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At least 289 records · Page 16Linked to original sources
PubMed2002

Grating-based real-time polarization phase-shifting interferometry: error analysis.

A phase Ronchi grating-based real-time polarization phase-shifting method can be efficiently used for dynamic phase measurement in optical interferometry. A thorough error analysis is required for exhibiting how error sources influence phase-measurement results. We analyze the phase-measurement errors that are induced by the retardation error and azimuth angle error of the quarter-wave plate, the azimuth angle error of polarizers, the phase and intensity aberrations of diffractive wave fronts, and pixel mismatch of the interferometric patterns. The results will also be useful for evaluating the phase-measurement accuracy of other similar systems.

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PubMed2002

Real-time speckle interferometry fringe formation with an adaptive phase mask.

We describe what we believe is a novel speckle-pattern interferometry method of applying a spatial light modulator (SLM) as an adaptive phase mask to obtain real-time fringes of a deformed object without using conventional correlation methods of electronic subtraction or addition. The method is to use a SLM to cancel initial phase in the speckled image before the object is deformed. The fringes from the deformed object can be visualized directly after the initial phase has been canceled. A commercial liquid-crystal television is used as a SLM. The performance of using this SLM in an out-of-plane speckle interferometer is demonstrated.

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PubMed2002

Focus retrocollimated interferometry for focal-length measurements.

Focus retrocollimated interferometry is developed for the measurement of focal lengths of optical lenses and systems, and achievable accuracy is discussed. It is shown that this method can be used to measure both short and long focal lengths simply and with high accuracy.

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PubMed2002

Direct measurement of the group refractive index of air with interferometry between adjacent femtosecond pulses.

The group refractive index of air in laboratory conditions is measured directly between adjacent femtosecond laser pulses by a new interferometry technique. Measurement of the repetition rate of the mode-locked pulse train that gives the maximum amplitude of the interference-signal envelope enables us to determine the group refractive index of air within a standard deviation of 2 x 10(-7). This simple method without vacuum reference is attractive for measuring the group refractive index needed for precise distance measurements in open fields.

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PubMed2002

Extended-range temporal electronic speckle pattern interferometry.

In recent years the availability of high-speed digital video cameras has motivated the study of electronic speckle pattern interferometry (ESPI) in the time domain. To this end a properly sampled temporal sequence of N-fringe patterns is used to analyze the temporal experiment. Samples of temporal speckle images must fulfill the Nyquist criteria over the time axis. When the transient phenomena under study are too fast, the required sampling frequency over time may not be fulfilled. In that case one needs to extend the measuring range of the algorithm used to extract the modulating phase. We analyze how to use short laser pulses or short video acquisition times with fairly long temporal separation among them to estimate the modulating phase of a dynamic ESPI experiment. The only requirement is that the modulating phase being estimated be properly sampled in the spatial domain.

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PubMed2002

Nulling interferometry without achromatic phase shifters.

In the infrared wavelength region, a typical star is approximately a million times brighter than the planet that surrounds it, which is a major problem when we attempt to detect exoplanets in a direct manner. Nulling interferometry is a technique that one can use to solve this problem by attenuating the stellar light and enhancing that of the planet. Generally, deep nulling is achieved by use of achromatic phase shifters (APSs). Unfortunately, the technology needed to build these APSs is not yet fully developed. We show that deep nulling can also be achieved by using delay lines only. We investigate the nulling depth as a function of the width of the wavelength interval and the number of telescopes. We also show that we can obtain nulling depths of less than 10(-6), which are required for exoplanet detection. Furthermore, we investigate the properties of the transmission map and make a comparison between our system and an APS system.

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PubMed2002

Fast surface profiler by white-light interferometry by use of a new algorithm based on sampling theory.

We propose a fast surface-profiling algorithm based on white-light interferometry by use of sampling theory. We first provide a generalized sampling theorem that reconstructs the squared-envelope function of the white-light interferogram from sampled values of the interferogram and then propose the new algorithm based on the theorem. The algorithm extends the sampling interval to 1.425 microm when an optical filter with a center wavelength of 600 nm and a bandwidth of 60 nm is used. The sampling interval is 6-14 times wider than those used in conventional systems. The algorithm has been installed in a commercial system that achieved the world's fastest scanning speed of 80 microm/s. The height resolution of the system is of the order of 10 nm for a measurement range of greater than 100 microm.

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PubMed2002

Effect of wave-number error on the computation of path-length delay in white-light interferometry.

We analyze the error in computed optical path-length delay when using a phase-shifting interferometry (PSI) algorithm with an error in the operating wavelength. The delay error decomposes into two terms. The first is the error in the conversion from a phase measurement to the delay because of the incorrect wavelength, and the second is the error made in the phase measurement itself that is due to the wavelength error. The most important aspect of this investigation is to ascertain this latter error. A general characterization is obtained, and a particularly simple formula is developed for the special case of least-squares estimation involving only the ratio of the wave-number error to the wave number and a multiplicative factor that is an a priori computable nonlinear function of the ratio of the modulator stroke length to the operating wavelength. Because the ultimate path-length error is a function of the two terms, a new set of PSI algorithms that compensate the computed phase error to cancel the conversion error is developed. Numerical simulations are presented to validate the analysis and establish the insensitivity of the new algorithms to wave-number error.

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PubMed2002

Real-time color holographic interferometry.

A new optical technique based on real-time color holographic interferometry has been developed for analyzing unsteady aerodynamic wakes in fluid mechanics or for measuring displacements and deformations in solid mechanics. The technique's feasibility is demonstrated here. It uses three coherent wavelengths produced simultaneously by a cw laser (mixed argon and krypton). Holograms are recorded on single-layer panchromatic silver halide (Slavich PFG 03C) plates. Results show the optical setup can be adjusted to obtain a uniform background color. The interference fringe pattern visualized is large and colored and exhibits a single central white fringe, which makes the zero order of the interferogram easy to identify. An application in a subsonic wind tunnel is presented, in which the unsteady wake past a cylinder is recorded at high rate.

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PubMed2002

Comparative study with double-exposure digital holographic interferometry and a shack-hartmann sensor to characterize transparent materials.

We compare wave-front measurements using double-exposure digital holography and a Shack-Hartmann sensor. A voltage-driven liquid-crystal wedge modulates the optical wave front and provides a refractive-index gradient typical of interesting transparent materials. Measurement accuracy and reliability are similar for both methods. In our opinion, digital holographic interferometry has several advantages for both laboratory and field environments. When compared with Shack-Hartmann methods, these advantages include hardware simplicity and robustness, relative insensitivity to sample dynamic range, and less computational demanding and more straightforward data evaluation algorithms. We believe that digital holography provides the methodology of choice for field studies of transparent materials such as microgravity protein crystal growth experiments.

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PubMed2003

Optimal re-referencing rate for in-plane dynamic speckle interferometry.

We investigate experimentally the optimal rate at which the reference speckle pattern should be updated when dynamic speckle interferometry is used to measure transient in-plane displacement fields. Images are captured with a high-speed camera and phase shifting and phase unwrapping are done temporally. For a wide range of in-plane velocities, up to a maximum of 40% of the Nyquist limit, the random errors in the calculated displacement field are minimized by updating the reference speckle pattern after a speckle displacement of 1/10 of the pixel spacing. The technique is applied to measurements of microscale deformation fields within an adhesive joint in a carbon-fiber epoxy composite.

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PubMed2003

Measurement of thermally induced vibrations of microelectronic devices by use of a heterodyne electronic speckle pattern interferometry imaging technique.

An imaging technique to measure modulated surface displacements on microelectronic devices is presented. A device is supplied by a sinusoidal current that creates a modulated variation of temperature. To measure the induced normal surface displacement, we use an electronic speckle pattern interferometry setup in which we introduce a secondary modulation using an electro-optic modulator. To extract the displacement information, we then analyze the term atthe blinking frequency, which is equal tothe difference between the frequency of the surface displacement and the frequency of the secondary modulation. As the photodetector is a visible CC D camera, weapply heterodyne detection byusing a multichannel lock-in scheme. We have experimented with this new technique on a membrane to measure the amplitude of modulated surface displacement induced by the Joule effect.

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PubMed2003

Theoretical measurement uncertainty of white-light interferometry on rough surfaces.

A great advantage of the white-light interferometry is that it can be used for profile objects with a rough surface. A speckle pattern that arises in the image plane allows one to observethe interference; however, this pattern is also the source of the measurement uncertainty. We derive the theoretical limits of the longitudinal uncertainty by virtue of the first-order statistics of thespeckle pattern. It is shown that this uncertainty depends on the surface roughness of the measured object only; it does not depend on the setup parameters.

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PubMed2003

Direct measurement of refractive-index dispersion of transparent media by white-light interferometry.

We report on a technique for measuring the refractive indices of nonabsorbing media over a broad spectral range from 0.5 to 5 microm. White-light interferometry based on a double-interferometer system consisting of a fixed Mach-Zehnder interferometer and a Fourier-transform spectrometer is used for direct measurement of the absolute rotation-dependent phase shift induced by an optical element. Refractive index n(lambda) over the whole investigated spectral range is thus obtained directly to an accuracy of 10(-4) without the need for any specific assumption about dispersion. Results for synthetic fused silica are presented and discussed.

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PubMed2003

Compensation for anamorphotic distortion in grazing-incidence interferometry testing planar specimens.

One important feature of grazing-incidence interferometry is the anamorphotic distortion or the fore-shortened view of the interference pattern along the optical axis caused by the geometry of theinterferometer. To compensate for the consequential lower resolution along the optical axis, a setup was built in which the object plane is imaged onto a rectifying grating, ensuring sharp mapping of the whole specimen onto the detector. A volume hologram and a diffraction grating serve as rectifying elements and are applied to measure various types of planar objects such as mirrors and structured plastic samples. Accuracy, image sharpness, and residual distortion for both cases are discussed.

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PubMed2003

Increasing the range of unambiguity in step-height measurement with multiple-wavelength interferometry--application to absolute long gauge block measurement.

An instrument for step-height measurement by multiple-wavelength interferometry is described. The addition of a 1152-nm wavelength to a multiple-wavelength scheme applying wavelengths of 633, 612, and 543 nm relaxes the tolerance range of the required preliminary measurement to +/- 140 microm, if the total uncertainty in the fringe fraction measurement can be kept below 2%. For larger fringe fraction measurement uncertainty, numerical simulations show that the integer number of interference orders can still be determined unambiguously if the range in the preliminary knowledge of the length has been correspondingly reduced. The interferometer instrument is described, and experimental data are presented in the context of long gauge block calibration at the National Research Council of Canada.

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PubMed2003

Application of schlieren interferometry to temperature measurements during laser welding of high-density polyethylene films.

Schlieren interferometry is found to be an alternative tool for temperature measurement during thermoplastic laser welding with regard to methods based on thermocouples or optical pyrometers. In fact, these techniques are not easily applied when materials to be processed have reduced thickness, negligible heat conduction, and low emissivity, as is the case of welding high-density polyethylene films with 10.6-microm CO2 laser radiation, even if the method reaches its applicability limit after approximately 1 s of the interaction process. The schlieren method provides the means and the results to probe the thermal variations of the laser-thermoplastic interaction on both the surface and the interface between the sample material and the air.

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