Change In Polarization State Research Articles

Polarization effects in microwave diversity imaging of perfectly conducting cylinders

A theoretical vector development is derived for the directly and mutually scattered wavefield of two cylinders in a bistatic measuring system. The approach is first to give an expression for the field scattered from a single cylinder illuminated by a right-hand circularly polarized plane wave. This expression is then extended to the case of the directly scattered (or first-order) field of two cylinders. The mutually scattered (or second-order) field of two cylinders is then formulated in terms of a coherent summation of the scattered field from each cylinder due to the incident wave scattered from the other cylinder. In order to simplify computational tasks, only the second-order scattered field is analytically derived; this is called the mutually scattered field. The total wavefield scattered by two cylinders becomes the coherent superposition of the directly scattered component and the mutually scattered component for each polarization. The use of an automated microwave imaging facility employing frequency, polarization and angular diversity to verify the results of theoretical analysis is described. The analytical and experimental results are shown to be in good agreement. The results show that the effects of polarization state transformation or change in the mutually scattered field component are more pronounced than in the directly scattered component. >

Combined interferometric and polarimetric fibre optic temperature sensor with a short coherence length source

We describe an optical sensor utilising a birefringent monomode fibre as the sensing element. The optical arrangement comprises a Mach-Zehnder and Michelson interferometer configured in tandem. A short coherence length source is used, and the path difference in one interferometer is balanced by the second. Measurand-induced changes in both polarisation state and phase are determined, thus extending the unambiguous measurement range over that of a simple interferometer. The inherently balanced optical arrangement reduces the effects of noise and drift in the optical frequency of the source. The device was demonstrated as a temperature sensor.

Depolarization and cross polarization in ellipsometry of rough surfaces

In ellipsometry of rough surfaces, the commonly used parameters, psi and delta, are insufficient to characterize completely the changes in polarization state which occur when light is reflected from a rough surface. When an experimentally determined Mueller matrix is available, parameters indicative of depolarization, cross polarization, and change in ellipticity can be found. When the Mueller matrix is regarded as an operator mapping input polarization states depicted on a Poincaré sphere to output states in a similar coordinate system, these new parameters can be illustrated in terms of their effects on the Poincaré sphere. The depolarization and cross polarization parameters correlate with specimen roughness and reflectance.

Monomode fiber-optic strain gauge with simultaneous phase- and polarization-state detection

A fiber-optic sensor for the measurement of strain is described, in which the measurand-induced changes in both phase and polarization state are simultaneously recovered. A simple optical arrangement based on the fiber Fabry-erot interferometer is used to realize a high-resolution instrument with enhanced measurement range.

Optical bistability based on self-induced polarization-state change in anisotropic Kerr-like media

A new scheme is proposed for the realization of all-optical ultrafast functional devices, such as self-modulators and multistable elements, utilizing a nonresonant chi((3)) process. The basic physical mechanism involved is the intensity-dependent self-induced change in the polarization state of light during propagation in an optically anisotropic Kerr-like medium with a large linear birefringence.

Angular dependence of scattering intensity from thin films in total reflection Raman spectroscopy

The Raman intensity from thin films on a hemicylinder glass prism (refractive index n=1.88) was measured by the total reflection method as a function of the Raman scattering angle (α) and the polarization of the exciting laser beam. The results were in good agreement with an electromagnetic theory. The Raman intensity is a sharp function of α and the maximum intensity is obtained near the critical angle of α c = sin -1(1/ n) of the prism side. The change in polarization state of the exciting laser beam resulted in remarkable variations of the Raman intensity, and besides, the intensity changed depending upon molecular orientations and symmetry of vibrational modes. These facts indicate that the sensitivity of total reflection Raman spectroscopy can be remarkably improved by exciting surface species with a suitably polarized laser beam and by measuring Raman scattered light through the prism at α c.

Origins and control of polarization effects in single-mode fibers

The polarization state of light in single-mode fibers is very sensitive to any perturbation which is not symmetric about the fiber axis. While this is a source of noise, drift, or signal fading in some applications, it can also be exploited in novel guided-wave devices. The basic birefringences that couple the two modes and change the polarization state along the fiber are reviewed. The three cases of uniform, phase-matched, and random coupling are considered. Polarization preservation in both low- and high-birefringence fibers is achieved by reducing this coupling. In addition to polarization-state changes, bireftingent fibers can quickly reduce the polarization degree of nonmonochromatic light if both modes are excited, a characteristic that greatly simplifies evaluation of the degree of polarization preservation in these fibers. Current evaluations of the birefringence and the polarization-holding ability of state-of-the-art fibers are discussed, and it is concluded that fibers with good polarization-holding properties are becoming available.

Ellipsometry in Thin Film Analysis

This is not meant to be another review of ellipsometry, since a number of detailed works (1-5) have been published in recent years, including a very complete book by Azzam & Bashara (I). Rather we discuss why this technique, nearly a century old, has received so much recent attention (about 500 references for the period 1976-1979) and will probably continue to develop even faster. The method consists of measuring and interpreting th� change of polarization state that occurs when a polarized light beam is reflected at non-normal incidence from a sample surface. Its sensitivity lies in the submonolayer range, it is nondestructive, and it can be used in adverse ambient conditions including high temperature or high pressure, through gases or liquids, etc. Thus it appears perfectly adequate for tackling many of the problems of the semiconductor industry, which begins with well-polished reflecting substrates such as Si and GaAs and deposits on them thin films in the 1000 A range by various methods such as plasma reactions, chemical vapor deposition, and electrochemistry. Here lies the main reason for th�: growing interest in ellipsometry. The recent development of minicomputers and microprocessors provided the necessary hardware to allow widespread deVelopment of the method. It is now possible to build fully automatic instruments that can perform in real time both data-acquisition and data-reduction functions. Applications are discussed in two categories. Real-time analysis of thin film processing is presented first. In this case, measurements are made at a fixed wavelength and a fixed angle of incidence. The emphasis is on

Propagation of high-frequency electromagnetic waves through a magnetized plasma in curved space-time. II. Application of the asymptotic approximation

This is the second of two papers on the propagation of high-frequency electromagnetic waves through an inhomogeneous, non-stationary plasma in curved space-time. By applying the general two-scale W.K.B. method developed in part I to the basic wave equation, derived also in that paper, we here obtain the dispersion relation, the rays, the polarization states and the transport laws for the amplitudes of these waves. In an unmagnetized plasma the transport preserves the helicity and the eccentricity of the polarization state along each ray; the axes of the polarization ellipse rotate along a ray, relative to quasiparallely displaced directions, at a rate determined by the vorticity of the electron fluid; and the norm of the amplitude changes according to a conservation law which can be interpreted as the constancy of the number of quasiphotons. In a magnetized plasma the polarization state changes differently for ordinary and extraordinary waves, according to the angle between the wavenormal and the background magnetic field, and under specified approximation conditions the direction of polarization of linearly polarized waves undergoes a generalized Faraday rotation.

Polarization-insensitive integrated-optical switches: a new electrode design

The combination of two different types of electrodes in integrated optical switches is shown to permit polarization- insensitive operation. The ability to process arbitrarily polarized light is critical to the use of integrated optical devices with glass fiber transmission lines, due to the rapid changes in polarization state of light propagating in the fibers. We examine the performance of a Deltabeta reversal switch which uses two different types of electrodes and show that polarization-insensitive operation is readily achievable.

Design of integrated optical switches for use in fiber data transmission systems

We consider the design of polarization-insensitive multiple section microoptical switches/modulators. The ability to process arbitrarily polarized light is critical to the use of integrated optical devices with glass fiber transmission lines, due to the rapid changes in polarization state of light propagating in glass fibers. We show that multiple-section <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\Delta\beta</tex> -reversal switches have less severe fabricational tolerances than single-section switches, and can be made polarization independent. We present design criteria and describe the tradeoffs involved in fabricating practical devices.

The mechanism of passivation of nickel as determined by an automatic recording ellipsometer

The process of anodic passivation of nickel in acid solutions has been investigated by means of an automatic recording ellipsometer in which the light emerging from a rotating analyzer has been electronically processed to yield a continuous read-out of the changes of polarization state of light reflected from a nickel anode during a galvanostatic transient. Digital computer analysis of the ellipsometric data has revealed: (a) the existence of an induction time for film formation; (b) a linear growth law for the film during a time interval commencing with the induction time and ending before the passivation time; (c) the occurrence of passivation when the absorption coefficient of the film jumps from a near-zero value to a fractional value; and (d) continuous changes of the refractive index of the film during the period of film growth. These phenomena are consistent with a model in which (a) a pre-passive film forms and thickens by a dissolution–precipitation mechanism, (b) the growth of the film is accompanied by its electrochemical oxidation, (c) the film growth–oxidation process continues until the pre-passive film is transformed into an electronically-conducting material, and (d) the conversion of the pre-passive film into an electronic conductor constitutes the essence of the so-called active–passive transition.

The demarcational potential difference as a characteristic of the changing state of polarization of skeletal muscles at different age periods

Experiments were carried out on rabbits of various ages. The authors investigated the value of the demarcation potential difference in the gastrocne mius muscle and the time interval until it vanishes. In adult rabbits the average value of the demarcation potential difference was 45–65 mv. The time interval until it disappeared equalled 6–8 hours. In rabbits under 10–12 days old this average value amounted to 6–11 mv, with the average time interval until its complete disappearance equalling 50–80 minutes. In rabbits aged from 12 days to 1–11/2 months these two figures were 25–35 mv and 3–4 hours.