Fig. 1. (a) Time domain and frequency domain spectra of the optical frequency comb^[3]; (b) measuring the offset frequency of the optical comb using a self-reference method^[2].

Fig. 2. (a) Micrograph of a fabricated lithium niobate microring resonator. (b) Output spectrum of the EO comb generated from the microring resonator, the bandwidth exceeding 80 nm and more than 900 comb lines. The left inset shows a magnified view of the dotted. The right inset shows the measured transmission spectrum for several different modulation indices ^[9].

Fig. 3. (a) Initial mode frequencies, and , separated by ; (b) final frequencies resulting from four-wave mixing, with the two sidebands at and shown in green^[44].

Fig. 4. (a) Calculated clamped gain and total loss as function of frequency for lasers with different ridge widths; (b) total GVDs at different ridge widths. The shaded area from 4.05 to 4.35 THz represents the lasing range of the THz QCL^[50].

Fig. 5. (a) Calculated gain cross-section g_c. Blue curves: individual designs. Green curve: total active region. Inset: design of the laser active region. (b) Octave-spanning spectrum of laser^[58].

Fig. 6. (a) Schematic of the chirped corrugation. The red wave has longer wavelength, while the blue wave has shorter wavelength. (b) Spectrum of the THz QCL comb at a temperature of 50 K. Atmospheric absorption is shown in yellow^[61]. (c) Schematic of the device in a two-section configuration. The DC section is shown in blue; part of the FP section is in red. (d) Current-voltage characteristics for each section. The pink-shaded area indicates the entire dynamic range of lasing. The inset shows the SEM photo for the air gap in the real device^[63].

Fig. 7. Experimental setup of THz QCL active mode-locking. The emitting frequency of THz QCL is 2.5 THz and its repetition frequency is 13.3 GHz^[69].

Fig. 8. (a), (b) In the case of simultaneous injection and phase-locking of THz QCL, the beat-note signal diagram obtained by changing the RF power and the current. (c), (d) The waveforms are measured in the time domain under the corresponding conditions. The black dots in the figure are experimental values. The red curves are the result of theoretical calculations by assuming that all modes have equal phase^[69].

Fig. 9. (a) Experimental setup of RF modulation to THz QCL; (b) THz emission spectra under different modulation current. The water absorption lines in the frequency range from 3.9 to 4.4 THz extracted from the HITRAN database^[70]

Fig. 10. (a) Experimental setup for achieving the carrier phase fixed in QCL by injecting coherent THz pulse. (b) Measured fields of the QCL output for various input THz pulse amplitudes. The THz pulse amplitude is proportional to the antenna voltage with 1 V (green curve), 0.25 V (blue curve) and 0.06 V (grey curve)^[78].

Fig. 11. (a) Schematic of the graphene-coupled QCL. Inset: Illustration of the terahertz light propagation in the compound cavity. (b) Dual-comb and linewidth with and without GiSAM^[82].

Fig. 12. (a) Schematics of the dual-comb on chip. (b) Optical spectrum (blue curve). The inset shows that the modes consist of two peaks corresponding to the two combs. In red is the corresponding multi-heterodyne spectrum extracted from the current bias of the LO laser^[87]. (c) Schematics of the on-chip dual-comb system under double injection. The inset shows an optical photo of the mounted dual-comb device. (d) The down-converted dual-comb spectra in free-running mode and under a microwave double injection^[88].

Fig. 13. (a) Experimental setup for separating dual-comb system. Inset shows real laser frequency combs on the copper mount, both of which are silicon lens-coupled. (b) Multiheterodyne signal obtained from the HEB^[89]. (c) Experimental simulation diagram for compact dual-comb system. The illustration shows the actual experimental device^[91].

Fig. 14. (a) Dual-comb hyperspectral imaging system. (b) Beat note spectra acquired with (red) or without (blue) a silicon wafer inserted in the beam path. (c) Transmission spectra calculated from the beat note spectra in (b). (d) Beat note spectra acquired with zero gas (blue) and atmospheric water vapor at 23% relative humidity (red). (e) Transmission spectra calculated from (d); the blue curve is extracted from the HITRAN 2016 database^[92].