A short history is given of the advances of interferometric optical testing that have occurred during the past 15 years. The present state of the art is discussed and a prediction is made as to the advances which will be made in interferometric optical testing during the next 10 years.

An infrared laser interferometer operating at 1.06 micrometers is fully described. Starting with a discussion of the applications of infrared interferometery, this paper examines the features of the interferometer and explains its operation. The effect of laser coherence length on obtaining high-quality fringe patterns is explored. Various techniques for setting up test configurations and aligning the system using the infrared radiation are described. The system has fringe photography capability and is fully interfaceable with existing microporcessor-based data reduction systems. The emphasis is on the practicality and usability of this infrared laser interferometer.

The Shack interferometer described i this paper is an unequal-path instrument which has demonstrated its utility in an optical shop environment. Its light weight and compact size make it a favored test instrument. In its standard configuration it is used in a variety of optical shop tests. ty the addition of a knife blade or wire, the instrument is converted to a monochromatic Foucault or wire tester thereby increasing its versatility. Its simplicity and low cost allow it to be readily built into special purpose test fixtures.

The theory and operation of the laser interferometer is explained in this paper. The laser system is widely used in the metal working industry, IC manufacturing, RED and a growing new area of disc drive track writing. The type of measurements made include length, angle, flatness, straightness and squareness. The advantage of the laser over conventional equipment includes high speed, versatility and the inherent accuracy of the system.

A Fourier optics approach is used to explain both the fabrication and theory of operation of a scatterplate interferometer. With these principles in mind, an understanding of the limitations and tolerances of the interferometer can be achieved. Some of these include limitations on the f/No. of the test system, quality of auxiliary optics, and tolerance on the scatterplate flip symmetry.

The basic principles of operation of the point-diffraction interferometer are briefly discussed. Properties of this interferometer such as formation of the reference wavefront at the image (allowing its use in a single-pass mode) and relative insensitivity to vibration make it especially convenient for testing large complex optical systems in situ. Examples of such kinds of tests are described.

The Zygo Interferometer System is designed to be suitable for a large spectrum of measurement applications. The fact that it has been widely accepted and is being used in its various configurations establishes the viability of the system architecture upon which it is based. The purpose of this paper is to provide a comprehensive description of this system so as to establish a base-line for what this state-of-the-art and off-the-shelf system can provide.

Modern opticians are finding that yesterday's state-of-the-art design requirements are quickly becoming commonplace and are generating surface figure tolerances of X/8 to A/20 or approximately one to two millionths of an inch. Here it becomes apparent that the classical test plate must give way to a more quantitative method of surface measurement; one that will give the optician a more accurate idea of the true surface geometry which is being polished. A digital minicomputer used in conjunction with a Twyman-Green interferometer is being used as an in-process production tool yielding favorable results.

Testplate testing is reviewed with special emphasis on limitations imposed by the use of these methods.

Fabrication of high quality optical surfaces requires both sensitive methods for measuring surface characteristics and delicate control of the fabrication process. Conventional metrology interferometers have been augmented with computer controlled interference phase measuring techniques to give greater accuracy and provide real-time data analysis. Key elements of a Phase Measuring Interferometer (PMI) system are a Twyman-Green interferometer; piezoelectric-driven reference mirror; image detector; and a minicomputer system with a graphic display, keyboard, and hard copier. Surface quality data is obtained by a direct measurement of intensity and computation of phase difference over an aperture containing a 32 x 32 element array. System functions include: isometric contour mapping at variable sensitivity, array subtraction, surface calibration, time averaging, determination of tilt and power over subapertures with respect to a clear aperture, and deconvolution of wavefront data to yield aberration coefficients. The system is being used to produce high-quality reflecting spheres, flats, mirrors, and refractive elements. A measurement uncertainty of <0.01A peak to valley and 0.001 Xrms has been achieved in the evaluation of a large spherical reflector.

Described is a technique for accurately measuring the radius of curvature of very long radius concave optical surfaces in a relatively short working length. Presented is the basic theory of the technique and the detailed information necessary to provide for its practical application. Using the technique, radii of up to 175 meters can be measured to an accuracy of better than 0.01 percent with a one-meter long scale and slide, and in an overall working length of less than 5 meters. Generalizations of the technique allow for the measuremt of convex surfaces and even longer radius concave surfaces. Photographs, interferograms, calculations, and an error analysis illustrate the technique.

This paper discusses the use of interferometers and interferogram evaluation equipment in the production environment of an optical shop. An analysis of the economics for utlizing this instrumentation to support optical fabrication is presented. Based upon the use of this instrumentation in the Zygo Optical Shop, the advantages of this equipment for increasing productivity and improving quality are examined.

The in situ testing of an unpolished off-axis parabolic mirror with a vertex radius of 12.8 m, a diameter of 1.42 m, and an off-set of 1 m is described. Interferograms are shown that were obtained during the fabrication process by means of a lona-wavelength interferometer that uses a CW CO2 laser operating at a wavelength of 10.6 µm as the coherent light source.

A 24 inch diameter segment of a 60 inch diameter, 10 meter focal length parabolic mirror was fabricated at Space Optics Research Labs. Due to the long focal length, a standard null test using a 24 inch flat could not be set up on our 12 foot air table. The test was set up in a long room which was subject to vibrations. A modification was made to a LUPI (Laser Unequal Path Interferometer) to allow the collimated reference beam to follow approximately the same path as the test beam. Adjustable flat mirrors for the reference beam were attached to the sides of the mounts for the parabola and flat. This combination allowed the r-,f,erence beam to "follow along" and pick up the same vibrations as the test beam. Good high contrast fringes were produced by this method. This paper will describe the test procedure and equipment. Additional problems of temperature, turbulence, and coherence will be summarized.

Machine-tool manufacturers have traditionally used position-feedback devices such as resolvers, encoders, Inductosyns, etc, to provide slide-displacement data to the control servo system. The location of the cutting tool and workpiece are thereby inferred by measuring a secondary characteristic such as the angle of the motor drive shaft or lead screw. Undesirable features of this technique arise from unstable elements that are outside the measurement loop as well as from inherent accuracy considerations. The laser interferometer position-measuring system offers an accuracy that is unsurpassed by conventional feedback systems; and, while it is more expensive, it should represent only a mall fraction of the total cost of a precision, numerically controlled machine tool. Also, it is frequently possible to measure more closely the location of the cutting tool and workpiece from a common reference, thereby avoiding some potential thermal-instability problems.

Metal mirrors of conventional and nonconventional design can be machined by the diamond-turning process. A wide variety of optical test equipment is needed to aid in the production of these mirrors and to provide final certification. Optical testing procedures require quality assurance surveillance. This paper reports on the repeatability of some routine tests.

A precision mirror fiducializinq system for use in large optics fabrication has been developed and demonstrated. The arrangement is compatible with Perkin-Elmer's Computer Controlled Polishing (CCP) facility and Coaxial Reference Interferometer (CORI). The absolute position of figure errors on the mirror surface can be determined through measurements on the interferogram to within lmm on mirrors in the 1-2 meter diameter range.

The Zygo interference Pattern Processor was designed to evaluate either hard copy interferograms or real time interference patterns rapidly and accurately. Since its introduction one year ago, it has undergone an evolution leading t two new systems which shall be described here. In addition, continuing software development has led to a highly flexible operating system, satisfying many users requirements. These new features have been stimulated in the course of their development by various use requirements leading to the refinement and definition of the new systems. For those interested only in the basic analysis of an interference pattern, a minimum system has been designed and built. Those requiring not only the entire analysis capability but multiple systems can now time share one processor. Continuing developments for the reduction of very complex fringe patterns will on offer the industry a tool for measuring virtually all of its interference pattern's., Before providing more details of the nature of these new systems, a brief description of the basic system and its operation will be given first.

An interferogram analysis system has been developed that uses a standard television camera interfaced to a microcomputer to acquire the coordinates of fringe centers. Following single-frame capture at a resolution of 480 x 512 x 1, continuous lines appear at fringe centers in less than 12 seconds. Editing software provides for the addition and removal of data points and smoothing. Selection of data for analysis and order assignment are under full operator control. The system provides elaborate graphics options based upon a polynomial fit to the data. Under ideal conditions a detailed analysis is obtained in less than 10 minutes.

A new method of testing mirrors with stretched and/or decentered aspheric surfaces is described. By utilizing Phase Measuring Interferometric techniques, the null lens can be replaced with a computer-generated reference file containing the equivalent OPD information. An example of the procedure used is given for a typical complex aspheric.

The concept of an inexpensive large aperture high quality interferometer using a low-resolution holographic corrector plate is defined in theory and experiment. Examples are given of various interferometers that may be constructed using the principle of the holographic corrector plate in combination with a read-out device and a computer to provide contrast enhancement and correction of residual fringe aberrations.

Parts that are manufactured with diamond tools on precision machines typically exhibit high-quality surfaces and contours. One technique for characterizing these machines is to fabricate a spherical teEt part from the material of interest so that the multiaxes contouring capability can be evaluated. Because most materials that can be diamond turned easily are also damaged easily, a noncontact, rotary inspection machine has been developed that utilizes a focused beam from a laser interferometer to measure perturbations in the surface of reflective parts. Digital output data from the laser display is converted to an analog signal that is available for additional processinkT to provide contour and/or surface texture information. A mastering sequence is also available that utilizes an air-bearing linear variable differential transformer (LVDT) to permit noncontact measurement of part size.

An electronic heterodyne technique is being investigated for video (i.e., television rate and format) recording of interference patterns. In the heterodyne technique electro-optic modulation is used to introduce a sinusoidal phase shift between the beams of an interferometer. For phase modulation frequencies between 0.1 and 15 MHz an image dissector camera may be used to scan the resulting temporally modulated interference pattern. Heterodyne detection of the camera output is used to selectively record the interference pattern. An advantage of such synchronous recording is that it permits recording of low-contrast fringes in high ambient light conditions. In addition, the use of a vector scanned image dissector camera permits one to scan only certain selected points for rapid acquisition of selected interferometric data. Results will be presented for the recording of simple interference patterns. The application of this technique to the recording of holograms will be discussed.

An interferometric approach to the calculation of the two-dimensional MTF of an optical system is proposed. The technique, in some ways analogous to that of speckle interferometry used in astronomical situations, is based on the computation of the second-order spatio-temporal statistics of a fluctuating speckle pattern. The theorum of Van Cittert-Zernike is invoked to relate the speckle, due to illumination of a perfect diffuser by the point spread function of an optical system, to the two-dimensional MTF of the system. The computed MTF is displayed in the form of a contour map and can also be represented in the conventional form of a one-dimensional vertical cut. Preliminary measurements have yielded qualitatively useful results and clearly illustrate the suitability of two-dimensional maps for the detection of transfer function anisotropies.

A simple technique employing two correlated diffusers has been used to measure the degree of spatial coherence of light sources. We present a description of the method as well as experimental measurements on quasimonochromatic sources of various sizes. Good agreement between experimental and theoretical results has been demonstrated.

A one-step rainbow holographic process by using an imaging lens is presented. Results of both the pseudoscopic and orthoscopic rainbow holographic imagings are discussed. One of the most interesting and important applications of the one-step rainbow process is the holographic interferometry. Demonstrations of double exposure, time averaging, and contour generation by this one-step holographic interferometric process are provided. Extension of the one-step process to multiwavelength and multislit holographic interferometry is also included. As compared with the conventional holographic interferometry, the one-step rainbow holograph-ic process is a very simple and versatile technique, which yields the advantages of white light readout. The one-step process offers a brighter image, a lesser speckle noise, and a better fringe visibility. For multi-wavelength and multislit rainbow holographic interferometry, the techniques enable different physical holographic fringe patterns to be displayed in different colors. For multiwavelength technique, they are fringe patterns due to different physical effects, and for multislit technique, they are fringe patterns from different perspectives.

Two-wavelength holography has been demonstrated to be a useful tool for contour mapping of three-dimensional objects. The combination of a tunable dye laser with a thermoplastic device allows the mapping of a diffused object in any desirable contour spacing in near real-time measurement. This paper describes such a combination using a newly developed thermoplastic holographic camera. The performance of this camera will be demonstrated by showing several examples of TWH contouring.

We could perform real-time holographic interferometry with Q-switched ruby laser. When real-time holographic interferometry is done using pulsed laser light, we can freeze high speed events and observe their changes of states as interference fringes, because Q-switced laser pulse has an extremely short pulse duration and high intensity. Further we could get a double-pulse and observe serial two images of an event as real-time holographic interfer-ence fringes using a high speed streak camera to separate them.

A single-mode optical liner can De used as a leg of an interterometer, making possible new applications of interferometry. Since this kind of fiber does not allow higher-order modes to propagate, it acts as a spatial filter and provides a smooth wavefront at its output end. A practical method of providing optically good in-put and output faces for a fiber core only a few micrometers in diameter is described, including a means of stripping away cladding modes. The problem of coupling from a laser into a single-mode fiber is discussed, including optimal matching of a gaussian beam to the Bessel function field distribution of the HEll mode. Theoretical coupling efficiency can be as high as 99.7 percent, and experimental efficiency is 70 percent, not corrected for Fresnel reflections. Experimental results are presented showing the change in the optical length of a fiber with temperature and the use of single-mode fibers in two types of interferometer, a Fabry-Perot etalon and an unequal-path Mach Zehnder interferometer with 128 meters of optical fiber in one of the paths.

Laser interferometry provides adequate range and resolution for thermal expansion measurements. However, accuracy can be achieved only if the relative motions of the sample and the optics are known and/ or accounted for. The system described incorporates self compensating features into the optics. Signal processing techniques for real time length change recording are also discussed.

Time-averaged holography is applied for vibration analysis of the tympanic membrane of frog, guinea-pig and human cadaver. Vibration. characteristics of human stapes which acts as a transmitter of vibration from tympanic membrane to internal ear were studied. Vibration of human skull by means of bone conduction. hearing and bone conduction hearing aid were analysed. Then, bone conduction. mechH.nism of skull with bone conduction hearing aid was investigated. Finally, possibility of holographic recording and holographic interferometry used with optical fiber system was studied theoretically,and it is verified with experiments.

Differential optical interferometry, initially developed to characterize optical recording media surfaces and subsequently used to detect continuous and pulsed ultrasonic elastic waves, has been applied to the measurement of the interface properties of bonded transparent solids. Acousto-optic interaction between dual laser probe beams and the periodic particle displacements induced by acoustic waves on the boundary permits the qualitative identification of surface roughness defects at the interface. In this paper, the theory and operation of the real-time interferometric measurement system, an analysis of the acousto-optic interaction, and surface defect testing results for bonded glass specimens are presented.

The use of Nomarski differential interference contrast (DIC) microscopy has been extended to provide nondestructive, quantitative analysis of a sample's surface topography. Theoretical modeling has determined the dependence of the image intensity on the micro-scope's optical components, the sample's optical properties, and the sample's surface orientation relative to the microscope. Results include expressions to allow the inversion of image intensity data to determine sample surface slopes. A commercial Nomarski system has been modified and characterized to allow the evaluation of the optical model. Data have been recorded with smooth, planar samples that verify the theoretical predictions.

The difficulty in the measurement of the strain at the joints of members of wooden trusses has made the evaluation of theoretical strengths uncertain. This is particularly important if one desires to economically assure adequate roof strength for installation of solar collectors and other similar roof loading. This measurement problem has been solved by using interferometric techniques to determine small strains at truss joints. A modified Michelson interferometer has been adapted for this purpose. The interferometer, including He-Ne laser source and related optics, has been fabricated into two parts: One of the interferometer mirrors is floated independently on one truss member (two-point bearing), and the remainder (including source) in one piece on a two-point bearing on the other joint member. Using photographic techniques, both angular and lateral relative displacements of the two members have been measured. Design and details of the interferometer use are outlined.

A technique is described for using a commercially available scanning Fabry-Perot interferometer and a low resolution monochrometer to measure solar rotation produced doppler shifts. A Spectra Physics, Model 380, Scanning Fabry-Perot Interferometer with a reflectivity finesse of greater than 150 was followed with a Heath EU-700, 350 mm Czerny-Turner monochrometer, used as an order separator. The output of the photomultiplier was displayed on an oscilloscope, and the spectral data was obtained photographically. The horizontal sweep of the oscilloscope was used as the driving ramp voltage for the z-axis pizzoelectric stack of the scanning interferometer. Using the known wavelengths of atmospheric 02 lines for calibration, doppler shifts of less than .001 nm were detectable. In order to obtain a flat output without interferometer order overlap, an interferometric mirror separation of .75 mm (0.265 nm free spectral range) was used with a monochrometer slit width of 125 um (2.0 nm/mm dispersion). The monochrometer was centered on the .2 nm spectral region of interest at 630.2 nm. An apparent overall finesse of about 100 was realized.

Current cryogenic targets for laser fusion experiments consist of glass Microshell® pellets with deuterium-tritium fuel frozen inside. The fuel forms a uniform layer in the optimum configuration. Until very sophisticated injection systems are developed, these targets must be prepared in the experimental target chamber. This situation imposes re-straints upon any technique used to characterize the quality of the targets before they are irradiated by the high power laser. This paper describes a wavefront-shearing interferometer for characterizing cryogenic targets in situ. The interferometer, which is based on the Saunder's shearing cube, is easy to adjust and is very stable in the presence of mechanical vibration; it produces fringes of high contrast without sacrificing intensity. The interferometer has been used with a low power pulsed laser to measure both the effect of amplified spontaneous emission, produced by the high power laser, and the effect of room-temperature thermal radiation on cryogenic targets.

Holographic exposure and selective etching in silicon are combined to fabricate high-precision gratings with nearly arbitrary blaze angle and flat surfaces. The process starts with commercially available silicon wafers, the surfaces of which are oriented at the desired angle to a {1 1 1} plane. Gratings between 0.5μm and 5 μm have been realized with holographic and mask exposure. Efficiencies up to 90% have been measured. Furthermore, simple tools for quick and high-precision alignment of the holographic equipment are described.