Free-space Laser Communication Systems Research Articles

Multi-Gb/s free-space laser communication at 4.6-μm wavelength using a high-speed, room-temperature, resonant-cavity infrared detector (RCID) and a quantum-cascade laser

Research has shown that free-space laser communication systems may experience fewer outages due to atmospheric impairments such as haze, fog, clouds, and turbulence by operating at a longer wavelength in the mid-wave or long-wave infrared, if disadvantages such as lower-performance transceiver components may be overcome. Here we report a resonant cavity infrared detector (RCID) with 4.6-µm resonance wavelength that enables 20-dB larger link budget than has been reported previously for ∼ 5 Gb/s operation. The device combines high responsivity, 1.97 A/W, with a low noise equivalent power (NEP) of 0.7 pW/Hz at room temperature, and a high bandwidth of 6.7 GHz at 3-dB. The relatively large surface-normal-incidence device with 30-µm diameter simplifies the coupling relative to intra-subband quantum cascade detectors. Although the RCID NEP is expected to increase with frequency to ∼ 1.5 pW/Hz, we estimate that the total equivalent noise power in a 2.5-GHz bandwidth is less than 200 nW. When combined with a relatively high power (∼100-mW) distributed-feedback quantum cascade laser, the difference of > 50 dB between modulated laser power and RCID noise significantly outpaces that of existing devices.

Secrecy analysis of a free-space laser communication system with a coherent main channel

Physical layer security of a free-space optical (FSO) communication link is investigated for certain eavesdropping scenarios in which the eavesdropper exploits the divergence of the transmitted laser beam, which is subject to atmospheric turbulence, to compromise the security of the transmission link. A solution for physical layer security in which the legitimate receiver is positioned to receive only the coherent component of the transmitted laser beam is proposed. The irradiance fluctuations are modeled with separate distributions for the coherent and non-coherent components of the transmitted beam, corresponding to the main channel and the eavesdropper’s channel, respectively. Complete secrecy analysis of this setup is presented, and closed form expressions of the probability of strictly positive secrecy capacity, average secrecy capacity, and secrecy outage probability are derived. The analytical results are verified by Monte-Carlo simulations. Secrecy performance for worst-case scenarios corresponding to an adversary with powerful resources to capture all of the leaked information is explored. Results are analyzed for different turbulence regimes, and it was found that secure communication is possible even in the region of strong turbulence.

Full mode power spectrum for Laguerre-Gauss beams in strong Kolmogorov turbulence.

We analyze the effects of atmospheric turbulence on the mode power spectrum of beams carrying orbital angular momentum represented by Laguerre-Gauss (LG) modes. For an input (p,m) LG mode, i.e. pump, we calculate the power transferred to other modes (p',m') due to turbulence. Our analysis is validated against split-step beam propagation simulations and shows agreement into the strong turbulence regime. These results have applications for the design and characterization of free-space laser communication systems.

Investigation of turbulence-tolerant free-space optical communications via multiplane light conversion

Multiplane light conversion (MPLC) offers an alternative to adaptive optics for coupling a turbulence-corrupted free-space optical beam into single-mode fiber or waveguide. Recent published test results suggest that such a conversion device has comparable or better performance than an adaptive optics system. To better understand the device characteristics, simulations were conducted to quantify the power loss for different turbulence strengths and the number of Hermite–Gaussian modes used in the conversion process. The specific case studied is a prototype free-space laser communication system developed by the U.S. Army Research Laboratory. Simulation and statistical results of the conversion performance are reported. Analysis of a beam power combiner after MPLC is also discussed.

Mode power spectrum for Laguerre-Gauss beams in Kolmogorov turbulence.

We analyze the effects of atmospheric turbulence on the mode power spectrum of beams carrying orbital angular momentum represented by Laguerre-Gauss (LG) modes. For an input (0, m) LG mode, we calculate the power transferred to other modes (0, m') due to turbulence. The analysis is validated against split-step beam propagation simulations and shows agreement into the strong turbulence regime. These results have applications for the design and characterization of free-space laser communication systems.

Multi-target free-space laser communication system based on a rotating double prism with an RF signal beacon.

Traditional free-space laser communication systems use beacon and signal lights for target detection and alignment. However, these approaches are inaccurate owing to signal dispersion errors. To overcome this difficulty, we propose a new method using transient radio frequency (RF) signals to achieve highly accurate target detection and alignment. To validate the feasibility of our proposed method, we built an experimental multi-target space-laser communication system based on a rotating double prism and applied it to achieve multi-target space-laser communication. The results demonstrate the efficiency of the proposed method to capture multi-target positions in the field of view using wireless RF signals and a rotating double prism. In addition, we show that the system is capable of rapid scanning and accurate pointing as well as establishing a one-way stable communication with multiple targets. When the target is 36 cm away, the pointing accuracy of the system motor is less than 0.8°, the pointing time is 1.2 s, and the average pointing lateral error is 0.666 mm.

Field Demonstrations of Wide-Beam Optical Communications Through Water–Air Interface

The connectivity of undersea sensors and airborne nodes across the water-air interface has been long sought. This study designs a free-space wireless laser communications system that yields a high net data rate of 850 Mbit/s when perfectly aligned. This system can also be used for an extended coverage of 1963 cm 2 at the receiver while sustaining a net data rate of 9 Mbit/s over 10 m. The utility of this system was verified for direct communications across the water-air interface in a canal of the Red Sea based on a pre-aligned link as well as a diving pool under a mobile signal-searching mode. The canal deployment measured a real-time data rate of 87 Mbit/s when pre-aligned in turbid water over 50 min, which confirms the system robustness in harsh water environments. In the pool deployment, a drone configured with a photodetector flew over the surface of the water and recorded the underwater signals without a structure-assisted alignment. Using a four-quadrature amplitude-modulated orthogonal frequency-division multiplexing (4-QAM-OFDM) modulation scheme provided a net data rate of 44 Mbit/s over a 2.3-m underwater and 3.5-m air link. The results validated the link stability and mitigated problems that arise from misalignment and mobility in harsh environments, which paves the way for future field applications.

Highly efficient coherent conformal projection system based on adaptive fiber optics collimator array

Coherent beam combination (CBC) of fiber laser array, which is a promising approach to overcome the power scaling limitations of the single fiber laser and to achieve high-brightness laser with good beam quality, can be employed in coherent conformal projection system (CCPS) for beam projection and free space laser communication system with enormous potential. To date, all the demonstrated CBC system through several kilometers atmospheric turbulence have employed collimated beams, which significantly decrease the final CBC effect because of the inefficient wavefront overlap. On the other hand, focused CCPS has lots of advantages over collimated projection system since the wave front of the projected beam is closer to that of a convergent spherical wave. In this paper, we design and manufacture focused CCPS based on an adaptive fiber optics collimator array for the first time, which is achieved by introducing controllable spherical aberration through the adaptive fiber optics collimator. A CBC system is setup to evaluate and validate its performance. Results show that compared with the collimated CCPS, the energy portion of the central lobe could be increased by a factor of 44%, which agrees well with the theoretical analysis.

Laser communication based on a multi-channel single-photon detector

We demonstrate a free-space laser communication system with a multi-channel single-photon detector to improve the efficiency of transmission. On account of the photon-number-resolving capability, the discrimination level is adjustable to optimize the extraction of effective signals from the background noise. This method greatly reduces the error bit rate even under the high intensity of the background noise of 6×106 counts/s in daylight. Meanwhile, combined with the pulse position modulation, the bandwidth of the laser communication system can be improved greatly. This study paves the way to the high accuracy and high speed free-space optical communication for 24 h a day.

Evaluation of the communication quality of free-space laser communication based on the power-in-the-bucket method.

Atmospheric turbulence seriously affects the quality of free-space laser communication. The Strehl ratio (SR) is used to evaluate the effect of atmospheric turbulence on the receiving energy of free-space laser communication systems. However, the SR method does not consider the area of the laser-receiving end face. In this study, the power-in-the-bucket (PIB) method is demonstrated to accurately evaluate the effect of turbulence on the receiving energy. A theoretical equation is first obtained to calculate PIB. Simulated and experimental validations are then performed to verify the effectiveness of the theoretical equation. This work may provide effective guidance for the design and evaluation of free-space laser communication systems.

Ground Simulation Method for Arbitrary Distance Optical Transmission of a Free-Space Laser Communication System Based on an Optical Fiber Nanoprobe

Simulation technologies for optical transmission are used to simulate the course of laser long-distance transmission in space and reception by a free-space optical (FSO) communication terminal, which determines the receiving power and is one type of core technology for ground validation and detection. The simulated distance of the current optical transmission simulation methods is limited, and these methods fail to meet the continually increasing distance of FSO communication. In this paper, a method called “relay transmission” is proposed based on an optical fiber nanoprobe that can simulate an optical transmission at an arbitrary distance. Using this method, direct pointto- point performance tests using two optical terminals are realized in the laboratory, whose results are consistent with on-orbit results. The far-field pattern of the incident light is obtained by an off-axis multiband reflecting telescope system with a large aperture, and wave sampling is performed by an optical fiber nanoprobe whose powercollection aperture corresponds to that of the receiving antenna of the on-orbit terminal. The feasibility of the proposed method is demonstrated through theoretical analysis and a series of validation experiments.

Fiber-coupling efficiency for a Gaussian-beam wave passing through weak fluctuation regimes

The efficiency of Gaussian-wave light propagating in weak atmospheric turbulence coupled into a single model fiber is studied, and an approximation to the original mutual coherence function (MCF) in the coupling-efficiency expression is given to simplify the complexity of the efficiency calculation. The influence of the transmitting beam and turbulence parameters on the Gaussian-beam coupling-efficiency has also been evaluated. The numerical results show that the relative error of the expression with the approximate MCF referenced to the original Gaussian-wave fiber coupling efficiency representation is not more than 3.69%. It is anticipated that this work will be helpful in estimating the receiving performance and optimizing effective beam parameters at the transmitter in the design of a free-space laser communication system.

Optical multiaccess free-space laser communication system

With urgent demand for an integrated information network and development of free-space laser communication technology, research on high-rate laser communication networking technology is vital. This study analyzed the technical difficulties related to space laser communication networking and proposed a laser communication networking solution. A wide-angle beam expander and dual-rotating prism group were incorporated into a multiaccess optical laser communication antenna. The wide-angle beam expander collects signal light from different directions; the dual-rotating prism group tracks different targets simultaneously. This paper presents an overall scheme allowing multiaccess free-space laser communications based on the optical antenna described and the associated relay optics and transceiver subsystems.

Free-space laser communication system with rapid acquisition based on astronomical telescopes

The general structure of a free-space optical (FSO) communication system based on astronomical telescopes is proposed. The light path for astronomical observation and for communication can be easily switched. A separate camera is used as a star sensor to determine the pointing direction of the optical terminal's antenna. The new system exhibits rapid acquisition and is widely applicable in various astronomical telescope systems and wavelengths. We present a detailed analysis of the acquisition time, which can be decreased by one order of magnitude compared with traditional optical communication systems. Furthermore, we verify software algorithms and tracking accuracy.

Performance analysis of a free-space laser communication system with a Gaussian Schell model

In practice, due to reasons related to the characteristics of the laser device and the inevitable error of the processing technique, a laser beam emitted from a communication terminal is represented by the Gaussian–Schell model. The incident optical intensity at the receiver aperture is affected by the source coherence parameter through atmospheric turbulence. With full consideration of both the average optical intensity and the scintillation, the statistical distribution of the optical intensity and the average bit error rate (BER) of an on-off keying (OOK) receiver is obtained. The results indicate that the effect on the degradation of the average optical intensity is reduced with a smaller beamwidth. The performance of the OOK receiver degrades drastically with the increasing source coherence parameter. Moreover, when the beamwidth becomes larger, the BER0 with a consistent source coherence parameter shifts to the side of lower BER. The goal of this work is to improve the redundancy design of the laser communication receiver system.

通用自适应光学波前实时处理机的设计

In order to satisfy the requirement of high speed and real-time processing for 1000-unit class adaptive optics(AO) system in telescope and free space laser communication system, a generic high speed and real-time processing platform on one board is proposed. Multi-core center processing unit(CPU) is arranged for wave-front real-time computing, combined with high speed frame-grabber, to achieve the purpose of high frame-frequency.Xenomai real-time operation system combined with Linux operation system is used to satisfy the requirement of low latency. A compact real-time controller is assembled in one computer. Computing power of 51 giga floatingpoint operations per second(GFLOPS) and communication band-width of 102 GB/s are achieved in a computer with 6-core, via vector instruction optimization and processing in parallel with multi-core CPU. The emulation result shows that processing frequency can be up to 2000 Hz for 1145-unit AO system with 949 sub-apertures, and the delay is below 240 ms. This processor is introduced to AO system for laser communication via adjusting the parameters, and Strehl ratio of 0.61 is achieved after correction. This indicates the scheme can satisfy the requirement of high speed and low latency for AO system with actuators below 1145 unit, with the merit of general purpose.

Design and Implementation of MOEMS Based Ground to Satellite Free Space Optical Link Under Turbulence Condition

Non-mechanical beam steering techniques are commonly used in free space laser communication systems to avoid the use of massive opto-mechanical components. In this paper, we demonstrate an array-of-arrays of micro-opto-electro-mechanical system (MOEMS) based mirrors as a replacement for a large aperture reflective single-element mirror for ground to satellite optical links along with its associated electronic drive circuitry and FPGA based interface. The performance of the free space optical (FSO) communication link in terms of beam variance under atmospheric turbulence conditions is discussed.

自由空间激光通信系统中多通道滤光膜的研制

自由空间激光通信系统中多通道滤光膜的研制

Implementation of a Phase Only Spatial Light Modulator as an Atmospheric Turbulence Simulator at 1550 nm

Modeling and simulating atmospheric turbulence in a controlled environment have been a focus of interest for scientists for decades. The development of new technologies allows scientists to perform this task in a more realistic and controlled environment and provides powerful tools for the study and better understanding of the propagation of light through a nonstatic medium such as the atmosphere. Free space laser communications (FSLC) and studies in light propagation through the atmosphere are areas which constantly benefit from breakthroughs in technology and in the development of realistic atmospheric turbulence simulators, in particular (Santiago et al. 2011). In this paper, we present the results from the implementation of a phase only spatial light modulator (SLM) as an atmospheric turbulence simulator for light propagation in the short-wave infrared (SWIR) regime. Specifically, we demonstrate its efficacy for its use in an FSLC system, at a wavelength of 1550 nm.

Transmission characteristics of a Raman-amplified atomic optical filter in rubidium at 780 nm

The transmission characteristics of a Raman-amplified atomic filter that can be used to detect fairly weak signals in free-space quantum-key distribution or laser communications are analyzed and discussed in the coherent and incoherent pump fields respectively. The theoretical model for the calculation of the transmission characteristics of a ground-state Raman-amplified Faraday dispersion atomic optical filter based on Raman gain and Faraday rotation is presented. The results show that the filter in a coherent pump field can achieve higher transmission and larger tunability than that in an incoherent pump field due to elimination of pumping detuning. In addition, the filter has a large scale tunability over 3.5 GHz via the Faraday transmission peak adjusted while its bandwidth is only 66 MHz, which is useful for free-space laser communication and lidar systems.