Yuhei Ishizaka

Design of a Compact Two-Mode Multi/Demultiplexer Consisting of Multimode Interference Waveguides and a Wavelength-Insensitive Phase Shifter for Mode-Division Multiplexing Transmission

A compact two-mode (de)multiplexer (TM-MUX) based on Si nanowire for mode-division multiplexing is designed. The TM-MUX is composed of two multimode interference (MMI) waveguides and a butterfly-shape tapered phase shifter between two MMI waveguides. Numerical simulations show that the designed device is compact compared with conventional-mode (de)multiplexer and operates as a TM-MUX in the whole C-band. In addition, we show that the designed TM-MUX has relatively good fabrication tolerance.

Design of Optical XOR, XNOR, NAND, and OR Logic Gates Based on Multi-Mode Interference Waveguides for Binary-Phase-Shift-Keyed Signal

We present a novel design method and potential application for optical XOR, XNOR, NAND, and OR logic gates for binary-phase-shift-keyed signal processing devices. The devices are composed of multi-mode interference waveguides and convert the phase information of the input signal to amplitude at the output. We apply the finite element method for numerical simulations, and the evaluated least ON to OFF logic-level contrast ratios for the XOR, XNOR, NAND, and OR logic gates are 21.5 dB, 21.5 dB, 22.3 dB, and 22.3 dB, respectively. The proposed logic gates are extremely promising signal processing devices for binary-phase-shift-keyed signals in packet switching systems.

PLC-based LP 11 mode rotator for mode-division multiplexing transmission

A PLC-based LP11 mode rotator is proposed. The proposed mode rotator is composed of a waveguide with a trench that provides asymmetry of the waveguide. Numerical simulations show that converting LP11a (LP11b) mode to LP11b (LP11a) mode can be achieved with high conversion efficiency (more than 90%) and little polarization dependence over a wide wavelength range from 1450 nm to 1650 nm. In addition, we fabricate the proposed LP11 mode rotator using silica-based PLC. It is confirmed that the fabricated mode rotator can convert LP11a mode to LP11b mode over a wide wavelength range.

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.

Three-Dimensional Finite-Element Solutions for Crossing Slot-Waveguides With Finite Core-Height

A three-dimensional vectorial finite element method (3-D-VFEM) for waveguide discontinuity problems is newly formulated and implemented. Through the use of the 3-D-VFEM, transmission characteristics of two types of crossing slot-waveguides with finite core-height are investigated for the first time. One is a direct crossing and the other is a crossing filling up the slots locally. Results show that three-dimensional analysis considering the finite core-height is indispensable.

Demonstration of PLC-based six-mode multiplexer for mode division multiplexing transmission

We demonstrated a six-spatial-mode multiplexer using effective index matching and adiabatic mode transition. Six-mode multiplexing including the equal effective indices of LP21b and LP02 was successfully achieved with a fabricated PLC on one chip.

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.

Ultra-Broadband Silicon-Wire Polarization Beam Combiner/Splitter Based on a Wavelength Insensitive Coupler With a Point-Symmetrical Configuration

An ultrabroadband silicon wire polarization beam combiner/splitter (PBCS) based on a wavelength-insensitive coupler is proposed. The proposed PBCS consists of three identical directional couplers and two identical delay lines. We design the PBCS using the 3-D finite element method. Numerical simulations show that the proposed PBCS can achieve the transmittance of more than 90% over a wide wavelength range from 1450 to 1650 nm for both TE and TM polarized modes.

Transmission-Efficient Structures of Bent and Crossing Silicon Slot Waveguides

We present transmission-efficient bent and crossing silicon slot waveguides for compact silicon slot waveguide-based circuits. To begin with, using the 3-D vector finite-element method, we investigate the transmission characteristics of a bent silicon slot waveguide with an asymmetric slot that is connected to straight waveguides. Numerical results show that the transmission efficiency can be improved by optimizing a taper structure that is introduced between the bend of an asymmetric slot waveguide and its straight input/output waveguides. Next, we investigate the transmission characteristics of a 3-D crossing silicon slot waveguide that has a vertical coupler. Numerical results show that our crossing structure can achieve a high transmission efficiency compared with conventional structures.