Stimulated Raman photothermal spectroscopy for gas sensing in a hollow-core optical fiber

Raman spectroscopy is an essential optical tool for tracing gases that exhibit weak or no infrared absorption. Existing Raman spectroscopic methods are deficient for precision sensing applications due to the extremely low Raman cross-section of gas. Herein, we report an approach, named stimulated Raman photothermal spectroscopy (SRPTS), to indirectly probe stimulated Raman scattering (SRS) via detecting the induced photothermal phase modulation in a gas-filled hollow-core fiber (HCF). Photothermal interferometry enables a linear increase of the SRPTS signal with the product of the pump and Stokes power while maintaining low background noise at the idle probe wavelength. The HCF tightly confines the light beams and the gas sample to significantly enhance both the SRS gain and the photothermal phase modulation efficiency. Preliminary experimentation with a 3.9-m-long HCF demonstrates hydrogen detection with a noise equivalent concentration of 6.8 ppm (parts-per-million) under 100 s averaging time and 6 bar gas pressure, indicating the potential for high-precision gas detection in chemical, medical, and energy industries.