Dynamic Mechanical Properties and Modified Material Constitutive Model for Hot Forged Ti2AlNb over Wide Ranges of Temperature and Strain Rate.

In this paper, the stress-strain curves of Ti2AlNb are established based on uniaxial impact tests over wide ranges of temperature and strain rate. The Ti2AlNb exhibited the work hardening effect but did not show an obvious yield stage during a quasi-static compression test. In the SHPB test, an obvious temperature softening effect was found, the strain rate strengthening effect was detected when the strain rate was 4000-8000 s-1, and the strain rate softening effect was detected in the range of 8000-12,000 s-1. A function describing the effect of strain rate on the strain rate strengthening parameters under various temperatures was proposed to modify the basic J-C constitutive model. The relative errors between the experimental measured value and predicted values in various experimental conditions with a modified J-C model were less than 5.0%. The results verified that the modified J-C model could accurately describe the dynamic mechanical properties of Ti2AlNb at high temperatures and strain rates. The research could help to illustrate the cutting mechanism and finite element simulation of Ti2AlNb alloy.

Automatic Epileptic Seizure Detection Using Graph-Regularized Non-Negative Matrix Factorization and Kernel-Based Robust Probabilistic Collaborative Representation.

Automatic seizure detection system can serve as a meaningful clinical tool for the treatment and analysis of epilepsy using electroencephalogram (EEG) and has obtained rapid development. An automatic detection of epileptic seizure method based on kernel-based robust probabilistic collaborative representation (ProCRC) combined with graph-regularized non-negative matrix factorization (GNMF) is proposed in this work. The raw EEG signals are pre-processed through the wavelet transform to obtain time-frequency distribution of EEG signals as preliminary feature information and GNMF is further employed for dimension reduction, retaining and enhancing the productive feature information of EEG signals. Then, the test sample is represented using robust ProCRC that can decide whether the testing sample belongs to each class (seizure or non-seizure) by jointly maximizing the likelihood. In addition, the kernel trick is applied to improve the separability of non-linear high dimensional EEG signals in robust ProCRC. Finally, post-processing techniques are introduced to generate more accurate and reliable results. The average epoch-based sensitivity of 96.48%, event-based sensitivity of 93.65% and specificity of 98.55% are acquired in this method, which is evaluated on the public Freiburg EEG database.

Compact polarization splitter based on a silicon angled multimode interferometer structure.

The difference between transverse electric (TE) and transverse magnetic (TM) mode-effective indices in a wave-guided angled multimode interferometer structure is found to produce practical polarization splitting (PS) in the silicon-on-insulator platform at 1550 nm. Simulations show that this PS offers competitive performance in low insertion loss (0.4 dB for TE and 0.8 dB for TM), high extinction ratio (ER) (27.6 dB for TE and 26.5 dB for TM), low cross talk (-27.3 dB for TE and -28.0 dB for TM), and a 53-nm bandwidth for ER>20 dB. The compact footprint (∼25 µm2), the identical single-mode input/output waveguides for integration without altering the cross section, and the simplicity in implementation are prominent advantages compared with prior art designs.

Electro-optical phase-change 2 × 2 switching using three- and four-waveguide directional couplers.

Theoretical modeling and numerical simulation have been performed at λ=2100 nm on silicon-on-insulator channel-waveguide directional couplers in which the outer two Si waveguides are passive and the central waveguide(s) are electro-optical (EO) "islands." The EO channel(s) utilize a 10 nm layer of Ge2Sb2Te5 phase-change-material sited at midlevel of a doped Si channel. A voltage-driven phase change produces a large change in the effective index of the TE(o) and TM(o) modes, thereby inducing crossbar 2×2 switching. A mode-matching method is employed to estimate EO switching performance in the limit of strong interguide coupling. Low-loss switching is predicted for cross-to-bar and bar-to-cross coupling lengths. These "self-holding" switches had active lengths of 500-1000 µm, which are shorter than those in couplers relying upon free-carrier injection. The four-waveguide devices had lower cross talk but higher loss than the three-waveguide devices. For the crystalline phase we sometimes used an active length that was smaller than that for the amorphous phase.

Electro-optical switching at 1550 nm using a two-state GeSe phase-change layer.

New designs for electro-optical free-space and waveguided 2 x 2 switches are presented and analyzed at the 1.55 µm telecoms wavelength. The proposed devices employ a ~10 nm film of GeSe that is electrically actuated to transition the layer forth-and-back from the amorphous to the crystal phase, yielding a switch with two self-sustaining states. This phase change material was selected for its very low absorption loss at the operation wavelength, along with its electro-refraction Δn ~0.6. All switches are cascadeable into N x M devices. The free-space prism-shaped structures use III-V prism material to match the GeSe crystal index. The Si/GeSe/Si "active waveguides" are quite suitable for directional-coupler switches as well as Mach-Zehnder devices-all of which have an active length 16x less than that in the free-carrier art.

Electrically actuated phase-change pixels for transmissive and reflective spatial light modulators in the near and mid infrared.

Transmissive and reflective spatial light modulators have been designed and simulated for the 1.55 to 2.10 µm spectral region. An electrically actuated layer of phase-change material (PCM) was employed as the electro-optical medium for two-state self-holding "light-to-dark" intensity modulation of free-space light beams. The PCM was sandwiched between transparent conductive N-doped Si or indium tin oxide contact layers in a simple planar structure. A 100 to 500 nm PCM layer of Ge2Sb2Te5 (GST) was employed for optimum performance at 1.55 µm where the transmissive-modulator insertion loss was around 4.5 dB. The GST light-dark contrast was found to be 32 dB. For the GST reflection device, an included metal film (Ag) improved the 1.55 µm performance metrics to 0.7 dB of insertion loss with a contrast around 26 dB. The calculated performance for both types of spatial light modulators was robust to changes in the input incidence angle near normal incidence. Applications include infrared scene generation and signal processing.

Long range mid-infrared propagation in Si and Ge hybrid plasmonic-photonic nano-ribbon waveguides.

We have investigated a hybrid plasmonic-photonic mode in Si and Ge channel waveguides over the 1.55-8.0 µm wavelength range. A 10-nm Cu ribbon was buried midway within a Si3N4 "photonic slot" centered in the semiconductor strip. For the TMo mode, propagation lengths L of several millimeters are predicted for a waveguide cross-section of about 0.7λ/n x 0.7λ/n which offers optical confinement mainly within the ~λ²/400-area slot. The L increased strongly with λ. For 0.4λ/n x 0.4λ/n channels, we found multi-centimeter propagation, but there ~60% of the propagating energy had leaked out into the thick, all-around Si3N4 cladding.

Insights into complex Berenger modes: a view from the weighted optical path distance perspective.

We present a simple and efficient approach for higher-order Berenger mode computation. We establish the physical mapping between radiation modes and complex Berenger modes, and theoretically prove that the higher-order substrate Berenger modes and cladding Berenger modes can converge to a cluster of complex modes with the same phase angle. This model can be explained by weighted optical path distance in both cladding and substrate, and can be implemented by adjusting parameters of perfectly matched layers. A germanium (Ge) photodetector is utilized to evaluate the merits of this method in terms of robustness, efficiency, and accuracy.

Generation of two-cycle pulses and octave-spanning frequency combs in a dispersion-flattened micro-resonator.

We show that octave-spanning Kerr frequency combs with improved spectral flatness of comb lines can be generated in dispersion-flattened microring resonators. The resonator is formed by a strip/slot hybrid waveguide, exhibiting a flat and low anomalous dispersion between two zero-dispersion wavelengths that are separated by one octave from near-infrared to mid-infrared. Such flattened dispersion profiles allow for the generation of mode-locked frequency combs, using relatively low pump power to obtain two-cycle cavity solitons on a chip, associated with the octave-spanning comb bandwidth. The wavelength dependence of the optical loss and of the coupling coefficient and thus wavelength dependent Q-factor are also considered.

Complex coupled-mode theory for tapered optical waveguides.

A coupled-mode formulation based on complex local modes is developed for tapered and longitudinally varying optical waveguides. Different from the conventional coupled-mode theory that requires integration over the entire spectrum of radiation modes, the new formulation treats the radiation fields via discrete complex modes similarly to the guided modes. Accuracy, convergence, and scope of validity for the solutions of the complex coupled-mode equations are investigated in detail for a typical single-mode waveguide taper. It is demonstrated that the complex coupled-mode theory has overcome the difficulties of the conventional theory in simulation of radiation field effects while preserving the simplicity and intuitiveness of this popular method.

Compact Bragg grating with embedded metallic nano-structures.

A compact Bragg grating with embedded gapped metallic nano-structures is proposed and investigated theoretically. The Bragg grating consists of periodic planar metallic strips embedded in a dielectric waveguide. The grating exhibits distinct polarization characteristics due to its underlying working mechanisms of the metallic nano-strips. The grating can be considered as insulator-metal-insulator surface plasmonic polariton waveguide grating with improved light confinement for TM polarized waves. For the TE waves, significant field mismatch between metal and non-metal sections of the grating results in strong reflection. Comparison with the conventional deeply-etched grating on the same waveguide structures reveals interesting characteristics. It is concluded that the two types of grating structures share similar guidance, reflection and loss mechanisms for the TE modes. The spectral characteristics and their dependences on grating duty cycle are drastically different for the TM modes, mainly due to the SPP effect for the metal. Although the proposed grating performs slightly worse comparing to the deeply-etched grating for TE waves, its fabrication process should be easier since there will be no narrow trench (in sub-microns) deep-etching process (up to a few microns in depth) involved.

An optical power combiner/wavelength demultiplexing module for hybrid WDM FTTX.

An optical device scheme that serves simultaneously as a power combiner for upstream and wavelength demultiplexer for downstream signals is presented. The design concept is validated experimentally by an optical module based on off-the-shelf discrete optical components. An integrated device based on planar lightwave circuit (PLC) is proposed and analyzed in which a multi-mode interference (MMI) device is utilized to separate the upstream 1310 nm signal from the downstream 155x nm signals. The dense WDM function is realized through an arrayed-waveguide-grating (AWG). Design guidelines and optimization procedure for the device are discussed by way of examples.

Complex coupled-mode theory for optical waveguides.

A coupled-mode formulation is described in which the radiation fields are represented in terms of discrete complex modes. The complex modes are obtained from a waveguide model facilitated by the combination of perfectly matched boundary (PML) and perfectly reflecting boundary (PRB) condition. By proper choice of the PML parameters, the guided modes of the structure remain unchanged, whereas the continuous radiation modes are discretized into orthogonal and normalizable complex quasi-leaky and PML modes. The complex coupled-mode formulation is identical to that for waveguides with loss and/or gain and can be solved by similar analytical and numerical techniques. By identifying the phase-matching conditions between the complex modes, the coupled mode formulation may be further simplified to yield analytical solutions. The complex coupled-mode theory is applied to Bragg grating in slab waveguides and validated by rigorous mode-matching method. It is for the first time that we can treat guided and radiation field in a unified and straightforward fashion without having to resort to cumbersome radiation modes. Highly accurate and insightful results are obtained with consideration of only the nearly phase-matched modes.

Simulation of three-dimensional waveguide discontinuities by a full-vector mode-matching method based on finite-difference schemes.

A rigorous full-vector analysis based on the finite-difference mode-matching method is presented for three-dimensional optical wave propagation problems. The computation model is facilitated by a perfectly matched layer (PML) terminated with a perfectly reflecting boundary condition (PRB). The complex modes including both the guided and the radiation fields of the three-dimensional waveguide with arbitrary index profiles are computed by a finite-difference scheme. The method is applied to and validated by the analysis of the facet reflectivity of a buried waveguide and the power exchange of a periodically loaded dielectric waveguide polarization rotator.