Fractality of speckle intensity fluctuations

Coherent-light diffraction on random phase screens with fractal properties leads to the formation of speckle patterns with peculiarities in correlation characteristics in the small-scale region. Such peculiarities are manifested in asymptotic behavior in intensity autocorrelation and structure functions in the vicinity of the zero values of their arguments. Intensity fluctuations in the far and the near diffraction zones are also characterized by values of fractal (Hausdorff-Besicovitch) dimensions D(HB), differing from the corresponding Euclidean dimension. Relationships between the exponential factors of the structure functions of boundary field phase and scattered-light intensity fluctuations as well as between values of D(HB) have been obtained as a result of speckle-formation analysis for different conditions. Their dependencies on the illumination and observation conditions obtained in experiments with fractallike scatterers (rough glass plates) are in satisfactory agreement with theoretical results.     

Phase-correction of turbulent distortions of an optical wave propagating under conditions of strong intensity fluctuations

Phase correction of a plane wave and a spatiolimited beam propagating through a turbulent layer of atmosphere were considered. The required adaptive corrector element size and the system bandwidth were found by numerical simulation. These requirements were determined to be the same as for a weak-intensity scintillation approximation. The size of the required segmented mirror element was found to be equal to Fried length r0, whereas the tolerable time lag was r0/V, where V is the wind velocity. However, the local slope sensors then became impractical, as did tip-tilt correction over the corrector subapertures.     

Finite-mode analysis by means of intensity information in fractional optical systems

It is shown how a coherent optical signal that contains only a finite number of Hermite-Gauss modes can be reconstructed from the knowledge of its Radon-Wigner transform-associated with the intensity distribution in a fractional-Fourier-transform optical system-at only two transversal points. The proposed method can be generalized to any fractional system whose generator transform has a complete orthogonal set of eigenfunctions.     

Analog measurement of scattered optical fluctuations

Astatistical model that describes the analog measurement of a fluctuating light intensity that arises from a non-Gaussian scattering process is developed. The higher-order statistical moments are derived for a p-i-n diode receiver model and gamma-distributed intensity fluctuations. Criteria for the accurate measurement of the scattering fluctuations are found, and these are used to analyze data derived from an on-line scatterometer system. Implications for future on-line measurement technology are discussed.     

Reflection intensity optical fiber sensors for the mid-infrared

Two kinds of reflection intensity sensor made of chalcogenide glass fiber for the mid-IR region are demonstrated. One is a double-fiber reflection sensor based on two tied fibers with a gold-coated hollow metal waveguide connected to the far end of the fibers. The other is a single-fiber reflection sensor based on contact couplers. These reflectance sensors were coupled to a Fourier-transform IR spectrometer by a unique accessory based on nonimaging concentrators. This setup was built to measure absorption spectra of a polymer coating of an aluminum can and a sheet of drafting paper. A theoretical model treating the ratio between the signal from the target and the background is introduced. This model was helpful in deriving the sensitivity characteristics of the sensors from experimental absorption peak heights. Hence, the absorption peaks heights that we obtained using a single-fiber reflection sensor with a symmetric coupler were nearly 50% relative to those obtained with a double-fiber reflection sensor.     

Effect of anisotropy on intensity fluctuations in oceanic turbulence

For an optical spherical wave propagating in an oceanic turbulent medium, the effect of anisotropy on the received intensity fluctuations is investigated. For different anisotropy factors, the variations of the scintillation index vs. the ratio that determines the relative strength of temperature and salinity in the index fluctuations, the rate of dissipation of the mean squared temperature, the rate of dissipation of the turbulent kinetic energy, viscosity, link length and the wavelength are plotted. It is found that, for all the oceanic turbulence and the link parameters of interest, as the medium becomes more anisotropic, the intensity of the optical spherical wave fluctuates less. It is concluded that the performance of an optical wireless communication systems (OWCS) operating in anisotropic oceanic turbulence is better than the performance of OWCS operating in isotropic oceanic turbulence.     

Maximum-likelihood curve-fitting scheme for experiments with pulsed lasers subject to intensity fluctuations

Evaluation schemes, e.g., least-squares fitting, are not generally applicable to any types of experiments. If the evaluation schemes were not derived from a measurement model that properly described the experiment to be evaluated, poorer precision or accuracy than attainable from the measured data could result. We outline ways in which statistical data evaluation schemes should be derived for all types of experiment, and we demonstrate them for laser-spectroscopic experiments, in which pulse-to-pulse fluctuations of the laser power cause correlated variations of laser intensity and generated signal intensity. The method of maximum likelihood is demonstrated in the derivation of an appropriate fitting scheme for this type of experiment. Statistical data evaluation contains the following steps. First, one has to provide a measurement model that considers statistical variation of all enclosed variables. Second, an evaluation scheme applicable to this particular model has to be derived or provided. Third, the scheme has to be characterized in terms of accuracy and precision. A criterion for accepting an evaluation scheme is that it have accuracy and precision as close as possible to the theoretical limit. The fitting scheme derived for experiments with pulsed lasers is compared to well-established schemes in terms of fitting power and rational functions. The precision is found to be as much as three timesbetter than for simple least-squares fitting. Our scheme also suppresses the bias on the estimated model parameters that other methods may exhibit if they are applied in an uncritical fashion. We focus on experiments in nonlinear spectroscopy, but the fitting scheme derived is applicable in many scientific disciplines.     

Determining the aberration characteristics of optical systems containing acousto-optical diffraction elements

Technical Physics Letters - We describe a method and present an example of the calculation of aberration images that arise in optical systems (OSs) containing acousto-optical (AO) diffraction...     

Assessment of electrical and optical properties of heavily Fe-implanted semi-insulating InP

We report total compensation of InP layers n-doped to levels as high as n=2×10¹⁸ cm⁻³ by high temperature (>200°C) MeV Fe implantation and annealing. The electronic density and the active Fe (in the form of the Fe²⁺/Fe³⁺ deep acceptor states) profiles are obtained from the comparison between the current–voltage (I–V) characteristics and the outcome of a numerical simulation. These results are confirmed by photo-induced current transient spectroscopy (PICTS) experiments, which show a correlation between the Fe activation and the background doping concentration. A deeper insight into the properties of the Fe²⁺/Fe³⁺ centers is gained by Fourier transform infrared (FTIR) photoluminescence measurements, showing intense and sharp emission peaks at 3.5 μm, associated with Fe intracenter transitions. The corresponding lifetimes have been studied by time resolved integrated photoluminescence measurements.     

Methods for evaluating the accuracy characteristics of the range finder channel of quantum optical systems

Methods for estimating the instrument error for quantum-optical systems using active and passive optical measures of length are suggested. One is based on simulation of measured pulses reflected from the object and entering the receiving device; another is based on using a section of the fiber optical path. __________ Translated from Izmeritel’naya Tekhnika, No. 5, pp. 19–22, May, 2006.     

Photoconductive arrays for monitoring motion of spatial optical intensity patterns

We describe a photodetector array based on photoconductance-monitoring by four-point probing. This detection scheme is aimed specifically at detecting changes within a speckle or microscopic fringes within a larger nonuniform optical intensity distribution. One specific application is the detection of lateral displacements of these speckles or fringes, for example, in laser light reflected from optically rough vibrating surfaces. With a prototype, we have detected subnanometer surface displacements interferometrically. We also demonstrate speckle-based, noninterferometric detection of a guitar body's vibrations at a standoff distance of 6 m with nanowatt power. We observe and explain the prototype's limited frequency response by considering space-charge effects. This detection scheme is most useful in low-power, low-frequency applications.     

Describing functions for nonlinear optical systems

The concept of describing functions is useful for analyzing and designing nonlinear systems. A proposal for using the idea of describing functions for studying the behavior of a nonlinear optical processing system is given. The describing function can be used in the same way that a coherent transfer function or optical transfer function is used to characterize linear, shift-invariant optical processors. Two coherent optical systems for measuring the magnitude of the describing function of nonlinear optical processors are suggested.     

Non-redundant apertures for optical interferometric systems

Abstract

A method of constructing non-redundant apertures (NRAs) for optical interferometric systems which is based on using cyclic difference sets is suggested, and with it a maximization of the number of NRA elements on square and hexagonal grids of given sizes is performed. Also, tables of elements of NRAs thus obtained are presented.     

Numerical analysis for eddy current characteristics of magnetically saturated ferromagnetic tubes

Magnetic saturation is applied to ferromagnetic tubes inspected by the encircling or inner coil because suppressing magnetic noise is important for the eddy current testing technique. Eddy current signal characteristics in magnetically saturated tubes are different from those in nonmagnetic tubes. In ferromagnetic tubes, defect signal phase angle is not useful for estimating defect depth because it does not depend on the defect depth. In this paper, numerical eddy current analysis has been done in order to explain the relationship between the defect depth and the phase angle in magnetically saturated tubes. This analysis is performed as follows: 1) The magnetic permeability near the defect is calculated as the non-linear magnetostatic problem. 2) The eddy current distribution is calculated as the linear magnetodynamic problem using the incremental permeability value calculated in step 1. The numerical analysis results reveal that the permeability around the defect remains inhomogeneous and it causes the unique eddy current characteristics. Based on these calculated results, a quantitative evaluation method of determining defect depth is proposed. After determining the defect shape by using signal characteristics obtained from the strongly magnetizing state, the defect depth can be estimated by using the signal amplitude.     

Self-adaptive optical systems for long-distance flying optics

The focusing characteristics of long-distance flying optics were studied systemically for TEM(mn) Gaussian beams. The results show that the ABCD law of parameter q can be extended to Gaussian modes of any order when waist radius w in the imaginary part of parameter q is replaced by Rayleigh range Z(R) of a certain resonator in the equation. The difference between the real focal length and the geometric focal length, defined as Df, was calculated for laser applications. A novel self-adaptive optical system was demonstrated for precisely controlling the focusing characteristics of long-distance flying optics. Theoretical analyses and experimental results were consistent.