It is well known that one can achieve optical cooling with anti-Stokes fluorescence (ASF) by pumping a laser material at a wavelength longer than its mean fluorescence wavelength. Accordingly, this well-known fact has been effectively applied to cool almost exclusively bulk samples placed in a vacuum, which reduces the heat introduced into the sample by convection of the surrounding air and helps produce larger temperature drops. However, for the development of radiation-balanced fiber lasers and fiber optic coolers, it is of the foremost importance to investigate ASF cooling of fibers at atmospheric pressure; a much more challenging problem that remains unexplored. Current high-power fiber lasers (>10 kW) require bulky fluid-based cooling systems to remove waste heat. These systems add both intensity and frequency noise to the laser output, as well as significant cost. ASF cooling provides a promising alternative that is compact and vibrationless.
A previous publication comprehensively reported on a model that quantifies analytically and numerically the heat that can be extracted per unit time by ASF from a fiber doped with a quasi-two-level laser ion such as Yb3+. The report showed that since ASF cooling is a relatively weak effect, it is vital to carefully design the fiber so that cooling due to ASF dominates over the heating caused by concentration quenching or absorption by impurities. To this end, Stanford University researchers: Jennifer Knall (PhD candidate), Dr. Arushi Arora and Professor Michel Digonnet together with Dr. Martin Bernier at the Laval University and Dr. Solenn Cozic at Le Verre Fluoré in France; proposed to utilize Yb-doped ZBLAN fibers. Their work is currently published in the research journal, Optics Letters.
The researchers adopted Yb3+ due to its short radiative lifetime, which increases the probability of ASF. Additionally, the low phonon energy in ZBLAN results in a high critical quenching concentration and, therefore, the possibility of using high Yb concentrations without suffering from quenching. Using a specialized slow-light fiber Bragg grating sensor, temperature changes of −5.2 mK and −0.65 K were measured in a single-mode (1 mol.% YbF3) and multimode (3 mol.% YbF3) ZBLAN fiber placed at atmospheric pressure with respective cooling efficiencies of 2.2% and 0.90%. The temperature changes measured in the multimode fiber were two orders of magnitude larger than in the single-mode fiber, highlighting the importance of a large doped area for maximal cooling.
In summary, for the first time, cooling through ASF was reported in a single-mode and a multimode fiber at atmospheric pressure. This tremendous achievement marked a fundamental milestone in the pursuit of radiation-balanced fiber lasers. Additionally, achieving low-error fits confirmed the accuracy of the presented model and validated it as a tool to provide quantitative predictions for the design of radiation-balanced fiber lasers, amplifiers, and coolers.
Jennifer Knall, Arushi Arora, Martin Bernier, Solenn Cozic, Michel J. F. Digonnet. Demonstration of anti-Stokes cooling in Yb-doped ZBLAN fibers at atmospheric pressure. Volume 44, Number 9 / 2019 / Optics Letters.Go To Optics Letters