Search

The physics of passively modelocked Ti:Sapphire femtosecond lasers is a topic of current interest. In the broadband case (short pulses) the net intracavity dispersion is an important limit in determining the pulse width and tuning behavior. We report a remarkably simple and powerful technique for the characterization of dispersion in broadband tunable passive modelocked lasers. A CW-pumped passive modelocked laser chooses its cavity roundtrip time independent of the pump laser cavity length, of course. If we simply note that the cavity roundtrip time at any particular frequency is exactly the total group delay of a quasi-monochromatic wave packet at that frequency, then clearly by accurately measuring the cavity repetition rate as a function of frequency while tuning the laser, we obtain the group delay as a function of frequency directly. The changes are small: a 100 fs group delay change in a 10 ns period corresponds to a frequency shift of only 1 kHz, which is easily measured with a high precision frequency counter. Using this simple technique, we demonstrate the effect of translating a Brewster prism, and show the magnitude of the cumulative cubic phase in an actual operating laser. The signals contain contributions from mirrors, materials, air and refraction. This appears to be a powerful and simple technique which should provide useful information for understanding of the new class of broadband tunable femtosecond lasers.