ACTIVE-MODE LOCKING OF BROAD-BAND CONTINUOUS-WAVE LASERS

We extend Kuizenga and Siegman's theory on active mode locking to the case of broad bandwidth continuous-wave tunable lasers; in particular, we analyze the effects of intracavity group-velocity dispersion and bandwidth control caused by intracavity optical elements, such as the solid-state hosts of the gain medium, modulators, and birefringent tuning filters. In contrast with simpler theoretical models on active mode locking which predict a monotonic decrease in the laser pulsewidth with increasing intracavity bandwidth, the present theory predicts that in the presence of significant group-velocity (material) dispersion (as is particularly relevant for the active mode locking of broad band solid-state lasers), there exists an optimal value of the intracavity bandwidth at which the shortest pulsewidths can be obtained. This optimal value of the intracavity bandwidth is inversely proportional to the square root of the intracavity group-velocity dispersion, i.e., to square-root[d2P / d-lambda-2\lambda = lambda-a], where P is the round-trip optical path length of the laser cavity, lambda-is the optical wavelength, and lambda-a is the center wavelength of the laser radiation. The results of this theory are compared with our previous experiments on active mode locking of a CW Ti:Al2O3 laser; for (d2P / d-lambda-2\lambda = lambda-a) = 4.86 x 10(3)-mu-m-1 and a near optimal intracavity bandwidth of 13.8 THz, a nearly transform-limited pulsewidth of 7.2 +/- 0.3 ps was observed in the experiments, in agreement with a value of 7.9 ps predicted by the above theory. The theory also predicts the possibility of obtaining pulsewidths less than 4 ps from appropriately designed mode-locked CW Ti:Al2O3 lasers without the use of intracavity dispersion compensating elements; pulse-widths of subpicosecond duration should be obtainable directly from actively mode-locked Ti:Al2O3 lasers with suitable dispersion compensation.