提升啁啾脉冲激光信噪比的扫描滤波方法

基于扫描滤波原理, 提出了一种新的用以提升啁啾脉冲激光信噪比(signal-to-noise, SNR)的光学扫描滤波方法. 针对内置电光晶体的Fabry-Perot标准具的扫描滤波方案, 定量分析了其扫描滤波谱特性, 详细讨论了Fabry-Perot标准具平行平板的镜面反射率和内置电光晶体类型对输出脉冲信噪比的影响, 并进一步探讨了控制电压变化对信噪比提升效果的影响. 结果表明, 扫描滤波器的透射窗口带宽越窄, 信噪比的提升效果越好; 为了保证输出效果, Fabry-Perot标准具平行平板反射率应大于0.9; 对于内置二次电光晶体钽铌酸钾晶体的Fabry-Perot标准具扫描滤波方案, 其外加电压较内置普通线性电光晶体的方案更低, 且更易于控制; 钽铌酸钾晶体构成组分的不同对滤波器效果的影响并不大, 而控制电压和信号光啁啾率的变化则会对滤波效果产生较大影响.     

Methods of filtering densities reconstructed from line projections

We present details important for practical applications of a new algorithm for filtering densities reconstructed from their line projections. The method is applied to densities reconstructed via the Cormack method from both model projections and 2D angular correlation of positron annihilation radiation spectra in CeIn₃ and ErGa₃. It is shown that this algorithm filters the experimental noise as well as enhances some details of the reconstructed densities, which then reveals their fine features. We also compare this way of filtering with the effects of imposing symmetry conditions on densities reconstructed for cubic systems. PACS 78.70.Bj; 87.59.Fm; 74.25.Jb     

A STUDY ON REAL-TIME LOW-FREQUENCY SPATIAL FILTERING PROCESS OF AN OPTICAL IMAGE USING LiNbO3:Fe CRYSTAL

The optimum image processing result of low-frequency filtering was obtained when LiNbO₃:Fe crystal as a spatial filter was placed at a certain position behind the spatial-frequency spectral plane of an optical Fourier transform system, which corresponds to the minimum transmittance ratio in Z-scan curve. The experimental results show that self-defocusing due to photorefractive negative-lens effect is the main factor responsible for the low- frequency filtering.     

Spatial filtering using poled polymer etalon light modulators

Spatial filtering using poled polymer light modulators is studied. The use of a resonator structure is proposed to minimize the driving voltage. Side-chain polymer poly-orange tom-1 isophoronedisocyanate is used as a material. The measured electro-optic coefficient r ₃₃ is 23 pm/V at the wavelength of 633 nm. Spatial filtering of the input images are realized with 30 V applied voltage.     

Elimination of zero-order and conjugate images in off-axis digital holography

In this paper, a new digital method has been developed for eliminating the zero-order and conjugate image in off-axis digital holography. The method consists of filtering the spatial frequencies associated with these undesired terms (the zero-order and conjugate images) in the computed Fourier transform plane of the hologram multiplied with computer simulated original reference wave (written as (RI_h)). An important conclusion is that, without the need for additional shift-spectrum process, the spectrum of real image always appears in the center of the spectrum plane and never shifts. This brings us some convenient to design a filtering window. The theoretical analysis and the corresponding simulation and experiment results are given. As the zero-order and conjugate image are eliminated, both the contrast and brightness of the real image are enhanced and the image quality is improved. Meanwhile, the process of designing a filtering window is simple and convenient.     

Scattering of light by a periodic structure in the presence of randomness IV. Limit of detection by curve fitting

In the context of scattering of light, we determine the extent of randomness within which a hidden periodic part can still be detected. The detection is carried out using a technique called the extended matched filtering, first introduced by us in this context. The earlier prediction, before our technique was introduced, had placed the limit of detection, by intensity measurements alone, at (r ₀/Λ) ∼ 0.33, where r₀ is the coherence length of light for scattering by the rough part of the surface and Λ is the wavelength of the periodic part of the surface. In our earlier works we have shown that by intensity measurements alone, the limit of detection can be taken to a much lower value of (r ₀/Λ), when the extended matched filtering method is employed. In this paper we follow the extended matched filtering method, and try to reach the lowest possible value of detection in (r ₀/Λ) by fitting the data to a polynomial. It is concluded by our numerical work that the lowest possible limit for detection from intensity measurements alone is (r ₀/Λ) = 0.11.     

Engineering of carbon-based superlight spin filter with negative differential resistance

Based on density functional theory and non-equilibrium Green's function, we investigate the edge hydrogenation and oxidation effects on the spin transport of devices consisting of a zigzag C₂N nanoribbon (ZC₂NNR) embedded in zigzag graphene nanoribbons in parallel (P) and antiparallel (AP) spin configurations. The results show that device with edge hydrogenation exhibits dual spin filtering effect in AP spin configuration and obvious negative differential resistance in both P and AP spin configuration. By substituting oxygen for hydrogen as passivation atoms of ZC₂NNR, the spin filtering efficiency is as high as 100% in the P spin configuration, and the negative differential resistance is largely enhanced with a peak to valley ratio in excess of 4×10³. Our theoretical studies suggest that zigzag C₂N nanoribbon modulated by edge substitution has great potential in the design of future multifunctional spin devices.     

New materials research for high spin polarized current

The author reports here a thorough investigation of structural and magnetic properties of Co₂FeAl_0.5Si_0.5 Heusler alloy films, and the tunnel magnetoresistance effect for junctions with Co₂FeAl_0.5Si_0.5 electrodes, spin injection into GaAs semiconductor from Co₂FeAl_0.5Si_0.5, and spin filtering phenomena for junctions with CoFe₂O₄ ferrite barrier. It was observed that tunnel magnetoresistance ratio up to 832%(386%) at 9 K (room temperature), which corresponds to the tunnel spin polarization of 0.90 (0.81) for the junctions using Co₂FeAl_0.5Si_0.5 Heusler electrodes by optimizing the fabrication condition. It was also found that the tunnel magnetoresistance ratio are almost the same between the junctions with Co₂FeAl_0.5Si_0.5 Heusler electrodes on Cr buffered (1 0 0) and (1 1 0) MgO substrates, which indicates that tunnel spin polarization of Co₂FeAl_0.5Si_0.5 for these two direction are almost the same. The next part of this paper is a spin filtering effect using a Co ferrite. The spin filtering effect was observed through a thin Co-ferrite barrier. The inverse type tunnel magnetoresistance ratio of −124% measured at 10 K was obtained. The inverse type magnetoresistance suggests the negative spin polarization of Co-ferrite barrier. The magnetoresistance ratio of −124% corresponds to the spin polarization of −0.77 by the Co-ferrite barrier. The last part is devoted to the spin injection from Co₂FeAl_0.5Si_0.5 into GaAs. The spin injection signal was clearly obtained by three terminal Hanle measurement. The spin relaxation time was estimated to be 380 ps measured at 5 K.     

Signal-to-noise enhancement techniques for quantum cascade absorption spectrometers employing optimal filtering and other approaches

Optical feedback to the laser source in tunable diode laser spectroscopy (TDLS) is known to create intensity modulation noise due to elatoning and optical feedback (i.e. multiplicative technical noise) that usually limits spectral signal-to-noise (S/N). The large technical noise often limits absorption spectroscopy to noise floors 100-fold greater than the Poisson shot noise limit due to fluctuations in the laser intensity. The high output powers generated from quantum cascade (QC) lasers, along with their high gain, makes these injection laser systems especially susceptible to technical noise. In this article we discuss a method of using optimal filtering to reduce technical noise. We have observed S/N enhancements ranging from ∼20% to a factor of ∼50. The degree to which optimal filtering enhances S/N depends on the similarity between the Fourier components of the technical noise and those of the signal, with lower S/N enhancements observed for more similar Fourier decompositions of the signal and technical noise. We also examine the linearity of optimal filtered spectra in both time and intensity. This was accomplished by creating a synthetic spectrum for the species being studied (CH₄, N₂O, CO₂ and H₂O in ambient air) utilizing line positions and linewidths with an assumed Voigt profile from a commercial database (HITRAN). Agreement better than 0.036% in wavenumber and 1.64% in intensity (up to a 260-fold intensity ratio employed) was observed. Our results suggest that rapid ex post facto digital optimal filtering can be used to enhance S/N for routine trace gas detection. Received: 1 April 2002 / Revised version: 7 May 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-509/376-6066, E-mail: robert.disselkamp@pnl.gov     

High brightness 735 nm tapered lasers – optimisation of the laser geometry

High brightness tapered laser diodes with different resonator geometries were fabricated and analysed. The devices consist of an index-guided straight section and a gain-guided tapered section. Lasers with a total length L = 2 and 4 mm and different length of the ridge waveguide L _RW (500 μm ≤ L _RW ≤ 1250 μm for L = 2 mm and 500 μm ≤ L _RW ≤ 2000 μm for L = 4 mm) were processed to study the influence of the straight section on the spatial mode filtering. The power–voltage–current-characteristics, the beam waist, the far field, and the beam propagation factor M ² were measured. From the experiments, it can be stated that the lasers with a small L _RW reach higher output powers compared to those with larger L _RW. Concerning the beam quality the length L _RW should exceed a minimal value to guarantee efficient spatial mode filtering. Devices optimised concerning maximum output power and excellent beam quality reach a beam propagation factor smaller than 2.1 at an output power P = 2 W.     

Picosecond pulse of variable duration by phase matching frequency doubling crystal

Report on a semiconductor saturable absorber mirror (SESAM) passively mode-locked picosecond Nd:YVO₄ laser, with adjustable durations between 8.7 and 70 ps. The pulse duration was adjustable by varying the length of type II phase matching KTiOPO₄ (KTP) crystal. Type II phase matched second harmonic interaction in combination with the anisotropic emission characteristic of Nd:YVO₄ introduced additional bandwidth limiting through birefringent filtering.     

Belavkin filtering with squeezed light sources

We derive the filtering equation for Markovian systems undergoing homodyne measurement in the situation where the output processes being monitored are squeezed. The filtering theory applies to case where the system is driven by Fock noise (that is, quantum input processes in a coherent state) and where the output is mixed with a squeezed signal. It also applies to the case of a system driven by squeezed noise, but here there is a physical restriction to emission/absorption coupling only. For the special case of a cavity mode where the dynamics is linear, we are able to derive explicitly the filtered estimate π _t (a) for the mode annihilator a based on the homodyne quadrature observations up to time t.'     

Electron-momentum filtering in antiparallel asymmetric double δ-magnetic-barrier nanostructure

We present a theoretical investigation on wave-vector filtering (WVF) effect for electrons in an antiparallel asymmetric double δ-magnetic-barrier nanostructure, which can be realized experimentally by patterning two asymmetric ferromagnetic (FM) stripes on top and bottom of GaAs/Al_xGa_1-xAs heterostructure, respectively. It is shown that a considerable WVF effect appears due to an essentially two-dimensional (2D) process for electrons across this nanostructure. It is also shown that WVF efficiency is associated with incident energy, incident direction and structural parameters. In particular, WVF efficiency can be tuned by applying a negative voltage, which maybe results in an electrically-controllable electron-momentum filter for nanoelectronics device applications.     

Optical pumping and modulation techniques with a molecular Ramsey–Bordé interferometer

Using the example of a matter-wave interferometer with K₂ molecules, we present different methods to simplify the observed interference structures within a hyperfine pattern for improving phase measurements of the interference structure. We consider optical pumping for depletion of specific components to simplify the observed spectra. In connection with a lock-in amplifier, amplitude modulation as well as frequency modulation of laser beams in the interferometer are investigated for better detection of the interference pattern. As a follow-up treatment, digital filtering is demonstrated. Received: 22 December 2000 / Revised version: 30 April 2001 / Published online: 18 July 2001     

Selective detection of minor isotope lines in saturated absorption spectra by absorption filtering of major isotope lines

We demonstrate a simple method that can be used to detect minor isotope lines in a saturated absorption spectrum by the absorption filtering of major isotope lines. We investigate this method for use in the spectroscopy of the ¹S₀-¹P₁ transition in Yb at 399 nm by controlling the density of Yb atoms by varying the discharge current of a hollow cathode lamp. The performance of an extended-cavity laser diode using a high-power ultraviolet diode chip is also analyzed.     

Preparation and characterization of GIAR-films for monochromatization of synchrotron radiation

The use of GIAR-films for nuclear resonant filtering of synchrotron radiation has been proposed. A deposition setup was developed to produce suitable layer systems. Various alloy coatings containing⁵⁷Fe were prepared. The hyperfine splittings have been investigated by CEMS. The layer systems have been analyzed in grazing incidence geometry with respect to thickness, density and boundary roughness using Mo K_α radiation.     

Tunable multilayer narrowband filter containing an ultrathin metallic film and a lithium niobate defect

By using lithium niobate (LiNbO₃) as a defect layer, a tunable multilayer narrowband reflection-and-transmission filter containing an ultrathin metallic film is proposed. Due to the voltage dependence of the refractive index for LiNbO₃, tunable optical filtering properties have been theoretically investigated based on the calculated wavelength-dependent reflectance and transmittance. The dependences of peak wavelength on the applied voltage and the angle of incidence are numerically illustrated. The results reveal that in addition to working as a tunable filter, it can also be expected to act as a refractometric optical sensor.     

Sensitivity- and Gradient-Enhanced Hetero (ω1) Half-Filtered TOCSY Experiment for Measuring Long-Range Heteronuclear Coupling Constants

An enhanced version of the X(ω₁) half-filtered TOCSY experiment for measurement of long-range heteronuclear coupling constants is proposed which yields high-quality spectra with substantially increased sensitivity and resolution. The modified method features gradient-enhanced X filtering sequences, broadband homonuclear decoupling duringt₁, optional¹J_XHscaling in theF₁domain, and gradient coherence selection in combination with the sensitivity-enhanced protocol for the TOCSY transfer. These modifications extend the applicability of the method—coupling constants can be measured accurately for natural abundance samples at low concentrations and for compounds yielding complex spectra. Computer-aided analysis of E.COSY-type multiplets is applied for the determination of heteronuclear long-range coupling constants.     

基于超长周期光纤光栅的高灵敏度扭曲传感器

利用高频CO₂激光脉冲写入的周期达数毫米的超长周期光纤光栅(ULPFG)，实验研究了这种新型ULPFG的扭曲特性，发现它的某些高阶谐振波长漂移与扭曲率之间具有良好的线性关系和方向相关性，其灵敏度可达0.2244nm/(rad/m)，是高频CO₂激光脉冲写入法写入的普通LPFG扭曲灵敏度的4倍.初步的理论分析表明，新型ULPFG横截面折变的非对称性以及导模与高阶包层模之间发生的耦合使得扭曲具有方向相关性和很高的灵敏度.基于这种ULPFG独特的扭曲特性，设计了一种可 关键词： 光纤传感 光纤光栅 2激光')" href="#">CO₂激光 扭曲测量 双折射     

Positron annihilation study of the electronic structure of CeCu6 and LaCu6

Summary The results of a prelimianry study of the electronic structure of the heavy-fermion system CeCu₆ via the two-dimensional angular correlation of electron-positron annihilation radiation (2D ACAR) technique were compared with those obtained for the isostructural reference non-f-electron material LaCu₆. The spectra relating to the 2 compounds displayed extremely similar anisotropic structures, with features related to the symmetry of the Brillouin zone. At present, these structures cannot however be attributed to Fermi-surface or positron wave function effects or to anisotropies of some full valence band. In the case of LaCu₆, data processing prior to the LCW procedure via new filtering techniques revealed a reasonably good agreement between the LCW data and LAPW Fermi-surface calculations while in the case of CeCu₆ it appeared difficult to relate the results, still of controversial interpretation, to any theoretical model.