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

Coherent detection can provide enhanced receiver sensitivity and spectral efficiency in free-space optical (FSO) communications. However, turbulence can cause modal power coupling effects on a Gaussian data beam and significantly degrade the mixing efficiency between the data beam and a Gaussian local oscillator (LO) in the coherent detector. Specifically, for widely used single-mode-fiber (SMF)-coupled coherent detectors, such degradation is mainly caused by the significantly reduced efficiency when coupling the multi-mode data beam into the SMF. Optical phase conjugation (OPC) in a photorefractive crystal can “automatically” mitigate turbulence by (a) recording a back-propagated turbulence-distorted probe beam, and (b) creating a phase-conjugate beam that has the inverse phase distortion of the medium as the transmitted data beam. However, previously reported crystal-based OPC approaches for FSO links have demonstrated either: (1) a relatively fast response time of 35 ms but at a relatively low data rate (e.g., <1Mbit/s), or (2) a relatively high data rate of 2-Gbit/s but at a slow response time (e.g., >60s). Here, we report an OPC approach for the automatic mitigation of dynamic turbulence that enables both a high data rate (8 Gbit/s) data beam and a rapid (<5ms) response time. For a similar data rate, this represents a 10,000-fold faster response time than previous reports, thereby enabling mitigation for dynamic effects. In our approach, the transmitted pre-distorted phase-conjugate data beam is generated by four-wave mixing in a GaAs crystal of three input beams: a turbulence-distorted probe beam, a Gaussian reference beam regenerated from the probe beam, and a Gaussian data beam carrying a high-speed data channel. We experimentally demonstrate our approach in an 8-Gbit/s quadrature-phase-shift-keying coherent FSO link through emulated dynamic turbulence. Our results show an up to ∼10-dB improvement in the free-space-to-SMF coupling efficiency for the data beam under dynamic turbulence with a bandwidth of up to ∼260Hz (Greenwood frequency). Our approach has the potential to significantly increase the resilience of high-performance coherent FSO links to turbulence.

The performance of ground to satellite Free Space Optical (FSO) communication link is degraded due to the presence of atmospheric turbulence that causes fluctuations in both intensity and phase of the received optical signal. In this paper, bit error rate (BER) performance of FSO link using transmit diversity is investigated for coherent (sub-carrier BPSK and sub-carrier QPSK) and non-coherent (OOK and Q-PPM) modulation schemes in weak atmospheric turbulence channel. It is seen that multiple transmitting antennae help to overcome the turbulence induced fading effect. It is observed that sub-carrier BPSK modulation outperforms the other modulation schemes in terms of minimum SNR requirement for a given BER. Among non-coherent modulation schemes, Q-PPM performs better than OOK. However, as the level of Q in Q-PPM increases, its performance degrades and it becomes worse than OOK for certain values of atmospheric turbulence. Also, the effect of correlation among different transmitting antennae beams is analysed on the link performance.

One of the major factors which degrades the Free Space Optics (FSO) link performance is the atmospheric turbulence. Turbulence manifests in increased Bit Error Rate (BER) leading to degradation in the link performance. Adaptive Coding and Modulation (ACM) technique is used to mitigate this effect. This technique is an adaptive transmission scheme wherein transmission parameters such as modulation scheme, code rate, etc., are adjusted based on Channel State Information (CSI). In this paper, we have considered lognormal channel model in low turbulence conditions. Three adaptive transmission schemes viz., variable data rate, variable modulation schemes and variable coding rate are considered. The entire work has been carried out in MATLAB. We see improvement in FSO link performance in terms of average transmitted data rate when variable data rate adaptation scheme is used. Improvement in spectral efficiency is observed when the other two adaptation schemes are used. Design of adaptive transmitter for all the three schemes is presented and data rate of RF feedback channel is determined.

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

Performance analysis of modulating retro-reflector FSO communication systems over Málaga turbulence channels

Modulating retro-reflectors (MRR) couple passive optical retro-reflectors with electro-optic modulators to allow free-space optical communication with a laser and pointing/acquisition/tracking system required on only one end of the link. In operation a conventional free space optical communications terminal, the interrogator, is used on one end of the link to illuminate the MRR on the other end of the link with a cw beam. The MRR imposes a modulation on the interrogating beam and passively retro-reflects it back to the interrogator. These types of systems are attractive for asymmetric communication links for which one end of the link cannot afford the weight, power and expense of a conventional free-space optical communication terminal. Recently, MRR using multiple quantum well (MQW) modulators have been demonstrated using a large area MQW placed in front of the aperture of a corner-cube. For the MQW MRR, the maximum modulation can range into the gigahertz, limited only by the RC time constant of the device. This limitation, however, is a serious one. The optical aperture of an MRR cannot be too small or the amount of light retro-reflected will be insufficient to close the link. For typical corner-cube MQW MRR devices the modulator has a diameter between 0.5-1 cm and maximum modulation rates less than 10 Mbps. In this paper we describe a new kind of MQW MRR that uses a cat's eye retro-reflector with the MQW in the focal plane of the cat's eye. This system decouples the size of the modulator from the size of the optical aperture and allows much higher data rates. A 50 Mbps device has been demonstrated.

Effect of atmospheric turbulence on the BER performance of an optical CDMA FSO link with SIK receiver

The WiMax (Worldwide Interoperability for Microwave Access) is a technology in<strong> </strong>broadband wireless access, which employs OFDM (Orthogonal Frequency Division Multiplexing) as an alternative transmission to enable high speed data in communication system. This research aim is to analyze the performance system of the OFDM-Based WiMAX, which used the cyclic prefix. The model was designed in four schemes of simulation method, the BPSK (Binary Phase Shift Keying, QPSK (Quadrature Phase Shift Keying), 16 QAM (Quadrature Amplitude Modulation) and 64 QAM. Each scheme was investigated BER (Bit Error Rate) on AWGN (Additive White Gaussian Noise) channel and multipath Rayleigh fading channel, which had applied the cyclic prefix. By simulation of the cyclic prefix was produced the modulation measurement of the BPSK, QPSK, 16 and 64 QAM. The performance result of Eb/No 15 dB was obtained the BER of BPSK and QPSK of 1.11E-11, the BER of 16 QAM and 64 QAM of 8.69E-06 and 0.00333 respectively. Those results indicated much smaller BER value than EbNo 0 dB which BPSK and QPSK of 1 BER, 1.5 and 1.75 BER for 16 QAM and 64 QAM respectively. In conclusion, a higher value of EbNo, hence the BER value would be lower.

Joint mitigation of frequency offset and phase noise for coherent free-space optical communication in atmospheric turbulence

Availability analysis of terrestrial free space optical (FSO) link using visibility data measured in tropical region

This paper proposes an ultra-high capacity free space optical communication (FSO) system, employing optical dual polarization quadrature phase shift keying (DP-QPSK) modulation format. It is a 16-channel dense wavelength division multiplexing (DWDM) system with 120 Gbps/channel bit rate. The FSO system is analyzed with three optical advanced modulation schemes; such as 4-quadrature amplitude modulation (4-QAM), quadrature phase shift keying (QPSK) and DP-QPSK. The main focus of the projected system is to improve the FSO setup performance in terms of eye diagram, bit error rate (BER), high received power and Q-factor. The center channel wavelength used for the proposed system is 1550 nm. In the present wireless communication world, the proposed system will be highly helpful as an ultrahigh capacity wireless system.

Performance of free-space optical communication links can be affected by spatial and temporal fields of the refractive index created by atmospheric turbulence. They result in wave front distortions and lead to performance degradation, which can be expressed quantitatively by reduction of the signal power and increase of the bit-error-rate (BER). Under severe turbulence conditions, these effects can be profound even in short-range links. While it is impossible to obtain closed-form solutions for instantaneous realizations of wave front distortions, several spectral models of atmospheric turbulence can be used to study statistical properties of the refractive index fluctuations and to find the scintillation index. This analysis can be further expanded to include communication performance of free-space optical links. From a practical stand point, it would be very beneficial to find a relationship between the expected system performance and specific variables responsible for wave front distortions, such as air temperature and pressure, temperature gradient, wind speed, cross wind, net radiation, soil-heat flux, etc. In this paper, we present the results of an experimental study and subsequent analysis that mathematically justifies some of these relationships after conducting a series of tests over an extended time period. This measurement data was obtained for a free-space laser communication link established between two fixed-point terminals and designed for transmitting a data stream using 1550 nm as the operating wavelength.

We propose a novel method to generate photonic quadrature-phase-shift-keying (QPSK) vector signal by using only one Mach-Zehnder modulator (MZM). Because of the 'square-law' by the photo-detector, the amplitudes and the phase of the driving radio-frequency (RF) signal changed after detection. So the amplitude and the phase of the driving radio-frequency (RF) signal should be pre-coded before used to drive the MZM. Turbo equalization is investigated in vector signal system to improve the system transmission performance. We experimentally demonstrate and investigate the transmission performance with rates of 1-Gbaud over 40-km single-mode fiber (SMF) transmission. Our experimental results shows that this concept of the generation of QPSK vector signal can be employed in our transmission system architecture based on Turbo equalization. The received sensitivity of QPSK vector signal with code rate 1/2 and 2/3 Turbo equalization at the bit error rate (BER) of 10−4 after 40km SMF transmission are 4dB and 3dB better than the original QPSK signal, respectively. The experimental results show that the QPSK signal with Turbo equalization has better dispersion compensation and error-correcting ability.

We experimentally demonstrate turbulence-resilient free-space optical (FSO) coherent communications with multiple multiplexed data channels in different orthogonal domains, including mode, polarization, and wavelength. The turbulence resiliency is enabled by utilizing pilot-assisted optoelectronic mixing of the received beams, wherein one pilot is being mixed with its corresponding beam. By transmitting additional continuous-wave (CW) pilot tones, whose frequencies are offset from the data-carrying beams, and mixing all pilot tones and beams in a single free-space-coupled photodetector (PD) at the receiver, the turbulence-induced modal coupling could be efficiently suppressed. The paper first discusses an experimental demonstration of a turbulence-resilient mode-division-multiplexed (MDM) FSO communication link using two orbital angular momentum (OAM) beams. Each OAM beam carries an independent 2-Gbit/s quadrature-phase-shift-keying (QPSK) data channel. Experimental results indicate that (i) the channel crosstalk under both the weaker and stronger turbulence distortions are measured to be lower than ∼−25.7 dB, (ii) the turbulence-induced OAM modal coupling could be effectively suppressed for the emulated 15 random turbulence realizations, and (iii) the error-vector magnitudes (EVM) of the multiplexed QPSK data channels are measured to be lower than by 17.4% and 19.8% under the relatively weaker and stronger turbulence effects, respectively. The paper subsequently describes an experimental demonstration of a turbulence-resilient FSO link with eight 0.5-Gbit/s QPSK data channels multiplexing two OAM beams ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">l</i> = +1 & <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">l</i> = −2), two polarizations, and two wavelengths. The measured bit-error rates (BERs) of all data channels under turbulence effects are below the 7% forward-error-correction (FEC) limit, with power penalties less than ∼3.7 dB.

To meet the needs of future wireless optical networks, this paper introduces a high-speed, hybrid multiplexed, coherent free-space optical (FSO) communication system that integrates an orbital angular momentum (OAM) multiplexed signal with an orthogonal frequency division multiplexing (OFDM) technique. Two independent QAM polarized beams, each carrying in-phase and quadrature (I/Q) phase 16-QAM-OFDM modulated data, are combined using mode division multiplexing (MDM) to increase the capacity of the proposed system. The reason of choosing OFDM is its capability to support higher data rate, and mitigating intersymbol interference (ISI). The signal is detected using a coherent detection-based digital signal processing (DSP) algorithm at the receiver end. The proposed hybrid FSO system is evaluated in low and heavy dust environments using bit error rate (BER), link distance, optical signal-to-noise ratio (OSNR), and received optical power performance matrices. The simulation results demonstrate the successful transmission of a 120 Gb/s single carrier over the longest link ranges of 1.5 and 0.40 km, respectively, under low and heavy dust weather environments below the signal degradation threshold value (forward error correction (FEC) limit) of BER 2.2 × 10–3 in strong turbulent conditions.