Optical fiber refractometers provide an electrically passive platform that is immune to electromagnetic interference and deployable in harsh environments. Conducted within the framework of the ANR FRAME project, this thesis presents the development of intensity-based capillary fiber refractometers for in situ methane detection in aquatic environments. A central contribution is the physics-based modeling of the tapered collapse zone formed during the laser fusion splicing of solid and air-core fibers. By tailoring this transition, optical power is redistributed to match the refractive index window of functional coatings. Combined with a differential optical interrogation scheme to suppress common-mode noise, the sensor achieves high resolution and dynamic range. When functionalized with a supramolecular Cryptophane-A-based coating, methane trapping induces refractive index variations that translate into optical intensity modulation. The platform demonstrates rapid response dynamics and low limits of detection in both gaseous and dissolved phases.