HIGH-EFFICIENCY BLUE GENERATION BY FREQUENCY-DOUBLING OF FEMTOSECOND PULSES IN A THICK NONLINEAR CRYSTAL

We have demonstrated highly efficient frequency doubling of femtosecon d pulses in a thick, noncritically phase-matched KNbO3 crystal under c onditions of large group-velocity mismatch. At low power we observed a slope efficiency of similar to 300% nJ(-1) for harmonic conversion, a nd at higher powers we generated 170 mW of second-harmonic blue output for 300 mW of input light. Furthermore, we have shown that the focusi ng dependence for our conditions of large group-velocity mismatch is c onsiderably different from that obtained for frequency doubling of con tinuous-wave light. We have also demonstrated that one can tune the sp ectral width of the generated blue light by varying the focusing condi tions. (C) 1998 Optical Society of America.