Mid-Infrared Sensing Using a Hollow–Core Fiber in a Nonlinear Interferometer

Increasing the interaction path length is a well-known method for enhancing the sensitivity of the optical detection system. Hollow–core fibers (HCFs) represent a viable alternative to the traditional multi-path cells offering low optical losses and strong confinement of the optical field. Here, the incorporation of an Antiresonant Hollow–core Fiber (AR-HCF) section into a nonlinear interferometer, where the AR-HCF section serves as a gas-sensing cell operating in the IR range is presented. By exploiting the effect of nonlinear interference, the detection is brought into the more operation-friendly visible range. The detection of methane (CH₄) gas at mid-IR wavelengths within a half-meter section of AR-HCF, with an estimated concentration accuracy of 200 ppm·m is demonstrated. These results represent the combination of two research fields within a single instrument and pave the way for further advancement of quantum-inspired gas sensing techniques.