Theoretical Research Of The Amplified Spontaneous Emission Effect In Yb:YAG Thin Disk Laser

The thin disk laser is a kind of new diode pump solid state laser,the unique construction of the gain medium can effectively cooling the heat generated during the laser operation,which allows the realization of high output lasers with good beam quality.For the developing of the ultra-high power laser system,besides the thermal problems,the Amplifier Spontaneous Emission(ASE)effect becomes one of the main reasons to cause upper laser level loss especially in large size,high gain thin disk laser.In this paper,a model combining ASE effect and non-uniform temperature distribution in the thin disk crystal is presented.Considering the gain distribution influence and frequency integral Lorentzian lineshape at all the fluorescence branching,the space distribution of ASE effect and the spectrum properties of ASE effect of the thin disk laser are analyzed.On the other hand,Yb:YAG crystal has lots of advantages which make it become an ideal laser medium for thin disk laser.The ytterbium-doped materials have a quasi-threelevel character that makes them intrinsically more sensitive to the temperature.So it is important to analyze the processes of heat production during the laser operation.A model about quasi-three level Yb:YAG thin disk laser is established to evaluate the fractional thermal load by fluorescence heating fraction,laser heating fraction,the ASE heating fraction and non-radiative relaxation heating fraction.Combining the rate equations and the ASE model,a complete model about the dynamic behavior of the thin disk laser is presented.Because the parameters are strongly coupled with each other,the iterative technique is used to solve the dynamic behavior of quasi-three level Yb:YAG thin disk laser.The model shows that the temperature distribution in the thin disk crystal will seriously affect the space distribution of gain coefficient and there are different amplified length at different locations of the disk laser.As a result,there is a three-dimensional ASE effect distribution in the disk laser.The reflectivity of coupling mirror and the size of the pump region are the main factors to influence ASE effect,and ultimately affect the output power and threshold of laser system.Furthermore the optimum coupler reflectivity of the disk laser under different condition can be obtained by the model.In order to decrease the ASE effect,the coupler reflectivity can be adjusted to control the gain coefficient to increase output power.Theoretical analysis shows the ASE effect is an important reason to affect the total heat generation and operation properties of thin disk laser,which cannot be neglected for high power thin disk laser especially under non-lasing condition.