Active-passive mode-licking of a pulsed Nd:YAG laser, which was designed and constructed in the laboratory, has been achieved by using an acousto-optic modulator and saturable absorbers such as BDN/iodoethane, Kodak dyes #26 and #9860/1, 2-dichloroethane solutions to improve the shot-to-shot stability and reproducibility of the mode-locked pulses.
The active-passive mode-locked laser was found to display a sharp lasing threshold and a high success rate of mode-locking(1 or 2 miss during 200 laser shots) and the shot-to-shot stability considerably improved compared to purely passive mode-locking, where the cavity length should be adjusted within ±0.1mm to obtain best results.
The effect of the saturable absorber position has been investigated for active- passive mode-locking in a Fabry-Perot cavity configuration. The saturable absorber placed at the center of the laser cavity was found to produce more stable mode-locked pulses without satellite pulses, which were very frequently observed when attempting purely passive mode-locking, than a contacted dye cell configuration. This improvement was concluded to be attributed to the complete suppression of the satellite pulses by the maximum loss of the acousto-optic modulator.
The mode-locked pulse width was measured by an autocorrelator using a well-known second harmonic generation (SHG) method, where a collinear type I $KH_2PO_4$ crystal was utilized for SHG, and found to be 23ps. The shot-to shot energy stabilities of the mode-locked pulse and its frequency-doubled output was within ±1.9% and ±2.7%, respectively.
Also, the displacement of the saturable absorber position from the exact center of the laser cavity was found not to be critical. When the saturable absorber was moved ±3cm from the center of the cavity, the pulse widths remained unchanged within the standard deviation of the second harmonic signals, which was in the range of ±3~4%.
