Fractional Thermal Loading Research Articles

Modeling of energy-transfer upconversion and thermal effects in end-pumped quasi-three-level lasers.

An analytical model of cw quasi-three-level lasers that includes the influence of energy-transfer upconversion (ETU) has been developed. The results of the general output modeling were applied to a laser with Gaussian beams, and rigorous numerical calculations have been made to study the influence of ETU on threshold, output power, spatial distribution of population-inversion density, and fractional thermal loading. The model was applied to a laser operating at 946 nm in Nd:YAG, where the dependence of laser-beam size on laser performance was investigated in particular. A simple model for the degradation of laser-beam quality from a transversally varying saturated gain is proposed that is in good agreement with measurements of the laser in a plane-plane cavity.

Thermal considerations on the design and operation of lens ducts

In this paper, heat effects due to powerful pump beam in the lens duct delivery system is investigated and subsequent thermal effects are described. Temperature raise in the device, thermal loading, and material considerations for the lens ducts fabrication are reported. For an absorbed intensity of 10 W/cm 2 in a 4 cm -long lens duct, the temperature difference between the midpoint and its faces, for a lens duct made from borosilicate is 7.05°C, for sapphire 4.01°C, and for synthetic fused silica is the smallest value of 0.54°C. For a typical absorbed intensity of 1.4 W/cm 2 in a 4 cm -long lens duct, with a steady-state time constant of 120 s , the fractional thermal loading is 1.98% for BK-7, 1.89% for sapphire, and only 0.16% for synthetic fused silica. By considering different glass materials, the overall results show that synthetic fused silica is superior to some other materials for the fabrication of a lens duct. The reported analytical method and the results of parametric studies can be helpful in material selection and construction of an effective lens duct.

Thermal lensing measurements and thermal conductivity of Yb:YAB

By using holographic shear interferometry, we have measured the thermal lens in a diode-pumped Yb:YAB crystal, under lasing and non-lasing conditions, as a function of absorbed power. From these purely optical measurements we estimate the fractional thermal loading and the thermal conductivity of Yb:YAB. Knowledge of these thermal parameters is essential for the optimisation of Yb:YAB laser cavity designs.

Self-quenching of Nd3+ emission in laser garnet crystals

Abstract The emission self-quenching mechanism in Nd 3+ :YAG at low pump intensities is identified as a cross-relaxation on intermediate levels inside the system of active ions. Up to 1.5 at.% Nd 3+ this down-conversion process is a direct donor–acceptor process, without important contribution from migration and is induced by a multiple interaction picture consisting of superexchange and dipole–dipole contributions. The parameters measured from non-selectively excited global decay under 532 nm excitation at room temperature are verified by this consistent explanation of the characteristics of decay of the first-, second- and third-order Nd 3+ ion pairs that induce resolved spectral satellites at low temperatures. A further check is provided by the agreement of the experimental data on emission quantum efficiency or on fractional thermal load with those calculated by using the characteristic parameters of the energy transfer determined in this work. It is thus shown that self-quenching of emission is a limiting factor for the quantum efficiency of Nd 3+ :YAG. The extension of the theoretical formalism based on a discrete, random and equiprobable occupation of crystallographic sites by Nd 3+ ions, used in the analysis of down-conversion to the case when up-conversion is also present is discussed.

Laser demonstration of Yb/sub 3/Al/sub 5/O/sub 12/ (YbAG) and materials properties of highly doped Yb:YAG

We have demonstrated the first stoichiometric Yb/sup 3+/ laser based on Yb/sub 3/A/sub 5/O/sub 12/ (YbAG). The laser operated in pulsed mode with a highest possible duty cycle of 85%. A slope efficiency of 27%, with respect to absorbed energy, was measured and the free-running lasing wavelength was 1048 nm for a 10% duty cycle. In a systematic analysis, measurements of spectroscopic and materials properties of (Yb/sub x/Y/sub 1-x/)/sub 3/Al/sub 5/O/sub 12/ for nominal x values of 0.05, 0.1, 0.15, 0.18, 0.25, 0.5, and 1 are reported. We also present a formalism to calculate the intrinsic fluorescence quantum efficiency (free of radiation trapping) and the fraction of reabsorbed light, based on measurements of the bulk and intrinsic emission lifetimes and the fractional thermal loading. Our best YbAG sample has an intrinsic lifetime of 0.664 ms at 94% quantum efficiency and a thermal conductivity at room temperature of 0.072 W/(cm-K).

Pump-to-mode size ratio dependence of thermal loading in diode-end-pumped solid-state lasers

The fractional thermal loading in a diode-end-pumped solid-state laser is deduced from experimental measurement of the critical pump power. The results show that fractional thermal loading critically depends on the pump-to-mode size ratio. This dependence is attributed to the effect of the pump-to-mode size ratio on the population inversion and to the effect of energy-transfer upconversion. Rigorous modeling calculations have been performed to yield a comprehensive understanding of the thermal loading in diode-end-pumped solid-state lasers.

Repetition-rate dependence of thermal loading in diode-end-pumped Q-switched lasers: influence of energy-transfer upconversion

The fractional thermal loading in diode-end-pumped Q-switched lasers was estimated from the theory of cavity stability. The experimental results show that the thermal loading depends on the pulse-repetition rate. This dependence is attributed to the effect of energy-transfer upconversion (ETU). A theoretical model including the ETU effect was developed to analyze the fractional thermal loading in end-pumped Q-switched lasers. The theoretical predictions were found to be in good agreement with the experimental data.

Emission dynamics of the4F3/2level ofNd3+in YAG at low pump intensities

The room-temperature high-resolution luminescence decay investigation of YAG crystals doped with ${\mathrm{Nd}}^{3+}$ in concentrations up to 1.5 at.% shows that under nonselective weak 532-nm pump, which prevents the up conversion, the emission dynamics of ${}^{4}{F}_{3/2}$ level is influenced by a cross relaxation inside the system of ${\mathrm{Nd}}^{3+}$ ions, dominated by superexchange ${(R}_{0}=5.43\AA{},$ $L=0.54\AA{})$ for the nearest-neighbor ${\mathrm{Nd}}^{3+}$ ion pair and by dipolar coupling ${(C}_{\mathrm{DA}}=1.85\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}40}{\mathrm{cm}}^{6}{\mathrm{s}}^{\ensuremath{-}1})$ for all other pairs. These parameters are in strong contradiction with other recent reports. The energy-transfer parameters are verified in the quasiselectively excited decay of the first-, second-, and third-order ${\mathrm{Nd}}^{3+}$ ion pairs by using a proper adaptation of the theory of energy transfer for discrete, equiprobable, and random placement of ${\mathrm{Nd}}^{3+}$ ions in the crystalline lattice. They give also a very good description of the data on quantum efficiency and fractional thermal load for ${\mathrm{Y}\mathrm{A}\mathrm{G}:\mathrm{N}\mathrm{d}}^{3+}$ samples of various concentrations. The absence of the quadrupolar interaction in the mechanism of transfer is consistent with the restrictions imposed by the selection rules for the transitions involved in cross relaxation.

Experimental determination of fractional thermal loading in an operating diode-pumped Nd:YVO_4 minilaser at 1064 nm

A practical in situ method is described and used for determination of the fractional thermal-loading parameter eta(h) in an operating diode-pumped Nd:YVO(4) minilaser at 1064 nm. Readily applicable to the thermal characterization of other solid-state media, the method is based on the fact that thermally induced lensing will cause the laser oscillation to be quenched at a critical pump power whose magnitude depends on the cavity configuration, thermo-optical properties of the gain medium, and, in particular, on the value of eta(h). In the experiments described here, a 0.5-mm-long coated Nd:YVO(4) crystal with 3-at. % Nd concentration was used to construct the diode-pumped laser with a flat highly reflecting end mirror and an intracavity lens. For the method to be effective, the resonator was set up close to the edge of the stability range. Above the oscillation threshold, the pump power at which lasing was quenched because of the onset of the thermally induced resonator instability was measured as a function of the intracavity lens position. A numerical model that accounted for absorption saturation and pump-induced thermal lensing was then used to analyze the experimentally measured data with eta(h) as an adjustable parameter. The average best-fit value of eta(h) was determined to be 0.40 with an estimated statistical variation of 8%.

Design criteria for concentration optimization in scaling diode end-pumped lasers to high powers: influence of thermal fracture

A systematic investigation of a series of Nd:YVO/sub 4/ crystals with different dopant concentrations is conducted to scale the performance of diode-end-pumped lasers to higher powers. From the theoretical analysis, the fracture-limited pump power is expressed as a function of the thermal shock parameter, fractional thermal loading, and the absorption coefficient. The thermal shock parameter of Nd:YVO/sub 4/ crystals is determined from the laser experiments. Using the thermal shock parameter and the space-rate-equation model, we calculate the maximum output power in Nd:WO/sub 4/ crystals at the optimum pump condition as a function of the dopant concentration. The theoretical calculations are in good agreement with the experimental results. The influence of lowering the absorption coefficient on the TEM/sub 00/ output efficiency is also studied through the overlapping integral.

Determination of the thermal loading of diode-pumped Nd:YVO_4 by use of thermally induced second-harmonic output depolarization

A new method of using thermally induced second-harmonic output depolarization to determine the fractional thermal loading of a diode-pumped Nd:YVO(4) laser is proposed. The experimental results show that the fraction of the green output power polarized along the extraordinary axis of a KTP crystal, f(e) , is an oscillatory function of the absorbed pump power. The fractional thermal loading can be determined by measurement of the difference in the absorbed pump power between the peaks of f(e) .

Emission quantum efficiency and heating effects in YAG:Nd<sup>3+</sup> lasers

The intrinsic limitations of emission quantum efficiency of Nd 31 in YAG that lead to heating of active element are analyzed. The main factors characteristic of the system of active ions that lead to heat are the quantum defect and the emission quenching due to energy trans- fer by cross-relaxation and multisite structure (especially for laser or di- ode pumping). It is shown that for concentrations of interest for lasers, the reduction of emission quantum efficiency by energy transfer is due to two types of interactions between active ions. The high-resolution data are used to explain the global behavior of metastable level emission kinetics and the concentration reduction of quantum efficiency. An ana- lytic expression for the fractional thermal load is obtained, imposing a lower limit at each concentration. The estimated values of emission quantum efficiency and fractional load are in agreement with reported measured values. © 1996 Society of Photo-Optical Instrumentation Engineers. Subject terms: laser crystals; YAG:Nd; emission quantum efficiency; heating effects.

Heat generation in Nd:YAG and Yb:YAG

The fractional thermal loading, the ratio of heat generated to absorbed energy, at a 6.5 at.% doped Yb:YAG crystal diode pumped at 0.943 mu m has been measured to be >