This article presents an innovative mechatronic design of a high-accuracy pointing mechanism for orbital laser communication terminals.The system is based on a dual-stage architecture and is miniaturized to fit nanosatellite-class spacecraft, aiming to enable optical communication on small-size space platforms. The focus is on control design aspects and on the performance assessment of an experimental prototype under emulated external environmental disturbances.

High-Precision Dual-Stage Pointing Mechanism for Miniature Satellite Laser Communication Terminals

Microelectromechanical systems (MEMS) have produced high-quality, high-bandwidth, small form factor, and inexpensive fast steering mirror (FSM) devices potentially suitable for a large variety of applications, such as image stabilization and beam pointing in satellite-based and ground-based, free-space optical communication systems. However, one outstanding question for this application is power handling. The absorption of the mirror substrate is low, but non-negligible, so the question remains of whether thermal loading from laser radiation on a MEMS mirror will deform its surface and, if so, to what extent. We show experimental results of optical performance changes due to thermal loading for MEMS two-axis FSM devices from Mirrorcle Technologies, Inc. Results and reproducible behavior are reported and compared in ambient versus vacuum conditions, where the benefits of convective cooling are absent. Finite element analyses corroborate the experimental results and show that the mirror substrate can deform due to thermal expansion imbalances. The deformation changes the focusing characteristics of the mirror, with a peak to valley defocus (second-order Zernike mode) of up to 50 nm when the mirrors are tested in ambient and up to approximately 450 nm when under vacuum. Such defocusing negatively impacts the link budget for laser-based satellite communications.

https://www.spiedigitallibrary.org/journals/optical-engineering/volume-59/issue-5/056109/Characterization-of-laser-thermal-loading-on-microelectromechanical-systems-based-fast/10.1117/1.OE.59.5.056109.pdf

Characterization of laser thermal loading on microelectromechanical systems-based fast steering mirror in vacuum

LaserCube optical communication terminal for nano and micro satellites

Laser communications can enable more efficient and higher bandwidth communications across longer distances than conventional radio frequency (RF) systems. However, beam divergence angles for laser systems are narrower than typical RF systems, and require precise pointing, acquisition, and tracking systems to establish and maintain the link. In addition, typical lasercom links are point-to-point, and not capable of multicast or broadcast. Conventional pointing and tracking (PAT) systems use mechanical gimbals or fast-steering mirrors. Mechanical gimbals may not meet the size, weight, and power (SWaP) constraints for small spacecraft, particularly for multiple concurrent spatially diverse beams. Fast-steering mirrors while compact and efficient have limited aperture size, and many would be needed to provide multiple links over a hemisphere. The Miniature Optical Steered Antenna for Intersatellite Communications (MOSAIC) aims to provide nonmechanical pointing and tracking using liquid lenses, allowing a wide field-of-view and support for multiple concurrent links. Initial work with commercially available liquid lenses showed that liquid lenses can be used in a space environment and assessed spatial coverage. In this work, we model a transmitter using three liquid lenses. One on-axis lens provides focusing capability. Two off-axis and perpendicular lenses provide beam steering, with a fisheye lens amplifying the effect. This provides near-hemispherical pointing up to 170 degrees. We investigate beam quality and divergence using a Zemax model and conduct a link analysis dependent on the beam steering angle and rotation angle. A 25 Mbps link with 200 mW transmit power at 1550 nm (optical C band) and 16-ary pulse position modulation (16-PPM) can be maintained up to 28 km separation with 3 dB margin for an Optotune EL-16-40-TC liquid lens. Losses are primarily due to the liquid lenses limiting aperture size to 16 mm. We also consider the impact of diffusers for increasing numerical aperture through a simple ray transfer analysis and experimental results.

Link analysis for a liquid lens beam steering system, the miniature optical steered antenna for intersatellite communication: MOSAIC

This paper presents a state-of-the-art overview of fine beam steering mechanisms for free-space optical communication on satellites. Precise beam pointing is a critical task for the successful operation of free-space optical communication systems. Based on past research and ongoing projects, the use of fast steering mirrors (FSMs) is still the most popular solution for free-space optical communication applications. Although a variety of commercial off-the-shelf (COTS) FSM solutions exist, there is limited publicly available data on these solutions in the space environment. Three main actuation principles are considered (electro-static force actuated, magnetic force actuated, piezo-effect actuated) and reviewed using available data from past space missions. The article describes the most important criteria in the choice of a fine beam steering solution for free-space optical communication in space.

A Review of Mechanical Fine-Pointing Actuators for Free-Space Optical Communication

The Cubesat Laser Infrared CrosslinK (CLICK) mission is a technology demonstration of <1.5U nanosatellite laser communications terminals, with downlinks from low-Earth orbit altitudes of 400-600 km to a portable optical ground station, and crosslinks over ranges from 25-580 km. The mission has two separate launches and three total 3U CubeSats. The first flight, with a single 3U CubeSat, CLICK-A will demonstrate a >10 Mbps downlink. On the second flight, with two identical 3U CubeSats, CLICK-B/C, a >20 Mbps crosslink will be demonstrated in addition to downlinks. In this paper representative link budgets for the crosslink are presented, including both communications and beacon lasers. The payload Pointing, Acquisition and Tracking (PAT) system is introduced, and the performance of the second stage closed loop tracking signal processing is assessed. Errors below 1 μrad are reported from test and simulation. The communication detector of the payload is a 200 μm InGaAs Avalanche PhotoDetector (APD), with a 1 GHz bandwidth and a dynamic range of more than 40 dB provided by programmable gain amplifiers. The APD performance enables a data rate of 17.7 Mbps at a range of 520 km. The timing accuracy of the detector is better than 130 ps.

/pdf/optical-communications-crosslink-payload-prototype-50nnrxrlhn.pdf

Optical Communications Crosslink Payload Prototype Development for the Cubesat Laser Infrared CrosslinK (CLICK) Mission

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Optical Communications Crosslink Payload Prototype Development for the Cubesat Laser Infrared CrosslinK (CLICK) Mission