The method of autocorrelation low coherence interferometry is proposed for diagnostics of layered media inner structure.
The possible applications of this method in technology and biomedicine are presented. In this method the low coherence
optical field is reflected from the object's structure and then analyzed using the Michelson interferometer. Since the
object is outside of the Michelson interferometer the axial position of the object is not important and thus the object can
move during the measurements. The theoretical background of this autocorrelation method for a media with discrete and
continuous optical structure modification is presented.
A new technique of Low-Coherent Interferometry (LCI) is discussed. In this method partially coherent optical field reflected from the layered medium directs one's steps into longitudinal shear scanning interferometer for autocorrelation analysis of low-coherent object field. Subject of inquiry is outside of interferometer here and reference beam is not used. Theoretical model for discrete transparent phantom and experimental results for multilayered media are presented.
It is well known that the spectral properties of the light source relate explicitly to the correlation properties of the source field. Thus it is possible to have an action on the correlation function of the source affecting on emission spectrum. It is shown in this paper that correlation analysis of the light source field allows to perform control of the spectral properties of the source. Availability of Michelson interferometer for investigation of the thin structure of emission spectrum of superluminescent diodes (SLD) and appreciation of the use of its in low coherence interference schemes are discussed in this work.
Effects of spatial coherence of optical field with wide frequency and angular spectra in the Michelson interferometer are studied. Manly effects of longitudinal spatial coherence are considered. It is shown that in classical configuration of the Michelson interferometer the effects of not temporal, but spatial coherence are taken place. Experimental data of observation of spatial and temporal coherence effects in image domain behind a lens at the output of the Michelson interferometer are shown.
Features of simultaneous manifestation of longitudinal spatial coherence and temporal coherence under different viewing conditions - angular spectrum width and form, frequency spectrum width and thickness of nondispersive layer in one of the arms of the interferometer - are discussed.
The influence of an angular spectrum of light fields (formed by extended light sources or at sharp focusing using high numerical apertures) on the signal of a low-coherence interferometer is discussed. The reduction of amplitude of the interferometer signal at probing of layered object owing to a broad angular spectrum has been shown both experimentally and by computer modeling.
The phenomenon of longitudinal spatial coherence of light and conditions of manifestation of this type of coherence in
the interference experiments with Michelson interferometer are discussed. Manifestation of longitudinal spatial
coherence of light of an extended thermal source at large (in comparison with length of longitudinal spatial coherence)
and short lengths oftemporal coherence oflight has been experimentally shown.
Two laser interferometric methods with focused laser probing beam for thickness measurements of transparent films and layers are discussed. In both methods the exact focusing of probing beam on the objects surface permit to determine the parameters of random inhomogeneous objects, such as coatings on the rough surfaces. The first of the methods is a so-called focused oblique laser beam (FOLB) method. In this method at illumination by focused oblique laser beam of transparent layer with rough rear surface the reflected field performs superposition of beams with smooth and speckle-modulated wave fronts. The quasiregular interference pattern in reflected field is observed when the waist of focused laser beam overlap a rough rear surface of tested layer. Knowing the angular period of observed interference fringes and refractive index of layer medium one can determine the local geometrical thickness of a layer. The second method is based on the so-called Laser Wave Front Matching Interferometer (LWFMI) which is constructed under scheme of Michelson interferometer with focusing microobjectives in the interferometer branches (Linnik scheme) and wide integrating detector aperture in common exit branch. In the envelope of interference LWFMI signal as a function of object displacement relatively to a focal point of an interferometer focusing objective the peaks are observed when the probing beam is focused on the front and rear surfaces of a layer. The distance bewteen the interference signal peaks is proportional to the geometrical thickness of a layer and can be measured by the technique of interference fringe counting. As in the first method geometrical thickness can be obtained, if the refractive index of layer medium is known.
The so-called Laser Wave Front Matching Interferometer (LWFMI) is developed for thickness measurements of transparent films and layers with high spatial resolution. This interferometer is constructed under scheme of Michelson interferometer with focusing microobjectives in the interferometer arms (Linnik scheme) and wide integrating detector aperture in common exit branch. The peaks are observed in the envelope of interference LWFMI signal as function of object displacement relatively to the focal point of an interferometer focusing objective when the probing beam is focused on the front and rear surfaces of a layer. The distance between the interference peaks is proportional to the geometrical thickness of a layer and can be measured by the technique of interference fringe counting, if the refractive index of layer medium is known. The results of geometrical thickness measurements of some objects by this interferometer for high numerical aperture of probing focused beam are discussed.
The idea of a new low coherence interferometric system with axial resolution better than coherence length providing simultaneous measurement of the geometrical thickness and refractive index of transparent layers by sharp focusing of light on the measured object is considered. The presented interferometric system consists of two parts -- the so-called Wave Front Matching Interferometer (WFMI) and a low coherence Michelson interferometer (LCMI) as a light source for the first. The WFMI provides high separation of interference signal peaks from demarcations in layer object at high numerical aperture focusing of light on the object. The tandem optical scheme of these interferometers allows to make this system very compact and mobile.
Two laser interferometric methods with a focused laser probing beam for thickness measurements of transparent films and layers are discussed. In both methods the exact focusing ofprobing beam on the objects surfaces permits one to determine the parameters of random inhomogeneous objects, such as coatings on the rough surfaces. One is the so-called focused oblique laser beam (FOLB) method. In this method at illumination by a focused oblique laser beam of a transparent layer with rough rear surface the reflected field performs superposition of beams with smooth and speckle-modulated wave fronts. A quasiregular interference pattern in the reflected field is observed when the waist of the focused laser beam overlap a rough rear surface of the layer being tested. Knowing the angular period of the observed interference fringes and refractive index of layer medium one can determine the local geometrical thickness of the layer. The latter method is based on the so-called Laser Wave Front Matching Interferometer (LWFMI), which is constructed under scheme of Michelson interferometer with focusing microobjectives in the interferometer branches (Linnik scheme) and a wide integrating detector aperture in the common exit branch. In the envelope of a interference LWFMI signal as a function of object displacement relative to the focal point of an interferometer focusing objective, peaks are observed when the probing beam is focused on the front and rear surfaces of the layer. The distance between the interference signal peaks is proportional to the geometrical thickness of the layer and can be measured by the technique of interference fringe counting. As in the first method, geometrical thickness can be obtained, if the refractive index of the layer medium is known. The results of geometrical thickness measurements of some objects by both methods are compared and discussed.
The presence of two or more distinct local maxima in the power spectrum of radiation results under certain conditions in appearance of peculiarities in interferometer temporal signal. These peculiarities are manifested as a single coherent trough (minimum) in envelope of interferometer signal and as a (pi) -jump of phase of this signal in the point of envelope's trough. The coherent trough can be used as a measuring signal in the partial coherence interferometry. The conditions for observation of single coherence trough in envelope of interferometer temporal signal are discussed.
In this paper a few speckle techniques is discussed from the point of view of their application for medical diagnosis in dermatology and ophthalmology using monitoring of tissue structure and blood microcirculation. The basic principles of proposed techniques and corresponding hard and software descriptions are presented. Results of model (tissue phantoms), in vitro and in vivo measurements for the human eye tissues and skin are discussed.
On the base of spatial-temporal analogy in properties of the optical fields the equivalence of temporal and spatial photodetector averaging of light interference field is shown. In the low-coherence interferometry the interferential signal is vanished when optical path difference becomes more than light coherence length lc approximately equals (lambda) 2/(Delta) (lambda) . This value determines the accuracy of the testing of the reflecting layer location. The analogous vanishing of the interferential signal can be realized by a spatial averaging of the interference field in the case of the sufficiently wide receiving photodetector aperture. For such averaging Michelson interferometer with a laser beam focussed on the object surface can be used. In this case interferential signal is vanished when the surface displacement is more than a focussed beam parameter (delta) ((Delta) z) equals (lambda) /(NA)2, where NA is a numerical aperture of the objective. With NA approximately equals 1 only a few oscillations of interferential signal is observed. It is corresponding to advanced achievements of the low-coherence interferometry. The experimental results show a high testing accuracy of the position of smooth and rough surfaces, the thickness of transparent coating attached to the rough surface and multilayered scattering objects. Thus, interferometers with a sharply focussed probing beam can be consider as a good alternative for the low-coherence interferometry and can be used in coherent optical tomography.
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