Uncertainty of optical feedback linear cavity ringdown spectroscopy

Cavity ring-down spectroscopy (CRDS) is a highly sensitive molecular absorption spectroscopic technology,which has been widely used in mirror reflectance measurement, atmospheric trace gas detection, molecularprecision spectroscopy and other fields. It deduces the intracavity absorption by measuring the rapid variationof the ringdown signal. As a result, detector with high linearity, broad bandwidth and low electrical noise isindispensable. Additionally, owing to the large noise in laser frequency, low laser-to-cavity coupling efficiency isobtained. Consequently, the cavity transmission is faint, which deteriorates the detection sensitivity. Opticalfeedback can address this problem by locking the laser to the cavity longitudinal mode. Then, the laserfrequency noise is suppressed and hence better detection sensitivity is expected. Optical feedback CRDS with V-shape cavity has been widely studied. Compared with Fabry-Perot cavity, this cavity geometry is very sensitiveto mechanical vibration and possesses low degree of fineness due to an additional mirror. In this paper, opticalfeedback linear cavity ring-down spectroscopy based on a Fabry-Perot cavity with a degree of fineness of 7800 ispresented. The principle of the combination of optical feedback and linear cavity is explained from theperspective of the light phase, which shows that the reflection will not generate efficient optical feedback if thefeedback phase is appropriately controlled and laser to cavity locking can be therefore realized. And then, thefactors influencing the stability of ring-down signal are analyzed, including the feedback ratio, the triggervoltage for the ringdown event, and the distance between the light spot and the detector center. Theexperimental results show that a superior fractional uncertainty of the empty ringdown time of 0.026% can beobtained with a low feedback rate (3% FSR), a high ringdown signal trigger threshold (90% cavity modeamplitude) and superposition of the light spot with the detector center. With Allan variance analysis, the whitenoise response of 1.6 x 10-9 cm-1middotHz-1/2 and the detection sensitivity of 1.3 x 10-10 cm-1 for trace gas detectioncan be achieved in an integration time of 180 s, corresponding to the lowest CH4 concentration detection of 0.35x 10-9 at 6046.9 cm-1. This robust spectroscopic technique paves the way for constructing high-sensitive and stable-cavity based instrument for trace gas detection