|
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
counterpropagating 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 4x4
mode-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.
|
