Fig. 1. The schematic to show the operation principle of proposed EO-DCS with real-time interleaved spectrum. CW laser: continuous-wave laser; EOFCG: electro-optic frequency comb generation; DUT: device under test; RF: radio frequency; FFT: fast Fourier transformation.

Fig. 2. Experimental setup of the proposed EO-DCS system. FL: fiber laser; MZM: Mach-Zehnder modulator; AWG: arbitrary waveform generator; AOM: acousto-optic modulator; DDMZM: dual-drive Mach-Zehnder modulator; DUT: device under test; BPD: balanced photodetector.

Fig. 3. (a) Experiment setup of swept lightwave characterization. (b) Sweep range and nonlinear error of the ultra-linearly swept optical source measured by using an unbalanced Mach-Zehnder interferometer. (c) Optical spectrum of the probe comb with a stabilized optical source and a linearly-swept optical source. (d) Optical spectrum of the local comb with a stabilized optical source.

Fig. 4. (a) Temporal waveform of dual-comb interferometer recorded in 1.6 ms with a sampling rate of 1 GS/s. (b) Zoom-in figure of (a) in 0.4 µs. (c) Electrical spectrum of reference branch containing total 25 lines centered at about 200 MHz. (d) Temporal waveform and (e) zoom-in figure of the 3-rd channel obtained by digital filtering. (f) Envelopes of the 3-rd channel for measurement and reference branches obtained by digital Hilbert transformation. Demodulated spectrum without DUT at 1 MHz spectral resolution for (g) the 3-rd channel in 16 GHz bandwidth and (h) all channels covering 400 GHz bandwidth, in which the standard deviation is calculated to be 2.316 %.

Fig. 5. (a) Reflectance spectrum measured in 1.6 ms with 1 MHz resolution. (b) Experimental setup of the fiber F-P cavity with 3 GHz FSR. (c) Measurement results of the deepest resonance with 17 MHz FWHM by using proposed method and ultra-fine EOFC. (d) Experimental setup of UFEOFC-based high resolution spectroscopy for comparison.

Fig. 6. Spectra of H¹³CN gas cell obtained by (a) single-shot measurement and (b) 25 times averaged measurement, containing the absorption lines of P9-13 in the 2v₃ band. (c) Measurement results and Voigt fitting curve of the P11 line. (d) Residual errors between the measurement results and fitting curve, and the standard deviations are calculated to be 0.378% and 0.087% for the single-shot measurement and 25-averaged measurement. The spectral resolution is 50 MHz by reducing the filter bandwidth to 200 kHz.

Fig. 7. (a) Unwrapped phase and nonlinear error of the swept optical source with 18 GHz sweep range and 90 ms sweep time. Electrical spectrum of (b) measurement branch and (c) reference branch, in which six lines circled are demodulated. (d) Demodulated reflectance spectrum of a fiber F-P cavity with 20 kHz spectral resolution. The points during the phase hopping circled in red are removed. (e) A resonance with 600 kHz FWHM measured by using the proposed method and UFEOFC.