We use the fiber-pumped MgO:PPLN crystal to realize the MIR CW-OPO operation, and observe the nonuniform temperature distribution on the central axis of the crystal. Then we use the heat transfer model in COMSOL software to simulate the temperature distribution in the crystal and find the near linear temperature gradient on the central axis of the crystal. Input the axial temperature distribution to our SRO model based on MATLAB and the simulation results show that the linear axial temperature gradient distribution will not only cause the center wavelength shift of the signal light, but also reduce the parametric gain of the signal light, the uniform temperature along the crystal axis will get the maximum gain. This feature limits the prospect of the single OPO in high-power narrow linewidth laser.
The spread of the pump model, established based on MATLAB, simulates the distribution of the pump in End-Pumped single crystal fiber. Simulation results show that the pump in the rod single crystal fiber will converge again. By changing the crystal absorption coefficient, it can be found that smaller the absorption coefficient is, more uniform the pump distribution is; when it is greater, the pump will concentrate to the pump end more seriously. Establish End- Pumped Experimental platform in the experiment, the crystal is 1 mm in diameter and length of 30 mm, Nd<sup>3+</sup> doping concentration is 1%. Change the position of the pump light's focus in the crystal, we can see different distribution of the pump light by different focus location in the crystal and find that the pump light has the most homogeneous distribution when the focus is on the crystal axis and has 1mm distance to the pump end face. At this time, the second convergence of the pump is clearly visible. By changing the pump wavelength, crystal absorption coefficient changes. It is found that under the same pump power, absorption coefficient is greater, the pump will concentrate to the pump end more seriously. And the temperature of crystal pump end rises, which is identical with the simulation results. The results indicate that for the single crystal fiber, the higher absorption coefficient is not better, low absorption coefficient leads to the uniform distribution of the pump, there will be a better absorption in a relatively long length of single crystal fiber. And due to the lower end face temperature, end pump power upper limit will also increase.