Calibration- and baseline-fitting-free all-fiber oscillating-interference laser spectroscopy for gas sensing

Laser absorption spectroscopy is a promising technique for sensing trace gases, with the advantages of being noninvasive, in situ, highly selective, and having a high temporal resolution. However, its performance under harsh environmental disturbances remains limited because of the necessity for laser intensity calibration and complex baseline correction. In this study, all-fiber oscillating-interference absorption spectroscopy (AOIAS) was developed, which utilizes a fiber-optic interferometer based on an antiresonant hollow-core fiber (AR-HCF) to overcome the previous limitations and achieve advanced spectroscopic analysis. Laser intensity calibration-free and baseline-fitting-free gas detection of CH₄ was demonstrated. The light intensity of AR-HCF in the absorption arm was attenuated, enabling the visibility (V) of the interference pattern to be utilized for determining the CH₄ absorption coefficients. The experimental results demonstrated high linearity between the V value and CH₄ concentration (R² > 0.999) and between absorbance and concentrations (R² > 0.999), confirming reliability and accuracy. Allan variance analysis reveals that the system achieves a minimum detectable limit of 26.5 ppm and the volume of gas required is less than 2 μL. The proposed AOIAS provides a new avenue for compact optical sensors, featuring immunity to light-source fluctuations, no baseline processing requirement, and high adaptability.