Optimal performances of a mode-locking technique: Theoretical and experimental investigations of the frequency-doubling nonlinear mirror

We present a detailed analysis of the mode-locker consisting of a nonlinear crystal and a dichroic mirror, introduced by Stankov and known as the frequency-doubling nonlinear mirror (FDNLM). Our aim is to determine optimal values for two physical parameters: the conversion efficiency η₀ of the nonlinear crystal, and the reflection coefficient R_ω of the output mirror. The optimization is based on a set of three figures of merit: the reflected intensity, the pulse shortening ratio and the Gaussian shape factor, which are combined to yield a final decision factor. Experimental investigations of η₀ and R_ω carried out using a FDNLM mode-locked Nd:YAG laser show good agreement with the theoretical predictions. In addition, a comparative study with other available experimental results is presented. This work demonstrates the capacity of this method to evaluate the best performance of this mode-locking not only for the steady-state pulse domain but also in the transient one.