18 Gchips/s electronically-processed OCDMA spread spectrum for access networks

We demonstrate for the first time coding/decoding for OCDMA networks using electronic transversal filters at 18Gchips/s-a ten-fold improvement over previous demonstrations. The chip rate allows users at Gb/s rates in access applications. © 2007 Optical Society of America.

10 Gb/s transmitter-based equalization for extended-reach multimode-fiber datacommunication links

Transmitter-based equalization is investigated for enhanced performance in 10 Gb/s multimodefiber links. Rigorous simulations and proof-of-principle experiments over 500 m of FDDI-grade fiber confirm for the first time the potential superiority of the technique relative to receiver-based schemes. © 2007 Optical Society of America.

Doping dependence of the Raman spectrum of defected graphene.

We investigate the evolution of the Raman spectrum of defected graphene as a function of doping. Polymer electrolyte gating allows us to move the Fermi level up to 0.7 eV, as directly monitored by in situ Hall-effect measurements. For a given number of defects, we find that the intensities of the D and D' peaks decrease with increasing doping. We assign this to an increased total scattering rate of the photoexcited electrons and holes, due to the doping-dependent strength of electron-electron scattering. We present a general relation between D peak intensity and defects valid for any doping level.

Non-compressive wideband spectrum sensing with sub-Nyquist sampling rates

In this paper, we propose a low complexity and reliable wideband spectrum sensing technique that operates at sub-Nyquist sampling rates. Unlike the majority of other sub-Nyquist spectrum sensing algorithms that rely on the Compressive Sensing (CS) methodology, the introduced method does not entail solving an optimisation problem. It is characterised by simplicity and low computational complexity without compromising the system performance and yet delivers substantial reductions on the operational sampling rates. The reliability guidelines of the devised non-compressive sensing approach are provided and simulations are presented to illustrate its superior performance. © 2013 IEEE.

Fourier spectrum based extraction of an equivalent trap state density in indium gallium zinc oxide transistors

Segregating the dynamics of gate bias induced threshold voltage shift, and in particular, charge trapping in thin film transistors (TFTs) based on time constants provides insight into the different mechanisms underlying TFTs instability. In this Letter we develop a representation of the time constants and model the magnitude of charge trapped in the form of an equivalent density of created trap states. This representation is extracted from the Fourier spectrum of the dynamics of charge trapping. Using amorphous In-Ga-Zn-O TFTs as an example, the charge trapping was modeled within an energy range of ΔEt 0.3 eV and with a density of state distribution as Dt(Et-j)=Dt0exp(-ΔEt/ kT)with Dt0 = 5.02 × 1011 cm-2 eV-1. Such a model is useful for developing simulation tools for circuit design. © 2014 AIP Publishing LLC.

Classical two-site fidelity and quantum phase transitions in the Ising model and the extended Hubbard model

In this Letter, the classical two-site-ground-state fidelity (CTGF) is exploited to identify quantum phase transitions (QPTs) for the transverse field Ising model (TFIM) and the one-dimensional extended Hubbard model (EHM). Our results show that the CTGF exhibits an abrupt change around the regions of criticality and can be used to identify QPTs in spin and fermionic systems. The method is especially convenient when it is connected with the density-matrix renormalization group (DMRG) algorithm. (C) 2008 Elsevier B.V. All rights reserved.

GaAs-Based Metamorphic Long-Wavelength InAs Quantum Dots Grown by Molecular Beam Epitaxy

A bilayer stacked InAs/GaAs quantum dot structure grown by molecular beam epitaxy on an In0.05Ga0.95As metamorphic buffer is investigated. By introducing a InGaAs Sb cover layer on the upper InAs quantum dots (QDs) layers, the emission wavelength of the QDs is extended successfully to 1.533 mu m at room temperature, and the density of the QDs is in the range of 4 x 10(9) -8 x 10(9) cm(-2). Strong photoluminescence (PL) intensity with a full width at half maximum of 28.6 meV of the PL spectrum shows good optical quality of the bilayer QDs. The growth of bilayer QDs on metamorphic buffers offers a useful way to extend the wavelengths of GaAs-based materials for potential applications in optoelectronic and quantum functional devices.

Experimental Demonstration on Structure-Parameter Dependence of Photonic Crystal Optical Spectrum

We present fabrication and experimental measurement of a series of photonic crystal waveguides and coupled structure of PC waveguide and PC micro-cavity. The complete devices consist of an injector taper down from 3 mu m into a triangular-lattice air-holes single-line-defect waveguide. We fabricated these devices on a silicon-on-insulator substrate and characterized them using tunable laser source. We've obtained high-efficiency light propagation and broad flat spectrum response of photonic-crystal waveguides. A sharp attenuation at photonic crystal waveguide mode edge was observed for most structures. The edge of guided band is shifted about 31 nm with the 10 nm increase of lattice constant. Mode resonance was observed in coupled structure. Our experimental results indicate that the optical spectra of photonic crystal are very sensitive to structure parameters.

Fourier series expansion method for gain measurement from amplified spontaneous emission spectra of Fabry-Perot semiconductor lasers

A gain measurement technique, based on Fourier series expansion of periodically extended single fringe of the amplified spontaneous emission spectrum, is proposed for Fabry-Perot semiconductor lasers. The underestimation of gain due to the limited resolution of the measurement system is corrected by a factor related to the system response function. The standard deviations of the gain-reflectivity product under low noise conditions are analyzed for the Fourier series expansion method and compared with those of the Hakki-Paoli method and Cassidy's method. The results show that the Fourier series expansion method is the least sensitive to noise among the three methods. The experiment results obtained by the three methods are also presented and compared.

Calculation of the current noise spectrum in mesoscopic transport: A quantum master equation approach

Based on our recent work on quantum transport [X. Q. Li , Phys. Rev. B 71, 205304 (2005)], we show how an efficient calculation can be performed for the current noise spectrum. Compared to the classical rate equation or the quantum trajectory method, the proposed approach is capable of tackling both the many-body Coulomb interaction and quantum coherence on an equal footing. The practical applications are illustrated by transport through quantum dots. We find that this alternative approach is in a certain sense simpler and more straightforward than the well-known Landauer-Buttiker scattering matrix theory.

Measurement of chirp parameter and modulation index of a semiconductor laser based on optical spectrum analysis

A novel and simple method for measuring the chirp parameter, frequency, and intensity modulation indexes of directly modulated lasers is proposed in a small-signal modulation scheme. A graphical approach is presented. An analytical solution to the measurement of low chirp parameters is also given. The measured results agree well with those obtained using the conventional methods.

Optimization of mechanically induced long-period fibre grating transmission spectrum

A new double-layer grating template is designed to reduce the out-of-band loss as much as 1.8dB when the loss of LP03 reaches 10.2 dB. Meanwhile, we propose a method to remove the sidelobes in the transmission spectra by the adjustment of the thickness of pressure plates. The plate-thickness-induced shift of resonant wavelength and the attenuation of loss peak intensity when removing sidelobes can be modified by the fibre distance and contact point on the pressure plates.

Biaxial stress dependence of the electrostimulated near-band-gap spectrum of GaN epitaxial film grown on (0001) sapphire substrate

The biaxial piezospectroscopic coefficient (i.e., the rate of spectral shift with stress) of the electrostimulated near-band-gap luminescence of gallium nitride (GaN) was determined as Pi=-25.8 +/- 0.2 meV/GPa. A controlled biaxial stress field was applied on a hexagonal GaN film, epitaxially grown on (0001) sapphire using a ball-on-ring biaxial bending jig, and the spectral shift of the electrostimulated near-band-gap was measured in situ in the scanning electron microscope. This calibration method can be useful to overcome the lack of a bulk crystal of relatively large size for more conventional uniaxial bending calibrations, which has so far hampered the precise determination of the piezospectroscopic coefficient of GaN. The main source of error involved with the present calibration method is represented by the selection of appropriate values for the elastic stiffness constants of both film and substrate. The ball-on-ring calibration method can be generally applied to directly determine the biaxial-stress dependence of selected cathodoluminescence bands of epilayer/substrate materials without requiring separation of the film from the substrate. (c) 2006 American Institute of Physics.

Comparison of valence band x-ray photoelectron spectrum between Al-N-codoped and N-doped ZnO films

The valence band structures of Al-N-codoped [ZnO:(Al, N)] and N-doped (ZnO:N) ZnO films were studied by normal and soft x-ray photoelectron spectroscopy. The valence-band maximum of ZnO:(Al, N) shifts up to Fermi energy level by about 300 meV compared with that of ZnO:N. Such a shift can be attributed to the existence of a kind of Al-N in ZnO:(Al, N), as supported by core level XPS spectra and comparison of modified Auger parameters. Al-N increased the relative quantity of Zn-N in ZnO:(Al, N), while N-N decreased that of Zn-N in ZnO:N. (c) 2006 American Institute of Physics.

Measurement of gain spectrum for semiconductor lasers utilizing integrations of product of emission spectrum and a phase function over one mode interval

A technique based on the integrations of the product of amplified spontaneous emission spectrum and a phase function over one mode interval is proposed for measuring gain spectrum for Fabry-Perot semiconductor lasers, and a gain correction factor related to the response function of the optical spectrum analyzer (OSA) is obtained for improving the accuracy of measured gain spectrum. The gain spectra with a difference less than 1.3 cm(-1) from 1500 to 1600 nm are obtained for a 250-mum-long semiconductor laser at the OSA resolution of 0.06, 0.1, 0.2, and 0.5 nm. The corresponding gain correction factor is about 9 cm(-1) at the resolution of 0.5 nm. The gain spectrum measured at the resolution of 0.5 nm has the same accuracy as that obtained by the Hakki-Paoli method at the resolution of 0.06 nm for the laser with the mode interval of 1.3 nm.