Compact silicon-on-insulator-based multimode interference coupler with bilevel taper structure

A novel compact silicon-on-insulator- (SOI-)based multimode interference (MMI) coupler with bilevel taper structures was designed. The MMI section and the S-bend sections of the input-output waveguides are deeply etched. The input-output waveguides connecting to single-mode fibers or other photonic light circuits are etched shallowly to yield single-mode operation. A bilevel taper is introduced in the transition region between the shallowly and deeply etched regions. It is predicted theoretically that this design will not only improve the quality of the self-imaging in the MMI section but will also make the structure compact. Both the excess loss and the nonuniformity of the MMI coupler are reduced. By use of a three-dimensional beam propagation method, the performance of a 1 x 4 MMI coupler based on a SOI is simulated as a numerical example of the novel design. The simulated nonuniformity and the excess loss are approximately 0.0285 and 0.2 dB, respectively.     

Electro-optical logic application of multimode interference coupler by multivalued controlling

Electro-optical hybrid logic is a potential solution to implement both electrical and optical signal processing, which receives analog or digital, electrical or optical signals and produces logic signals in a desired manner. In light of the transfer matrix theory, we found that one can steer light into different output ports of a multimode interference coupler by controlling the phases in a multivalued manner on the image-extended arms. This implementation acts as an analog-to-digital convertor from electric domain to optical domain. Also, an electrical-to-optical 2-to-2(2) binary-coded decoder is described and examined by the 3D beam propagation method.     

Design of an ultrashort Si-nanowaveguide-based multimode interference coupler of arbitrary shape

A design procedure for an arbitrarily tapered multimode interference (MMI) coupler based on Si nanowaveguides is presented. First a series of the effective indices of the zeroth and first eigenmodes in multimode waveguides are obtained as the core width increases by using a full-vectorial finite-difference method. Two polynomial functions are used to fit the two relations between the effective indices and the core width. The phase difference Delta phi between the zeroth and first eigenmodes can then be easily calculated when the light goes through any given arbitrarily tapered MMI section. By making the phase difference Delta phi equal to a certain value Delta phi 0 required for an N-fold self-imaging, the length of a MMI coupler is determined. With the present design procedure, an ultrashort 2 x 2 parabolic MMI coupler is designed as an example. The size of the designed ultrashort MMI section is only approximately 1.4 microm x 4.7 microm.     

Three-dimensional power splitter based on self-imaging effect in multimode layer-by-layer photonic crystal waveguides

The self-imaging effect based on symmetrical interference in multimode layer-by-layer photonic crystal waveguides (PhCWs), is numerically studied with finite-difference time-domain simulations. With the properties of twofold images, a kind of three-dimensional (3D) PhCW-based power splitters with an ultracompact size using complete photonic bandgaps is proposed, calculated, and analyzed. The presented structure can be extended for the design of M×N power splitters for 3D photonic integrated circuits applications.     

Fabrication of a concentric-circular focusing grating coupler by a conic-wave-front interference method and light-convergence experiments using the coupler

An interference method utilizing conic-wave-front light for the fabrication of a concentric-circular and chirped grating is proposed. The design method and fabrication of an interference lens that generates conic-wave-front light are also shown. A focusing element is constructed from a concentric-circular grating coupler with a 0.4-mm diameter and a concentric-circular focusing grating coupler with an annular aperture of 2-mm focal length and 2.0-4.0-mm diameter. Light-convergence experiments using the focusing element were able to obtain a focusing spot of 0.5 μm × 0.7 μm at half-intensity widths for a wavelength of 820 nm in combination with liquid-crystal polarization elements.     

Double S-bend structure for a compact two mode interference coupler

A double S-bend structure has been proposed for the reduction of coupling length of the 2 × 2 two mode interference (TMI) coupler. The coupling characteristics of the proposed structure are compared with those of conventional TMI structures using a simple mathematical model based on sinusoidal modes. The longitudinal beat length for the proposed TMI coupler is reduced by 15% of the beat length of a conventional TMI coupler. The effect of power imbalance on fabrication tolerance of the proposed TMI coupler is also studied and compared with other TMI couplers.     

Analytical characteristics of stimulated Raman scattering in a multimode fiber obtained with an optical time-domain reflectometer

The nonlinear behavior of stimulated Raman scattering (SRS) in a low-loss optical fiber has been analytically and experimentally investigated by the assumption of certain restraint conditions. As a result, it has been found that the SRS light propagating along the optical fiber is expressed in a simple formula by the introduction of nonlinear parameters, which are experimentally identified and whose maximum value is restricted by the restraint conditions. The parameters, moreover, are available for compensating the SRS intensity in the presence of noninteracting light. Therefore the formula for SRS intensity presented is useful for investigating the SRS effect in a multimode fiber.     

Measuring a spectral bandpass of a fibre-optic spectrometer using time-domain and spectral-domain two-beam interference

Abstract

Spectral bandpasses of a miniature spectrometer with three read fibres of different core diameters are obtained using the visibility functions for both spatial and spectral fringes resolved in the interference of two light beams from a laser diode operated below the threshold. From a width of the central peak of the visibility function for the spatial interference fringes measured in the Michelson interferometer configuration with a broadband detector, the source spectral width is evaluated. From widths of the visibility functions for the spectral interference fringes measured in the Michelson interferometer configuration by the spectrometer with the different read fibres, the overall spectral bandpasses are evaluated. Subtracting the effect of the source spectral width, the spectral bandpasses of the fibre-optic spectrometer are determined. These are compared with the directly measured bandpasses using the delta-function spectrum of the same laser diode operated far above the threshold.     

Two-dimensional model for three-dimensional index-guided multimode plasmonic waveguides and the design of ultrasmall multimode interference splitters

We demonstrate that a three-dimensional (3D) index-guided multimode plasmonic waveguide can be approximated to a two-dimensional (2D) lossy slab waveguide by using an effective-index method. It is found that this 2D approximation is more accurate when the width of the multimode waveguide increases. Such a 2D approximation can be used for a quicker and more efficient design of complicated multimode plasmonic devices. 1 x N ultrasmall multimode interference splitters based on multimode surface plasmon waveguides are designed by using this 2D model and the designs are validated with a 3D finite-difference time-domain method.     

Design of phased-array wavelength division multiplexers using multimode interference couplers

Novel designs for phased-array wavelength-division multiplexers based on self-imaging properties of multimode interference (MMI) couplers are presented. These devices, which operate on N equally spaced wavelength channels, consist of two MMI couplers connected by an array of N monomode waveguides. The MMI couplers function as power splitters/combiners, and the waveguide array is the dispersive element. The excellent characteristics of MMI couplers offer the possibility of designing small-size devices with low loss and with high uniformity among different channels. A general theoretical formulation for an N-channel multiplexer is presented, and a simple procedure for finding an optimum set of lengths for the array guides is given. We show that these multiplexers can function as N x N wavelength-selective interconnecting components. The simulated performance of three variations of a five-channel device, designed in a rib waveguide system, is given. It is demonstrated that sidelobes in the multiplexer spectral response can be suppressed by weighting the power samples in the array waveguides through appropriate design of a nonuniform MMI power splitter.     

Scanning near-field optical microscopy as a tool for the characterization of multimode interference devices

We report the scanning near-field optical microscopy (SNOM) characterization of a 4 x 4 multimode interference (MMI) device working at a wavelength of 1.55 microm and designed for astronomical signal recombination. A comprehensive analysis of the mapped propagating field is presented. We compare SNOM measurements with beam-propagation-method simulations and thus are able to determine the MMI structure's refractive-index contrast and show that the measured value is higher than the expected value. Further investigation allows us to demonstrate that good care must be taken with the refractive-index profile used in simulation when one deals with low-index contrast structures. We show evidence that a step-index contrast is not suitable for adequate simulation of our structure and present a model that permits good agreement between measured and simulated propagating fields.     

轮毂轴承单元过盈量理论设计及试验研究

针对第3代轿车轮毂轴承单元过盈量的设计,提出了一种考虑铆接装配影响的过盈量理论设计方法.首先,基于弹性力学和平面应力假设推导了轮毂轴承单元内圈与内法兰轴配合的过盈量理论计算式;然后,采用有限元软件建立了轮毂轴承单元过盈装配、铆接装配及铆接卸载回弹过程仿真模型,并通过数值模拟分析获取了其过盈配合面的压力分布及其轴向预紧力...     

Finite-difference time-domain numerical simulation study on the optical properties of silver nanocomposites

The effects of the nanoparticle geometry and the host matrix on the optical properties of silver (Ag) nanocomposites were investigated. The spatial intensity distribution and absorption spectra were obtained by solving Maxwell equations using the finite-difference time-domain method. Local enhancement of the optical field was produced near the surface of the Ag nanoparticle. As the nanoparticle size increased, the plasmon-induced absorption increased and the surface plasmon resonance (SPR) wavelength of the Ag nanocomposite was redshifted. As the nanoparticle geometry was transformed from a sphere to an ellipsoid, two plasmon peaks appeared and their spectral spacing became larger with increasing the aspect ratio. The effects of the nanoparticle size and the anisotropic geometry on the optical properties of the Ag nanocomposites can be described by the Maxwell-Garnett theory and the Drude model. From the absorption spectra of the Ag nanocomposites with five different host matrices (SiO2, Al2O3, ZnO, ZrO2, and TiO2), it was found that the SPR wavelength of the Ag nanocomposite was redshifted with increasing the refractive index of the host matrix.     

Synthesis of Hadamard transformers by use of multimode interference optical waveguides

We propose a synthesis method of optical Hadamard transformer using multimode interference (MMI) couplers. By using the signal transfer matrix of 2 x 2, 4 x 4, and 8 x 8 MMI couplers, we show that sum and difference units of input signals can be synthesized. An interchange unit of two signals can also be synthesized. One synthesis method of Hadamard transformers is a combination of only 2 x 2 units, and the other is a combination of N x N(N > or = 4) units as well as 2 x 2 units. The design examples of operation units are shown, and the size and the output power of Hadamard transformers are estimated.     

Novel approach for design of low index contrast multimode interference devices

Self-imaging theory is widely accepted as a good method in designing multimode interference (MMI) couplers, but it is also true that self-imaging theory is not suitable for low-contrast structures. An improved self-imaging theory is proposed in this paper for the optimal design of low-contrast 1 × N MMI couplers. The average effective width of the MMI waveguide and the average effective propagation constant of the MMI waveguide are used in the improved self-imaging theory. An approach is given to find the average effective width. We use this approach in the optimal design of a 1?×?4 silica MMI coupler, and the results show that the improved self-imaging theory is more accurate than conventional self-imaging theory for low-contrast structures.     

An integrated fluorescence array as a platform for lab-on-a-chip technology using multimode interference splitters

We present the design and fabrication of 1-to-N multimode interference (MMI) splitters, suitable for use in integrated optical fluorescence array sensing, with particular applications in lab-on-a-chip (micro-TAS) technologies. Electron beam irradiation of germanium-doped flame hydrolysis deposited silica was used to define the MMI waveguide regions. The splitters were integrated with microfluidic channels to form direct-excitation fluorescence sensor chips for use at visible wavelengths. Characterization of the waveguides shows that predictable splitting ratios can be achieved. Two devices are presented: a 1/spl times/2 splitter integrated with one analytical chamber and a 1/spl times/4 array device for multipoint excitation. A photomultiplier tube was used to assess the analytical performance of the chip, in response to standard aliquots of fluorophore (31 nM to 1.25 /spl mu/M).     

Design and fabrication of an optical interleaver with cascaded multimode interference couplers

A lattice-form optical interleaver was designed and fabricated with a silicon-based silica waveguide. Cascaded multimode interference couplers were first employed in the lattice circuit and helped to relax the fabrication tolerance. The device shows good performance, the insertion loss is less than 2.25 dB, the passband ripple is less than 0.15 dB, the cross talk is less than -18.9 dB, and the 0.5 dB passband is more than 100 GHz.