Single-mode fiber-linearly tapered planar waveguide tunable coupler

We developed a simple system of tunable fiber-film coupler using a linearly tapered thin-film planar waveguide (PWG) evanescently coupled by a single-mode distributed fiber half-coupler. We investigate the characteristics of the coupler theoretically and experimentally taking into consideration the refractive index (n(f)) of nonuniform films, the magnitude of nonuniformity (m) of the films, and the source wavelength (lambda). The thickness variation of the nonuniform film is along the direction of propagation of optical power. Tapered and plano-concave thin films of a mix of oils as well as a plano-concave poly(methyl methacrylate) film were fabricated to serve as nonuniform PWG's. Similar to single-mode fiber with a uniform thickness PWG coupler, such a coupler also provides light modulation with a change of n(f). However, position shifting of a half-coupler in a tapered PWG structure along the direction of propagation exhibits the variation of fiber throughput power. This action serves as a simple system for a tunable fiber-film coupler. Wavelength-dependent throughput fiber power for such a coupler also behaves as a filter. The center wavelength can be controlled by shifting the position of the half-coupler. A coupling fiber as a half-coupler can be used for efficient coupling. We performed a theoretical analysis of the structure using Marcuse's model and observed good agreement with the experimental results.