Fig. 2: Schematic and results of the QEMR-PAS setup.

a Experimental setup of QEMR-PAS. A continuous-wave (CW) laser is split into two beams. Each beam is modulated by an acousto-optic modulator (AOM) and an electro-optic modulator (EOM). 99% of the combined light is amplified by an erbium-doped fiber amplifier (EDFA), and the rest (1%) is sent to a photoreceiver. A lock-in amplifier (LIA-1) records the comb intensity and phase fluctuations. The signal from a QTF is amplified by a pre-amplifier (PA) and demodulated by a lock-in amplifier (LIA-2). The data is acquired by a personal computer (PC). b Piezoelectric signal from the QTF. It has a sinusoidal wave shape since the QTF just measures the resonant sound wave from the multiple heterodyne sound waves. c Audio frequency comb generated through the down-conversion of the optical dual comb. The audio frequency comb is obtained by a spectrum analyzer using the output signal of the photoreceiver. d Normalized absorption spectrum of acetylene gas acquired by QEMR-PAS (light blue dots) and contrast with the Hitran model (solid black line) at normal pressure and temperature