An Acousto-optic mode-locking system is designed for active mode-locking of an iodine photodissociation laser. The acousto-optic modulator consistes of a BK-7 optical glass block (2cm x 2cm x 1cm) and an X-cut quartz transducer (20MHz). The acoustic resonance in the BK-7 block is obtained every 287±1 kHz and the full width at half maximum of an acoustic resonance is found to be 16 kHz. The peak-to-background ratio (PBR) of the acousto-optically mode-locked pulse is 85% for RF power of 5 W. Also, the passive mode-locking is performed by using BDN II saturable absorber, and it is succeeded only under the condition of pressure broadened iodine laser line. The passive mode-locking gives 75% of PBR and has inferior stability.
The combined method of active-passive mode-locking is useful for the pressure broadened iodine laser line and gives PBR of 91%. The stability, reproducibility, and reliability are as good as the case when the active mode-locking is alone employed. The minimal RF power required for 100% of PBR active-passive mode-locking is dependent on the homogeneous line broadening of the iodine laser line and the saturable dye, and it is reduced to 7 W from 23 W required in sole active mode-locking without Ar buffer gas.
