Four cases of the ball mirror are investigated and the fractional energy of the covering range are obtained.

We describe the development of a portable NRL seeing monitor which consists of a 12 inch Meade f/10 telescope with a Dalsa Cad6 260x260 camera having 10 micron pixels. This seeing monitor is capable of up to 700 frames per second. We have three different techniques to measure Fried's r0 parameter: full aperture, two-hole mask, and two-hole mask with in-line intensifier. For the observations done at the Anderson Mesa, Arizona site in January-July 2004, we present comparison of Fried's r0 obtained. Calibration, observing techniques, and data analysis techniques are described. Comparison of the three different techniques is discussed.

The field reflected of a telescope focal plane highlighted by a laser is calculated. The method of physical optics is used. The size of dispersion spot on the receiving aperture plane is determined on depending of parameters of a line and telescope, coordinates of the maximum reflected signal and level of signal in maximum. Correlation of received signal level and quantity of the information in it also are received. On the basis of analysis of these correlation the losses in the communication laser channel caused by not an optimality of laser and channel parameters are designed. It is offered to consider a complex parameters of system at which dispersion spot contains the given quantity of the information as criterion of orientation.

Laser satellite communication uses free space as a propagation medium for various applications, such as inter-satellite communication or satellite networking. This system includes a laser transmitter and an optical receiver. In order to communicate, the line-of-sight of the transmitter and the receiver must be aligned. However, mechanical jitter reduce alignment between the transmitter laser beam and the receiver field of view (FOV), which results in pointing errors. The outcome of pointing errors is fading of the received signal which leads to degrade link performance. We consider system where the optical preamplifier is incorporated into the receiver, and derive a bit error probability (BEP) model that takes into account the statistics of the pointing error,

One of the most promising remote sensing technologies for the coming years is a network of sensors. The system under consideration includes a network of sensors and a remote base-station. The base-station communicates with the sensors using optical wireless links and acquires and identifies sensors based on the unique subcarrier frequency. The sensors use active retro-reflectors to communicate with the base-station in order to make the sensor simple, cost-effective, and with minimum power consumption. The base-station employs an imaging receiver (detector matrix), in which signals arriving from different directions are detected by different pixels. The imaging receiver mitigates ambient light noise and interference between simultaneous uplink transmissions from different sensors, provided that the transmissions are imaged onto disjoint sets of pixels. Our scheme allows simultaneous acquisition and identification of a sensor in a network by an imaging receiver. A probability model of erroneous acquisition due to noise is derived.

Because PAT (pointing-acquisition-tracking) parameters and integrated technical specifications of laser communication terminals for inter-satellite link must be pre-verified and assessed thoroughly on a ground-based test-bed before launched into the space, it is necessary to develop a system as a primary part of the test bed to simulate the relative trajectory between the satellites. In this paper, an original high-precision satellite relative-trajectory simulating servosystem is introduced in detail as well as its structures and characteristics. The system is used to simulate the motion of relative-trajectory between satellites in different orbits. The principle of the system is to import the data of two satellites’ orbits into a computer-based control system in advance. After processed and analyzed, the data is transformed into the angular displacement of the servomotor which drives the gimbal directly. The angular displacement of the two axes of the gimbal can simulate precisely the relative-trajectory, namely elevation angle and azimuth angle of the two satellites in communication. A laser communication terminal mounted on the gimbal then performs the PAT mechanisms to evaluate the system’s capacity.

The development of a photovoltaically (PV) powered, laser communication system that constitutes a miniature, highly energy-efficient wireless communication technology is described. The technology is based on the direct integration of a multi-quantum well (MQW) modulating retroreflector (MRR) optical communication node and a monolithically integrated module (MIM) PV power source. The MQW MRR optical data link exploits the shift in the MQW absorption peak under an applied reverse bias to modulate incident laser light enabling binary encoding of data for transfer. A MIM consists of many individual solar cells monolithically integrated on a single substrate and offers the design versatility necessary to allow efficient electrical conversion of both incident sunlight and the system laser-light and the ability to match the voltage output to the MRR requirements. A description of the development of the MRR and MIM components of the system along with the power management and distribution circuitry is given. Results of bench-top demonstrations of the operational system are presented.

A novel high accuracy all electronic technique for phase locking large arrays of optical fibers is demonstrated. We report the first demonstration of RF phase modulation phase locking for fiber arrays and measure a fringe visibility of 97%. Phase locking was maintained for phase disturbances of 20,000 waves/sec.