Optical Communication Terminals Research Articles

Steering Mirror System with Closed-Loop Feedback for Free-Space Optical Communication Terminals

Precision beam pointing plays a critical role in free-space optical communications terminals in uplink, downlink and inter-satellite link scenarios. Among the various methods of beam steering, the use of fast steering mirrors (FSM) is widely adopted, with many commercial solutions employing diverse technologies, particularly focusing on small, high-bandwidth mirrors. This paper introduces a method using lightweight, commercial off-the-shelf components to construct a custom closed-loop steering mirror platform, suitable for mirror apertures exceeding 100 mm. The approach involves integrating optical encoders into two off-the-shelf open-loop actuators. These encoders read the signal reflected on purposefully diamond-machined knurled screw knobs, providing maximum contrast between light and dark lines. The resulting steering mirror has the potential to complement or replace FSM in applications requiring a larger stroke, at the expense of motion speed. In the presented setup, the mirror tilt resolution achieved based on the encoder closed-loop signal feedback is 45 μrad, with a mean slew rate of 1.5 mrad/s. Importantly, the steering assembly is self-locking, requiring no power to maintain a steady pointing angle. Using the mirror to actively correct for a constantly moving incoming beam, a 5-fold increase in concentration of the beam spot on the center of the detector was obtained compared to a fixed position mirror, demonstrating the mirrors ability to correct for satellite platform jitter and drift.

Continuous daytime and nighttime forecast of atmospheric optical turbulence from numerical weather prediction models.

Future satellite-to-ground optical communication systems will benefit from accurate forecasts of atmospheric optical turbulence; namely for site selection, for the routing and the operation of optical links, and for the design of optical communication terminals. This work presents a numerical approach based on the Weather Research and Forecasting software that enables continuous forecast of the refractive index structure parameter, C n2, vertical profiles. Two different C n2 models are presented and compared. One is based on monitoring the turbulent kinetic energy, while the other is a hybrid model using the Tatarskii equation to depict the free atmosphere region, and the Monin-Obukhov similarity theory for describing the boundary layer. The validity of both models is assessed by using thermosonde measurements from the Terrain-induced Rotor Experiment campaign, and from day and night measurements of the coherence length collected during a six-day campaign at Paranal observatory by a Shack-Hartmann Image Motion Monitor. The novelty of this work is the ability of the presented approach to continuously predict optical turbulence both during daytime and nighttime, and its validation with measurements in day and night conditions.

LEAPing Ahead – The Space Development Agency's Method for Planning for the Future

AbstractThe Space Development Agency (SDA) is a constructive disruptor within the Department of Defense, tasked with rapidly acquiring and delivering space‐based capabilities. SDA delivers dozens of satellites on two‐year cycles and this pace requires defined processes to ensure that the technology exists to support the required capabilities. SDA has developed a process called List, Evaluate, Achieve, Procure (LEAP) which is used to identify technologies that require additional development resources to meet both current Warfighter needs and those that will occur in the future. This paper provides an illustrative example of SDA applying the LEAP process to the development of SDA's optical communications terminals, demonstrating how it is used to identify critical technologies to be supported through investment opportunities that augment the system acquisitions.

Data-driven estimation, tracking, and system identification of deterministic and stochastic optical spot dynamics.

Stabilization, disturbance rejection, and control of optical beams and optical spots are ubiquitous problems that are crucial for the development of optical systems for ground and space telescopes, free-space optical communication terminals, precise beam steering systems, and other types of optical systems. High-performance disturbance rejection and control of optical spots require the development of disturbance estimation and data-driven Kalman filter methods. Motivated by this, we propose a unified and experimentally verified data-driven framework for optical-spot disturbance modeling and tuning of covariance matrices of Kalman filters. Our approach is based on covariance estimation, nonlinear optimization, and subspace identification methods. Also, we use spectral factorization methods to emulate optical-spot disturbances with a desired power spectral density in an optical laboratory environment. We test the effectiveness of the proposed approaches on an experimental setup consisting of a piezo tip-tilt mirror, piezo linear actuator, and a CMOS camera.

Pointing-error correction of optical communication terminals on motion platforms using a parameter model and kernel weight function estimation.

Optical communication terminals (OCTs) with high pointing accuracy on motion platforms are highly important for establishing a global communication network. The pointing accuracy of such OCTs is seriously affected by linear and nonlinear errors generated by various sources. A method based on a parameter model and kernel weight function estimation (KWFE) is proposed to correct the pointing errors of an OCT on a motion platform. Initially, a parameter model, which has a physical meaning, is established to reduce the linear pointing errors. Then, the nonlinear pointing errors are corrected using the proposed KWFE method. Tracking star experiments are conducted to verify the efficiency of the proposed method. The parameter model reduces the initial pointing error associated with the stars used for calibration from 1311.5µrad to 87.0µrad. After applying parameter model correction, the KWFE method is applied to further reduce the modified pointing error associated with the stars used for calibration from 87.0µrad to 70.5µrad. Additionally, based on the parameter model, the KWFE method reduces the actual open-loop pointing error associated with the target stars from 93.7µrad to 73.3µrad. The sequential correction using the parameter model and KWFE can gradually and effectively improve the pointing accuracy of an OCT on a motion platform.

Pointing-Error Correction of Optical Communication Terminals on Motion Platforms Based on a K-Nearest Neighbor Algorithm

High-precision pointing is a key technology in free-space optical communication, as it improves the acquisition probability and reduces the acquisition time. High-precision pointing of an optical communication terminal (OCT) on a motion platform is particularly important for establishing a global communication network. An OCT on a motion platform is subjected to both linear and nonlinear pointing errors. The present paper analyzes both types of pointing errors in detail. Linear pointing errors can be well corrected by the parameter model. This paper proposes a K-nearest neighbor (KNN) algorithm that corrects nonlinear pointing errors and verifies its effectiveness through tracking star experiments. After correcting the nonlinear pointing errors through KNN algorithm, the modified and actual open-loop pointing errors were 69.0 and 70.8 μrad, respectively, reduced from 87.3 and 91.9 μrad, respectively, in the parameter model. Thus, the proposed KNN algorithm can effectively improve the pointing accuracy of an OCT on a motion platform.

Approach of recognition and precision location for the beacon in satellite optical communications

Satellite optical communications have high communication speed and good confidentiality, which can establish high-speed and large-capacity communication links between satellites and the ground or other satellites. The requirements for the PAT (pointing, acquisition and tracking) system of satellite optical communication terminals are very high. In satellite optical communications, the biggest influence on the beacon’s recognition and tracking is the background light on CCD (Charge Coupled Device). Therefore, denoising work is required during the recognition or tracking process. This paper uses corner detection method to extract the beacon frame by frame. Mark the spot, and use the optical flow method to calculate the optical flow displacement based on the two consecutive frames. In addition, calculation accuracy of the beacon’s centroid is compared with that of the tradition method. The feasibility is verified by simulation experiment, the tracking process is completed, and the noise can be precisely eliminated.

Возможность построения модульной системы квантового распределения ключей в атмосфере

The possibility of quantum key distribution in the atmosphere has been experimentally demonstrated by coupling commercially available quantum key distribution units designed for fiber-optic communication lines with atmospheric optical communication terminals. For distances up to 3100m, data on losses in a quantum channel on an optical path were obtained and the influence of systems for intelligent adjustment of atmospheric communication terminals on the synchronization system of quantum communication blocks was studied. It has been established that failures of synchronization systems in the case of quantum key distribution in the atmosphere at distances greater than 10m are due to the peculiarities of the algorithm implemented in the quantum communication unit.

Possibility of creating a modular system for quantum key distribution in the atmosphere

The possibility of quantum key distribution in the atmosphere has been experimentally demonstrated by coupling commercially available quantum key distribution units designed for fiber-optic communication lines with atmospheric optical communication terminals. For distances up to 3100 m, data on losses in a quantum channel on an optical path were obtained and the influence of systems for intelligent adjustment of atmospheric communication terminals on the synchronization system of quantum communication blocks was studied. It has been established that failures of synchronization systems in the case of quantum key distribution in the atmosphere at distances greater than 10 m are due to the peculiarities of the algorithm implemented in the quantum communication unit. Keywords: quantum key distribution, atmospheric optical communication lines, FSO, decoy-state BB84 protocol, polarization coding.

Modeling and Correction of Pointing Errors in Gimbals-Type Optical Communication Terminals on Motion Platforms

In free-space optical (FSO) communication, high-precision pointing is a critical technology required for rapid acquisition to reduce link establishment time and increase communication time. FSO communication on a motion platform is necessary to expand the communication area and to promote the establishment of a global communication network. However, the pointing accuracy of an optical communication terminal on a motion platform is low due to numerous error sources and error coupling. This paper evaluates the error sources and proposes a pointing model to avoid problems resulting from error coupling. This proposed pointing model was designed to improve the pointing accuracy of a gimbals-type optical communication terminal (GOCT) on a motion platform. The effectiveness of the proposed pointing model was verified by tracking star experiments. The modified residual error of the proposed point model was 94.8 μrad compared to 1324.2 μrad without correction. Additionally, the modified residual error was 94.8 μrad of the proposed pointing model compared to 140.2 μrad of the existing model. The actual open-loop pointing error was reduced from 150.4 μrad of the existing model to 101.3 μrad of the proposed model. Thus, the pointing accuracy of a GOCT on a motion platform was significantly improved after correction by the proposed pointing model.

High-bandwidth fine tracking system for optical communication with double closed-loop control method

A fine tracking system is crucial for maintaining the accuracy of the optical communication terminals that aim at each other. To ensure the reliability of the communication link, the fine tracking system requires high bandwidth to mitigate the effect of arrival angle fluctuation caused by atmospheric turbulence. Traditionally, the fine tracking system includes only a single feedback loop; a high bandwidth is obtained by increasing the high gain of the fine tracking system, which usually suffers considerably from the time delay engendered by sampling and data processing, the hysteresis nonlinearity of a fast steering mirror (FSM), and the limitations of dynamic response of FSM. To track the beacon in real time and with high precision, a pioneering control method is presented in our paper, namely, double closed-loop control (DCC), which performs better in a tracking system compared with a traditional single-feedback loop. In the inner feedback loop, the response of FSM is measured by a strain gauge sensor (SGS) and used as the inner feedback signal. Thus, by co-operating with the outer CCD-based feedback loop, a DCC scheme is proposed for the fine tracking system. With the SGS signal, the inner loop controller is designed to obtain a rapid response without overshooting; meanwhile, the hysteresis nonlinearity is diminished. Experimental results indicate that the static hysteresis nonlinearity of FSM is reduced from 15.6% to 1.4% by an inner feedback loop, and the dynamic response and stability of FSM is greatly improved, thereby simplifying the outer loop controller design. Then, with the SGS signal, the time delay of the outer loop can be compensated accurately with a predicted signal compensation method. The experimental results show that the −3 dB error rejection bandwidth is increased from 76 to 85 Hz, and the coupling efficiency in our optical communication system is improved by 16.87% after using the DCC fine tracking method. These results indicate that the DCC method can effectively achieve the goal of fast and accurate tracking for optical communications systems.

Using top-hat beam to improve the performance of the inter-satellite laser communication

The profile of transmitting beams in inter-satellite optical links significantly impacts communication performance, shaping the general Gaussian transmitting beam into a far field top-hat profile may improve the performance of the inter-satellite laser communication system. The calculations show that this scheme would at least reduce beaconless acquisition time by nearly 63%, and may reduce the outage probability of communication link several orders of magnitude. In addition, the tracking error allowance of communication terminals would at least increase to 1.65 times while keep communication link stable. Use of top-hat beam is quite helpful to build and operate robust inter-satellite optical communication terminals at lower technical risks and cost.

Antenna gain of actively compensated free-space optical communication systems under strong turbulence conditions.

Current Strehl ratio models for actively compensated free-space optical communications terminals do not accurately predict system performance under strong turbulence conditions as they are based on weak turbulence theory. For evaluation of compensated systems, we present an approach for simulating the Strehl ratio with both low-order (tip/tilt) and higher-order (adaptive optics) correction. Our simulation results are then compared to the published models and their range of turbulence validity is assessed. Finally, we propose a new Strehl ratio model and antenna gain equation that are valid for general turbulence conditions independent of the degree of compensation.

Research of Image Processing Based on High Frame-Rate Beacon for Free Space Optical Communication

It is the key step for real-time image processing of beacon to complete the pointing, acquisition and tracking course between the optical communication terminals. The segmentation and extraction of beacon is the most difficult problem for the sequence image processing, a kind of adaptive threshold segmentation method is put forward on the basis of Otsu algorithm , in the improved algorithm, the image processing is divided into two parts: firstly, it is to identify potential target in a single frame; the secondly, it is to detect beacon image in a region of interesting . Finally, the kalman model is applied to accomplish the tracking of beacon, the experimental result shows that the Otsu algorithm overcomes the disadvantage of incomplete information for traditional algorithm, therefore, it is beneficial to improve the tracking ability of system．

Effect of plane-mirror installation errors on pointing errors in periscope-type satellite optical communication terminals

In a satellite optical communication system, the pointing error is an important factor that strongly influences the system performance. For periscope-type satellite optical communication terminals, installation errors of 45° plane mirrors in the coarse point apparatus (CPA) can lead to pointing errors. In this respect, we classify the installation errors of two 45° plane mirrors in the system and derive the corresponding “transfer” matrices to describe them. Furthermore, we establish mathematical models of the pointing error caused by installation errors of 45° plane mirrors. These models allow one to calculate and compensate the pointing errors in periscope-type satellite optical communication terminals. The results of our simulations show that the pointing errors vary with the elevation axis angle but do not depend on the azimuth axis angle. Compared to the side-angle error, the slope-angle error brings a larger pointing error. However, the mathematical model we developed allows one to compensate more than 99.9 % of the overall pointing error. The present work provides a theoretical basis for the design of periscope-type satellite optical communication terminals.

A Magnetically Levitated Precise Pointing Mechanism for Application to Optical Communication Terminals

Increasing data bandwidth requirements from spacecraft systems is beginning to pressure existing microwave communications systems. Free-Space optical communications allows for larger bandwidths for lower relative power consumption, smaller size and weight when compared to the microwave equivalent. However optical communication does have a formidable challenge that needs to be overcome before the advantages of the technology can be fully utilized. In order for the communication to be successful the transmitter and receiver terminals need to be pointed with a high accuracy (generally in the order of ≤10 μradians) for the duration of communication. In this paper we present a new concept for the precise pointing of optical communications terminals (termed the Precise Pointing Mechanism). In this new concept we combine the separate pointing mechanisms of a conventional optical terminal into a single mechanism, reducing the complexity and cost of the optical bench. This is achieved by electromagnetically actuating the whole telescope assembly in 6 degrees-of-freedom with an angular resolution of less than ±3 μradians within a 10 (Az. El.) field of view and linear resolution of ±2 μm. This paper presents the new pointing mechanism and discusses the modelling, simulation and experimental work undertaken using the bespoke engineering model developed.

Discussion on design aspects for free-space optical communication terminals

Free-space optical communications is an emerging solution to increase link capacity of nondetectable links to multi gigabits per second. FSO is widely believed as to overcome radio frequency spectrum and data rate limitations. However, one of the biggest challenges facing FSO deployment is its optical signal propagation in different atmospheric conditions. This effect causes variable link degradation due to variable attenuation and fading. Atmospheric turbulence produces temporary pockets of air with slightly different indices of refraction. Therefore the laser beam phase-front varies randomly, producing intensity fluctuation. One further key challenge with FSO systems is maintaining transceiver alignment. Using highly directional and narrow beams of light, variable mispointing of the transmit beam, and tracking errors of the receiver will generate additional signal fading. In this article terminal design aspects are discussed, and problems that occur when building FSO terminals are highlighted.