Future space observatory missions require controlling wave front error and system alignment stability to picometer scale. Picometer stability performance demands precision knowledge of the mirror and metering structure materials to the same level. A high-speed electronic speckle pattern interferometer was designed and built to demonstrate measurements of both static and dynamic responses of picometer level amplitudes in mirror and structural materials subjected to very low energy disturbances. This paper summarizes the current status of tests to impart a dynamic disturbance of picometer scale and measure the response of specular and diffuse materials. The results show that subpicometer scale effects can be accurately measured in an open test environment outside a vacuum chamber.
One of the most time-consuming aspects of measuring the radius of curvature of a test article is repeatedly moving between the cateye and confocal positions for stage readings and measurements. Here we introduce a unique short coherence dynamic Fizeau interferometer with internal path matching that facilitates long term stability of the cateye reference position relative to the confocal position, eliminating the need to take repeated cateye measurements. The path matching assembly is equipped with a high-quality encoder and has an athermalized design for long-term stability of the path match locations. In addition, the interferometer uses a polarization based pixelated camera for single frame acquisition of the measurements. Combined with the short coherence source, the single frame measurements facilitate rapid scans of the fringe modulation as a function of stage position for accurately locating the relative position of the test article along the optical axis. Radius of curvature measurement repeatability of less than a micron has been demonstrated. This paper will discuss the theory of operation and present the results of measurement studies.
The largest limitation of phase-shifting interferometry for optical testing is the sensitivity to the environment, both vibration and air turbulence. An interferometer using temporal phase-shifting is very sensitive to vibration because the various phase shifted frames of interferometric data are taken at different times and vibration causes the phase shifts between the data frames to be different from what is desired. Vibration effects can be reduced by taking all the phase shifted frames simultaneously and turbulence effects can be reduced by averaging many measurements. There are several techniques for simultaneously obtaining several phase-shifted interferograms and this paper will discuss two such techniques: 1) Simultaneous phase-shifting interferometry on a single detector array (PhaseCam) and 2) Micropolarizer phase-shifting array. The application of these techniques for the testing of large optical components, measurement of vibrational modes, the phasing of segmented optical components, and the measurement of deformations of large diffuse structures is described.
Over the last two decades the use of single-frame interferometric techniques, known as Dynamic Interferometry, has
become widely available in commercial interferometer systems and they have been used extensively in the production of
state-of-the-art space-based optical systems. This paper presents an overview of the techniques and configurations used
to build dynamic interferometers and measurement results for a variety of space-based optical components as well as the
structures that hold them under simulated space-flight conditions. These techniques and configurations have applicability
for many non-space applications as well.
A method for reducing the coherent noise, by a factor of two, in dynamic interferometry measurements is presented. Reducing coherent noise is particularly important in "on-machine" metrology applications where residual noise can be polished into the surface under test. Both theory and experimental measurements are discussed.
We present a dual mode interferometer system based on a single-frame phase acquisition
sensor that is capable of measuring the dynamic displacement of both specular and
diffuse components. The single frame acquisition allows the interferometer to freeze the
motions of the test articles in both configurations and examine the dynamic nature of the
surface figure under dynamic stress. The system has applications in the testing of
dynamic optical components such as deformable mirrors as well as defect detection and
structural stability of composite materials. This paper will provide an overview of the
interferometer design, outline the different measurement configurations and present
measurement results of dynamically excited test articles.
The largest limitation of phase-shifting interferometry for optical testing is the sensitivity to the environment, both vibration and air turbulence. In many situations the measurement accuracy is limited by the environment and sometimes the environment is sufficiently bad that the measurement cannot be performed. Recently there have been several advances in dynamic interferometry techniques for reducing effects of vibration. This talk will describe and compare two dynamic interferometry techniques; simultaneous phase-shifting interferometry and a special form of spatial carrier interferometry utilizing a micropolarizer phase-shifting array.
The technique for measuring changes in diffuse surfaces using Electronic Speckle Pattern Interferometry (ESPI) is well known. We present a new electronic speckle pattern interferometer that takes advantage of a single-frame spatial phase-shifting technique to significantly reduce sensitivity to vibration and enable complete data acquisition in a single laser pulse. The interferometer was specifically designed to measure the stability of the James Webb Space Telescope (JWST) backplane. During each measurement the laser is pulsed once and four phase-shifted interferograms are captured in a single image. The signal is integrated over the 9ns pulse which is over six orders of magnitude shorter than the acquisition time for conventional interferometers. Consequently, the measurements do not suffer from the fringe contrast reduction and measurement errors that plague temporal phase-shifting interferometers in the presence of vibration. This paper will discuss the basic operating principle of the interferometer, analyze its performance and show some interesting measurements.
The measurement accuracy of an interferometric optical test is generally limited by the environment. This paper discusses two single-shot interferometric techniques for reducing the sensitivity of an optical test to vibration; simultaneous phase-shifting interferometry and a special form of spatial carrier interferometry utilizing a micropolarizer phase-shifting array. In both techniques averaging can be used to reduce the effects of turbulence and the normal double frequency errors generally associated with phase-shifting interferometry.
The benefits of using two-wavelength measurements to extend the dynamic range of an interferometric measurement are well known. We present a new multi-wavelength interferometer that uses two successive single frame measurements obtained rapidly in time to significantly reduce sensitivity to vibration. At each wavelength, four phase-shifted interferograms are captured in a single image. The total acquisition time for both wavelengths is 100 microseconds, over three orders of magnitude shorter than conventional interferometers. Consequently, the measurements do not suffer from the fringe contrast reduction and measurement errors that plague temporal phase-shifting interferometers in the presence of vibration. In this paper we will discuss the basic operating principle of the interferometer, analyze its performance and show some interesting measurements.
We demonstrate a new type of spatial phase-shifting, dynamic interferometer that can acquire phase-shifted interferograms in a single camera frame. The interferometer is constructed with a pixelated phase-mask aligned to a detector array. The phase-mask encodes a high-frequency spatial interference pattern on two collinear and orthogonally polarized reference and test beams. The phase-difference between the two beams can be calculated using conventional N-bucket algorithms or by spatial convolution. The wide spectral response of the mask and true common-path design permits operation with a wide variety of interferometer front ends, and with virtually any light source including white-light.
The advantages of common path interferometers for reducing effects of vibrations are well known. A scatterplate interferometer is one common-path interferometer that is well suited for the testing of large concave mirrors, however due to the common path characteristics it is difficult to perform phase-shifting. This paper describes a phase-shifting scatterplate interferometer where the phase-shifting is achieved by making use of the polarization characteristics of a birefringent scatterplate. The major advantage of this design is that it does not require any optical components to be placed near the surface under test. The theory of the interferometer is presented and experimental results are shown.
A new phase shifting scatterplate interferometer is realized by exploiting the polarization characteristics of a birefringent scatterplate. The advantages of this design are that it does not require any optical components to be placed near the surface under test and, the hot spot and background intensity, which are inherent to scatterplate interferometers, are eliminated. The theory of the interferometer is presented.
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