4,646 research outputs found
Performance analysis of atmospheric field conjugation adaptive arrays
System configurations based on single monolithic-apertures that are immune to atmospheric fluctuations are being
developed. Main goal is the improvement of the performance achievable in coherent, free-space optical communication
systems using atmospheric compensation techniques such as adaptive optics. As an alternative to a single monolithicaperture
coherent receiver with a full-size collecting area, a large effective aperture can be achieved by combining the
output signal from an array of smaller receivers. We study the communication performance of field conjugation adaptive
arrays applied in synchronous laser communication through the turbulent atmosphere. We assume that a single
information-bearing signal is transmitted over the atmospheric fading channel, and that the adaptive array coherent
receiver combines multiple dependent replicas to improve detection efficiency. We consider the effects of log-normal
amplitude fluctuations and Gaussian phase fluctuations, in addition to local oscillator shot noise. We study the effect of
various atmospheric parameters and the number of branches combined at the receiver.Postprint (published version
Adaptive Subcarrier PSK Intensity Modulation in Free Space Optical Systems
We propose an adaptive transmission technique for free space optical (FSO)
systems, operating in atmospheric turbulence and employing subcarrier phase
shift keying (S-PSK) intensity modulation. Exploiting the constant envelope
characteristics of S-PSK, the proposed technique offers efficient utilization
of the FSO channel capacity by adapting the modulation order of S-PSK,
according to the instantaneous state of turbulence induced fading and a
pre-defined bit error rate (BER) requirement. Novel expressions for the
spectral efficiency and average BER of the proposed adaptive FSO system are
presented and performance investigations under various turbulence conditions
and target BER requirements are carried out. Numerical results indicate that
significant spectral efficiency gains are offered without increasing the
transmitted average optical power or sacrificing BER requirements, in
moderate-to-strong turbulence conditions. Furthermore, the proposed variable
rate transmission technique is applied to multiple input multiple output (MIMO)
FSO systems, providing additional improvement in the achieved spectral
efficiency as the number of the transmit and/or receive apertures increases.Comment: Submitted To IEEE Transactions On Communication
Digital equalization of time-delay array receivers on coherent laser communications
© [2017 Optical Society of America.]. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper are prohibited.Field conjugation arrays use adaptive combining techniques on multi-aperture receivers to improve the performance of coherent laser communication links by mitigating the consequences of atmospheric turbulence on the down-converted coherent power. However, this motivates the use of complex receivers as optical signals collected by different apertures need to be adaptively processed, co-phased, and scaled before they are combined. Here, we show that multiple apertures, coupled with optical delay lines, combine retarded versions of a signal at a single coherent receiver, which uses digital equalization to obtain diversity gain against atmospheric fading. We found in our analysis that, instead of field conjugation arrays, digital equalization of time-delay multi-aperture receivers is a simpler and more versatile approach to accomplish reduction of atmospheric fading.Peer ReviewedPostprint (author's final draft
Experimental and Analytical Investigations of an Optically Pre-Amplified FSO-MIMO System With Repetition Coding Over Non-Identically Distributed Correlated Channels
This paper presents theoretical and experimental bit error rate (BER) results for a freespace optical (FSO) multiple-input-multiple-output system over an arbitrarily correlated turbulence channel.
We employ an erbium-doped fiber amplifier at the receiver (Rx), which results in an improved Rx’s sensitivity
at the cost of an additional non-Gaussian amplified spontaneous emission noise. Repetition coding is used
to combat turbulence and to improve the BER performance of the FSO links. A mathematical framework
is provided for the considered FSO system over a correlated non-identically distributed Gamma-Gamma
channel; and analytical BER results are derived with and without the pre-amplifier for a comparative study.
Moreover, novel closed-form expressions for the asymptotic BER are derived; a comprehensive discussion
about the diversity order and coding gain is presented by performing asymptotic analysis at high signal-tonoise ratio (SNR). To verify the analytical results, an experimental set-up of a 2 × 1 FSO-multiple-inputsingle-output (MISO) system with pre-amplifier at the Rx is developed. It is shown analytically that, both
correlation and pre-amplification do not affect the diversity order of the system, however, both factors have
contrasting behaviour with respect to coding gain. Further, to achieve the target forward error correction
BER limit of 3.8 × 10−3
, a 2 × 1 FSO-MISO system with a pre-amplifier requires 6.5 dB lower SNR
compared with the system with no pre-amplifier. Moreover, an SNR penalty of 2.5 dB is incurred at a higher
correlation level for the developed 2×1 experimental FSO set-up, which is in agreement with the analytical
findings
Modern optical astronomy: technology and impact of interferometry
The present `state of the art' and the path to future progress in high
spatial resolution imaging interferometry is reviewed. The review begins with a
treatment of the fundamentals of stellar optical interferometry, the origin,
properties, optical effects of turbulence in the Earth's atmosphere, the
passive methods that are applied on a single telescope to overcome atmospheric
image degradation such as speckle interferometry, and various other techniques.
These topics include differential speckle interferometry, speckle spectroscopy
and polarimetry, phase diversity, wavefront shearing interferometry,
phase-closure methods, dark speckle imaging, as well as the limitations imposed
by the detectors on the performance of speckle imaging. A brief account is
given of the technological innovation of adaptive-optics (AO) to compensate
such atmospheric effects on the image in real time. A major advancement
involves the transition from single-aperture to the dilute-aperture
interferometry using multiple telescopes. Therefore, the review deals with
recent developments involving ground-based, and space-based optical arrays.
Emphasis is placed on the problems specific to delay-lines, beam recombination,
polarization, dispersion, fringe-tracking, bootstrapping, coherencing and
cophasing, and recovery of the visibility functions. The role of AO in
enhancing visibilities is also discussed. The applications of interferometry,
such as imaging, astrometry, and nulling are described. The mathematical
intricacies of the various `post-detection' image-processing techniques are
examined critically. The review concludes with a discussion of the
astrophysical importance and the perspectives of interferometry.Comment: 65 pages LaTeX file including 23 figures. Reviews of Modern Physics,
2002, to appear in April issu
SLM-based Digital Adaptive Coronagraphy: Current Status and Capabilities
Active coronagraphy is deemed to play a key role for the next generation of
high-contrast instruments, notably in order to deal with large segmented
mirrors that might exhibit time-dependent pupil merit function, caused by
missing or defective segments. To this purpose, we recently introduced a new
technological framework called digital adaptive coronagraphy (DAC), making use
of liquid-crystal spatial light modulators (SLMs) display panels operating as
active focal-plane phase mask coronagraphs. Here, we first review the latest
contrast performance, measured in laboratory conditions with monochromatic
visible light, and describe a few potential pathways to improve SLM
coronagraphic nulling in the future. We then unveil a few unique capabilities
of SLM-based DAC that were recently, or are currently in the process of being,
demonstrated in our laboratory, including NCPA wavefront sensing,
aperture-matched adaptive phase masks, coronagraphic nulling of multiple star
systems, and coherent differential imaging (CDI).Comment: 14 pages, 9 figures, to appear in Proceedings of the SPIE, paper
10706-9
Advanced deep space communication systems study Final report
Deep space communication system requirements for period 1970 to 198
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