Recently, 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 new 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 and priority work in the cavity enhanced spectroscopy measurement system.

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

In this study, by leveraging 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, 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.

Further, 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.