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We demonstrate dual-comb spectroscopy in the vicinity of 2 µm wavelength based on a single dual-wavelength dual-comb Thulium-doped fiber laser. The shared laser cavity ensures passively maintained mutual coherence between the two combs due to common mode environmental noise rejection. In a proof-of-principle experiment, the absorption characteristics caused by the water in the optical path that composes the dual-comb spectrometer are measured. The retrieved spectral positions of the water absorption dips match with the HITRAN database.

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We show that the relative intensity noise (RIN) of a mode-locked fiber laser can be suppressed below a -140  dB/Hz level for the entire >20  Hz offset frequency range by a proper combination of intra-cavity and extra-cavity optical bandpass filters. When a 12-nm-bandwidth intra-cavity filter and a 16-nm-bandwidth extra-cavity filter are employed for a polarization-maintaining-nonlinear-amplifying-loop-mirror (PM-NALM)-based Er-fiber laser, the RIN spectrum level is suppressed by ∼30  dB in the low offset frequency range. The resulting integrated rms RIN is only 0.0054% [1 Hz-1 MHz]-to our knowledge, one of the lowest integrated RIN performances for any mode-locked laser reported so far. Besides the simplicity, this double-filtering approach has an additional advantage: unlike active pump-laser feedback methods, it does not have any resonant peaks in the stabilized RIN spectrum. In addition to the RIN suppression, with intra-cavity bandpass filtering, the integrated rms timing jitter is also reduced from 7.29-fs (no-filter) to 2.95-fs (12-nm intra-cavity filter) [10 kHz-1 MHz] in the soliton PM-NALM laser.