PASSIVELY MODE-LOCKED Nd-GLASS LASER WITH PARTIALLY SUPPRESSED NATURAL MODE SELECTION

The spectral narrowing of emission in the linear region of passively mode-locked lasers is thought to increase the final pulse duration obtained in the nonlinear mode-locking region. Properly adjusted Fabry— Perot interferometers were applied for partial suppression of natural mode-selection in mode-locked ruby [ 1,2] Nd-glass [3,4], and Nd—YAG lasers [5]. Pulse shortening factors between 1.5 and 3 have been achieved. In this letter the natural mode selection of a modelocked Nd—glass laser is partially suppressed by inserting a 0.1 mm thick uncoated fused silica etalon into the laser oscillator. A large tilting angle around 43° is used to adjust the etalon transmission to the gain curve of the active medium. Fine angle tuning allows to position the transmission minimum of the etalon to the gain maximum of the active medium in order to flatten the net gain distribution. The spectra at the end of the linear phase are broadened by a factor of 4 to 5. The final pulse durations are shortened by factors between 1.5 and 2. The pulse shortening is limited by reduced suppression of spikes within the opening time of the modelocking dye and by pulse broadening due to two-photon absorption in the active medium at elevated intensities. The experimental findings are compared with theoretical calculations of the temporal pulse development in the mode-locking region. Good agreement between theory and experiment is found.