Fig. 1. Schematic of the mid-IR comb. The mode-locked fiber oscillator serves as an NIR comb whose repetition rate is locked at 75 MHz. The CPA with two-stage fiber amplifiers scales the average power to 6.7 W. After the compressor, the system emits a pulse train with an average power of 4 W and a pulse duration of 194 fs, which corresponds to a pulse energy of 53 nJ. In the DFG module, the mid-IR pulse laser is generated in the PPLN by quasi-phase matching. SM LD, single-mode laser diode; YDF, Yb-doped fiber; WDM, wavelength division multiplexer; Col, collimator; FR, Faraday rotator; PBS, polarization beam splitter; ISO, isolator; MM LD, multimode laser diode; DC-YDF, double-cladding Yb-doped fiber; PCF, photonic crystal fiber; DM, dichroic mirror; PPLN, periodically poled lithium niobate.

Fig. 2. Characterization of the chirped pulse amplification. (a) Normalized optical spectrum of NIR oscillator (blue curve) and amplified pulse (green line), centered at 1030 and 1038 nm with a spectral width of 30 and 12 nm, respectively. (b) Measured autocorrelation trace (blue line) of the amplified pulse with corresponding sech fitting (dotted green line).

Fig. 3. (a) The spectrum of the broadened signal laser after a long-pass filter at 1100 nm. (b) The spectrum and corresponding average power of the mid-IR comb. The mid-IR comb has a tunable coverage of 2.7–4.0 μm. The average powers are 30, 130, 190, 240, 250, and 187 mW centered at 2.7, 3.0, 3.3, 3.5, 3.7, and 4.0 μm, respectively.

Fig. 4. The autocorrelation of mid-IR pulse at 3.5 μm. The pulse duration is 174 fs with Gaussian fitting.

Fig. 5. (a) Phase noise PSD and (b) relative intensity noise (RIN) of the repetition rate signal corresponding to NIR comb (origin), CPA (green), and mid-IR comb at 3.5 μm (blue).

Fig. 6. (a) The measured repetition rate stability of NIR comb (blue) and mid-IR comb (orange) for 3 h. (b) The Allan variance of the NIR comb (blue) and mid-IR comb (orange).