An experimental campaign for the propagation of laser modes carrying orbital angular momentum (OAM) over 1 kilometer has been established at UANDES university campus. We describe our method for estimating OAM spectra and the average topological charge values from the images delivered by Shack-Hartmann sensor. For OAM beams transmitted with a single topological charge we analyze the average departure of the measured charge with respect to the intended one and the spread of these values as a function of turbulence strength.
In order to improve laser communication link performance, turbulence strength is an important parameter to characterize the system’s correcting limits and detection availability. By setting an experimental optical link through free space horizontal propagation in a 1-km path, we study the strength of different turbulence scenarios through the Rytov approximation and scintillation of the beam, and compare methods of experimental detection of the refractive-index structure constant of the turbulence, C2n . Results show that, under low and medium turbulence regimes, both methods behave similiarly as a way to predict C2n ; however, with larger turbulence strength, the beam’s displacements in the focal plane are more sensitive than the intensity fluctuactions.
Vector vortex beams (VVB) combine a nontrivial phase structure and a transverse polarization pattern that can be used for optical free-space or optical fiber communication links. In this presentation we will show, using numerical propagation, the effects of atmospheric turbulence on the estimation of the pixel-by-pixel Stokes parameters, and how the concepts of Optimal Transport allow an optimal selection of basis symbols and a more accurate detection of the VVBs in moderate turbulence.
Spatial diversity is a technique widely used in wireless communications to enhance the signal quality at the receiver. We propose a multiple-input single-output system that utilizes this technique to enhance a free-space optics quantum communication link by reducing the amount of photon losses caused by atmospheric turbulence, thus increasing the capacity of the quantum channel. The system consists of two transmitters with uncorrelated optical paths, and a single receiver. A 515-nm quantum signal is transmitted through the transmission path with the highest gain, dynamically chosen by comparing the signal distortions of a 660-nm classical signal. Preliminary experiments with a single transmitter have been conducted in a laboratory environment with atmospheric turbulence generated via heat guns. We observed that the single-photon channel is highly correlated with the fluctuations of the 660-nm classical signal, so that an improvement in the former is expected when selecting the path with highest gain. The number of photon counts received was compared with the turbulence-free scenario, revealing that the mean number of counts decreased, and its standard deviation increased when turbulence is present, as expected.
We present an experimental campaign consisting on the propagation of laser modes with orbital angular momentum (OAM), carried out in our campus, for a total propagation distance of 1 kilometer. In this proceedings article we describe our experiment and describe some preliminary results, using topological charges up to |ℓ| = 45. We also demonstrate that a Shack-Hartmann sensor may be used for OAM sensing in the presence of weak to intermediate turbulence without the help from adaptive optics.
We propose the use of orbital angular momentum (OAM) states to form modulation symbols in a free-space laser communication link affected by atmospheric turbulence. A collection of superpositions of 2 and 4 active OAM modes are used for transmitting digital information. To sense and decode the data we compare three candidate architectures, based on a Mode Sorter, a Shack-Hartmann and phase flattening holograms. In this work we use concepts of Optimal Transport, particularly the Wasserstein distance and barycenter, for an optimal selection of OAM superpositions and a more appropriate processing of OAM spectra, leading to more accurate detection, achieving a classification error smaller than 1/1000 in intermediate to strong turbulence conditions.
When propagated through atmospheric turbulence, Orbital Angular Momentum (OAM) modes suffer a loss of orthogonality that can compromise their detection and classification. The problem is more challenging when user information encoded on multi-state OAM superpositions needs to be detected with high probability. Optical sensors like the Shack-Hartmann detector or the Mode Sorter are candidates for such task. We describe how OAM histograms derived from such detectors can be used for decoding the original data symbols. We propose Machine Learning strategies for a reliable classification of the histogram patterns obtained with 4-mode superpositions propagated over a 1 km range in weak to intermediate turbulence.
The increase of data rate and bandwidth efficiency of free-space optical communication links may be supported by the use of dense orbital angular momentum (OAM) states, carrying several information bits per transmission. Using machine-learning decoding, the performance of 32-OAM and 64-OAM signal constellations –designed using 4-state superpositions– are studied using numerical propagation models. Using two candidate architectures for detection –Shack-Hartmann and Mode Sorter– we evaluate the performance of the modulation in a simulated optical atmospheric channel by means of the detection accuracy.
We propose and demonstrate a communication system based on compact optics, using Raspberry Pi and a FPGA Basys3 boards at a speed of 50 Mbps through a propagation distance of 500 meters in medium to strong turbulence. Results in various scenarios and remaining challenges are presented.
We propose new techniques for the reconstruction of complex fields through the development of new iterative algorithms based on propagation equations and the representation of complex objects on orthogonal bases. These techniques use the advantages of the latest phase retrieval techniques and single pixel (SP) detection to retrieve the complex field. These techniques as a whole form a suitable tool for the characterization of dynamically perturbed scalar beams with singular phase profiles.
We review and compare recent methods for generating and detecting orbital angular momentum states in the context of free-space optical data transmission, considering their actual contribution in terms of aggregate capacity, photon efficiency, and flexibility. Based on simulation-based and experimental evaluations, we propose practical metrics to evaluate the performance of such systems under different scenarios, including those that require quantum security.
We propose an FSO receiver with adaptive-gain stage and associated filtering to reduce signal amplitude excursion and noise for decoding the received symbols with low error rate. A digital synchronization method together with a sampling strategy is developed for a FPGA-based receiver. The proposed techniques are evaluated through simulation and laboratory experiments under real turbulence.
An Adaptive Optics (AO) system may offer an alternative to compensate and correct for beam degradation by reducing turbulence distortions that affect signal detection over horizontal propagation. Based on an experimental testbed placed in the laboratory, we simultaneously study the effects of the communication signal detection, beam wavefront and image quality using a continuous membrane-type deformable mirror and Shack- Hartmann wavefront sensor. By inducing distorting effects on the beam with a Spatial Light Modulator and turbulence masks that are Rytov variance-equivalent to that of actual atmospheric scenarios, and by employing a Zernike polynomials decomposition, beam correction was achieved and signal detection improved. Our results show that both beam-spreading and beam-wandering were reduced after correction, but more significantly, the beam's intensity percentage over detector surface increased in 164%. Future improvements are discussed as an experimental campaign is being prepared to evaluate a closed-loop AO setup for an FSO communication link over a 400-m range at the university campus to evaluate the effectiveness of such approach at different hours of the day and weather conditions.
Free-space optical communications are highly sensitive to distortions induced by atmospheric turbulence. This is particularly relevant when using orbital angular momentum (OAM) to send information. As current machine learning techniques for computer vision allow for accurate classification of general images, we have studied the use of a convolutional neural network for recognition of intensity patterns of OAM states after propagation experiments in a laboratory. The effect of changes in magnification and level of turbulence were explored. An error as low as 2.39% was obtained for a low level of turbulence when the training and testing data came from the same optical setup. Finally, in this article we suggest data augmentation procedures to face the problem of training before the final calibration of a communication system, with no access to data for the actual magnification and level of turbulence of real application conditions.
This research is based in the analysis of the decomposition of Laguerre-Gaussian modes into Hermite-Gaussian modes when propagating through turbulence. This effect can be clearly observed by cancelling the original Laguerre-Gaussian beam with another of different topological charge. The results of the propagations of these beams through Kolmogorov phase masks were analyzed, identifying which turbulences achieved the decomposition. The turbulences were then decomposed into the first 15 Zernike polynomials and used as new "Zernike turbulences". We discovered that the turbulence internal process that allows the decomposition has a high correlation with a phase distortion based on a single Zernike polynomial or a combination of them.
Wavefront reconstruction of Laguerre-Gaussian (LG) beams presents a great challenge due to the singularities in their phase profiles. Interferometric methods are difficult to implement since they require a reference beam and they are very sensitive to mechanical perturbations. Deterministic methods such as those based on the phase-transport or intensity-transport equation are limited to the paraxial approximation. Furthermore, if such beams propagate in atmospheric turbulence, complex dynamic characteristics are added to the problem. We seek to find an implementation of a technique that is capable of dynamically recovering the singular phases of LG beams under such random conditions. In this work, we will demonstrate a phase-retrieval technique that allows the recovery of LG wavefronts in turbulence and will characterize its effectiveness under a range of atmospheric parameters and propagation length. This technique is based on binary amplitude modulation and is suitable for dynamic applications.
The Shack-Hartmann wavefront sensor has proven to be a valuable detector particularly in the context of turbulence and adaptive optics. In this work we take advantage of its capacity of characterizing orbital-angular-momentum (OAM) states under certain conditions, in the context of a free-space optical communication link. First, we propose a method to compute the locations of the light spots created by the lenslet array that is more robust than the simple centroid formula when atmospheric turbulence is present. Second, we propose a “local OAM” estimation that avoids the computation of a circulation integral in the discrete Shack-Hartmann array. Our proposal does not require prior knowledge of beam diameter or OAM state. We show simulations and laboratory experiments for OAM beams in turbulence conditions at which reliable detection is feasible. We analyze the quality of detection as a function of turbulence strength, Shack-Hartmann resolution and number of acquisitions. These ideas can be applied to coherent coaxial superpositions of two or more OAM states if the light rings are non-overlapping. Using the concept of “optical ferris-wheels” introduced by Franke-Arnold et al. (2007), the detection can be implemented for a limited set of pairs of vortices. We propose a generalization of the recipe for generation of ferris-wheels now for pairs of orthogonal vortices with arbitrary OAM states. The proposal is supported by simulations in turbulence conditions. An extension of our work considers the use of error-correcting codes that take advantage of the large set of available combinations of OAM states.
We demonstrate the communication of three coaxial laser channels using single transmit and receive apertures based on a set of distinct orbital-angular-momentum (OAM)-carrying modes. Each channel is individually modulated using OOK at 40 Mb/s and made incident onto an arrangement of spatial-light modulators (SLMs) that impose OAM. The three generated modes are combined before the transmit aperture. The received beam with superimposed OAM modes is filtered using SLMs to separate the channels, which are subsequently photo-detected. The effects of OAM crosstalk among channels and the selection of OAM modes are analyzed.
KEYWORDS: Free space optics, Modulation, Optical networks, Wireless communications, Telecommunications, Free space optical communications, Modulators, Data communications, Internet, Data centers
In order to solve capacity and energy-efficiency problems of future Internet technologies simultaneously, in this paper,
we propose the use of energy-efficient N-dimensional (ND) orbital angular momentum (OAM) coded-modulation. The
energy-efficient signal constellation is obtained by employing the energy-efficient signal constellation design algorithm.
This scheme can achieve beyond 100 Gb/s transmission while employing the state-of-the-art 10 Gb/s technology. The
proposed scheme significantly outperforms conventional M-ary PAM. The proposed scheme represents a promising
candidate for indoor optical wireless communication, terrestrial free-space optical (FSO) communication, data center
applications and can be used as enabling technology for heterogeneous optical networking, thanks to its transparency to
both free-space optical and few-mode/multimode fiber links.
The vast majority of large telescopes are now equipped with Adaptive Optics (AO) systems, and many use lasers to
create artificial stars (laser guide stars, LGS). Despite the significant advances in the use of LGS for AO, some problems
persist during the operations. In particular, achieving a satisfactory performance in terms of on-sky laser power and beam
quality usually requires frequent and complex alignments of the laser system, beam transfer optics and launch telescope.
To provide easier calibrations and faster pre-setting of the LGS facility during routine operations, we propose the
introduction of active elements (deformable mirrors) in the laser beam before it is propagated to the sky. The paper
studies an AO configuration with two deformable mirrors to correct for quasi-static and dynamic aberrations. The
problem of determining the correction phases to apply to the deformable mirrors is particularly challenging due to the
highly nonlinear problem and the possible appearance of branch points. We propose an iterative method based on a
phase retrieval algorithm that uses a weighted least squares unwrapper to avoid branch points. Simulations are performed
aiming to a future implementation in the Gemini Multi-conjugate-adaptive-optics System (GeMS). Results show that the
technique is accurate and robust, with a reasonable convergence speed.
In a laboratory experiment we generate, propagate, and detect laser vortex beams carrying orbital angular
momentum (OAM) by means of spatial light modulators (SLMs). We show that beams with OAM states from -
20 to +20 can be effectively generated using different types of phase gratings, and that excellent contrast between
adjacent OAM modes is achieved. A weak turbulent air flow is induced on the propagation path to emulate
the effects of atmospheric turbulence. By characterizing the effects of optical turbulence on the modal crosstalk
among received OAM states we show that it is possible to distinguish them for the purpose of increasing the data
throughput of a laser communication link in weak turbulence. It is also demonstrated that by increasing the
complexity of the receiver, optical separation of the OAM modes is possible at stronger turbulence conditions.
We study communication over atmospheric turbulence channels based on LDPC-coded, multidimensional OAM signal
constellations. Multidimensional signal constellation is obtained as the Cartesian product of one-dimensional signal
constellation X={(i-1)d, i=1,2,...,M} (where d is the Euclidean distance between neighboring signal constellation points
and M is the number of amplitude levels) as XN={(x1, x2,...,xN)|xi is from X, for every i}. This scheme represents an
energy efficient alternative, since log2(M N) bits/symbol can be transmitted. We describe two possible implementations
of N-dimensional OAM modulator and demodulator: (1) volume holograms based, and (2) multimode fibers (MMFs)
based. We evaluate the performance of this scheme by determining conditional symbol probability density functions
(PDFs) from numerical propagation data. Two cases of practical interest are studied: (i) when conditional PDFs are
known on the receiver side, and (ii) when conditional PDFs are not known and Gaussian approximation is used instead.
We show that in case (ii) an early error floor occurs because of inaccurate PDF assumption, which is caused by OAM
crosstalk introduced by the atmospheric turbulence. We also show that the OAM modulation is more sensitive to
atmospheric turbulence as the number of dimensions increases. Finally, we evaluate the BER performance for different
amplitude levels and different number of OAM dimensions.
In a laboratory experiment we propagate a continuous-wave, expanded laser beam over a surface with evenly
distributed heat and record the received distorted optical signal. Temperature is continuously measured in two
fixed points to determine turbulence strength. Through a number of trials covering a wide range of turbulence
conditions we demonstrate that the temporal correlation of the received signal fluctuations has a strong dependence
on the turbulence strength at the link path. Power spectra of the received signals show a clear increase
in both slope and maximum frequency as the temperature gradient increases. Measurements suggest that scintillation
also correlates with temporal correlation at weak turbulence conditions and such correlation decays in
stronger conditions.
We study the effects of optical turbulence on the energy crosstalk among constituent orbital angular momentum
(OAM) states in a vortex-based multi-channel laser communication link and determine channel interference in
terms of turbulence strength and OAM state separation. We characterize the channel interference as a function of
C2n
and transmit OAM state, and propose probability models to predict the random fluctuations in the received
signals for such architecture. Simulations indicate that turbulence-induced channel interference is mutually
correlated across receive channels.
KEYWORDS: Receivers, Scintillation, Free space optics, Sensors, Transmitters, Atmospheric optics, Error control coding, Signal to noise ratio, Turbulence, Telecommunications
We consider a terrestrial free-space optical communication link affected by atmospheric turbulence. By means of numerical simulations we estimate the reduction in scintillation achieved by the combined effect of aperture averaging and four transmitters when mutually-incoherent sources and a realistic detector size are used. We evaluate two space-time coding schemes: the first scheme uses a repetition code operating on four transmitters, and the second scheme uses a size-four rate-one space-time block code, originally proposed for wireless radio-frequency links. Both schemes deliver large signal-to-noise ratio gains compare to a single-beam system. The first scheme gives the best performance in all cases because of the natural incoherence found among optical sources.
We evaluate two error correction systems based on low-density parity-check (LDPC) codes for free-space optical (FSO) communication channels subject to atmospheric turbulence. We simulate the effect of turbulence on the received signal by modeling the channel with a gamma-gamma distribution. We compare the bit-error rate performance of these codes with the performance of Reed-Solomon codes of similar rate and obtain coding gains from 3 to 14 dB depending on the turbulence conditions.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.