Design criteria in high power diode-pumped, mode-locked laser with nonlinear mirror

The nonlinear mirror combines a dichroic mirror with a nonlinear crystal. A dichroic mirror totally reflects the second-harmonic (SH) wavelength and output coupler of fundamental wavelength (FW). If the SH wavelength experiences the appropriate phase shift with respect to the FW between the two passes in the nonlinear crystal, the reflectivity of FW increases and the system losses decreases. It is so called nonlinear mode locking. We will discuss the influence of cw mode locking with respect to nonlinear crystal lengths and the reflectivity of the dichroic output couplers. Based on the thorough experimental result, the optimum values of nonlinear crystal length and reflectivity of output coupler were found to 9 mm long and 74%. We established a theoretical model to calculate the threshold of output power for CML by means of the rate equation, the nonlinear reflection coefficient R_NL, power conversion efficiency η and KTP length. Theoretical prediction and tendency were sustained by experiments results.