Changhong Zhu

Modeling of end-pumped Yb:YAG thin-disk lasers with nonuniform temperature distribution

A plane wave model with nonuniform temperature distribution in the thin-disk crystal is developed to describe the dynamic behavior of an end-pumped Yb:YAG thin-disk laser. A set of couple-rate equations and 2D stationary heat-conduction equations are derived. The stable temperature distribution in the disk crystal is calculated using a numerical iterative method. The analytic expression is capable of dealing with more practical laser systems than previous works on this subject as it allows for nonuniform temperature distribution in the disk crystal. Based on these results, we examined laser output intensity as a function of pump intensity, dopant concentration, resonator coupler reflectivity, crystal thickness and temperature of cooling liquid.

Analytical model of thermal effect and optical path difference in end-pumped Yb:YAG thin disk laser

An analytical model of the thermal effect and optical path difference (OPD) of a thin disk laser is developed with the combination of the analytical method and commercial finite element analysis software. The distributions of temperature, stress, strain, and OPD caused by temperature gradient, axial thermal strain (bulging), thermal strain-induced birefringence, and deformation are obtained. Based on the analytical model, the production mechanisms, features, and influence of OPD in an end-pumped thin disk laser are discussed, which make the causes of spherical and aspherical parts of the OPD more obvious. Furthermore, the OPD including the spherical and aspherical parts of the thin disk crystal is discussed for various pumping intensities.

Numerical analysis of an end-pumped Yb:YAG thin disk laser with variation of a fractional thermal load

An analytical model is developed to describe the dynamic behavior of an end-pumped Yb:YAG thin disk laser. Within the model, the rate equations, including the nonradiative relaxation process, are calculated taking into account the dependence of the fractional thermal load on the temperature of the thin disk crystal and intracavity laser intensity. The fractional thermal load is analyzed, or can be evaluated clearly, under lasing or nonlasing conditions. The stable temperature and fractional thermal load in a thin disk crystal for different radiative quantum efficiencies are obtained using the numerical iterative method. Furthermore, the dependence of the laser output intensity on variables such as pumping intensity, coupler reflectivity, radiative quantum efficiency, and the temperature of thin disk crystal is discussed.

Analytical approach of laser beam propagation in the hollow polygonal light pipe

An analytical method of researching the light distribution properties on the output end of a hollow n-sided polygonal light pipe and a light source with a Gaussian distribution is developed. The mirror transformation matrices and a special algorithm of removing void virtual images are created to acquire the location and direction vector of each effective virtual image on the entrance plane. The analytical method is demonstrated by Monte Carlo ray tracing. At the same time, four typical cases are discussed. The analytical results indicate that the uniformity of light distribution varies with the structural and optical parameters of the hollow n-sided polygonal light pipe and light source with a Gaussian distribution. The analytical approach will be useful to design and choose the hollow n-sided polygonal light pipe, especially for high-power laser beam homogenization techniques.

Analytical model of optical path difference in an end-pumped Yb:YAG thin-disk laser with nonuniform pumping light

An analytical model of the thermal effect and optical path difference (OPD) of a thin-disk laser is developed by an arbitrary form pumping spot. Based on the analytical model, the distribution of temperature, stress, and strain can be derived using a super-Gaussian form pumping spot. The total OPD caused by temperature gradient, axial thermal strain, thermal strain-induced birefringence, and deformation of the thin-disk crystal is discussed for different super-Gaussian factors and is separated into spherical and aspherical parts. The analytical results show that the aspherical part of the OPD is the main reason leading to the decrease of the laser beam quality and it is closely related to super-Gaussian factors, which are very useful for thin-disk laser design and evaluation.

Optical model and optimal output coupler for a continuous wave Yb:YAG thin-disk laser with multiple-disk configuration

This article presents the fundamental principles of operational performance of a continuous wave (cw) thin-disk laser with multiple disks in one resonator. Based on the model of an end-pumped Yb:YAG thin-disk laser with nonuniform temperature distribution, the effect of the multiple disks in one resonator is considered. The analytic expressions are derived to analyze the laser output intensity, laser intensity in the resonator, threshold intensity, and the optical efficiency of a thin-disk laser with multiple disks arranged in series. The dependence of output coupler reflectivity and the number of thin disks on various parameters are investigated, which are useful to determine the optimal output coupler reflectivity of the thin-disk lasers and control the laser intensity in the resonator.

Deicing with Nd:YAG and CO 2 lasers

A model of deicing with Nd:YAG and CO2 lasers for simulation using ANSYS software is presented. Experiments with a 300-W, 1-ms, 60-Hz Nd:YAG laser and a 500- to 2000-W cw CO2 laser are reported. The Nd:YAG laser is considered as a volume thermal source, and the CO2 laser as a plane thermal source. The model and the simulation results can describe both Nd:YAG and CO2 laser deicing well. The results of the simulation and experiments suggest that the melting rates for the two lasersarealmostequalatthesamelaserpowerdensity.Soarethemelting efficiencies. The hard and transparent ice irradiated by the Nd:YAG laser becomes opaque and loose, because the thermal stress is distributed in the body of the ice, while the ice irradiated by the CO2 laser is still transparent and hard, because thermal stress hardly occurs. So the laser with characteristics of high output power and large ice absorbing length can be selected for the power line laser deicing system, and Nd:YAG laser is more appropriate for power-line deicing than CO2 laser. C 2010 Society of

Influence of beam spatial distribution on the laser damage of optical material

Based on the theoretical model of uniform pulsed laser beam irradiating on the optical material, the stress distribution of the transparent material when laser with different beam spatial distribution - circle beam spot and dark-cross beam spot - irradiated to the inner of optical material was discussed. The experiment that 532nm Q-switched laser pulse was focused to the inner of silicate glass was taken, which the beam spatial distribution of the output laser was dark-cross beam spot. According to the theory, the experimental results and phenomena were analyzed and the damage micro-configurations were explained. The theory and experiment could be accordant well.

UV-laser-assisted processing of ceramics in air and in water

Liquid water acts as an important role in laser processing. Water can be added on purpose gain better result: to avoid redeposition of debris, to cool the material, to increase plasma pressure or to conduct light. On the other hand water is the most common, cheap and safe medium and has an exceptionally high heat capacity. So many researches have been carried out in water-assisted laser processing. In this article, UV laser (wavelength: 355nm, pulse width: 10ns THG Nd:YAG laser) assisted processing of ceramics in air and in water is studied and compared with the processing quality in different environment.

Practical applications of femtosecond laser micromachining and fabrication

Femtosecond laser is suitable to machine a variety of materials, such as metals, semiconductors, polymers, oxide ceramics, silica aerogels, optical glasses, crystals, deep sea sands and even explosives because of its high peak power density and low heat affected zone. In this paper, the femtosecond laser micromachining of different materials and for different processing is presented, including structuring in optical glasses, and the cutting of metals and the deep-sea (South China Sea) sands. The laser used in the experiment is a commercial Ti:Sapphire laser with the pulse width of 50 and 100 fs, wavelength of 800 nm, maximum pulse energy up to 2 mJ and the repetition rate of 1 kHz. The evolution of material eruption as a function of the number of laser pulses and intensity is studied. The dependence of ablation rate with laser intensity and the number of the pulses is characterized by measuring the maximum laser penetration depth in different materials.