Qian Wang

A low-loss Y-branch with a multimode waveguide transition section

A low loss Y-branch structure (symmetric or asymmetric) with a multimode waveguide transition section is introduced and designed. A wide range of the branching ratio can also be achieved for the asymmetric case.

Iterative finite-difference method for calculating the distribution of a liquid-crystal director

Iterative finite-difference method for calculating the distribution of a liquid-crystal director

Optimal design of a multimode interference coupler using a genetic algorithm

Abstract The optimal design of an N × N coupler of multimode interference (MMI) type is considered. The geometrical structure of the whole N × N coupler can be determined effectively by three parameters when the width of the MMI section is given. A genetic algorithm is used to determine these three parameters to achieve good results for the uniformity and excess loss. The optimal design is verified by a numerical simulation using the beam propagation method. Compared with the conventional design based on the self-image theory, the present method improves the uniformity and excess loss greatly.

An effective and accurate method for the design of directional couplers

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. Results obtained with the 3-D model are compared with those from 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 experiments. The new design method is shown to be effective and accurate.

Optimal design method of a low-loss broadband Y branch with a multimode waveguide section

An optimal design method for a low broadband silica-on-silicon Y branc is considered. A multimode waveguide section, which was used earlier to reduce the excess loss, is designed optimally when the light distribution at the end of the multimode waveguide section is matched to the profile of the symmetric supermode for the structure of the two branching waveguides. An optimization method that combines the genetic algorithm and a gradient-based search method is used to obtain the optimal geometrical parameters for the multimode waveguide section as well as the widths for the input and branching waveguides.

Optimal design of planar wavelength circuits based on Mach-Zehnder Interferometers and their cascaded forms

Optimal design of planar wavelength circuits based on Mach-Zehnder interferometers and their cascaded forms is presented with the wavelength dependence of the directional couplers considered. A simple approximate formula is presented to calculate quickly the wavelength-dependent phase difference for a directional coupler. By using this simple and effective formula and the genetic algorithm, a broadband coupler (1450-1650 nm) based on a Mach-Zehnder interferometer and a flat-top passband coarse wavelength division demultiplexer (with a channel spacing of 20 nm) based on cascaded Mach-Zehnder interferometers are optimally designed.

A new effective model for the director distribution of a twisted nematic liquid crystal cell

A new approximate model is proposed to describe effectively the director distribution of a twisted nematic liquid crystal (TNLC) cell with a strong surface anchoring when some of the physical parameters of the liquid crystal (LC) cell are unknown. The three model parameters for the liquid crystal cell (sandwiched between a polarizer and an analyser) are determined with an adaptive simulated annealing method by using the electro-optical transmissions for three configurations. The present three-parameter model is compared with the existing two-parameter model and is shown to be more stable in predicting electro-optical behaviours for LC cells of arbitrary twist angles with different thicknesses.

A simple, fast and accurate method of designing directional couplers by evaluating the phase difference of local supermodes

The local supermode formalism and a three-dimensional beam propagation method (BPM) are combined in designing directional couplers. The accumulated phase difference of the two local supermodes in the input or output region is evaluated quickly with the assistance of the BPM. The design method is simple, fast and accurate. A comparison of the present method with some other design methods is given. The design results are verified both numerically and experimentally.

Analysis and design of variable optical attenuators based on nematic liquid-crystal cells

Variable optical attenuators based on nematic liquid-crystal cells are considered. The attenuation curves of variable optical attenuators based on the 90° twisted and the parallel-aligned liquid-crystal cells are analysed explicitly with an approximate model, and the electro-optical behaviours of parallel-aligned liquid-crystal cells with a weak surface anchoring strength are modelled. Simulation results show that a large attenuation range and a shallow attenuation slope can be achieved simultaneously by using the parallel-aligned liquid-crystal cell with appropriate surface anchoring. The attenuation is nearly wavelength insensitive in a broad range of wavelengths.

Y-branch spot-size converter for a buried silica waveguide with large index difference

Coupling loss occurs between a standard single-mode fiber and a silica waveguide when the difference between the refractive indices of the core and the cladding of the silica waveguide is high. We designed a Y-branch structure for use as a spot-size converter to reduce this coupling loss. The structure was tested with a three-dimensional beam-propagation method and was shown to exhibit a significantly reduced coupling loss, a low polarization-dependent loss, and a good tolerance of imprecision in fabrication. No additional fabrication steps are required for this proposed spot-size converter.