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.
In free-space optical (FSO) communications, conditions may be met when laser links suffer from solar background radiation (SBR).
There are four types of such conditions
Direct sunlight hitting a photodetector
Reflected sunlight (glints)
Sunlight scattered by hydrometeors
Sunlight scattered by surrounding objects (walls, etc.)
Direct sunlight may cause total break of communications (link outage), and thus affect the link availability. However, experiments prove that the sunlight does not cause irreversible degradation of semiconductor photodetectors used in FSO systems.
Estimations are made of the link outage periods duration for various types of SBR conditions, also other effects caused by SBR have been considered. Recommendations are presented for the link directivity optimization to avoid (or to minimize the probability of) communication interrupts caused by SBR. A nomographic chart has been developed to forecast periods of time when direct or scattered solar radiation may cause link outage. With this chart, a user in any point of the globe, knowing the link orientation (azimuth and elevation angles), can see when and for how long (if at all) may the link operation be affected by unfavorable SBR conditions, also in many cases it is possible to recommend insignificant modifications in the link orientation causing material improvement in FSO system performance.
One important challenge to implementation of efficient free- space optical (FSO) systems is optical signal scintillation and fade caused by atmospheric turbulence and optical aberration in output beam shaping devices and windows. A new method for mitigation of these harmful effects to delivery of optical radiation to remote subscriber terminals, such as aberration and refraction index non-uniformity in a free- space path, has been developed and tested in field experiments. A known approach to damping optical signal scintillation caused by turbulence in a free-space path was based on forming several substantially parallel optical beams modulated by the same transmit signal and overlapping such beams on a receive optical aperture. The beams transmitted through different free-space paths with uncorrelated optical inhomogenity have different, uncorrelated, transverse distribution of light intensity. Their overlapping provides for averaging out the light intensity non-uniformity and efficient suppression of the signal scintillation. The existing approach to mitigation of optical aberration in atmosphere requires targeting several beam shaping telescopes at a subscriber. This is not always practical. For example, in point-multipoint FSO systems servicing multiple subscribers it is advisable to allocate one telescope per subscriber to achieve highest compactness and cost effectiveness of a system. Also the existing method has limitations in solving a problem of window glass optical inhomogenity and aberration in the telescope itself. A new method for optical aberration mitigation is based on using an extended light source with sufficiently large emitting surface and properly selected width of output radiation angular spectrum coupled to the telescope targeted at a subscriber terminal. The method has been implemented in a point-multipoint base terminal having multiple output beams that could be independently targeted at different subscriber terminals. Results of the trial are presented in this paper. The extended source with given light emitting surface diameter d and angular spectrum width (Theta) may be implemented with an optical fiber having core diameter d and numerical aperture NA equals sin((Theta) /2) installed in optical path between a light source with compact light emitting surface, such as a semiconductor laser, and the telescope. Exit end of such fiber coupled to the telescope acts as an extended light source with angular size (alpha) determined by the fiber core diameter and a focal length of the telescope via a formula (alpha) equals d/f. It has been proven in our field experiments, that by using the source with properly selected angular size and angular spectrum width the following results may be achieved with single telescope targeted at a subscriber terminal: (a) damping of optical signal scintillation at a remote photo-detector (the signal standard deviation has been decreased by several times for wide range of scintillation indexes); and (b) elimination of the signal fade caused by aberration in the telescope and output window (in our experiments the extended source provided 5 to 30 times increase in average signal power at the photodetector for a variety of window glass samples used in residential construction).
Based on analysis of the meteorological visibility statistical data, the probability P of degradation (unavailability) of a free-space optical communication (FSOC)link caused by the light scattering in fog and precipitation versus distance of communication D has been investigated. It has been found that for a variety of geographical locations, P(D) dependence averaged over entire year is nearly proportional. Such linear dependence is valid for a wide range of communication distances, from tens of meters to kilometers. It can be shown that the averaged P(D) function is linear if the density of the light scattering partices number statistics is driven by Pareto distribution, and the distribution of particle sizes does not depend on the density of particles number. It is important that averaging of the unavailability data for any separate month of a year gives non-linear and significantly different for different months P(D)functions.
The basic principles of nonreciprocal optical systems with phase -conjugating mirrors design and their main properties are presented. Such system can form high quality images by means of telescope with distorting large primary mirror. The demands to quality of the mirror are hundreds of times less strict, than for the common type telescope. Several different realizations of such systems were investigated experimentally and numerically.
The results cI experimental realizaticu of the 1as telescce with the phase-axijugation axnpensaticii 1 segmented main mirrcr w ith the diameter 300 mm, focal length 1200 mm are presented. The expiment results, which are correlated witti the numerical model results, show the pcssibility of diffraction limited image formation while segments angle irusmatch is up to 13X 1O and step mistake up to several dozen of micrometers.
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