Free-space Laser Communication Research Articles

Experimental research on transmission performance on OAM beam and Gaussian beam in atmospheric turbulence channel

The transmission performance of orbital angular momentum(OAM) beam and Gaussian beam in atmospheric turbulent channel was studied and compared experimentally. The beam spreading, pointing error, power jitter and bit error rate of the two beams were measured respectively, the OAM beam loaded with modulated signals and the Gaussian beam were as carrier. The experimental results show that in the atmospheric turbulence channel, compared with Gaussian beam, beam spreading ratio of the OAM beams reduces by 10.5%, variance of power jitter falls 0.13, dispersion circle diameter of the pointing error decreases by 30.4%, and more focused on the distribution of the center of the beam axis. The lowest detection sensitivity of the OAM beam carrier system reaches −28.97 dBm, which improved 2.5 dB compared with that of Gaussian beam. The experimental results verify that OAM beam is less affected by turbulence than Gaussian beam in atmospheric turbulence channel, and the deterioration of beam quality of OAM beam is much less than Gaussian beam with the increase of turbulence intensity. The results of the experiment provide a reference for the development and application of free space laser communication in atmospheric turbulent channel.

Tip of the hat

Researchers from China have proposed a hollow top-hat beam (HTB) diffractive optical element (DOE) shaping method, for use in space uplink chaotic communications systems (SUCCS). Simulation results show that by shaping the beam into a HTB, the bit error-rate (BER) improvement is substantial when compared to the traditional hollow Gaussian beam (HGB). The results could be significant for the optimisation of designs relating to SUCCS. Laser communication is a type of communication technique that, unsurprisingly, uses laser beams as a signal carrier, allowing the transmission of images, sound and other data in free space. It has been conceptualised since the development of the first lasers in the 1960s. When compared to more traditional microwave communication, laser communication has the advantages of allowing faster transmission due to higher data-rate (in principle anywhere from 1,000 to 100,000 times faster), a larger communication capacity, smaller terminal size and lower power consumption. It is anticipated that, due to its higher data-rate, laser communication will be the preferred candidate for the problem of bottlenecking when it comes to space uplink communications. Prof Mi Li (left of centre) and his research group. (a) original Gaussian distribution of laser beam (b) hollow Gaussian beam (HGB) shaped by DOE (c) hollow top-hat beam (HTB) shaped by DOE. Space chaotic laser communication system. Reducing the complexity of designing laser communications systems is a hot topic in the research field of free-space communication. Optical antennas play an important role, where they function as beam collators, expanders and convergers – all of which can improve the performance of free-space communication. One of the most commonly used antennas is the Cassegrain reflective optical antenna, featuring a short mirror body, high magnification and elimination of spherical aberrations (which result in blurry images due to numerous undesired focal points). Cassegrain antennas do come with a drawback, however, in that the secondary mirror can block part of the transmission beam, affecting the transmission efficiency of the optical system. The work presented in this issue of Electronics Letters is applicable to space chaotic laser communications systems, which are more complex than ground optical fiber chaotic communication systems. The authors use a DOE to shape a carrier beam into a hollow, top-hat shape (see inset images). DOEs have been widely used in the field of beam-shaping due to the advantages of high diffraction efficiency, flexible design and simple structure. Intelligent shaping of the beam can eliminate the problem of secondary mirrors in a Cassegrain antenna blocking part of the transmission beam. It can also be used to improve the transmission efficiency of the optical sub-system, meaning that the design pressure on other sub-system parameters is eased. DOEs can be used to shape and split laser beams in an energy-efficient manner and are often used in production facilities for laser material processing, or in medical laser treatments and diagnostics. They also see use in lighting, printing and lithography. In short, DOEs are frequently used in areas where shaping and splitting with minimal light loss is necessary. In order to effectively demonstrate how their suggested HTB improves the quality of the communication system, the authors use BER to evaluate their work. They also compare the HTB to a HGB, which they used in previous research works, to see if there is any difference in quality when considering the two shapes for SUCCS. The research considers strong and weak atmospheric conditions, as atmospheric turbulence also plays a role in the BER of signals moving between Earth and orbit. The work shows that, at higher beam power, BER is reduced by around 10−0.9 when using the HTB shaping method at a low chaotic mismatch and 10−1.5 at a high chaotic mismatch. In both scenarios the HTB shaping outperforms the HGB shaping, showing that the HTB is more suitable for SUCCS. In recent years there have been a number of exciting achievements in the free-space laser communication field. Laser communication has moved from concept to practice, Mbps rates to Gbps rates and we are now seeing the next iteration of communication technology. Research scientists are carrying out research work on small-size, high-speed laser communication stations. The work presented here not only improves the efficiency of the beam, but also makes steps towards reducing the design difficulty of other sub-systems within the communication terminal. The authors hope that this work will also allow a meshing of 5G ground wireless communications and space laser communications networks, and even pave the way for easier realisation of 6G in the future.

Design and evaluation of a high-sensitivity digital receiver with the finite impulse response filter algorithm for free-space laser communication

We discuss the design of a digital laser communication receiver and its sensitivity test process. Laser communication is especially suitable for long-distance low-power micro–nano satellite links due to its narrow beam width, but the electro-optical noise situation of the micro–nano satellite platform is complicated and bad. Hence, it is necessary to introduce a digital laser communication receiver to flexibly suppress interference and noise to improve communication sensitivity. A digital laser communication receiver based on the avalanche photodiode (APD) is designed. Based on the principle of digital communication error probability, a high-sensitivity APD device is selected, and a digital receiving channel based on field programmable gate array is designed. The finite impulse response, the dynamic threshold, the baseband median, and other filtering algorithms are used to optimize the signal-to-noise ratio and enhance the detection sensitivity. At the same time, an 800-nm laser communication receiver sensitivity test experimental architecture is designed and the detection sensitivity experiment is carried out. The experiment indicates that the APD receiver achieves detection sensitivity at an Mbps rate better than −60 dBm. The digital filtering algorithm provides a filtering signal-to-noise ratio improvement of about 15 dB, which supplies a reference for the receiver design of micro–nano satellite laser communication.

Field experiment and analysis for free-space laser transmission characteristic in turbulent path based on MWIR and NIR

It is hard to achieve low bit error rate (BER) and high-quality signals in free-space optical (FSO) communication systems due to atmospheric link interference. For the sake of seeking solutions, we proposed an experimental scheme of free-space laser communication based on middle-wave infrared (MWIR) and near infrared (NIR) for obtaining atmospheric transmission characteristic in 1 km turbulence path. Compared with NIR waveband, MWIR-based scheme is more suitable for atmospheric FSO communication since it owns higher anti-interference ability for atmospheric attenuation, turbulence and skylight background noise. The experimental results may offer a significant reference for FSO communication in atmosphere link.

Investigation of Inverse Solutions for Tilting Orthogonal Double Prisms in Laser Pointing With Submicroradian Precision

Laser pointing unit using tilting orthogonal double prisms (TODP) opens up a promising avenue for fine pointing in free-space laser communication, which can steer the laser beam with a pointing precision upon submicroradian order. How to obtain accurate inverse solutions is a basic issue for TODP systems. In this paper, a lookup-table method combined with the binary search procedure is demonstrated, and an iterative method that converges to the inverse solution by gradually reducing the search region is also investigated. The simulation results show that both methods can reach the pointing precision of micrometer order (namely submicroradian order after unit conversion) at a pointing distance of 400 mm, which can well meet the requirement of high-accuracy laser beam pointing applications.

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.

Beam steering with ultracompact and low-power silicon resonator phase shifters.

Photonic integrated circuit (PIC) phased arrays can be an enabling technology for a broad range of applications including free-space laser communications on compact moving platforms. However, scaling PIC phased arrays to a large number of array elements is limited by the large size and high power consumption of individual phase shifters used for beam steering. In this paper, we demonstrate silicon PIC phased array beam steering based on thermally tuned ultracompact microring resonator phase shifters with a radius of a few microns. These resonators integrated with micro-heaters are designed to be strongly coupled to an external waveguide, thereby providing a large and adjustable phase shift with a small residual amplitude modulation while consuming an average power of 0.4 mW. We also introduce near-field and far-field characterization techniques to enable the calibration and programming of resonator phase shifters in the phased array. With such compact phase shifters, we demonstrate beam steering with a 1x8 PIC phased array. The small size of these resonator phase shifters will enable low-power and ultra-large scale PIC phased arrays for long distance laser communication systems.

Shipborne Acquisition, Tracking, and Pointing Experimental Verifications towards Satellite-to-Sea Laser Communication

Acquisition, tracking, and pointing (ATP) is a key technology in free space laser communication that has a characteristically high precision. In this paper, we report the acquisition and tracking of low-Earth-orbit satellites using shipborne ATP and verify the feasibility of establishing optical links between laser communication satellites and ships in the future. In particular, we developed a shipborne ATP system for satellite-to-sea applications in laser communications. We also designed an acquisition strategy for satellite-to-sea laser communication. In addition, a method was proposed for improving shipborne ATP pointing error. We tracked some stars at sea, achieving a pointing accuracy of less than 180μrad.We then acquired and tracked some low-Earth-orbit satellites at sea, achieving a tracking accuracy of about 20μrad. The results achieved in this work experimentally demonstrate the feasibility of ATP in satellite-to-sea laser communications.

Inter-satellite laser communication system based on double Risley prisms beam steering.

We designed an intensity modulation direct detection free-space laser communication terminal. In this system, the output power of the signal-modulated semiconductor laser amplified by two-stage fiber amplifier is 2W, and the modulation contrast is 15dB. Tracking of the incident beam and demodulation of the communication signal are realized by using a quadrant avalanche photodiode (APD). By means of rotating double Risley prisms, the ranges of the pitching angle and azimuth angle are ±35 and ±180 deg, respectively. The focal length of the transceiver lens is 125mm, and the final communication field of view is 4mrad. Without error correction coding, the sensitivity of -44.5 dBm was achieved, while the bit-error rate was below 10-3 at 70Mbps. We also established a laser communication experiment at the laboratory; the platform jitter below 5Hz can be effectively suppressed. The laser communication system has a compact structure, low power consumption, and effective suppression of low-frequency and high-amplitude angle jitter of the platform. The experiments demonstrate that the laser communication terminal can be used in an inter-satellite laser communication with a distance of thousands of kilometers. The theoretical analysis and experimental results are presented in the paper.

Optical adaptive filtering for Doppler shift simulator

Based on free space laser communication, this article describes the working principle of electro-optical frequency shifting, designs an optical adaptive filtering module, and builds the core module of the dynamic optical Doppler shifting simulator for laser channel. It is expected to be applied to the heaven-ground integrated communication link. In this article, we adopt the electro-optical frequency shifting technique combined with the microwave-light wave. In the 1 550 nm band, the negative feedback algorithm is used to complete the adaptive filtering, which realizes optical Doppler frequency shifting and high-precision locking. The frequency shift range reaches +5.5–+32 GHz, and the analog precision is better than 645 Hz. When the microwave frequency is greater than 13.5 GHz, the signal-to-noise ratio (SNR) of the output optical power reaches 20 dB, which lays the foundation for the next stage space laser communication.

Aperture scalable liquid crystal optically duplicated array of phased array

To achieve non-mechanical laser beam steering in the scenario of long distance propagation such as free-space laser communication between satellites, large aperture size is an inevitable issue to be considered to narrow the divergence angle of the output beam. Liquid crystal optical phased array, to be one of the solutions of non-mechanical beam steering, has already shown its obvious potential to achieve a relative large aperture on the order of centimeter. To achieve even larger, its driving matrix becomes squared larger. In this paper, we proposed a novel architecture to realize an optical phased array with a scalable aperture. Meanwhile the driving matrix is almost not increased. It provides the feature of a cascade system with a device of spatial phased modulation and an array of duplicating units. Each unit of the duplicating array is consist of a polarization beam splitter and a half wave plate to have the same output optical field distribution as the input beam whose phase front is modulated by a small size spatial modulator. Not only the property of beam deflection is numerically simulated and experimentally verified, but also the property of divergence angle compression and grating lobes limitation are evaluated. Meanwhile, due to the high precision of the experimental alignment, the non-mechanical beam deflection property is still maintained no matter how many the duplicated unit number is. The relative standard derivation steering error is 0.025.

Validation of pre-compensation under point-ahead-angle in a 1 km free-space propagation experiment

Adaptive optical pre-compensation is seen as crucial for free-space laser communication in order to overcome the influence of atmospheric turbulence, particularly with respect to Earth-to-GEO feederlinks. This paper presents an experimental investigation into adaptive optical pre-compensation under large point-ahead-angles. We detail the design and realization of a free-space laser communication experiment over a 1.0 km horizontal path using a divergent beacon beam and a focussed signal beam, propagating in opposite directions. We describe the design and development of our experimental setup and measurement campaign using real turbulence. The median isoplanatic angle was calculated to be 0.16 mrad, while an increase in the received optical power through pre-compensation could be demonstrated for point-ahead-angles in the range of 0.13 mrad to 0.27 mrad.

Nonmechanical two-user tracking method of space-polarization division using the liquid crystal optical phased array

We propose a two-user tracking method called space-polarization division to enable simultaneously multiple terminals tracking in the field of free space laser communications. This method is based on the nonmechanical beam steering device of liquid crystal optical phased array, which is capable of continuously laser beam pointing and multiple beams generating. An optical architecture of two-user access is presented to show a one-to-two communication link between the relay satellite terminal and user terminals. The two-user tracking strategy is modeled numerically and experimentally verified to show the feasible and agile two-user tracking with a good accuracy less than 50 microradians which meet the requirements of coarse tracking.

Beam spreading, kurtosis parameter and Strehl ratio of partially coherent cosh-Airy beams in atmospheric turbulence

In order to reduce turbulence-induced degeneration or resist the turbulence effect in non-diffraction partially coherent Airy optical fields, we introduce the cosh function to partially coherent Airy beam in turbulence, and focus our attention on the dependence of beam spreading, kurtosis parameter and Strehl ratio on cosh modulation parameter in the condition of the fixed input power. The results show that the resulting beam possesses more side lobes, and it can make main lobe to acquire more peak intensity in atmospheric propagation. Beam spreading of non-zero cosh modulation parameter is less sensitive to turbulence than that for zero value of cosh modulation parameter. The kurtosis parameter or Strehl ratio of partially coherent cosh-Airy beams can be further improved by increasing cosh modulation parameter. It means that partially coherent cosh-Airy beams with a larger cosh modulation parameter bring advantages in beam spreading, sharpness and peak intensity when compared with partially coherent Airy beams, which could have potential application in optical radar detection and free-space laser communication.

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.

Investigation of isolation for free space laser communication in the mono-wavelength optical T/R channels

Favorable isolation performance in optical transmitting-receiving (T/R) channels is key to establishing full-duplex bi-directional laser communication links when designing mono-wavelength laser communication terminals. This study proposes a novel design scheme for T/R channels that use a single laser wavelength. The system includes measures such as the deflection at λ/4 wave plate angles, light trap installation at a polarization beam splitter, and removal of polarization analyzers in optimized systems. A mathematical model describing isolation in the optical T/R channels is presented. Three types of factors that affect the isolation of optical T/R channels were analyzed and simulated in detail. The present simulation results show that the roughness of the λ/4 wave plate and the solid angle of the detector field of view are exponentially related to T/R isolation, which can thus be regarded as the most significant factors affecting isolation. According to the present design scheme, the prototype T/R channel was developed for measuring isolation. The measured value (77.8 dB) agrees well with the result determined using the established mathmatical model (77.3 dB). Conclusively, the present theoretical model can provideguidance for optimizing isolation in a mono-wavelength optical channel. Thus, it can be applied extensively in laser communication systems, including inter-satellite networks.

Experimental Study on PAT System for Long-Distance Laser Communications Between Fixed-Wing Aircrafts

Free-space laser communication is characterized by high communication speed, strong anti-jamming ability, high confidentiality, and flexible configuration. In this paper, a pointing, acquisition, and tracking (PAT) system based on a two-stage (i.e., coarse and fine) composite tracking mechanism is proposed to solve the optical axis alignment problem, which is common in free-space laser communications. The acquisition probability of the PAT system is ensured by designing two tracking modules, a coarse tracking module which combines passive damping with active suppression and a fine tracking module based on an electromagnetic galvanometer. Both modules are combined by using a dynamic scanning mechanism based on the gyroscope signal. Finally, a free-space laser communication test with a long range and a high speed is conducted by two fixed-wing Y12 aircrafts equipped with the proposed PAT system. Experimental results show that the coarse tracking precision of the airborne PAT system is 10 μrad (1σ), and the fine tracking precision is 8 μrad (1σ) during flights which are much improved as compared with the indoor tests. This indicates that the system can achieve a high precision for PAT during high-speed and long-range laser communications in the free-space. This also verifies the tracking capability and the environmental adaptability of the proposed laser communication PAT system.

Wavefront Sensing in Deep Turbulence

Applications ranging from lidar to free-space laser communications to directed energy require ever-better ways to overcome the distortions of a churning atmosphere. Emerging wavefront-sensing technologies are stepping up to the challenge.

Demonstration of horizontal free-space laser communication with the effect of the bandwidth of adaptive optics system

Abstract The performance of free-space laser communication (FSLC) is greatly decreased by horizontal atmospheric turbulence. Adaptive optics is used to overcome horizontal turbulence. This study investigated the effect of the 3 dB bandwidth of an adaptive optics system (AOS) on FSLC performance by using power-in-bucket (PIB) method, which considers the effect of receiving aperture and then, can accurately evaluate of the effect of system bandwidth on the communication performance. A 9 km horizontal maritime link experiment was performed through adaptive correction to validate the theoretical analysis. Results indicate that the effect of system bandwidth can be accurately evaluated using PIB method. This work provides a basis for designing and evaluating AOSs for FSLC.

Gain-switched seed laser for Deep Space communication applications

Deep Space (DS) communication plays an important role in space exploration programs, especially in interplanetary flights projects. To resolve limitations of a well-known microwave link, an optical communication is considered. In the article a gain-switched seed laser for high power transmitter in a Master Oscillator Power Amplifier (MOPA) architecture is presented. This optical signal source is able to generate picosecond pulses on demand and is suitable for high speed data link over a long range. The laser is dedicated to the pulse position modulation (PPM) scheme for low power consuming, high capacity and energy efficient optical communication. Full Text: PDF ReferencesH. Hemmati, Deep Space Optical Communications (Jet Propulsion Laboratory 2005), John Wiley & Sons, 2006. CrossRef T. Tolker-Nielsen and G. Oppenhauser, “In-Orbit Test Result of an Operational Intersatellite Link between ARTEMIS and SPOT 4,” FreeSpace Laser Communication Technologies XIV, Proceedings of SPIE, vol. 4639, pp. 1–15, San Jose, California, January 2002. CrossRef D. M. Boroson, et al., "Overview and results of the Lunar Laser Communication Demonstration", Proc SPIE 8971, 89710S (2014). CrossRef D. M. Boroson, A. Biswas, B. L. Edwards, "MLCD: overview of NASA's Mars laser communications demonstration system", P. Soc. Photo-Opt. Ins. 5338 (2004). CrossRef H. Hemmati, A. Biswas, I. B. Djordjevic, "Deep-Space Optical Communications: Future Perspectives and Applications", Proc. of the IEEE 99.11, (2011). CrossRef H. Kaushal, G. Kaddoum, "Optical Communication in Space: Challenges and Mitigation Techniques", IEEE Commun Surv Tut. 19.1, 57 (2017). CrossRef B. Moision, J. Hamkins, M. Cheng, "Deep-space optical communications downlink budget: modulation and coding", IPN Prog. Rep. 42.154, 1 (2005). DirectLink