KEYWORDS: Silica, Glasses, Luminescence, High power lasers, Optical fibers, Optical amplifiers, High power fiber amplifiers, Absorption, High power fiber lasers, Fiber lasers
Anti-Stokes fluorescence cooling of a Yb-doped silica glass optical fiber preform is achieved using a high-power laser in a double-pass configuration. The coherent laser beam illuminates the silica glass preform in the red tail of its absorption spectrum, and the heat is carried out by anti-Stokes fluorescence of the blue-shifted photons. The high-purity Yb-doped silica glass preform has low parasitic absorption and is codoped with modifiers to mitigate the quenching-induced non-radiative decay for sufficiently high concentrations of Yb ions in silica glass. Therefore, sufficiently large laser absorption could be achieved to observe cooling while maintaining a near-unity external quantum efficiency.
In radiation balanced lasers, anti-Stokes fluorescence is used to minimize the heat generated by the quantum defect and other non-radiative processes. Thermo-optics distortions can be minimized, enabling the scaling to high power. Here, experimental results of radiation balanced operation in various disk gain materials (i.e. YLF:Yb and LLF:Yb ) are presented. Different multipass pumping schemes are investigated for pump beam area scaling towards high power CW operation. Laser cavity design and thermal management issues are also discussed.
The first demonstration of optical refrigeration in 1% Ho:BYF crystal with near unity external quantum efficiency (ɳ_{ext}~99.7%) is reported. Laser cooling efficiency, ɳ_{ext}, and background absorption (α_b) of 1% Ho:BYF crystal is obtained from laser induced temperature modulation spectroscopy (LITMoS) test, and its minimum achievable temperature (MAT) and optimum laser cooling wavelengths are extracted via temperature-dependent spectroscopic analysis. MAT of 120 K at the optimum laser cooling wavelengths of 2056 nm or 2063 nm is estimated for 1% Ho:BYF crystal with measured ɳ_{ext}=99.7% and α_b~1e-3 cm^-1. Additionally, MATs of 80 K and sub-80 K are estimated for this crystal considering improved background absorptions of α_b~2e-4 cm^-1 and α_b~5e-5 cm^-1 respectively.
We report the observation of anti-Stokes fluorescence cooling of Yb-doped silica glass by 0.7 degrees Celsius. We conduct a detailed investigation of the cooling parameters of this glass, including the wavelength dependence of the cooling efficiency as a function of the wavelength and also the parasitic absorption of the pump laser. The measurements are performed on three different glass samples with different compositions and cooling is observed in all samples to varying degrees. The results highlight the possibility of using Yb-doped silica glass for radiation-balancing in fibers. Radiation-Balancing is a viable technique for heat mitigation in lasers and amplifiers.
Abstract: The use of temperature-sensitive fluorescence up-conversion in Ho doped crystals for non-contact Differential Luminescence Thermometry (DLT) is investigated. The up-conversion fluorescence spectra of Ho3+-doped YLF crystals, subject to pumping within the 5I8 - 5I7 manifold (1899-2066 nm), are measured versus the temperature in a cryostat. Considerable changes in the measured photoluminescence spectra, and in the red, yellow and green upconversion spectra is observed as the temperature is reduced from 300 K to 80 K
The latest efforts in performing high-power mid-IR optical refrigeration in Tm-doped crystals are reported. A Thuliumdoped fiber amplifier (TDFA) seeded by a mid-IR Continuous Wave Optical Parametric Oscillator (CW-OPO) and diodepumped at 793 nm is developed to obtain a high-power source in the mid-IR for laser cooling and RBL experiments. Using the TDFA, experiments are underway for implementing a Herriott cell geometry for laser cooling to low temperatures, as well as demonstrating the first mid-IR RBL in Tm and Ho-doped crystals.
Using vibronic transitions in rare-earth doped crystals as a case-study, we present detailed analysis for the optimum operation of radiation-balanced lasers. In particular, conditions for achieving highest output power and highest optical-to-optical efficiency in Yb:YAG and Yb:YLF thin disc RBLs are given. Finally, we extend our analysis to two-tone RBL systems involving Ho-Tm doped crystals.
Detailed characterization of Tm- and Ho-doped crystals is performed to identify optimum operation parameters for reaching cryogenic temperatures. The energy-gap scaling law states that these materials can deliver double the cooling efficiency compared with the Yb-doped systems. Here, we show our recent measurements of external quantum efficiency 𝜂ext and background absorption 𝛼b in Ho- and Tm-doped YLF and BYF crystals. Together with temperature-dependent spectroscopy, these data are then used to determine the minimum achievable temperature and the optimum cooling wavelengths for each crystal. Finally, the potential of these crystals for implementing mid-IR radiation balanced lasers is discussed.
Optical refrigeration of rare-earth doped crystals has exceptional qualities that can be used for building a compact and vibration-free all-solid-state optical cooler. Estimating the lowest achievable temperature and cooling power of such a device requires accurate measurements of external quantum efficiency, mean fluorescence wavelength, and parasitic absorption. Here we discuss temperature dependent measurements of these parameters for a high quality Yb:YLF sample by performing a LITMoS test (Laser Induced Temperature Modulation Spectrum) combined with contact-free differential luminescence thermometry. These measurements are challenging at low temperatures, but by integrating these two methods, we can perform LITMoS test at any temperature.
Laser cooling in Tm:YLF and Tm:BYF crystals has recently been reported. We investigate high power laser cooling of Tm doped crystals under high vacuum using multiple-pass Herriott cell configuration. We also model potential mid-IR Radiation Balanced Lasers (RBLs) in available Tm:YLF and Tm:BYF crystals. Our experiments and modelling shows that our 1% wt. Tm:BYF sample is a promising 2 µm RBL candidate, since it has high gain and high external quantum efficiency as well as good room temperature cooling efficiency. We will attempt to demonstrate the first mid-IR RBL experimentally in Tm:BYF crystal as well.
We investigate high power laser cooling in Tm:YLF crystals both in ambient pressure and high vacuum. For this purpose, we have constructed a high power CW OPO broadly tunable from 1755 nm to 2000 nm. By using this tunable source, laser cooling for (3 mm3) 1% doped Tm:YLF crystal was observed from 1801 nm to 2000 nm. Cooling efficiency of the sample, external quantum efficiency (EQE), background absorption and optimum laser cooling wavelength are extracted from laser induced temperature modulation spectrum (LITMoS) test on the cooling sample. To improve cooling performance, we have designed multiple pass non-resonant cavities to maximize the absorption of the laser light inside the sample. We setup multiple pass cavities in a high vacuum chamber to reduce convective heat load and enhance laser cooling results.
We report the first observation of laser cooling in 1% doped Tm:YLF by 0.5 K and in 0.8% doped Ho:YLF crystals by
0.1 K starting from room temperature in air. To achieve this, we designed and constructed a high power, broadly tunable
(1735 nm-2086 nm) continuous wave singly-resonant optical parametric oscillator. (OPO). The cooling experiments were
performed at ambient pressure, and temperature changes were measured using a thermal camera.
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