Mode-locked Raman laser in H_2 pumped by a mode-locked external-cavity diode laser

We experimentally demonstrate a far-off resonant mode-locked Raman laser at 1196 nm pumped by an actively mode-locked external-cavity diode laser (ML-ECDL) at 799 nm. Using the Pound-Drever-Hall locking technique, we simultaneously frequency locked all the longitudinal modes from the ML-ECDL to a high-finesse Raman cavity filled with diatomic hydrogen (H2). When operating at an average power level slightly above the mode-locked (ML) threshold (which is comparable to the cw threshold), each of the nine pump modes, taken on its own, is below the cw lasing threshold. However, since the modes are in-phase, they can augment each other through four-wave-mixing processes, causing all of them to lase. The measured threshold for this process is approximately 5.4 mW. The FWHM of the ML Stokes output is 310 ps.