Empirical Analysis of Widely Tunable Fused Fiber Coupler Assisted by External Medium of High Thermo-Optic Coefficient

Abstract

This article reports the fused fiber coupler whose transmission spectrum is widely tuned or switched with temperature. With a proper choice of the external medium applied on the fused region of the coupler, the tuning range could be expanded up to the free spectral range of the coupler itself. A polymer with a matched refractive index and high thermo-optic coefficient was used as the external medium, and a micro-coil heater was installed to tune or switch the coupler. As the elongation length of the coupler increased in the fabrication process, the tuning range increased also. With the polymer having a thermo-optic coefficient of −1.8 × 10−4/°C and a 1.440 room temperature refractive index, wavelength tuning as wide as 210 nm was experimentally obtained. The fabrication process of the tunable fused coupler and the temperature-dependent spectral properties of the implemented couplers in terms of the elongation length and the polymer index are presented in detail. The feasibility of the proposed device as a tunable filter, optical switch, and variable optical attenuator are also presented.