Figure | Mixed-frequency heterodyne demodulation method. (IMAGE)
Caption
Figure | Mixed-frequency heterodyne demodulation method. a, Schematic circuit diagram of the mixed-frequency heterodyne demodulation system. Two modulated laser beams are combined by a wavelength division multiplexer (WDM) and focused onto a single quartz tuning fork (QTF). The piezoelectric signal generated by the QTF is amplified to obtain the mixed signal Va, which is then separated into a high-frequency signal Vc and a low-frequency signal Vb through a high-pass filter (HPF) and a low-pass filter (LPF), respectively. The two signals are mixed with local oscillator signals Ve and Vd to produce Vg and Vf, which are simultaneously demodulated by a single lock-in amplifier. b, Schematic diagram of the frequency spectrum shifting principle. The original mixed signal contains two frequency components f0 (fundamental mode) and f1 (first overtone mode). After being separated by filters, the two signals are mixed with the same local oscillator frequency and converted to an identical intermediate frequency f1+f0, enabling synchronous demodulation with only one reference source. c, Time-domain waveforms at key nodes of the demodulation system. From top to bottom: the input mixed signal Va, the filtered low-frequency signal Vb and high-frequency signal Vc, and the final demodulated DC output signals Vm and Vn, which correspond to the concentration information of the two target gases respectively.
Credit
Yufei Ma et al.
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CC BY