Impact of site-selective spectroscopy on laser cooling parameter characterization

Thomas J., Meyneng T., Tehranchi A., Gregoire N., Monet F., Seletskiy D., Messaddeq Y., Kashyap R.

AbstractWe demonstrate laser induced cooling in ytterbium doped silica (SiO2) glass with alumina, yttria co-doping (GAYY-Aluminum: Yttrium: Ytterbium Glass) fabricated using the modified chemical vapour deposition (MCVD) technique. A maximum temperature reduction by − 0.9 K from room temperature (296 K) at atmospheric pressure was achieved using only 6.5 W of 1029 nm laser radiation. The developed fabrication process allows us to incorporate ytterbium at concentration of 4 × 1026 ions/m3 which is the highest value reported for laser cooling without clustering or lifetime shortening, as well as to reach a very low background absorptive loss of 10 dB/km. The numerical simulation of temperature change versus pump power well agrees with the observation and predicts, for the same conditions, a temperature reduction of 4 K from room temperature in a vacuum. This novel silica glass has a high potential for a vast number of applications in laser cooling such as radiation-balanced amplifiers and high-power lasers including fiber lasers.

Topper B., Neumann A., Albrecht A., Flores A., Kuhn S., Haessner D., Hein S., Hupel C., Nold J., Haarlammert N., Schreiber T., Sheik-Bahae M., Mafi A.

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.

Wilke S.K., Benmore C.J., Menon V., Ilavsky J., Rezikyan A., Youngman R.E., Carson M.P., Weber R.

Topper B., Neumann A., Albrecht A.R., Flores A.S., Kuhn S., Häßner D., Hein S., Hupel C., Nold J., Haarlammert N., Schreiber T., Sheik-Bahae M., Mafi A.

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.

Vigneron P., Meehan B., Cahoon M.A., Hawkins T.W., Ballato J., Dragic P.D., Engholm M., Boilard T., Bernier M., Digonnet M.J.

The first observation of cooling by anti-Stokes pumping in nanoparticle-doped silica fibers is reported. Four Yb-doped fibers fabricated using conventional modified chemical vapor deposition (MCVD) techniques were evaluated, namely, an aluminosilicate fiber and three fibers in which the Yb ions were encapsulated in CaF2, SrF2, or BaF2 nanoparticles. The nanoparticles, which oxidize during preform processing, provide a modified chemical environment for the Yb3+ ions that is beneficial to cooling. When pumped at the near-optimum cooling wavelength of 1040 nm at atmospheric pressure, the fibers experienced a maximum measured temperature drop of 20.5 mK (aluminosilicate fiber), 26.2 mK (CaF2 fiber), and 16.7 mK (SrF2 fiber). The BaF2 fiber did not cool but warmed slightly. The three fibers that cooled had a cooling efficiency comparable to that of the best previously reported Yb-doped silica fiber that cooled. Data analysis shows that this efficiency is explained by the fibers’ high critical quenching concentration and low residual absorptive loss (linked to sub-ppm OH contamination). This study demonstrates the large untapped potential of nanoparticle doping in the current search for silicate compositions that produce optimum anti-Stokes cooling.

Vicente R., Cittadino G., Di Lieto A., Tonelli M., Gardelein A., Nogues G.

We report on the optical cooling of a 7.5% Y b : L i Y F 4 crystal down to 125 K in a multi-pass Herriott absorption cell, coupled via a single mode polarization maintaining optical fiber to the laser source. This configuration, never exploited before, is more practical for potential applications, in particular for spaceborne cryogenics setups. Moreover, the temperature reached is exactly the one needed in many setup embarked in small and medium satellites. We evaluate the heat load on the crystal at the minimum attainable temperature, which is comparable to state of the art systems.

Ballato J., Dragic P.D.

Optical fiber-based lasers and amplifiers are ubiquitous tools across many practical applications including communications, metrology, sensing, manufacturing, machining, and directed energy. Even for the most efficient cases where only a few percent of the optical power is converted into heat, this heat generation can have wide-ranging detrimental effects on overall system performance. In homage to the 2022 International Year of Glass, this paper provides a new look into the enabling roles that glass can play in the fully passive thermal management of active fiber lasers and amplifiers. Partially didactic, complemented with new insights and novel approaches and results, specific topics include means by which glass compositions can reduce the amount of heat generated by an active dopant and how the glass’ response to heat generation, for example, thermo-optic and thermal expansion coefficients, can be minimized or otherwise used to largely mitigate parasitic thermal effects.

Topper B., Peysokhan M., Albrecht A.R., Flores A.S., Kuhn S., Hässner D., Hein S., Hupel C., Nold J., Haarlammert N., Schreiber T., Sheik-Bahae M., Mafi A.

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.

Knall J.M., Engholm M., Boilard T., Bernier M., Digonnet M.J.

We report what we believe to be the first radiation-balanced fiber amplifier - a device that provides optical gain while experiencing no temperature rise. The gain medium is a silica fiber with a 21 um-diameter core highly doped with Yb3+ (2.52 wt.%) and co doped with 2.00 wt.% Al to reduce concentration quenching. The amplifier was core-pumped with 1040 nm light to create anti-Stokes fluorescence (ASF) cooling and gain in the core at 1064 nm. Using a custom slow-light FBG sensor with mK resolution, temperature measurements were performed at multiple locations along the amplifier fiber. A 4.35-m fiber pumped with 2.62 W produced 17 dB of gain while the average fiber temperature remained slightly below room temperature. This advancement is a fundamental step toward the creation of ultra-stable lasers necessary to many applications, especially low-noise sensing and high-precision metrology.

Knall J., Engholm M., Boilard T., Bernier M., Vigneron P.-., Yu N., Dragic P.D., Ballato J., Digonnet M.J.

In optically pumped lasers, heat generated by the quantum defect causes detrimental fluctuations in the output mode, frequency, and power. Common heat-mitigation techniques use bulky mechanical coolers that introduce vibrations, leading to laser frequency and amplitude noise. Here, we present a radiation-balanced fiber laser, optically cooled by anti-Stokes fluorescence (ASF). The gain medium is a silica fiber with a 21-µm-diameter core doped with 2.06 wt. % ${{\rm Yb}^{3 +}}$ and co-doped with ${{\rm Al}_2}{{\rm O}_3}$ and F- to reduce concentration quenching. The laser was core-pumped at 1040 nm to create both gain at 1065 nm and ASF cooling at atmospheric pressure. We demonstrate a maximum output power of 114 mW with a slope efficiency of 41% while maintaining near-zero average temperature change. This result could enable the development of fiber lasers with unprecedented coherence and stability.

Knall J.M., Digonnet M.J.

A model of laser cooling in a fiber with a doped cladding shows that a radiation-balanced fiber laser (RBFL) made of silica can produce substantial output powers. Bidirectional pumping is found to reduce the average temperature of the laser, enabling higher output powers while maintaining radiation-balanced operation. For a large-mode-area silica fiber doped with Yb in the core and cladding, simulations predict that output powers as large as 115 W can be achieved by bidirectionally pumping the doped cladding. This is not only slightly higher than with a Yb-doped ZBLAN fiber with the same dimensions, but the temperature gradient in silica is also about half as large. Since silica is the most common host in fiber lasers, these predictions are very promising for the near-term realization of practical RBFLs.

Püschel S., Kalusniak S., Kränkel C., Tanaka H.

We revisit the spectroscopic characterization of ytterbium-doped LiYF4 (Yb:YLF) for the application of laser cooling. Time-dependent fluorescence spectroscopy reveals a temperature dependence of the radiative lifetime which we explain by the Boltzmann distribution of excited ions in the upper Stark levels. The emission cross sections of Yb:YLF from 17 K to 440 K are revised using the temperature-dependent radiative lifetimes from fluorescence spectra. We provide fit equations for the peak values of important transitions as a function of temperature, which is also useful for the design of Yb:YLF laser oscillators and amplifiers operated at cryogenic temperatures. Based on our spectroscopic data, we show the prerequisite crystal purity to achieve laser cooling below liquid nitrogen temperatures.

Peysokhan M., Rostami S., Mobini E., Albrecht A.R., Kuhn S., Hein S., Hupel C., Nold J., Haarlammert N., Schreiber T., Eberhardt R., Flores A., Tünnermann A., Sheik-Bahae M., Mafi A.

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.

Peysokhan M., Mobini E., Mafi A.

A detailed formalism to achieve an analytical solution of a lossy high power Yb-doped silica fiber laser is introduced. The solutions for the lossless fiber laser is initially attained in detail. Next, the solution for the lossy fiber laser is obtained based on the lossless fiber laser solution. To examine the solutions for both the lossless and lossy fiber laser, two sets of values are compared with the exact numerical solutions, and the results are in a good agreement. Furthermore, steps and procedures for achieving the final solutions are explained clearly and precisely.

Knall J., Vigneron P., Engholm M., Dragic P.D., Yu N., Ballato J., Bernier M., Digonnet M.J.

For the first time, to the best of our knowledge, laser cooling is reported in a silica optical fiber. The fiber has a 21-µm diameter core doped with 2.06 wt.% ${{\rm Yb}^{3 + }}$Yb3+ and co-doped with ${{\rm Al}_2}{{\rm O}_3}$Al2O3 and ${{\rm F}^ - }$F− to increase the critical quenching concentration by a factor of 16 over the largest reported values for the Yb-doped silica. Using a custom slow-light fiber Bragg grating sensor, temperature changes up to $ - {50}\;{\rm mK}$−50mK were measured with 0.33 W/m of absorbed pump power per unit length at 1040 nm. The measured dependencies of the temperature change on the pump power and the pump wavelength are in excellent agreement with predictions from an existing model, and they reflect the fiber’s groundbreaking quality for the radiation-balanced fiber lasers.

Topper B., Tolliver J., Kuhn S., Haessner D., Hein S., Hupel C., Nold J., Haarlammert N., Mafi A., Neumann A., Schreiber T.

A thorough investigation of the spectroscopic properties of ytterbium-doped silica as a function of temperature (77-420 K) is carried out. Whitelight absorption and fluorescence collected under 915 nm excitation are used to calculate the temperature-dependent laser cross-sections. These datasets are made publicly available in this work. Factors influencing the acquisition and interpretation of Yb-doped glass spectroscopic data are discussed, including spectrum fitting ambiguities, site-selective excitation, lifetime decay versus spectral integration, vibronic features, and the validity of the McCumber theory over the studied temperature range. Site-selectivity affects the measurement of the emission lineshape at standard pump wavelengths of 915, 940, and 976 nm at room temperature. Lifetime measurements under 915 nm excitation vary by up to 10%, depending on the choice of bandpass or long pass filter employed and hence the spectral region integrated over. The McCumber transform yields reasonable agreement with measured spectra over the range of ∼200-420 K and then diverges rapidly at lower temperatures. Considering the measured cross-section data in the range applicable to contemporary fiber laser system operation, between 300-420 K, the cross-sections for some spectral regions change by more than 10%, including the absorption cross-section at 977 nm and the emission cross-section at 1030 nm. Some regions are essentially unchanged over the same temperature range, such as absorption at 940 nm and emission at 1045 nm. The provided data will be useful for future modeling and simulation efforts to consider the temperature-dependence of relevant quantities including, but not limited to, lifetime, cross-section, gain, and intensity saturation.

Topper B., Neumann A., Wilke S.K., Alrubkhi A., Mafi A., Weber R.

AbstractYtterbium‐doped lanthanum titanate glasses were prepared by levitation melting for the detailed characterization of the spectroscopic properties in the rare‐earth titanate glass host. Low‐temperature fluorescence spectroscopy reveals distinct site‐selectivity in both static and lifetime fluorescence measurements suggesting an absence of clustering as well as significant variation of local ytterbium environments. Typical site‐selectivity behavior of a shrinking Stark manifold with lower excitation energy is observed. At 77 K, both the mean emission frequency and the fluorescence lifetime initially increase as the excitation energy decreases from about 11100 to 10750 and then slightly decrease at lower excitation energy. Temperature‐dependent lifetime measurements between 77 and 420 K show a decreasing lifetime with increasing temperature and are well described by a two‐level thermal activation model. The temperature‐dependent fluorescence spectroscopy coupled with a room temperature white light absorption measurement allow the determination of the Stark energy levels of in lanthanum titanate glass as well as the calculation of the laser cross‐sections.

Topper B., Neumann A., Kuhn S., Hässner D., Hein S., Hupel C., Nold J., Haarlammert N., Mafi A., Schreiber T.

Topper B., Kuhn S., Neumann A., Albrecht A., Flores A., Haessner D., Hein S., Hupel C., Nold J., Haarlammert N., Schreiber T., Mafi A., Sheik-Bahae M.

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.