Coherent Free-space Optical Communication Research Articles

High-speed modulating retro-reflectors with optical phase conjugation compensation

A modulating retro-reflector (MRR) with optical phase conjugation (OPC) compensation is presented and demonstrated to mitigate the turbulence-induced channel distortion for high-speed coherent free-space optical (FSO) communications. The impact of the turbulence on the beam spot, the phase fluctuation and the bit error rate (BER) is analyzed for the MRR FSO link carrying quadrature phase-shift keying (QPSK) signals with spatial or single-mode fiber (SMF)-based OPC. The mitigation of the atmospheric turbulence is verified by comparing the transmission performance of the MRR FSO links with and without OPC. The compensation using SMF-based OPC is experimentally demonstrated in a turbulence-tunable atmospheric chamber. The BER is improved by two orders of magnitude in the turbulent channel with the temperature difference of 250 °C, and the power penalty is reduced by about 2.0 dB, which agree well with the simulation results. A larger receiving aperture size will improve the compensation effect

Dual-mirror adaptive-optics fiber coupling for free-space coherent optical communication

The wavefront distortion caused by atmospheric turbulence leads to the decrease of the coupling efficiency of spatial light to optical fiber. Adaptive optics technology was employed to improve the coupling efficiency of free-space coherent optical communication. Accordingly, the effect of the discrete distorted wavefront on the coupling efficiency could be determined. The experimental results showed that the average coupling power was −31.97, −31.42, −30.79, −32.05, and −35.31 dBm when turbulence strength D / r0 was 0.2, 0.5, 1, 2, and 5, respectively. After eliminating the noncommon path aberration, the fiber coupling optical power increased from −35 to −27 dBm. After the optical wavefront was transmitted at indoor (0.5 m), 600 m, 1, 5, 10, and 100 km, the coupling efficiency increased from 10.1%, 1.3%, 11.4%, 2.8%, 7.2%, and 8.2% to 95.2%, 83.4%, 93.5%, 56.7%, and 42.4%, respectively, after wavefront correction. This will improve the performance of free-space coherent optical communication system.

Design and experimental demonstration of 8-QAM coherent free-space optical communication using amplitude compensation and phase recovery.

In this paper, a new, to the best of our knowledge, scheme is proposed to mitigate the atmospheric turbulence effect in coherent free-space optical (FSO) communications with 8-quadrature amplitude modulation by employing amplitude compensation and phase recovery. The amplitude compensation and phase recovery algorithm in the scheme can significantly improve system performance without acquiring instantaneous channel state information or probability density function of a turbulence model. Numerical studies show that the bit error rate of the proposed scheme is four orders of magnitude lower than that of the system without any algorithm over a lognormal turbulence channel with normalized standard deviation of irradiance σ=0.25 and phase noise with normalized variance σϕ2=0.07, when the frequency offset fo=20MHz, combined linewidth Δf=10kHz, and average signal-to-noise ratio γ=20dB. Experiments are also carried out to investigate the performance of the scheme, and the results prove its superiority. Hence, this scheme can contribute to practical realization of the FSO system.

Design of 90-deg hybrid based on birefringent crystal for coherent optical communication system

Based on the birefraction of crystals and phase retardation of wave plate, a 2 × 4 90-deg crystal spatial optical hybrid is designed. It is comprised of three birefringent optical plates, λ / 4 wave plate and two λ / 2 wave plates. By splitting and recombining the signal beam and local oscillator beam, four mixed beams with phase shift of 90 deg are obtained. The mathematical model of the mixer is established by Jones matrices. The results show that the hybrid has low insertion loss, and by rotating the wave plate, the phase difference and split ratio between in-phase and quadrature-phase can be changed. It proved that the crystal hybrid design method we proposed has good performance; the phase compensation and splitting ratio adjustment scheme is simple, effective, and feasible; and it can be applied to free-space coherent optical communication system well.

Error performance analysis of optical communication over Lognormal-Rician turbulence channel using Gram-Charlier Series

Abstract In this paper, the symbol-error rate (SER) performance of a coherent free-space optical (FSO) communication system in lognormal-Rician turbulence channel is analyzed using the generalized Gram-Charlier (GCC) series. We proposed the differential evolution (DE) algorithm to solve the parameters in GCC efficiently. It is shown that highly accurate closed-form SER expressions are obtained for M-ary phase-shift keying (MPSK) and M-ary quadrature amplitude modulation (MQAM) schemes with maximum ratio combining (MRC) technique. The asymptotic error rate analysis is presented to reveal the performance behavior in the high signal-to-noise (SNR) regime. The effects of imperfect phase noise compensation on the error rate performance are also studied, and it is found that the impact of phase compensation error can be small enough with loop SNR ρ c more than seven.

Free-Space Communication Turbulence Compensation by Optical Phase Conjugation

A new scheme is proposed to mitigate the atmospheric turbulence effect in coherent free-space optical (FSO) communications by employing optical phase conjugation (OPC) to compensate the signal distortion. The compensation performance of the dual-hop FSO transmission link is simulated using the phase screen method for quadrature phaseshift keying (QPSK) signals. The results show that the bite error rate (BER) can be effectively reduced and the OPC correction is tightly affected by the OPC receiving aperture in strong turbulence regimes. The dual-hop FSO link with the OPC correction is experimentally demonstrated in a turbulence-tunable atmospheric cell, where the OPC is achieved by using degenerate four-wave mixing (DFWM) in a 110-m-long highly nonlinear fiber. At the BER of 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , the power penalty for dual-hop link with OPC correction decreases about 1.2 dB and 2 dB compared with the single-trip link and the non-compensation dual-hop link. The result demonstrates that the OPC unit can improve the performances of the coherent FSO communication, and it has the potential to be extended to higher-order modulation formats and higher bit rate processing.

Performance analysis of amplify and forward parallel relaying free-space optical system over M distribution

We propose a parallel relaying coherent orthogonal frequency division multiplexing free-space optical (FSO) communication system model with amplify-and-forward channel-state-information-assisted relaying in an M distribution. Considering the combined effects of path loss, pointing error, and atmospheric turbulence, the closed-form expression of moment generating function, probability density function, and cumulative distribution function of the upper limit of equivalent signal-to-noise ratio are derived. Furthermore, the Meijer G closed-form expressions for outage probability and symbol error rate (SER) over M aggregated channel are obtained. The influences of turbulence intensity, the number of parallel paths, normalized beam bandwidth, normalized jitter error, and pointing error on the outage probability and SER of the proposed FSO system are analyzed by simulation. Simulation results indicate that controlling the normalized beam width to a moderate range and reducing the normalized jitter error can reduce the outage probability and improve system reliability. After the pointing error exceeds 7, the SER decline tends to be flat, which provides a theoretical reference for the precision of the pointing technology, the selection of the optical antenna transmission aperture and the receiver sensitivity index in engineering design.

Low-complexity parallel real-valued weight adaptive digital combining algorithm for coherent FSO communications employing modes diversity reception under atmospheric turbulence channel

Diversity reception is an effective technology to mitigate the atmospheric turbulence effect on the performance of the coherent free-space optical (FSO) communications. Furthermore, considering high-speed optical communications and the state-of-art clock frequency of state-of-the-art field-programmable gate array (FPGA) or application-specific integrated circuits (ASIC), reducing the computational complexity of digital combining algorithm and analyzing the performance of parallel structure algorithm has become important research issues for diversity reception system. In this paper, we experimentally investigate the performance of a low complexity digital combining algorithm (CA) based on adaptive filter By replacing complex weight with real-valued weight, the number of real multiplications required in digital CA can be reduced by 62.5%. real-valued weight CA also ensuring the similar BER and outage probability performance in terms of serial structure under three different turbulence strengths. For parallel structure algorithm, the performance of real-valued weight CA is superior to that of complex-valued weight CA.

Development of adaptive optical correction and polarization control modules for 10-km free-space coherent optical communications

ABSTRACT In this work, we developed an adaptive optical correction module and polarization control module to compensate for the effects of atmospheric turbulence on long-distance free-space coherent optical communications. We established a coherent optical communication system based on heterodyne differential binary phase-shift keying modulation in Xi’an University of Technology, China, to achieve video transmissions over a long distance of 10.2 km. Indeed, the modules resulted in a baseband signal with a more regular pattern with a significant decrease in burrs and the polarization state of the optical signal was effectively controlled, which verifies the usability of the modules.

Influence of atmospheric turbulence on coherent source in a horizontal long-distance laser link

In coherent optical communication, the application of linearly polarized light can greatly improve communication sensitivity and communication capacity. However, when a laser transmits in an inferior atmospheric layer, the polarization character is influenced by atmospheric turbulence. This study proposes a measurement experiment of the characteristics of linearly polarized light over an 11.16-km turbulent path. We also present a theoretical approach and its operation for investigating the polarization characteristics of a laser. The variations of polarization under different atmosphere link distances and turbulence intensities are also discussed. The experimental results were in agreement with the theoretical analysis results. The theoretical analysis and experimental results show that the effect of atmospheric turbulence on polarization direction is stronger than that of atmospheric turbulence on the polarization state. This work will benefit coherent free-space optical communication system design efforts.

Performance analysis of 349-element adaptive optics unit for a coherent free space optical communication system

As a continuation of our previous work [Optics Express.25, 15229(2017)] in which we have verified the performance of a coherent free space optical communication (FSOC) system with a 97-element adaptive optics (AO) system, in this paper, we evaluated the performance improvement of the coherent FSOC system using a large-scale high-speed AO system with a 349-element continuous surface deformable mirror. The mixing efficiency (ME) and bit-error-rate (BER) under different Greenwood frequency (GF) were calculated as the performance metric of coherent FSOC system. The performance of FSOC system using such a large-scale AO system was quantitatively verified for the first time. The obtained results showed that the performance was obviously improved when a larger-scale high-speed AO system is employed in coherent FSOC system. This analysis result provides a performance verification for large-scale high-speed AO systems used in FSOC system which is beneficial for coherent FSOC system parameters design.

An experimental study of wavefront correction for a heterodyne detection system for optical communication

The turbulence effect is the most significant factor limiting the performance of coherent optical communication systems. In this paper, we report on wavefront correction for a coherent free-space optical communication system comprising a Shack–Hartmann wavefront sensor (SH-WFS), a deformable mirror, and a wavefront controller. With this system, distorted wavefronts of laser beams transmitted through the atmosphere can be corrected in real time. To demonstrate this real-time correction, we simulated atmospheric turbulence using phase screens of different intensities, based on a modified Hill spectrum. These laboratory-based simulations indicate that the peak-to-valley value obtained using the wavefront correction system in a closed-loop configuration can be reduced to less than 0.8 μm, the root-mean-squared error to less than 0.05 μm, and the Strehl ratio can be increased to greater than 0.8. Finally, the adaptive optics system described above was applied to a heterodyne detection system in an experiment investigating optical communication at a distance of 1.3 km, from which we were able to demonstrate real-time video transmission. The results of this experiment verified that a closed AO system can improve signal beam coupling power and increase the amplitude of the intermediate frequency signal, making the waveform of the baseband signal more ideal.

Adaptive digital combining for coherent free space optical communications with spatial diversity reception

Spatial diversity reception is an effective technology to mitigate the atmospheric turbulence effect on the performance of the coherent free space optical (FSO) communications. However, considering the real-time and fast atmospheric channel fading variation, how to combine the multiple receiving signals efficiently and adaptively has become one of the key technologies for multiple-aperture receivers. In this paper, to eliminate the time-consuming and complex estimation process of random and time-varying channel fading, a novel adaptive digital combining algorithm is proposed for coherent FSO communications with binary phase-shift keying (BPSK) and quadrature phase-shift keying (QPSK) modulation. For verification of the feasibility of the proposed combining algorithm, corresponding simulations of the proposed adaptive digital combining diversity based and traditional equal gain combining (EGC) diversity based four-aperture receiver system are carried out. The simulation results show that, with the proposed adaptive digital combining, signal quality of the combining output can be improved effectively and the required transmit power can be reduced by up to 3-10 dB under different turbulence conditions for the same BER compared to EGC.

OAM Mode Selection and Space–Time Coding for Atmospheric Turbulence Mitigation in FSO Communication

Orbital angular momentum (OAM) multiplexing has recently received considerable interest in free space optical (FSO) communications. Propagating OAM modes through free space may be subject to atmospheric turbulence (AT) distortions that cause intermodal crosstalk and power disparities between OAM modes. In this paper, we are interested in multiple-input-multiple-output (MIMO) coherent FSO communication systems using the OAM. We propose a selection criterion for the OAM modes to minimize the impact of the AT. To further improve the obtained performance, we propose a space-time (ST) coding scheme at the transmitter. Through numerical simulations of the error probability, we show that the penalty from AT is completely absorbed for the weak AT regime, and considerable coding gains are obtained in the strong AT regime.

基于数字相位恢复算法的正交相移键控自由空间相干光通信系统

基于数字相位恢复算法的正交相移键控自由空间相干光通信系统

Highly sensitive fiber coupling for free-space optical communications based on an adaptive coherent fiber coupler

Abstract In this paper, we present the result that the performances of the highly sensitive adaptive coherent fiber coupling (ACFC) technology, which consists of a movable fiber tip and a coherent detector. The ACFC technology is applied for the free-space coherent optical communication systems especially with a long distance link such as the satellite to ground, and it is used for compensating the tiny random angular jitter caused by the vibration of the receiver and the residual jitter error of the adaptive optics unit which is used for alleviating the effect of turbulence. Compared with the traditional fiber coupling using direct detection, the advantages of ACFC technology has been verified by analyzing the SNR value. The result shows that the receiver sensitivity can be enhanced by up to 20 dB. The residual jitter error of a free-space optical receiver with a 357-element AO unit is acquainted, and it is used for verifying the performance of the ACFC technology. The result shows that the average coupling efficiency is 73.2% with a mean square error (MSE) 1.12%, increasing significantly compared with the average coupling efficiency 50.1% and a MSE 7.43% without ACFC correction.

Channel correlation and BER performance analysis of coherent optical communication systems with receive diversity over moderate-to-strong non-Kolmogorov turbulence.

In this paper, new expressions of the channel-correlation coefficient and its components (the large- and small-scale channel-correlation coefficients) for a plane wave are derived for a horizontal link in moderate-to-strong non-Kolmogorov turbulence using a generalized effective atmospheric spectrum which includes finite-turbulence inner and outer scales and high-wave-number "bump". The closed-form expression of the average bit error rate (BER) of the coherent free-space optical communication system is derived using the derived channel-correlation coefficients and an α-μ distribution to approximate the sum of the square root of arbitrarily correlated Gamma-Gamma random variables. Analytical results are provided to investigate the channel correlation and evaluate the average BER performance. The validity of the proposed approximation is illustrated by Monte Carlo simulations. This work will help with further investigation of the fading correlation in spatial diversity systems.

Experimental results of 5-Gbps free-space coherent optical communications with adaptive optics

In a free-space optical communication system with fiber optical components, the received signal beam must be coupled into a single-mode fiber (SMF) before being amplified and detected. The impacts analysis of tracking errors and wavefront distortion on SMF coupling show that under the condition of relatively strong turbulence, only the tracking errors compensation is not enough, and turbulence wavefront aberration is required to be corrected. Based on our previous study and design of SMF coupling system with a 137-element continuous surface deformable mirror AO unit, we perform an experiment of a 5-Gbps Free-space Coherent Optical Communication (FSCOC) system, in which the eye pattern and Bit-error Rate (BER) are displayed. The comparative results are shown that the influence of the atmospheric is fatal in FSCOC systems. The BER of coherent communication is under 10−6 with AO compensation, which drops significantly compared with the BER without AO correction.

BER Analysis of Coherent Free-Space Optical Communication Systems with a Focal-Plane-Based Wavefront Sensor

A wavefront sensor is one of most important units for an adaptive optics system. Based on our previous works, in this paper, we discuss the bit-error-rate (BER) performance of coherent free space optical communication systems with a focal-plane-based wavefront sensor. Firstly, the theory of a focal-plane-based wavefront sensor is given. Then the relationship between the BER and the mixing efficiency with a homodyne receiver is discussed on the basis of binary-phase-shift-keying (BPSK) modulation. Finally, the numerical simulation results are shown that the BER will be decreased obviously after aberrations correction with the focal-plane-based wavefront sensor. In addition, the BER will decrease along with increasing number of photons received within a single bit. These analysis results will provide a reference for the design of the coherent Free space optical communication (FSOC) system.

Average BER of coherent optical QPSK systems with phase errors over M turbulence channels

The average bit-error-rate (BER) performance is studied for a coherent free-space optical communication system employing differentially encoded quadrature phase-shift keying (QPSK) with the Mth-power phase estimation method. A closed-form expression, considering the combined effects of the Malaga (M) turbulence fading, pointing errors, and phase estimation errors, is derived in terms of Meijer’s G function. Numerical and Monte Carlo simulation results are presented to verify the derived expression.