The Mars Laser Communications Demonstration Project completed a preliminary system design for sending data at 1-30 Mbps from a spacecraft orbiting Mars. The flight transceiver diameter was 30.6 cm, transmitting 5 W average laser power at 1064 nm and using 32- and 64-ary pulse position modulation (PPM). A ground network comprised of two receive terminals (5-m and 1.6-m effective diameter) and two transmit terminals for sending 1076 nm lasers would have been used to communicate with the transceiver.

We present the results of the on-orbit free-space laser communications between the Optical Inter-orbit Communications Engineering Test Satellite (OICETS) and the Advanced Relay and Technology Mission (ARTEMIS) geostationary satellite. We first introduce the history of developing OICETS and add some descriptions on characteristics of OICETS, where the performance of the optical communication terminal named the Laser Utilizing Communications Equipment (LUCE) is also stated. We summarize the on-orbit investigation results of LUCE's function and finally make a report on the inter-orbit laser communication experiment carried out by ARTEMIS and OICETS.

As alternatives to the traditional gimbaled terminal design, future satellite based laser communications terminals are envisioned that utilize a wide field of view or field of regard (WFOV/WFOR). This approach can be advantageous in situations requiring rapid switching between user terminals, support for multiple terminals simultaneously (via TDMA, SDMA or WDMA) or other non-standard mission requirements. However, a traditional gimbaled terminal has the capability to continuously track a single user over very large angles, such as the 18-20° spanned by a LEO satellite as seen from GEO. WFOV/WFOR designs face increasing cost and/or complexity issues with each incremental increase in angular coverage. The methodology and inputs for a trade study are presented here that attempts to maximize the available connectivity to a LEO satellite while minimizing cost and complexity metrics by choosing an optimal FOV/FOR size for a GEO terminal.

A simplified laser communications (lasercom) system architecture, primarily for a deep-space flight transceivers, can be realized by decoupling the lasercom optical components from the host spacecraft using a disturbance-free platform (DFP) developed by Lockheed Martin Space System Company. Unlike conventional lasercom system architectures where a high bandwidth control loop is used to stabilize the optical line-of-sight in the presence of platform disturbance, the DFP package isolates the optical train from the high frequency platform jitter produced by the host. By preventing the vibration from coupling into the optics train, the need for a high bandwidth beam stabilization control loop (including fast steering mirror, detectors, controls and the associated relay optics) is eliminated with possible mass savings. Effective isolation of the platform jitter also enables the optical focal plane array to operate at a much longer integration time, thus enabling the use of either faint stars or a weak beacon as a pointing reference. This feature can allow the same lasercom system architecture to be employed for deep space and some near Earth applications, and can potentially enable deep space return signal pointing without the need of an uplink beacon.

Next generation laser communication systems will require small optical antennas and fine tracking system. Such optical communication systems might be applied not only to space communications such as optical feeder links, intersatellite links and stratospheric platforms but also optical links for long distance communication on the ground. We have developed a high-speed laser communication system including a small optical antenna which utilizes off-axis free form surface optical system, and a fine tracking system which feeds back an incident angle signal detected by quadrant detector to a small fine positioning mirror. In particular, the available mirror actuator response frequency for tracking is approximately 2kHz. Accordingly, even though the received 1.5μm laser beam experiences angle-of-arrival fluctuation as a result of atmospheric turbulence the antenna with this fine tracking function has a capability coupling the laser beam directly to the single mode fiber.

The results of field experiments designed to demonstrate key enabling concepts of optical array receivers are presented, in a field environment similar to actual operating conditions. The theoretical framework and analytical evaluation of optical array receivers have been developed and documented in previous articles. This article describes the implementation and field evaluation of a two-element optical array receiver incorporating photon-counting, signal conditioning, high-speed digital sample distribution, adaptive delay compensation and sample combining operations required for array reception of intensity-modulated optical pulse-position modulated (PPM) signals. Data collected in the field was processed offline to determine communications performance of a two-element optical array, and demonstrate the inherent advantages of optical array reception.

We investigated both experimentally and by simulation, non-return-to-zero (NRZ) and return-to-zero (RZ) 4-ary PAM operating at 20 Gb/s. A simple scheme to realize the quadratic signal leveling by suitably driving a MZ modulator is proposed, which provides greater than 6 dB improvement in receiver sensitivity as compared to equal level spacing. We experimentally demonstrated NRZ and RZ 10 Gbaud/s 4-ARY PAM transmissions over an 80 km standard single mode fiber (SSMF) link as a proof of concept and more detailed experimental results over longer reach will follow. Numerical simulations for the 4-ary PAM performance over longer distances (>200 km) are also presented.

A free-space optical link based on homodyne BPSK (binary phase shift keying) has been established between two of the Canary islands, La Palma and Tenerife, to transmit 5.625 Gpbs across 142 km. This link verifies homodyne BPSK as a robust modulation scheme even for the transmission through the atmosphere.

Free-space laser communications systems experience fading due to quasi-static pointing error and tracking error that impairs communications performance. Under a block fading model we show that using forward error correcting codes and interleaving - a simple non-mechanical data processing technique - much of the harmful effect of tracking error on communications performance can be removed. Using the concepts of fading capacity and outage capacity, we provide analytical tools that quantify the effects of fading on communications performance with and without interleaving. A link budget based on this analysis includes three loss terms due to pointing and tracking error: 1) static loss, which is primarily a function of pointing error; 2) capacity loss, which is the power difference between fading capacity and fade-free capacity; and 3) finite interleaver loss, which is the power difference between the probability of outage curve and fading capacity. Assuming pulse-position modulation and a Poisson channel, we derive closed-form solutions for the probability of outage of interleaved systems using a Gaussian beam with circularly symmetric Gaussian tracking error.

Laser communication systems hold great promise for broadband applications. This technology uses much higher-than-RF region of the spectrum and allows concentration of the signal within a very small spatial angle, thus offering unsurpassed throughput, information security, reduced weight and size of the components and power savings. Unfortunately, these intrinsic advantages do not come without a price: small beam divergence requires precise positioning, which becomes very critical at high bit rates. Complex motion patterns of the communicating platforms, resident vibrations, and atmospheric effects are known to cause significant signal losses through the mechanisms of the pointing errors, beam wander and other higher-order effects. Mitigation of those effects is achieved through the multiple means of fast tracking and wavefront control. In this paper we focus on the application of a beam steering technology and its effect on the communication performance of the system. We present the results of an experimental study of a laser communication link subjected to pointing distortions. These distortions are generated by a special disturbance element in the optical setup, which recreates specific operation environments with particular spectral characteristics. The acousto-optic technology is used to build an agile tracking system to assure the maximum signal reception in spite of the harsh operational conditions. The received communication signal is recorded and statistically analyzed to calculate the bit-error-rates. This paper presents the synthesis of a tracking system and the experimental results characterizing the communication performance under uncompensated pointing disturbance and with tracking.

A star tracker based beaconless (a.k.a. non-cooperative beacon) acquisition, tracking and pointing concept for precisely pointing an optical communication beam is presented as an innovative approach to extend the range of high bandwidth (> 100 Mbps) deep space optical communication links throughout the solar system and to remove the need for a ground based high power laser as a beacon source. The basic approach for executing the ATP functions involves the use of stars as the reference sources from which the attitude knowledge is obtained and combined with high bandwidth gyroscopes for propagating the pointing knowledge to the beam pointing mechanism. Details of the conceptual design are presented including selection of an orthogonal telescope configuration and the introduction of an optical metering scheme to reduce misalignment error. Also, estimates are presented that demonstrate that aiming of the communications beam to the Earth based receive terminal can be achieved with a total system pointing accuracy of better than 850 nanoradians (3 sigma) from anywhere in the solar system.

Infrared (IR) Earth thermal image tracking has potential to enable optical communications throughout the solar system and is a promising alternative to traditionally proposed laser beacon tracking. Image blurring due to finite receiver aperture size introduces distortions to IR Earth image in the presence of Earth's non-uniform emissivity and reduces the centroiding accuracy in identifying the center of the Earth. The impact is largest in the 0.5 to 2 AU range. We demonstrate that a deconvolution algorithm can mitigate the effect of blurring associated with IR Earth non-uniformity and improve centroiding edge detection accuracy.

Tests at the 200-inch Hale Telescope on Palomar Mountain have demonstrated this telescope's ability to withstand considerable thermal stress, and subsequently produce remarkably unaffected results. During the day of June 29, 2005, the Hale telescope dome was left open, and the telescope was exposed to outside air and direct sunlight for 8 hours. During this time, portions of the telescope structure in the telescope's optical path experienced temperature elevations of 30 C, while the primary mirror experienced unprecedented heating of over 3 C. The telescope's measured blind pointing accuracy after this exposure was not noticeably degraded from the measurements taken before exposure. More remarkably, the telescope consistently produced stellar images which were significantly better after exposure of the telescope (1.2 arcsec) than before (1.6 arcsec), even though the conditions of observation were similar. This data is the first step in co-opting astronomical telescopes for daytime use as astronomical receivers, and supports the contention that deleterious effects from daytime exposure of the telescope can be held to an acceptable level for interleaved communications and astronomy.

A novel transmitter for directed-beam infra-red wireless that utilizes a combination of both gain-guiding and index guiding mechanism to ameliorate the shortcoming of the state-of-the-art technology is proposed and demonstrated. The 3mm long tapered laser consists of an index-guided ridge straight section and a gain-guided tapered section with a full angle of 6°. By implementing multiple contact with a sufficiently high inter-contact resistance, discrete switching between different angles can be obtained by selectively pumping current to different contact (gain-guiding effect), while fine-tuning of a given angle can be achieved by adjusting the injection current of nearby contacts (index-guiding effect). The tapered laser's metal cover is removed using focus ion beam etching technique to form three separate sections: base section as filter, left section and right section for beam steering. The device is biased by current pulses of 1μs width and 0.1% duty cycle. With a 1.6A injection current, an output power of 2W is achieved, which would be suitable to overcome large free space optical loss and facilitate the use of transmitting methods. The beam profile steered by 3.2° and -5.4° from the central lobe when injection current is limited to the left and right section respectively is measured. It is also observed that as injection current increases, the beam profile is steered towards the central position. This is because as the injection current increases, the local refractive index is decreased, thereby shifting the beam profile towards the opposite direction.

We report the most recent advances of optical beamsplitters for laser communications developed at Barr Associates. The challenges for meeting the two key requirements, polarization control and wavefront control, of the lasercom beamsplitters are discussed. It is demonstrated that in order to meet the two key requirements, the repeatability of the refractive indices and the thickness of the coatings need to be controlled within 0.3% and 0.1% respectively from run to run, and the coating stress need to be controlled to less than 25 MPa. The latest result of various lasercom beamsplitters will be presented. For a typical 50% reflection and 50% transmission beamsplitter at 22.5 degree angle of incidence, the s and p polarization split is less than 0.2% from 1540nm to 1575nm and the surface flatness of the beamsplitter after coating is less than 1/40^th waves peak to valley at 632.8nm.

An experimental demonstration of a quantum-optimal receiver for optical binary signals, developed as a joint effort by the Jet Propulsion Laboratory and the California Institute if Technology, is described in this article. A brief summary of the classical, quantum-optimal, and quantum near optimal solutions to detecting binary signals is first presented. The components and experimental setup used to implement the receivers is then discussed. Experimental performance and results for both optimal and near-optimal receivers are presented and compared to theoretical limits. Finally, experimental shortcomings are discussed along with possible solutions and future direction.

Optical communications systems have been gaining momentum with the increasing demand for transmission bandwidth in the last several years. Optical cable based solutions have become an attractive alternative to copper based system in the most bandwidth demanding applications due to increased bandwidth and longer inter-repeater distances. The promise of similar benefits over radio communications systems is driving the research into free space laser communications. Along with increased communications bandwidth, a free space laser communications system offers lower power consumption and the possibility for covert data links due to the concentration of the energy of the laser into a narrow beam. A narrow beam, however, results in a requirement for much more accurate and agile steering, so that a data link can be maintained in a scenario of communication platforms in relative motion or in the presence of vibrations. This paper presents a laser beam tracking system employing an acousto-optic cell capable of deflecting a laser beam at a very high rate (order of tens of kHz). The tracking system is subjected to vibrations to simulate a realistic implementation, resulting in the increase of BER. The performance of the system can be significantly improved through digital control. A constant gain controller is complemented by a Kalman filter the parameters of which are optimized to achieve the lowest possible BER for a given vibrations spectrum.

In this paper we present some preliminary results of an ultra-light weight telescope manufactured entirely with Carbon Fiber Reinforced Polymer (CFRP), including the optics, coupled with a light weight Adaptive Optics (AO) system. This research has many scopes, ranging from long baseline interferometry to laser communications. In this paper we will examine some of the mechanical properties of the telescope and describe the testing that the system is undergoing.

When a free-space optical beam propagates through the atmosphere it experiences deterioration and deformation of its wave-front caused from small scale, randomly localized changes in the atmospheric index of refraction. This results in beam wander and scintillation effects which can reduce the link availability and may introduce burst errors. This paper outlines experimental work on a free-space optical (FSO) communication system which connects an optical beam directly to a single-mode fiber (SMF) without any optical-to-electrical (O-E) conversion. In order to effectively couple the 1550 nm transmitted optical beam to a SMF it is necessary to be able to track and control the beam angle-of-arrival (AOA) changes. To achieve this, we have developed an optical antenna which uses a fine positioning mirror (FPM) capable of performing high-speed beam tracking and steering thus reducing to a great extent the optical power fluctuations of the received beam coupled to the SMF. This optical power fluctuation is partly a result of beam angle-of-arrival fluctuations caused by atmospheric turbulence. In our experiments we have tried to measure and quantify the magnitude of atmospheric turbulence experienced by an optical beam propagating through the atmosphere. First we demonstrate the relation between the AOA fluctuations and the frequency characteristics of the scintillation effects for a free-space optical beam propagating through a turbulent atmosphere. We use this information to determine the optimum antenna FPM tracking speed for improved performance and error free transmission. The results in the improved fiber received power as well as continuous error free transmission are presented.

With the goal of reducing the surface wavefront error of low-cost multi-meter-diameter mirrors from about 10 waves peak-to-valley (P-V), at 1μm wavelength, to approximately 1-wave or less, we describe a method to compensate for slowly varying wavefront aberrations of telescope mirrors. A deformable mirror is utilized in an active optical compensation system. The RMS wavefront error of a 0.3m telescope improved to 0.05 waves (0.26 waves P-V) from the original value of 1.4 waves RMS (6.5 waves P-V), measured at 633nm, and the Strehl ratio improved to 89% from the original value of 0.08%.

Free space optical (FSO) communication is a promising candidate for emerging broadband applications, considering that RF spectrum is already congested, rendering accommodation of additional RF broadband channels difficult and costly. Communications via RF signals are reliable but cannot support emerging high data rate services unless they use a large portion of the costly radio spectrum. FSO communications offer enormous data rates but operate much more at the mercy of the atmospheric environment, such as scintillation and multi-scattering through fog and clouds. Since RF paths are relatively immune to these phenomena, combining the attributes of a higher data rate but bursty link (FSO) with the attributes of a lower data rate but reliable link (RF) could yield attributes better than either one alone, enabling a high availability link at high data rates. This transmission configuration is typically called a hybrid RF/FSO wireless system. The focus of this paper will be on applying well-known equalization techniques to FSO, to further enhance availability of RF/FSO wireless system.

Laser beam propagating through the atmosphere is subjected to severe wavefront distortions due to the optical turbulence. This leads to reduction in the received power, ultimately resulting in the BER degradation, even for short ranges. Optical properties of the atmospheric channel change over time; hence, maintaining a reliable link requires dynamic wavefront control to mitigate the effects of the atmospheric turbulence. An electrically addressed programmable nematic liquid crystal spatial light modulator (SLM) is proposed to perform this task. Wavefront correction is achieved by computing a phase shift for each pixel of the SLM, which could be a rigorous and time-consuming procedure. Hence, the goal is to obtain a stable and relatively simple approach to dynamically control the modulator elements. The phase profile of the distorted beam can be approximated using Zernike formalism or another type of wavefront polynomial, which provides efficient mapping between a large number of SLM pixels and a much smaller number of approximation coefficients. Furthermore, wavefront correction needs to be performed in real-time; hence the Simplex method by Nelder and Mead, known for fast improvement of an optimization metric, is used to adjust the approximation coefficients. The phase profile obtained from the optimization procedure is imposed on the received beam by the SLM. This facilitates the reduction of the optical path difference (OPD) present in the distorted wavefront by applying an inverse OPD, and mitigating the effects of the optical turbulence. This paper presents a basic algorithm as well as the experimental results.

The performance of a coherent free-space optical communications system operating in the presence of turbulence is investigated. Maximum Likelihood Detection techniques are employed to optimally detect Pulse Position Modulated signals with a focal-plane detector array, and reconstruct the turbulence-degraded signals. The experimental demonstration of this project and results may be divided in three parts; two of which have already been explained in previous publications [1]. This latest paper shows the final experimental results, including investigation of performance of the Coherent Optical Receiver Experiment (CORE) performed at the laboratory facilities at JPL. Bit Error Rate (BER) is presented for single and multichanel optical receivers, where quasi-shot noise limited performance is achieved, under simulated turbulence conditions using non-coherent post-detection processing techniques. Theoretical BER expressions are compared with experimental obtained BER results and array combining gains are presented. Receiver sensitivity in terms of photons per bit (PPB) is examined; BER results are shown as a function of signal to noise ratios, (SNR), as well as a function of photons per symbol, and photons per bit.

The use of free-space optical (FSO) communications links are envisioned as a viable option for providing a temporary high-bandwidth communications link between a ground station and an unmanned aerial vehicle (UAV). The presence of atmospheric turbulence causes three different phenomena to occur, namely beam wander, scintillation and beam spread, each of which is a wavelength dependent phenomenon. In this paper, simulation tools are used to investigate the effects of atmospheric turbulence on a wavelength diversified ground-to-UAV FSO communications link. This paper compares the effects of atmospheric turbulence on three different wavelengths, 1.55 μm, 0.85 μm and 10 μm. Each of these wavelengths has different advantages depending on prevalent weather conditions and atmospheric turbulence conditions. Based on the effects of atmospheric turbulence on each wavelength, a wavelength diversity scheme is proposed in order to optimize the performance of the FSO link. The largest problem associated with establishing a ground-to-UAV FSO link is alignment and tracking of the FSO link. For this reason, the wavelength diversity scheme is further analyzed as a method to optimize link acquisition and tracking of the ground-to-UAV link by exploiting various characteristics of each wavelength.

It is well known that atmospheric turbulence diminishes the performance of laser communications systems. Among the multiple degradations caused by turbulence is fading and surging of the received signal, usually referred to as scintillation. If a minimum probability of error receiver is employed for on-off keying (OOK), it is necessary to understand the two conditional probability densities (pdfs) corresponding to the transmission of ones and zeros. These probability densities are the distributions of signals received when the laser is on when sending binary ones and when the laser is off sending binary zeros. Many theoretical studies have determined the expected forms of the pdfs. An ongoing experimental study operating a low-power, low data rate link over a range of 9.3 Km has been started at Colorado State University-Pueblo to carefully examine the effects of atmospheric turbulence on laser communications. Experimental models of actual, true and typical pdfs have been obtained. The results do not always match theoretical predictions. The non-stationary nature of these pdfs is also a problem that must be addressed. This paper summarizes the experimental testing and shares a number of its conclusions.

The intensity fluctuations of random electromagnetic beams propagating in the atmosphere are studied. For such beams it is shown that when the atmospheric fluctuations are weak then the scintillation index (the normalized variance of intensity fluctuations) of the beam at any distance from the source depends not only on the state of coherence but also on the degree of polarization of the beam in the source plane. In particular, we found that for initially unpolarized beams the scintillation index generally takes on smaller values than that for completely polarized beams. The presented analysis might be useful in the applications (e.g. communications, laser radars) where atmospheric effects can be mitigated by adjusting the coherence properties and the polarization properties of the source.