Radiation-balanced Lasers Research Articles

Anti-Stokes cooling in highly ytterbium doped phase separated aluminium-yttrium oxide glass by 4 K

Silica glass is a strong candidate in the development of radiation balanced fiber lasers and amplifiers due to its properties such as high stability, high mechanical robustness and chemical durability, thermal resistance, high purity and so on. However, low rare-earth ion solubility lead to inter-ionic energy transfers processes which cause multi-phonon relaxation through a non-radiative decay process and hinder it from being used for aforementioned applications as well as for optical cooling. In the present work, we report cooling to a record low temperature of −4 K from room temperature at atmospheric pressure in a novel silica based highly Yb doped phase separated oxide glass (named as PS: GAYY glass - aluminium: yttrium: ytterbium) using 20 W of 1037 nm pump wavelength. We further theoretically show a global minimum achievable temperature of ∼165 K in our glass. Here, the basic limitation for laser induced cooling in silica glass associated with the existence of clustering of rare earth ions is significantly mitigated. This was achieved by the addition of suitable co-dopants such as aluminum and yttrium which modified the local rare-earth ion environment to avoid clustering as well as reducing the phonon energy. The fabricated glass using modified chemical vapor deposition process in combination with a solution doping technique achieved a maximum doping concentration of 5.57 × 1026 ions/m3 and exhibits better performance than other silica-based host glasses in terms of both optical properties and laser cooling characteristics. We believe PS-GAYY glasses may open new avenues in the field of solid-state laser cooling, promising exciting and broad applications in radiation balanced lasers and amplifiers.

Radiation-balanced lasing in Yb3+:YAG and Yb3+:KYW.

Radiation-balanced lasing and thermal profiling is reported in two Yb-doped laser crystals at room temperature. In 3% Yb3+:YAG a record efficiency of 30.5% was achieved by frequency-locking the laser cavity to the input light. Both the average excursion and axial temperature gradient of the gain medium were maintained within 0.1 K of room temperature at the radiation balance point. By including saturation of background impurity absorption in the analysis, quantitative agreement was obtained between theory and the experimentally measured laser threshold, radiation balance condition, output wavelength, and laser efficiency with only one free parameter. Radiation-balanced lasing was also achieved in 2% Yb3+:KYW with an efficiency of 2.2% despite high background impurity absorption, losses from Brewster end faces that were not parallel, and non-optimal output coupling. Our results confirm that relatively impure gain media can be operated as radiation-balanced lasers, contrary to earlier predictions which ignored background impurity properties.

Laser cooling of Yb3+:LuLiF4 crystal below cryogenic temperature to 121 K

Optical cooling techniques of solid-state refrigerators, especially those toward the cryogenic temperature range, have attracted considerable attention in the fields of space exploration, precise measurement, material sciences, and so forth. Here, we report the laser cooling of the 7.5% Yb3+-doped LuLiF4 crystal down to 121 K reaching NIST's designated range of cryogenic temperatures (<123 K). Further results based on the cooling window indicate a promising cooling limit of 59 K, provided with enhancement in pump absorbance and heat load management of the sample. Our work, therefore, can motivate an all-solid-state optical refrigeration application beyond the liquid nitrogen boiling point, thus bringing great opportunity to realize cryogenic coolers and radiation-balanced lasers in miniaturized systems.

Radiation-Balanced Lasers: History, Status, Potential

The review of history and progress on radiation-balanced (athermal) lasers is presented with a special focus on rare earth (RE)-doped lasers. In the majority of lasers, heat generated inside the laser medium is an unavoidable product of the lasing process. Radiation-balanced lasers can provide lasing without detrimental heating of laser medium. This new approach to the design of optically pumped RE-doped solid-state lasers is provided by balancing the spontaneous and stimulated emission within the laser medium. It is based on the principle of anti-Stokes fluorescence cooling of RE-doped low-phonon solids. The theoretical description of the operation of radiation-balanced lasers based on the set of coupled rate equations is presented and discussed. It is shown that, for athermal operation, the value of the pump wavelength of the laser must exceed the value of the mean fluorescence wavelength of the RE laser active ions doped in the laser medium. The improved purity of host crystals and better control of the transverse intensity profile will result in improved performance of the radiation-balanced laser. Recent experimental achievements in the development of radiation-balanced RE-doped bulk lasers, fibre lasers, disk lasers, and microlasers are reviewed and discussed.

Optical refrigeration of the Yb3+-doped YAG crystal close to the thermoelectric cooling limit

The Yb3+:YAG crystal has been one of the most widely used active media in the solid-state lasers of high power, mainly thanks to its excellent thermal, mechanical, and optical properties. Thermal effect due to heat deposition in the active medium, however, greatly deteriorates the beam quality of the laser output and sets a limit on its maximum power available. Although the cooling proposal of anti-Stokes fluorescence can help realize the heat-free high-power lasers with good beam quality, so-called radiation-balanced lasers, there is no substantial advancement in the optical cooling of Yb3+:YAG crystals since its latest experimental report with a temperature drop of about 9 K. Here we demonstrate experimentally a remarkable temperature drop of about 80 K in a 3% Yb3+-doped YAG single crystal pumped by a fiber laser at 1030 nm. Further analysis predicts that the cooling limit of the titled crystal can reach as low as 180 K from the room temperature. Our work therefore reveals a key pathway to facilitate the optical refrigeration of the Yb3+:YAG crystal down to the thermoelectric cooling limit, thus offering a unique entry point to practical radiation-balanced lasers.

Temperature distribution inside a double-cladding optical fiber laser or amplifier

The temperature distribution inside a double-cladding optical fiber laser or amplifier is examined in detail. Traditionally, the quantum defect in the core is taken to be the main source of heating in an active optical fiber. However, contributions from the parasitic absorption of the signal and the pump may also play an important role, especially for low quantum defect or radiation-balanced lasers and amplifiers. The contributions to the heating in both the core and the inner-cladding are considered and analyzed in general terms in this paper. In particular, it is shown that if the maximum tolerable surface temperature of the fiber relative to the ambient is taken to be 300°C to avoid damaging the fiber’s outer polymer cladding, the core temperature rises only in the range of 0°C–5°C relative to the inner-cladding for an air-cooled fiber. However, for a water-cooled fiber, the core temperature can be higher than the inner-cladding by as much as 50°C, potentially changing a single-mode core to multimode due to the thermo-optic effect.

Characterization of Yb-doped ZBLAN fiber as a platform for radiation-balanced lasers

Recent advances in power scaling of fiber lasers are hindered by the thermal issues, which deteriorate the beam quality. Anti-Stokes fluorescence cooling has been suggested as a viable method to balance the heat generated by the quantum defect and background absorption. Such radiation-balanced configurations rely on the availability of cooling-grade rare-earth-doped gain materials. Herein, we perform a series of tests on an ytterbium-doped ZrF 4 – BaF 2 – LaF 3 – AlF 3 – NaF (ZBLAN) optical fiber to extract its laser-cooling-related parameters and show that it is a viable laser-cooling medium for radiation balancing. In particular, a detailed laser-induced modulation spectrum test is performed to highlight the transition of this fiber to the cooling regime as a function of the pump laser wavelength. Numerical simulations support the feasibility of a radiation-balanced laser, but they highlight that practical radiation-balanced designs are more demanding on the fiber material properties, especially on the background absorption, than solid-state laser-cooling experiments.

Optimum Operation of Radiation-Balanced Lasers

The operation of radiation-balanced lasers (RBLs) is analyzed for practical considerations of cavity losses, optimum out-coupling, and radial distribution of pump and laser modes. We derive expressions for laser efficiency under arbitrary heat loads, discuss the limitations on high-power scaling imposed by thermal gradients in radiation-balanced disk geometries, and propose potential remedies. The effects of parasitic absorption of the pump and fluorescence are evaluated. The analysis is intended primarily for disk lasers but is also applicable to rod laser geometries.

Demonstration of anti-Stokes cooling in Yb-doped ZBLAN fibers at atmospheric pressure.

For the first time, to the best of our knowledge, optical cooling is demonstrated in a fiber at atmospheric pressure. Using a specialized slow-light fiber Bragg grating temperature sensor, -5.2 mK and -0.65 K were measured in a single-mode (1% YbF3) and multimode (3% YbF3) ZBLAN fiber with respective cooling efficiencies of 2.2% and 0.90%. Fitting a recently reported quantitative model of optical cooling in fibers to the measured temperature change dependence on the pump power per unit length validates the model and allows us to infer the fibers' absorptive loss and quenching lifetime, key parameters that are scarce in literature. These values are necessary for accurate cooling predictions and will aid in the development of fibers for application in optical coolers and radiation-balanced lasers.

Observation of optical refrigeration in a holmium-doped crystal

We report the first demonstration of solid-state optical refrigeration of a Ho-doped material. A 1 mol% Ho-doped Yttrium Lithium Fluoride (YLF) crystal is cooled by mid-IR laser radiation, and its external quantum efficiency and parasitic background absorption are evaluated. Using detailed temperature-dependent spectroscopic analysis the minimum achievable temperature of a 1% Ho:YLF sample is estimated. Owing to its narrower ground- and excited-state manifolds, larger absorption cross-section, and coincidence of optimum cooling wavelength of 2070 nm with commercially available high-power and highly efficient Tm-fiber lasers, Ho^3+-doped crystals are superior to Tm^3+-doped systems for mid-IR optical refrigeration. With further improvement in material purity and increased doping concentration, they offer great potential towards enhancing the cooling efficiency nearly two-fold over the best current Yb:YLF systems, achieving lower temperatures as well as for realization of eye-safe mid-IR high-power radiation balanced lasers.

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 110K 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.

High-resolution slow-light fiber Bragg grating temperature sensor with phase-sensitive detection

This Letter reports a slow-light fiber Bragg grating (FBG) temperature sensor with a record temperature resolution of ∼0.3 m°C/√Hz, a drift of only ∼1 m°C over the typical duration of a measurement (∼30 s), and negligible self-heating. This sensor is particularly useful for applications requiring the detection of very small temperature changes, such as radiation-balanced lasers and the measurement of small absorptive losses using calorimetry. The sensor performance is demonstrated by measuring the heat generated in a pumped Yb-doped fiber. The sensor is also used to measure the slow-light FBG's very weak internal absorption loss (0.02 m-1), which is found to be only ∼2% of the total loss.

Special Section Guest Editorial: Optical Refrigeration and Radiation-Balanced Lasers

Richard I. EpsteinThermoDynamic Films, LLC1313 Madrid RoadSanta Fe, New Mexico 87505E-mail: richard.epstein@gmail.comMansoor Sheik-BahaeUniversity of New MexicoDepartment of Physics and AstronomyAlbuquerque, New Mexico 87131-0001E-mail: msb@unm.eduThe laser cooling of solids by anti-Stokes emission has beenadvancing on many fronts. Work on the laser cooling of rareearth–doped solids has expanded to include materials com-prised of ytterbium, thulium, holmium, and erbium dopantsin a variety of crystal and glass hosts. The growth techniquesand quality of these cooling materials have greatly improved.With these improvements, laser cooling may soon approachliquid nitrogen temperatures. Rare earth–based cryogenicoptical refrigerators may find applications in cooling infraredcameras,low-noiseelectronics,high-puritygermaniumgammaray spectrometers, and high-temperature superconductors.Laser cooling in semiconductor devices has also been pur-sued intensively, and net cooling in CdS nanostructures wasrecently reported. In parallel, “athermal” or radiation-balancedlasers, which exploit anti-Stokes fluorescence to balance theheatgeneratedbyquantumdefectinrareearth–dopedlasers,may enable the development of extremely high-power laserswith excellent beam quality.Severalpapersinthisspecialsectionexplorethegrowthofrare-earth doped crystal and the use of these crystals in lasercooling. Cittadino et al. describe the Czochralski technique asdeveloped at the University of Pisa for growing large, defect-free cooling crystals. They explain the growth parameter andthe processes that are necessary to avoid contaminationinside crystals like OH− ion and how to avoid reduction ofYb3þ to Yb2þ. Volpi et al. and Zhong et al. report on the opti-cal cooling of LiLuF

Phonon-Assisted Anti-Stokes Lasing in ZnTe Nanoribbons.

Phonon-assisted anti-Stokes emission and its stimulated emission in polar semiconductor ZnTe are demonstrated via the annihilation of phonons as a result of strong exciton-phonon coupling. The findings are not only important for developing high-power radiation-balanced lasers, but are also promising for manufacturing ultraefficient solid-state laser coolers.

Laser induced cooling of solids

AbstractLaser induced cooling of solids, also known as optical refrigeration, is different to laser cooling of atoms, and was originally proposed in 1929 by Pringsheim. It was not achieved experimentally until 1995 at the Los Alamos National Laboratory by Epstein's group. Since that time, several proposal for cooling have been made and demonstrated, pushing the achievable coldness towards cryogenic temperatures. In addition, radiation balanced lasers, in which heat is mitigated by laser cooling has also been implemented. This powerful technique enables the scaling of optical powers in lasers, has no moving parts and the electrical connections are entirely remote to the laser. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)