This PDF file contains the front matter associated with SPIE Proceedings Volume 7091, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

The capacity to integrate RF and free space optical hybrid communications now feasible given advances in adaptive optics and optical automated gain control. The ORCA program is developing on operationally capable of highly reliable hybrid communications. This paper provides an overview of the ORCA systems and discusses some of the key developments in making the systems a reality.

For free-space optical (FSO) communications systems, sensitive optical receivers are the key to closing the link over long distances in inter-satellite transmission scenarios, or to overcome large atmospheric attenuation in terrestrial FSO systems. We present a 10.7 Gb/s optical transmitterreceiver pair operating at 1550-nm, based on return-to-zero, differential phase-shift keying (RZDPSK). The receiver is pre-amplified and uses an optical delay interferometer and a balanced photo-receiver. The outer dimensions, the weight, and power consumption are 44×44×18 cm³, 14.1 kg, and 35 W, respectively. This optical receiver is single-mode fiber coupled. At 10.7 Gb/s, a receiver sensitivity of 27 photons/bit was measured, which yields a bit error rate of 1e-9. This is less than 1 dB from the quantum limit (22 photons/bit). Coupled with a commercial optical booster amplifier having an output power of about +37 dBm, a link loss of more than 80 dB can be bridged. In an inter-satellite communications scenario, this corresponds to several tens of thousands of kilometers. Additionally, high link losses can also be experienced in terrestrial systems as the result of atmospheric scintillation. To study this effect, the transmitter and receiver combination were tested with simulated turbulence (scintillation). A turbulence box was used to emulate different levels of scintillation under which the pre-amplified RZ-DPSK system was investigated. Results of these tests are presented.

Efficient use of a deep-space optical communications channel requires changes to the duty cycle of the modulation as the signal and noise powers change. This can be facilitated by modulating the signal with pulse-positionmodulation (PPM), supporting multiple PPM orders. To implement iterative demodulation, which is required by certain error-correction-codes to obtain near-capacity performance, would nominally require a distinct hardware implementation for each PPM order. In this paper we describe a method to utilize a single hardware implementation of an iterative demodulator for any PPM order. The method may be applied to any coded modulation that utilizes iterative demodulation and maps to multiple modulation orders.

Liquid Crystal Optical Phased Arrays (LCOPA) capable of steering optical beams over large angles require very large number of individually addressable electrodes that can be reduced by grouping the electrodes into periodic pattern to modulate phase profiles with consequent stepwise phase corrections made by an additional LCOPA. Such phase ramp-corrector configuration allows for reductions in the total number of the addressed electrodes and results in lower costs of development and manufacturing of LCOPA devices. Characterization of the device made by Teledyne Scientific for an experimental RF/EO antenna has been accomplished. Issues concerning optical beam steering efficiency, incident angle dependency and transparent electrodes alignment were investigated.

The rapid improvement in the efficiency of solid-state lighting has led to predictions that it will be the dominant source used for most indoor lighting applications in the future. At present an attractive candidate for generating white-light are blue LEDs that excite a yellow phosphor, with a resultant colour emission. Such solid state sources can be used for both illumination and communications simultaneously, offering the possibility of creating wireless broadcasting within a room or office space. In this paper we outline a typical basic configuration, and the performance available using simple modulation schemes. Unmodified LEDs typically have modulation bandwidths of several MHz, but typical lighting levels provide a communications channel with a Signal to Noise Ratios in excess of 40dB. Techniques such as equalisation can be used to improve available data rate significantly, and in this paper we outline several approaches that have the potential to offer data rates of 100Mb/s and above.

Omnidirectional free space optical communication receivers can employ multiple non-imaging collectors, such as compound parabolic concentrators (CPCs), in an array-like fashion to increase the amount of possible light collection. CPCs can effectively channel light collected over a large aperture to a small area photodiode. The aperture to length ratio of such devices can increase the overall size of the transceiver unit, which may limit the practicality of such systems, especially when small size is desired. New non-imaging collector designs with smaller sizes, larger field of view (FOV), and comparable transmission curves to CPCs, offer alternative transceiver designs. This paper examines how transceiver performance is affected by the use of different non-imaging collector shapes that are designed for wide FOV with reduced efficiency compared with shapes such as the CPC that are designed for small FOV with optimal efficiency. Theoretical results provide evidence indicating that array-like transceiver designs using various non-imaging collector shapes with less efficient transmission curves, but a larger FOV will be an effective means for the design of omnidirectional optical transceiver units. The results also incorporate the effects of Fresnel loss at the collector exit aperture-photodiode interface, which is an important consideration for indoor omnidirectional FSO systems.

Recent progress at the Applied Physics Laboratory in high data rate communications technology development is described in this paper. System issues for developing and implementing high data rate downlinks from geosynchronous earth orbit to the ground, either for CONUS or in-theater users is considered. Technology is described that supports a viable dual-band multi-channel system concept. Modeling and simulation of micro-electro-mechanical systems (MEMS) beamsteering mirrors has been accomplished to evaluate the potential for this technology to support multi-channel optical links with pointing accuracies approaching 10 microradians. These models were validated experimentally down to levels in which Brownian motion was detected and characterized for single mirror devices only 500 microns across. This multi-channel beamsteering technology can be designed to address environmental compromises to free-space optical links, which derive from turbulence, clouds, as well as spacecraft vibration. Another technology concept is being pursued that is designed to mitigate the adverse effects of weather. It consists of a dual-band (RF/optical) antenna that is optimally designed in both bands simultaneously (e.g., Ku-band and near infrared). This technology would enable optical communications hardware to be seamlessly integrated with existing RF communications hardware on spacecraft platforms, while saving on mass and power, and improving overall system performance. These technology initiatives have been pursued principally because of potential sponsor interest in upgrading existing systems to accommodate quick data recovery and decision support, particularly for the warfighter in future conflicts where the exchange of large data sets such as high resolution imagery would have significant tactical benefits.

Next generation communication networks are becoming increasingly complex systems. Previously, we presented a novel physics-based approach to model dynamic wireless networks as physical systems which react to local forces exerted on network nodes. We showed that under clear atmospheric conditions the network communication energy can be modeled as the potential energy of an analogous spring system and presented a distributed mobility control algorithm where nodes react to local forces driving the network to energy minimizing configurations. This paper extends our previous work by including the effects of atmospheric attenuation and transmitted power constraints in the optimization problem. We show how our new formulation still results in a convex energy minimization problem. Accordingly, an updated force-driven mobility control algorithm is presented. Forces on mobile backbone nodes are computed as the negative gradient of the new energy function. Results show how in the presence of atmospheric obscuration stronger forces are exerted on network nodes that make them move closer to each other, avoiding loss of connectivity. We show results in terms of network coverage and backbone connectivity and compare the developed algorithms for different scenarios.

Free-space directional communication links (FSO) experience short-term link outages or fades because of atmospheric turbulence, and longer term link outages because of obscuration resulting from either atmospheric conditions, beam pointing errors, or temporary line-of-sight obstructions for links from mobile or static nodes. Various approaches can be used to mitigate these effects. Physical-layer techniques, such as dynamic thresholding, time delayed diversity, and data encoding can significantly reduce the effects of short term (millisecond scale) outages caused by deep turbulenceinduced fades. Outages on a longer term (second scale) producing large data loss can be mitigated by packet-layer largeblock, error protection techniques. In this paper, we will first introduce physical-layer mitigation techniques. Second we present experimental data comparing the latencies and throughput of different means of packet-based error protection techniques. We will discuss the influence of error protection techniques to quality of service issues like error probability and delay and further compare this with service requirements given by the application.

The Free Space Optic (FSO) communication is a daily reality used by an increasing number of companies. For indoor environment, optical wireless communication becomes a good alternative with respect to radio proposals. For both technologies, the architecture is similar: emission/reception base station (Gateway or Bridge) are installed to cover zones, which are defined to ensure a quality of service. The customers may be connected to the Wireless Local Area Network (WLAN) with an adapter or module that emits and receives on this network. But due to its specific characteristics, wireless optical technology could present important advantages such as: Transmitted data security, medical immunity, high data rate, etc... Nevertheless, the optical system may have a limit on the network management aspect and link budget. The scope of this paper is to present a proposal at crossroads between optical fibre telecom system and data processing. In this document, we will present a prototype developed in Brittany during a regional collaborative project (Techim@ges). In order to answer to the management aspect and the link budget, this prototype uses an optical multiplexing technique in 1550 nm band: the Wavelength Division Multiple Access (WDMA). Moreover it also proposes a new class 1 high power emission solution. This full duplex system transmits these various wavelengths in free space, by using optical Multiplexer/Demultiplexer and optical modules. Each module has a defined and personal wavelength associated to the terminal identification (addresses MAC or IP). This approach permits a data rate at a minimum of a ten's Mbit/s per customer and potentially hundred Mbps for a line of sight system. The application field for the achieved and proposed prototype is potentially investigated from WLAN to WPAN.

High speed free space optical data links are currently finding limited use in military aircraft; however the technology is slowly starting to diffuse to civilian applications, where they could be used to provide a high bandwidth connection. However there are several issues that have to be resolved before the technology is ready for deployment. An important part of these are physical layer issues which deal with the ability to transmit and receive the optical signal reliably, as well as mechanical issues which focus on the construction of high performance, small and lightweight terminals for the optical transceiver. The later in conjunction with the cost of such a terminal create a significant limitation on the number of such equipment that any aircraft might carry on board. This paper attempts to evaluate how various such parameters affect the capability of an aircraft to take part in and help form a mesh network. The study was conducted by modeling the aircraft into a custom built SystemC based simulator tool and evaluating the connectivity achieved for varying several parameters, such as the pointing and acquisition time of the terminal and the number of terminals on board.

This work is part of an effort to investigate methodologies for active turbulence control of laser beam propagation through separated flows relevant to airborne laser applications as well as to detect the turbulenceinduced laser aberrations with Shack-Hartmann sensing. Large scale turbulence suppression control in separated compressible flows is investigated as a means to directly reduce aero-optical aberrations in laser propagation for airborne directed energy capabilities. Experiments are conducted on forced and unforced large-Reynolds-number compressible separated shear layers. Flow forcing is realized using a custom-built dielectric barrier discharge pulsed plasma actuator that can operate at elevated pressures. Results from flow control experiments show significant reductions in the root-mean-square optical path difference depending on the pulsed plasma actuator forcing frequency. Shack-Hartmann wavefront sensor laser profiling is conducted to measure directly the aero-optical aberrations. The flow conditions used in this research are Reynolds number of 6 million, based on the visual thickness of the turbulent separated shear layer, a freestream Mach number of 0.9, and an elevated test section pressure of 3 atm. The Shack-Hartmann sensor provides pathintegrated information regarding the turbulent refractive field and interfaces that the laser wavefront propagated through. Experimental comparison of control on vs. off cases indicates evidence showing the effectiveness of pulsed plasma forcing for the direct reduction of the laser aberrations. Since the dominant contributions to the aberrations, in unforced flows, are caused by large-scale organized structures, our findings indicate that the mechanism by which the significant reduction is observed in the present forced experiments is due to large-scale organized structure suppression effected by pulsed plasma forcing.

We have compared experimentally the transmission performance of on-off keying (OOK), differential phase-shift keying (DPSK), and pulse position modulation (PPM) optical waveforms at 2.5 Gb/s for free-space optical communication applications. We show that each technique is advantageous depending on the desired system complexity, cost, transmission data rate, optical aperture used (single mode vs. multimode), optical receiver used, availability of optical preamplifier, and ambient conditions. RZ format offers a 1- to 2-dB receiver sensitivity advantage over NRZ for both OOK and DPSK. With an optical preamplifier, an APD receiver offers no advantage over a PIN receiver. If the light can be collected efficiently in single-mode fiber without phase noise and an optical preamplifier is available, RZ-DPSK has definite performance advantages over other data formats. We measured an unoptimized balanced receiver sensitivity of 58 photons/bit for 10^-9 BER for RZ-DPSK. For multimode received optical signals, RZ-OOK combined with an APD receiver is an optimum choice for 2.5 Gb/s.

In this work the statistical distribution of the photocurrent generated at the receiver by detecting a Gaussian beam wave after propagating through the turbulent medium was investigated. The results are presented as photocurrent's relative frequencies obtained by means of computer simulation. It was taken into account the cases where random beam misalignment was present. With knowledge of the photocurrent statistical distribution the system bit error rate (BER), which is an important parameter of a digital communication system, was obtained. An experimental set up was mounted in order to collect data for comparisons with the theoretical results. The normalized relative frequencies of both simulated and measured current were plotted in the same graphic. The distributions have shown to be very similar to each other, what leads to the conclusion that the theory provides reliable results.

In free space optical communication systems, atmospheric turbulence makes it very difficult to focus transmitted laser power onto small, low capacitance photodetectors. The obvious challenge, therefore, is to take advantage of larger area photodiodes without sacrificing a great deal of bandwidth and sensitivity in the process. In this work, we report on a high sensitivity, high speed adaptive avalanche photodetector array for free-space optical communication. The receiver consists of a 2×2 InGaAs APD array with each 100um element in the array having its own dedicated trans-impedance amplifier and buffering stage. The corresponding voltage outputs for each element are processed through a four channel digital, fast switching and summation circuit. The resulting signal is selectable to be either that of the element in the array with the greatest signal response or the sum of multiple or all channels. Design requirements, laboratory sensitivity measurements, and field testing results are presented.

B-PPM formatting for trans-atmospheric optical communication is compared experimentally to OOK (NRZ) at a single channel data rate of 1.25Gbps in deep fading conditions. Unlike low data rate transmission using M-ary PPM formatting, high-speed B-PPM formatting does not benefit from the theoretical improvement that has been realized at low data rate. Although B-PPM can indeed benefit from a threshold set to near-zero, the high speed transmission precludes the implementation of a traditional Maximum Likelihood Detection circuit that compares the integrated power of each slot. At high speed, one has to rely on signal strength alone within the bit period which degrades the contrast between a "one" and a "zero." Moreover, the need for twice the bandwidth for B-PPM significantly limits available components such as APDs. More important, however, is the fact that during deep fades clock resynchronization dominates at high data rate. The primary question to be addressed is: Does B-PPM formatting really provide sufficient margin compared to NRZ to merit its use in deep fading atmospheric conditions? By building a special dual transceiver system, we have been able to propagate both B-PPM and NRZ formatted signals co-linearly on two C-band wavelengths centered close to 1550nm. Under field testing we measured the BER, including signal resynchronization, using special InGaAs, high-speed, multimode pigtailed, APD-based detectors in the receiver. The data were collected on fully instrumented horizontal paths of 1km and 500m with C_n ² [m^-2/3] ranging from 10^-15 to 10^-13.

Recent advances in ultraviolet (UV) semiconductor sources and detectors have inspired significant research activities in short-range UV communications, particularly in non-line-of-sight (NLOS) channel conditions due to atmospheric scattering. However, a scattering channel involves complex interactions of photons with atmospheric particles. This paper presents a parametric channel model that greatly simplifies channel characterization. For a short range link, single scattering may dominate in some scenarios. We model the channel impulse response with a gamma function as well as its variants to better fit the prediction by a widely adopted analytical single scattering model. Normalized mean square fitting error is adopted to validate our parametric model. Path losses and channel bandwidths are subsequently studied under different geometrical link configurations.

The performance of free space optical (FSO) links in a clear atmosphere is affected by the non-ideal characteristics of the communication channel. Atmospheric turbulence causes fluctuations in the received signal level, which increase the bit errors in a digital communication link. In order to quantify performance limitations, a better understanding of the effect of the intensity fluctuations on the received signal at all turbulence levels is needed. Theory reliably describes the behavior in the weak turbulence regime, but theoretical descriptions in the intermediate and strong turbulence regimes are less well developed. We have developed a flexible empirical approach for characterizing link performance in strong turbulence conditions through image analysis of intensity scintillation patterns coupled with frame aperture averaging on an FSO communication link. These measurements are complemented with direct measurements of temporal and spatial correlation functions. A He-Ne laser beam propagates 106 meters in free-space over flat terrain about a meter above the ground to provide strong atmospheric turbulence conditions. A high performance digital camera with a frame-grabbing computer interface is used to capture received laser intensity distributions at rates up to 30 frames per second and various short shutter speeds, down to 1/16,000s per frame. A scintillometer is used for accurate measurements of the turbulence parameter C_n². Laboratory measurements use a local strong turbulence generator, which mimics a strong phase screen. Spatial correlation functions are measured using laterally separated point detectors placed in the receiver plane. Correlations and captured image frames are analyzed in Labview to evaluate correlation functions, C_n², and the aperture averaging factor. The aperture averaging results demonstrate the expected reduction in intensity fluctuations with increasing aperture diameter, and show quantitatively the differences in behavior between various strengths of turbulence. This paper will present accurate empirical data in the strong turbulence regime. Such results can help build upon existing empirical data and lead to the development of new theories.

In this paper we present experimental beam wander measurements and compare them with theory developed from the Kolmogorov model for the atmosphere. Data from a collimated beam over a 1km horizontal terrestrial path were recorded with a high-speed camera. Several sets of image data were taken before, during, and after the quiescent period of the atmosphere in June 2006. The propagation path was instrumented with a commercial scintillometer to measure the Cn 2 and a weather station to monitor temperature and wind velocity. The data were analyzed for centroid movement and hot spot movement.

The Aerospace Corporation has developed a testbed for studying pointing, acquisition, and tracking systems for lasercom terminals. The testbed consists of two configurable terminals that are currently set up to represent a GEO-to- GEO link. Each terminal has the ability to point open-loop, execute scan patterns, and track a received beam. The system operates in small-beam space and consists of a far-field space simulator and two lasercom terminals operating at 473 nm and 633 nm with representative hardware (fast steering mirrors, optical detectors, etc.). This paper discusses the software developed for the testbed and the characterization of its performance, which includes open-loop pointing accuracy and residual tracking error in the presence of applied disturbances. Analytical predictions are compared to experimental results. Each terminal has the ability to progress from acquisition to tracking mode and the two terminals together demonstrate the cooperative acquisition process.

There is transmitting optics of 250mm aperture with about 8 microradians in SILEX system. This is often large aperture and diffraction-limited laser beam in the laser communications. Large-Optics white light interferometer using double-shearing structure has been submitted to analysis the laser wavefront before. Six optical plates of 490 millimeters apertures are manufactured now one of which is also aperture-divided so that the precision of measured wave front is higher than the full aperture design. It is suitable for measurement of minimum diffraction-limited laser wave front and any wavelength. The interference is happened between equal optical path of the reflection and the other. The plates are the basic structures which are precisely parallel or perpendicular needed for either two plates. There are several tools equipped with the interferometer including white light test source and collimators and so on to confirm the precision of several seconds angle. The apparatus and application is explained in detail in this paper. The adjustment is important for the realization of white light test.

Satellite laser communication systems offer many advantages such as high data rate, small sized equipment, low consumption electric power and others. Recent successful demonstrations of laser communications have demonstrated the feasibility of some of the key aspects of this technology. Sub-microradian pointing, acquisition, and tracking (PAT) functions are key issue to establish the laser communication system in space. The terminals must perform a series of onground test to evaluate characteristics of The PAT performances before flight test. So an optical dynamical PAT test bed is developed to perform ground test of a laser terminal. In this paper, we detail the separate test and the system test results of the optical dynamical PAT test bed. The test was carried out by auto-collimation method, the precision deflect accuracy, scan accuracy and deflect scan accuracy are obtained.

For testing the large-aperture and the diffraction-limited wavefront, we have proposed a revised Mach-Zehnder double-shearing interferometer with an aperture of 500 millimeters. This interferometer is composed of six same size plates. One plate is used as the beam splitter. Another one plate is acted as the beam combiner. Other plates are used as reflectors and one of them is cut to two parts. In this interferometer, two reflectors can be moved as a whole to change the shearing amount. The reflector cut to two parts is positioned to form the double shearing and it can be move to make the interferometer become an equal optical path system. In fitting process of this interferometer, the six plates are placed in the correct positions and directions by adjusting of the mechanical structure. The mechanical structures is also demanded to maintain the shape and position of the six plates in the application process. Thus the mechanical design of the interferometer is very important. The mechanical design is described in detail in this paper. The beam splitter and the beam combiner are mounted in one plate frame using strap-style support. A reference plane is generated on the lateral surface of the frame to act as a reference for the adjustment of all plates. The mounting structure of the reflectors is similar to the one of the beam splitter and their difference is the addition of the direction adjustment. With the mechanical design, this interferometer can meet the request of the wavefront testing.

Compared with direct detection, homodyne binary phase shift keying receivers can achieve the best sensitivity theoretically, and became the trend of the research and application in inter-satellite coherent laser communications. In coherent optical communication systems an optical hybrid is an essential component of the receiver. It demodulates the incoming signal by mixing it with the local oscillator. We present a design of a 2*6 optical hybrid. 4 output ports of the hybrid give the narrow mixed beams of the incoming signal and the local oscillator shifted by 90°for communication, and the others give the wide mixed beams with a shifted degree of 180°for position errors detection. CCD captures the interference pattern from the wide beams, and then the pattern is processed and analyzed by the computer. Target position information is obtained from characteristic parameter of the interference pattern. The position errors as the control signals of PAT (pointing, acquisition and tracking) subsystem drive the receiver telescope to keep tracking to the target. The application extends to coherent laser rang finder.

Degree of coherence of partially coherent beam is degraded when it propagates through atmospheric turbulence, remaining a very small coherent area inside the beam. Such small coherent area is not enough for the applications of laser communications and laser radar where a rather large coherent area is necessary for the optical imaging, especially for the heterodyne detection. In this paper, we start from cross-spectral density matrix of partially coherent beam on propagation through turbulence. Ratio of energy degradation is defined to represent the effect of residual coherent area inside the partially coherent laser beam. And we also give the evolvement of the residual coherent area inside partially coherent laser beam through channels involving atmospheric and sea water turbulences, respectively.

Transmission property of light through rain is discussed in this paper. Instead of the scattering methods usually used in predicting rainfall attenuation, the principle of the blotting-out effect of raindrops on light transmission is introduced. Based on this principle and an accurate raindrop size distribution, an initial model for predicting the attenuation due to rain is first set up. However, when it rains heavily, this initial model is not accurate enough because the overlap effect between the projected circles of raindrops is not considered. Therefore, by considering the blotting-out and overlap effect of raindrops together with an accurate raindrop size distribution, a more accurate model for predicting the attenuation due to rain is finally derived. Numerical simulations show that this model is in good agreement with experimental results.

A dual optical wireless test link works with two different optical wavelengths in spectral windows of 850 nm and 1550 nm. The transceivers of the link are placed on the highest peak of the Czech Central Mountains (Milesovka Mountain) so that the transmission path is oriented almost in a vertical direction. The installation site is situated in a locality with the harshest climate in the Czech Republic with extreme attenuation conditions. The almost vertically oriented path of the link allows analyzing the impact of the different atmospheric layers on the signal transmission. The monitoring of the received power and the archiving of the appropriate data are constantly provided. The relationship between the link attenuation and the atmospheric visibility has been investigated. The results of this experiment are presented.

We describe characterization of a hybrid Free-Space Optical (FSO) and Radio Frequency (RF) link for efficient switching between the two links. The monitoring and switching are controlled by a program that checks the FSO connection health using echo packets. The switching program was tested using a fiber optic link that can simulate atmospheric attenuation and scintillation effects by using an optical modulator. The sensitivity to connection quality degradation and momentary connection outages can be optimized for a given system. Connection quality dependent switching rather than continuous operation of both FSO and RF links is desired in situations where RF use is to be minimized, while maintaining high reliability.