We demonstrate and characterize both asynchronous optical sampling terahertz time-domain spectroscopy (AOS THz-TDS) and terahertz frequency comb spectroscopy (TFCS) as high-speed, high-resolution terahertz (THz) spectroscopy. Two mode-locked femtosecond (fs) lasers with slightly different repetition frequencies are used without a mechanical delay stage to generate and detect THz pulses. The repetition frequencies of the two fs lasers are stabilized by use of two phase-locked loops sharing the same reference oscillator. For AOS THz-TDS, the difference frequency between the repetition frequencies is optimized, and the signal-to-noise ratio is measured as a function of the measurement time. The spectra of a THz frequency comb and its individual modes are measured using TFCS. A spectral resolution of 100 MHz is obtained in both types of spectroscopy.
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