Interferometric metrology is now experiencing a major resurgence of interest due to several new developments. This paper presents an overview of current trends and a brief discussion of future prospects.
Methods for testing aspheric surfaces fall into two categories: null tests and non-null tests. In a null test, accessory optics are included in the test setup to cancel the asphericity of the aspheric optics under test. Thus, if the aspheric optics under test are perfect, an interferometric test will give straight equally-spaced fringes. In a non-null test, non-straight, non-equally-spaced fringes will be obtained even if the optics under test are perfect. As long as computer analysis is available and the fringe spacing is sufficiently large that the fringes can be detected, non-null tests are as good as null tests, and they are generally easier to perform. Non-null tests can measure the wavefront directly, or a shearing interferometer test can be performed which measures the slope of the wavefront, rather than the wavefront itself. A shearing interferometer test has the advantage that for a given wavefront aberration, fewer interference fringes are obtained; however it has the disadvantage that the measurement accuracy is less than if the wavefront is measured directly. This paper discusses and compares various aspheric optics testing schemes. Null tests that are discussed include the use of real and computer-generated holograms and the use of refractive and reflective null optics. Non-null tests that are discussed include lateral and radial shearing interferometry, direct phase measurement using large linear and two-dimensional solid-state detector arrays, sub-Nyquist interferometry, long-wavelength interferometry, and two-wavelength holography (or interferometry). The importance of combining lens analysis (design) software with interferogram analysis software is discussed.
Measurements of optical phase differences have been obtained with resolutions of 1/500 wave rms at 100 kHz data rates using frequency shift interferometry. One implementation of this approach, called the digital heterodyne interferometer, uses Bragg cells to generate the frequency shift, and has been applied extensively to real-time, high precision measurements as well as in limited applications to optical testing. We discuss the fundamentals of this approach and review the applications.
Analysis of a fringe pattern to obtain quantitative information, i. e. fringe reduction, has been an important area of research. Numerous techniques have been developed for fringe reduction with varying degrees of automation. The goal of a fully automated fringe reduction technique, however, remains elusive. We have carried out a survey to compare and contrast the various techniques and to identify the important problem areas and their possible solutions. In particular, it has been possible to identify each technique as a combination of three generic operations, namely, noise management, fringe position determination and fringe order assignment. A discussion of various fringe reduction techniques and the role of these operations is presented.
Research efforts for interferometric techniques and their applications in Japan are reviewed with an emphasis on recent developments. We will describe briefly digital fringe analysis techniques and theoretical approaches to precision phase analysis, and then practical applications of novel interferometers, holography, moire topography and optical fiber interferometry.
An account is given on how to analyze the measurement accuracy of a laser interferometer system. Each component of the system accuracy budget is discussed, including the magnitude of these components. Two important components of this budget; atmospheric affects and optics thermal drift are discussed in more detail. The effects of these two error components have been significantly reduced on the Hewlett-Packard laser interferometer systems by the use of Wavelength Tracking Compensation and a new interferometer.
This paper will discuss some of the recent advances in the field of interferometry that have been made at Zygo. In the past year Zygo has introduced five major advances in the interferometry field: the 5500 Heterodyne Profiler, the AXIOM 2/20 distance measuring interferometer, a 450mm phase modulator for large aperture phase interferometry, a phase measuring interferometer at 351nrn and the Mark IV programmable phase measuring interferometer. The presentation will conclude with a general discussion of what Zygo sees for the future of interferometry.
This paper presents measurements of surface features from a variety of surfaces using a non-contact, long scan surface profiler. Data is presented emphasizing the waviness and roughness of the surface. Several different types of samples are presented, including the surface of a diamond turned mirror, computer hard disk, machine tool surface, and film. The instrument used for these measurements has a resolution of 1 Angstrom and has a scan length up to 100 millimeters.
WYKO produces interferometer systems for a number of optical metrology applications. Systems to be discussed include: WISP, an interferogram analysis package that uses input from a graphics digitizing tablet or video camera; TOPO, a microscope system for profiling surfaces such as mirrors, hard disks, magnetic tape, and paint; SIRIS, used to test optical components ranging in size from a millimeter to meters; IR3, a 10.6 μm Twyman-Green interferometer for testing up to 16-in, diameter objects; LADITE, a wavefront analysis system whose major application is the testing of diode lasers; HOLOCAM, for quantitative holographic interferometry; and MAX, a projected fringe contouring system. All products but WISP use phase-measurement interferometry principles to obtain data. This paper gives the general operational principles of these systems and describes some of their applications and results.
One of the most important applications of holographic interferometry is in the observation and analysis of the class of objects known as phase objects, which modulate the phase of a reflected or transmitted light wave and not the intensity. Phase objects include all transparent optical elements, and any volume in which the refractive index or optical path-length varies in the volume. This field of application has evolved significantly in recent years, partially owing to evolution of the general field of holography, and also aided by the developments in computer technology that simplify image analysis and laser technology that improves data recording quality and rates. The individual techniques that make possible the on-line recording and analysis of holographic interferograms are described. These include new recording materials and techniques such as thermoplastics and TV holography. The holography laser state of the art is reviewed. Advanced data reduction techniques including the most recent algorithms for fringe analysis and tomography are discussed. The high sensitivity techniques such as phase shift and heterodyne interferometry are presented. Finally, some recent applications are presented. This will include a discussion of a holographic system that was flown in the Spacelab 3 spaceflight and plans for its refinement and reflight in an upcoming mission.
Recent developments in the field of quantitative interpretation of fringes in hologram interferometry are reviewed, with special emphasis on automated data reduction. More specifically, methods for determination of three-dimensional information on object's displacements and strains, directly from holograms, are presented.
Measurements of distance obtained interferometrically are modulo(X) but can be made with accuracy of a fraction of the wavelength of the light used. Various techniques have been developed to overcome this limitation and obtain an absolute distance measurement with interferometric accuracy. To accomplish this, it is necessary to make measurements with more than one wavelength. The basic principles of absolute distance interferometry are discussed and the current state of the technology is reviewed.
The University of Arizona is uniquely fortunate to include both the Optical Sciences Center and Steward Observatory, where a great deal of work has been done in the past and is continuing today in the areas of optical design, fabrication, and testing. These organizations, literally across the street from each other, have over the years cooperated on numerous telescope and related projects. Interferometry has played a key role in the fabrication of the optics for these projects and has been an area of constant development as requirements for these optics have become more stringent.
Shearing interferometry has been under development at Itek for well over a decade for a variety of applications. Principal among these has been the high speed measurement of optical beams transmitted through atmospheric turbulence. The technique has also been applied successfully to measurement of complex highly aspheric optical surfaces, control or camera systems, and diagnostic measurement of laser beams. Shearing interferometry has several features that make it highly advantageous for these applications. First, it is self referencing and so can be used to measure wavefronts from remote sources without the usual interferometric requirement of a separate high quality coherent reference beam. Second, it can be configured to operate with broadband or 'white light' sources. Third, provided the source has some spatial coherence, it can operate with extended sources. Fourthly, it can provide linear wavefront measurement over as large a dynamic range as required (hence its value in measuring steep aspherics), and finally, it can give highly accurate wavefront measurement with the bare minimum of available light. These features make shearing interferometry a flexible and valuable technique. Itek has developed several different implementations of shearing interferometers. A particularly successful one for atmospheric turbulence measurement has been the rotating grating shearing interferometer. However in recent years, applications involving pulsed sources have led to the development of some DC (direct current) techniques, including the use of spatial carrier frequency gratings, and the use of polarization. In this paper, we describe the technique and principals of shearing interferometry and give examples of the various implementations, as well as providing results and data on their performance.
During recent years, Lockheed has been researching and developing many of the key optical technologies required by modern optical systems for military and civil space programs. This paper summarizes recent progress made in interferometry as a tool for wavefront sensing in beam control and diagnostics and other precision optical metrology. The areas include: (1) optical design, modeling, and analysis of interferometers; (2) computer-assisted opto-mechanical design and layout; (3) real-time interferometry by digital heterodyne and phase-shifting techniques for various applications; and (4) data processing algorithms.
This review paper discusses interferometric applications and developments within Eastman Kodak Company. Four different areas are examined. The first is the testing of mass-produced lenses, particularly molded glass lenses. Specific attention is given to testing of aspheric surfaces. The second general area is the measurement of large optics. An example is the method used to test large mirrors, such as that produced for the Space Telescope program. The third area deals with other uses of interferometry within Kodak. Many of these applications look at surfaces that are not optical surfaces in the traditional sense. The final portion of this paper examines current research and development activities in interferometry. A major need is to test aspheric surfaces that have large departures from a best-fit reference sphere. Progress in implementation of the new sub-Nyquist interferometry technique for extending the measurement range of existing interferometers is reviewed. An underlying theme of this paper is the general move towards aspherics throughout Kodak, with the accompanying need to test these surfaces quickly and easily.
Interferometric metrology is used in a number of different modes at Perkin-Elmer. In addition to its wide application of evaluating optical surfaces for figure it is also used to determine the inhomogeneity of optical materials, examine the polishability of surfaces in terms of surface roughness, determine the mechanical and thermal stability of materials and aid in the assembly of optical systems. There is installed a large base of conventional interferometers to perform these metrology operations. Many have been designed and built to measure very specific optical components.