Research On Fluorescence Cooling Of Solid And Radiation Cooling Of Yb-doped Laser Crystals

Some materials emitted light at shorter wavelengths than that with which the material was illuminated due to thermal (phonon) interactions with the excited atoms. This process is termed anti-Stokes fluorescence cooling.This thesis consists of two parts: the first part focuses on the mechanism and experiments of solid fluorescence cooling, and the second parts focuses on the radiation cooling of Yb-doped laser mediums. The main contents are as follows:â… . Principles and experiment of fluorescence cooling of Yb-doped mediumThe mechanism of fluorescence cooling is discussed from the points of view of spectroscopy and entropy, and the expression and thermodynamics limitation of cooling efficiency are achieved.Fluorescence cooling capabilities of Yb-doped medium are analyzed and pump wavelength are optimized. The absorption and emission spectrums of Yb³⁺:KGW crystal, Yb³⁺:KLuW crystal, and Yb-doped phosphate glass are measured, and the absorption and emission cross sections are calculated with reciprocity methods and F-L equation. The results show that both crystals' absorption cross sections are about ten times over those of the Yb-doped phosphate glass. By use of the absorption and emission spectrums, the pump wavelengths are optimized with the optimization function of absolute cooling efficiency. The results show that the optimal pump wavelength for Yb-doped phosphate glass is near 1025nm (cooling efficiency is 0.18%), and the optimal pump wavelentgh for both crystal is near 1022nm, and the polarizaiton is E//m (cooling efficiency is 1.68%).The influences of fluorescence reaborption are calculated and analyzed. Because of a spectral overlap between absorption and fluorescence, reaborption-reemission events cause a redshift in the observed fluorescence spectrums, so as to decrease the cooling efficiency. The effects on fluorescence cooling of fluorescence reaborption are analyzed with Monte Carlo method. The side and back emission spectrums of the samples with 1mm and 2mm thickness (diameter 10mm) are measured with Fluorolog Tan 3-21 fluorescence spectrometer. The fluorescence shape calculated by Monte Carlo method is agreed with the measured data.Validation experiments of fluorescence cooling of Yb-doped crystals are carried out. The thermal radiation spectrums of 1～2mm thickness samples are measured by use of the step-scan technology of Fourier transform spectrometer with the pump source of tunable OPO laser whose pulse duration is about 10 ns. The fluorescence cooling of the Yb³⁺:KGW and Yb³⁺:KLuW crystals are observed. The time resolved process of the solid fluorescence cooling is acquired, which is very difficult to get by the reportorial methods in literature (such as photothermal deflection method, fluorescence method, and etc) because of their principle limitations.â…¡. Radiation cooling of Yb-doped laser mediumThe processes of excitation and stimulated emission always results in heat generation within the lasing medium. This produces increased temperatures and stresses in the lasing medium which limit beam quality and average power. The fluorescence cooling removes heat from the medium directly, which can be used in solid-state laser cooling.A new approach to the design of Q-switched solid-state lasers is proposed which offsets heating loads by anti-Stokes fluorescence. In this ideal system, the pump will cool the gain media when the Q switch is off and removes the thermal loads that generated by the laser pulse when the Q switch is on. The operation of this model with Yb³⁺ :KGW crystal is simulated and the parameters and requirements is educed.Principles and cell technology of radiation-balanced lasers are researched. The relations among pump intensity, laser intensity, gain medium parameters and resonance cavity parameters are derived, and the output performances are discussed for radiation-balanced laser.The output laser wavelength and polarization are derived by use of rate equations. Pump wavelength, laser wavelength and polarization optimized based on small signal gain coefficiency and optics - optics efficiency function. The results show that the optimal pump and laser wavelenthes are 1001nm and 1039 nm, and the optimal pump and laser polarizations are E//m and E//p(b) for Yb³⁺:KGW crystal radiation-balanced laser.The LD bar-hollow duct-gain medium coupling calcalation model is proposed for the requirements of radiation-balanced laser. The 3D pump intensity is calculated and the initial gain distribution is achieved in the gain medium. The duct parameters are optimized based on the transmission efficiency.The forming process of self-reproductive mode is calculated with combining the rate equations and angular spectrum propagation theory. Thespatio-temporal dynamics of the mode formation are analyzed, and output laser characteristics are achieved for the Yb³⁺:KGW gain medium.The mechanism and application in Yb-doped laser mediums cooling by anti-Stokes fluorescence in this thesis have references to low-heat solid-state lasers.