Free-space Optical Communication Links Research Articles

Free-space optical communication based on hybrid optical mode array encoding

The generation, propagation and application of optical vortex have been hot research topics in recent years. Optical vortex carries orbital angular momentum (OAM) that potentially increases the capacity and the spectral efficiency of optical communication system as a new degree of freedom. The optical vortex can be used not only as information carrier for space-division multiplexing, but also for encoding/decoding. We present a novel free-space optical communication system based on hybrid optical mode array encoding/decoding. The array includes four modes that can easily be identified by image processing. The four modes are Gaussian beam, single optical vortex, and two different composite optical vortices. In this paper, the computer generated hologram (CGH) of the hybrid optical mode array is generated based on the object-oriented conjugate-symmetric extension Fourier holography. When the CGH is loaded onto the electronic addressing reflection-type spatial light modulator (SLM), a single light beam illuminates the SLM, and the desired hybrid optical mode array is generated. In the experiment, a m 32 pixel32 pixel Lena gray image is transferred. At the transmitter, the Lena gray image is scanned line by line. The gray value (0-255) of each pixel with 8-bit information is extracted from the image and converted into a 22 hybrid optical mode array, which is encoded into the CGH. Hence, the m 32 pixel32 pixel Lena gray image is corresponding to a sequence with 1024 CGHs. By switching the CGHs loaded onto the SLM, the Lena gray image is transmitted in the form of the hybrid optical mode array. At the receiver, each hybrid optical mode array is decoded to a pixel value. To distinguish different modes conveniently, two cross lines are set at the center of each mode. By counting the peaks of two intensity distribution lines, the modes can easily be identified. We demonstrate the image reproduction of Lena with zero bit error rate (BER). The experimental result shows the favorable performance of the free-space optical communication link based on hybrid optical mode array encoding/decoding. Compared to that of the traditional single-vortex encoding communication system, the information capacity of our system with 22 hybrid optical mode array increases by four times. In addition, the presented experimental system is feasible and has strong expansibility. The information capacity can increase by 16 times with a 44 hybrid optical mode array based on the same experimental setup. Therefore, the presented free-space optical communication system using hybrid optical mode array encoding/decoding has great significance for improving the capacity of free-space optical communication system.

Subcarrier multiplexing multiple-input multiple-output quantum key distribution scheme with orthogonal quantum states

Quantum key distribution (QKD) system is presently being developed for providing high-security transmission in future free-space optical communication links. However, current QKD technique restricts quantum secure communication to a low bit rate. To improve the QKD bit rate, we propose a subcarrier multiplexing multiple-input multiple-output quantum key distribution (SCM-MQKD) scheme with orthogonal quantum states. Specifically, we firstly present SCM-MQKD system model and drive symmetrical SCM-MQKD system into decoherence-free subspaces. We then utilize bipartite Werner and isotropic states to construct multiple parallel single photon with orthogonal quantum states that are invariant for unitary operations. Finally, we derive the density matrix and the capacity of SCM-MQKD system, respectively. Theoretical analysis and numerical results show that the capacity of SCM-MQKD system will increase $${\log _2}(N^2+1)$$log2(N2+1) times than that of single-photon QKD system.

Improved Performance Analysis of Free Space Optics Communication Link under Rain Conditions using EDFA Pre-amplifier

Abstract Free Space Optics (FSO) also known as Optical Wireless Communication (OWC) is a communication technology in which free space/air is used as the propagation medium and optical signals are used as the information carriers. One of the most crucial factors which degrade the performance of FSO link is the signal attenuation due to different atmospheric weather conditions such as haze, rain, storm, and fog. In this paper, an improved performance analysis of a 2.5 Gbps FSO link under rain conditions has been reported using Erbium-Doped Fiber Amplifier (EDFA) as a pre-amplifier. The results show that the maximum link distance for an FSO link under rain weather conditions with acceptable performance levels (Q ∼ 6 $$\sim6$$ and BER ≤ 10 − 9 ) $$ \le {10^{- 9}})$$ in the absence of EDFA pre-amplifier is 1,250 m which increases to 1,675 m with the use of EDFA pre-amplifier.

Investigation on the Performance of FSO Communication Link Under Various System Parameters

Optical Wireless Communication (OWC) also known as Free Space Optics (FSO) technology has many advantages such as availability of license-free spectrum, low deployment cost, large modulation bandwidth, low consumption of power, and small size. In this paper, the investigation on the performance of free space optical communication link has been done for different system parameters and transmission windows using OOK (on-off keying) modulation scheme. The performance of the system has been analyzed using Q Factor and SNR of the received signal as the performance metric. Also, the effect of atmospheric attenuation on the performance of the system has been analyzed by varying the link distance between the transmitter and the receiver for a specified transmission power level and bit rate. The performance of the system has also been investigated under different data transmission rates and transmission power levels using OPTISYSTEM simulation software.

Turbulence mitigation scheme based on multiple-user detection in an orbital–angular-momentum multiplexed system**Project supported by the National Natural Science Foundation of China (Grant Nos. 61271238 and 61475075), the Open Research Fund of Key Lab of Broadband Wireless Communication and Sensor Network Technology, Ministry of Education, China (Grant No. NYKL2015011), the Postgraduate Innovation Research Plan of Jiangsu Province, China (Grant No. CXZZ13_0489),

Atmospheric turbulence (AT) induced crosstalk can significantly impair the performance of a free-space optical (FSO) communication link using orbital angular momentum (OAM) multiplexing. In this paper, we propose a multiple-user detection (MUD) turbulence mitigation scheme in an OAM-multiplexed FSO communication link. First, we present a MUD equivalent communication model for an OAM-multiplexed FSO communication link under AT. In the equivalent model, each input bit stream represents one user’s information. The deformed OAM spatial modes caused by AT, instead of the pure OAM spatial modes, are used as information carriers, and the overlapping between the deformed OAM spatial modes are computed as the correlation coefficients between the users. Then, we present a turbulence mitigation scheme based on MUD idea to enhance AT tolerance of the OAM-multiplexed FSO communication link. In the proposed scheme, the crosstalk caused by AT is used as a useful component to deduce users’ information. The numerical results show that the performance of the OAM-multiplexed communication link has greatly improved by the proposed scheme. When the turbulence strength is 1 × 10−15 m−2/3, the transmission distance is 1000 m and the channel signal-to-noise ratio (SNR) is 26 dB, the bit-error-rate (BER) performance of four spatial multiplexed OAM modes lm = +1,+2,+3,+4 are all close to 10−5, and there is a 2–3 fold increase in the BER performance in comparison with those results without the proposed scheme. In addition, the proposed scheme is more effective for an OAM-multiplexed FSO communication link with a larger OAM mode topological charge interval. The proposed scheme is a promising direction for compensating the interference caused by AT in the OAM-multiplexed FSO communication link.

Probability density function of Hermite–Gaussian beam intensity in the presence of spatial error

The probability density function of the intensity of a Hermite–Gaussian (HG) beam is studied. It is assumed that the beam is used in a free-space optics communication link or laser ranging system. In particular, it is assumed that the captured optical radiation pattern assumes an HG beam intensity profile and that the beam is impaired by the residual pointing error cause by the spatial tracking loop responsible for tracking the atmospheric beam wander and platform sway. The mean and variance of the beam profile are also obtained to underscore the impact of spatial jitter on the overall beam profile.

Free space optical communications system performance under atmospheric scattering and turbulence for 850 and 1550 nm operation

In this work, a free space optical communication (FSO) link is proposed and utilized to explore and evaluate the FSO link performance under the joint occurrence of the atmospheric scattering and turbulence phenomena for 850 and 1550 nm operation. Diffraction and nondiffraction-limited systems are presented and evaluated for both wavelengths' operation, considering far-field conditions under different link distances. Bit error rate, pointing error angles, beam divergence angles, and link distance are the main performance indicators that are used to evaluate and compare the link performance under different system configurations and atmospheric phenomena combinations. A detailed study is performed to provide the merits of this work. For both far-field diffraction-limited and nondiffraction-limited systems, it is concluded that 1550 nm system operation is better than 850 nm for the whole presented joint occurrences of atmospheric scattering and turbulence.

Availability analysis of free-space-optical links based on rain rate and visibility statistics from tropical a climate

Availability is one of the major parameters to measure the performance of Free Space Optics (FSO) communication link. In tropical climate, rainfall acts as a dominant factor that affects the FSO link availability; whereas haze has no significant effect compare with rain. But for longer FSO link distances, the effect of haze on the overall FSO link availability becomes serious and need to be considered. In this paper, availability of an FSO link is estimated under the impact of rain, haze and the combination of the two phenomena. The availability estimations based on predicted attenuation due to rain and haze using long-term statistical measurements of rain rate (mm/hr) and visibility (km) data measured in Malaysia. The effect of individual weather phenomena was demonstrated over FSO link ranges from 1 to 5km. The combine effect of both rain and haze also has been analyzed using the cumulative distribution function (CDF) of predicted atmospheric attenuations. In tropical regions, for short FSO link distance, the influence of haze on the availability of FSO signal is negligible compare with rain; whereas for longer FSO distances, haze attenuation will reduce the overall link availability. This effect of haze needs careful consideration for the deployment of FSO systems especially in Malaysian tropical climates.

Free-Space Optical Communications in Vehicular Networks Using Rectangular Guiding Models

This letter introduces Free-Space Optical (FSO) Communication links in vehicular applications using potential guiding structures around a vehicle. An optical wireless communication system simulation is described which delivers received power, bandwidth, root mean square delay spread channel impulse response for purely diffuse and diffuse-specular materials with omnidirectional and directed transmitters. In the former case, a bandwidth of 225 MHz with a power deviation of 25% results at the exit. For the latter, a 75GHz bandwidth is available at best but with a power deviation of over 99% making receiver positioning critical. The impulse response is calculated using a Modified Monte Carlo algorithm taking into account up to 15 reflections. The effect of the pipe bend angle on the path loss is also presented and the simulation is supported with experimental work.

Saturation in cascaded optical amplifier free‐space optical communication systems

The performance of a free-space optical (FSO) communication system in a turbulent atmosphere employing an optical amplifier (OA) cascade to extend reach is investigated. Analysis of both single and cascaded OA FSO communication links is given and the implications of using both adaptive (to channel state) and non-adaptive decision threshold schemes are analysed. The benefits of amplifier saturation, for example in the form of effective scintillation reduction when a non-adaptive decision threshold scheme is utilised at the receiver for different atmospheric turbulence regimes, are presented. Monte Carlo simulation techniques are used to model the probability distributions of the optical signal power, noise and the average bit error rate due to scintillation for the cascade. The performance of an adaptive decision threshold is superior to a non-adaptive decision threshold for both saturated and fixed gain preamplified receivers and the ability of a saturated gain OA to suppress scintillation is only meaningful for system performance when a non-adaptive decision threshold is used at the receiver. An OA cascade can be successfully used to extend reach in FSO communication systems and specific system implementations are presented. The optimal cascade scheme with a non-adaptive receiver would use frequent low gain saturated amplification.

Both channel coding and wavefront correction on the turbulence mitigation of optical communications using orbital angular momentum multiplexing

A free-space optical (FSO) communication link with multiplexed orbital angular momentum (OAM) modes has been demonstrated to largely enhance the system capacity without a corresponding increase in spectral bandwidth, but the performance of the link is unavoidably degraded by atmospheric turbulence (AT). In this paper, we propose a turbulence mitigation scheme to improve AT tolerance of the OAM-multiplexed FSO communication link using both channel coding and wavefront correction. In the scheme, we utilize a wavefront correction method to mitigate the phase distortion first, and then we use a channel code to further correct the errors in each OAM mode. The improvement of AT tolerance is discussed over the performance of the link with or without channel coding/wavefront correction. The results show that the bit error rate performance has been improved greatly. The detrimental effect of AT on the OAM-multiplexed FSO communication link could be removed by the proposed scheme even in the relatively strong turbulence regime, such as Cn2=3.6×10−14m−2/3.

Feasibility study of free-space optical communication for South Africa

This paper aims to determine the feasibility of deploying free-space optical communication (FSOC) technology in South Africa. In order to achieve this aim, visibility, wind speed, and altitude data for several potential deployment locations over a period of 4 years have been used to compute the FSOC-based atmospheric losses under average and worst case atmospheric conditions. Results have shown that Ermelo has the highest optimal FSOC link distance of 7.5 km at an overall atmospheric loss of 2.8 dB under average conditions, while Durban has the shortest FSOC link distance at 2.6 km at an overall atmospheric loss of 12 dB under worst case conditions. It has also been found that the refractive index structure parameter is mainly altitude dependent. The parameter is larger at lower altitudes due to the more significant heat transfer between the air and the surface. Overall, this study has shown that FSOC technology deployment in South Africa is largely feasible (i.e., deployable with good reliability) for last mile broadband access networks, where link distances between transceivers measure 10 km on average. These results have been based on theoretical models, which take into account reasonable realistic assumptions of worst case atmospheric and the transceiver system parameter losses.

Rain rate intensity model for communication link design across the Indian region

A study on rain statistical parameters such as one minute rain intensity, possible number of minute occurrences with respective percentage of time in a year has been evaluated for the purpose of communication link design at Ka, Q, V bands as well as at Free-Space Optical communication links (FSO). To understand possible outage period of a communication links due to rainfall and to investigate rainfall pattern, Automatic Weather Station (AWS) rainfall data is analysed due its ample presence across India. The climates of the examined AWS regions vary from desert to cold climate, heavy rainfall to variable rainfall regions, cyclone effective regions, mountain and coastal regions. In this way a complete and unbiased picture of the rainfall statistics for Indian region is evaluated. The analysed AWS data gives insight into yearly accumulated rainfall, maximum hourly accumulated rainfall, mean hourly accumulated rainfall, number of rainy days and number of rainy hours from 668 AWS locations. Using probability density function the one minute rainfall measurements at KL University is integrated with AWS measurements for estimating number of rain occurrences in terms of one minute rain intensity for annual rainfall accumulated between 100mm and 5000mm to give an insight into possible one minute accumulation pattern in an hour for comprehensive analysis of rainfall influence on a communication link for design engineers. So that low availability communications links at higher frequencies can be transformed into a reliable and economically feasible communication links for implementing High Throughput Services (HTS).

Investigating Wavelength Dependency of Terrestrial Free Space Optical Communication Link

Investigating Wavelength Dependency of Terrestrial Free Space Optical Communication Link

Optical Attenuation & Atmospheric Visibility of Free Space Optics

The study was conducted at University of Gujrat during 2015 with an objective to explore the effects of dust on Free Space Optics (FSO) communication. Growing pollution and dust gales motivated studies we presented in this paper. In this paper analysis of the influence of dust on free space optics communication is presented. We focused our studies on Lahore as it is polluted city of Punjab and made a comparison with dust effects in Rahimyar Khan Region. Dense dust consists of large dust particles that raise from Earths sphere and whose atmospheric intensification values are high enough to affect FSO communication links. Dust storm research is based on visibility data. In order to test visibility data, we made calculations of atmospheric conditions for different dust conditions. Visibility of wave is calculated using weather spark. As it is related with the optical attenuation, so we also calculated optical attenuation of dust particles due to influence of dust particles and compared it with that of Rahimyar Khan.

Modeling and Simulative Analysis of High-Speed FSO Link using Different System Parameters

In this paper, modeling and simulative analysis of a Free Space Optical (FSO) communication link has been performed using different system parameters. Analysis has been performed by observing the Q Factor and SNR of the received signal under different data transmission rates, link length, divergence angle, and transmitted power levels of the information signal using OPTISYSTEM simulation software. It has been observed from the results that as the FSO link distance increases, the Q Factor of the received signal decreases and thus BER value increases. Also, as the data transmission rate of the system in increased, the Q Factor of received signal decreases. On increasing the value of divergence angle, the Q Factor of received signal decreases and thus degrades the performance of the system.

Orbital-angular-momentum-multiplexed free-space optical communication link using transmitter lenses.

In this paper, we explore the potential benefits and limitations of using transmitter lenses in an orbital-angular-momentum (OAM)-multiplexed free-space optical (FSO) communication link. Both simulation and experimental results indicate that within certain transmission distances, using lenses at the transmitter to focus OAM beams could reduce power loss in OAM-based FSO links and that this improvement might be more significant for higher-order OAM beams. Moreover, the use of transmitter lenses could enhance system tolerance to angular error between transmitter and receiver, but they might degrade tolerance to lateral displacement.

Mode-Division-Multiplexing of Multiple Bessel-Gaussian Beams Carrying Orbital-Angular-Momentum for Obstruction-Tolerant Free-Space Optical and Millimetre-Wave Communication Links.

We experimentally investigate the potential of using ‘self-healing’ Bessel-Gaussian beams carrying orbital-angular-momentum to overcome limitations in obstructed free-space optical and 28-GHz millimetre-wave communication links. We multiplex and transmit two beams (l = +1 and +3) over 1.4 metres in both the optical and millimetre-wave domains. Each optical beam carried 50-Gbaud quadrature-phase-shift-keyed data, and each millimetre-wave beam carried 1-Gbaud 16-quadrature-amplitude-modulated data. In both types of links, opaque disks of different sizes are used to obstruct the beams at different transverse positions. We observe self-healing after the obstructions, and assess crosstalk and power penalty when data is transmitted. Moreover, we show that Bessel-Gaussian orbital-angular-momentum beams are more tolerant to obstructions than non-Bessel orbital-angular-momentum beams. For example, when obstructions that are 1 and 0.44 the size of the l = +1 beam, are placed at beam centre, optical and millimetre-wave Bessel-Gaussian beams show ~6 dB and ~8 dB reduction in crosstalk, respectively.

Polarization-insensitive PAM-4-carrying free-space orbital angular momentum (OAM) communications.

We present a simple configuration incorporating single polarization-sensitive phase-only liquid crystal spatial light modulator (SLM) to facilitate polarization-insensitive free-space optical communications employing orbital angular momentum (OAM) modes. We experimentally demonstrate several polarization-insensitive optical communication subsystems by propagating a single OAM mode, multicasting 4 and 10 OAM modes, and multiplexing 8 OAM modes, respectively. Free-space polarization-insensitive optical communication links using OAM modes that carry four-level pulse-amplitude modulation (PAM-4) signal are demonstrated in the experiment. The observed optical signal-to-noise ratio (OSNR) penalties are less than 1 dB in both polarization-insensitive N-fold OAM modes multicasting and multiple OAM modes multiplexing at a bit-error rate (BER) of 2e-3 (enhanced forward-error correction (EFEC) threshold).

Performance evaluation of turbulence‐accentuated interchannel crosstalk for hybrid fibre and free‐space optical wavelength‐division‐multiplexing systems using digital pulse‐position modulation

A hybrid fibre and free-space optical communication link using digital pulse-position modulation (DPPM) in a wavelength-division-multiplexing system is proposed. Such a system, which could provide a power efficient, robust and flexible solution to high-speed access networks, is a contender for a passive optical network solution and could readily be deployed in areas with restrictions in optical fibre installation, or alternatively as a disaster recovery network. Interchannel crosstalk and atmospheric turbulence are major impairments in such a system and could combine in some cases to degrade the system. Both impairments are investigated here and the results are presented in the form of bit error probability, required optical transmission power and power penalties. Depending on the position of the interferer relative to the desired user, power penalties of about 0.2–3.0 dB for weak turbulence and above 20 dB for strong turbulence regimes are reported for bit error rate of 10−6. DPPM scheme with a coding level of 2 show about 2 dB improvements over on–off-keying scheme.