Optomechanical Design and Analysis for Nanosatellite Laser Communications

The CubeSat Laser Infrared CrosslinK (CLICK) mission is a technology demonstration of a 1.5U laser communications terminal for an intersatellite link. The terminal is deployed on a pair of 3U CubeSats in Low Earth Orbit (LEO). The pointing, acquisition, and tracking (PAT) approach includes both coarse and fine systems. The coarse tracking system uses a beacon laser transmitter and receiver camera. The fine tracking system uses a fast steering mirror and quadrant photodiode. The communications transmit and receive paths include a refractive telescope, transmit laser collimator, and avalanche photodetector (APD) receiver. The communications laser full-width, half maximum (FWHM) beam divergence angle is 14.6 arcseconds, and the beacon laser FWHM divergence is 0:75° (2700 arcseconds). The opto-mechanical design process includes prediction & verification of assembly alignment & calibration, thermoelastic effects, structural modes & static loading, and fastener analysis. The opto-mechanical assembly has the sensors and laser transmitters kinematically mounted to enable on-ground calibration to less than 25.4 mm decenter, or 0.1° tip/tilt. The thermoelastic alignment error between the payload and bus star tracker is estimated via finite element analysis to be less than 9 arcseconds. The payload optical bench is designed with custom thermal isolation and control to maintain 20 ± 10 ° C. The thermal modeling of the payload is described in detail. Structural static loading and fastener analyses of the CLICK payload under launch loads of 30 G verify margins of safety are greater than 10 and above the recommended values. Modal analyses predict the first resonant frequency to be 888 Hz, above typical vehicle structural vibration ranges with a factor of safety greater than 3.5

Design and Prototyping of a Nanosatellite Laser Communications Terminal for the Cubesat Laser Infrared CrosslinK (CLICK) B/C Mission

The CubeSat Laser Infrared CrossLink (CLICK) mission goal is to demonstrate a low cost, high data rate optical transceiver terminal with fine pointing and precision time transfer in aleq1.5U form factor. There are two phases to the technology demonstration for the CLICK mission: CLICK-A downlink, and then CLICK-B/C crosslink and downlink. The topic of this paper is the design and prototyping of the laser communications (lasercom) terminal for the CLICK-B/C phase. CLICK B/C consists of two identical 3U CubeSats from Blue Canyon Technologies that will be launched together in Low Earth Orbit to demonstrate crosslinks at ranges between 25 km and 580 km with a data rate of ≥20 Mbps and a ranging capability better than 0.5 m. Downlinks with data rates of ≥10 Mbps will also be demonstrated to the Portable Telescope for Lasercom (PorTeL) ground station. Link analysis using current parameters & experimental results predicts successful crosslink & downlink communications and ranging. Moreover, closed-loop 3σ fine pointing error is predicted to be less than 39.66 μrad of the 121.0 μrad 1/e² transmit laser divergence. The status of the payload EDU and recent developments of the optomechanical and thermal designs are discussed

Testing of the CubeSat Laser Infrared CrosslinK (CLICK-A) Payload

The CubeSat Laser Infrared CrosslinK (CLICK-A) is a risk-reduction mission that will demonstrate a miniaturized optical transmitter capable of ≥10 Mbps optical downlinks from a 3U CubeSat to aportable 30 cm optical ground telescope. The payload is jointly developed by MIT and NASA ARC, and is on schedule for a 2020 bus integration and 2021 launch. The mission purpose is to reduce risk to its follow-up in 2022, called CLICK-B/C, that plans to demonstrate ≥20 Mbps intersatellite optical crosslinks and precision ranging between two 3U CubeSats. The 1.4U CLICK-A payload will fly on a Blue Canyon Technologies 3U bus inserted into a 400 km orbit. The payload will demonstrate both the transmitter optoelectronics and the fine-pointing system based on a MEMS fast steering mirror, which enables precision pointing of its 1300 μrad full-width half-maximum (FWHM) downlink beam with anestimated error of 136.9 μrad (3-σ) for a pointing loss of -0.134 dB (3-σ) at the time of link closure. We present recent test results of the CLICK-A payload, including results from thermal-vacuum testing, beam characterization, functional testing of the transmitter, and thermal analyses including measurement of deformation due to the thermal loading of the MEMS FSM

On-orbit beam pointing calibration for nanosatellite laser communications

Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. We describe techniques developed to optimize beam pointing control for a CubeSat laser downlink demonstration mission being developed at the MIT Space Telecommunications, Astronomy, and Radiation Laboratory. To fine-point its downlink beam, the mission utilizes an uplink beacon signal at 976 nm captured by an on-board ±5-deg field-of-view detector and tracked by a 3.6-mm commercial, off-the-shelf MEMS fast steering mirror. As these miniature actuators lack feedback sensors, the system design is augmented with an optical calibration signal to provide the mirror's pointing feedback. We describe the system and introduce calibration algorithms utilizing the feedback signal to achieve higher fidelity beam pointing control. A demonstration in the laboratory is conducted to obtain a quantitative performance analysis using quasi-flight hardware with simulated spacecraft body pointing disturbances. Experimental results show that beacon tracking errors of only 16 μrad root-mean-square are feasible for both axes, significantly exceeding the mission pointing requirement of 0.65 mrad and indicating the feasibility of narrower beams and higher data throughputs for next-generation downlink demonstration missions

Miniature Optical Steerable Antenna for Intersatellite Communications Liquid Lens Characterization

Laser communication (lasercom) can enable more efficient links across larger distances compared with radio frequency (RF) systems. However, lasercom systems are typically point-to-point connections that would have difficulty interacting with several concurrently active spatially diverse users, where RF systems can more easily support such scenarios. Lasercom pointing, acquisition and tracking (PAT) systems have traditionally relied on mechanical beam steering devices, such as fast steering mirrors (FSMs) or gimbals, both of which are subject to potential mechanical failure. In this work we investigate an alternative steering solution using liquid lenses. Liquid lenses are tunable lenses that can non-mechanically alter focal length based on an applied voltage or current. A series of liquid lenses, one on-axis to control beam divergence, and one each offset in the x and y-axes to steer, could be used to achieve laser pointing control. Currently available commercial off the shelf (COTS) liquid lenses are based on electrowetting (manufactured by Corning [1]) or pressure-driven (manufactured by Optotune [2]) operation. In this work, we analyze the suitability of both types of liquid lenses for use in a space-based multiple access lasercom terminal. Early liquid lens technology first surfaced in 1995 with the control of the shape of an oil droplet through electrowetting [3]. The technology then started to become commercially available with the founding of Varioptic in 2002. However, there is limited data on liquid lens survivability and operation in a space-like environment. Through vacuum testing, we have found that electrowetting-based liquid lenses not only survive, but continue to operate nominally in a very low-pressure environment. The pressure-driven liquid lenses appeared to have issues initially in vacuum testing, with gas bubbles forming in the lens aperture during pump-down. However, after extended exposure to vacuum of approximately two weeks, the gas bubbles diffuse through the lens membrane, and the lenses operate in vacuum. Steering transfer functions were developed both in ambient and in vacuum conditions for both lens types, and in each case, the differences between the two curves were largely negligible. The electrowetting lenses provide a steering range of 2.7°, both in and out of vacuum, with an approximate slope of 0.046°/V. In testing the Optotune lenses, the steering was limited by the camera detector size, but for a range of -92 mA to 144 mA on the steering lens, the lenses provided for approximately 8.6° of steering with a slope of 0.0367°/V. These steering ranges can be extended to near hemispherical coverage with the addition of a diffuser and wide-angle fisheye lens [4]. Maximum hysteresis error, the difference in steering angle response when increasing lens voltage or current as opposed to decreasing lens voltage or current, was identified at 0.02° for the Corning lenses and 0.05° for the Optotune lenses. A Zemax beam quality analysis was conducted to see how transmit gain would be affected by refraction through the liquid lenses. Through this analysis, the worst-case link penalties were determined to be -0.5 dB for the Corning lenses at _0.8° steering and -0.4 dB for the Optotune lenses at _1.0° steering. Thus, we see that liquid lenses are likely good candidates for space applications and may perform well in nonmechanical beam steering. We discuss next steps in environmental testing as well as optical layout and control approaches for using liquid lenses in PAT systems for a nanosatellite based optical antenna

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

The Cubesat Laser Infrared CrosslinK (CLICK) mission is a technology demonstration of10 Mbps downlink. On the second flight, with two identical 3U CubeSats, CLICK-B/C, a \u3e20 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 urad are reported from test and simulation. The communication detector of the payload is a 200 um 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