Plate Interferometer Research Articles

The dynamic zone plate interferometer for measuring aspherical surfaces (II)

The dynamic zone plate interferometer (DZPI) realized by changing the wavelength of the illuminating light has the possibility of measuring aspherical surfaces without the need of either a standard surface or a computer generated hologram (CGH). This paper shows a fringe analysis for the DZPI for improving the accuracy of surface error measurements to about λ/100. An experimental result obtained by the fringe analysis is also presented.

Servocontrol of a Fabry-Perot interferometer

A multiloop servocontrol system is described which establishes and then maintains the parallelism and spacing of the interferometer plates in a tunable, 55 mm aperture, triple Fabry-Perot interferometer to an accuracy of lambda /500. Control is maintained even under adverse conditions; in the event of control being lost it is automatically re-established by a secondary servosystem.

Scatter plate interferometer using two correlated diffusers

A scatter plate interferometer that uses two discrete correlated diffusers as a scatter plate is described. The diffusers consist of mirror images of a speckle distribution.

Symmetry error of scatter plate and its effect on fringe contrast in a scatter plate interferometer

Unnecessary Young's fringes due to symmetry error in the speckle distribution, and their influence on the desired interferogram in a scatter plate interferometer are discussed and it is found that symmetry errors cause the decrease of fringe contrast. This difficulty can be avoided by narrowing the aperture of the scatter plate, and a condition to determine the appropriate aperture of the scatter plate is proposed.

Aperture of scatter plate and its effect on fringe contrast in a scatter plate interferometer

The excess optical path difference due to an off-axis scatterpoint and its influence on the desired interferogram in a scatter plate interferometer are discussed, and a limit to the aperture of the scatter plate is proposed.

Capillary rise, wetting layers, and critical phenomena in confined geometry

We have used an interferometric technique to measure the capillary rise of sulfur hexafluoride (SF6) between closely spaced, nearly parallel plates. The data span the range of plate separations 0.8–3.5 μm and span temperatures from 2 mK below the critical temperature (Tc=319 K) to 540 mK below Tc. The data demonstrate that a thin wetting layer of liquid SF6 intrudes between each interferometer plate and the SF6 vapor. Close to Tc, where the capillary rise is quite small, the wetting layer becomes as thick as 0.7 μm. These layers are a factor of 3 thicker than our theoretical estimates which are based on very simple models which assume that the thickness is governed by a competition between the gravitational force which tends to thin the layers and long-ranged van der Waals forces which tend to thicken the layers. Although our data are close to Tc, the correlation length in the SF6 is always much less than the separation between the interferometer plates; thus, we did not expect to observe strong departure from three-dimensional critical behavior towards two-dimensional critical behavior. In agreement, we find that the sample of SF6 between the interferometer plates has the same critical temperature as that of bulk SF6 (±1 mK). Furthermore the capillary rise data are consistent with the hypotheses that the surface tension and the difference between the liquid density and the vapor density of this confined sample of SF6 are the same as those of bulk SF6 at the same temperature. These results concerning critical phenomena contrast with results obtained in certain earlier studies of binary liquid mixtures near their consolute temperatures. The earlier experiments were interpreted without consideration of wetting layers to indicate that large critical temperature shifts and a crossover to two-dimensional behavior did occur in a confined geometry and temperature range similar to the one we use. We briefly discuss the effects that wetting layers have on other experiments near the critical point of SF6.

An improved reference interferometer, incorporating an angular positioning servo, used to tune a narrow bandwidth optical filter

A reference interferometer based on an earlier design has been developed, in which a reference wedge is rotated by a direct-drive position servo-system. The reference interferometer is used to set the spacing of Fabry-Perot interferometer plates in a narrow bandwidth optical filter. The instrument is designed to contain a nitrogen atmosphere and to operate under adverse conditions.

MTF measurement via diffraction shearing with optically superimposed gratings

The use of double grating diffraction shearing for measurement of MTF is demonstrated. By superimposing two gratings optically we avoid the problem of grating contact and subsequent degradation. This simple and economical method is especially advantageous for optical systems with large pupil for which a rotating plate interferometer becomes prohibitively expensive.

Asymmetrical fringe broadening in a Lummer-Gehrcke interferometer

This paper analyzes the effects of random surface irregularities on the intensity distribution of a Lummer-Gehrcke plate interferometer. These surface defects are distributed asymmetrically about the plane of the L-G plate and are usually caused by scratches during its use. Calculations have been conducted for typical values of parameters, and results of some specific examples have been presented graphically.

Holographic interferometer to test optical surfaces

A new interferometric method for testing optical surfaces is given. It is related to Murty's Fresnel zone plate interferometer and suggests the fruitful applications of holography to this branch of optics. In this paper a theoretical model that explains both interferometers is developed. Experimental results and pertinent comments are included.

A Focused-Beam Type Laser Interferometric Dilatometer

Using a laser beam focused near interferometer plates, an improved interferometric dilatometer has been developed. It has a couple of features, i.e. it has a much greater latitude in parallelism adjustment of the interferometer plates than that of the conventional Fizeau-type ones and enables, on the other hand, automatic recording with a simple device just composed of a solar cell and a recorder to measure reflected beams. Moreover, collimation of the optical system is very easy. Therefore, it is applicable even to routine determination of dilatation for quality control in a factory. The dilatometer is capable of determining the thermal dilatation of materials with low and/or negative expansion as well as those with transition, extremum or inflection point in expansion curves. Experimental results on some metallic, ceramic, semiconductor and glassy materials are presented.

Wavelengths and Energy Levels of Ar I and Ar II based on New Interferometric Measurements in the Region 3 400-9 800 Å

Wavelengths of 233 lines in the Ar I spectrum and 354 lines in the Ar II spectrum, excited in a water-cooled hollow cathode at a pressure of 0.2 torr, have been measured by means of a Fabry-Perot interferometer in the vacuum. Fourteen different etalon spacers between 5 and 70 mm in length, two pairs of interferometer plates and three different coatings have been used during the investigation. Reference lines have been obtained from a calibrated electrodeless lamp containing Kr86 at a pressure of 0.2 torr. By means of the measured wavelengths the values of 80 energy levels in Ar I and 153 levels in Ar II have been derived, the uncertainty of the relative values varying from a few tenthousandths to a few thousandths of a cm-1. For both spectra these level values have then been used to obtain calculated wavelengths, extending down to 1 557 Å in the case of Ar II. The levels of 3d and 5s of ArI, most of which are inaccessible in the photographic region, have been redetermined relative to the new values of the 4p-levels by using the interferometric measurements in the photo-electric infrared published by Humphreys and Paul, Littlefield and Rowley, and Peck et al.

A Tilted Plate Interferometer for Heat Transfer Studies

This paper describes a tilted plate interferometer suitable for studies in natural convection. The basic element consists of a pair of highly reflecting plates; these plates are separated by about 15 cm. Collimated light from a He–Ne laser is normally incident on the first plate; Fizeau fringes appear at the second plate. The plates are highly reflecting, resulting in sharp fringes. Despite the complicated nature of fringe formation, due to multiple reflection, fringe displacement is simply proportional to the change in index of refraction of the medium between the plates if terms of order (Δμ/gm∞)2 are neglected. Thus the form of the fringe shift vs temperature relationship is the same as for the Mach-Zehnder interferometer. An experimental program of calibration has been conducted, and the fringe shift vs temperature relationship is confirmed.

Reflection Plate Interferometer

The reflection plate interferometer, which is similar to a Schlieren system but with a small glass shearing plate replacing the knife edge, has been investigated and the necessary relations to understand its operating characteristics are presented. It was found that the optimum operating range is for incident angles between 40 degrees and 65 degrees , and that the fringe spacing is approximately inversely proportional to the shearingplate thickness. The undisturbed fringe direction can be adjusted at any angle with respect to a test bject. The interferograms are of very good quality and can be interpreted to give the density gradient of the test fluid.

Interferometrische homogenitätsprüfung mit kompensation

When testing glassplates for inhomogeneities, surface deviations must be eliminated. The proposed method achieves the suppression of surface deviations by adding 2 compensating plates to the testpiece. Even unpolished plates with deviations as great as 10μm can be tested with sensitivities up to 10 -6/cm. The deviations introduced by the compensating and interferometer plates are eliminated by a compensating hologram taken of these compensating plates in the same interferometer. The resulting interferogram gives the variations of the refractive index directly.

A Differential Scanning Fabry-Perot Interferometer

In Fabry-Perot interferometry, a method has been investigated to eliminate the influence of an intense Rayleigh line in spectroscopy of scattered light in a region of 0.01–10 cm-1. In order to eliminate the background, a transmitted spectral line is modulated by vibrating one of the interferometer plates, and only the modulated signal is detected. By using an optimum vibrating amplitude, detectivity can be increased by about 10 times than that in a usual Fabry-Perot spectrometer without decreasing spectral resolution.

The Use of a Single Plate Interferometer in the Hook Method of Roschdestvensky

The theory of a simple inexpensive single plate interferometer coupled to a spectrograph is developed and the feasibility of this instrument in the hook method is demonstrated by observing the anomalous dispersion of sodium vapor. This instrument appears especially attractive for use in the vacuum ultraviolet region of the spectrum.

The Coupling of Conduction With Laminar Natural Convection From a Vertical Flat Plate With Arbitrary Surface Heating

A method has been developed for coupling conduction in a solid with natural convection in a surrounding fluid. The problem investigated is that of steady, constant-property, two-dimensional, laminar natural convection from a vertical, heat-conducting flat plate of finite thickness with an arbitrary heating distribution in its surface. Using this method it is possible to predict the variation of temperature in the plate and the velocity and temperature profiles in the boundary layer as a function of the heating distribution and the thermal properties of the plate and fluid. The equations for conduction in the plate and convection in the boundary layer are written in finite difference form, coupled through the common heat flux at the plate-fluid interface, and solved numerically by an iterative technique. Experimental corroboration of the numerical results is provided by measuring temperatures, both with thermocouples and a laser holographic interferometer, along ceramic and glass plates heated by thin film resistance heating elements vacuum deposited on their surface. The results indicate that the degree of coupling between conduction in the plate and natural convection in the fluid is greatly influenced by the plate-fluid conductivity ratio.

Field Mach-Zehnder Interferometer for Heat-Transfer Studies

The use of interferometry for heat transfer studies enables a complete field of view to be quantitatively examined at one time without disturbing the temperature distribution being measured. The Mach-Zehnder interferometer described has an elliptical field, with a 15cm minor axis and is capable of accommodating test chambers 102cm in length using the full field of view. The sensitivity of the instrument for various air pressures and temperature distributions within the test cell has been evaluated. The use of the instrument for identifying isotherms in the neighbourhood of a heated slab supported on skid bars is described. A Moiré technique which eliminates the effects of imperfections in the interferometer plates has also been used.

On Automatic Parallelism Control in a Scanning Fabry-Perot Interferometer

A method of keeping the optical plates of a scanning Fabry-Perot interferometer parallel when setting on fringe maxima is described. The method needs no additional instrumentation other than the duplication of some of the electronic detection equipment needed for normal use of the instrument. Small oscillatory rotational movements about two axes at right angles are applied to one of the interferometer plates, while the other is moving axially in an oscillatory fashion. The transmitted light flux from the interferometer is detected with the single photodetector, and the electrical signal contains components at different frequencies that can be used to measure and control precisely the plate separation and plate parallelism of the interferometer. Experiment and analysis show that for path differences up to 200 mm using the mercury 198 green line as the light source, the parallelism of the plates can be controlled to better than 10(-8) rad, if parameters in the interferometer system are chosen suitably. For path differences up to 100 mm, the accuracy approached 10(-9) rad. With laser light, the method should be applicable for long path differences. Vibration and medium stability would be the limiting factors.