Abstract
Optical feedback cavity ringdown spectroscopy is presented with a linear Fabry–Pérot cavity and a cost-effective DFB laser. To circumvent the low coupling efficiency caused by the broad laser linewidth, an optical feedback technique is used, and an enhanced coupling efficiency of 31%, mainly limited by impedance mismatch and mode mismatch, is obtained. The trigger of the ringdown event is realized by the shutoff of the laser driving current, and a novel method with the aid of one electronic switch is applied to avoid the ringdown events excited by the unexpected cavity modes during the process of laser current recovery. As a result, the ringdown signal with a signal-to-noise ratio of 2500 is achieved. Through continuous monitoring, the fractional uncertainty of the empty cavity ringdown times is assessed to be 0.04%. An Allan variance analysis indicates a detection sensitivity of 4.3 × 10−10 cm−1 is resulted at an integration time of 120 s, even with a moderate finesse cavity. To further improve the long-term stability, we regularly rectify the empty cavity ringdown time, and an improvement factor of 2.5 is demonstrated.
Highlights
Laser absorption spectroscopy is an effective way for trace gas detection due to its advantages of high sensitivity and high stability
The ringdown event is excited by changing the laser current below the threshold of emitting quickly
Special measures to avoid the trigger of unwanted ringdown events during the laser current recovery have been taken, and their effect is illustrated in Figure 2 by the time sequence of process signals
Summary
Laser absorption spectroscopy is an effective way for trace gas detection due to its advantages of high sensitivity and high stability. To improve the detection sensitivity, a variety of methods, such as cavity ringdown spectroscopy [1, 2], cavity-enhanced absorption spectroscopy [3, 4], thermoelastic spectroscopy [5, 6], and photoacoustic spectroscopy [7, 8], have been proposed. CRDS deduces the intracavity absorption information from the variation of the ringdown times, rather than the amplitudes of the cavity transmission modes. It is immune to laser intensity noise and frequency-to-amplitude noise, which is the main limitation of cavity-enhanced absorption spectroscopy (CEAS). In recent decades, CRDS has been widely applied in a variety of application fields [11,12,13]
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