Determination of the refractive index and size distribution of aerosol from dual-scattering-angle optical particle counter measurements

A method is presented for inferring both the refractive index and the size distribution of aerosol from observations of a dual-scattering-angle optical particle counter (OPC). An existing prototype of an OPC with 60 degree and 90 degree dual-scattering angles was used for the experiments. Based on the high sensitivity of the OPC response to the refractive index of particulates, two families of size distribution curves may be calculated. The solution of the refractive index corresponds to the superposition of the two size distributions. This method was applied to the simulation and to the field measurements conducted in Beijing and Hefei, and the results of both are presented.     

High-accuracy calculations of the refractive indices of optical materials in a wide spectral range

The interaction between light and matter is studied thoroughly by means of the quantum theory. On the basis of it, the refractive indices of types of optical materials are calculated by several formerly experienced formulas. Finally,anew, tested approximate formula with the highest calculated accuracy is obtained, and some groups of valuable parameters are given.     

Determination of refractive indices of quarter-wavelength Bragg reflectors by reflectance measurements in wavelength and angular domains

We show that all the structural properties of periodic dielectric multilayers can be accurately determined by a combined measurement of the transmission as a function of the wavelength and of the reflection as a function of the angle of incidence when the wavelength of the incident light is fixed. This method is applied to determine the structural properties of two commercial dielectric mirrors, and the results obtained are compared with a measurement of the same structural parameters by use of another technique based on the more standard optical guiding method.     

Determination of refractive indices of biconvex lenses by use of a Michelson interferometer

Measurements of lens parameters such as focal length, radius of curvature, and refractive index are important. We describe a measurement method that utilizes a Michelson interferometer to determine parameters of thin, convex lenses. The real fringe system formed by a Michelson interferometer is used to determine the focal lengths and the radii of curvature of the lenses. The refractive index of the lens material is determined from the thin-lens formula. We were able to determine the refractive indices to an accuracy as great as 99.97%. A detailed theoretical and experimental analysis is given.     

Laboratory measurements and T-matrix calculations of the scattering matrix of rutile particles in water

We present experimentally determined scattering matrix elements of birefringent rutile particles in water as a function of the scattering angle for a wavelength of 633 nm (in air). These elements are compared with the results of T-matrix calculations for prolate spheroids. For the diagonal matrix elements the results of the T-matrix calculations are in good agreement with those of the measurements. A good fit for the whole matrix, including the off-diagonal elements, is obtained when we compensate for the birefringence of the rutile particles by performing the computations for spheroids with a slightly larger length/width ratio than measured.     

White-light optical particle spectrometer for in situ measurements of condensational growth of aerosol particles

A new optical particle counter was developed to provide fast in situ sizing of cloud droplets in the Leipzig Aerosol and Cloud Interaction Simulator (LACIS). The new instrument features white light for the illumination of the sampling volume: two off-axis elliptical mirrors, providing a wide angle of collection for light scattered by particles; and an optically defined sampling volume. The smooth unambiguous response characteristic for water droplets allows direct conversion of the measured signal amplitudes into droplet diameters. Preliminary response measurements for dry polystyrol microspheres and water droplets, grown in the LACIS on NaCl particles, have shown good agreement with the corresponding calculated response curves.     

Determination of cloud microphysical properties by laser backscattering and extinction measurements

The extinction and backscattering of 514-nm laser radiation in polydisperse water droplet clouds have been studied in the laboratory. Three cloud size distributions with modal diameters of 0.02, 5, and 12 microm have been investigated. The relationships between the cloud optical parameters (attenuation coefficient sigma and volume backscattering coefficient beta(pi)) and the cloud water content C have been measured for each size distribution. It has been found that a linear relationship exists between sigma and C and between beta(pi) and C for cloud water content values up to 3 g/m3. The linear relationships obtained, however, have slopes which depend on the droplet size distribution. For a given water content both sigma and beta(pi) increase as the modal diameter decreases. The measured data are compared with existing theoretical analyses and discussed in terms of their application to lidar measurements of atmospheric clouds. It is concluded that the empirical information obtained can serve as a basis for quantitative lidar measurements.     

Determination of cloud effective particle size from the multiple-scattering effect on lidar integration-method temperature measurements

A method is presented that permits the determination of the cloud effective particle size from Raman- or Rayleigh-integration temperature measurements that exploits the dependence of the multiple-scattering contributions to the lidar signals from heights above the cloud on the particle size of the cloud. Independent temperature information is needed for the determination of size. By use of Raman-integration temperatures, the technique is applied to cirrus measurements. The magnitude of the multiple-scattering effect and the above-cloud lidar signal strength limit the method's range of applicability to cirrus optical depths from 0.1 to 0.5. Our work implies that records of stratosphere temperature obtained with lidar may be affected by multiple scattering in clouds up to heights of 30 km and beyond.     

Structural and optical properties of high refractive indices lead vanadate thin films

Glasses with the chemical formula xWO₃–50PbO–(50 − x)V₂O₅, (0 ≤ x ≤ 10 mol%) were prepared by the usual melt quenching technique. FTIR analysis revealed that, the incorporation of WO₃ into the lead-vanadate network increases the number of bridging oxygens, and replaces low-coordinated structural units such as PbO₄, and VO₄ with higher coordinated structural units WO₆ and VO₅ which increases the number of bonds and the average cross-link density. Thin films of these glasses onto quartz substrates have been obtained by thermal evaporation technique. Based only on the measured transmittance spectra in the wavelength range 200–2500 nm, both the film thickness and the complex index of refraction have been calculated precisely. The absorption edge was shifted toward the long wavelength side (i.e. red shift of the optical band gap) as the WO₃ content increases. The dispersion of the refractive index was discussed in terms of the single oscillator model. The allowed non-direct transitions successfully describe the absorption mechanism in these films. Based on generalized Miller's rule, the third-order non-linear optical susceptibility χ⁽³⁾ has been investigated. The obtained values of χ⁽³⁾ values are rather large, indicating that the films under study are interesting materials for non-linear optical devices.     

Theoretical development of in situ optical particle separator: cross-type optical chromatography

An in situ separation system, cross-type optical chromatography, is developed theoretically, and an analytic solution of the retention distance is derived. Particle trajectories in the cross-type optical chromatography are calculated for various sizes and materials of the particles and for flow velocities. Further, cross-type optical chromatography assisted by a particle beam generation system is designed.     

Determination of the refractive indices of layers in a multilayer stack by a guided-wave technique

The m-lines technique is used to measure the refractive indices and thicknesses of layers embedded in a multilayer stack. The multilayer considered is deposited by ion plating. Its formula is silica-H-L-H-L-H-air, where H and L denote Ta(2)O(5) and SiO(2)lambda/4 layers, respectively, with lambda = 514.5 nm. Measurements indicate that the refractive index of Ta(2)O(5) is 5 x 10(-3) greater when the layer is close to air than when the layer is inside the coating and that the Ta(2)O(5) is slightly more birefringent.     

Determination of refractive index, thickness, and the optical losses of thin films from prism-film coupling measurements

We present a method of analysis of prism-film coupler spectroscopy based on the use of transfer matrix and genetic algorithm, which allows the simultaneous determination of refractive index, thickness, and optical losses of the measured layer.     

Simultaneous determination of size and wavelength-dependent refractive indices of thin-layered droplets from optical resonances

A technique for determining the size and wavelength-dependent refractive indices of a droplet coated with a thin layer is presented. The existence of a layer on the droplet is identified by a procedure that involves separate alignments of independently measured TE- and TM-mode resonances with computed homogeneous-sphere resonances. The procedure also yields the mode and the order numbers associated with the measured resonances. The observed resonances are then aligned with layered-sphere resonances of the same mode and order numbers to determine the core radius, layer thickness, and constants of core and shell dispersion formulas that minimize the difference between the observed and the calculated positions of resonances. The technique has been tested with synthetic data with various levels of random errors as well as with experimental data from two droplets under identical conditions. The results show that the core radius, layer thickness, and core and layer refractive indices can be determined with relative errors of 3.5 × 10(-4), 4.5 × 10(-2), 2.3 × 10(-4), and 4.4 × 10(-3), respectively, with the technique.     

Straightness measurements by use of a reflection confocal optical system

Straightness measurement is a very important technique in the field of mechanical engineering. A particular application for straightness measurement is high-accuracy machining on a diamond-turning lathe. We propose a novel, to our knowledge, optical method for measuring the straightness of motion, and its mathematical analysis is outlined. The technique is based on measurement of the lateral displacement of point images by use of reflection confocal optical systems. The advantages of this method are that (i) the lateral displacements in the direction of the two axes perpendicular to the optical axis can be measured, (ii) the rotation angles around all three axes can be measured, and (iii) reflection optical systems are more compact in length than are transmission optical systems.     

In situ and ex situ stress measurements of YF3 single layer optical coatings deposited by electron beam evaporator

Yttrium fluoride (YF₃) is a material with good potential as a single-layer anti-reflection (AR) coating deposited onto glass substrates. YF₃ is a possible candidate to replace more studied fluorides such as MgF₂ or ThF₄ the latter being radioactive. For thin-film photovoltaic solar cells, depositing such layers could be a cost-effective way of improving the transmission of light to the p–n junction. However, long-term stability of these AR coatings is an important issue to be considered. This paper is concerned with the residual stress of YF₃ single layers deposited onto glass substrates, being a potential failure mechanism. The measured stress values are correlated to the structure of the layers, using ex situ characterization techniques, such as X-ray diffraction (XRD) and atomic force microscopy (AFM), in an attempt to explain the observed changes. Three substrate temperatures were investigated during deposition, namely 25, 115, and 210 °C. Intrinsic stress of up to 197 MPa has been observed in amorphous YF₃ single layers deposited by electron beam evaporation, at ambient temperature. At a substrate temperature of 210 °C, the intrinsic stress decreased to 67 MPa in the YF₃ layer, developing an orthorhombic structure. Adsorptive stress is an important issue encountered in YF₃ layers, directly related to a low packing density. The in situ stress measurements were carried out using a novel optical approach with a laser-fiber system, briefly introduced here. The residual stress values measured with this novel optical system were compared to two ex situ stress measurement techniques. The optical performance of the YF₃ single layers was also assessed using a spectrometer ex situ and in situ by the interferometry capability of the novel in situ monitoring device.     

Determination of optical constants of thin films and multilayer stacks by use of concurrent reflectance, transmittance, and ellipsometric measurements

Using measurements of reflectance, transmittance, and the ellipsometric parameter D, we have determined the thickness, refractive index, and the absorption coefficient of various thin films and thin-film stacks. (D, the relative phase between the p- and s-polarized components, is measured for both reflected and transmitted light.) These optical measurements are performed with a specially designed system at the fixed wavelength of lambda = 633 nm over the 10 degrees -75 degrees range of angles of incidence. The examined samples, prepared by means of sputtering on fused-silica substrates, consist of monolayers and trilayers of various materials of differing thickness and optical constants. These samples, which are representative of the media of rewritable phase-change optical disks, include a dielectric mixture of ZnS and SiO(2), an amorphous film of the Ge(2)Sb(2.3)Te(5) alloy, and an aluminum chromium alloy film. To avoid complications arising from reflection and transmission losses at the air-substrate interface, the samples are immersed in an index-matching fluid that eliminates the contributions of the substrate to reflected and transmitted light. A computer program estimates the unknown parameters of the film(s) by matching the experimental data to theoretically calculated values. Although our system can be used for measurements over a broad range of wavelengths, we describe only the results obtained at lambda = 633 nm.     

Quantitative optical image reconstruction of turbid media by use of direct-current measurements

We present a detailed experimental study concerning quantitative optical property reconstruction of heterogeneous turbid media by use of absolute dc data only. We performed experiments by using tissuelike phantoms in both single-target and multitarget configurations in which variations in target size and optical contrast with the background were explored. Our results show that both scattering and absorption images can be reconstructed quantitatively by use of dc data only, whereas it was impossible to obtain such quantitative information in previously reported studies. We believe that this improvement is primarily a result of the realization of a novel data preprocessing/optimization scheme for accurately determining several critical parameters needed for reconstruction. The use of this data preprocessing/optimization scheme also eliminates the calibration reference measurement previously required for reconstruction. Experimental confirmation of this scheme is given in detail.     

In situ measurements of H2O from a stratospheric balloon by diode laser direct-differential absorption spectroscopy at 1.39 microm

A distributed-feedback InGaAs laser diode emitting near 1.393 mum is used in conjunction with an optical multipass cell that is open to the atmosphere to yield ambient water-vapor measurements by infrared absorption spectroscopy. To obtain the high dynamic range for the measurements that is required for continuous water-vapor monitoring in the upper troposphere and the lower stratosphere, we used a simple circuit that combined differential and direct detection. Furthermore, the laser emission wavelength was tuned to balance the steep decrease in H(2)O concentration with altitude by sweeping molecular transitions of stronger line strengths. The technique was implemented by use of the Spectromètre à Diodes Laser Accordables (SDLA), a tunable diode laser spectrometer operated from a stratospheric balloon. Absorption spectra of H(2)O in the 5-30-km altitude range obtained at 1-s intervals during recent balloon flights are reported. Water-vapor mixing ratios were retrieved from the absorption spectra by a fit to the full molecular line shape in conjunction with in situ pressure and temperature measurements, with a precision error ranging from 5% to 10%.     

Direct measurement of substrate refractive indices and determination of layer indices in slab-guiding structures

We present a new method for accurate and nondestructive measurement of the refractive indices of substrates and guiding layers in slab waveguides. This method is based on the excitation of leaky waves in substrates and guided waves in guiding layers owing to the etching of grating couplers on the top of structures. It is particularly applicable to high refractive-index materials and to in situ measurements near the energy band gap of semiconductor waveguides. We present results that were obtained for an InP substrate with an InGaAsP epitaxial layer with regard to their refractive indices and temperature coefficients.