Upon entering graduate school, I wanted to ensure real world experience prior to graduation. Fortunately, Jim Wyant was willing to accept me as a student, working with him at Wyko Corporation. After graduating I stayed on as an optical engineer, product manager, and eventually engineering and research director as the company was acquired by Veeco and eventually Bruker. Meanwhile Jim Wyant had brought another optics company to Tucson, 4D Technology, with unique technology developed by James Millerd and Neal Brock. That organization was growing well, with many former Wyko/Veeco employees, and as Bruker's Tucson business changed, I moved to help grow it further, staying in the Wyant ecosystem. This talk will focus on my journey under Jim's companies, including key milestones and stories from my own experience and those of some of the other long-term employees of Wyant’s optical enterprises.
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.
The advent of pixelated micropolarizer arrays (MPAs) has facilitated the development of polarization-sensitive focal plane arrays (FPAs) based on charge-coupled devices (CCDs) and active pixel sensors (APSs), which are otherwise only able to measure the intensity of light. Polarization sensors based on MPAs are extremely compact, light-weight, mechanically robust devices with no moving parts, capable of measuring the degree and angle of polarization of light in a single snapshot. Furthermore, micropolarizer arrays based on wire grid polarizers (so called micro-grid polarizers) offer extremely broadband performance, across the optical and infrared regimes. These devices have potential for a wide array of commercial and research applications, where measurements of polarization can provide critical information, but where conventional polarimeters could be practically implemented. To date, the most successful commercial applications of these devices are 4D Technology's PhaseCam laser interferometers and PolarCam imaging polarimeters. Recently, MPA-based polarimeters have been identified as a potential solution for space-based telescopes, where the small size, snapshot capability and low power consumption (offered by these devices) are extremely desirable. In this work, we investigated the performance of MPA-based polarimeters designed for astronomical polarimetry using the Rochester Institute of Technology Polarization Imaging Camera (RITPIC). We deployed RITPIC on the 0.9 meter SMARTS telescope at the Cerro Tololo Inter-American Observatory and observed a variety of astronomical objects (calibration stars, variable stars, reflection nebulae and planetary nebulae). We use our observations to develop calibration procedures that are unique to these devices and provide an estimate for polarimetric precision that is achievable.
A camera capable of obtaining single snap-shot, quantitative, polarimetric measurements is investigated to determine
performance characteristics. The camera employs a micropolarizer array with linear polarizers oriented at 0, 45, 90, and,
135 degrees. Micropolarizer arrays with elements as small as 7.4 microns and arrays as large 4 million pixels have been
fabricated for use across the visible spectrum. The pixelated polarization camera acquires the four polarization
orientations in a single video frame, which enables instantaneous measurements of the linear Stokes parameters.
Examples of calibration methods and the results of controlled experiments are presented. Error sources and methods for
minimizing them are discussed and demonstrated. A practical example of measuring stress induced birefringence is
demonstrated.
A dynamic profiler is presented that is capable of precision measurement of surface roughness in the presence of significant vibration or motion. Utilizing a special CCD camera incorporating a micro-polarizer array and a proprietary LED source, quantitative measurements were obtained with exposure times of <100 μsec. The polarization-based interferometer utilizes an adjustable input polarization state to optimize fringe contrast and signal to noise for measurement of optical surfaces ranging in reflectivity from 1 to 100%. A new phase calculation algorithm is presented that nearly eliminates phase-dependent errors resulting in shot noise limited performance. In addition to its vibration immunity, the system’s light weight, <5 kg, compact envelope, 24 x 24 x 8 cm, integrated alignment system, and multiple mounting options facilitate use both directly resting on large optical surfaces and directly mounted to polishing equipment, stands, gantries and robots. Measurement results presented show an RMS repeatability <0.005 nm and an RMS precision < 0.1nm which are achieved without active vibration isolation.
A dynamic profiler is presented that is capable of precision measurement of surface roughness in the presence of
significant vibration or motion. Utilizing a special CCD camera incorporating a micro-polarizer array and a proprietary
LED source, quantitative measurements were obtained with exposure times of <100 μsec. The polarization-based
interferometer utilizes an adjustable input polarization state to optimize fringe contrast and signal to noise for
measurement of optical surfaces ranging in reflectivity from 1 to 100%. A new phase calculation algorithm is presented
that nearly eliminates phase-dependent errors resulting in shot noise limited performance. In addition to its vibration
immunity, the system's light weight, <5 kg, compact envelope, 24 x 24 x 8 cm, integrated alignment system, and
multiple mounting options facilitate use both directly resting on large optical surfaces and directly mounted to polishing
equipment, stands, gantries and robots. Measurement results presented show an RMS repeatability <0.005 nm and an
RMS precision < 0.1nm which are achieved without active vibration isolation.
A pixel-level micropolarizer array bonded to a scientific camera has been developed for use in commercial dynamic
interferometers. The pixelated array includes the 0, 45, 90, and, 135 degree polarization orientations. Micropolarizer
arrays with elements as small as 7.4 microns and array sizes as large 4 Mega-pixels have been fabricated for use across
the visible spectrum. The pixelated polarization camera acquires the four polarization orientations in a single video
frame, which enables instantaneous interferometric or polarimetric measurements. Examples of each type of
measurement are presented. Details of how the pixelated camera is used in interferometry are reviewed and the spatial
resolution performance of the camera when used in interferometry is discussed.
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.
We demonstrate an instantaneous phase-shift, point diffraction interferometer that achieves high accuracy and is capable of measuring a single pulse of light at NA greater than 0.8.
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 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 phase-shifting, point diffraction interferometer that achieves high accuracy and is capable of measuring a single pulse of light. The measurement system utilizes a polarizing point diffraction plate to generate a synthetic reference beam that is orthogonally polarized to the transmitted test beam. The plate has very high polarization contrast, works over an extremely broad angular and spectral range, and is only 100 nanometers thick. The unique features of the polarizing element make the system amenable to measuring strongly convergent light from high numerical aperture optics without the need to use a point reference source to calibrate the system. Results of measuring optics with numerical apertures as high as NA 0.8 are presented.
We report on a technique to measure the surface figure of mirrors under extreme vibrational conditions. Measurements are presented of the surface figure changes of Zerodur primary mirrors with both spherical and parabolic shapes, manufactured for the NASA Deep Impact program. Conditions ranged from room temperature to 130K. The interferometer was located outside the cryogenic vacuum chamber and did not require any active or passive vibration isolation. We show measurement repeatability of better than 1/500 waves RMS at 633nm.
A real-time holographic interferometer has been developed for quantitative flow and wavefront diagnostics. The interferometer employs a new variety of the non-linear recording material, Bacteriorhodopsin, to not only record interferograms in real-time, but to analyze them in real- time as well, using an innovative adaption of Phase Shift Interferometry. The versatile interferometer can be configured as a real-time holographic interferometer for general applications and also as a high-speed, multiple pulsed interferometer for time differential applications, such as analyzing unsteady flow and turbulence. The versatility and relative low cost of the hardware components make the interferometer an attractive option for upgrading current schlieren flow visualization systems.
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