Dynamics of cw intra-cavity second harmonic generation by PPKTP

We report on quasi-phase matched intra-cavity doubling of a cw 1064 nm Nd:YVO₄ laser. In a laser pumped by a 14 W fiber coupled laser diode array, and containing a periodically poled KTP (PPKTP) doubling crystal we obtain more than 2.3 W of 532 nm radiation. The second harmonic generation (SHG) is stable for short and long times with less then 5% intensity noise (RMS). SHG in PPKTP was measured for both extra-cavity and intra-cavity configurations and it was found that intra-cavity SHG vs. temperature allows for a wider temperature range, as compared with the extra-cavity SHG. In addition, the intra-cavity SHG curve shows an abrupt decrease in second harmonic intensity and hysteresis as a function of the crystal temperature. This observation can be explained by self-suppression of mode hopping that exists in intra-cavity SHG systems. The broadening and hysteresis are due to the laser locking in on a particular wavelength at the fundamental although the loss is increased. Because of self-suppression mode hopping to a neighborhood longitudinal mode does not occur until the optimum phase matching wavelength is detuned by several tens of nm from the peak of the gain curve. When the jump in lasing wavelength finally occurs the detuning is much larger than the longitudinal mode spacing, so that the generated green power is very low corresponding to the abrupt decrease in SHG.