Laser induced cooling for space communications and sensor systems

Laser cooling of transparent materials is a technique that uses light emission to transport and manage heat dissipation in lasers and amplifiers by converting optical phonons into light of shorter wavelength than a pump source through an anti-Stokes process. Elucidated by Pringsheim in 1929 [1], it led to a heated debate on whether the third law of thermodynamics was being violated. Landau settled the argument by invoking that entropy increased as the emitted light had no specified directionality. Laser cooling was only first demonstrated by Epstein et al in 1995 [2], when a sample of Yb:ZBLAN was cooled by 0.3C from room temperature, the long delay from theory to practice being indicative of challenges in achieving the purity of the material system required for the effect to be observed. As absorption of any photon strongly counters the effects of cooling by the emission of another photon adding to the heat load, extremely low absorption is a fundamental requirement for the material. Another high purity route is the Modified Chemical Vapour Deposition (MCVD) technique that has been successful in the production of ultra-low-loss optical fibers, fiber lasers and amplifiers for optical communication systems. While amplification and lasing require low intrinsic loss, the impurity content needed for laser cooling is at least an order magnitude lower; one which has recently been met. Laser cooling in MCVD glasses has thus been achieved in several material combinations, of which our focus has been on a particular all-oxide glass system of an active rare earth and inactive lanthanides as a predominant host in silica. This route has allowed the highest dopant concentration of active rare-earth ions in the form of nano-crystalline glasses. These glasses offer excellent opportunities for the fabrication of optically cooled high-power lasers and amplifiers for communication systems and sensing in space where payload of a solution strongly influences the choice for deployment. Heat generated through laser action causes thermal lensing and beam wander in high power fiber lasers. This can lead to beam pointing instabilities, detrimentally affecting optical sensing, power beaming, and bit error rate, which over astronomical distances is greatly magnified. Laser cooling may be configured to cool high power fiber lasers via radiation balancing, thereby mitigating this difficult problem.