This paper presents power budget predictions for retroreflective, free-space optical communication systems and examines options for producing retroreflectors with very wide field of view. Power budgets containing data representative of practical conditions show that operational ranges of many kilometres can be expected. Novel graded-index (GRIN) spherical retroreflectors have been examined in comparison with other types of retroreflector, and it is shown that they can offer technical advantages if they can be fabricated with suitable sizes and focal lengths. It is also shown that realisable high-index corner cube reflectors may be technically acceptable for this application.

Retrocommunication is a new technique for asymmetric free-space optical communication that has attracted interest during recent years. Novel technologies such as multiple quantum well (MQW) optical modulators and non-mechanical laser beam steering and tracking have been studied for implementation in a retrocommunication link. Large and small aperture reflective AlGaAs/GaAs MQW modulators were optimised and fabricated. The modulators exhibit high contrast ratios (from 5 to 100) and high modulation rates (up to 16 Mbit/s). A retroreceiver consisting of four large aperture MQW modulators, associated optics and drive electronics was fabricated. Nematic liquid crystal spatial light modulators have been evaluated, characterised for beam steering and tracking and implemented in a transceiver. Small area MQW modulators, used in focal plane configurations, were studied for static communication links. Results from a novel retrocommunication link utilising a retroreceiver and non-mechanical laser beam steering and tracking will be presented. Bit rates of 8 Mbit/s were observed during non-mechanical tracking of a moving retroreceiver over 100 m range. The demonstrator system was capable of transferring audio-, real-time images or bit streams. The demonstrated principles show promising features for future low weight free-space communication links. Performance calculations including requirements for a retrocommunication link using MQW modulators and non-mechanical beam steering are discussed.

A summary is presented of some of the design criteria relevant to the realisation of silicon micromachined modulator arrays for use in free-space optical communication systems. Theoretical performance levels achievable are compared with values measured on experimental devices produced using a modified Multi-User MEMS Process (MUMPS). Devices capable of realising modulation rates in excess of 300 kHz are described and their optical characteristics compared with published data on devices based on multiple quantum well technology.

A retro-reflective communications system comprises a laser transmit/receive station and a remote retroreflector that can be switched between "on" and "off" states. The laser illuminates the remote station and a collection telescope directs the reflected light to an associated detector the output of which is interpreted as logic 1 or 0. Atmospheric turbulence affects the outgoing illumination beam, resulting in beam spreading and in fluctuations in the intensity (scintillation)1. The reflected beam undergoes further turbulence induced spreading and there is an enhancement in the intensity fluctuations. These fluctuations mean that the logic level of signals may be wrongly identified, leading to bit errors. The signal may fade below detectable levels for periods of time, leading to sections of the bit stream being lost. We develop a description of the intensity fluctuations in terms of the Gamma-Gamma distribution2, and incorporate the effect of "aperture averaging" associated with the retro-reflector and the collection aperture. We characterise signal-to-noise ratios and calculate bit error rates as a function of retro-reflector cross-section and contrast for a variety of ranges, turbulence levels and system configurations. We also identify the characteristic timescale over which the atmosphere causes changes of intensity and discuss the implications.

BAE SYSTEMS reports on a program to characterize the performance of MEMS corner cube retroreflector arrays under laser illumination. These arrays have significant military and commercial application in the areas of: 1) target identification; 2) target tracking; 3) target location; 4) identification friend-or-foe (IFF); 5) parcel tracking, and; 6) search and rescue assistance. BAE SYSTEMS has theoretically determined the feasibility of these devices to learn if sufficient signal-to-noise performance exists to permit a cooperative laser radar sensor to be considered for device location and interrogation. Results indicate that modest power-apertures are required to achieve SNR performance consistent with high probability of detection and low false alarm rates.

FSO communication by retro-modulation is an innovative way to convey information between an optical transceiver and a semi-passive unit that reflects signals denoted by Modulating Retro Reflector (MRR). This concept is completely different from typical FSO communication systems consisting of two transceivers operating in full-duplex mode. In this paper, we review this new concept, analyze atmospheric effects due to folded paths propagation, evaluate different optical modulator technologies suitable for these applications and their impacts on system performance, and compare analysis results to the commonly implemented active intelligent RFID tags.

Strong optical injection and optical frequency matching have been used to effect an experimental demonstration of dual-mode synchronisation using a multi-mode external-cavity chaotic master laser and two single-mode stand-alone slave lasers. It is shown by means of synchronisation diagrams and measured cross-correlation functions that the longitudinal modes of the slave lasers have been successfully synchronised to frequency matched modes of the master laser operating in the low frequency fluctuation regime. The time lag between the lasers identifies that injection-locked synchronisation is achieved.

Optical synchronisation of chaos in unidirectionally coupled external cavity vertical-cavity surface-emitting semiconductor lasers (VCSELs) has been experimentally achieved. The polarisation of the injected beam is perpendicular to that of the free-running receiver (x-polarisation). Normal (positive-slope) synchronisation is observed between the injected beam and the y-polarized component of the receiver. However, due to the anti-phase dynamics of the receiver, an inverse (negative-slope) synchronisation is found between the injected beam and the x-polarized component of the receiver. A 200 MHz message has been successfully encoded in an external-cavity VCSEL transmitter and decoded in a receiver. Message recovery has been achieved with about 9 dB signal-to-noise ratio.

Protecting signals is one of the main tasks in information transmission. A large number of different methods have been employed since many centuries ago. Most of them have been based on the use of certain signal added to the original one. When the composed signal is received, if the added signal is known, the initial information may be obtained. The main problem is the type of masking signal employed. One possibility is the use of chaotic signals, but they have a first strong limitation: the need to synchronize emitter and receiver. Optical communications systems, based on chaotic signals, have been proposed in a large number of papers. Moreover, because most of the communication systems are digital and conventional chaos generators are analogue, a conversion analogue-digital is needed. In this paper we will report a new system where the digital chaos is obtained from an optically programmable logic structure. This structure has been employed by the authors in optical computing and some previous results in chaotic signals have been reported. The main advantage of this new system is that an analogue-digital conversion is not needed. Previous works by the authors employed Self-Electrooptical Effect Devices but in this case more conventional structures, as semiconductor laser amplifiers, have been employed. The way to analyze the characteristics of digital chaotic signals will be reported as well as the method to synchronize the chaos generators located in the emitter and in the receiver.

The growing awareness of the vulnerability of information transmitted on communication systems within the government, military and commercial sectors, has stimulated a number of areas of research within the optical community to design optical hardware encryption systems providing inherent immunity to espionage techniques. This paper describes a hardware optical encryption technique that utilises off the shelf telecommunication equipment and negates the necessity for an independent key distribution system with respect to the data transmission system, as is common with alternative encryption system implementations. This method also lends itself easily to fiber optic or free space communication and is applicable within any optical waveband. The encryption-decryption of the optical signal is achieved through low coherence optical interferometry. This requires the instantaneous processing and analysis of the signal, optically, to retrieve the relevant optical phase information hidden in the transmitted optical noise. This technology allows an authorised user to transmit encrypted information at a high data rate securely, while maintaining opaqueness to an unauthorised observer that data transmission is occurring. As the instantaneous optical field properties of the signals present in the system are essential to the optical encryption - decryption process, the system is inherently protected against electronic recording and advances in computational decryption algorithms. For organisations wishing to protect sensitive data and levels of communication activity these are highly desirable features.

Information transmitted by free-space optical (FSO) communication systems is generally well protected from unauthorized access (eavesdropping) by narrow divergence of laser beams and absence of narrow-band (about 20 nm) optical filters tunable within a wide wavelength range. However, in certain circumstances such an access is still possible, due to the fact, that part of the communication beam radiation is reflected and/or scattered by solid objects within the beam, dust and/or water droplets on window panes, and particles of atmospheric aerosol. If the wavelength is known a priory, and if specially designed equipment is used, detection and eavesdropping may be implemented under some specific conditions at distances up to several hundred meters. Recommendations are presented on measures that should be taken to prevent unauthorized access to FSO communication links.

Adaptive Optics (AO) is a critical underpinning technology for future laser delivery (including free-space optical communications), target illumination and imaging systems. It measures and compensates for optical distortion caused by transmission through the atmosphere, resulting in the ability to deploy smaller lasers and identify targets at greater ranges. One of the key components in an AO system is the wavefront modifier, which acts on the incoming or outgoing beam to counter the effects of the atmosphere. BAE SYSTEMS Advanced Technology Centre is developing multi-element bimorph deformable mirrors for such an applications. Our initial designs were based on a standard construction and exhibited a resonant frequency of 1kHz with a maximum stroke of ±20μm for an active aperture of 50mm. These devices were limited by the necessity to have a 'dead space' between the inner active area and the mirror boundary; this ensured that both the requirements for the stroke and the fixed boundary conditions could be met simultaneously. However, there was a significant penalty to pay in terms of bandwidth, which is inversely proportional to the square of the full mirror diameter. In a series of iteration steps, we have created novel mounting arrangements that reduce dead space and thus provide the optimum trade-off between bandwidth and stroke. These schemes include supporting the mirror from underneath, rather than at its edge. As a result, models of 60mm active diameter mirrors predict a resonance in excess of 5kHz, combined with a maximum stroke greater than ±40μm. This paper will discuss a number of different mirror designs and present experimental results for recently assembled devices.

This paper investigates some of the issues involved in creating viable free-space optical (FSO) communication links for mobile platforms. Within the aerospace industry much of the work to-date has been theoretical, entailing very sophisticated and expensive technology. In contrast, the work here focused on adding tracking and alignment technology to commercially available FSO equipment to create a practical technology demonstrator. The technology demonstration successfully took commercial FSO units designed for fixed static links and added a tracking system. A closed-loop tracking system to sense the infrared beam and actuate pan and tilt heads was developed. The results of a series of individual tests into the viability of the tracking and control technology are reported. The developed tracking system actively tracked each end of the link and remained continuously aligned while the mobile platform travelled a random 100m route, and also while the mobile unit rotated on the spot. The tracking system was independent of the main FSO link or the vehicle, requiring no knowledge of the absolute or relative location, speed or rotation of either end of the link.

Free space optical communications (FSO) requires receivers with a wide field of view, large collection area and high bandwidth, as well as good rejection of unwanted ambient illumination. At present most of the optoelectronic components used in these systems are designed for fibre-optic systems and as such are not optimal for this application. Work at the Universities of Oxford, Cambridge, Huddersfield and Imperial College has produced receivers incorporating detectors and preamplifiers specifically optimised for FSO and these show performance beyond that available commercially available. In this paper we describe the design, fabrication and performance of these integrated components. Further, we describe how this performance might scale with further optimisation, and future directions for optical receiver design.

We have experimentally demonstrated message broadcasting and decoding using a chaotic external-cavity DFB laser transmitter and two stand-alone DFB laser receivers. A GHz message has been successfully broadcast to the two receivers. Message recovery was achieved with greater than 14 dB signal-to-noise ratio.

In this paper Free Space Optics (FSO) in combination with Wireless LAN and Satellite Communications in Civil-Military-Cooperation (CIMIC) is presented. At first a modular communication system developed at the TU Graz is shown, which allows worldwide access to the Internet or other networks by combining Satellite Communications, FSO and Wireless LAN. Wireless LAN offers connectivity to mobile users in a network cell, Free Space Optics allows quick installation of broadband fixed wireless links instead of cables and Satellite Communications provides a backbone between distant locations in the world. In the second part results of Free Space Optics- and Satellite applications installed at a civil-military exercise in Spring 2004 in Styria (southern part of Austria) are presented. In this civil-military cooperation a mobile Satellite Earth Station (equipped with FSO and Wireless LAN) was used for Videoconferencing between military and civil organisations.