Planar add/drop two-wavelength filter employing a blazed Bragg grating and a 3x3 asymmetric coupler

Abstract

ABSTRACT We propose a novel 6-port planar waveguide coupler device for adding and/or dropping two different wavelengths from a WDM channel using a single grating. By writing a blazed Bragg grating into a few-modedcore at a slight angle to the waveguide propagation direction, power can be coupled from the funthmental modeinto higher-order, backward-propagating modes and vice-versa. Each such mode is channelled into a particularoutput port using an adiabatic splitter. Modelling results indicate that more than 99.99% of the power in thefundamental mode can be coupled into the higher-order modes at their respective Bragg wavelengths.Keywords: gratings, couplers, multiplexing, demultiplexing, planar waveguides, asymmetric couplers L INTRODUCTION Typically fibre Bragg gratings are written into optical fibres designed to support just one mode - the fundamental. In order to maximise the reflection of the fundamental mode at its Bragg wavelength, these gratings are writtenwith the index modulation phase fronts perpendicular to the optical fibre axis and, therefore, parallel to the modalphase front. However, if the Bragg grating written in the single-moded fibre is tilted slightly away from theperpendicular to the optical fibre axis, i.e. is blazed, power can be coupled, and hence reflected, into any higher-order, backward-propagating cladding modes '.