Cavity phase engineering for shape-invariant ultrashort pulse trains

Summary form only given. The use of dispersion control has made possible the generation of light pulses with durations shorter than 6 fs directly from a mode-locked Ti:sapphire laser oscillator. Such pulses contain only two or fewer cycles of the optical frequency oscillation. In this "single-cycle" regime, the Gouy phase shift plays an important role in pulse shaping during propagation. Moreover, the absolute phase of the pulse, defined as the phase of the carrier relative to the envelope, can drastically affect the pulse temporal profile and determine the efficiency of certain light-matter interactions. As is generally recognized, the presence of dispersive elements within the laser cavity results in a difference between the phase and group velocities of the pulse and causes the carrier to slide underneath the envelope. This can lead to a pulse-to-pulse instability in the temporal profile and hence there is much current interest in the control of this absolute phase. What is perhaps not so readily apparent is that this temporal instability is also an inherent feature of an empty resonator with no dispersive or nonlinear elements. In an empty resonator the Gouy shift of focused beams also results in a difference between phase and group velocities, which leads to pulse to pulse variations in absolute phase and intensity profile. We show that proper cavity design can eliminate the contribution of the Gouy shift to this pulse shape variation.