In a study published in Optics Letters, a research group led by Prof. WANG Qiang from Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) of the Chinese Academy of Sciences (CAS) proposed a convenient approach to directly calibrate the cavity-mirror reflectivity for cavity-enhanced gas sensing using Pound-Drever-Hall (PDH) signals.

Trace gas detection based on cavity-enhanced spectroscopic is widely used in many fields due to its high sensitivity to intracavity absorption. With high reflectivity cavity mirror (>99%), a small change in its value will cause a large range of fluctuation in the detection sensitivity of the cavity enhanced spectroscopy. Therefore, the reflectivity calibration of the cavity mirror has always been a critical work and the priority in the cavity enhanced spectroscopy measurement system.

Specific gas sample-assist technologies and cavity ringdown technologies are commonly used in mirrors reflectivity calibration. However, they have limited applications because of the availability of standard reference gas and high-speed optical/electrical devices, and they are time consuming. Therefore, a simple, convenient, effective, and fast real-time calibration technique to determine reflectivity is needed to promote the application of optical cavity-enhanced gas sensing.

In this study, by usinging an electro-optic phase modulator (EOM) to generate sidebands, the researchers recorded the PDH error signal waveforms shaped by cavity modes. A LabVIEW program was developed to process the PDH error signals, and the cavity-mirror reflectivity can be obtained by fitting the processed signals with the Levenberg-Marquardt algorithm.

An external cavity diode laser (ECDL) with a narrow linewidth was used as the laser source. By scanning the laser wavelength, the researchers measured the effective reflectivity of a pair of cavity mirrors over 80 nm with a free spectral range-limited resolution, and a reflectivity uncertainty of as low as 2×10^-5.

Furthermore, the researchers explored the spectral response in a cavity-enhanced absorption spectroscopy (CEAS) manner to investigate the reliability of this reflectivity calibration approach, choosing the C₂H₂ as target gas, which achieves a minimum detectable absorption coefficient of 9.1×10^-9 cm^-1.

This method, by providing convenient calibration in an almost real-time manner, holds promising application prospects, and benefits the development of practical cavity-enhanced techniques for many scenarios.