Feedback Control of Colliding-Pulse Mode-Locking Operations in a Nd:YAG Laser

The pulse width of a flashlamp pumped Nd:YAG laser in 120cm long linear cavity was significantly shortened by joint operation of CPM (Colliding Pulse Mode-locking) and FCM (Feedback Controlled Mode-locking). First, only CPM was operated with a saturable absorber cell of 0.5mm thickness set at one quarter of the cavity length apart from the end mirror. The average and shortest pulse widths were 15.8ps and 8.0ps in Gaussian shape (14.4ps and 7.3ps in sech2), respectively, while about 40 pulses of 2ns interval were emitted out within a one-shot pulse train of about 80ns duration. The average energy per pulse was about 40μJ. Next, in addition of CPM, FCM was done by a piece of 0.45mm thick GaAs wafer near the coupling mirror. The average and shortest pulse widths were 5.4ps (4.9ps) and 5.1ps (4.6ps), respectively, while the number of pulses (≈10μJ) increased by several times within the elongated train of about 500ns duration. Last, the pulse shortening mechanisms were also discussed by observing the down-chirping effect with a grating pair.