Partial bandgap characteristics of parallelogram lattice photonic crystals are proposed to suppress theradiation modes in a compact dielectric waveguide taper so as to obtain high transmittance in a largewavelength range.Band structure of the photonic crystals shows that there exists a partial bandgap.Thephotonic crystals with partial bandgap are then used as the cladding of a waveguide taper to reduce theradiation loss efficiently.In comparison with the conventional dielectric taper and the complete bandgapphotonic crystal taper,the partial bandgap photonic crystal taper has a high transmittance of above 85%with a wide band of 170 nm.
A two-dimensional (2D) optimized nanotaper mode converter is presented and analyzed using the finitedifferencetime-domain (FDTD) method.It can convert the mode size in a silicon pillar waveguide (PWG)from 4 μm to 1 μm over a length of 7 μm and achieve a transmission efficiency of 83.6% at a wavelengthof 1.55 μm.The dual directional mode conversion of the nanotaper and its ability to perform modecompression and expansion are also demonstrated.The broadband with high transmittance is satisfied inthis structure.Using this silicon-based nanotaper,mode conversion between integrated photonic devicescan be more compact and efficient.
We discuss the optimal design of line-tapered multimode interference (MMI) devices using a genetic algorithm (GA).A 1×4 MMI device is designed as a numerical example.Compared with the conventional design based on self-imaging theory,the present method demonstrates superior performance with low insertion loss and small non-uniformity.
