Effects of phase change on reflection in phase-measuring interference microscopy

We show by analytical and numerical calculations that the phase change on reflection that occurs in interference microscopy is almost independent of the numerical aperture of the objective. The shift of the microscope interferogram response due to the phase change on reflection, however, increases with the numerical aperture. Measurements of the interferogram shift are made with a Linnik interference microscope equipped with various numerical-aperture objectives and are reported and compared with theory.     

Suppression of phase ambiguity in digital holography by using partial coherence or specimen rotation

In this paper we present two approaches for extracting the surface profile as well as obtaining 3D imaging of near field objects by usage of partial coherence and digital holography. In the first approach a light source with given temporal partial coherence is used to illuminate a near field object. The reflected light is interfered with the reference source. By computing the local contrast of the generated fringes one may estimate the 3D topography and the profile of the object. This approach extracts the 3D information from a single image, and its accuracy does not depend on triangulation angle like in fringe projection methods. The second approach is tomography based. There we illuminate the object from several slightly different angles, and for each we compute the wrapped phase using digital holography techniques. Combining the wrapped phase estimation from several points of projection allows calculating the unwrapped phase and therefore the true profile of even a phase-only object. Increasing the number of points of view decreases the relative error of the estimated profile.     

Raising the precision of phase measurements at ultrahigh frequencies with a mismatched phase shifter

Conclusions The errors due to mismatching in phase-difference measurements consist of those produced by the mismatching of the measurement element and of the phase shifter. In the above case we evaluate the errors due to the mismatching of the phase shifter and suggest that the normal measuring method should be changed in order to reduce the error of a given mismatch by a factor of 5–10.Translated from Izmeritel'naya Tekhnika, No. 3, pp. 30–33, March, 1967.     

High accuracy, wide range, rotation angle measurement by the use of two parallel interference patterns

Based on the idea of measuring small rotation angles with a parallel interference pattern (PIP), a method is developed to measure large rotation angles accurately. Two parallel PIP's that have different periods are used to measure a rotation angle of an object. The measurement made with a small-period PIP provides a high accuracy, and the measurement made with a large-period PIP provides a wide range. An accurate measurement for wide-range angles is made by combining the two measured values. The accuracy of the phase detection is determined by the periods of two PIP's. Rotation angles from approximately -30 to 30 arc min can be measured with an accuracy of 0.2 arc sec. Analytical results are supported by experimental results.     

Quantitative phase and refractive index analysis of optical fibers using differential interference contrast microscopy

A systematic and straightforward image processing method to extract quantitative phase and refractive index data from weak phase objects is presented, obtained using differential interference contrast (DIC) microscopy. The method is demonstrated on DIC images of optical fibers where a directional integration routine is applied to the DIC images to extract phase and refractive index information using the data obtained across the whole DIC image. By applying the inverse Abel transform to the resultant phase images, an accurate refractive index profile is obtained. The method presented here is compared to the refracted near-field technique, typically used to obtain the refractive index profile of optical fibers, and shows excellent agreement. It is concluded that through careful image processing procedures, DIC microscopy can be successfully implemented to obtain quantitative phase and refractive index information of optical fibers.     

Achromatic phase retarder by slanted illumination of a dielectric grating with period comparable with the wavelength

We discuss some properties of dielectric gratings with period comparable with the illuminating wavelength for slanted illumination (this illumination geometry is often referred to as concical mounting). We demonstrate the usefulness of such an illuminating geometry. We show that the threshold period (under which only the zeroth transmission and reflection orders are nonevanescent) can be significantly higher, thereby easing fabrication constraints, and that this illumination setup makes it possible to design achromatic phase retarders. Such a design, for an achromatic quarter-wave plate with lambda/60 uniformity of the retardation phase in the 0.47-0.63-mum wavelength interval, is demonstrated.     

Reconstruction in interferometric synthetic aperture microscopy: comparison with optical coherence tomography and digital holographic microscopy

It is shown that the spatial frequencies recorded in interferometric synthetic aperture microscopy do not correspond to exact backscattering [as they do in unistatic synthetic aperture radar (SAR)] and that the reconstruction process based on SAR is therefore based on an approximation. The spatial frequency response is developed based on the three-dimensional coherent transfer function approach and compared with that in optical coherence tomography and digital holographic microscopy.     

Linear analog rotation angle converter with an error of 0.001%

Conclusions The above linear analog converter can be used for nonperiodic transformation of an angle into a voltage over a wide angular range. The simple design, simple manufacturing requirements, and reliable operation inherent to analog devices are combined with a high conversion precision.The converter can be used in compensated circuits owing to the small phase difference between its output and supply voltages. The converter has a considerable output power combined with a high precision. The converter's insensitivity to noise is very high owing to its very low (of the order of 100 m) output resistance. The low noise level simplifies the design of null indicators. This converter can be usefully and effectively applied to programmed control systems, remote transmissions, and autocompensating systems.     

Design of a vector-sum integrated microwave photonic phase shifter in silicon-on-insulator waveguides

An orthogonal vector-sum integrated microwave photonic phase shifter (IMWPPS), consisting of mode-order converter multiplexers (MOCMs), a variable optical power splitter (VOPS), an optical switch (OS) and fixed time delay lines (FTDLs), was theoretically demonstrated in a silicon-on-insulator wafer. MOCMs, as a key element of our device, were employed to generate orthogonal vector signals and served as lossless optical combiners. Combining with the thermo-optical VOPS, OS and FTDLs, the microwave phase shift of 0～2π could be achieved by a refractive index variation of 0～15×10(-3) in the millimeter wave band. The corresponding tuning resolution was about 1.64°/°C. This work, for the first time to our knowledge, provides an attractive solution to transferring a vector-sum method based bulk MWPPS into a integrated one, which is very important for large-scale optically controlled phase array antenna.     

Partial spatial coherence effects in digital holographic microscopy with a laser source

We investigate a digital holographic microscope that permits us to modify the spatial coherence state of the sample illumination by changing the spot size of a laser beam on a rotating ground glass. Out-of-focus planes are refocused by digital holographic reconstruction with numerical implementation of the Kirchhoff-Fresnel integral. The partial coherence nature of the illumination reduces the coherent artifact noise with respect to fully coherent illumination. The investigated configuration allows the spatial coherence state to be changed without modifying the illumination level of the sample. The effect of the coherence state on the digital holographic reconstruction is theoretically and experimentally evaluated. We also show how multiple reflection interferences are limited by the use of reduced spatial coherent illumination.     

Determination of fringe order in white-light interference microscopy

Combining phase and coherence information for improved precision in white-light interference microscopy requires a robust strategy for dealing with the inconsistencies between these two types of information. We correct for these inconsistencies on every measurement by direct analysis of the difference map between the coherence and the phase profiles. The algorithm adapts to surface texture and noise level and dynamically compensates for optical aberrations, distortions, diffraction, and dispersion that would otherwise lead to incorrect fringe order. The same analysis also provides the absolute height data that are essential to relational measurements between disconnected surfaces.     

Approximate phase conjugation with a retroreflecting array in microscopy

Some experiments on image recovery in microscopy are described. The aperture stop of a microscope objective is partially obscured, and a retroreflecting array is used as an approximate phase conjugator to return the beam through the unobstructed part of the aperture. An application is made to the measurement of the absolute reflectance of small samples by use of a variant of Strong's method.     

Evaluation of coherence interference in optical wireless communication through multiscattering channels

Optical wireless communication has been the subject of much research in recent years because of the increasing interest in laser satellite-ground links and urban optical wireless communication. The major sources of performance degradation have been identified as the spatial, angular, and temporal spread of the propagating beam when the propagation channel is multiscattering, resulting in reduced power reception and intersignal interference, as well as turbulence-induced scintillations and noise due to receiver circuitry and background illumination. However, coherence effects due to multipath interference caused by a scattering propagation channel do not appear to have been treated in detail in the scientific literature. We attempt a theoretical analysis of coherence interference in optical wireless communication through scattering channels and try to quantify the resultant performance degradation for different media. We conclude that coherence interference is discernible in optical wireless communication through scattering channels and is highly dependent on the microscopic nature of the propagation medium.     

Digital measurements of the shaft rotation angle and the phase shift by the conversion of the parameters into time intervals

The feasibility of a single direct-reading meter making a digital measurement of two physically inhomogeneous parameters, shaft rotation angle and phase shift, is examined. A generalized block diagram of such a device, based on conversion of the parameters into time intervals and then into code, is given. Methods of increasing the accuracy of measurement and speed, as well as expanding the functional capabilities of the direct-reading meter are analyzed. Translated from Izmeritel’naya Tekhnika, No. 1, pp. 22–26, January, 1998.     

Characterization of low angle grain boundaries in yttrium orthovanadate

Single crystals of Nd:YVO₄ grown with the Czochralski technique frequently exhibit light scattering defects that are detrimental to their lasing and optical properties. Defects in the form of low angle grain boundaries have been characterized in what are nominally ‘single crystals’. The misorientation angles of the boundaries were determined to be typically < 1°, which corresponds to formation energies of approximately 1 Jm⁻². It was found that dislocations generated during crystal growth and cooling have enough mobility in certain growth directions to form low angle grain boundaries through polygonization. Despite the relatively high energies the boundaries were stable, being immobile at annealing temperatures up to 97% of the melting point (2083 K). Suggestions are made to reduce or eliminate polygonization, including the addition of atoms with a size either much larger or smaller than Y³⁺.