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.
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.
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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