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At least 1,441 records · Page 80Linked to original sources
PubMed2006

Stroboscopic white-light interference microscopy.

The principle of stroboscopic motion freezing of oscillating objects extends directly to interference microscopes that use coherence as part of the measurement principle. Analysis shows, however, that the fringe contrast loss for out-of-plane motion in stroboscopic interferometry is a wavelength-dependent phenomenon, which can alter the apparent nominal center wavelength of the white-light source. As in monochromatic systems, the key adjustable parameter is the duty cycle, equal to the product of the vibrational frequency and the pulse width. This theoretical study provides detailed graphs of expected errors as a function of the duty cycle, including fringe contrast loss, apparent wavelength shift, and measurement error.

Algorithms↗
PubMed2006

Surface-mount sapphire interferometric temperature sensor.

A fiber-optic high-temperature sensor is demonstrated by bonding a 45 degrees -polished single-crystal sapphire fiber on the surface of a sapphire wafer, whose optical thickness is temperature dependent and measured by white-light interferometry. A novel adhesive-free coupling between the silica and sapphire fibers is achieved by fusion splicing, and its performance is characterized. The sensor's interference signal is investigated for its dependence on angular alignment between the fiber and the wafer. A prototype sensor is tested to 1,170 degrees C with a resolution of 0.4 degrees C, demonstrating excellent potential for high-temperature measurement.

Journal Article↗
PubMed2006

Homogenized Fourier transform holographic data storage using phase spatial light modulators and methods for recovery of data from the phase image.

We present a method for homogenizing the Fourier spectrum for holographic digital data storage by use of a phase spatial light modulator (SLM), and methods for the recovery of data from a phase image are implemented and discussed. Binary digital data are displayed on a phase SLM operating in 0 and pi phase modes to optimally remove the intense dc peak so as to obtain a homogenized Fourier spectrum. Methods based on holographic interferometry have been developed and employed for recovery of the original amplitude data page from the phase-data page. A new edge-detection-based method also has been demonstrated and analyzed for reconstruction of the original data. Experimental results are presented to confirm the feasibility of these novel techniques.

Journal Article↗
PubMed2006

Folding mirror schlieren diffraction interferometer.

We demonstrate the use of a mirror as a viewing diaphragm to generate a built-in diffracted reference beam in schlieren diffraction interferometry (SDI). The use of a mirror edge as a diffracting element instead of a conventional knife edge considerably enhances the contrast of the schlieren pattern, and it is shown to be equal to that of a phase knife edge. This increase in contrast is due to the fact that the otherwise unutilized diffracted beam in SDI is recombined in the described folding mirror geometry.

Journal Article↗
PubMed2006

Phase geographical map for determining the material type of a right-angle prism.

A phase geographical map for determining a right-angle prism is presented. The proposed method is based on total-internal-reflection effects and chromatic dispersion. Under the total-internal-reflection condition, the phase difference between the S and P polarizations, as a function of the wavelength and refractive index, can be extracted and measured using heterodyne interferometry. Various wavelengths correspond to various refractive index values. The proposed map is convenient in ensuring the prism material using a specific V number. The method has the following merits: high stability, ease of operation, and rapid measurement.

Journal Article↗
PubMed2006

Experimental results on piston detection by using the classical Ronchi test.

We present some experimental results of piston detection by using a classical Ronchi test in a segmented spherical surface. We compare the Ronchi fringes frequency of both segments (reference and under test). When the piston term is present at the segment under test, it is moved iteratively until we obtain a minimum piston error. When the surface is almost cophased, both segments have the same fringe frequency. The fringes of each segment are compared by correlating the intensity versus position in each segment. The Ronchi test is used at the same time as Shack interferometry; this second method is used as a reference to avoid segments tilts. With the Ronchi test we have detected experimentally a piston of the order of 45 nm (lambda/14 with lambda=632.8 nm). This result was compared with images of simulated Ronchigrams.

Journal Article↗
PubMed2006

Comparison of annular wavefront interpretation with Zernike circle polynomials and annular polynomials.

A general wavefront fitting procedure with Zernike annular polynomials for circular and annular pupils is proposed. For interferometric data of typical annular wavefronts with smaller and larger obscuration ratios, the results fitted with Zernike annular polynomials are compared with those of Zernike circle polynomials. Data are provided demonstrating that the annular wavefront expressed with Zernike annular polynomials is more accurate and meaningful for the decomposition of aberrations, the calculation of Seidel aberrations, and the removal of misalignments in interferometry. The primary limitations of current interferogram reduction software with Zernike circle polynomials in analyzing wavefronts of annular pupils are further illustrated, and some reasonable explanations are provided. It is suggested that the use of orthogonal basis functions on the pupils of the wavefronts analyzed is more appropriate.

Journal Article↗
PubMed2000

Analytical model of Doppler spectra of coherent light backscattered from rotating cones and cylinders.

The interaction between a rotating object and laser beams has been studied in the field of laser Doppler velocimetry, where two incident laser beams are focused on one small spot of the rotating surface and interferometry is used. In the case of a single incident laser beam illuminating a relatively large area of the rotating surface, both the Doppler broadening and the reflected-power level are dictated by points distributed over a wide curved area at varying angles of incidence. An analytical model of spectra in backscatter from cones and cylinders rotating around their axes is presented. This analytical solution may contribute to laser Doppler velocimetry as well as ladar applications.

Laser-Doppler Flowmetry↗
PubMed2001

Two-dimensional phase unwrapping with use of statistical models for cost functions in nonlinear optimization.

Interferometric radar techniques often necessitate two-dimensional (2-D) phase unwrapping, defined here as the estimation of unambiguous phase data from a 2-D array known only modulo 2pi rad. We develop a maximum a posteriori probability (MAP) estimation approach for this problem, and we derive an algorithm that approximately maximizes the conditional probability of its phase-unwrapped solution given observable quantities such as wrapped phase, image intensity, and interferogram coherence. Examining topographic and differential interferometry separately, we derive simple, working models for the joint statistics of the estimated and the observed signals. We use generalized, nonlinear cost functions to reflect these probability relationships, and we employ nonlinear network-flow techniques to approximate MAP solutions. We apply our algorithm both to a topographic interferogram exhibiting rough terrain and layover and to a differential interferogram measuring the deformation from a large earthquake. The MAP solutions are complete and are more accurate than those of other tested algorithms.

Journal Article↗
PubMed2001

Wave-front measurement errors from restricted concentric subdomains.

In interferometry and optical testing, system wave-front measurements that are analyzed on a restricted subdomain of the full pupil can include predictable systematic errors. In nearly all cases, the measured rms wave-front error and the magnitudes of the individual aberration polynomial coefficients underestimate the wave-front error magnitudes present in the full-pupil domain. We present an analytic method to determine the relationships between the coefficients of aberration polynomials defined on the full-pupil domain and those defined on a restricted concentric subdomain. In this way, systematic wave-front measurement errors introduced by subregion selection are investigated. Using vector and matrix representations for the wave-front aberration coefficients, we generalize the method to the study of arbitrary input wave fronts and subdomain sizes. While wave-front measurements on a restricted subdomain are insufficient for predicting the wave front of the full-pupil domain, studying the relationship between known full-pupil wave fronts and subdomain wave fronts allows us to set subdomain size limits for arbitrary measurement fidelity.

Journal Article↗
PubMed2001

High-precision fringe tracking invariant to systematic model errors.

We present a new approach to processing of interferometric data, which dramatically improves operation on the smallest fraction of the fringe (approximately 1/1,000 of a wavelength and beyond). In particular, this approach, dubbed the variation-invariant subspace tracking approach (VISTA), makes it possible to operate a Michelson interferometer in a highly stable mode in which the estimation of optical path delay becomes invariant (insensitive) to a large class of systematic model errors. This previously unknown invariance property of interferometry follows from the fundamental odd-even symmetry of the channeled spectrum and its derivatives. VISTA offers a powerful algorithmic alternative for alleviating technological challenges in the design of high-precision long-baseline spaceborne interferometers.

Journal Article↗
PubMed2002

Adaptive restoration of a partially coherent blurred image using an all-optical feedback interferometer with a liquid-crystal device.

A liquid-crystal adaptive optics system using all-optical feedback interferometry is applied to partially coherent imaging through a phase disturbance. A theoretical analysis based on the propagation of the cross-spectral density shows that the blurred image due to the phase disturbance can be restored, in principle, irrespective of the state of coherence of the light illuminating the object. Experimental verification of the theory has been performed for two cases when the object to be imaged is illuminated by spatially coherent light originating from a He-Ne laser and by spatially incoherent white light from a halogen lamp. We observed in both cases that images blurred by the phase disturbance were successfully restored, in agreement with the theory, immediately after the adaptive optics system was activated. The origin of the deviation of the experimental results from the theory, together with the effect of the feedback misalignment inherent in our optical arrangement, is also discussed.

Journal Article↗
PubMed2002

Use of single-mode waveguides to correct the optical defects of a nulling interferometer.

In the area of long-baseline nulling interferometry, high rejection ratios are needed to cancel out the light of a bright central source and look for nearby faint structures or companions. These rejection requirements directly translate into drastic optical constraints. We discuss the possibility of using single-mode waveguides for such applications. Conversely to simple pinholes, single-mode waveguides efficiently correct wave-front defects of both high- and low-order spatial frequencies, ensure a perfect matching of the amplitude profiles coming from the various beams, and can be used with almost optimum coupling efficiency over a broad optical bandpass (typically an octave). They then appear to greatly enhance the feasibility of high-dynamic interferometric coronagraphs.

Journal Article↗
PubMed2003

Cramér-Rao sensitivity limits for astronomical instruments: implications for interferometer design.

Multiple-telescope interferometry for high-angular-resolution astronomical imaging in the optical-IR-far-IR bands is currently a topic of great scientific interest. The fundamentals that govern the sensitivity of direct-detection instruments and interferometers are reviewed, and the rigorous sensitivity limits imposed by the Cramér-Rao theorem are discussed. Numerical calculations of the Cramér-Rao limit are carried out for a simple example, and the results are used to support the argument that interferometers that have more compact instantaneous beam patterns are more sensitive, since they extract more spatial information from each detected photon. This argument favors arrays with a larger number of telescopes, and it favors all-on-one beam-combining methods as compared with pairwise combination.

Journal Article↗
PubMed2003

Adaptive phase-shifting algorithm for temporal phase evaluation.

Most standard temporal-phase-shifting (TPS) algorithms evaluate the phase by computing a windowed Fourier transform (WFT) of the intensity signal at the carrier frequency of the system. However, displacement of the specimen during image acquisition may cause the peak of the transform to shift away from the carrier frequency, leading to phase errors and even unwrapping failure. We present a novel TPS method that searches for the peak of the WFT and evaluates the phase at that frequency instead of at the carrier frequency. The performance of this method is compared with that of standard algorithms by using numerical simulations. Experimental results from high-speed speckle interferometry studies of carbon fiber panels are also presented.

Journal Article↗
PubMed2005

Low-coherence light-scattering calculations for polydisperse size distributions.

The calculation of angular light-scattering distributions is considered for low-coherence light incident on a polydisperse particle size distribution of scatterers. As low-coherence light is now commonly used in interferometry schemes when applied to biomedical imaging, the difference between detecting scattered intensity and interferometrically detecting the scattered field is examined. An expression is derived that allows the presence of multiple wavelengths lambda and particle sizes d to be described by a single distribution in the size parameter x = pi d/lambda, which simplifies numerical calculations. The applicability of this expression is examined numerically.

Journal Article↗
PubMed2005

Global analysis of the phase calibration operation.

A global approach to phase calibration is presented. The corresponding theoretical framework calls on elementary concepts of algebraic graph theory (spanning tree of maximal weight, cycles) and algebraic number theory (lattice, nearest lattice point). The traditional approach can thereby be better understood. In radio imaging and in optical interferometry, the self-calibration procedures must often be conducted with much care. The analysis presented should then help in finding a better compromise between the coverage of the calibration graph (which must be as complete as possible) and the quality of the solution (which must of course be reliable).

Journal Article↗
PubMed2005

Fabry-Perot metrology for displacements up to 50 mm.

A system designed to apply Fabry-Perot interferometry to the measurement of displacements is described. Two adjacent modes of a Fabry-Perot cavity are probed, and both the absolute optical frequencies and their difference are used to determine displacements via changes in cavity length. Light is coupled to the cavity via an optical fiber, making the system ideal for remote sensing applications. Continuous interrogation is not necessary, as the cavity length is encoded in the free spectral range. The absolute uncertainty is determined to be below 10 pm, which for the largest displacement measured corresponds to a relative uncertainty of 4 x 10(-10). To my knowledge this is the smallest relative uncertainty in a displacement measurement ever demonstrated.

Journal Article↗