Ultrafast spectroscopy is an effective tool in investigating molecular dynamics in quantum mechanical systems of different molecules and clusters on the femtosecond timescale. Ultrafast cavity-enhanced transient absorption spectroscopy is a relatively new technique to carry out transient absorption spectroscopy on samples in molecular beams. The technique requires ultrafast frequency comb lasers and ours is a home-built modelocked ytterbium fiber laser. Rare-earth doped fiber lasers require less pump power but have narrow gain bandwidth, hence, achieving the stable sub-100 femtosecond pulses is challenging. In this project, we characterize a new ytterbium fiber laser design to achieve reproducibly ultrashort, sub-45 fs pulses; specifically the inclusion of an intracavity spectral filter. We characterize the laser dispersion at different intracavity double-pass transmission grating compressor separations. By optimizing the grating separation, the magnitude of the group delay dispersion is minimized to yield the most compressed pulse. Adjusting degrees of intracavity optical interference filters and tuning quarter- and half-wave plates resulting in cutoff frequency with stable mode-locking regimes. We successfully compress a Yb-doped fiber laser pulse to 41-43 fs with low relative intensity ratio. The effects of intracavity filters are investigated and compared to filters at an external position to the cavity. In the future, the pulse will be used in the cavity-enhanced transient absorption spectrometer (CETAS).

Building ytterbium fiber lasers for ultrafast spectroscopy

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