Development of the critical technological building blocks of a cryocooler for space and embedded applications – LASCO2

Early detection of missile firing, intelligence and greenhouse gas monitoring are major defense and civil challenges of our century. All share in common the need for infrared imagers embedded on satellites. Performances of those systems come from cryogenic cooling of sensors. The need for constant monitoring of Earth globe imply the use of mini-satellites constellations. This induce constraints for coolers which need to be miniature, vibration-free and cost effective.
The project LASCO2 propose to proceed toward a new space cryocooler generation able to answer those specifications. It is based on a technological revolution: laser-cooling. In such system, the cold source is built upon a crystal excited by a laser beam that absorb the energy and re-emit in fluorescence. Excitation conditions are such that radiated power is greater than absorbed power. Compared with a conventional mechanical cooler (like a pulse-tube), a laser-cooler shows numerous advantages: no vibrations, low electromagnetic perturbation and very small volume for the cold head – the cooling crystal fitting in less than 0.25 cubes centimeters. The stake of this technology development is to reach an efficient system.
The record of temperature is at 87 K with an Ytterbium doped YLF crystal pumped with a laser at 1 µm. Theory predicts a direct dependency of pump wavelength and energy efficiency: the higher the former, the better the later. To answer the efficiency stake this project propose to operate at 2 µm with holmium doped YLF crystals.
A laser-cryocooler includes three critical items: the crystal where the Anti-Stokes effect occurs; the thermal link which conduct the heat from the thermal load to the crystal; the laser source which provide the power to the crystal.
Capitalizing on latest developments, the objective of the project is to break several technological locks on critical items:
• Integration of laser-cooler on space-borne application. This includes a mechanical pre-qualification of thermal link and a pre-design of an integrated cooler-detector assembly.
• Comparison of cooling performance of crystals at 1 µm and 2 µm. This is performed first by spectroscopy by measuring the absorption and fluorescence, and by cooling efficiency measurement as a function of temperature.
• Development of a new fiber laser source both powerful and efficient, operating at 2 µm. This includes the development of a highly tunable laser source for crystal characterization, and a laser source optimized both in wavelength and in efficiency.
• Tolerance to ionizing radiations (protons, electrons) of cooling crystal and fibers for lasers. Comparing the properties before and after irradiation those measures allow to infer the evolution of cooling efficiency and of pump laser power in space radiation environment.
This project enables to seal the alliance of major actors in cryogenics and power laser source namely AL-aT, Institute Néel, ONERA and ISL. By maturing those items, LASCO2 allows on the short run to hold a technological basis paving the way toward disruptive applications, both civil and defense.