We propose and investigate numerically a new kind of integrated optical waveguide coupler. Two waveguides with tilted Bragg gratings couple selectively to a discrete counter-propagating cladding mode which acts as an intermediary to transfer power between the waveguides. This gives a highly versatile platform that can act as a low-loss coupler with arbitrary coupling ratio between 0 and 100%, as a narrowbandwidth wavelength filter, and as a phase-selective switch that transmits two light waves of a fixed relative phase but reflects the orthogonal phase within a single device.

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We propose and numerically simulate a new and highly compact integrated 4x4 mode coupler based on two single-mode waveguides exploiting both propagation directions to double the number of modes. The two parallel waveguides are coupled via long and short-period gratings to the co-and counterpropagating directions, respectively, of a single, isolated cladding mode of the device which acts as a bus to transfer light between the waveguides. By connecting all end facets of the two waveguides to optical circulators we construct a device with four input and four output ports but only using two single-mode waveguides.

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We propose and investigate theoretically the use of bi-directional propagation of light in silica integrated coupled waveguide structures for linear optical processing of fiber-coupled photons. We show that the class of linear operations that can be implemented in such a system is given by symmetric and unitary matrices. We present how an arbitrary 4×4 coupler of this form can be realized by a linear sequence of more fundamental 2 × 2 couplers and single-waveguide phase shifters and discuss in detail the implementation in a silica integrated platform utilizing direct UV-written waveguides and long-and short-period grating couplers with tilted gratings for optimized coupling efficiency.

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We have recently developed and experimentally demonstrated the technology of tilted Bragg gratings in integrated optical waveguides to achieve polarisation selective coupling between parallel waveguides. Here I will review our ideas, designs and simulations for novel devices based on this platform. Multimode devices, tailored phase matching, and methods for significantly improved coupling efficiencies will be discussed that will be of interest for applications in quantum information processing and multimode optical telecommunication.