Miniature Optical Steerable Antenna for Intersatellite Communications Liquid Lens Characterization

Abstract

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.