Evolution of gain and absorption in a cw mode-locked dye laser

We have measured the evolution of the gain (Rhodamine) and absorber (DODCI) dyes inside a cw mode-locked dye laser. The recovery time of each dye after the passage of the intracavity pulse was much longer than 1 psec. However, a probe pulse passing through one dye and then through the other dye experienced a net gain that lasted only ~1 psec, the duration of the pulse. The results are consistent with the mechanism of pulse formation in a dye laser mode locked by a slow saturable absorber proposed by New and Haus [IEEE J. Quantum Electron. QE-10, 115 (1974); QE-11, 736 (1975)].