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.     

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.     

Transfer-matrix method based on a discrete coupling model for analyzing uniform and nonuniform codirectional fiber grating couplers

A simple and efficient transfer-matrix method based on a discrete coupling model is presented to analyze uniform and nonuniform fiber grating couplers between copropagating core and cladding modes. Uniform and piecewise-uniform long-period gratings were fabricated by a point-by-point arc discharge technique. Their measured transmission spectra were compared with the transmission spectra calculated by the presented method.     

Bessel-function analysis of the optimized star coupler for uniform power splitting

An optimized N x N planar optic star coupler that utilizes directional coupling of arrayed waveguides for uniform power splitting is analyzed on the basis of special properties of the involved Bessel-function series. The analysis has provided a remarkably simple, novel basic design formula for such a device with much needed physical insights into the unique diffraction properties. For the analysis of diffraction from the end of directionally coupled arrayed waveguides, many useful formulas around the Bessel functions, such as the addition theorem and the Kepler-Bessel series, have been given in new forms.     

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.     

Theoretical time-domain study of self-imaging properties in a multimode interference coupler

We report a theoretical and numerical study of self-imaging properties, including time domain and pulse spreading, caused by modal group-delay dispersion in generalized N x N multimode interference devices achieved by using a mode-propagation analysis and finite-difference time-domain method. It was found that the spatial self-imaging condition does not realize temporal self-imaging but lets waveforms separate whose shape depends on input position and input field distribution. Pulse spreading, which is sensitive to beam diameter, has a very large variation (420 fs) among input positions as well as rising to a very high 900 fs in response to a 21 fs and spatially Gaussian pulse for the conveniently smallest size with 10 channels.     

Hierarchic finite elements with selective and uniform reduced integration for Reissner-Mindlin plates

It is well known that the numerical solution of Reissner-Mindlin plates degenerates very rapidly for small thickness (locking phenomenon) when standard finite elements are used for the approximation. We have introduced a family of hierarchic high order finite elements in order to assess reliability and robustness with respect to the locking behavior. In a previous note we have given numerical results obtained with exact numerical integration. In this paper we present the results obtained with selective and uniform reduced integration. The results show that, compared with exact integration, selective reduced techniques improve the quality of the numerical performance and are preferable since computational cost is made smaller.     

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.     

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).     

Establishment of a nonuniform mode of fluidization with a uniform distribution of the flow of the cooling medium

Using the principle of the mechanical stability of a system, a criterion for the disturbance of the uniformity of fluidization is obtained and the initial properties of the nonuniform fluidization state of a layer of fine particles close to a distributed grating (the porosity of the dense phase, and the concentration and dimensions of the drops which appear) are considered.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 36, No. 3, pp. 416–425, March, 1978.     

Frequency and phase stabilization of a multimode gyrotron with megawatt power by an external signal

We have numerically studied capturing of the oscillation frequency and phase of the TE28.12 operating mode by an external monochromatic signal for a multimode gyrotron with a close to real regime of activation. It is shown that, even at a low external-signal power, in a wide range of the magnetic field and voltage, a highly efficient single-mode generating is set at the frequency of the external signal.     

A digital TDC with a reduced number of delay line cells

In nuclear physics experiments, a decision maker named as “trigger” gives a bit pattern which allows the fired detectors identification. As the data acquisition dead time is greater than the time between physical events, timing information is essential. We add to the trigger function a Time to Digital Converter (TDC) in order to make a separation between events. The paper describes the architecture chosen for the TDC and illustrates the contribution of each element to the TDC performance. An eight-bit counter is used for the dynamic range of the TDC (in microsecond) associated to a delay line improving the resolution (in nanosecond). The study shows that exploiting the two system clock states (high and low) allows to reduce the number of delay line cells. The Differential Nonlinearity Measurements are given for different resolutions (1, 2 and 5 ns) and illustrate the clock period, the clock duty cycle and the delay line contributions to the TDC performances.     

Active power decoupling with reduced converter stress for single-phase power conversion and interfacing

Single-phase DC–AC power electronic converters suffer from pulsating power at double the line frequency. The commonest practice to handle the issue is to provide a huge electrolytic capacitor for smoothening out the ripple. However, the electrolytic capacitors having short end of lifetime limit the overall lifetime of the converter. Another way of handling the ripple power is by active power decoupling (APD) using the storage devices and a set of semiconductor switches. Here, a novel topology has been proposed in implementing APD. The topology claims the benefit of (1) reduced stress on converter switches and (2) using smaller capacitance value, thus alleviating the use of electrolytic capacitor and in turn improving the lifetime of the converter. The circuit consists of a third leg, a storage capacitor and a storage inductor. The analysis and the simulation results are shown to prove the effectiveness of the topology.     

Flexible reduced graphene oxide paper with excellent electromagnetic interference shielding for terahertz wave

Flexible and lightweight shielding materials recently have attracted increasing attention for terahertz wave due to the deteriorated performance of electronic equipment interferenced by electromagnetic yielding. Herein, flexible reduced graphene oxide (rGO) papers were prepared by solution evaporation method to attenuate the electromagnetic wave in terahertz field. They exhibited excellent electrical conductivity and remarkable electromagnetic interference shielding effectiveness. The electrical conductivity of the rGO papers was improved from 54.07 to 78.67 S/cm after annealing at 400 °C. The rGO paper annealed at 400 °C was only ~?370 µm in thickness and particularly exhibited an outstanding absorption performance of 17.6 dB at 0.7 THz. It has been found that the maximum absorption performance of rGO papers was basically equivalent to that of the metamaterials. Moreover, electromagnetic interference shielding effectiveness of 72.1 dB of the rGO papers was obtained at 0.6 THz, which indicated that ~?99.99% of the power was shielded. Thus, the flexible rGO papers maybe a promising candidate for optimized electromagnetic shielding materials in the terahertz band.     

Superposition of displaced number states and interference effects

Abstract

We introduce a new state of light field by superposing two displaced number states having a phase difference between them in the phase space. The influence of this phase upon its various non-classical properties is investigated, including a discussion on the generation of this state, which contains as particular cases important states studied in the literature.     

Effect of beam sizes on the amplitude and phase of photothermal deflection signals for both uniform and nonuniform heating

A detailed theoretical treatment of a one- (1D) and three-dimensional (3D) photothermal deflection (PTD) technique is presented. Important effects of the probe beam size occur in PTD experiments when the radius of this beam is of the order of magnitude of the thermal diffusion length. The calculation of this effect is checked by experiments in paraffin oil at low modulation frequency as well as for 1D and for 3D. In this last case, we have considered two kinds of deflection: normal and transverse, and we have studied their variation for different values of the pump beam radius. The coincidence between theoretical and experimental curves confirms the validity of our theoretical model.     

Real-time ultrasound process tomography for two-phase flow imaging using a reduced number of transducers

In this paper a novel ultrasound tomography imaging system is presented. It employs a relatively small number of transducers that produce fan-shaped beam profile to effectively insonify the cross-section. The impact on the image quality due to the reduction of the number of transducers is discussed, and different approaches such as multiple receiver data acquisition and nonlinear thresholding are explored. A prototype of a tomographic imaging system with only 36 transducers has been constructed, and processing rates of up to 100 frames per second have been achieved using a parallel processing technique. Good image reconstructions based on simulations and real objects also are provided to confirm the principles of the theoretical analysis.