The output characteristics of a CW mode-locked Nd:YLF laser was investigated for cavity length detunings. The pulsewidth of the laser was measured by a real-time optical autocorrelator constructed by using second harmonic generation in noncollinear type I phase matching. The cavity length producing the shortest mode-locked pulses was regarded as zero detuning, where the pulsewidth was measured to be 20 ps. The pulsewidth increased as the cavity length was detuned. Negative detuning of 0.4㎛ resulted in periodic relaxation oscillations of the laser output. As detuning increased, the modulation depth also increased and the laser output showed periodic modulations consisting of several spikings. For negative detunings of 2.4 to 3.4㎛, regular undamped spikings occurred, which showed a similar effect as Q-switching without an additional modulator. The pulsewidth of these self Q-switched mode-locked pulses was 30 to 47 ps, which is as short as those of mode-locking without self Q-switching. The stability of the self Q-switched envelope was excellent and the peak-to-peak fluctuation was less than 3% on a ~ ms time scale. The peak power was 10 times as large as that of mode-locking without self Q-switching. Such self Q-switching effect was also reported to be found in Nd:YAG lasers by Eichler et al. and Golyaev et al., but the pulsewidths were much broader to be 200 to 700 ps and the stabilities were poor. Further studies are being performed on the mechanisms of the self Q-switching behavior.
Also an active-passive mode-locked Nd:YAG laser was constructed and the pulsewidth was measured by an optical autocorrelator constructed by using second harmonic generation in collinear type I phase matching. The autocorrelator was also used to obtain the spectral width of the laser output by measuring the coherence time. The pulsewidth of the laser pulse was measured to be 125 ps, 91 ps, and 88 ps for saturable absorbers of BDN/iodoethane, Kodak dye # 26/1,2 dichloroethane, and Kodak dye # 9860/1,2 dichloroethane, respectively. The spectral width was found to broaden by self phase modulation as the unsaturated transmission of the saturable dye decreased, which implies that transmissions higher than 60% are adequate to obtain transform-limited pulses.
