Jingbo Cai

Ultracompact high-efficiency polarizing beam splitter with a hybrid photonic crystal and conventional waveguide structure.

We propose an ultracompact high-efficiency polarizing beam splitter that operates over a wide wavelength range and is based on a hybrid photonic crystal and a conventional waveguide structure. Within a small area (15 microm x 10 microm), this polarizing beam splitter separates TM- and TE-polarized modes into orthogonal output waveguides. Results of simulations with the two-dimensional finite-difference time-domain method show that 99.3% of TM-polarized light is deflected by the photonic crystal structure (with a 28.0-dB extinction ratio), whereas 99.0% of TE-polarized light propagates through the structure (with a 32.2-dB extinction ratio). Wave vector diagrams are employed to explain the operation of a polarizing beam splitter. Tolerance analysis reveals a large tolerance to fabrication errors.

Parallel microgenetic algorithm design for photonic crystal and waveguide structures

We have developed a powerful parallel genetic algorithm design tool for photonic crystal and waveguide structures. The tool employs a small-population-size genetic algorithm (microgenetic algorithm) for global optimization and a two-dimensional finite-difference time-domain method to rigorously design and optimize the performance of photonic devices. We discuss the implementation and performance of this design tool. We demonstrate its application to two photonic devices, a defect taper coupler to connect conventional waveguides and photonic crystal waveguides, and a sharp 90 degrees waveguide bend for low index contrast waveguides.

Hybrid integration of conventional waveguide and photonic crystal structures

We propose the hybrid integration of conventional index-guided waveguides (CWGs) and photonic crystal (PhC) regions of very limited spatial extent as a promising path toward large-scale planar lightwave circuit (PLC) integration. In CWG/PhC structures the PhC regions do not perform the function of waveguiding, but instead augment the CWGs to permit a drastic reduction in the size of photonic components. For single mode waveguides with a refractive index contrast of only 2.3%, simulation results show a 90 degree bend with 98.7% efficiency, a compact beamsplitter with 99.4% total efficiency, and a planar Mach-Zender interferometer (MZI) with 97.8% efficiency. The MZI occupies an area of only 18microm m x 18 microm.

New ring resonator configuration using hybrid photonic crystal and conventional waveguide structures

We propose a new method of realizing ring resonators based on hybrid photonic crystal and conventional waveguide structures. The proposed ring resonator configuration is advantageous compared with general ring resonator structures for its controllability of the quality (Q) factor, free spectral range (FSR), and full width at half maximum (FWHM) over a wide range. We show ring resonator structures based on a single mode waveguide with core and clad refractive indices of 1.5 and 1.465, respectively. A 35mum x 50mum ring resonator has a free spectral range (FSR) of 14.1nm and a quality (Q) factor of 595 with high optical efficiency (92.7%). By decreasing the size of the ring resonator to 35mum x 35mum, the FSR is increased to 19.8nm. Modifying the splitting ratio of the beam splitters permits the Q factor to be increased to 1600.

High efficiency 90/spl deg/ silica waveguide bend using an air hole photonic crystal region

We propose the hybrid integration of an air hole photonic crystal (PhC) structure with a high /spl Delta/ (0.75%) single-mode silica waveguide to achieve an ultracompact high efficiency 90/spl deg/ bend for transverse-magnetic polarized light. Diffraction from the periodic boundary between the PhC and silica waveguide regions is shown to seriously degrade the optical efficiency of the bend. A microgenetic algorithm (/spl mu/GA) combined with a two-dimensional finite-difference time-domain method is used to modify the PhC and its boundary layer to suppress this diffraction which in turn maximizes bend efficiency. The final optimized structure has a 99.4% bend efficiency at a wavelength of 1.55 /spl mu/m and occupies an area of only 27 /spl times/ 27 /spl mu/m.

Stratified waveguide grating coupler for normal fiber incidence

We propose a new stratified waveguide grating coupler (SWGC) to couple light from a fiber at normal incidence into a planar waveguide. SWGCs are designed to operate in the strong coupling regime without intermediate optics between the fiber and the waveguide. Two-dimensional finite-difference time-domain simulation in conjunction with microgenetic algorithm optimization shows that approximately 72% coupling efficiency is possible for fiber (core size of 8.3 microm and delta=0.36%) to slab waveguide (1.2-microm core and delta=3.1%) coupling. We show that the phase-matching and Bragg conditions are simultaneously satisfied through the fundamental leaky mode.

Three-dimensional analysis of a hybrid photonic crystal-conventional waveguide 90° bend

We present a three-dimensional (3D) analysis of a hybrid photonic crystal-conventional waveguide 90° bend proposed previously [Opt. Express10, 1334 (2002)] as an ultracompact component for large-scale planar lightwave circuit integration. Both rigorous 3D finite-difference time-domain modeling and a simple perfect mirror model analysis were carried out for different Si post heights in the photonic crystal region. Results show that the bend efficiency increases rapidly with Si post height. For a post height of 6.5 μm, this structure yields a bend efficiency of 97.3% at a wavelength of 1.55 μm for 90° bends in 2 μm × 2 μm square channel conventional waveguides with a refractive index contrast of 3.55%, which is very close to the bend efficiency of 98.2% for the corresponding two-dimensional problem. Our 3D analysis permits the examination of issues such as out-of-plane scattering loss and the effects of finite Si post height that are not considered in two dimensions.

Microgenetic algorithm design of hybrid conventional waveguide and photonic crystal structures

We previously proposed the hybrid integration of photonic crystals (PhCs) and conventional index-guided waveguides (CWGs) as a potentially attractive method of realizing compact waveguide elements for large-scale planar lightwave circuits (PLCs). We now examine 90-deg bends and beamsplitters in PhC/CWG structures in which the waveguide core has a high refractive index (3.25) and yet a low refractive index contrast (1.54%) with the clad material. A PhC structure composed of a triangular or square array of air holes is placed at the intersection of input and output waveguides to obtain high efficiency 90-deg bends. We find that diffraction from the boundary of the PhC region with CWG limits the optical efficiency of the bend. To overcome this we use a rigorous design tool based on a microgenetic algorithm (µGA) and a finite difference time domain (FDTD) method to optimize the boundary layer to suppress the unwanted diffraction. We find that this approach yields improvements in the bend efficiency at a wavelength of 1.55 µm from 56.2 to 92.5% (for a triangular PhC structure, TE polarization) and from 72.0 to 97.4% (square PhC structure, TM polarization).

Ultra-short waveguide polarization converter using a sub-wavelength grating

We proposed a new approach for realizing ultra-short waveguide polarization converter by employing form birefringence sub-wavelength air trench gratings slanted etched across waveguide. A rigorous and efficient design approach and a design example are presented.

Microgenetic-algorithm-based design of combined conventional waveguide and photonic crystal devices

We have proposed the hybrid integration of photonic crystals (PhCs) and conventional index-guided waveguides (CWGs) as a potentially attractive method of realizing compact waveguide elements for large-scale planar lightwave circuits (PLCs). In this paper we briefly review the properties of PhC/CWG 90° bends in low index, low index contrast waveguides and then extend them to waveguides with high index and low index contrast. We find that diffraction from the boundary of the PhC region limits the optical efficiency of the bend. To overcome this we use a rigorous design tool based on a micro-genetic algorithm (microGA) and a finite difference time domain (FDTD) method to optimize the boundary layer to suppress the unwanted diffraction. We find that this approach yields an improvement in the bend efficiency for light at 1.55 micron from 80% to 95%.