Micro-structured fiber Bragg gratings. Part I: Spectral characteristics

In this paper, the authors present a detailed theoretical and numerical analysis of an in-fiber photonic device based on micro-structured fiber Bragg gratings for sensing and telecommunication applications. The investigated structure consists in a fiber Bragg grating with a localized stripping of the cladding layer. The thinning of the cladding layer changes the core propagation features and thus it induces a phase delay on the guided mode, leading to the formation of a defect state inside the original grating band-gap in agreement with the phase-shift grating theory. The behaviour of the defect state is ruled by three main parameters identified in the length and depth of the stripped region and the surrounding refractive index. This special characteristic leads to the possibility to realize a new class of active and passive photonic devices. In particular, here, the theoretical and numerical analysis of the spectral behaviour exhibited by the device and its dependence on the perturbation features have been carried out. Particular emphasis has been focused on the effect of the SRI on the spectral response, providing an exhaustive analysis of the potentiality of the device both for sensing and telecommunication applications. In addition, the investigation of standard uniform and strong fiber Bragg gratings has been carried out. Finally, the case of real devices realized by wet chemical etching in hydrofluoric acid solutions was analysed and a spectral equivalence was found to extend the design rules identified for the ideal case.     

光纤相位光栅（布喇格光栅）的制作

讨论了光纤的光敏性，并着重综述了光纤相位光栅（布喇格光栅）的制作方法。     

磁流体填充的光子晶体光纤Bragg光栅三参量同时传感特性研究

基于光子晶体光纤(PCF)包层空气孔灵活的结构 设计,将磁流体填充的PCF(MF-PCF)和普通PCF进行焊接, 并在其纤芯区域刻蚀Bragg光栅,提出一种能够同时测量温度、磁场和应变的PCFBG传感器。 基于模式耦 合理论以及传输矩阵法,结合PCF中基模以及高阶光波模式的不同传感特性,通过分析热光 效应、磁光效 应以及弹光效应对模型不同光波模式的影响,探究谐振波峰反射谱响应特性。实验结果表明 ,本文的PCFBG传感器, 温度灵敏度最高可达13.97pm/℃,磁场灵敏度最高达10.04pm/mT,应变灵敏度最高达1.223pm/με;且谐振反 射光谱的漂移量与温度、磁场以及应变的变化具有良好的线性关系。 通过设计相应的三参量矩阵解调算法,可实现温度、应变以及磁场的同时测量,从而有效 降低传感成本,拓宽传感思路,具有一定的应用价值。     

Dispersion-flattened Bragg photonic crystal fiber for large capacity optical communication system

A novelty dispersion ultra-flattened Bragg photonic crystal fiber (PCF) has been fabricated in this paper.The fiber is composed of compound cores and periodical claddings with 11 coaxial rings.It has flattened dispersion of 8.54±1.3 ps-(nm· km)-1 in the communication wavelength range of 1460-1625 nm.Its dispersion slope alters from -0.0428 to 0.0392ps·nm-2·km-1.The low attenuation of 0.52 dB/km and low bending loss of 0.09 dB at 1550 nm of the fiber are also achieved.The Bragg PCF has enormously potential application in the fields of dense wavelength division multiplexing systems because of its superior dispersion properties and easy splicing performances.     

光子晶体光纤长周期光栅制作技术与应用最新进展

详细介绍了光子晶体光纤长周期光栅几种典型制作方法,包括相位掩模法、CO2激光脉冲激励法,机械压力诱导法、电弧放电法、液晶法和声波诱导法,并且分析了各种方法的优缺点,简单介绍了光子晶体光纤长周期光栅的温度、应变、折射率传感特性。简要介绍了光子晶体光纤长周期光栅在光通信和光传感领域的应用情况,并且展望了基于光子晶体光纤长周期光栅的新型光子器件的未来发展情况。     

Nanofaceting and alloy decomposition: From basic studies to advanced photonic devices

Most of the modern epitaxial structures for semiconductor lasers serving the needs of optical storage and fiber pumping are grown on misoriented GaAs(0 0 1) substrates. It has been found in metal-organic vapor-phase epitaxy that surface misorientation helps to achieve better epitaxial quality of the alloy layers. On the other hand, these misoriented or, in other definition, high-index surfaces are known to undergo phase transformations, depending on the misorientation angle, from nanofaceting (like (3 1 1)A, (3 1 1)B, (3 3 1), (2 1 1)B GaAs surfaces) to arrays of step bunches (like (7 7 5) GaAs, etc.). In the present paper, we consider growth-related effects during growth of both standard and advanced laser structures on GaAs substrates which are typically used for growth of 650 nm GaAlInP devices. We show that the active region of the laser structures represents a corrugated superlattice with a 25 nm in-plane periodicity, while the surrounding layers are natural superlattices with a 5 nm vertical periodicity. Corrugated superlattice used as an active region manifests itself through a strong modification of optical properties. Strong in-plane polarization evidences the formation of arrays of quantum wires. Both standard and advanced red laser have been grown and processed. The advanced lasers have demonstrated a vertical beam divergence of only 7–8° full-width at half-maximum (FWHM). The advanced lasers with 10 μm-wide stripes demonstrate continuous wave (CW) power up to 200 mW and the lateral beam divergence of 4°, and those with 4 μm-wide stripes show CW power up to 120 mW and the lateral beam divergence of 6.5°. No facet passivation has been applied and the power is limited by the catastrophic optical mirror damage (COMD). 20 W pulsed power has been achieved in 100 μm-wide stripes for the advanced design and 6 W has been obtained for the standard design. The advantage comes from the design optimization of the laser waveguide using the concept of the longitudinal photonic bandgap crystal. We believe that also the optimization of the active region with better utilization of the nanofaceting effects may enable, in addition, a dramatic extension of the emission wavelength towards bright red (620–630 nm) and, probably, yellow (580 nm) spectral ranges.     

Microwave V-I transmission matrix formalism for the analysis of photonic circuits: application to fiber Bragg gratings

We propose the use of V-I matrices well known in microwave engineering to the analysis of photonic devices, especially those based on multilayer dielectrics. As an application we present a novel fast effective index method for the analysis of fiber Bragg gratings based on the use of V-I transmission matrices. It combines the exactitude of traditional effective index methods and the speed of coupled-mode methods.     

Efficiency enhancement of organic light-emitting devices by using honeycomb metallic electrodes and two-dimensional photonic crystal arrays

In this paper, a facile fabrication technique called nanosphere lithography combining with two-step reactive ion etching method for patterning honeycomb metallic electrode with high transparency and excellent uniformity is demonstrated. The patterning silver electrode with 15-nm film thickness and 68.6% fill-factor was used as the organic light-emitting diode (OLED) anode, which showed an average transmittance of 77.4% and sheet resistance of 30.7 Ω/□. The current efficiency is 8.35 cd/A for the OLED with patterned silver anode under 100 cd/m² operation brightness, which was 47% higher than the device with indium tin oxide (ITO) anode. After applying the polystyrene nanosphere to form a photonic crystal array onto the device, the extracted light from organic mode can be further coupled out from device substrate mode. The overall luminous enhancement of the device with the combination of internal honeycomb metallic anode and external photonic crystal array is 115% higher than the traditional ITO-based OLED.     

Optimization of large-scale fiber sensor arrays incorporatingmultiple optical amplifiers. Part II: pump power

For part I see, ibid., p. 218, 1998. We discuss how to minimize the pump power required for large-scale fiber sensor arrays employing erbium-doped fiber amplifier (EDFA) telemetry with respect to the number of amplifiers per bus, number of sensors per rung, and the gain per amplifier. For a large array, the pump power requirement is dominated by passive component losses along the array. We investigate several methods, including alternative array topologies, to reduce the power requirement while minimizing the impact on the signal-to-noise ratio (SNR). We define an optimum topology which requires less than 1 W of 1480 nm pump power per bus to support 200 high sensitivity (1 μrad/√(Hz)) sensors on a pair of fiber buses, a power requirement that is reasonable and attainable with available laser diodes     

Molecular image-processing devices based on chemical reaction systems. 2: Implementation of blum-type algorithms

Potentialities for impliementing Blum-type algorithms based on chemical and biochemical dynamical media are discussed. These media proved to be efficient for performing some primitive operations important for image processing.     

Molecular image-processing devices based on chemical reaction systems. I: General principles for implementation

Potentialities for implementing simple neural information-processing devices based on chemical and biochemical dynamical media are discussed. Pilot ‘hardware’ models of neural molecualr devices that able to perform image-processing operations were constructed.     

Molecular image-processing devices based on chemical reaction systrems. 3: Some operational characteristics of excitable light-sensitive media used for image processing

Operational features of excitable Belousov–Zhabotinsky-type light-sensitive media that can be used to implement image-processing operations are discussed. They embrace (i) the main primitive responses of such media to light excitation as a function of medium acidity, temperature and light exposure, (ii) the characteristics of temporal evolution of stored media and (iii) the resolution and information content inherent in images stored by excitable media. It is shown that controlling the acidity of the medium, its temperature and the duration of light exposure enables one to get reproducible results.     

Molecular image-processing devices based on chemical reaction systems. 5: Processing images with several levels of brightness and some application potentialities

Excitable media of Belousov–Zhabotinsky type were used for processing images with several levels of brightness. In this case the image-processing operations proved to be somewhat more complicated than in the case of black-and-white pictures. Excitable media seem to be useful tools for handling some practical applications such as aerial picture processing. Belousov–Zhabotinsky-type media are also simple and convenient experimental models for investigation of the information-processing capabilities of biomolecular systems with complicated non-linear dynamics. © 1997 John Wiley & Sons, Ltd.     

Molecular image-processing devices based on chemical reaction systems. 4: Image-processing operations performed by active media functioning in the oscillating mode

Basic primitive image-processing operations performed by chemical dynamic media functioning in the oscillating mode are discussed. These operations have rather high computational complexity and can be considered as simulation of human vision capabilities.     

Structure oriented compact model for advanced trench IGBTs without fitting parameters for extreme condition: Part II

Compact model for expressing turn-off waveform for advanced trench gate IGBTs is proposed even under high current density condition. The model is analytically formulated only with device structure parameters so that no fitting parameters are required. The validity of the model is confirmed with TCAD simulation for 1.2–6.5 kV class IGBTs. The proposed turn-off model is sufficiently accurate to calculate trade-off curve between turn-off loss and saturation collector voltage under extremely high current conduction, so that the model can be used for system design with the advanced trench gate IGBTs.     

Inverted vs standard PTB7:PC70BM organic photovoltaic devices. The benefit of highly selective and extracting contacts in device performance

In this work we compare the photovoltaic performance of different cell designs, standard and inverted, for one of the most promising systems to achieve power conversion efficiencies over 10% in polymer:fullerene single cells, namely PTB7:PC70BM. Impedance spectroscopy, charge extraction and transient photovoltage are used in order to assign the electrical losses initially observed in the current density–voltage curve and understand the main limitation of every design. While inverted devices show competitive performance in terms of charge generation, transport of carriers and also for charge collection at electrodes, standard devices present additional resistive losses that are assigned to charge transfer issues at the active layer/anode interface. This additional resistance increase the overall series resistance of devices, lowers the fill factor and it is the ultimate responsible for the observed reduced device performance of standard cells in comparison to inverted ones. In this way, devices over 7.2% are reported with ZnO and MoO₃ as interlayer electrodes that act as improved highly selective and extracting contacts in comparison to standard PEDOT:PSS and Ca/Ag. Contacts are thus electrically optimized. Additional improvement of device performance must consider enhancement of intrinsic recombination properties of the blend. Lower molecular weights and/or any residual catalyst impurities with respect to other batches are the only limitation to reach record efficiencies as those shown in recent works.     

Coarsening of three-dimensional droplets by two-dimensional diffusion: Part II. Theory

Theoretical modeling of coarsening among a finite cluster of precipitates is implemented, using the multipole expansion method. This method requires the diffusion field to behave quasi-statically. Two approximate solutions were developed, one to monopolar order, and other to the dipolar order. The conventional Gibbs-Thomson equilibrium relationship was used as the boundary condition at the precipitate-matrix interface. Part I of this paper considers a liquid-liquid system in a mixed-dimensional geometrical configuration, wherein three-dimensional precipitates interact via a diffusion field constrained in two dimensions. This kind of geometric configuration is often encountered in island evolution dynamics and phase segregation in thin films. The initial experimental configuration of droplets provides the initial condition for the simulation. Both monopole and dipole approximations closely follow the experimentally observed scaling laws, characteristic for the mixed-dimensional coarsening (N^−4/3 and ⁴, varied linearly with time, where N is the number of droplets in the experimental field of view, and is the average droplet radius). Good agreement is observed for time evolution of radii of some individual precipitates. Certain deviations appearing among the two approximate solutions and the experimental data are discussed.     

Crystallographic evolution of microstructure in thin film processing: Part II. Grain boundary structure

The technique of electron backscattered diffraction is used to charactcrize the microtexture and mesotexture of grain neighborhoods in an annealed thin film of Al₉₇Ge₃ on thermally oxidized silicon. Of the microstructural features present in this material, a “sunken” (or “collapsed”) grain neighborhood is examined in terms of its mesotexture. The representation of crystallographic orientation between a “sunken” grain and its surrounding neighbors is assessed using inverse pole figures and Rodrigues-Frank (R-F) space orientation mapping. The two types of mappings are compared and detailed calculations of R-F space are shown. The advantages of the R-F space representation are illustrated. The total number of grains examined is 78 while the number of axis-angle pairs is 72. The microtexture is a strongly preferred < 111> parallel to the substrate normal while the mesotexture is comprised of low angle boundaries and fiber mesotexture as indicated by a R-F map.     

Discrete realization for receivers detecting signals over random dispersive channels. Part II: Doppler-spread channel

We propose a new suboptimal receiver for detecting signals transmitted over random doppler-spread channels. We discuss in particular the first order channel and a transmitted signal of a constant envelope. The proposed receiver is composed of an integrator, a sampler and a quadratic filter. We give numerical results concerning the performances of this receiver. In particular, we give the evolution of error probability versus the number of samples (or sampling rate). We show the important role of the ratio of signal duration to channel coherence time and we suggest how to choose the transmitted signal duration and the sampling rate for a given signal to noise ratio.     

System Implications of 14/11 GHz Path Depolarization. Part II: Reducing the impairments

Dual-polarized co-channel operation via international communications satellites places stringent requirements on the amount of interference that can be allowed from all sources. System design must allow for the depolarization caused by rain and ice crystals, the principal source of additional interference. In the 14/11 and 14/12 GHz bands, some paths are depolarization limited, particularly at lower path elevation angles, or become so with up-link power control. Others are attenuation limited. Therefore, a means must be found to reduce the level of path depolarization to meet the requirements for path availability. In Part I or this paper, reference joint statistics of cross-polarization discrimination/attenuation statistics were derived for a number of paths and climates to establish the additional fade and cross-polarization discrimination margins required. This second paper addresses methods for reducing the system effects of path depolarization. Appropriate orientation of the polarization vectors at the spacecraft is shown to be an attractive alternative for spot-beam applications. For all other situations, it is concluded that single-parameter, phase-only compensation at the earth terminal will meet all practical system requirements.