There has been increased interests and an increasing demand in recent years from optical communication systems for optical fibers which combine low transmission loss (<0.5 dB/km), wide bandwidth (~ THz), small size, light weight, cross talk immunity to field interference, electronic isolation, signal security, abundant of raw material, and very cheap. Especially wavelength- or subwavelength-scale dimensions tapered optical fibers can generate easily evanescent fields with short decay distance which offer tapered optical fibers promising candidates as the basis of coupling devices for micro-/nano-structures, optical sensors, and optical communication components.
This thesis presents a fabrication method for low-loss tapered optical fibers (0.014 dB/mm) and twisted fiber-optic couplers (0.34 dB excess loss) using a two-step wet-etching method with hydrofluoric acid-based droplets and surface tension driven flows. The droplet-based wet etching technique allows simple and cost-effective fabrication of tapered fibers as well as 2x2 fiber couplers. It is observed that the power split ratio of the optical couplers can be monitored in real time by monitoring the transmitted optical powers with respect to the etching time. The optical fiber couplers based on two tapered optical fibers show wavelength- and polarization-independent power split ratios in the wavelength range of 1525~1555 nm (optical communication C-band) due to the twisted geometric shape. In addition, the coupling length of the two twisted tapered optical fibers, which form a directional coupler device, can be affected by surrounding refractive index. Thus, the coupler can measure the water droplet volume by detecting the refractive index changes of the surrounding medium around the etched fiber core region.