T. T. Sultanov

Application of speckle interferometry to the analysis of refractive-index gradients in flows

A version of a scheme of speckle interferometry is considered, which is intended for the analysis of the distribution of refractive-index gradients in optically transparent media. The application of this approach to problems of hydrodynamics and gas dynamics makes it possible to determine the structure of the distribution of optical inhomogeneities in liquid and gaseous flows. The method is based on the recording of patterns of two speckle structures obtained in the plane of spatial frequencies by using illumination with an auxiliary speckle structure. One speckle structure is formed in the absence of an object, and the other one is formed by transilluminating an object. In comparison with other known phase methods, the speckle-interferometry methods make it possible to change the measurement sensitivity in a wide range and substantially simplify the interpretation of the results obtained.

Analysis of the amplitude and phase structure of optical nonuniformities in transmitting media with registration in the spatial frequency plane

A solution to the phase problem in optics is considered within the context of the registration and analysis of the amplitude-phase structure of optical nonuniformities in stationary transmitting media or in investigated objects. To solve the problem, the object or the medium is tested by radiation with a known structure. For a certain selected direction of testing, the structural change due to the interaction with the object is registered. Stationary media and objects can be tested along several directions The three-dimensional structure of the optical nonuniformities under study can be analyzed using preliminary information on the symmetry of the medium or the object. To obtain information on the amplitudes and phases of the light field and on their change resulting from the testing of the object, the modulation-spectral method is used. To solve the problem, the intensity distribution is directly detected for the spatial spectrum of the field and for that of the field additionally modulated in a special way. The modulation is performed in the plane of the analyzed filed. It should provide a visualization of the phase information contained in the light field. The obtained intensity distributions and the known initial field make it possible to calculate the two-dimensional structure of the analyzed field and therefore the effect of the optical nonuniformities of the medium or of the object on the field. It is important that the method requires no iteration procedures in solving the problem. This allows one to expect substantial speeding up of the processing and analyzing of the information if compared with the known methods. The paper deals with two variants of the influence of the medium or object on the testing radiation. The first one is connected with the spatial modulation of the field and is described by multiplication. In the second case, the effect of the object leads to redistribution of the radiation in the studied plane and is described by the operation of convolution.

Measurement and analysis of optical inhomogeneities with steep refractive-index gradients in stationary transparent media by speckle-interferometry methods

A version of a scheme of speckle interferometry is considered, which is intended for the analysis of the stationary distribution pattern of steep refractive-index gradients in optically transparent media. The application of this approach to problems of hydrodynamics and gas dynamics allows one to determine the structure of distribution of optical inhomogeneities in liquid and gaseous flows or media. The method is based on the recording of patterns of two speckle structures obtained in the plane of image formation of the object by an optical system when the object is illuminated by radiation having an auxiliary speckle structure. The patterns correspond to the two positions of an object shifted somewhat in the direction of the optical axis of the system. Contrary to the common methods of speckle interferometry, only the state of the object being investigated for two different conditions of illumination by the probe radiation is used for the detection in this case. The difficulties arising in the measurements of the objects with steep refractive-index gradients by the known methods of speckle interferometry are accordingly practically eliminated in the scheme under consideration. In comparison with the schlieren phase methods, this method offers higher possibilities in the variation of sensitivity.

Measurement of small phase shifts of radio signals using acoustooptic reduction

Results are presented of model-based experimental investigations of an interference correlator intended for the measurement of small phase shifts of radar signals received from two antennas. The correlator is based on a Mach-Zender interferometer system. Introduction of a radio signal for the data reduction was simulated by phase diffraction gratings placed in the interferometer arms and displaced relative to one another. The operation of the system was investigated in a regime with measurements in the region of the extrema of the correlation function and in a regime with measurements near zeros (extrema of the derivative) of the correlation function. The investigations were carried out for signals formed in first, second, and third order of diffraction. It is shown that the sensitivity of the system for measurements in the region of a correlation-function zero is higher by an order of magnitude than in the extrem~ m region. It is shown that for radio signals in the 100-MHz band, with allowance for the experimental measurement error, the accuracy of determining the time delays reaches 10−11 sec. The accuracy with which geometric displacements of diffraction gratings are determined reaches 5·10-5 mm.

Application of the correlation method to analyze deformations in transparent materials

The scheme of elastic stress visualization in transparent materials (Plexiglas) by a change in optical path is proposed. The optical path gradient was measured using the correlation method. A model experiment was performed at various configurations of transverse static loads on a Plexiglas beam. It was shown that the displacement field of the background structure corresponds to stress diagram at a given beam load scheme. For the process of point heating of quartz glass, the correlation method is compared to the method based on the photoelasticity effect.

Digital Photography in Measuring the Inclination of the Object Surface with Formation of a Speckle Structure

Optical schemes for measuring surface inclination due to shift or deformation are considered. Speckle-interferometry methods are used. The distinguished features of these schemes are connected with using a digital photocamera and a forming speckle structure in nonmonochromatic light. Several technical solutions are proposed making it possibile to measure inclination with an accuracy up to tenths of degree using standard cheap equipment. To form the speckle structure, the pattern of a random binary structure with unit size of several millimeters is used in the base fixed plane. The digital camera was placed in the analyzed plane and turned together with the plane. It registered a reduced image of the pattern. The registration was done for two positions of the surface, the pattern being illuminated by the camera flash lamp. Measuring the width and orientation of the interference fringes formed for the two speckle structures allows one to determine the inclination and orientation of the rotation axis.

Acoustooptical Spectral Processing of Radar Signals Taking into Account the Signal Spectra of Local Heterodyne and Optical Radiation

We describe the formation of a radio signal designed for acoustooptical processing based on the radar signal analyzed which is characterized by a spectrum, a heterodyne signal with a spectrum of known structure, and a mixer. The signal formed fits the working frequency of the acoustooptical cell. In the cell, the signal produces a traveling acoustic wave due to propagating inhomogeneities of the dielectric constant or refractive index. Within the framework of physical optics, we consider how the acoustooptical cell used for processing the signal works in the regime of the Raman–Nath diffraction. The cell is illuminated by quasimonochromatic radiation of known arbitrary spatial and spectrum structures. One works in the first and higher diffraction orders. Using an original method proposed earlier, we analyze the functioning of the acoustooptical cell in the Bragg diffraction regime at small radiation transformation coefficients. The description is done for a probing signal of known spatial and spectrum structures. The method of acoustooptical analysis of the spectrum structure of a radio signal or radar signal is considered under the decreased influence of the spectra of the probing optical radiation and heterodyne signal. The effect of the spectrum structure of the probing radiation used when processing the radio signal which propagates within the acoustooptical cell is studied in complete analogy with taking into account the exciting radiation spectrum in spectroscopy. For signals with a low level of noise, we use the method based on the Fourier transformation of the intensity distribution registered and on that of the intensity distribution of the initial probing radiation spectrum. Effects of the width and spectrum structure of the heterodyne can also be taken into account similar to the method of excluding the spread function used in investigating spectrograms. The processing uses Fourier transforms of the registered spectrum intensity distribution and of the known spectrum intensity distribution of the heterodyne signal.

Using Speckle Interferometry for Controlling the Units of Multi‐Mirror

Applications of speckle interferometry for measuring displacements of mirrors of multi‐mirror centimeter radiotelescopes are considered and analyzed in modeling experiments. The general principles of speckle photography, speckle interferometry, and the use of two-exposure speckle patterns for the analysis of displacements of rough surfaces (units of the radiotelescope mirrors) are worked out. Experiments are carried out for three possible displacements of the object surface: parallel shifts along the surface itself, rotations, and shifts in the perpendicular direction. The shape and structure of the displaced units do not change. The experiments prove that speckle interferometry is a rather effective and practical tool for measuring shifts and rotations of composite radiotelescope mirrors.

Testing and Analysis of the Structure of Transmitting Media or Objects with Registration of the Field Structure in the Image Plane

A solution to the phase problem in optics is considered within the context of the registration and analysis of the amplitude–phase structure of optical nonuniformities in stationary transmitting media or in investigated objects. To solve the problem, the object or the medium is tested by radiation with a known structure. For a certain selected direction of testing, the structural change due to the interaction with the object is registered. To obtain information on the amplitudes and phases of the testing light field, an original development of the modulation‐spectral method put forward by the authors is used. To solve the problem, the intensity distribution is detected in the image plane both for an unmodulated field and for that subjected to an additional two‐dimensional modulation specially formed in the plane of spatial frequencies. The modulation should provide a visualization of the phase information contained in the light field. The intensity distributions obtained make it possible to determine the two‐dimensional structure of the testing field at the output of the medium or the object. In the proposed variant of the method, the testing field should not be affected in the investigated plane. The interpretation of the results is easier, since it is the image that is registered. The two intensity distributions can be registered simultaneously, provided the light beam is divided into two channels after the optical system. It is significant that the method requires no iteration procedures in solving the problem. This allows one to expect speeding‐up of the processing of the information and analyzing it in almost real time. Two variants of optical schemes are considered in the paper. The first one deals with media or objects with a modulation effect described by multiplication by a complex function characterizing the effect. In the second case, the effect of the object leads to redistribution of the radiation in the investigated plane and is described by the operation of convolution of the testing signal and the function characterizing the effect.

Analysis of phase relationships between radar signals in acoustooptic processing systems

The performance and the salient operation features of an interference correlator with a modified optical twin-wave Rayleigh interferometer system in the mode of measurements of phase shifts between radar signals are considered. Such measurements for signals received from two antennas provide information on the angular position of an object or its displacement. A system wherein radio signals are fed for processing into a correlator with the aid of a double acoustooptical cell with a paratellurite acoustic line is experimentally realized and investigated. We consider the modes of system operation when a signal is generated in the regions of the extrema of the correlation functions and in the regions of their strongest variation (extrema of the derivatives). The accuracy of measurements obtained experimentally for the phase shifts between radio signals reaches 0.003 rad to give an accuracy of determining the signal time delays of the order of 0.005 ns in the frequency region of 100 MHz.