Experimental Spectral Response Research Articles

Thermographic cameras for thermal comfort applications: simulated and experimental spectral response errors of various long-wave infrared detectors

Modern remote temperature sensing in the form of infrared imaging has become a widely used and important technique, with the ability to measure and characterize important but unseen Radiant heat. As more Long-wave Infrared (LWIR) detectors come to market aiming to meet a wide array of needs and goals, there is a need to differentiate and appraise LWIR detectors based upon the specific needs of thermal comfort research. While most detectors measure in the range of 8-14 μm, only 37.6% of the emitted energy of a blackbody at 300 K is contained in this spectral range. Thus, inherent to the operation of nearly all infrared detectors and cameras is an assumption about the emission curves of the objects sensed. Many materials in the built environment deviate significantly from the blackbody assumption, and the error due to this deviation is one which generic gray-body emissivity corrections are unable to fix — it is akin to taking black-and-white images with only the red channel of a camera, and using exposure compensation to correct the image to attempt a true monochrome rendition: there is simply missing information and the adjusted image will still be very clearly wrong. In this paper, we aim to evaluate by simulation and experiment the potential errors in infrared thermography used to drive thermal comfort heat transfer calculations due to intrinsic spectral assumptions of LWIR detectors.

Detection of Crack Locations in Aluminum Alloy Structures Using FBG Sensors.

This study investigated the reflected spectral deformation mechanism of fiber Bragg grating (FBG) sensors with crack propagation. This analysis was performed based on the simulated FBG response by applying modified-transfer matrix modeling (TMM) with the strain states, which were extracted by the finite element method (FEM) analysis. Experimental data were obtained from FBG sensors bonded in an aluminum alloy structure and subjected to multiple crack lengths, and the strain values were obtained by digital image correlation (DIC) technology. Based on the simulations and the experimental full spectral response, we compared the performance of two damage features: The full width at half maximum (FWHM) and the spectral difference. In addition, results showed that the two features were insensitive to experimental noise and were highly sensitive to the complex strain field caused by crack propagation. Moreover, the damage features changes in the crack propagation process also provided a way for crack position measurement. Ultimately, the 10 mm grating lengths sensors showed better performance to the crack detection with longer sensitivity distance. According to the research in this paper, the crack position was quantitatively determined by evaluating different damage features of the reflected spectrum.

Charge layer optimized 4H-SiC SACM avalanche photodiode with low breakdown voltage and high gain

An n-type charge layer was introduced into the 4H-SiC separated absorption and multiplication avalanche photodiode. The effect of the charge layer on the optoelectronic characteristics of the photodiode was modeled and studied to optimize the performance of the photodiode. According to the modeling results, 4H-SiC photodiodes were fabricated. A low breakdown voltage of 77.6 V and a significant gain of more than 105 were obtained. The peak responsivity for 270 nm illumination of the photodiode biased at −40 V was 83 mA W−1, corresponding to an external quantum efficiency over 38.2%. Both the simulated and experimental spectral responses are almost identical.

Modeling of the tandem III–V dilute nitride bulk or quantum engineered/silicon solar cells

Tandem silicon solar cells are designed in order to lower the levelized cost of solar electricity by increasing the cell’s efficiency to more than 30%. Dual junction thin film III–V/Si cell efficiencies are limited to 20% because of high dislocation densities (>106 cm−2) due to lattice mismatch between them. GaAsyP1−x−yNx has a lattice constant matched with silicon at y = 4.7*x − 0.1 and has an optimal bandgap i.e. between 1.7 and 1.9 eV for tandem III–V’s with Si. This work designs a p-i-n GaAsPN/GaP symmetric or asymmetric (resonant thermos tunneling) multi-quantum well (MQW) device for a top sub-cell of a dual junction silicon tandem to minimize the effect of the degraded minority carrier transport properties of bulk dilute nitride layers and presents the simulated solar cell properties using a realistic drift-diffusion approach. Using extracted experimental spectral responses as well as other solar cell parameters of various silicon solar cell like: HIT, PERC, IBC etc, the tandem efficiencies of designed p-i-n MQWs devices in conjunction with silicon solar cells have been evaluated. The tandem efficiency of an optimized p-i-n MQWs device under AM 1.5 G spectrum can exceed 33% when in conjunction with a 25.6% efficient HIT silicon solar cell.

Ultrasonic wave velocity measurement in concrete using the impact-echo method

An ultrasound wave measurement technique suitable for large compact-sized (where measured thickness is comparable with other dimensions) concrete structures is described. The technique is based on ultrasonic bulk wave velocity measurement Cl. The Cl value is determined by the correlation method: correlation is established between the experimental spectral responses of a real sample and the series spectral responses of a finite element model calculated with different model ultrasonic velocity parameter values Cmod.j. To increase the accuracy of the velocity measurement the multiplicative correlation method is used: cross-correlation is calculated between the spectral response curves obtained at different points on the sample and the spectral response curves of the finite element model calculated for the same points of the model.

Fluctuation-dissipation relation and stationary distribution of an exactly solvable many-particle model for active biomatter far from equilibrium.

An exactly solvable, Hamiltonian-based model of many massive particles that are coupled by harmonic potentials and driven by stochastic non-equilibrium forces is introduced. The stationary distribution and the fluctuation-dissipation relation are derived in closed form for the general non-equilibrium case. Deviations from equilibrium are on one hand characterized by the difference of the obtained stationary distribution from the Boltzmann distribution; this is possible because the model derives from a particle Hamiltonian. On the other hand, the difference between the obtained non-equilibrium fluctuation-dissipation relation and the standard equilibrium fluctuation-dissipation theorem allows us to quantify non-equilibrium in an alternative fashion. Both indicators of non-equilibrium behavior, i.e., deviations from the Boltzmann distribution and deviations from the equilibrium fluctuation-dissipation theorem, can be expressed in terms of a single non-equilibrium parameter α that involves the ratio of friction coefficients and random force strengths. The concept of a non-equilibrium effective temperature, which can be defined by the relation between fluctuations and the dissipation, is by comparison with the exactly derived stationary distribution shown not to hold, even if the effective temperature is made frequency dependent. The analysis is not confined to close-to-equilibrium situations but rather is exact and thus holds for arbitrarily large deviations from equilibrium. Also, the suggested harmonic model can be obtained from non-linear mechanical network systems by an expansion in terms of suitably chosen deviatory coordinates; the obtained results should thus be quite general. This is demonstrated by comparison of the derived non-equilibrium fluctuation dissipation relation with experimental data on actin networks that are driven out of equilibrium by energy-consuming protein motors. The comparison is excellent and allows us to extract the non-equilibrium parameter α from experimental spectral response and fluctuation data.

Investigation on the impact of irregular fringe patterns of a single-fiber Mach-Zehnder interferometer on its sensing capabilities

Abstract A novel single-mode single-fiber (SMSF) MZI formed by cascading of two non-adiabatic fiber tapers, with stable and repeatable spectrum, has been found to be useful in sensing applications in recent times. A multimode interference based novel simulation approach is proposed to predict the sensing characteristics of SMSF-MZI and is validated with experimental observation. The proposed method includes solving of simultaneous non-homogenous equations for determining the amplitudes of the interfering modes excited in the tapered section of the interferometer. The simulated fringe pattern and the experimental spectral response converge to some important comprehension reported for the first time. A linear shift in output spectral response, of SMSF-MZI, due to change in optical path length induced by temperature/strain etc., is likely to be characterized by three modes interference occurring in the interference region of the interferometer. Whereas if the spectral shift starts saturating at moderately higher temperature/strain, then the formation of interference fringes are possibly governed by two modes interference. Further, it was also explained that a SMSF-MZI with variable fringe widths in its spectral pattern exhibits higher sensitivity than that of the SMSF-MZI having wavelength spectrum with uniform free spectral range. These findings are useful in selecting and predicting the sensitivity of a given SMSF-MZI, based on its spectrum, for sensing applications.

Six-port optical switch for cluster-mesh photonic network-on-chip

AbstractPhotonic network-on-chip for high-performance multi-core processors has attracted substantial interest in recent years as it offers a systematic method to meet the demand of large bandwidth, low latency and low power dissipation. In this paper we demonstrate a non-blocking six-port optical switch for cluster-mesh photonic network-on-chip. The architecture is constructed by substituting three optical switching units of typical Spanke-Benes network to optical waveguide crossings. Compared with Spanke-Benes network, the number of optical switching units is reduced by 20%, while the connectivity of routing path is maintained. By this way the footprint and power consumption can be reduced at the expense of sacrificing the network latency performance in some cases. The device is realized by 12 thermally tuned silicon Mach-Zehnder optical switching units. Its theoretical spectral responses are evaluated by establishing a numerical model. The experimental spectral responses are also characterized, which indicates that the optical signal-to-noise ratios of the optical switch are larger than 13.5 dB in the wavelength range from 1525 nm to 1565 nm. Data transmission experiment with the data rate of 32 Gbps is implemented for each optical link.

Analysis of the relation between spectral response and absorptivity of GaAs photocathode

In order to study the relation between spectral response and absorptivity of GaAs photocathode, two kinds of GaAs photocathodes are prepared by molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD), respectively. The samples grown by the MBE include varying doping GaAs photocathodes with different values of emission layer thickness from A to E. The thickness of GaAs emission layer is 1.6 μm or 2 μm. The Al component is 0.5 or 0.63. The samples grown by the MOCVD include varying doping or various component GaAs photocathodes with different values of emission layer thickness and different window layer components from F to J. The thickness values of GaAs emission layer are 1.4 μm, 1.6 μm or 1.8 μm, respectively. The Al component is 0.7 or varies from 0.9 to 0. The doping concentration of the GaAs emission layer is divided into 8 sections between 1×1018 cm-3 and 1×1019 cm-3. The experimental spectral response curves for all samples are obtained by the optical spectrum analyzer. And the experimental reflectivity and transmittivity curves are measured by the ultraviolet visible near infrared spectrohootometer. Based on the law of energy conservation, the absorptivity curves are obtained according to the experimental reflectivity and transmittivity. In the same coordinate system, both the curves are obtained by unitary processing according to the max. A similar surface barrier can be given by dividing the normalized absorptivity by the normalized spectral response, and those are termed the similar I barrier and the similar Ⅱ barrier, respectively. The results indicate that for both the GaAs photocathodes, the experimental spectral response curves both tend to move to the infrared band compared with the experimental absorptivity curves. The average energy differences between absorptivity and spectral response are calculated to be 0.3101 eV for the MBE sample, and 0.3025 eV for the MOCVD sample, respectively. The red-shifts of the photocathodes grown by MBE are a bit bigger than those of the photocathodes grown by MOCVD. In the shortwave region, the absorptivity is very large, but the spectral response cuts off nearby 500 nm. In the visible wavelength region, the peak position of the spectral response curve shifts toward the infrared band for several hundred meV in comparison with the absorptivity curve. In the near infrared region, a red shift of several meV appears at the cut-off position of the spectral response curve in comparison with the absorptivity curve. The results have the guiding significance for improving the photoemission performance of wide-spectrum GaAs photocathode by optimizing the optical performance.

Five-port silicon optical router based on Mach—Zehnder optical switches for photonic networks-on-chip**Project supported by the National High Technology Research and Development Program of China (Nos. 2015AA010103, 2015AA010901) and the National Natural Science Foundation of China (Nos. 61575187, 61235001, 61505198, 61377067).

With the continuous development of integrated circuits, the performance of the processor has been improved steadily. To integrate more cores in one processor is an effective way to improve the performance of the processor, while it is impossible to further improve the property of the processor by only increasing the clock frequency. For a processor with integrated multiple cores, its performance is determined not only by the number of cores, but also by communication efficiency between them. With more processor cores integrated on a chip, larger bandwidths are required to establish the communication among them. The traditional electrical interconnect has gradually become a bottleneck for improving the performance of multiple-core processors due to its limited bandwidth, high power consumption, and long latency. The optical interconnect is considered as a potential way to solve this issue. The optical router is the key device for realizing the optical interconnect. Its basic function is to achieve the data routing and switching between the local node and the multi-node. In this paper we present a five-port optical router for Mesh photonics network-on-chip. A five-port optical router composed of eight thermally tuned silicon Mach—Zehnder optical switches is demonstrated. The experimental spectral responses indicate that the optical signal-to-noise ratios of the optical router are over 13 dB in the wavelength range of 1525–1565 nm for all of its 20 optical links. Each optical link can manipulate 50 wavelength channels with the channel spacing of 100 GHz and the data rate of 32 Gbps for each wavelength channel in the same wavelength range. The lowest energy efficiency of the optical router is 43.4 fJ/bit.

Estimation of Thickness and Cadmium Composition Distributions in HgCdTe Focal Plane Arrays

Mercury cadmium telluride (HgCdTe) is one of the most commonly used material systems for infrared detection. The performance of infrared focal-plane arrays (IRFPAs) based on this material is limited by several noise sources. In this paper, we focus on the fixed pattern noise, which is related to disparities between the spectral responses of pixels. In our previous work, we showed that spectral nonuniformities in a HgCdTe IRFPA were caused by inhomogeneities of thickness and cadmium composition in the HgCdTe layer, using an optical description of the pixel structure. We propose to use this bidimensional dependence combined with experimental spectral responses to estimate disparities of thickness and cadmium composition in a specific HgCdTe-based IRFPA. The estimation methods and the resulting maps are presented, highlighting the accuracy of this nondestructive method.

Wavelength-Selective Infrared Metasurface Absorber for Multispectral Thermal Detection

A wavelength-selective metasurface absorber suitable for use in multispectral microbolometer focal plane arrays in the long-wavelength infrared (LWIR) region is theoretically and experimentally investigated. We show that a thin metal metasurface, which is characterized by effective surface impedance, as an absorbing layer can be integrated with an asymmetric Fabry–Perot cavity to construct a wavelength-selective perfect absorber. The absorbed infrared energy is mainly dissipated in a thin metal metasurface, in contrast to a multilayer metamaterial absorber. The calculated and experimental spectral responses of the metasurface absorbers show excellent wavelength-selective narrowband absorption to allow multispectral imaging in LWIR.

Techniques for estimating the unknown functions of incomplete experimental spectral and correlation response matrices

In this paper, we propose analytical and numerical straightforward approximate methods to estimate the unknown terms of incomplete spectral or correlation matrices, when the cross-spectra or cross-correlations available from multiple measurements do not cover all pairs of transducer locations. The proposed techniques may be applied whenever the available data includes the auto-spectra at all measurement locations, as well as selected cross-spectra which implicates all measurement locations. The suggested methods can also be used for checking the consistency between the spectral or correlation functions pertaining to measurement matrices, in cases of suspicious data. After presenting the proposed spectral estimation formulations, we discuss their merits and limitations. Then we illustrate their use on a realistic simulation of a multi-supported tube subjected to turbulence excitation from cross-flow. Finally, we show the effectiveness of the proposed techniques by extracting the modal responses of the simulated flow-excited tube, using the SOBI (Second Order Blind Identification) method, from an incomplete response matrix 11This paper is an extended version of the work presented at the International Conference on Condition Machinery in Non-Stationary Operations (CMMNO13), May 8–10, 2013, Ferrara, Italy..

Optimization of reversible LPFG for sensing application

We report here for the first time to our knowledge the characterization of mechanically induced long period fiber gratings in novel MSM fiber structure. Reversible grating of same period and length was induced in single mode fiber, multimode fiber and novel multimode-singlemode-multimode (MSM) fiber structure. The spectral response of reversible LPFG in SMF is verified experimentally as well as from simulation results and then compared with the experimental spectral response of reversible LPFG in multimode fiber and MSM fiber structure. Reversible LPFG in novel MSM fiber structure is the most optimized and suitable grating for sensing application. For this grating we have obtained single resonant wavelength over a wide wavelength range and maximum transmission loss peak of around 20dB.

Nonlinear dynamics of adhesive micro-spherical particles on vibrating substrates

The coupled (rocking and out-of-plane) vibrational dynamics of micro-spherical particles on vibrating flat substrates is studied experimentally, analytically, and computationally. In the experimental spectral responses of some of vibrating particles, in addition to their predicted rocking resonance frequencies, extra resonance peaks at the doubles of these predicted frequencies have been previously observed and reported. To understand and explain the rocking resonance frequency doubling effect, a two-dimensional mathematical model describing the coupled dynamics of a micro-spherical particle in the out-of-plane and rocking coordinates is developed and reported. Previously, the doubling effect was leading to ambiguity in adhesion characterization. It is determined that the frequency doubling observed in the spectral domain responses of particles is caused by nonlinear coupling between the out-of-plane and in-plane modes of motion while the particle is experiencing a whirling-like motion taking place around a nearly stationary axis tilted with respect to the substrate normal (as opposed to the substrate normal itself). In current study, the leaning angles of the rocking motion with whirling of a set of polystyrene latex particles are approximated by utilizing the coupled dynamic model. Also, the work-of-adhesion values extracted from the experimental resonance frequencies of a set of particles using the developed model are compared to those reported in the literature and a good agreement is found. The detection and analysis of the reported motion is relevant to adhesion bond characterization and particle manipulation.

Spectra distortion by the interstrip gap in spectroscopic silicon strip detectors

The NUSTAR experiments to be carried out as the part of the FAIR program (Facility for Antiproton and Ion Research) now under development in GSI, Germany, require unique spectrometers for heavy ions, for an energy range between a hundred keV up to hundreds of MeV. These spectrometers are constructed on the basis of silicon double sided detectors capable of providing simultaneously the energy spectrum of the particles and the position of hit points. The double sided Si strip detectors for high resolution ion spectroscopy and tracking were developed by the PTI-RIMST consortium. Reduced sized detectors were studied with alpha-particles from a 238Pu source to define the spectral response of their p+ side. The energy resolution was measured and found to be the highest, 9.6 keV, in the p+ strips area. The energy spectrum for the particles hit at the interstrip gap was shown to be much broader and have a maximum at the low energy edges. In this study the alpha-particle spectra were measured on the p+ side of strip detector and their shape was found to depend on the p+ strip structure and potential distribution under the strip and in the interstrip gap, where the surface is passivated by SiO2 layer. Therefore, the 2D potential distribution in the interstrip gap was simulated and interpreted through the effective entrance window for alpha-particles. The calculated spectrum of a detector from alpha-particle source has a shape specific to the experimental detector spectral response, i.e., the peak at low energies. These findings are to be taken into account in the analysis of short range particle spectra and may well contribute to further development of spectroscopic single sided and double sided Si strip detectors to be used in investigations in nuclear physics.

Optical simulations of microcrystalline silicon solar cells applying plasmonic reflection grating back contacts

Light trapping is a key issue for high efficiency thin-film silicon solar cells. The authors present three-dimensional electromagnetic simulations of an n-i-p substrate-type microcrystalline silicon solar cell applying a plasmonic reflection grating back contact as a novel light-trapping structure. The plasmonic reflection grating back contact consists of half-ellipsoidal silver nanostructures arranged in square lattice at the back contact of thin-film silicon solar cells. Experimental results of prototypes of microcrystalline silicon thin-film solar cells showed significantly enhanced short-circuit current densities in comparison to flat solar cells and, for an optimized period of the plasmonic reflection grating back contact, even a small enhancement of the short-circuit current density in comparison to the reference cells applying the conventional random texture light-trapping structure. The authors demonstrate a very good agreement between the simulated and experimental spectral response data when taking experimental variations into account. This agreement forms an excellent basis for future simulation based optimizations of the light-trapping by plasmonic reflection grating back contacts. Furthermore, from the simulated three-dimensional electromagnetic field distributions detailed absorption profiles were calculated allowing a spatially resolved evaluation of parasitic losses inside the solar cell.

Electrical characterisation of liquid crystals at millimetre wavelengths using frequency selective surfaces

The accurate measurement of the permittivity, loss tangent and dielectric anisotropy DC bias dependence for two different liquid crystal (LC) materials in the frequency range 140–165 GHz is described. The electrical characteristics are obtained by curve fitting computed transmission coefficients to the experimental spectral response of a new class of electronically reconfigurable frequency selective surface. The periodic structure is designed to yield bandpass filter characteristics with and without an applied bias control voltage in order to measure the tunability of the LC material which is inserted in a 705 µm-thick cavity.

Comparison of structure and performance between extended blue and standard transmission-mode GaAs photocathode modules

Extended blue and standard transmission-mode GaAs photocathode modules were prepared, respectively, by metal organic chemical vapor deposition. The experimental reflectivity, transmissivity, and spectral response curves were measured and compared separately. The integral sensitivities are 1980 μA/lm and 2022 μA/lm for both the modules. By use of the revised quantum yield formula, the experimental spectral response curves are fitted to obtain the structure parameters. The fitted results show that the Ga(1-x)Al(x)As window layer with varied aluminum components is beneficial to improve extended blue GaAs photocathode module. In addition, the layer-thickness and aluminum component in the window layer determine the extended blue performance, while the thickness of the GaAs active layer settles the long-waveband performance for the transmission-mode GaAs photocathode module.

A 320 Gb/s-Throughput Capable 2$\,\times\,$2 Silicon-Plasmonic Router Architecture for Optical Interconnects

We demonstrate a 2 × 2 silicon-plasmonic router architecture with 320 Gb/s throughput capabilities for optical interconnect applications. The proposed router platform relies on a novel dual-ring Dielectric-Loaded Surface Plasmon Polariton (DLSPP) 2 × 2 switch heterointegrated on a Silicon-on-Insulator (SOI) photonic motherboard that is responsible for traffic multiplexing and header processing functionalities. We present experimental results of a Poly-methyl-methacrylate (PMMA)-loaded dual-resonator DLSPP waveguide structure that uses two racetrack resonators of 5.5 μm radius and 4 μ m-long straight sections and operates as a passive add/drop filtering element. We derive its frequency-domain transfer function, confirm its add/drop experimental spectral response, and proceed to a circuit-level model for dual-ring DLSPP designs supporting 2 × 2 thermo-optic switch operation. The validity of our circuit-level modeled 2 × 2 thermo-optic switch is verified by means of respective full vectorial three-dimensional Finite Element Method (3D-FEM) simulations. The router setup is completed by means of two 4 × 1 SOI multiplexing circuits, each one employing four cascaded second order micro-ring configurations with 100 GHz spaced resonances. Successful interconnection between the DLSPP switching matrix and the SOI circuitry is performed through a butt-coupling design that, as shown via 3D-FEM analysis, allows for small coupling losses of as low as 2.6 dB. The final router architecture is evaluated through a co-operative simulation environment, demonstrating successful 2 × 2 routing for two incoming 4-wavelength Non-Return-to-Zero (NRZ) optical packet streams with 40 Gb/s line-rates.