Shuntaro Makino

Slow-Light-Enhanced Nonlinear Characteristics in Slot Waveguides Composed of Photonic Crystal Nanobeam Cavities

We investigate slow-light-enhanced nonlinear characteristics in slot waveguides composed of photonic crystal (PC) nanobeam cavities. To evaluate the effective nonlinear coefficient, numerical simulations are performed by using 3-D vector finite-element method for periodic waveguide analysis. Results show that the maximum nonlinear coefficient of 11 600 m-1W-1 is obtained by optimizing waveguide width and height. This high optical nonlinearity is realized owing to strong light confinement at slotted geometry and small group velocity in PC nanobeam cavities. Also, we show that there exists tradeoff between the effective nonlinear coefficient and the bandwidth in the case of changing one-period length of the proposed structure.

Structural Dependence of Group Velocity and Leakage Loss in 1-D Photonic Crystal Coupled Resonator Optical Waveguide With Modulated Mode-Gap

One-dimensional photonic crystal coupled resonator optical waveguide (1-D PC-CROW) based on mode-gap confinement has a possibility to reduce leakage losses compared with conventional 1-D PC-CROW constructed by placing defects periodically. However, transmission characteristics of 1-D PC-CROW based on mode-gap confinement have not been investigated so far. In this paper, we evaluate structural dependence of group velocity and leakage losses of 1-D PC-CROW by using the 3-D vector finite element method for periodic waveguide analysis. We show that such 1-D PC-CROW can realize small group velocity and low leakage losses simultaneously by investigating dispersion relationships of 1-D PC-CROW.

Design and Fabrication of Broadband PLC-Based Two-Mode Multi/Demultiplexer Using a Wavefront Matching Method

A broadband and low-loss two-mode (LP01/LP11a) multi/demultiplexer (MUX/DEMUX) based on a planar lightwave circuit (PLC) is designed by a wavefront matching method, which is an optimization algorithm of PLC based on the beam propagation method. The calculated transmission loss of the optimized structure is lower than −0.3 dB in the wavelength range from 1400 to 1700 nm. Numerical results show that the proposed structure has large fabrication tolerance compared to the conventional structure. The designed structure is fabricated and broadband characteristics are well-reproduced, showing the usefulness of the proposed MUX/DEMUX.

Low-loss, compact, and fabrication-tolerant Si-wire 90° waveguide bend using clothoid and normal curves for large scale photonic integrated circuits

Ultimately low-loss 90° waveguide bend composed of clothoid and normal curves is proposed for dense optical interconnect photonic integrated circuits. By using clothoid curves at the input and output of 90° waveguide bend, straight and bent waveguides are smoothly connected without increasing the footprint. We found that there is an optimum ratio of clothoid curves in the bend and the bending loss can be significantly reduced compared with normal bend. 90% reduction of the bending loss for the bending radius of 4 μm is experimentally demonstrated with excellent agreement between theory and experiment. The performance is compared with the waveguide bend with offset, and the proposed bend is superior to the waveguide bend with offset in terms of fabrication tolerance.

A rigorous definition of nonlinear parameter γ and effective area A_eff for photonic crystal optical waveguides

A rigorous definition of nonlinear parameter γ for high-index-contrast (HIC) periodic optical waveguides, such as photonic crystal (PC) optical waveguides, is proposed. The definition is fully vectorial, and approximations assumed in previous works are unnecessary. The γ values calculated by the proposed definition are compared with those obtained by previous definitions, the rigorous nonlinear mode solver, and measured values for weakly guiding optical fibers, HIC Si-nanowire waveguides, and PC waveguides. It is demonstrated that the γ values calculated by proposed definition agree well with those of the rigorous nonlinear mode solver and experiments for all three waveguides, showing the validity of the definition.

Three-Dimensional Finite-Element Time-Domain Beam Propagation Method and Its Application to 1-D Photonic Crystal-Coupled Resonator Optical Waveguide

A 3-D finite-element time-domain beam propagation method is described for the first time, to the best of our knowledge, for the analysis of transmission characteristics of any kind of photonic structures. In order to avoid nonphysical reflections from computational window edges, the perfectly matched layer boundary condition is introduced. Furthermore, the present method can treat wideband optical pulses owing to the use of Padé approximation. The validity of this method is verified by numerical simulations of a photonic crystal cavity and a microring resonator. In addition, in order to demonstrate the usefulness of the developed method, a taper waveguide for a highly efficient connection between a straight waveguide and a 1-D photonic crystal-coupled resonator optical waveguide is designed.

A Metal-Assisted Silicon Slot Waveguide for Highly Sensitive Gas Detection

A metal-assisted silicon slot waveguide for highly sensitive gas detection is proposed. To show the superiority of the proposed structure, the optical confinement factor in the gas region and the sensitivity are theoretically investigated using the 2-D vector finite element method. Numerical results show that the proposed structure can achieve a strong light confinement in the gas region compared with the conventional slot waveguide. The maximal optical confinement factor of 85% is obtained in the proposed structure, which is approximately 2.5 times larger than the conventional slot waveguide. Additionally, the results show that the proposed structure has a large sensitivity compared with the conventional slot waveguide. Finally, a ring resonator based on the proposed structure is analyzed to evaluate the Q factor and to increase the validity of results obtained by 2-D simulation.

Three-Dimensional Finite-Element Mode-Solver for Nonlinear Periodic Optical Waveguides and Its Application to Photonic Crystal Waveguides

A 3-D finite-element mode-solver, based on a self-consistent algorithm, is newly developed for nonlinear periodic optical waveguides. The developed method makes it possible to calculate various nonlinear characteristics based on optical Kerr effect rigorously, including the nonlinear phase shift, the nonlinear refractive index distribution, and γ-value. Its validity is confirmed by analyzing a silicon-nanowire waveguide and a slot waveguide. To show the usefulness of the method, the nonlinear characteristics of photonic crystal line-defect waveguides and 1-D photonic crystal coupled resonator optical waveguides are investigated. Numerical results show that in some cases, γ-value calculated by the newly developed method and approximate linear analysis differs two-orders of magnitude. For instance, γ-value calculated by developed method is 510 000 m-1W -1 for a 1-D photonic crystal coupled resonator optical waveguide, whereas γ-value calculated by conventional method is only 5000 m-1W-1. This difference indicates that, to estimate the nonlinear characteristics of 3-D waveguides precisely, the developed nonlinear guided mode solver is required.

Design of a reflection-suppressed all-optical diode based on asymmetric L-shaped nonlinear photonic crystal cavity

A simple design method for suppressing the reflection of the all-optical diode based on the L-shaped photonic crystal (PC) cavity is proposed. Analyzing the linear resonant characteristics of the PC cavity and using the nonlinear coupled-mode theory, the strategy for obtaining reflection-suppressed structure is illustrated. Based on the design rule, a “reflection-suppressed all-optical diode” is presented, and numerical solutions indicate that the designed structure has a small reflection as well as an extremely large nonreciprocal transmission ratio.

Broadband LP 01 /LP 11 PLC-based mode multi/demultiplexer designed by wavefront matching method

A broadband, large fabrication tolerance LP01/LP11 PLC-based mode multi/demultiplexer design is presented by using wavefront mathing method. The low-loss bandwidth of the optimized structure is 4-times wider than that of conventional structure.