This PDF file contains the front matter associated with SPIE Proceedings Volume 6877, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

Laser communication terminals based on homodyne BPSK are under in-orbit verification in LEO-to-ground and duplex LEO-LEO 5.65 Gbps links. With the LEO-to-ground link beacon-less acquisition has been verified as a reliable and quick acquisition procedure with acquisition times less than one minute.

NASA Goddard Space Flight Center is developing a direct-detection free-space laser communications transceiver test bed. The laser transmitter is a master-oscillator power amplifier (MOPA) configuration using a 1060 nm wavelength laser-diode with a two-stage multi-watt Ytterbium fiber amplifier. Dual Mach-Zehnder electro-optic modulators provide an extinction ratio greater than 40 dB. The MOPA design delivered 10-W average power with low-duty-cycle PPM waveforms and achieved 1.7 kW peak power. We use pulse-position modulation format with a pseudo-noise code header to assist clock recovery and frame boundary identification. We are examining the use of low-density-parity-check (LDPC) codes for forward error correction. Our receiver uses an InGaAsP 1 mm diameter photocathode hybrid photomultiplier tube (HPMT) cooled with a thermo-electric cooler. The HPMT has 25% single-photon detection efficiency at 1064 nm wavelength with a dark count rate of 60,000/s at -22 degrees Celsius and a single-photon impulse response of 0.9 ns. We report on progress toward demonstrating a combined laser communications and ranging field experiment.

A lasercom pointing, acquisition, and tracking (PAT) testbed has been developed at The Aerospace Corporation. The current setup represents a GEO-to-GEO link, but depending on the target PAT subsystem, this testbed can be reconfigured. No communication aspects are currently implemented. The system operates in small beam space and consists of a far field space simulator, and two identical lasercom terminals implemented with representative hardware (fast steering mirrors, optical detectors, etc.) and differentiated only by two laser sources operating at visible wavelengths of 473 nm and 633 nm. In this paper, the design process will be examined, and aspects of the pointing accuracy will be discussed.

The paper discusses a low power consumption, light weight and low cost 10MHz free-space optical communication (FSO). The system is designed for mounting on mobile platforms, with effective range of at least 50 meters. Light-Emitting Diodes (LEDs) and Laser diodes (LDs) are used in the transmitter for comparison for their relative performance in the system. The receiver features a beam splitter that feeds part of the data link optical signal to a quadrant photodetector and controls a tracking/stabilization system using microcontroller. Since almost all commercial FSO systems are mounted on fixed positions such as buildings, the paper explores the possibility and challenges of designing a FSO system for a mobile system such as remotely operated-vehicles or between ships.

Research toward returning high-rate data from deep-space using laser communications has been ongoing for nearly thirty years. Recently the Mars Laser Communication Demonstration (MLCD) Project completed the preliminary design of a system that would have allowed communicating at 1-Mb/s from Mars farthest range and at 30-Mb/s from the closest Mars range. In this paper we propose link improvements based on increased efficiencies and low-risk design modifications that will allow realizing a 30-dB improvement in return data rates from the farthest Mars range by the 2020 time frame.

Array-based architectures for deep-space photon counting lasercom links offer a powerful mechanism to lower the cost and improve the scalability of Earth- or space-based optical receivers. In this architecture, a large area receive telescope is constructed by using an array of small, inexpensive telescopes that are networked together. However, a limitation on how many small telescopes can be used comes from the minimum SNR requirement for synchronization. In general, the synchronization requirements are not difficult to meet for systems with enough SNR to achieve >Mbps performance. However, deep-space links often have large variations in their operational parameters due to varying link distances from orbital considerations as well as atmospherics. If the system is required to operate under such stressing conditions, albeit with a low (100's of Kbps) data rate, it is important to consider required SNR for synchronization as a design parameter. Furthermore, for very remote systems (e.g. beyond Mars), expected data rates may only be 100's of Kbps, in which case synchronization will be a critical design parameter. In this paper, we will examine the design trade space between number of arrayed telescopes and synchronization parameters. We will focus on the low SNR/ low data rate case as it is the most stressing.

This paper addresses various aspects for designing and implementing a bridging protocol for reliable data transfer between two local area networks over the high data rate slow-fading free-space optical channel. First the service requirements of widely spread applications of today's life like voice communication, video streaming or file transfer are given and the resulting constrains for data transfer protocols are compiled. Then, based on the physical characteristics of different communication scenarios and the compiled results, the feasibility of these services on optical free-space links is studied. Finally different protocol design aspects are discussed and an implementation concept for bridging data of different services over free-space optical links is presented.

Free-space optical (FSO) communications links are envisioned as a viable option for the provision of temporary high-bandwidth communication links between moving platforms, especially for deployment in battlefield situations. For successful deployment in such real-time environments, fast and accurate alignment and tracking of the FSO equipment is essential. In this paper, a two-wavelength diversity scheme using 1.55 μm and 10 μm is investigated in conjunction with a previously described tracking algorithm to maintain line-of-sight connectivity battlefield scenarios. An analytical model of a mobile FSO communications link is described. Following the analytical model, simulation results are presented for an FSO link between an unmanned aerial surveillance vehicle, the Global Hawk, with a mobile ground vehicle, an M1 Abrams Main Battle Tank. The scenario is analyzed under varying weather conditions to verify continuous connectivity is available through the tracking algorithm. Simulation results are generated to describe the performance of the tracking algorithm with respect to both received optical power levels and variations in beam divergence. Advances to any proposed tracking algorithm due to these power and divergence variations are described for future tracking algorithm development.

On/Off-Keying (OOK) data transmission in the atmospheric optical channel suffers from slow fading of the received power. Fading is produced by index of refraction turbulence and transmitter and receiver terminal alignment errors produced by tracking errors. Standard bit-error-ratio measurement techniques count the number of errors during a fixed time. Variation of measurement duration can change the measurement result significantly. Further measurement times are normally longer than fading events. Therefore no information about the distribution of bits in error in a data-stream can be measured with standard bit-error-rate testers. However the error distribution is important for the design of error correction methods like forward error correction. Therefore a special device has been developed to measure the error distribution. A description of this device is given in this paper. Further first measurements results are presented.

Laser beam propagating through the atmosphere encounters dynamic turbulence, which creates spatial and temporal fields of the refractive index. The resulting wavefront distortions lead to severe performance degradation in the form of reduced signal power and increased BER, even for short-range links. To alleviate this problem, an electrically addressed liquid crystal spatial light modulator (SLM) can be used to correct the wavefront by dynamically changing the optical path delays. Application of Zernike Formalism reduces the complexity of calculation of the SLM control signals by approximating the required phase profile. A real-time wavefront correction procedure utilizing Simplex optimization by Nelder and Mead was previously demonstrated. The performance of such procedure could be improved by proper re-initialization to avoid sub-optimum solutions. Interference-based phase estimation is proposed for this task and its potential was demonstrated in a proof-of-concept theoretical study. This paper presents the modification in the previously developed system and the corresponding experimental results, which show dynamic correction of the phase distortions.

Radio on Free-Space Optics (RoFSO) communication systems have attracted a considerable attention for a variety of applications where optical fibers are not feasible, especially in rural areas, to provide ubiquitous wireless services quickly and more effectively. RoFSO links can be used to transmit signals like cellular W-CDMA, terrestrial digital TV or WLAN signals. In spite of its potential, such links are highly dependent on the deployment environment characteristics in particular the weather conditions. Severity and duration of the atmospheric effects have direct impact on the availability of the links as well as on the quality of RF signal transmitted over it. Thus, the necessity of investigating the effects of various weather conditions on RF signal transmission using FSO links. In collaboration with several institutions, we are currently developing an advanced Dense Wavelength Division Multiplexing (DWDM) RoFSO antenna capable of transporting multiple RF signals. As preliminary work, we are conducting experiments on a 1 km link using an off-the-shelf Radio Frequency - FSO (RF-FSO) antenna, with the objective of obtaining and characterizing performance related parameters of RF-FSO transmission in operational environment. As an example, we examine the influence of atmospheric turbulence on the transmission quality of W-CDMA signal. Among the performance metric of interest is the Adjacent Channel Leakage Power Ratio (ACLR) which will be measured, analyzed and correlated with the weather conditions. An atmospheric fluctuation model for estimating the communication quality of RF signal transmission on FSO links is being developed. Also the obtained results will be used for the deployment environment characterization as well as baseline for the design and performance evaluation of new advanced DWDM RoFSO communication systems we are currently developing.

In the atmospheric optical channel amplitude modulated beam experience slow amplitude variations coming from refraction caused by atmospheric turbulence. The receiver introduces noise making the detection of the binary data subject to errors. Normally physical-layer proactive error protection is used to protect the system. Physical-layer communication is naturally a connection-orientated service because a synchronization of the code-symbols is necessary. Because of fading it is demanding to keep synchronization anytime and continuously alive. Therefore additional effort for synchronization has to be made in order to overcome fading. In this paper a multi-layer proactive error protection scheme is introduced and discussed. In this multi-layer approach each level of protection is addressing different problems.

We report on an application of a pulsed distributed feedback quantum cascade laser (QCL) for an open path data transmission. A pulse QCL in the 1046 cm^-1 range (28.7 THz) is used as a carrier signal source. The QCL is modulated with 50 ns pulses at repetition rate of 100 kHz, used as a sub-carrier. This sub-carrier is frequency modulated with a low frequency signal with bandwidth of 20 kHz (high quality sound signal). Data transmission experiments over 6 km open path were successfully completed. Pulse frequency modulation (PFM) technique instead of the usually used amplitude modulation was chosen because of its high immunity against amplitude noise and amplitude instabilities caused by atmospheric turbulence and aerosols. The quality of the demodulated signal is good enough, characterized by low distortion, low noise, high dynamic range and wide frequency band, even for detected signal variation of more than thousand times. The haze immunity of the Mid IR communication channel was studied in a laboratory and in a real open path conditions. The QCL beam was transmitted through a 60 cm cell, filled with water aerosol with high optical density in the visible. Despite the high aerosol optical density, sufficient to suppress completely a probe 20 mW 532 nm beam at a 5 cm distance, no distortion in the IR transmission was observed passing full 60 cm. The real 6 km open path transmission in a fog confirms the high haze immunity of IR beam propagation. The distance could be increased up to a few tens of kilometers. The bandwidth can be increased significantly up to a MHz range using a higher sub-carrier or up to a GHz range performing a direct frequency modulation of the laser frequency using CW QCL.

This paper presents an extensive insight into error protection techniques for free space optical links, focusing in particular in aeronautic stratospherically applications. The long distances present in these scenarios along with challenging atmospherically conditions present significant obstacles that degrade link performance. Thus it is imperative to apply highly efficient error protection scheme to avoid unacceptably high loss rates. The goal was to design a point-to-point data link layer error protection protocol that allows user-transparent bridging of Fast Ethernet data transmission over the optical fading channel in an high altitude inter platform link environment.

We present initial results on research and development of an optical antenna module suitable for Radio-on Free-Space Optics (RoFSO) links. This new optical communication system is envisaged to be an effective means of realizing a ubiquitous society and therefore eliminating the digital divide. The RoFSO system is a trial system applying Radio on Fiber (RoF) technology for transmission through free space. Based on the results of research of next generation high-speed free-space optical communication system conducted in the past two years at Waseda University, we have developed an optical antenna module with efficient laser receiving characteristics as well as simple adjustment. The tracking system adopts two phases including rough tracking by the beacon light at 0.85 μm wavelength and fine tracking using communication light at 1.55 μm wavelength to improve compensation precision for the atmospheric turbulence at the time of beam propagation. We present results on the evaluation of performance characteristics (static characteristics) of the separate functions for RoFSO antenna module we have developed and confirmed the coupling efficiency and fine tracking characteristics which were set as goals at the beginning.

Multi-axis displacement sensing technique with a single optical beam is proposed. Our system consists of propagating optical vortex beam and its imaging system. Since optical vortex beam have feature points known as zero-points or phase singularities in its beam profile, we can detect lateral and rotational displacement of the beam precisely by imaging and tracking the points. Unlike conventional laser displacement sensing techniques, our measurement scheme can be applied for the deformation sensing of civil infrastructures such as bridges and highways where triangulations can not be applied. The basic optical setup is presented and the results of the fundamental experiments are shown.

Pulse position modulation (PPM) is the preferred signaling format for deep space optical communications. Its high peak to average power ratio easily supports efficient forward error correction codes that operate within one dB of capacity at efficiencies better than one bit per detected photon when using photon counting detectors. Sub-100 picosecond slot widths are desired for efficient gigabit/second data links, but slot widths are limited by the jitter of available single photon detectors and laser modulators. Presently, the slot width must be larger than the receiver detector jitter and the transmitter modulator transition times. We show techniques whereby the slot time can be reduced, such that the jitter limitation is no longer the slot width but rather the PPM symbol duration.

Unique properties of ultrashort laser pulses open new possibilities for broadband optical communications in both space and terrestrial systems. Spectral slicing offers a promising approach to wavelength multiplexing using a coherent broadband source such as a modelocked femtosecond laser. We have realized a free-space spectral slicing and transmission system, with a spectrally sliced modelocked laser delivering ~100 fs pulses at 806 nm as the "frequency comb" source. Spectral slicing was performed using monolithic arrays of electro-absorption modulators (EAM) fabricated from quantum-well GaAs/AlGaAs semiconductor material with a bandgap energy falling within the fs pulse spectrum. The array bars contained between 2 and 10 individually addressable EAM channels and were packaged into modules with cylindrical micro-optics for efficient coupling of light into and from the semiconductor waveguide. By performing absorption measurements as a function of wavelength and voltage bias on the EAM, we identified the spectral region where modulation depth was the largest. Wavelength slicing was achieved by fanning out the fs pulse beam with a diffraction grating and coupling it across the full width of the EAM array. A modulation depth >12 dB was achieved by probing adjacent spectral channels using ON/OFF keying. In summary, we have demonstrated spectral slicing of femtosecond pulses with EAM arrays for free-space communications. The technology can find use in other areas, e.g., instant chemical analysis and remote sensing, as EAMs can modulate both the intensity and phase of randomly selectable spectral channels, allowing complex spectra and waveforms to be generated in real time.

A Two Terminal Laser Communication Test Bed has been developed at The Aerospace Corporation. This paper presents the design and preliminary results of a reprogrammable detector within the Test Bed for use in pointing, acquisition, and tracking between a Satellite-to-Satellite Laser Communication link. The detector may be commanded by an emulated spacecraft Command & Data Handling subsystem to switch between full-array scanning and "small sized" N x M pixel Field of View (FOV) for high-rate laser tracking. The approach follows a parallel path to implement the signal processing algorithm on two different hardware resources: a Field Programmable Gate Array (FPGA) and a Digital Signal Processor (DSP). The focus of this effort is to present a methodology for testing and evaluating various techniques for advanced focal plane array (FPA) hardware, as well as sensor FPA control, image processing and laser beam X & Y position algorithms.

Free-space optics (FSO), or Optical Wireless, is an unlicensed line-of-sight technology that uses modulated lasers to transmit information through the atmosphere. By using light beams, FSO can transmit and receive data, voice, and video, information through the air. FSO provides data rates ranging from 100Mbps to 2.5Gbps. In most applications, FSO transceivers normally remain in a static location to ensure continuous line of sight and to maintain accurate alignment. One current challenge facing FSO technology is the desire to implement mobility. As a potential solution, this study introduces an auto-tracking system that will achieve and maintain alignment between two mobile FSO nodes. This auto-tracking system can be used in many different applications, such as reducing the time needed to achieve alignment of an FSO link, and maintaining a link between an aircraft and a stationary command post to exchange real-time video and data with high-speed laser communications. After link initiation, the auto-tracking system application will send steering commands back to the positioning gimbal. These steering commands are determined by feedback from Position Sensing Diodes (PSDs). The proposed FSO auto-tracking system provides optical beam steering and capturing mechanisms to provide tracking between two transceivers, either fixed or mobile. In this paper, we illustrate the feasibility of such a system and present experimental results for a source aligned with a PSD in a mobile environment.