Couplings in spun twin-core optical fibers

Optical bandpass filter from a curved segment of a detuned two-core fiber

We present the use of a dual-core fiber as a bandpass filter. Our device is based on the fact that intrinsic detuning between the propagation constants of the two cores can be compensated for in a controlled manner by use of a loop. This compensation is made to occur for a specific frequency band, so that the resulting device acts as a bandpass filter.

Enhanced power transfer and mode coupling in spun twin-core optical fibers

In a twin-core optical fiber a propagating light pulse periodically transfers power between its two cores. Experiments by Tjugiarto et al. [Opt. Lett. 17, 1058 (1992)] have demonstrated that this coupling length is considerably reduced when the fiber is also spun. A coupled-mode analysis reveals a pitch resonance that couples a cladding mode with the circularly polarized core mode whose handedness matches that of the helical twist of the cores. This resonance mechanism explains the observation of enhanced coupling and claddingmode cross coupling.

Practical coupling device based on a two-core optical fiber

A practical coupling device that relies on a dual-core fiber in a loop configuration is presented. Its coupling properties are analyzed in terms of the optical path difference between the cores, which is controlled by the rotation of the fiber about its axis and by a small twist applied to it along the loop. The device actually acts as an anisotropic coupler, and the coupled power can be perfectly controlled from 0-100% by proper adjustment of the loop. A simple implementation of the device was used in the fabrication of a compact single-fiber Michelson interferometer.

Dual-core fiber as a tunable directional coupler

The bending-induced birefringence in a fiber loop is used to restore locally perfect coupling conditions between the two slightly detuned cores of a dual-core fiber. It is also shown that varying the angle between the birefringence and the core's axis allows one to obtain perfect control (0-100%) of the coupling ratio.