A simple approach of broadband mid-infrared pulse generation with a mode-locked Yb-doped fiber laser

Abstract: Broadband mid-infrared (MIR) molecular spectroscopy demands a bright and broadband light source in the molecular fingerprint region. To this end, intra-pulse difference frequency generation (IDFG) has shown excellent properties among various techniques. However, previous IDFG systems have mainly used unconventional long-wavelength 2-${\mu}$m ultrashort pulsed lasers. A few systems have been demonstrated with 1-${\mu}$m lasers, but they use bulky 100-W-class high-power Yb thin-disk lasers. In this work, we demonstrate a simple and robust approach of 1-${\mu}$m-pumped broadband IDFG with a conventional mode-locked Yb-doped fiber laser. We first generate 3.3-W, 12.1-fs ultrashort pulses at 50 MHz by a simple combination of spectral broadening with a short single-mode fiber and pulse compression with chirped mirrors. Then, we use them for pumping a thin orientation-patterned gallium phosphide (OP-GaP) crystal, generating 1.2-mW broadband MIR pulses with the -20-dB bandwidth of 480 cm$^{-1}$ in the fingerprint region (760-1240 cm$^{-1}$, 8.1-13.1 ${\mu}$m). The 1-${\mu}$m-based IDFG system allows for simultaneous generation of ultrashort pulses in the ultraviolet and visible regions, enabling, for example, 100-MHz-level high-repetition-rate vibrational sum-frequency generation spectroscopy or pump-probe spectroscopy.