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