CONTINUOUS-WAVE MODE-LOCKED SOLID-STATE LASERS WITH ENHANCED SPATIAL HOLE-BURNING .1. EXPERIMENTS

We systematically investigate the difference between both actively and passively mode-locked lasers with Gain-at-the-End (GE) and Gain-in-th e-Middle (GM) at the example of Nd:YLF lasers. The GE laser generates pulse widths approximately three times shorter than a comparable GM ca vity. This is due to enhanced Spatial Hole Burning (SHE) which effecti vely flattens the saturated gain and allows for a larger lasing bandwi dth compared to a GM cavity. We first investigate enhanced SHE by meas uring the cw mode spectrum, where we have observed that the mode spaci ng in GE cavities depends primarily on the crystal length. This was al so confirmed for a Nd:LSB crystal, where the pump absorption length wa s significantly shorter than the crystal length. In mode-locked operat ion, pulse widths of 4 ps for passive mode locking and 5 ps for active mode locking are demonstrated with GE cavities, compared to lips for passive and 17 ps for active mode locking with GM cavities. Additional ly, the time-bandwidth product for the GE cavity is approximately twic e the ideal product for a sech(2) pulse shape and cannot be improved b y dispersion compensation alone, while the GM cavity has nearly ideal time-bandwidth-limited performance. The results for the GM cavity comp are well to existing theories taking into account the added effect of pump-power-dependent gain bandwidth which increases the bandwidth of N d:YLF from 360 to > 500 GHz. In a following paper [1] (called Part II) a rigorous theoretical treatment of the effects due to SHE will be pr esented.