Free-space optical communications (FSOC) are rapidly becoming a key technology for terrestrial, aerial, and space communication, mainly because of its very high throughput capacity. To achieve multi-gigabit laser downstream, an efficient single-mode fiber coupling is required. However, atmospheric turbulence remains one of FSOC’s main limitations. The turbulence affects the communications performance by inducing wavefront distortions that develop into coupled power fluctuations. In regimes of very strong turbulence, the use of traditional adaptive optics systems is limited due to strong scintillation and higher number of phase singularities. These limitations could be solved by relying on systems based on the stochastic iterative maximization of the coupled power. The drawback of such systems is that a high number of iterations are required for signal optimization. We address this problem and propose a different iterative method that compensates the distorted pupil phasefront by operating directly on the focal plane. The technique works by iteratively updating the phases of individual speckles to maximize the received power coupled into a single-mode fiber. We show numerically and experimentally that the method can improve the quality of the received signal with reduced bandwidth utilization.
Due to expected capacity bottlenecks of exploited microwave technologies, feeder links for data relay or broadband access systems will require the implementation of high capacity optical communication links between space and ground. In this context, it is necessary a detailed investigation of the optical technologies and techniques that could enable the transmission of high data rates at optical frequencies through the Earth’s atmosphere, with regard to all kinds of atmospheric phenomena. In particular, the adverse effects of atmospheric turbulence fading are of special relevance to optical communication systems for ground-to-space uplink applications. Although previous studies and experiments have demonstrated the feasibility of such optical links at low data rate, research is still needed to identify technical solutions and strategies adapted to the specific constraints imposed to these high-speed links in order to ensure the required level of performance. Against this background, various test benches have been developed in order to characterize different modulation and detection techniques for optical communication systems prior to be incorporated in the conceptual design of future 1-Tb/s ground-space optical links. The expected performances of such an experimental demonstration are derived based on simulation models taking into account atmospheric turbulence effects, in order to prove the feasibility of reliable ground-space high date rate optical communication links. Our first simulation studies, considering On-Off Keying (OOK) and Differential Phase Shift Keying (DPSK) modulations, have allowed us to understand the complexity of the link and to optimize both the transmitter and the receiver to achieve acceptable performance levels.
The on-demand tailoring of the properties of light, such as phase, polarization or spatial shape, has completely changed the landscape of photonic-based applications. In this way, complex light fields have become an ubiquitous tool in areas of research such as classical and quantum communications, optical tweezers and super-resolution microscopy, among many others. Here we will present some novel applications to optical metrology. First we will show how appropriate tailoring of the properties of light interacting with chiral molecules can enhance their chiral response by two orders of magnitude compared to circularly polarized light. As a second application, we will present a highly sensitive digital technique capable to measure layer thickness in the nanometer regime. This technique is interferometric in nature and contrary to others based on the same principle does not require the highly engineered construction of holders. Finally, we will describe a novel laser remote sensing technique that enables the direct measurement of the transverse component of velocity, a measure that up to now has relied in complicated techniques based on measurements of the longitudinal component of the velocity. This technique offers the possibility to also measure in a direct way the vorticity in fluids, a measure that is commonly measured through the curl of the fluid velocity.
We have determined the optimal beams for free-space optical transmission through atmospheric turbulence. These are stochastic eigenmodes derived analytically from a canonical turbulence model, assuming known turbulence statistics. Under weak or strong turbulence, using these modes as transmit and receive bases minimizes signal degradation by turbulence, and minimizes the complexity of any signal processing method employed to compensate for turbulence. These modes can be mapped to/from single-mode waveguides by fundamentally lossless modal multiplexers and demultiplexers. Adaptive optics can be replaced by adaptive multi-input multi-output signal processing, enabling compensation of fast fluctuations of both phase and amplitude.
The need for an accurate time and position reference on orbiting platforms motivates the study of time transfer over satellite optical communication links. The transfer of precise optical clock signals to space would benefit many fields in fundamental science and applications. However, the precise role of atmospheric turbulence during the optical time transfer process is not well-known and documented. In free-space optical links, atmospheric turbulence represents a major impairment, since it causes degradation of the spatial and temporal coherence of the optical signals. We present possible link scenarios in which the atmospheric channel behavior for time transfer between ground and space can be investigated, and have identified the major challenges to be overcome. We found in our analysis that, despite the limited reciprocity in uplink and downlink propagation, partial two-way cancellation of atmospheric effects still occurs. We established that laser communication links make possible high-quality time transfer in most practical propagation scenarios and over a single satellite visibility period. Our results demonstrate that sharing of optical communication resources for optical time transfer and range determination is an effective and relevant scheme for space clock developments and enabling for future space missions.
In this work we will review some of the novel applications, recently proposed, where the use of structured light has played a crucial role. First, in the field of laser remote sensing, we discuss about a technique that allows to measure, in a direct way, the component of velocity perpendicular to the line of sight. This technique has found applications in the field of fluid dynamics, where an effective and simple optical technique capable to provide accurate measurements of ow vorticity, the tendency of a ow to rotate, was recently demonstrated. We then move to the field of profilometry to revise the key ideas behind a highly sensitive interferometric technique for thickness measurement, which is based on mode projection. We finally enter the field of optical activity to explore a novel proposal where an enhanced interaction between the handedness of structured light and chiral molecules was predicted.
In this work we present a practical, experimental analysis of the effects of adaptive optics compensation on the performance of free-space coherent optical receivers. In order to fulfill this objective, we have developed a laboratory test bed for simulating atmospheric turbulence using Kolmogorov statistics; we have implemented a digital-signal-processing-based phase shift keying heterodyne coherent receiver; and we have integrated a compact module operating a low-cost adaptive optics system that applies modal and zonal wavefront correction. We have checked our experimental results against previously reported analytical models describing the performance of coherent receivers using atmospheric compensation techniques.
The performances of satellite‐to‐ground downlink optical communications over Gamma–Gamma distributed turbulence are studied for a multiple‐aperture receiver system. Equal gain‐combining (EGC) and selection‐combining (SC) techniques are considered as practical schemes to mitigate the atmospheric turbulence under thermal‐noise‐limited conditions. Bit‐error rate (BER) performances for on‐off keying‐modulated direct detection and outage probabilities are analyzed and compared for SC diversity receptions using analytical results and for EGC diversity receptions through an approximation method. To show the net diversity gain of a multiple‐aperture receiver system, BER performances and outage probabilities of EGC and SC receiver systems are compared with a single monolithic‐aperture receiver system with the same total aperture area (same average total incident optical power) for satellite‐to‐ground downlink optical communications. All the numerical results are also verified by Monte‐Carlo simulations.
KEYWORDS: Receivers, Signal to noise ratio, Turbulence, Signal detection, Telecommunications, Atmospheric optics, Atmospheric propagation, Optical communications, Photons, Free space optics
Free-space laser downlinks have the potential to provide high data rates in space-to-earth communications and become a key enabler of future optical information systems in space. However, when a downlink passes through the atmosphere, clear-air turbulence induces serious phase distortion and fading. Here, we show how field conjugation adaptive arrays may function in downlink optical communications, overcoming the limitations imposed by the atmosphere by adaptively tracking and correcting atmospherically distorted signals. The goal of this analysis is to provide measures of performance related to practical downlink coherent receivers using adaptive compensation of atmospheric effects. The analysis reveals that for typical downlink conditions, substantial system performance gains can be obtained using only a small number of apertures, supporting use of the proposed array receivers in future space-based optical communication systems.
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 monolithic-aperture
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.
We introduce a realistic model for the impact of atmospheric phase and amplitude fluctuations on free-space links using
either synchronous or nonsynchronous detection. We compare options for atmospheric compensation, including active
modal methods and diversity combining techniques. 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 parameters,
including the ratio of receiver aperture diameter to wavefront coherence diameter, the scintillation index, the number of
modes compensated, and the number of independent diversity branches combined at the receiver. We analyze outage
Shannon capacity, placing upper bounds on the achievable spectral efficiency and enabling the performance of specific
system designs to be predicted.
We present recent studies on the impact of atmospheric turbulence on free-space links using either synchronous or
nonsynchronous detection. We compare options for atmospheric compensation, including phase compensation and
diversity combining techniques. We consider the effects of log-normal amplitude fluctuations and Gaussian phase
fluctuations, in addition to local oscillator shot noise, for both passive receivers and those employing active modal
compensation of wave-front phase distortion. We compute error probabilities for M-ary phase-shift keying, and evaluate
the impact of various parameters, including the ratio of receiver aperture diameter to the wave-front coherence diameter,
the number of diversity channels, and the number of modes compensated.
Free-space optical communications have distinct advantages over conventional RF and microwave systems by virtue of
their high carrier frequencies that permit high modulation bandwidth, enhanced security, freedom from interference, and
low power. However, the turbulent atmosphere causes phase variations along the path that are manifested in intensity
variations (scintillation) and beam profile distortion and boresight wander. These variations manifest as a multiplicative
noise source that reduces the ability of the receiver to determine the information contained in the modulation. For many
years, the emphasis throughout this area has been on elucidating those implications of the atmospheric propagation
problem that bear on the design and performance of optical communication systems. In this work, it is our intention to
elucidate how the addition of adaptive optics to the transmitter or receiver can reduce the effects of atmospheric
propagation and, in so doing, to quantify the improvement on the performance of optical communications systems
regarding coherent detection.
Free-space optical communications have distinct advantages over conventional RF and microwave systems by virtue of their high carrier frequencies that permit high modulation bandwidth, enhanced security, freedom from interference, and low powered. However, the turbulent atmosphere causes phase variations along the path that are manifested in intensity variations (scintillation) and high beam divergence. These variations are a noise source that reduces the ability of the receiver to determine the information contained in the modulation. For many years, the emphasis throughout this area has been on elucidating those implications of the atmospheric propagation problem that bear on the design and performance of optical communication systems. In this work, it is our intention to elucidate how the addition of adaptive optics to the transmitter or receiver can reduce the effects of atmospheric propagation and, in so doing, to quantify the improvement on the performance of optical communications systems regarding incoherent detection. Adaptive optics offers the potential for overcoming these limitations by adaptive tracking of the beam and correction of atmospherically-induced aberrations.
The application of scattering matrix theory in optics has usually been limited to multilayer propagation of plane waves but the analysis of spatial diffraction, the most characteristic effect of any optical element, has had to be considered through techniques much less competent. This paper describes the use of a generalized modal scattering matrix theory as a fast, efficient approach to the analysis of optical systems. In contrast with other methods, the new technique uses a type of optical vortices, called Bessel beams. This rigorous modelling technique has interest in areas of optics as diverse as optical communications, spectrometry, and remote sensing systems. The tactic allows solving both multilayered reflections problems and spatial diffraction phenomena using scattering parameters associated with the transmitted and reflected vortical spectrum.
Within the framework of the European Space Agency (ESA) SILEX project, aimed at experimentally demonstrating the feasibility of inter-satellite optical communications links, an Optical Ground Station (OGS) has been built by ESA in the premises of the Instituto de Astrofisica de Canarias (IAC, Institute of Astrophysics of the Canary Islands) Observatory of Teide, in the Tenerife island. The OGS is designed to test the optical communications payload on board the ESA's Artemis satellite and to perform ground-satellite optical communications experiments. As part of the OGS design study, an assessment of the impact of the atmosphere on the ground- satellite links was carried out. This assessment included experimental characterizations of the atmospheric effects through both measurements from stars in positions close to the Artemis one in bands comprising the SILEX wavelengths, using the IAC's Mons telescope in the Observatory of Teide, and measurements on a horizontal link with a transmitter near the IAC's Roque de los Muchachos Observatory in La Palma island, based on a laser diode similar to those to be used in SILEX, and a receiver in the Teide Observatory, almost 150 km apart, in the Tenerife island. The 830-nm wavelength horizontal measurements allowed checking the variations in the behavior of the atmospheric turbulence through the diurnal cycle. Besides the information relevant to assess the OGS performance, the horizontal-propagation experiments allowed to gather a considerable amount of propagation data on a very long path, most of it 2400 m above the sea.
This communication presents a new method to overcome some limitations in the simulation of the propagation of waves originating from a point source through a very long path in a turbulent medium. Existing propagation simulation algorithms suffer from either windowing or lack of resolution when applied to long paths. If Cartesian coordinates are used, the limited size of the numerical mesh originates undesired windowing errors in the long run. Casting the classical split-step Fourier algorithm in a spherically diverging coordinate system can solve this problem. In this way an angular mesh that adapts the source and the propagation algorithm to the geometry of the problem is used. But in long path propagation, this spherical divergent mesh causes a loss of resolution that can become a serious problem in the evaluation of the field statistical moments.
Atmospheric turbulence causes intensity and phase disturbances on the wavefront of electromagnetic waves propagation through it that can seriously degrade the reliability of free- space optical communication links. This paper deals with the estimation of the statistics for power fades resulting from the combined effects of distortion of the receiving system instantaneous point-spread function and from the fluctuations of the collected power arising from wavefront intensity fluctuations. Fractal techniques are employed to simulate the turbulence-induced point-spread function distortions, while a log-normal model is assumed for the collected-power fluctuations. The reduction in the cumulative probability of losses due to these two effects through spatial diversity using a multiaperture receiver configuration is assessed.
In planned intersatellite optical communication systems, the optical payload on board the geostationary satellite will be periodically pointed towards an Optical Ground Station. When the satellite-ground link is established, the turbulence-induced disturbances must be taken into account. The subject of this paper is to assess the statistics for the power fadings that result from the instantaneous point-spread function distortion. Results predicted by an approximate technique which considers the instantaneous point-spread function as a gaussian intensity distribution displaced from the focus due to the angle-of-arrival tilt are compared against results obtained from wavefront simulations produced by fractal generation techniques. The reduction in the cumulative probability of losses that can be obtained by spatial averaging using a multiaperture receiver is also assessed.
In planned intersatellite optical communication systems, the optical payload on board the geostationary satellite will be periodically pointed towards an Optical Ground Station. When the satellite-ground link is established, the turbulence-induced disturbances must be taken into account. The subject of this paper is to assess the statistics for the power fadings that result from the instantaneous point-spread function distortion. Results predicted by an approximate technique which considers the instantaneous point-spread function as a gaussian intensity distribution displaced from the focus due to the angle-of-arrival tilt are compared against results obtained from wavefront simulations produced by fractal generation techniques. The reduction in the cumulative probability of losses that can be obtained by spatial averaging using a multiaperture receiver is also assessed.
Adolfo Comeron, Javier Bara, Aniceto Belmonte, Juan A. Lopez Rubio, Joaquim Giner, Pedro Menendez-Valdes, Jose Diaz, Manfred Wittig, Paul Kremer, Chris Connor
Propagation effects at around 830 nm were evaluated in an intermountain scenario in the Canary Islands (Spain) in the framework of an ESA Free Space Optical Communication program. It is concluded that the extinction effects in clear weather conditions do not limit possible ground experiments. The different turbulence-induced effects are found to be reasonably consistent with simplified models. Power scintillation and point-spread function widening can be disturbing effects on the horizontal path.
Results are presented on laser-diode (LD) based intermountain tests carried out as a support activity to ESA's Free-space Optical Communications program to evaluate the Canary Islands (Spain) as an optical communicaton test range. The model predictions of low atmospheric attentuation and turbulence-induced effects were confirmed by a simple link test between observatories, showing that the Canary Islands are a proper site for performing terrestrial experiments of free-space optical communication systems.
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