KEYWORDS: Diodes, Silicon, Interfaces, Solar cells, Solar energy, Tandem solar cells, Compound semiconductors, Germanium, Structural design, Optical alignment
The electrical carrier transport of a tandem cell structure was evaluated by investigating the band alignment of
and carrier transport through a tunnel junction. The modeling structure of a tandem cell consists of a III-V (or
II-VI) top cell layer, a Si bottom cell layer and tunnel junction layers in-between which connect the top and
the bottom cells. The values of energy bandgap and electron affinity of each layer were varied to investigate
their effect on the energy barrier height at the interface between Si and compound semiconductors of interest.
The contour plots of barrier heights for majority and minority carriers at the hetero-interface are used as a
starting point to define the successful regions for electrical carrier transport through the tunnel junctions.
Long-period grating assisted Michelson interferometer has been proved to be a perspective device for chemical and
biochemical sensing based on refractive index measurement. However, the phase difference between the fiber core
propagation and the fiber cladding propagation in the in-fiber Michelson interferometer is correlated to the common
path length. As a result, the fringe spacing in the reflection spectrum drops with the interferometer cavity length, leading
to reduced fringe wave length shift in refractometry measurement. In this paper, we proposed a long-period grating
assisted Michelson interferometer with the core mode and the cladding mode propagation separated. By eliminating the
residue core mode after the long period grating coupling, an interferometer arm consists of cladding mode propagation
is realized. With the cavity length of the other arm controllable, the fringe spacing in the reflection spectrum of the
Michelson interferometer can be precisely controlled. The proposed Michelson interferometer allows use of long cavity
length of the cladding mode arm to accumulate phase change and large fringe spacing for wavelength interrogation in
refractometry sensing.
Many configurations of fiber optic evanescent wave sensors have recently been explored, with various structural and material modifications applied in attempt to increase their resolution and/or sensitivity. With the aid of long period gratings inscribed within the core of standard single mode fibers, fiber optic evanescent wave sensors with in-fiber interferometric configurations have been realized and have been shown to have excellent resolution due to sharp spectral features. The Michelson interferometer configuration, whereby a single long period grating acts as a beam splitter for the core and cladding modes, is of interest because it operates in reflection mode, which allows for easy signal detection schemes. In this work, it is experimentally demonstrated for the first time that the deposition of a nanoparticle-polymer composite high refractive index overlay film onto the cladding arm surface of such an interferometric sensor greatly increases its sensitivity. Film refractive indices of > 1.7 are achieved and can be further increased to > 2 upon repetition of the nanoparticle synthesis cycle. Sensitivity enhancement factors as large as 16.7 occur in the film index range of 1.9 - 2.1. Experimental data are presented and compared to the theoretical simulation results.
Deposition of a high refractive index (RI) overlay on long-period grating (LPG) was recently studied to enhance the
sensitivity of LPG sensor to the surrounding ambient RI changes and offset the sensitive operational range of LPG RI
sensor to low ambient RI. The enhancement mechanism relies on cladding mode reorganization due to the existence of
the overlay. In this paper, the reorganization of the cladding mode in a cladding radius reduced LPG coated with high RI
overlay is studied with full vector transfer matrix method. The dependence of the cladding modes effective indices on
the fiber parameters and ambient RI is systematically investigated. Based on the simulation result, an LPG refractive
index sensor optimization procedure is proposed, in which the grating period is set as an adaptive parameter and the
cladding radius of the LPG is reduced. It is found that by using the proposed optimization method, the notch band
corresponding to low order cladding mode resonance has higher sensitivity to the ambient refractive index. It is shown
that an ambient refractive index sensitivity as large as 4012nm/RI can be achieved, representing a 3-fold sensitivity
enhancement as compared to the best result obtained from the reported structure in which the high order cladding mode
resonate notch wavelength was employed in sensing.
A sensitivity enhanced long-period grating (LPG) refractive index sensor is proposed and studied by using the LP model. In the simulation, the cladding layer of the LPG is assumed to be partially removed and then deposit a sensitivity enhancement layer (SEL) with a higher refractive index. The effects of the thickness of the original cladding material, and the thickness and refractive index of the SEL layer on the LPG transmission spectrum notch wavelength shift as a function of the ambient refractive index change are reported. The LPG sensor performance depends on the phase match condition of the core mode and cladding mode coupling in the LPG structure and the dependence of the effective index of the cladding mode on the thickness of the original cladding material, and the thickness and refractive index of the SEL layer. The structure modified cladding layer moves the work point of the long period grating to the cladding mode reorganization zone, where the cladding mode effective refractive index changes rapidly upon the LPG waveguide parameter. Proper selection of parameters of the cladding layer can be applied to enhance the modulation of the effective index of the cladding mode by the ambient refractive index through the evanescent field and thus construct sensitivity enhanced ambient refractive index sensors.
Recently, multimode fiber (MMF) and components based on MMF have attracted much attention due to their potential applications in future optical access networks. Fiber Bragg gratings (FBGs) are considered to be key components in both telecommunication and sensing applications. Although single-mode fiber based FBGs (SMFBG) have been studied thoroughly, few studies have been reported on MMFBGs, mainly because of the complexity and multiple mode nature of the MMF.
In this paper, transmission and reflection spectra of MMFBGs are studied systematically. Relationships between transmission/reflection spectra and the excitation conditions are clearly demonstrated by observing the far-field pattern. Different launching methods including the lateral offset launching and angular offset launching and light sources with different spectrum width are used in experiments. Furthermore, the transmission/reflection spectra dependence on the polarization state of excitation light and asymmetric refractive index perturbation profile are studied in detail. Theoretical simulations are used to compare these experimental results.
A DNA aptamer sensor using a long-period grating (LPG) is proposed for the sensitive detection of the interaction between the DNA aptamer and its target. The DNA aptamer can be immobilized on the surface of the LPG, and the detection of the specific interaction between the DNA aptamer and its target is monitored optically. This sensor operates on the evanescent field interacting with the DNA aptamer immobilized over the grating region. The reaction between the DNA aptamer and its target alter the refractive index and thus change the LPG transmission spectrum, which can be monitored in real time. This technology eliminates the labeling of the DNA aptamer. In this paper, for the first time, the dependence of the LPG transmission spectrum on the characteristic of the LPG and the property of the immobilized DNA aptamer are studied both theoretically and experimentally. Experimental demonstration of the immobilization of DNA aptamer on LPG and its real-time effect on the transmission spectrum are reported.
In order to offer operating information to the monitoring system of a chip refiner, a neural network classifier is proposed for identifying the acoustic emission signals of wood species. In addition to classifying single wood species, the system is required to be able to recognize mixed species. The classification task is accomplished by a multilayer feedforward neural network in which both supervised and unsupervised learning are included. The simulations are run on the testing data, mixing two single species to represent mutually mixed wood species of five categories. If a signal is identified as a mixture, the network will indicate the corresponding component species according to a lookup table. Some expected classification accuracy is obtained on both single and mixed species identification and performance of classification is discussed based on the simulation results.
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