All-cavity-driven cw ringdown spectrometer with regulation of intracavity Doppler frequency shifts

This paper presents a new design of all-cavity-driven cw-CRDS to minimize the problem of cavity Doppler shift, as well as a firm theoretical background for the ringdown signal formation. The cavity output signal is modeled analytically, and a close inspection of the resulting expression reveals the clear evidence for the absence of intensity modulation. The intracavity Doppler shift is shown to occur only within the duration of cavity length excursion, terminating before the onset of a ringdown transient. Thus the only spectral influence is expected to be the frequency shift of an absorption feature, preserving its lineshape. The amount of frequency shift is shown to be determined by the cavity excursion distance, which directly indicates the condition required for the minimal Doppler shift.