A new method which combines the three-dimensional (3-D) beam propagation method with the overlap integral method is presented for the design of directional couplers. A backward beam propagation is calculated for the output region to reduce the computation time greatly. The design method is illustrated with buried silica-on-silicon waveguide couplers. The design results obtained with the 3-D model are compared with the results obtained with the two-dimensional model and it is shown that the 3-D model should be used. The design results are verified by both 3-D simulations and the experiment. The new design method is shown to be effective and accurate.

A numerical method based on the finite-difference time-domain (FDTD) scheme for computing defect modes in two-dimensional photonic crystals (with dielectric or metallic inclusions) is presented. Compared to the FDTD transmission spectra method, the present method reduces the computation domain significantly. By means of it one can find as many defect modes as possible, including those that are inactive to the incident plane wave in the FDTD transmission spectra method. The calculated eigenfrequencies and field patterns for a defect in a square array of dielectric rods are consistent with those obtained by the plane wave expansion method. Modes for a defect in a square array of copper rods are also studied, and the calculated eigenfrequencies are in a very good agreement with experimental results.

An analytic method of spatial phase modulation based on Fourier analysis is introduced for the design of a planar waveguide demultiplexer with a flat-top spectral response. An analytic formula for an etched diffraction grating demultiplexer is derived using the scalar diffraction theory. The spatial phase modulation is realized by slightly adjusting each grating facet's position according to the analytic formula to obtain a spectral response with a flat top and sharp transitions as well as a good dispersion characteristic. The analytic formula is characterized by two parameters: a transverse shift distance and a profile exponent for the phase modulation. A linear relation between the passband width and the transverse shift distance is given, and an optimal figure of merit of the spectral response is obtained by choosing an appropriate profile exponent. A numerical example of a typical SiO2 etched diffraction grating demultiplexer is used to demonstrate the advantages of this method.

General properties of N

Silica waveguides with diameters larger than the wavelength of transmitted light are widely used in optical communications, sensors and other applications1-3. Minimizing the width of the waveguides is desirable for photonic device applications, but the fabrication of low-loss optical waveguides with subwavelength diameters remains challenging because of strict requirements on surface roughness and diameter uniformity4-7. Here we report the fabrication of subwavelength-diameter silica 'wires' for use as low-loss optical waveguides within the visible to near-infrared spectral range. We use a two-step drawing process to fabricate long free-standing silicawires with diameters down to 50nm that show surface smoothness at the atomic level together with uniformity of diameter. Light can be launched into these wires by optical evanescent coupling. The wires allow single-mode operation, and have an optical loss of less than 0.1dBmm21. We believe that these wires provide promising building blocks for future microphotonic devices with subwavelength-width structures.