Mode-locked Raman laser in H2 pumped by a mode-locked external cavity diode laser

Abstract

In this thesis, a theory is first presented for a far-off resonance mode-locked Raman laser in H2 with high finesse cavity enhancement. The theoretical derivation for the mode-locked Raman laser is based on semiclasscial laser theory and time-dependent continuous wave (CW) Raman theory. Numerically calculated results, including the intracavity fields' amplitude and phase evolution and output Stokes power versus input pump power are discussed in three different regimes depending on the relationship between the coherence dephasing rate and the repetition rate of the mode-locked pump laser. Then experimentally, first an actively mode-locked external-cavity diode laser (ML-ECDL) was built along with a demonstration of how to frequency lock all the longitudinal modes from the ML-ECDL to a high finesse cavity. Then a tapered amplifier diode system was designed to increase the ML-ECDL power.

Finally, a far-off resonance mode-locked Raman laser pumped by a ML-ECDL was demonstrated. With nine longitudinal pump modes, when operating at an average power level slightly above the CW threshold (3.89 mW), each of the pump modes, taken on its own, is below the CW lasing threshold. If these pump modes are not in-phase, the threshold will be nine times the CW threshold, roughly 35 mW. However the measured threshold for the ML Raman laser is about 5.4 mW, much less than 35 mW, because the pump modes are in-phase, and they can augment each other through four-wave-mixing processes causing all of them to lase as expected from the theory. The full width half maximum of the ML Stokes output is 310 ps. The beat signals from the ML Stokes showed that the Stokes pulses were made of at least eight modes.