Optimal Refractive Index Research Articles

Modification in the Chemical Bath Deposition Apparatus, Growth and Characterization of CdS Semiconducting Thin Films for Photovoltaic Applications

Abstract In this paper, growth and characterization of CdS thin films by Chemical Bath Deposition (CBD) technique using the reaction between CdCl 2 , (NH 2 ) 2 CS and NH 3 in an aqueous solution has been reported. The parameters actively involved in the process of deposition have been identified. A commonly available CBD system has been sucessfully modified to obtain the precious control over the pH of the solution at 90°C during the deposition and studies have been made to understand the fundamental parameters like concentrations of the solution, pH and temperature of the solution involved in the chemical bath deposition of CdS. It is confirmed that the pH of the solution plays a vital role in the quality of the CBD–CdS films. Structural, optical and electrical properties have been analysed for the as-deposited and annealed films. XRD studies on the CBD–CdS films reveal that the change in Cadmium ion concentration in the bath results in the change in crystallization from cubic phase with (1 1 1) predominant orientation to a hexagonal phase with (0 0 2) predominant orientation. The structural changes due to varying cadmium ion concentration in the bath affects the optical and electrical properties. Optimum electrical resistivity, band gap and refractive index value are observed for the annealed films deposited from 0.8 M cadmium ion concentration. The films are suitable for solar cell fabrication. Further on, annealing the samples at 350°C in H 2 for 30 min resulted in an increased diffraction intensity as well as shifts in the peak towards lower scattering angles due to enlarged CdS unit cell. This in turn brought about an increase in the lattice parameters and narrowing in the band-gap values. The results are compared with the analysis of previous work.

Microanalytical investigations on optical phase gratings in K(TiO)PO 4 single crystals

The Rb/K ion exchange in micro-structured K(TiO)PO4 single crystals in contact with Rb containing nitrate melts results in ion-exchanged zones with enlarged refractive index which works as an optical phase grating. The knowledge of the diffusion behavior was the prerequisite for the generation of optimum refractive index profiles. Therefore, the local Rb/K concentration of ion-exchanged single crystals was determined with an electron probe microanalyzer (EPMA). The observed degree of ion exchange and the calculated Rb/K counterdiffusion coefficients were used to estimate the optimum ion exchange conditions. The diffusion-generated optical phase gratings show very narrow and deep ion-exchanged zones. The ratio of the ion penetration depths results in an extreme diffusion anisotropy DRb/K(c):DRb/K(b) ≈ 500:1.

Characterization and optimization of absorbing plasma-enhanced chemical vapor deposited antireflection coatings for silicon photovoltaics

We have optimized plasma-enhanced chemical vapor deposition (PECVD) of SiN-based antireflection (AR) coatings with special consideration for the short-wavelength (<600 nm) parasitic absorption in SiN. Spectroscopic ellipsometry was used to measure the dispersion relation for both the refractive index n and the extinction coefficient k, allowing a precise analysis of the trade-off between reflection and absorption in SiN-based AR coatings. Although we focus on photovoltaic applications, this study may be useful for photodetectors, IR optics, and any device for which it is essential to maximize the transmission of light into silicon. We designed and optimized various AR coatings for minimal average (spectrally) weighted reflectance (? R(w) ?) and average weighted absorptance (? A (w) ?), using the air mass 1.5 global solar spectrum. In most situations ? R (w) ? decreased with higher n, but ? A (w) ? increased because k increased with n. For the practical case of a single-layer AR coating for silicon under glass, an optimum refractive index of ~2.23 (at 632.8 nm) was determined. Further simulations revealed that a double-layer SiN stack with an n = 2.42 film underneath an n = 2.03 film gives the minimum total photocurrent loss. Similar optimization of double-layer SiN/SiO(2) coatings for silicon in air revealed an optimum of n = 2.28 for SiN. To determine the allowable tolerance in index and film thickness, we generated isotransmittance plots, which revealed more leeway for n values below the optimum than above because absorption begins to reduce photocurrent for high n values.

Formation of the refractive index profile in the graded index polymer optical fiber for gigabit data transmission

Bandwidth characteristics of the large core graded index polymer optical fiber (GI-POF) are theoretically and experimentally clarified. The refractive index profile of the GI-POF was controlled by interfacial-gel polymerization to investigate the relation between the index profile and the bandwidth characteristics. It was experimentally confirmed that the maximum bandwidth of the poly methyl methacrylate (PMMA) base GI-POF is at most 3 GHz for 100 m transmission using a typical laser diode emitting at 650-nm wavelength (3 nm source spectral width) when its refractive index profile is optimized. The maximum bandwidth theoretically estimated by considering both modal and material dispersions is approximately 3 GHz which is exactly the same as the measured value, while higher than 10 GHz for 100 m was expected if only modal dispersion was taken into account. The optimum refractive index profile of the PMMA base GI-POF is theoretically and experimentally clarified by considering the profile dispersion further.

Design of subpicosecond dispersion-flattened fibers

Dispersion flattened (DF) fibers are required for wide-band WDM systems. The DF fibers designed in the past have dispersion in the range of 2.0-3.0 ps/km-nm. In this letter, we define a generalized refractive index profile that can be characterized by few controlling parameters. An optimum refractive index profile is obtained by minimizing the maximum dispersion over the wavelength range of 1300-1600 nm with respect to profile parameters. The designed fiber gives dispersion less than 1.0 ps/km-nm over 1350-1590 nm wavelength range. Sensitivity of the dispersion performance to the profile parameters is also discussed.

Optimum refractive-index profile of the graded-index polymer optical fiber, toward gigabit data links

The optimum refractive-index distribution of the high-bandwidth graded-index polymer optical fiber (POP) was clarified for the first time by consideration of both modal and material dispersions. The ultimate bandwidth achieved by the POP is investigated by a quantitative estimation of the material dispersion as well as the modal dispersion. The results indicate that even if the refractive-index distribution is tightly controlled, the bandwidth of the graded-index POP is dominated by the material dispersion when the required bit rate becomes larger than a few gigabits per second. It is also confirmed that the material dispersion strongly depends on the matrix polymer and that the use of a fluorinated polymer whose material dispersion [-0.078 ns/(nm km)] is lower than that of poly(methyl methacrylate) [-0.305 ns/(nm km)] allows for a 10-Gb/s signal transmission.

Broadband dispersion-compensating fiber for high-bit-rate transmission network use

The optimum refractive-index profile and drawing temperature were investigated so as to maximize the figure of merit for multicladding broadband dispersion-compensating fibers. Based on the results of the investigation, the authors have fabricated a highly bend-resistant fiber with a 92.6-ps/(nm dB) figure of merit using the modified chemical-vapor deposition method for dispersion compensation in the 1.5-1.6-µm wavelength region. The manufactured dispersion compensator does not suffer bend loss at 1.55 µm for curvatures of radia of 6.3 and 3.3 cm, and it has a 1.1-dB/km bend loss at a curvature of radius of 1.6 cm. Codoping the germanium silicate core with fluorine diminishes the optical loss down to 0.70 dB/km at a 1.55-µm wavelength.

レーザー回折・散乱法における入力光学特性値が粒子径分布測定結果に及ぼす影響

A method for estimating the optimum refractive index input value was established to decrease the scattering of the measured results that come with different equipment using the laser diffraction and scattering method.The effect of the imaginary part of the refractive index input value in this estimation method was discussed with copper as the absorbing material and polystyrene, alumina as the non-absorbing material, respectively. For the copper powders which have a large value in the imaginary part, the scattering of the results was decreased with the optimum refractive index input value which is obtained when considering both the real and imaginary parts in this estimation method. For polystyrene and alumina powders which have a small value in their imaginary part, the optimum refractive index input value was obtained when the imaginary part was 0.On the other hand, the estimation method consists of some problems when measuring a bimodal sample. The validity of this estimation method was discussed for the bimodal sample of mono dispersed silica.

Estimation of the optimum refractive index by the laser diffraction and scattering method: On the raw material of fine ceramics

In the field of ceramics, the particle size distribution of submicron powders has been measured by using many different types of equipment, which are based on different measuring principles. However, at present, the laser diffraction and scattering method is the most popular method in particle size analysis. Although this method is good for operation, the refractive index of the particles is required in order to calculate the size distribution. Proper results will not be obtained if the refractive index is not considered in the calculation. For the measurement of an unknown sample, this is the weakest point of the laser diffraction and scattering method. In this study, an estimation method of the optimum refractive index was established by simple data arrangement and the accuracy of the results was improved.

レーザー回折・散乱法における最適屈折率の推定法 ファインセラミックス原料粉体の場合

In the field of ceramics, the particle size distribution of submicron powders has been measured by using many kinds of equipment which are based on the different measuring principles. However, at present, laser diffraction and scattering method is the most popular method in particle size analysis. Though this method is good for operation, the refractive index of a particle is necessary to calculate the size distribution. If the refractive index is not considered in the calculation, it will not be possible to obtain proper results. This is the weakest point of the laser diffraction and scattering method for the measurement of an unknown sample.In this study, a method for the estimation of the optimum refractive index was established by a simple arrangement of the data, and the accuracy of the results was improved.

Precise pattern delineation by refractive index matching in a trilayer resist system

In order to obtain precise pattern dimension control, optical indexes of the middle layer (ML) in a trilayer resist system are investigated. Photoabsorption in ML and bottom layer (BL) is important. However, optimization of photoabsorption in these layers is not sufficiently to suppress linewidth variations. From calculations of the reflection at each interface of the trilayer resist system, the matching of the refractive index between each layer [top layer (TL), ML, and BL] is found to be also important. The optimal refractive index of the ML for the novolac-type of TL and BL materials is estimated to be 1.7 at i line, which is the same value as that of novolac resin. To confirm this matching effect, an admixture material of Ti–alkoxide and silanol is used as the ML material. The mixing ratio is 50 wt %. The refractive index of this material with curing at 230 °C is 1.7. Using this material, the linewidth variation caused by the interference effect can be reduced as 0.04 μm, which is half that using spin-on glass as the ML. The material also shows a high O2 reactive ion etching resistance (1.7 nm/min) and good adhesion to the TL and BL. In the trilayer resist system, it is possible to fabricate a fine, precise pattern even on a large topography.

Analysis of two‐core single‐mode optical fibers for subscriber lines

AbstractMulticore optical fiber, consisting of several cores in the same clad, has been developed for use as optical fiber cable for subscriber lines, by which high‐density cable can be realized. One of the problems of multicore fiber is crosstalk between adjacent cores. In this paper, a two‐core single‐mode optical fiber, which is the basic structure of the multifiber, is analyzed and an optimum refractive index profile for desired crosstalk and possible high density is discussed. In the analysis, comparison was made between step‐type, W‐type, α‐power and inverse α‐power index profiles, considering the mode field diameter which is an important parameter for a single‐mode optical fiber. The results show that: (1) under the same mode field diameter, the crosstalk for α‐power or for inverse α‐power refractive index profile has little difference with that for the step‐type profile; (2) by the use of the W‐type refractive index profile, high density becomes possible. Based on these results, the design and evaluation of the multifiber can be carried out. To analyze the coupled fiber with a radially inhomogeneous refractive index profile, the point matching method and homogeneous multilayer division method are combined. Although scalar approximation is made in the method, in the case of a weak waveguide, the relative error of coupling coefficients was found to be less than 0.1% even for strong coupling.

Refractive index dispersion and related properties in fluorine doped silica

Refractive index dispersion has been determined for fluorine doped fused silica containing 1 and 2% fluorine. Addition of fluorine is found to reduce the refractive index and material dispersion. Fibers modeled for minimization of modal dispersion having fused silica cores and fluorine doped silica cladding show a strong composition dependence of the optimum refractive index profile. Comparison of results with those in the literature based on N.A. vs λ measurements reveal minor discrepancies probably due to the drawing process.

Selfoc microlens with a spherical surface

This paper presents the light-focusing properties of a gradient-index rod lens with a spherical end surface which aims at increasing numerical aperture and decreasing lens aberration. Optimum refractive-index profiles for obtaining a smaller focused spot are derived, and the increase of numerical aperture is clarified. Experimental results are described; the Selfoc microlens with a spherical end surface realizes a focused spot below 2 microm in diameter.

Numerical Calculation of Optimum α for a Germania-Doped Silica Lightguide

Using an exact numerical solution to Maxwell's equations, we determine the optimum refractive index profile parameter α for a germania-doped silica lightguide. Our results agree closely with the earlier work of Olshansky and Keck and with the work of Marcatili. However, differences exist that may be important in the manufacture of very high bandwidth lightguides.

Determination of the complex refractive index and size distribution of atmospheric particulates from bistatic‐monostatic lidar and solar radiometer measurements

A method is presented for inferring both the size distribution and the complex refractive index of atmospheric particulates from combined bistatic‐monostatic lidar and solar radiometer observations. The basic input measurements are spectral optical depths at several visible and near‐infrared wavelengths as obtained with a solar radiometer and backscatter and angular scatter coefficients as obtained from a bistatic‐monostatic lidar. The spectral optical depth measurements obtained from the radiometer are mathematically inverted to infer a columnar particulate size distribution. Advantage is taken of the fact that the shape of the size distribution obtained by inverting the particulate optical depth is relatively insensitive to the particle refractive index assumed in the inversion. Bistatic‐monostatic angular scatter and backscatter lidar data are then processed to extract an optimum value for the particle refractive index subject to the constraint that the shape of the particulate size distribution be the same as that inferred from the solar radiometer data. Specifically, the scattering parameters obtained from the bistatic‐monostatic lidar data are compared with corresponding theoretical computations made for various assumed refractive index values. That value which yields best agreement, in a weighted least squares sense, is selected as the optimal refractive index estimate. The results of this procedure applied to a set of simulated measurements as well as to measurements collected on two separate days are presented and discussed.

Tunnelling rays in graded-index fibres

An analysis of the role of tunnelling rays in pulse propagation in graded-index fibres is presented. Tunnelling ray attenuation is handled by using the generalized parameter technique which allows attenuation coefficients to be dispensed with and relevant families of tunnelling rays to be identified. Results are given for impulse response pulse widths and shapes. Incorporation of tunnelling ray pulse contributions into the formalism for determining optimum refractive-index profiles is presented. For most cases, tunnelling ray effects are small, but significant corrections may be necessary when measurements are made on short lengths of fibre.

Computer-Aided Synthesis of the Optimum Refractive-Index Profile for a Multimode Fiber

In a multimode optical fiber, the so-called multimode dispersion (mode-delay difference) is the principal cause that widens the transmitted pulse. The multimode dispersion can be controlled by the refractive-index profile. However, the optimum profile that minimizes the multimode dispersion has not yet been determined. This paper describes the computer-aided trial-and-error synthesis of the optimum refractive-index profile. It is shown that the group delay is reduced to about 10/sup -3/ times the value obtained with the uniform core fiber, to about 10 ps/km. This value is comparable to the material dispersion obtained with an ordinary fused-silica fiber and a typical semiconductor laser. It is also shown that the optimum profile is a smoothed W-shaped one.

Pulse dispersion in partially-excited graded-index fibres

Simple analytical results are obtained for the mode distribution and impulse response of graded index fibres under a variety of excitation conditions relevant to practical systems. It is found that the choice of an optimum refractive index profile depends quite strongly on the source distribution of or, that one can significantly improve the impulse response of a particular profile by appropriate choice of launching conditions. Furthermore the r.m.s. pulse width is not necessarily reduced simply by partially filling the numerical aperture of the fibre.

Aberrations and Design of Graded-Index (GRIN) Rods Used as Image Relays

In this paper, we consider the aberrations and design of graded-index (GRIN) rods for use as image relays. Aberration formulas are derived based on the ray path solutions previously published by the authors. The aberrations are then minimized by selecting an optimal radial refractive index variation. We assume no index variation along the axis and also consider only monochromatic aberrations. Design curves and data on the number of resolvable spots are included. The accuracy of the aberration formulas is compared with total aberrations computed numerically.