Four-port integrated waveguide coupler exploiting bi-directional propagation of two single-mode waveguides

Abstract

We propose and numerically simulate a new and highly compact integrated 4x4 mode coupler based on two single-mode waveguides exploiting both forward and backward propagating directions to double the number of modes. The two parallel waveguides are coupled via long and short-period gratings to the co- and counter-propagating directions, respectively, of a single cladding mode of the device which acts as a bus between the waveguides. By connecting all end facets to optical circulators we construct a device with four input and output ports but only using two single-mode waveguides. Such a device can be fabricated in a single micromachined silica ridge structure. A photosensitive raised index layer is used for vertical confinement that supports multiple modes horizontally. We UV-write the waveguides and the Bragg gratings and provide a tilt angle to improve coupling. We have demonstrated this technology before for a polarizing waveguide-to-waveguide coupler and have simulated other unidirectional devices. We use coupled mode theory to simulate the system. By tailoring the grating parameters, we can achieve a wide variety of coupling ratios. Analytically, we find a set of solutions in which no light escapes via the cladding modes through the ends of the device and we have calculated device parameters to achieve a wide range of splitting ratios including coupling light from one input port equally into all output ports. Moreover, we derived analytically a set of parameters to implement a Walsh-Hadamard transformation and are investigating further options to implement a universal 4x4mode-coupler on this platform. We envisage that the device can be used for quantum information processing where two qubits are encoded in the waveguides using a photon in each propagation direction.

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Identifiers

ORCID for Mathias, John Weisen: orcid.org/0000-0003-0387-972X

ORCID for James Gates: orcid.org/0000-0001-8671-5987

ORCID for Corin Gawith: orcid.org/0000-0002-3502-3558

ORCID for Peter G.R. Smith: orcid.org/0000-0003-0319-718X

ORCID for Peter Horak: orcid.org/0000-0002-8710-8764

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