Showing papers on "Interferometry published in 1997"

Phase-shifting digital holography

Abstract: A new method for three-dimensional image formation is proposed in which the distribution of complex amplitude at a plane is measured by phase-shifting interferometry and then Fresnel transformed by a digital computer. The method can reconstruct an arbitrary cross section of a three-dimensional object with higher image quality and a wider viewing angle than from conventional digital holography using an off-axis configuration. Basic principles and experimental verification are described.

Atmospheric effects in interferometric synthetic aperture radar surface deformation and topographic maps

Abstract: Interferogram images derived from repeat-pass spaceborne synthetic aperture radar systems exhibit artifacts due to the time and space variations of atmospheric water vapor Other tropospheric variations, such as pressure and temperature, also induce distortions, but the effects are smaller in magnitude and more evenly distributed throughout the interferogram than the wet troposphere term Spatial and temporal changes of 20% in relative humidity lead to 10 cm errors in deformation products, and perhaps 100 m of error in derived topographic maps for those pass pairs with unfavorable baseline geometries In wet regions such as Hawaii, these are by far the dominant errors in the Spaceborne Imaging Radar-C and X Band Synthetic Aperature Radar (SIR-C/X-SAR) interferometric products The unknown time delay from tropospheric distortion is independent of frequency, and thus multiwavelength measurements, such as those commonly used to correct radar altimeter and Global Positioning System (GPS) ionospheric biases, cannot be used to rectify the error In the topographic case, the errors may be mitigated by choosing interferometric pairs with relatively long baselines, as the error amplitude is inversely proportional to the perpendicular component of the interferometer baseline For the SIR-C/X-SAR Hawaii data we found that the best (longest) baseline pair produced a map supporting 100 m contouring, whereas the poorest baseline choice yielded an extremely noisy topographic map even at this coarse contour interval In the case of deformation map errors the result is either independent of baseline parameters or else very nearly so Here the only solution is averaging of independent interferograms, so in order to create accurate deformation products in wet regions many multiple passes may be required Rules for designing optimal data acquisition and processing sequences for interferometric analyses in nondesert parts of the world are (1) to use the longest radar wavelengths possible, within ionospheric scintillation and Faraday rotation limits, (2) for topography, maximize interferometer baseline within decorrelation limits* and (3) for surface deformation, use multiple observations and average the derived products Following the above recipe yields accuracies of 10 m for digital elevation models and 1 cm for deformation maps even in very wet regions, such as Hawaii

In vivo bidirectional color Doppler flow imaging of picoliter blood volumes using optical coherence tomography.

Abstract: We describe a novel optical system for bidirectional color Doppler imaging of flow in biological tissues with micrometer-scale resolution and demonstrate its use for in vivo imaging of blood flow in an animal model. Our technique, color Doppler optical coherence tomography (CDOCT), performs spatially localized optical Doppler velocimetry by use of scanning low-coherence interferometry. CDOCT is an extension of optical coherence tomography (OCT), employing coherent signal-acquisition electronics and joint time-frequency analysis algorithms to perform flow imaging simultaneous with conventional OCT imaging. Cross-sectional maps of blood flow velocity with <50-µm spatial resolution and <0.6-mm/s velocity precision were obtained through intact skin in living hamster subdermal tissue. This technology has several potential medical applications.

High-speed phase- and group-delay scanning with a grating-based phase control delay line.

Abstract: A rapid-scanning optical delay line that employs phase control has several advantages, including high speed, high duty cycle, phase- and group-delay independence, and group-velocity dispersion compensation, over existing optical delay methods for interferometric optical ranging applications. We discuss the grating-based phase-control delay line and its applications to interferometric optical ranging and measurement techniques such as optical coherence domain reflectometry and optical coherence tomography. The system performs optical ranging over an axial range of 3 mm with a scanning rate of 6m/s and a repetition rate of 2 kHz. The device is especially well suited for applications such as optical coherence tomography that require high-speed, repetitive, linear delay line scanning with a high duty cycle.

“Plug and play” systems for quantum cryptography

Abstract: We present a time-multiplexed interferometer based on Faraday mirrors, and apply it to quantum key distribution The interfering pulses follow exactly the same spatial path, ensuring very high stability and self balancing The use of Faraday mirrors compensates automatically any birefringence effects and polarization dependent losses in the transmitting fiber First experimental results show a fringe visibility of 09984 for a 23-km-long interferometer, based on installed telecom fibers

Differential interferometer system and lithographic step-and-scan apparatus provided with such a system

Abstract: A differential interferometer system for measuring the mutual positions and movements of a first object (WH) and a second object (MH) is described, which system comprises a first interferometer unit (1, 2, 3, 4) with a first measuring reflector (RW) and a second interferometer unit (5, 6, 7, 8) with a second measuring reflector (RM). Since a measuring beam (bm) passes through both the first and the second interferometer unit and is reflected by both the first and the second measuring reflector, and since the measuring beam and the reference beam (br) traverse the same path at least between the two interferometer units, accurate measurements can be performed very rapidly. The interferometer system may be used to great advantage in a step-and-scan lithographic projection apparatus.

Wavelength-tuning interferometry of intraocular distances.

Abstract: We describe basic principles of wavelength-tuning interferometry and demonstrate its application in ophthalmology. The advantage of this technique compared with conventional low-coherence interferometry ranging is the simultaneous measurement of the object structure without the need for a moving reference mirror. Shifting the wavelength of an external-cavity tunable laser diode causes intensity oscillations in the interference pattern of light beams remitted from the intraocular structure. A Fourier transform of the corresponding wave-number-dependent photodetector signal yields the distribution of the scattering potential along the light beam illuminating the eye. We use an external interferometer to linearize the wave-number axis. We obtain high resolution in a model eye by slow tuning over a wide wavelength range. With lower resolution we demonstrate the simultaneous measurement of anterior segment length, vitreous chamber depth, and axial eye length in human eyes in vivo with data-acquisition times in the millisecond range.

Phase-shifting point diffraction interferometer

Abstract: Disclosed is a point diffraction interferometer for evaluating the quality of a test optic. In operation, the point diffraction interferometer includes a source of radiation, the test optic, a beam divider, a reference wave pinhole located at an image plane downstream from the test optic, and a detector for detecting an interference pattern produced between a reference wave emitted by the pinhole and a test wave emitted from the test optic. The beam divider produces separate reference and test beams which focus at different laterally separated positions on the image plane. The reference wave pinhole is placed at a region of high intensity (e.g., the focal point) for the reference beam. This allows reference wave to be produced at a relatively high intensity. Also, the beam divider may include elements for phase shifting one or both of the reference and test beams.

On the characterization of optical fiber network components with optical frequency domain reflectometry

Abstract: The basic features of coherent optical frequency domain reflectometry are presented. The ultimate limits of range and sensitivity were discussed, as well as polarization effects. It is shown that this technique is very suitable for optical network components characterization, and accurate measurements of a number of such components are presented.

Three dimensional optical imaging colposcopy

Abstract: An optical coherence tomography(OCT) and spectral interferometry imaging probe for the automatic screening and diagnosis of cervical and skin cancer in vivo. The probe eliminates the old techniques of having to perform Pap smears followed by a biopsy, known as colposcopy. The novel probe is cylindrical in shape and has a disposable outer plastic shield. Inside the probe is a motor driven rotatable casing having a planar optical fiber bundle array therein. The fiber bundle array has plastic light coupling lenslet arrays on both ends. The exposed end of the probe has one lenslet array disc that couples light between the probe and an interior of the cervix area being examined. Both the casing and the bundle array rotate relative to the outer probe walls. Inside the casing is a rotatable motor driven scanning mirror which couples passes light from an incoming second fiber bundle array to the lenslet array on the inside end of the rotatable casing fiber bundle array. The incoming second fiber bundle array is attached to a detector located external and remote to the probe. The detector has a tunable light source and a spectrometer for providing 3-D images of the interior of the cervix. The proposed technique of simultaneous OCT imaging and spectral interferometry and analysis has potential use in either multi-layer optical disk read out using low coherence tomography and spectral interferometry. Alternatively, by employing spectral interferometry, the measured optical spectrum also provides a unique mapping of the recorded bits into the observed spectral modulation. The unique feature is that by simultaneously employing both spectral interferometry and low coherence tomography, the resultant data transfer rate is increased, but the combination also allows for error rate analysis.

An Imaging Nulling Interferometer to Study Extrasolar Planets

Abstract: Interferometric techniques offer two advantages for the detection and analysis of thermal radiation from planets: destructive interference to strongly suppress the stellar emission, and the possibility of high-resolution imaging to resolve planets and distinguish them from dust emission. This paper presents a new interferometric configuration in which the conflicting requirements for these goals are reconciled. It realizes a very strong, broad interference null, so high-resolution fringes can be used while maintaining good suppression of the stellar disk. Complex phase measurement is precluded by the need for destructive interference, but we find that a cross-correlation technique analogous to aperture synthesis can recover true images. When operated 5 AU from the Sun to escape background emission from local zodiacal dust, the interferometer's sensitivity will be limited fundamentally by noise in the photon flux from warm zodiacal dust in the planetary system under observation. In order to scale the interferometer for adequate sensitivity, the 10 μm emission from such dust could be determined early on by a ground-based interferometer. If stars at 10 pc distance have zodiacal clouds like our own, a 50 m long space interferometer with four 1 m elements should see individual planets like the Earth in images taken over 10 hours. Simultaneous infrared spectra of planets like Earth, Venus, Jupiter, and Saturn could be obtained during a 3 month integration, with the sensitivity to detect carbon dioxide, water, and ozone at the levels seen in Earth's spectrum.

Heterodyne methods in millimetre wave plasma diagnostics with applications to ECE, interferometry and reflectometry

Abstract: Basic principles of heterodyne techniques are introduced and the various components of a heterodyne system are summarized. Special applications in ECE, interferometry and reflectometry are discussed after introducing the diagnostic principles. Realized systems as described in the literature are briefly outlined. Ordering principles are radiometer types in the case of ECE, mixing scheme and generation and stabilization of local oscillator and intermediate frequency signals in the case of interferometry and reflectometry. Special techniques and their impact on the performance of the diagnostic instruments are discussed.

Deriving object visibilities from interferograms obtained with a fiber stellar interferometer

Abstract: A method is given for extracting object visibilities from data provided by a long baseline interferometer, where the beams are spatially filtered by single-mode fibers and interferograms are obtained as scans around the zero optical pathlength difference. It is shown how the signals can be corrected from the wavefront perturbations caused by atmospheric turbulence. If the piston perturbations are also removed, then the corrected data contain both spatial and spectral information on the source (double Fourier interferometry). When the piston cannot be removed, object phase and spectral information are lost, and the observable (free of detector noise bias) is the squared modulus of the coherence factor, integrated over the optical bandpass. In a fiber interferometer this quantity leads to very accurate object visibility measurements because the transfer function does not involve an atmospheric term. The analysis also holds for a more classical pupil plane interferometer which does not take advantage of the spatial filtering capability of single-mode fibers. In that case however, the transfer function includes a turbulence term that needs to be calibrated by statistical methods.

Rotation Sensing with an Atom Interferometer

Abstract: We have measured the phase shift induced by rotation of an atom interferometer at rates of $\ensuremath{-}2$ to $+2$ earth rates and obtained $1%$ agreement with the predicted Sagnac phase shift for atomic matter waves. The rotational rms noise of our interferometer was $42$ milliearth rates for $1$ sec of integration time, within $9%$ of shot noise. The high sensitivity and agreement of predicted and measured behavior suggest useful future scientific applications of atom interferometers as inertial sensors.

Optical Measurement Techniques and Applications

Abstract: Introduction (P.K. Rastogi). Optical Metrology of Engineering Surfaces -- Scope and Trends (H.J. Tiziani). Digital Processing of Fringe Patterns in Optical Metrology (W. Osten, W. J?ptner). Interferometric Optical Testing (D. Briers). Holographic Interferometry -- An Important Tool in Nondestructive Measurement and Testing (P.K. Rastogi). Speckle Photography, Shearography, and ESPI (C. Joenathan). Photoelasticity and Moire (A. Asundi). Optical Fiber Sensors (A.D. Kersey). Fiber Optic Smart Sensing (D. Inaudi). Holographic Metrology of Micro-Objects in a Dynamic Volume (C.S. Vikram). Particle Image Velocimetry (I. Grant). Surface Roughness Measurement (J.M. Bennet). Lidar for Atmospheric Remote Sensing (U.N. Singh). Some Other Methods in Optical Metrology (S. Krishnaswamy, T.C. Philip Chu, I. Saxena, H.G. Maas).

The Virgo interferometer

Abstract: The Virgo gravitational wave detector is an interferometer with 3 km long arms in construction near Pisa to be commissioned in the year 2000. Virgo has been designed to achieve a strain sensitivity of a few times at 200 Hz. A large effort has gone into the conception of the mirror suspension system, which is expected to reduce noise to the level of at 10 Hz. The expected signals and main sources of noise are briefly discussed; the choices made are illustrated together with the present status of the experiment.

Fiber Sagnac interferometer temperature sensor

Abstract: A modified Sagnac interferometer-based fiber temperature sensor is proposed. Polarization independent operation and high temperature sensitivity of this class of sensors make them cost effective instruments for temperature measurements. A comparison of the proposed sensor with Bragg grating and long-period grating fiber sensors is derived. A temperature-induced spectral displacement of 0.99 nm/K is demonstrated for an internal stress birefringent fiber-based Sagnac interferometer.

Generation of diffraction-free beams for applications in optical microlithography

Abstract: A new concept based on a Fabry–Perot interferometer for the generation of nondiffracting Bessel beams is described and proposed for potential applications in microlithography such as the fabrication of small isolated patterns. It was experimentally demonstrated that the depth of focus can be increased by a factor of about 2, and simultaneously the transverse resolution improved by a factor of 1.6, when using this technique to image contact holes. The properties of simultaneous imaging of two contact holes were also investigated. It was shown experimentally that, even in the most critical case (when the first diffraction rings overlap), undesirable interference effects between the adjacent contact holes can be eliminated by means of a phase shifting technique.

Optical sensor system utilizing bragg grating sensors

Abstract: An optical system for detecting perturbations indicative of the performance of the piece of equipment being monitored is disclosed. The optical system comprises sensors (12), each of which uses Bragg gratings (12A-12N), induced therein and wherein the Bragg gratings (12A-12N) are arranged into a preselected distribution and each Bragg grating returns, when subjected to incident light, a narrowbeam signal (22, 24, 26) identified by a predetermined wavelength. The optical system utilizes at least one interferometer (56) whose operation is interlinked with optical multiplexing techniques, such as differentiate-cross-multiplying and time-division multiplexing.

The processing of hexagonally sampled signals with standard rectangular techniques: application to 2-D large aperture synthesis interferometric radiometers

Abstract: In Earth observation programs there is a need of passive low frequency (L-band) measurements to monitor soil moisture and ocean salinity with high spatial resolution 10-20 km, a radiometric resolution of 1 K and a revisit time of 1-3 days. Compared to total power radiometers aperture synthesis interferometric radiometers are technologically attractive because of their reduced mass and hardware requirements. In this field it should be mentioned the one-dimensional (1D) linear interferometer ESTAR developed by NASA and MIRAS a two-dimensional (2D) Y-shaped interferometer currently under study by European Space Agency (ESA). Interferometer radiometers measure the correlation between pairs of nondirective antennas. Each complex correlation is a sample of the "visibility" function which, in the ideal case, is the spatial Fourier transform of the brightness temperature distribution. Since most receiver phase and amplitude errors can be hardware calibrated, Fourier based iterative inversion methods will be useful when antenna errors are small, their radiation voltage patterns are not too different, and mutual coupling is small. In order to minimize on-board hardware requirements-antennas, receivers and correlators-the choice of the interferometer array shape is of great importance since it determines the (u,v) sampling strategy and the minimum number of visibility samples required for a determined aliasing level. In this sense, Y-shaped and triangular-shaped arrays with equally spaced antennas are optimal. The main contribution of this paper is a technique that allows the authors to process the visibility samples over the hexagonal sampling grids given by Y-shaped and triangular-shaped arrays with standard rectangular FFT routines. Since no interpolation processes are involved, the risk of induced artifacts in the recovered brightness temperature over the wide held of view required in Earth observation missions is minimized and signal to noise ratio (SNR) is preserved.

Optical quadrature Interferometer

Abstract: An optical quadrature interferometer is presented. The optical quadrature interferometer uses a different state of polarization in each of two arms of the interferometer. A light beam is split into two beams by a beamsplitter, each beam directed to a respective arm of the interferometer. In one arm, the measurement arm, the light beam is directed through a linear polarizer and a quarter wave plate to produce circularly polarized light, and then to a target being measured. In the other arm, the reference arm, the light beam is not subject to any change in polarization. After the light beams have traversed their respective arms, the light beams are combined by a recombining beamsplitter. As such, upon the beams of each arm being recombined, a polarizing element is used to separate the combined light beam into two separate fields which are in quadrature with each other. An image processing algorithm can then obtain the in-phase and quadrature components of the signal in order to construct an image of the target based on the magnitude and phase of the recombined light beam. The system may further be used for lensless imaging.

The Processing of Hexagonally Sampled Signals with Standard Rectangular Techniques: Application to 2-D Large Aperture Synthesis

Abstract: In Earth observation programs there is a need of passive low frequency -band) measurements to monitor soil moisture and ocean salinity with high spatial resolution 10-20 Km, a radiometric resolution of 1K and a revisit time of 1-3 days (1). Compared to total power radiometers aperture syn- thesis interferometric radiometers are technologically attractive because of their reduced mass and hardware requirements. In this field it should be mentioned the one-dimensional (1-D) linear interferometer ESTAR developed by NASA (2) and MIRAS a two-dimensional (2-D) Y-shaped interferometer currently under study by European Space Agency (ESA) (3). Interferometer ra- diometers measure the correlation between pairs of nondirective antennas. Each complex correlation is a sample of the "visibility" function which, in the ideal case, is the spatial Fourier transform of the brightness temperature distribution. Since most receiver phase and amplitude errors can be hardware calibrated, Fourier based iterative inversion methods will be useful when antenna errors are small, their radiation voltage patterns are not too different, and mutual coupling is small. In order to minimize on-board hardware requirements—antennas, receivers and cor- relators—the choice of the interferometer array shape is of great importance since it determines the sampling strategy and the minimum number of visibility samples required for a determined aliasing level. In this sense, Y-shaped and triangular- shaped arrays with equally spaced antennas are optimal. The main contribution of this paper is a technique that allows us to process the visibility samples over the hexagonal sampling grids given by Y-shaped and triangular-shaped arrays with standard rectangular FFT routines. Since no interpolation processes are involved, the risk of induced artifacts in the recovered brightness temperature over the wide field of view required in Earth obser- vation missions is minimized and signal to noise ratio (SNR) is preserved.

Methods and arrangement for increasing contrast in optical coherence tomography by means of scanning an object with a dual beam

Abstract: In optical coherence tomography, a sectional image I(x,z) of an object is obtained in that a light beam scans the object along a x-line on the surface, and the depth z from which light of intensity I is reflected is measured by means of a partial-coherence interferometer In so doing, a strong medium intensity can cover and conceal small differences in adjacent object points The invention renders small differences in adjacent object points visible in that the object is illuminated by a dual beam which simultaneously illuminates the object at two adjacent points and whose two components are in opposite phase after traversing the arrangement, so that they cancel one another in a homogeneous object structure On the other hand, if changes are brought about in the two light bundles by means of the reflectance at the object structure, they do not cancel one another, but rather produce an image signal

Optical fiber interferometer with PZT scanning of interferometer arm optical length

Abstract: The invention is related to technical physics, in particular to devices for investigating the internal structure of objects and can be used in medicine for diagnostics of human organs and systems, in particular for optical coherence tomography and in technical diagnostics, for example technological process control The invention relates to the creation of an optical-fiber interferometer, which, being used in a device for optical coherence tomography, allows one to investigate media with short time of changing of optical characteristics or its position relative to the optical probe, for example biotissues in vivo The invention also relates to the creation of a piezoelectric modulator, suitable for use in the interferometer and for providing the necessary scanning depth in the mentioned media In the described optical fiber interferometer the piezoelectric modulator, constructed as a fiber optic piezoelectric controllable delay line, performs a function of the fiber part of the interferometer arm which allows one to change practically inertialess the optical path in the interferometer arm and consequently an optical path difference at least to several tens of the working wavelenghts

Fiberoptic interferometer and associated method for analyzing tissue

Abstract: The fiberoptic interferometer includes a broadband light source which is selected to illuminate tissue of predetermined organ with light having a wavelength within a predetermined range of wavelengths. Within the predetermined range of wavelengths, the attenuation characteristics of tissue of the predetermined organ define a region of minimum attenuation upon illumination with light having a first wavelength. As a result, the fiberoptic interferometer can precisely determine the optical properties of the tissue and can therefore identify the tissue by comparing the interferometric signal produced upon illumination of the tissue sample with predetermined interferometric signals corresponding to illumination of different types of tissue of the predetermined organ with light having the first wavelength. The fiberoptic interferometer can also include a light source which includes wavelength selection means for controllably selecting the predetermined wavelength of light emitted by the light source. Thus, the optical properties of the tissue can be analyzed at the different wavelengths emitted by the light source to individually determine the optimum wavelength of light for subsequent treatment of the tissue.

The GEO600 project

Abstract: GEO600, an interferometric gravitational-wave detector with an arm length of 600 m, is currently being built in northern Germany close to Hannover. GEO600 incorporates an externally modulated fourfold delay-line Michelson interferometer giving a round-trip optical length of 2400 m. A master - slave combination of a monolithic diode-pumped Nd:YAG ring laser and an injection-locked amplifier will give a light power of about 10 W at a wavelength of 1064 nm. Power recycling increases the light power inside the interferometer to a level of about 10 kW. The use of both power and signal recycling will yield a sensitivity of the same order of magnitude as the first stages of the other large-scale gravitational-wave detectors LIGO and VIRGO currently under construction. High signal recycling factors allow the sensitivity to be increased at a chosen frequency while reducing the bandwidth of the detector. This gives an advantage over broad-band detectors in detecting narrow-band periodic sources such as pulsars. The 25 cm diameter mirrors will be suspended as double pendulums from a platform supported by vibration-reduction systems. The passive filtering properties of this system sufficiently reduce the seismic noise in the frequency range of interest, i.e. 50 - 1000 Hz. The detector will start taking data in the year 2000.

Characterization of the polarization state of weak ultrashort coherent signals by dual-channel spectral interferometry.

Abstract: We demonstrate that dual-channel spectral interferometry in conjunction with a well-characterized reference pulse can be used to time resolve the polarization state of extremely weak ultrashort coherent signals from linear- and nonlinear-optical experiments by measuring the intensity and the phase of two orthogonal polarization components In this way the signal is completely characterized

Reconstruction of displacement waveforms with a single-channel laser-diode feedback interferometer

Abstract: Using a laser-diode feedback interferometer, we show how to reconstruct without ambiguity the displacement waveform of an external target from a single interferometric signal. We present the underlying theory with numerical simulations and report an example of actual reconstruction from experimental data. Reconstruction accuracy is on the order of tens of nanometers for displacements of a few micrometers.