A phase stable hybrid dual comb spectrometer for time-resolved spectroscopy

Dual comb spectroscopy (DCS) is a broadband technique offering high resolution and fast data acquisition. We describe a hybrid dual-comb spectrometer comprising a broadband commercial fiber laser and an actively mode-locked semiconductor laser with a tunable and relatively narrow spectrum. We have devised a direct locking method, which leads to high long-term absolute stability (5 x $10^{-12}$ at 1 second). Our hybrid DCS system exhibits a mutual coherence time of roughly 100 seconds, a significant improvement over current schemes. We demonstrate this by performing DCS on rubidium atoms at 313 K with different data integration times. Finally, we used the phase stable DCS system to perform time-resolved measurements, where the resolution is limited only by the laser repetition rate (4 ns) and observed Rabi oscillations of the rubidium atoms at room temperature. The high time resolution is achieved by performing a series of DCS measurements with varying numbers of laser pulses incident on the atoms. The temporal evolution of the population of atoms in the excited state is recovered by suitably combining the measurement data. This is the first demonstration of the DCS method with a few nanosecond resolutions without using pump-probe spectroscopy.