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 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
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.
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.
The spatial frequency response of the pixelated phase mask sensor has been investigated both theoretically
and experimentally. Using the small phase step approximation, it is shown that the instrument transfer
function can be approximated as the product of the system optical transfer function and the spatial carrier
processing filter transfer function. To achieve optimum performance it is important that the bandwidth of
the optical imaging system is adequate so that the limiting factor is the detector pixel width. Actual
measurements on a commercial Fizeau interferometer agree very well with the theory, and demonstrate
detector limited performance. The spatial resolution of the calculated phase map is algorithm dependent;
however, both the 2x2 and 3x3 convolution algorithms result in a frequency response that is significantly
more than what would be obtained by a simple parsing of the image. Therefore, a 1k x 1k sensor has a
spatial frequency response that is approximately equal to the detector limited resolution of a 700 x 700
array with its frequency response extending to the full Nyquist limit of the 1k x 1k array.
An on-axis, vibration insensitive, polarization Fizeau interferometer is realized through the use of a
pixelated polarization mask spatial carrier phase shifting technique in conjunction with a high
coherence source and a polarization frequency shift device. In this arrangement, differential motion
between the test and reference surfaces, in conjunction with the polarization frequency shift device, is
used to effectively separate the orthogonally polarized test and reference beam components for
interference. With both the test and the reference beams on-axis, the common path cancellation
advantages of the Fizeau interferometer are maintained. Additionally, the use of a high coherence
source eliminates the need to path match the test and reference arms of the interferometer. Using a 1
mW HeNe source, the optimum camera shutter speed, used when measuring a 4% reflector, was 250
usec, resulting in significantly reduced vibration sensitivity. Experimental results show the
performance of this new interferometer to be within the specifications of commercial phase shifting
An on-axis, vibration insensitive, polarization Fizeau interferometer is
realized through the use of a novel pixelated mask spatial carrier phase
shifting technique in conjunction with a low coherence source and a
polarization delay-line. In this arrangement, coherence is used to
effectively separate out the orthogonally polarized test and reference beam
components for interference. With both the test and the reference beams
on-axis, the common path cancellation advantages of the Fizeau
interferometer are maintained. The interferometer has the unique ability
to isolate and measure any surface that is substantially normal to the
optical axis of the cavity. Additionally, stray light interference is substantially reduced due to the source's short coherence. An expression
for the fringe visibility on-axis is derived and compared with that of a
standard Fizeau. Using a 15 mW source, the maximum camera shutter
speed, used when measuring a 4% reflector, was 150 usec, resulting in
very robust vibration insensitivity. We experimentally demonstrate the
measurement of both sides of a thin glass plate without the need to modify
the plate between measurements. Experimental results show the
performance of this new interferometer to be within the specifications of
commercial phase shifting interferometers.
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.
Digital Speckle Pattern Interferometry (DSPI) is a well-established method for the measurement of diffuse objects in experimental mechanics. DSPIs are phase shifting interferometers. Three or four bucket temporal phase shifting algorithms are commonly used to provide phase shifting. These algorithms are sensitive to vibrations and can not be used to measure large optical structures far away from the interferometer. In this research a simultaneous phase shifted interferometer, PhaseCam product of 4D Technology Corporation in Tucson Arizona, is modified to be a Simultaneous phase shifted Digital Speckle Pattern Interferometer (SDSPI). Repeatability, dynamic range, and accuracy of the SDSPI are characterized by measuring a 5 cm x 5 cm carbon fiber coupon.
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.
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.
We present a technique to characterize and quantitatively measure the vibrational mode shapes and amplitudes of mirrors concurrently with surface figure testing. The technique utilizes a fast interferometer that does not introduce any mass loading to the test structure. We present the fundamentals of the technique, discuss sevral modes of operation, such as resonant and transient response, and analyze the operational limits. The performance of the measurement system is characterized using a small ambient test mirror.
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.
Important advances of holographic memory. such as parallel input-output, high data transfer rate and ultimate density for optics storage have attracted a lot of interest. The principal feature of volume holography (known as very high selectivity of the diffracted beam intensity to reference beam deviation) is typically used for data multiplexing. Thus, most common techniques for data sampling are based on angular' and spectral2 selectivity resulting from the momentum conservation law (Bra law) for volume holograms3. Angular selectivity has been used more often as it is simpler to implement in practice. It was demonstrated4.5 that data multiplexing with up to 10.000 pages of information can be stored in one crystal as write-read/erase or read-fix. Refextnce bean arbitrary phase encoding also was demonstrated as a useful technique for this purpose, although the detailed analysis of the method was limited only by the conditions where the profile modulation across the reference beam was altered by arbitrary encoding. The first demonstration of shift selectivity of thick holograms with a speckle encoded reference beam was reported7, and later a theoretical explanation of the observed peculiarities8 was given suesting this type of selectivity for high density data storage9. A similar approach was suested for volume holograms with a spherical reference wave;10 however the existing methods do not explore the possibility of the effective use of an entire volume of the hologram as suested in the development for optical disc memory systems11. In this report we present the results of volume hologram recording with high density data obtained through a real volumetric encoding method that allows an increase in the data storage density.
Bacteriorhodopsin (BR) has been proven to be an effective non-linear media for a variety of applications, such as optically addressable spatial light modulators, volumetric memories, optical image processing systems, optical sensors, and optical correlators. However, practical realization of such systems with BR depends upon the specific characteristics of this material. In this report we present experimental results of the time evolution and intensity dependent characteristics of a BR gelatin film. In particular we studied the spectral dependence of the optical density/refraction index modulation. A holographic technique was used to investigate the exposure characteristics of photorefraction, recording versus storage time, as well as the connection between the diffraction efficiency of the recorded grating and light induced scattering (noise)--the parameters that are of primary importance for such applications as high density memory systems and optical correlators.
In this paper we demonstrate the scheme for volume holographic data multiplexing by using the features of shift selectivity of a random encoded speckle reference wave. The proposed recording method results in a more efficient use of the recording medium volume and increases the storage density in comparison with spherical or plane-wave reference beams. The mechanism of lateral and longitudinal shift selectivity are described theoretically and shown to agree with experimental measurements for holographic data multiplexing in the volume of the FetLiNbCE crystal.
Holographic interferometry can be used to examine minute changes in the surface of components as they undergo stress, thermal expansion, erosion, growth, and vibration. Such changes often can be used to identify the presence of a defect beneath the surface, because of the anomalous microscopic behavior of the surface. In addition, the mechanical characteristics of the component, such as vibrational modes, expansion, and residual stress can be identified through holographic inspection. Over the past 30 years a wide range of methods have evolved as new hardware and technology becomes available. The wide range of procedures, including electronic holography, multiwavelength recording thermoplastic recording, time-averaged holography, real-time holographic interferometry, cineholography, and other methods can be revisited each time a new development is made in lasers, computers, and recording materials. Methods that once held only academic interest often become practical with newly available hardware and software.
A modified laser Doppler flowmetry technique that significantly improves the performance of the current
technique in measuring pulpal blood flow is described. A preliminary model demonstrates that, by using a forward-scattered geometry, the detected signal will have a much higher signal-to-noise ratio and calibration capacity. The forward-scattered signal is readily detectable because teeth are relatively thin organs with moderate optical loss. Preliminary experiments comparing forward-scattered detection with conventional back-scattered detection were carried out using an extracted, perfused human molar. The results showed that: (1) the existing back-scattering method produced readings that fluctuated by as much as 187% in response to small changes in sensor position relative to the tooth and (2) the forward-scattered method produced consistent
readings (within 10%) that were independent of the sensor position, a signal-to-noise ratio that was at
least 5.6 times higher than that obtained by the back-scattering method, and a linear response to flow rate.
The results validated the findings of the preliminary model and clearly showed the superiority of the forward-scattering geometry.
We present a design for a compact residual stress measurement sensor based on laser annealing and laser speckle interferometry. The instrument integrates laser diodes, holographic optical elements, a compact CO2 laser, and advanced data reduction techniques to provide quantitative measurements. In this paper we present a review of residual stress measurement using laser annealing, details of our design and preliminary measurement data.
We have proposed and experimentally demonstrated a new configuration of laser Doppler flowmetry for dental pulpal blood flow measurements. To date, the vitality of a tooth can be determined only by subjective thermal or electric tests, which are of questionable reliability and may induced pain in patient. Non-invasive techniques for determining pulpal vascular reactions to injury, treatment, and medication are in great demand. The laser Doppler flowmetry technique is non-invasive; however, clinical studies have shown that when used to measure pulpal blood flow the conventional back-scattering Doppler method suffers from low signal-to-noise ratio (SNR) and unreliable flux readings rendering it impossible to calibrate. A simplified theoretical model indicates that by using a forward scattered geometry the detected signal has a much higher SNR and can be calibrated. The forward scattered signal is readily detectable due to the fact that teeth are relatively thin organs with moderate optical loss. A preliminary experiment comparing forward scattered detection with conventional back- scattered detection was carried out using an extracted human molar. The results validated the findings of the simple theoretical model and clearly showed the utility of the forward scattering geometry. The back-scattering method had readings that fluctuated by as much as 187% in response to small changes in sensor position relative to the tooth. The forward scattered method had consistent readings (within 10%) that were independent of the sensor position, a signal-to-noise ratio that was at least 5.6 times higher than the back-scattering method, and a linear response to flow rate.