Temperature-Insensitive Tunable Optical Filter Based on a Microsphere-Coupled Off-Core Spliced Fiber

A low-cost fiber-integrated temperature-insensitive tunable Fabry-Pérot filter fabricated from an off-core spliced segment of standard single mode fiber (SMF) and a commercially available barium titanate (BaTiO3) microsphere is proposed and demonstrated. The filter's cavity extends from the cleaved face of the launching SMF to the back face of the microsphere, thus forming a hybrid (air + BaTiO3) cavity. The device is robust against temperature variations due to the null thermo-optic coefficient of air, and a self-compensating optical path effect within the microsphere enabled by diffraction optics, which is credited to its spherical structure. Indeed, the filter's spectrum exhibits an ultra-low temperature-dependence, with experiments showing a temperature coefficient of 2 pm/°C for spectral shifts, agreeing well with theoretical calculations. Spectral contrasts higher than 35 dB were experimentally obtained, which were shown to depend on the longitudinal position of the microsphere. Such high contrasts are due to the high refractive index of the microsphere (1.9), which allows for greater reflectivity at the microsphere-air interface. Different from other off-core based filters, the one studied in this work offers wavelength tunability, which is achieved by changing the position of the microsphere either along (coarse tuning) or transversally (fine tuning) on the off-core segment. From its simplicity, low-cost, high-contrast, repeatability, temperature-independence, tunability and fiber-integration, it is expected that the proposed filter will find direct application in both research and industry.