Wavelength-tunable, mid-infrared ultrafast pulse generation through Raman self-frequency shift in an all-solid fluorotellurite fiber

We report a mid-infrared (mid-IR) fiber laser system that can deliver ultrafast soliton-like pulses with a wavelength tunable from 2.8 to 4.0 μm. The pump light source of the system is a mid-IR fiber laser mode-locked at 2.8 μm, which has an average output power of ∼500 mW, a repetition rate of ∼30 MHz, a pulse energy of ∼15 nJ, and a pulse duration of ∼150 fs. The pump light was then launched into a short (∼70 cm) sample of fluorotellurite (TeO₂-BaF₂-Y₂O₃) glass fiber as the high-nonlinearity waveguide with simultaneously good features of large Raman gain, tailored dispersion, and high-power capability. High-efficiency Raman soliton self-frequency-shift phenomenon can be obtained in this short fiber sample, leading to the generation of broadband tunable (3-4 μm), mid-IR pulses with hundreds-of-fs pulse duration and tens-of-mW average power, corresponding to a pulse energy level of ∼1 nJ. The frequency-conversion efficiency inside the nonlinear fiber was measured to be as high as ∼16%. The present system, combining the advanced techniques of fluorotellurite fiber fabrication and mid-IR ultrafast fiber laser, highlights its application potentials for generating low-noise, high-beam-quality, mid-IR ultrafast pulses with a compact fiber configuration.