Fig. 1. (a) Corresponding experimental concept with an all-waveguide implementation starting from a near-IR seed frequency comb. After amplification and compression to few-cycle duration, a waveguide extends the spectrum from visible to MIR. An f-2f interferometer records the carrier-envelope offset frequency fceo to prove the comb nature of the expanded spectrum experimentally. The free-running linewidth of the fceo is a direct indication of the optical phase noise in each comb line. (b) Building blocks of mid-infrared frequency-comb synthesizer: seed laser, amplifier (10× power), compressor (20 times less pulse duration), and frequency extender. The frequency extender synthesizes new frequency components covering absorption features of important molecules in the mid-infrared.

Fig. 2. (a) Experimentally recorded spectrum (red) and simulated (gray) spectrum after self-compression segment. (b) Experimentally recorded (blue) and simulated (gray) autocorrelation of the 25 fs pulse.

Fig. 3. (a) Wavelength-dependent nonlinear parameter estimated based on the core profile. Inset: W-shaped refractive index profile of the fiber core for enhanced mode confinement. The geometry parameters of the Ge-silica core (D) and F-silica layer (d) are measured using a microscope. The doping concentration is numerically scanned to reconstruct the fiber profile. (b) Simulated group velocity dispersion (β2) of the highly nonlinear fiber by reconstructing the fiber core profile numerically superimposed with values from the datasheet. Inset: fiber dispersion over the entire spectrum range. (c) Experimentally recorded and simulated spectra. Top: spectrum of a 3.2 cm fiber length from the experiment (blue) and simulation (black). Bottom: experimental (red) and simulated (black) spectrum of a 33 cm fiber piece.

Fig. 4. (a)–(d) Pulse temporal distribution when input pulse gets compressed down to various optical cycles. To match experiment parameters, we assume the initial condition to be 100 fs chirp-free, 2.8 nJ pulses, and fiber to be PM1550. (e) Center pulse energy versus the pulse duration. (f) Numerical simulation for scaling the power beyond 2300 nm as a function of fiber length and pump duration; the red mark is the combination of fiber length and pump duration in this work. (g) Similar simulation results for power beyond 2500 nm.

Fig. 5. (a) fceo and frep−fceo at 100 kHz RBW. Both signals show >40  dB SNR. (b) fceo of (a) at 1 kHz RBW with 33 MHz offset. Inset: zoom-in of the fceo shows 90 kHz linewidth.

Fig. 6. RIN measurement at different locations of the laser setup from 1 Hz to 100 kHz.