Our group in the Space Optics Department at INTA has been working during more than 20 years in the development of devices based on liquid crystals for optical payloads onboard space missions. Currently, there are three of our devices successfully in operation in the PHI and METIS instruments of the Solar Orbiter ESA/NASA mission. Therefore, they have the highest technology maturity level, TRL9 (Technology Readiness Level). To the best of our knowledge, we are pioneers of the use of polarization modulators based on liquid crystals in a telescope or a camera in a space platform.
Liquid crystal devices avoid using standard solutions that involve mechanisms with rotatory polarization optical parts. Instead, we use this technology that minimizes the size, mass and power consumption of the device while maximizing its useful aperture and performance. These new capabilities open up new possibilities for small satellites that were previously only attainable by larger satellites. Liquid crystal-based polarization modulator technology is highly versatile and can be configured in multiple ways to suit diverse applications. It is based on the ability of liquid crystal variable retarders to control, modify and measure the polarization state of light, be it in an image or in a spot beam.
The application fields are numerous, from Astrophysics to Earth Observation. This work will introduce some of the main instruments that we are working on: from the Vigil ESA mission for Space Weather to Quantum Communication Space Systems, and including the Miniature Absolute Magnetometer for the NanoMagSat mission of ESA’s SCOUT Program. Also, we will show the development status of other liquid-crystal devices for compact space instrumentation that we are developing as Liquid Crystal Tunable Filters (LCTFs) and Spatial Light Modulators (SLMs).
In October 2023, the INTA spinoff Eye4Sky was established to the exploitation and commercialization of this optical technology of liquid crystal devices for space applications. This deep-tech startup has been selected for the prestigious European Space Agency Business Incubation Centre (ESA BIC) program.
Liquid Crystal Polarization Rotators (LCPRs) have been developed for the Miniaturized Absolute Magnetometer (MAM) instrument in NanoMagSat project, an ESA’s SCOUT program mission. This project consists of a constellation of three nanosatellites aimed to study the Earth’s magnetic and ionospheric environment based on a 16U CubeSat-type structure. The MAM instrument is an optically pumped scalar and vector magnetometer derived from the ASM flown on the ESA Swarm mission. In this type of instruments, a device to rotate the direction of the incident linear polarization of the pumping beam injected into the helium-4 gas cell sensor is required. In NanoMagSat, the LCPRs will replace the sensor head rotor driven by a piezoelectric motor used in the ASM, allowing a very significant miniaturization of the sensor head. The LCPRs developed are miniaturized devices derived from the polarization modulators based on liquid crystals of PHI and METIS instruments on board the Solar Orbiter mission and optimized for the MAM instrument requirements. The key performance parameters of the devices have been evaluated in a validation test campaign, under the different environmental conditions expected in NanoMagSat, including the polarization rotation and the Polarization Extinction Ratio (PER) as a function of voltage, and the response times at the MAM polarization rotation scheme and will be presented in this work. Based on the results found, the LCPRs design and validation test campaign has been considered successful and they have been approved to be implemented for the NanoMagSat mission.
In Quantum Key Distribution (QKD), the emitter and receiver need to share an optical quantum channel - which can be optical fibre, terrestrial free-space or space-based links- to exchange the quantum states. However, with the future aim to achieve a quantum global communication network, communications links between small satellites in constellations will be required. In this context, the experience of INTA in the ANSER (Advanced Nanosatellite Systems for Earth Observation Research) small satellite constellation program will be exploited. This program develops a set of missions that will include groupings of a minimum of three CubeSats (a leader and two or more followers) flying in formation and in coordinated operation for a common mission. Therefore, the only difference between ANSER and Q-ANSER program will be the payload of the satellite. In Q-ANSER, in which a prepare-and-measure B92 QKD protocol will be used to generate the secret key, two optical systems will be introduced. In the emitter this system will be capable of sending polarized weak coherent laser pulses, attenuated to single-photon level, to the receiver, which will also be an optical system capable of receiving and detecting these single photons. Prepare-and-measure QKD schemes with polarization encoding require the minimization of polarization degradation both in the transmitter and receiver designs. In particular, the polarization extinction ratio (PER) should be maintained as high as possible to reduce the quantum bit error rate (QBER) . This polarization control will be done with the polarization modulators based on liquid crystals developed by INTA.
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