| The solid state laser with high average power and high beam quality has very important applications in the fields of military and industry,and now it is one of the hotspots in the field of laser technology.When the solid state laser is operating at high average power output,the waste heat of the laser medium will cause the thermal effects,such as the thermal lens,the thermal stress,the thermally induced birefringence and the depolarization,and so on,which will lead to the beam quality degraded,the output power limited,and even the solid state laser medium damaged.The thermal loading will be decreased effectively by using the laser diode pumping and the laser medium with low quantum defect,then the output power of the solid state laser will be enhanced evidently.The heat of the solid state laser mainly comes from the quantum defect of the laser medium,for example,the quantum defect of the Nd:YAG is about 24%with the pumping wavelength of 808nm and the laser wavelength of 1064nm,plus the non-radiative transition,the thermal loading ratio of the Nd:YAG laser is about 32%when the Nd:YAG laser is operating,so it is very important to select the solid laser medium with low quantum defect to decrease the thermal loading of the solid state laser.At the present stage the solid state laser mediums suited to high average power output include the Nd:YAG and the Yb:YAG mainly,but the quantum defect of the Yb:YAG is much lower than the Nd:YAG.The Yb:YAG is a quasi-three-level system at room temperature,so it is necessary to achieve high power and high efficiency laser output through the high intensity pumping and the high brightness laser injecting.The pumping intensity of the Yb:YAG laser is several times that of the Nd:YAG laser,and then the higher requirements to the pumping power of the LDA are proposed.The damage resistance performance of the Yb:YAG slab and the laser propagation chain,the cooling capability of the slab cooler will face greater challenges with high intensity pumping and high brightness laser injecting.Based on the basis of our group on the Nd:YAG slab MOPA technology for many years,this paper selects the Yb:YAG slab gain module as the study object and analyzes the key technologies of the high average power Yb:YAG slab gain module at room temperature,then explores the feasibility that the single Yb:YAG slab gain module achieves high average power over lOkW laser output at room temperature through theoretical and experimental research.The main contents of this paper are as follows:First,the design method on the multi concentration segmented doped slab with end pumping is established according to the absorption law of the slab with end pumping to the pumping light,and then the double concentration doped Yb:YAG slab with high energy storage density is designed.According to the energy level structure of the Yb:YAG and the quasi-three-level rate equation,the laser dynamics research on the Yb:YAG is developed,the mathematical expressions of main physical quantities such as the absorption coefficient of the pumping light,the laser gain coefficient and the laser extracting efficiency are derived,then the influences of the pumping intensity and the laser intensity to the laser extracting efficiency of the Yb:YAG are analyzed.The numerical simulation on the output laser intensity and the optical-to-optical efficiency of the Yb:YAG slab gain module under the coupling status of the pumping light and the laser are developed,then the influences of the pumping intensity,the laser intensity,the operating temperature and the propagating loss of the gain medium to the output laser intensity and the optical-to-optical efficiency of the Yb:YAG slab gain module are studied separately,then the design parameters for the optical-to-optical efficiency of the Yb:YAG slab gain module over 45%at room temperature is given in theory.Second,according to the energy flow of slab gain module,the mathematical expression of the optical-to-optical efficiency is derived,which shows the influences of all kinds of factors to the optical-to-optical efficiency of the slab gain module visually.The research on the asymmetrical end pump coupling technology is developed,which proves that the pump coupling efficiency can be enhanced evidently and the residual pumping light can be isolated effectively through the asymmetrical end pump coupling technology in theory,and then the service life of the LDA is extended.The theoretical research on the Yb:YAG slab gain module with the zero-phonon-line pumping is also developed,which proves that the optical-to-optical efficiency of the Yb:YAG slab gain module with the zero-phonon-line pumping is essentially the same as that of the Yb:YAG slab gain module with the 941nm pumping under the same thermal loading state,but the effective output power of the Yb:YAG slab gain module can be enhanced nearly 30%because the pumping intensity of the Yb:YAG slab gain module with the zero-phonon-line pumping can be enhanced over 30%.Third,the differences on the output characteristics between the LDA with spatial overlay and the LDA with polarized overlay are analyzed and compared,then the pump coupling system which can achieve the high pumping intensity and the high pumping uniformity is designed,and the uniformity of the fluorescence distribution at the end of the Yb:YAG slab is measured as 95%in the experiment,which fully proves the pumping uniformity of the pump coupling system with waveguide homogenization and beam expansion imaging in the slow axis direction,and then the Yb:YAG slab gain modules used in the preamplifier and the main amplifier are developed.The technology researches on suppressing the ASE and the PO in the Yb:YAG slab gain module are developed,then the technical method of bonding the Cr4+:YAG cladding on the sides of the Yb:YAG slab to suppress the ASE and the PO is selected,and the temperature gradient at the edge of the Yb:YAG slab may be decreased meanwhile,the output capability of the Yb:YAG slab gain module can be enhanced in theory.Fourth,the laser output experiments on the Yb:YAG slab gain modules are developed at room temperature,the output powers and the optical-to-optical efficiencies of the Yb:YAG slab gain modules used in the preamplifier and the main amplifier are measured respectively in the experiments,and the effective output power of the Yb:YAG slab gain module used in the main amplifier reaches 11.3kW.This is the highest power continuous wave laser output reported in the YbLYAG slab gain module at room temperature for the moment,which verifies the feasibility that the single Yb:YAG slab gain module achieves high average power over 1 OkW continuous wave laser output at room temperature fully.Fifth,the numerical simulations are developed according to the input parameters of the experiment,and the numerical simulations agree well with the experiment results,then the correctness of the theoretical model is proved,and then the foundation for further developing the optimal design,enhancing the output laser power,the ratio of power to volume and the optical-to-optical efficiency of the Yb:YAG slab gain module is established.The Yb:YAG slab gain modules have been applied in the Yb:YAG slab MOPA laser experiment,when the 1030nm fiber seed source propagates through one Yb:YAG slab gain module used in the preamplifier and one Yb:YAG slab gain modules used in the main amplifier,the output power of the Yb:YAG slab MOPA laser is 11.9kW,the continuous output time reaches 110s and the beam quality βis about 2.8 times diffraction limited after the wavefront correction at room temperature.On above basis,the output power of the Yb:YAG slab MOPA laser increases to 22.3kW by adding one Yb:YAG slab gain module used in the main amplifier,and the beam quality β is about 3.3 times diffraction limited after the wavefront correction.It is the highest power continuous wave laser obtained in the Yb:YAG slab laser at room temperature.The main technological progress points and the innovation points of this paper are as follows:First,the design method on the multi concentration segmented doped slab with end pumping is established,the average energy storage density of the double concentration doped slab designed in this paper is 30%higher than the conventional single concentration doped slab,the pumping uniformity of the pump coupling system with waveguide homogenization and beam expansion imaging in the slow axis direction reaches 95%.Second,the laser output experiments on the Yb:YAG slab gain modules are developed at room temperature,and the effective output power of the Yb:YAG slab gain module used in the main amplifier reaches 11.3kW,which is the highest power continuous wave laser output reported in the Yb:YAG slab gain module at room temperature for the moment.Third,the technical proposals on the asymmetrical end-pumping and the zero-phonon-line pumping to the Yb:YAG slab gain module are proposed in this paper,which establish the foundation for further enhancing the output power of the Yb:YAG slab gain module. |
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