Fig. 1: Experimental setup of the GHz-mid-IR DCS system.
From: Architecture for microcomb-based GHz-mid-infrared dual-comb spectroscopy
a Counter-propagating (CP) solitons at 1.55 μm are generated in a silica microcavity to provide two of four comb signals. These solitons are photo-detected and the resulting signals are processed to create the two other comb signals by electro-optic modulation at 1.06 μm. These near-IR combs are combined in pairs to pump PPLN crystals for generation of GHz line spacing mid-IR combs by interleaved difference frequency generation. These mid-IR comb sources pass through a gas cell and are detected for dual-comb spectroscopy. Fiber Bragg grating filters used to filter pump waves in the soliton microcomb spectra are omitted in the figure. \({f}_{\,{{\mbox{r}}}}^{{{\mbox{cw}}}\,}\) (\({f}_{\,{{\mbox{r}}}}^{{{\mbox{ccw}}}\,}\)) corresponds to the cw (ccw) soliton repetition rates. AOM: acousto-optical modulator, circ: circulator, PM: phase modulator, IM: intensity modulator, EDFA: erbium-doped fibre amplifier, YDFA: ytterbium-doped fibre amplifier, WDM: wavelength division multiplexer, PPLN: periodically poled Lithium Niobate. Scale bar: 1 mm. b Optical spectrum of 1.55 μm soliton comb. c Optical spectrum of 1.06 μm EO-comb. d Multi-heterodyne beat between two CP soliton microcombs (repetition rate difference, Δfr, is 80 kHz). The beat note produced by the counter-pumps is identified. One of the microcombs is shifted by 55 MHz using the AOM placed after the cavity (see diagram in panel a). e Peak LFR of the comb lines in panel (d) as a function of averaging time τ. The solid line is a fit of the \(\sqrt{\tau }\) trend. f Measured Allan deviation of Δfr is close to the stability of the AOM driver. The frequency of the AOM driver (a radio frequency function generator) was set to be Δfr in this measurement. The error bar corresponds to the standard deviation of the Allan deviation.
