Optical Performance and Prototyping of a Liquid Lens Laser Communications Transceiver

Abstract

Laser communications can enable more efficient and higher bandwidth communications than conventional radio frequency (RF) systems. Free-space optical communications systems' beams are typically narrower in divergence and require precise pointing, acquisition, and tracking (PAT) systems to establish and maintain links. Several technologies for beam steering exist, including MEMS fast-steering mirrors (FSMs), gimbals, and photonic integrated circuit (PIC) devices. However, these may not meet steering, aperture, power handling, or size, weight, and power (SWaP) requirements for small spacecraft.

The Miniature Optical Steered Antenna for Intersatellite Communications (MOSAIC) aims to utilize liquid lenses to provide miniaturized non-mechanical beam steering, allowing wide field-of-view communications and multiple access capabilities. MOSAIC uses three liquid total liquid lenses: one lens is on-axis to provide divergence control, whereas the other two are offset in +x and +y respectively to provide steering. Previous work has focused on qualifying liquid lenses for the space environment, showing a clear path for evaluating their optical performance and constructing prototypes. Lenses from Corning Varioptic (France) and Optotune (Switzerland) are both considered in this work.

The liquid lenses undergo environmental testing for liquid lenses, including microgravity testing, radiation exposure, and quantifying how much power the lenses can effectively couple. An analytic formulation of beam steering is presented, which can be used as a feedforward controller and to optimize system size to fit in constrained spaces, such as CubeSats. Simulation work in Zemax is presented to characterize the transmit and receive gain parameters, and a complete optical link budget is constructed from simulation results. Simulation results are validated with experiments showing beam profiles. An evaluation of diffusers is made, to evaluate the trade in increasing numerical aperture (NA) at the expense of beam quality. Transmit and receive capability is also demonstrated experimentally using two laboratory prototypes. Transceiver architecture trades are discussed, introducing the baseline design, strategies to incorporate diffusers, the benefits of apodization, optimizing the receive path, and strategies to beacon using nutation. Characterization of prototype 1550 nm (optical C-band) optimized lenses from Corning Varioptic are also characterized. Preliminary simulation results for steering multiple beam using a single optical train of variable focuses lenses is also presented.

The liquid lenses from both Corning Varioptic and Optotune show excellent power handling capabilities, with no visible damage to either lens with input powers of up to 2 W continuous wave (CW) at 1550 nm. Both sets of do not show increased degradation rate due to radiation exposure. The visible-spectrum Corning Varioptic and Optotune lenses decrease in transmission from 100% to 91% (Corning Varioptic) and 85% (Optotune), after exposure to radiation equivalent to 10 years in low Earth orbit (LEO) with 0.5 mm aluminum shielding. Additional tip/tilt and coma aberration is measured on the lenses in gravitational environments due to the optical fluid sagging. Tip/tilt changes by 0.74 mrad and 4.05 mrad for the Corning Varioptic A-39N0 and Optotune EL-16-40-TC lenses, respectively. Beam quality significantly improves in microgravity for the Optotune EL-16-40-TC lenses, with significantly decreased coma aberration. The Corning Varioptic A-39N0 lenses maintain excellent beam quality throughout gravitational regimes and show a slight decrease in coma aberration in microgravity. Extended environmental testing qualifies these lenses to a TRL 5-6 on NASA's Technology Readiness Level (TRL) scale. Optical link budgets show that with a reference system, the Corning A-39 and Optotune EL-16-40-TC lenses can maintain a 25 Mbps 16-PPM link with 4 W of input power at 1550 nm with hemispherical steering up to 40 km (Corning Varioptic) and 220 km (Optotune).