Dual-Core Optical Fiber as Beam Splitter With Arbitrary, Tunable Polarization-Dependent Transfer Function

Abstract

Abstract-We present the design of a microstructured dualcore optical fiber with integrated electrodes and filled with liquid crystals. The dual-core structure acts as a directional coupler whose properties depend on the liquid crystal alignment. We show that with four electrodes and two separate driving voltages below 30 V on the electrodes, the beam-splitting properties of the fiber can be controlled independently and continuously for the two polarization components, thus allowing for the realization of any arbitrary 2x2 transfer function, such as tunable polarizers, polarization-dependent attenuators, or polarization-independent beam splitting. Index Terms-Directional couplers, Liquid crystals, Optical fiber devices I. INTRODUCTIONOLARIZATION beam splitters, filters and attenuators are essential optical components for various applications such as polarization-division multiplexing in optical telecommunication networks [1], or polarization encoding in quantum information systems [2], [3]. Splitting between the TE-and TM-polarized modes of optical waveguides has been demonstrated by utilizing different photonic elements, including Mach-Zehnder interferometers [4], multi-mode interference devices [5], [6], directional couplers [3], [7] or waveguide grating structures [8]. Among them, the directional coupler is one of the most widely used structures to achieve a high polarization extinction ratio. It consists of two distinct waveguides placed close together and coupled through overlapping evanescent fields of propagating modes. The coupling strength can be precisely engineered by controlling the waveguide geometry and the length of the interaction region. To some limited degree, post-fabrication tuning can also be achieved by changing either the propagation constants of optical modes or the separation between waveguides.This work was supported through the U.K. National Quantum Technology Programme (EPSRC grants EP/M013243/1 and EP/M013294/1).N. Podoliak is with Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, U.K. (e-mail: n.podoliak@soton.ac.uk).P. Horak is with the Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, U.K. (e-mail: peh@orc.soton.ac.uk).Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org.Digital Object Identifier: 10.1109/JLT.2017.2729939Polarization beam splitters based on directional couplers can in principle be realized using different waveguide geometries and fabrication platforms. However, most of them so far were fabricated using planar waveguide structures on integrated photonic chips [3], [7], [9]. On the other hand, the realization of directional couplers with polarizing, and ideally continuously tunable, functionality in a fiber platform [10]- [13] would allow for a new generation of all-fiber photonic devices in particular for telecommunication, quantum information, and sensing applications.The possibility of selective filling of air holes in microstructured fibers with differen...