Laser cooling ytterbium doped silica by 67 K from ambient temperature.

Laser cooling of a 5 cm long, 1 mm diameter ytterbium doped (6.56×1025 ions/m3) silica rod by 67 K from room temperature was achieved. For the pump source, a 100 W level ytterbium fiber amplifier was constructed along with a 1032 nm fiber Bragg grating seed laser. Experiments were done in vacuum and monitored with the non-contact differential luminescence thermometry method. Direct measurements of the absorption spectrum as a function of temperature were made, to avoid any possible ambiguities from site-selectivity and deviations from McCumber theory at low temperature. This allowed direct computation of the cooling efficiency versus temperature at the pump wavelength, permitting an estimated heat lift of 1.42 W/m as the sample cooled from ambient temperature to an absolute temperature of 229 K.

Potential of ytterbium doped silica glass for solid-state optical refrigeration to below 200 K.

We report on the optical refrigeration of ytterbium doped silica glass by >40 K starting at room temperature, which represents more than a two-fold improvement over the previous state-of-the-art. A spectroscopic investigation of the steady-state and time-dependent fluorescence was carried out over the temperature range 80 K to 400 K. The calculated minimum achievable temperature for our Yb3+ doped silica sample is ≈150 K, implying the potential for utilizing ytterbium doped silica for solid-state optical refrigeration below temperatures commonly achieved by standard Peltier devices.

Impact of site-selective spectroscopy on laser cooling parameter characterization.

From laser design to optical refrigeration, experimentally measured fluorescence spectra are often utilized to obtain input parameters for predictive models. However, in materials that exhibit site-selectivity, the fluorescence spectra depend on the excitation wavelength employed to take the measurement. This work explores different conclusions that predictive models reach after inputting such varied spectra. Here, temperature-dependent site-selective spectroscopy is carried out on an ultra-pure Yb, Al co-doped silica rod fabricated by the modified chemical vapor deposition technique. The results are discussed in the context of characterizing ytterbium doped silica for optical refrigeration. Measurements made between 80 K and 280 K at several different excitation wavelengths yield unique values and temperature dependencies of the mean fluorescence wavelength. For the excitation wavelengths studied here, the variation in emission lineshapes ultimately lead to calculated minimum achievable temperatures (MAT) ranging between 151 K and 169 K, with theoretical optimal pumping wavelengths between 1030 nm and 1037 nm. Direct evaluation of the temperature dependence of the fluorescence spectra band area associated with radiative transitions out of the thermally populated 2F5/2 sublevel may be a better approach to identifying the MAT of a glass where site-selective behavior precludes unique conclusions.

Hybrid membrane-external-cavity surface-emitting laser.

We develop, analyze, and demonstrate an optically-pumped semiconductor disk laser using an active mirror architecture formed by sandwiching the semiconductor gain membrane between two heatspreaders, one of which is coated with a high-reflectivity multilayer. Thermal modeling indicates that this structure outperforms traditional VECSELs. Employing an InGaAs/GaAs MQW gain structure, we demonstrate output powers of approximately 30 W at a center wavelength of λ ≈ 1178 nm in a TEM00 mode using an in-well pumped geometry.

Optical refrigeration of payloads to T < 125†K.

A modified all-solid-state optical cryocooler prototype based on anti-Stokes fluorescence in a 10%-doped Yb:YLF crystal cooled a payload to temperatures below 125 K starting from room temperature. To achieve this record performance, the optical refrigerator employed a novel, to the best of our knowledge, textured-MgF2 thermal link to improve the thermal transport and fluorescence escape. Additionally, it used spectrally selective, high-reflection coatings in the pump circulator cavity to suppress parasitic lasing and amplified spontaneous emission.

Laser cooling experiments to measure the quantum efficiency of Yb-doped silica fibers.

A detailed investigation into the wavelength-dependent cooling efficiencies of two ultra-pure large core diameter ytterbium-doped silica fibers is carried out by means of the laser-induced thermal modulation spectroscopy (LITMoS) method. From these measurements, an external quantum efficiency of 0.99 is obtained for both fibers. Optimal cooling is seen for pump wavelengths between 1032 and 1035 nm. The crossover wavelength from heating to cooling is identified to be between 1018 and 1021 nm. The fiber with higher Yb3+ ion density exhibits better cooling, seen by the input power normalized temperature differential.

Open-aperture Z-scan study for absorption saturation: accurate measurement of saturation intensity in YLF:Yb for optical refrigeration.

Knowledge of saturation intensity of gain or absorption plays a fundamental role in a variety of applications ranging from lasers to many nonlinear optical processes. Here, we present an analytical expression for open-aperture Z-scan transmission for accurately measuring the saturation intensity in the low absorbance samples but at arbitrary pump intensities. We exploit this formalism to investigate the absorption saturation of LiYF4:Yb3+ (YLF:Yb) in the anti-Stokes excitation region for optical refrigeration at high pump intensities. An absorption saturation intensity of 14.5±1kW/cm2 was measured in YLF:Yb at 1020 nm (E||c) at room temperature.

Laser cooling of a Yb doped silica fiber by 18 Kelvin from room temperature.

An ytterbium doped silica optical fiber with a core diameter of 900µm has been cooled by 18.4 K below ambient temperature by pumping with 20 W of 1035 nm light in vacuum. In air, cooling by 3.6 K below ambient was observed with the same 20 W pump. The temperatures were measured with a thermal imaging camera and differential luminescence thermometry. The cooling efficiency is calculated to be 1.2±0.1%. The core of the fiber was codoped with Al3+ for an Al to Yb ratio of 6:1, to allow for a larger Yb concentration and enhanced laser cooling.

Radiation-balanced Yb:YAG disk laser.

A radiation-balanced Yb:YAG disk laser is demonstrated in an intracavity pumping geometry. Detailed analysis of the data reveals the feasibility of using the multi-kilowatt level "athermal" disk lasers with minimal modal instabilities, which arise from thermal lensing.

Optical refrigeration: the role of parasitic absorption at cryogenic temperatures.

Optical cooling of a YLF:Yb single crystal to 87 K, well below the minimum achievable temperature predicted from existing theory, has been observed. This discrepancy between theory and data has motivated us to revisit the current model of optical refrigeration, in particular the critical role of parasitic background absorption. Challenging experiments that measured the cooling efficiency as a function of temperature reveal that the background absorption coefficient decreases with temperature, resulting in a significant enhancement of the cooling efficiency at cryogenic temperatures. These discoveries emphasize the high sensitivity of optical cooling to impurity-mediated processes and show the necessity of formulating a cooling model that includes the temperature dependence of the background absorption. To properly characterize the cooling properties of any sample, it is necessary to measure its low-temperature performance.

Tm-doped crystals for mid-IR optical cryocoolers and radiation balanced lasers.

We report the complete characterization of various cooling-grade Tm-doped crystals including, to the best of our knowledge, the first demonstration of optical refrigeration in Tm:YLF crystals. Room temperature laser cooling efficiencies of 1% and 2% (mol) Tm:YLF and 1% Tm:BYF crystals at different excitation polarizations are measured, and their external quantum efficiency and background absorption are extracted. By performing detailed low-temperature spectroscopic analysis of the samples, global minimum achievable temperatures of 160 to 110 K are estimated. The potential of Tm-doped crystals to realize mid-IR optical cryocoolers and radiation balanced lasers (RBLs) in the eye-safe region of the spectrum is discussed, and a promising two-tone RBL in a tandem structure of Tm:YLF and Ho:YLF crystals is proposed.

Optically pumped DBR-free semiconductor disk lasers.

We report high power distributed Bragg reflector (DBR)-free semiconductor disk lasers. With active regions lifted off and bonded to various transparent heatspreaders, the high thermal impedance and narrow bandwidth of DBRs are mitigated. For a strained InGaAs multi-quantum-well sample bonded to a single-crystalline chemical-vapor deposited diamond, a maximum CW output power of 2.5 W and a record 78 nm tuning range centered at λ≈1160 nm was achieved. Laser operation using a total internal reflection geometry is also demonstrated. Furthermore, analysis for power scaling, based on thermal management, is presented.

Intra-cavity cryogenic optical refrigeration using high power vertical external-cavity surface-emitting lasers (VECSELs).

A 7% Yb:YLF crystal is laser cooled to 131 ± 1 K from room temperature by placing it inside the external cavity of a high power InGaAs/GaAs VECSEL operating at 1020 nm with 0.15 nm linewidth. This is the lowest temperature achieved in the intracavity geometry to date and presents major progress towards realizing an all-solid-state compact optical cryocooler.

Exploring ultrafast negative Kerr effect for mode-locking vertical external-cavity surface-emitting lasers.

We present analytical considerations of "self-mode-locked" operation in a typical vertical external-cavity surface-emitting laser (VECSEL) cavity geometry by means of Kerr lens action in the semiconductor gain chip. We predict Kerr-lens mode-locked operation for both soft- and hard-apertures placed at the optimal intra-cavity positions. These predictions are experimentally verified in a Kerr-lens mode-locked VECSEL capable of producing pulse durations of below 500 fs at 1 GHz repetition rate.

Implementation of Laser-Induced Anti-Stokes Fluorescence Power Cooling of Ytterbium-Doped Silica Glass.

Laser cooling of a solid is achieved when a coherent laser illuminates the material, and the heat is extracted by annihilation of phonons resulting in anti-Stokes fluorescence. Over the past year, net solid-state laser cooling was successfully demonstrated for the first time in Yb-doped silica glass in both bulk samples and fibers. Here, we report more than 6 K of cooling below the ambient temperature, which is the lowest temperature achieved in solid-state laser cooling of silica glass to date to the best of our knowledge. We present details on the experiment performed using a 20 W laser operating at a 1035 nm wavelength and temperature measurements using both a thermal camera and the differential luminescence thermometry technique.

First demonstration of an all-solid-state optical cryocooler.

Solid-state optical refrigeration uses anti-Stokes fluorescence to cool macroscopic objects to cryogenic temperatures without vibrations. Crystals such as Yb3+-doped YLiF4 (YLF:Yb) have previously been laser-cooled to 91 K. In this study, we show for the first time laser cooling of a payload connected to a cooling crystal. A YLF:Yb crystal was placed inside a Herriott cell and pumped with a 1020-nm laser (47 W) to cool a HgCdTe sensor that is part of a working Fourier Transform Infrared (FTIR) spectrometer to 135 K. This first demonstration of an all-solid-state optical cryocooler was enabled by careful control of the various desired and undesired heat flows. Fluorescence heating of the payload was minimized by using a single-kink YLF thermal link between the YLF:Yb cooling crystal and the copper coldfinger that held the HgCdTe sensor. The adhesive-free bond between YLF and YLF:Yb showed excellent thermal reliability. This laser-cooled assembly was then supported by silica aerogel cylinders inside a vacuum clamshell to minimize undesired conductive and radiative heat loads from the warm surroundings. Our structure can serve as a baseline for future optical cryocooler devices.

Solid-state optical refrigeration to sub-100 Kelvin regime.

Since the first demonstration of net cooling twenty years ago, optical refrigeration of solids has progressed to outperform all other solid-state cooling processes. It has become the first and only solid-state refrigerator capable of reaching cryogenic temperatures, and now the first solid-state cooling below 100 K. Such substantial progress required a multi-disciplinary approach of pump laser absorption enhancement, material characterization and purification, and thermal management. Here we present the culmination of two decades of progress, the record cooling to ≈ 91 K from room temperature.