Compact all-fiber light-induced thermoelastic spectroscopy for gas sensing.

To overcome the limitations of size, optical alignment, and integration into photonic circuits in previous light-induced thermoelastic spectroscopy (LITES) using free-space optics, a compact all-fiber LITES was proposed for gas sensing. A hollow-core photonic crystal fiber was employed as a waveguide and a microcapillary gas cell simultaneously. A single-mode fiber (SMF) tip was employed to guide light on the quartz tuning fork (QTF) surface. The distance between the SMF tip and the QTF, and the light excitation position on the QTF's surface were optimized experimentally. The detection performance of the all-fiber LITES was evaluated by detecting methane, and a normalized noise equivalent absorption coefficient of ${9.66} \times {{10}^{ - 9}}\; {{\rm cm}^{ - 1}} \cdot {\rm W}\,{{\rm Hz}^{ - 1/2}}$9.66×10-9cm-1⋅WHz-1/2 was realized at a 1 atm pressure and an environmental temperature of $ {\sim} 297\;{\rm K}$∼297K. The combination of fiber sensing and LITES allows a class of LITES sensors with compact size and potential for long-distance and multi-point sensing.