The CubeSat Laser Infrared Crosslink (CLICK) B/C mission seeks to demonstrate laser crosslinks for full-duplex communications and two-way ranging and time-transfer between two 3U CubeSats: CLICK-B and CLICK-C. Laser crosslinks between satellites can provide enhanced performance, with high data transfer rates and high precision range and timing information, using low size, weight, and power (SWaP) optical transceiver terminals. CLICK-B and CLICK-C will demonstrate laser crosslinks with data rates of at least 20 Mbps over separation distances ranging from 25 km to 580 km. CLICK-B/C will also demonstrate a ranging precision of better than 50 cm and a time transfer precision of better than 200 ps single shot over these distances. We present the design and development status and recent testing results of the laser transmitter and fine pointing, acquisition, and tracking (PAT) system, which are key to achieving these capabilities. The 1550 nm laser transmitter follows a master oscillator power amplifier (MOPA) design using an erbium-doped fiber amplifier (EDFA) for an average output power of 200 mW. A semiconductor optical amplifier (SOA) is used to achieve the pulse position modulation (PPM), ranging in order from 4 PPM - 128 PPM. The PAT system uses a microelectromechanical systems (MEMS)-based fast steering mirror (FSM) for fine pointing. A quadrant photodiode (quadcell) provides feedback for the actuation and steering of the FSM.
The CubeSat Laser Infrared CrosslinK (CLICK) mission is a technology demonstration of a low size, weight, and power (SWaP) crosslink optical communication terminal. The 3U CLICK-A spacecraft is the first phase of the mission with a 1.2U optical communication downlink terminal. The twin 3U CLICK-B/C spacecraft are the second phase of the mission each with a 1.5U crosslink optical communication transceiver terminal. This work discusses the flight functional and environmental testing for the CLICK-A terminal as well as the optomechanical design and testing for the CLICK-B/C terminals. The CLICK-A terminal serves as a risk reduction effort for the CLICK-B/C terminals, whose goal is to establish a 20 Mbps intersatellite link at separations from 25 to 580 km. The CLICK-B/C terminals communicate with M-ary pulse position modulation (PPM) using a 200 mW erbium-doped fiber amplifier (EDFA). The payloads are capable of ranging up to a precision of 50 cm. CLICK-B & C will both be deployed from the International Space Station (ISS) at the same time and fly in the same orbital plane. We begin by discussing the final integration and environmental testing results from the CLICK-A terminal, which was launched to the ISS in July 2022 and expected to be deployed in September 2022, as well as preparation of the CLICK optical ground station in Westford, MA. Second we present the CLICK-B/C flight terminal development. We describe the optomechanical design of the optical bench and its interface with the terminal. A prototype optical bench with the initial version of the CLICK-B/C optomechanical design has been built and tested. We also capture the lessons learned that have informed the building of an engineering development unit (EDU).
For optical links through deep turbulence, closed-loop adaptive optics (AO) can facilitate more efficient communication schemes, such as those based on coherent detection and optically pre-amplified receivers. Perturbation-based wavefront correction algorithms, such as stochastic parallel gradient descent (SPGD), are promising candidates for low size, weight, and power consumption (SWaP) alternatives to conventional AO based on direct wavefront sensing. However, limited actuator bandwidth combined with poor convergence rate can constrain the effective AO refresh rate, and degrade the performance when multiple atmospheric modes need correction. Here, we derive and test a new, generalized, non-stochastic, modal wavefront correction algorithm that utilizes either time- or frequency-division to correct multiple modes simultaneously. Using an end-to-end AO simulation, we show the new approach can relax the actuator bandwidth requirement by up to a factor of 8 in comparison to SPGD. Finally, we describe a hardware testbed that is being used to validate the developed approaches.
Recent advances in pointing and tracking capabilities of small satellite platforms have enabled adoption of capabilities such as high-resolution Earth Observation (EO), inter-satellite laser communications and, more recently, quantum communications. Quantum communications requires unusually narrow optical beams and tight pointing performance (on the order of ten microradians) to close an inherently brightness-limited quantum link. This limit is due to quantum communication protocols such as quantum key distribution and teleportation requiring individual quantum states to be transmitted with photon number restrictions. We examine an opportunity to combine quantum communications with laser communications in sharing an optical link. We discuss a combined quantum and laser communication terminal capable of performing space-to-ground entanglement-distribution and high data rate communications on a 12U CubeSat with a 95mm beam expander and an 60 cm aperture optical ground telescope. Photon pairs produced by the quantum terminal are entangled in polarization so the polarization must be maintained throughout the optical link. We discuss active and passive compensation methods in space and polarization reference frame correction using a polarized reference beacon at the ground station. The combined quantum and laser communication terminal approach enables secure communications over an optical channel with rates of 100 Mbps and sub-nanosecond time transfer.
Constellations of CubeSats will benefit from high data rate communications links and precision time transfer and ranging. The CubeSat Laser Infrared CrosslinK (CLICK) mission intends to demonstrate low size, weight, and power (SWaP) laser communication terminals, capable of conducting full-duplex high data rate downlinks and crosslinks, as well as high precision ranging and time transfer. A joint project between the Massachusetts Institute of Technology (MIT), the University of Florida (UF), and NASA Ames Research Center, CLICK consists of two separate demonstration flights: the initial CLICK-A, which will demonstrate a space-to-ground downlink and serve as a risk-reduction mission, and CLICK-B/C, a crosslink demonstration mission. The CLICK payloads each consist of laser transceivers and pointing, acquisition, and tracking (PAT) systems, and will fly on 3U CubeSat buses from Blue Canyon Technologies to perform their optical downlink and crosslink experiments in low Earth orbit (LEO). We present an update on the status of both the CLICK-A and CLICK-B/C payloads. At the time of writing, the final assembly and testing of the CLICK-A payload has been completed and the payload has been delivered for integration with the spacecraft bus. The final testing included the validation of the transmitter and the PAT system, the performance of both of which was characterized under various environmental test conditions and shown to meet their requirements for operation on orbit. On CLICK-B/C, the payload electronics have been designed and are under test. The optical bench of the payload has been assembled, the characterization of which is ongoing.
KEYWORDS: Signal to noise ratio, Mirrors, Microelectromechanical systems, Optical tracking, Free space optics, Signal detection, Receivers, Error analysis, Optoelectronics, Transceivers
Free-space laser links traditionally utilize an independent spatial tracking channel with a beacon laser and tracking sensors to meet stringent pointing requirements. In this work, we propose a miniaturized monostatic beaconless fiber transceiver that infers fine tracking information using existing receiver optoelectronics and a small injected pointing dither (nutation). A single MEMS steering mirror is used to both fine-point the beams and inject nutation. While this results in some additional link loss due to disturbed fiber coupling and transmit beam pointing, our analysis shows the loss becomes negligible for sufficient SNR. Links without point-ahead correction need an SNR of about 35 dB to minimize the dither loss below 0.1 dB and also maintain the RMS spatial tracking noise below a tenth of the beam divergence. Since the pointing and tracking bandwidth is typically many orders of magnitude slower than the receiver communication bandwidth, such SNR is usually achievable on the receiver with appropriate filtering. If point-ahead correction is needed, we show that depending on the available link margin, a transceiver based on single-mode fiber can reach up to about 1 beamwidth of correction, while a few-mode fiber design can reach up to about 1.75 beamwidths due to improved coupling sensitivity at higher point-ahead offsets. Finally, we propose the use of double-clad fiber with a secondary detector to help further minimize the incurred coupling loss.
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.
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.
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