This PDF file contains the front matter associated with SPIE Proceedings Volume 12018, including the Title Page, Copyright information, Table of Contents and Conference Committee lists.

Yb:YLF crystal with near-unity quantum efficiency attracts the attention of researchers in high-power laser development and laser-cooling communities. Here, we will review our recent efforts in energy and average/peak power scaling of room temperature and cryogenic Yb:YLF lasers and amplifiers. At first, we will present temperature dependence of important laser related parameters in Yb:YLF such as fluorescence lifetime, absorption cross section, emission cross section and gain in the 78-300 K range. We will then discuss the in situ optical temperature estimation methods that could be used to accurately estimate Yb:YLF crystal temperature. Later, we will review our recent lasing/amplification results with room temperature (RT) and cryogenic Yb:YLF systems, where we have achieved output powers exceeding 500 W in cw operation with efficiencies approaching 80% and pulse energies above 300 mJ at 10 Hz repetition rate.

The effects of amplified spontaneous emission (ASE) as well as absorption saturation in cryogenic optical refrigeration in rare-earth doped materials are analyzed theoretically. It is seen that ASE may pose a limitation on power scaling under strong feedback conditions (i.e. in high finesse pump circulator cavities). Absorption saturation may have similar effect depending on the value of the background absorption.

The feasibility of space-borne no-vibration optical cryocoolers based on GaAs for high precision petrology and optical clock applications will be discussed. In particular, theoretical analysis and reasonable estimations will be presented for a cryocooler device using GaAs at T=16.8K suitable for cooling the silicon reference cavity developed by NIST.

The recent reports of laser cooling in Yb-doped aluminosilicate fibers and silica preforms have opened up the field of optical refrigeration and radiation-balanced lasers to the enormous realm of silica fiber lasers and amplifiers. To increase the cooling efficiency achieved in these materials, it is critical to identify host compositions that improve the Yb³⁺-ion properties in the directions of low concentration quenching, short radiative lifetime, and a long-wavelength absorption tail that extends as far as possible above the zero-phonon line. In this on-going quest, nanoparticle-doped fibers offer a promising technique to modify the chemical environment of the Yb³⁺ ions and achieve some of these properties. In this work, three fibers in which the Yb³⁺ ions are initially encapsulated in CaF_2, SrF_2, or BaF₂ nanoparticles were fabricated using a solution-doping technique, and their laser-cooling properties evaluated experimentally and analyzed. The CaF₂ fiber and the SrF₂ fiber were successfully cooled at atmospheric pressure when pumped with a continuous-wave laser at the near-optimum wavelength of 1040 nm. The measured maximum temperature change from room temperature was -26.2 mK for the CaF₂ fiber at a pump power absorption level of 90 mW/m, and -16.7 mK at 66 mW/m for the SrF₂ fiber. The BaF₂ fiber did not cool, but it warmed only slightly, indicating that it was not far from cooling. Analysis of the measured dependence of the fiber temperature change on pump power with a model enabled extraction of the fiber’s critical quenching concentration and residual absorptive loss due to impurities. Comparison of these values to the values reported for an aluminosilicate fiber and fiber preforms that cooled shows that the CaF₂ and SrF₂ fibers faired as well as the fiber, and better than the preforms, in terms of quenching, but that they had a higher absorptive loss. This study establishes the significant research potential of nanoparticle-doped fibers in the search for efficient laser-cooling silica hosts.

The optimization of the fiber geometry, core composition & rare earth ion concentration in optical fibers are of great importance in obtaining high photoluminescence quantum yield (PLQY) for optical refrigeration applications. Presented herein are the important advancements in the development of a Y2O3: Yb (5 molars percent) nanoparticle doped fibers embedded in glass matrix with composition 95SiO_2-5GeO², with an in-depth investigation on structural and optical properties for laser cooling applications. The impurity absorption was minimized by using high-purity precursors to fabricate the fiber using modified chemical vapor deposition method (MCVD). Structural characterizations have evidenced the presence of Y²O₃ nanoparticles ranging from 25 to 50 nm. The optical properties such as transmission, refractive index, photoluminescence (PL) emission and lifetime were studied in detail. Finally, the background absorption as well as temperature change of the fibers using 1030 nm laser are measured using fiber Bragg grating (FBG) sensor.

We have successfully synthesized lithium yttrium fluoride (YLF) nanocrystals doped with ytterbium. The Yb content was varied between 1% and 10%. The nanocrystals emit in near-infrared, with the emission spectrum extending from 960 nm to 1060 nm when excited with the 900 nm light. Strong anti-Stokes photoluminescence was observed when using excitation wavelengths ranging from 1010 nm to 1020 nm. The temperature-dependence of the anti-Stokes photoluminescence was measured over the range from 10 °C till 70 °C. These nanocrystals have a high potential to be used in optical cooling applications.