In-situ refractive index measurement of ZIF-8 thin films on multimode optical fibers for methane sensing

Metal–organic frameworks (MOFs) are widely used as active layers in refractive index-based fiber optical sensors. Refractive index determination is almost always performed ex-situ rather than on the active sensor element. Reported values for ZIF-8 span a range of 1.13–1.55 depending on synthesis route, film density and surrounding atmosphere, which makes the transfer from a reference substrate to the actual sensor unreliable. Here we present a method that measures the refractive index of ZIF-8 thin films in-situ, directly on coated fibers, from far-field intensity patterns emitted by the fiber end face. The radius of the pattern yields the numerical aperture of the coated segment and thereby the refractive index of the surrounding medium. The method was validated against aqueous sodium metatungstate solutions of known refractive index on a cladding-stripped reference fiber, with a mean deviation of 0.034% across the accessible window between the cladding and core refractive indices. Applied to ZIF-8 films grown on four independently prepared multimode fibers, the method revealed a refractive index increase of (1.4 ± 0.7) × 10⁻⁴ RIU between vacuum and nitrogen and (5.4 ± 0.3) × 10⁻⁴ RIU between vacuum and methane at 1 bar, accompanied by a decrease in transmitted intensity to 99.1 ± 0.1% under nitrogen and 96.0 ± 0.1% under methane. Furthermore, far-field analysis shows that high mode orders are more susceptible to mode decoupling upon refractive index increases, providing a basis for targeted sensor optimization.