Generation of Tunable Frequency Combs for Cavity-Enhanced Ultrafast Spectroscopy*

Through the use of a frequency comb laser and optical enhancement cavities, we have previously demonstrated a detection limit of $\Delta$OD $= 1 \times 10^{-9}/\sqrt{\textrm{Hz}}$ in a time resolved measurement on a dilute molecular beam. However, in order to have a more widely applicable spectrometer, the pump and probe wavelengths must be tunable. Here we present a frequency conversion setup for the generation of high average power frequency combs across the ultraviolet, visible, and infrared. The initial comb is generated using an Er:fiber oscillator at 100 MHz which, after nonlinear amplification, is shifted in a highly nonlinear fiber to 1 $\mu$m and amplified to 10W in a home built Yb:fiber amplifier. This light is then used as a pump for several nonlinear processes including a dual-focus optical parametric oscillator for the generation of tunable visible frequency combs from 450 to 700 nm. Optical parametric amplifiers are used for infrared comb generation from 3 to 5 $\mu$m which are seeded by additional shifted erbium comb branches.