Development and investigation of a fiber-optic Fabry–Pérot interferometer with an air cavity between the resonator mirrors

Subject of study . A fiber-optic Fabry–Pérot interferometer (FPI) with an air cavity between the resonator mirrors is studied. Aim of study . A fiber-optic FPI with an air cavity between the resonator mirrors is designed and fabricated, achieving an increase in the visibility of the interference pattern and the free spectral range. Method . The FPI contains two collimating structures with resonator mirrors coaxially aligned and separated along the optical axis. Each structure includes a coaxially connected optical fiber and a gradient lens formed by a section of a multimode graded-index optical fiber. The opposite polished end face of the gradient lens with a reflective coating forms the reflective surface of the interferometer resonator. Main results . Interferometer samples with air cavity lengths ranging from 0.5 mm to 5 mm (increased in 0.5 mm steps) were fabricated. The visibility of the interference pattern in the wavelength range of 1300–1310 nm ranged from 0.989 to 0.679, and the free spectral range ranged from 1.707 nm to 0.178 nm. Practical significance . The developed interferometer can be applied as a sensing element in a refractive index sensor for measuring the refractive indices of gases and liquids in the range from 1 to 1.47 with a sensitivity of 190.4 pm/RIU.