Showing papers on "Interferometry published in 1993"

The displacement field of the Landers earthquake mapped by radar interferometry

Abstract: GEODETIC data, obtained by ground- or space-based techniques, can be used to infer the distribution of slip on a fault that has ruptured in an earthquake. Although most geodetic techniques require a surveyed network to be in place before the earthquake1–3, satellite images, when collected at regular intervals, can capture co-seismic displacements without advance knowledge of the earthquake's location. Synthetic aperture radar (SAR) interferometry, first introduced4 in 1974 for topographic mapping5–8 can also be used to detect changes in the ground surface, by removing the signal from the topography9,10. Here we use SAR interferometry to capture the movements produced by the 1992 earthquake in Landers, California11. We construct an interferogram by combining topographic information with SAR images obtained by the ERS-1 satellite before and after the earthquake. The observed changes in range from the ground surface to the satellite agree well with the slip measured in the field, with the displacements measured by surveying, and with the results of an elastic dislocation model. As a geodetic tool, the SAR interferogram provides a denser spatial sampling (100 m per pixel) than surveying methods1–3 and a better precision (∼3 cm) than previous space imaging techniques12,13.

Interferometric detection of optical phase shifts at the Heisenberg limit.

Abstract: We show that the uncertainty in the relative quantum phase of two fields propagating in the arms of a Mach-Zehnder interferometer can be reduced to the Heisenberg limit by driving the interferometer with two Fock states containing equal numbers of photons. This leads to a minimum detectable phase shift far below that of any interferometer driven by a coherent light source.

Recent advances in displacement measuring interferometry

Abstract: The present state of high-resolution displacement measuring interferometry is reviewed. Factors which determine the accuracy, linearity and repeatability of nanometre-scale measurements are emphasized. Many aspects of interferometry are discussed, including general metrology and alignment errors, as well as path length errors. Optical mixing and the nonlinear relation between phase and displacement are considered, as well as the influence of diffraction on accuracy. Environmental stability is a major factor in the repeatability and accuracy of measurement. It is difficult to obtain a measurement accuracy of 10-7 when working in air. Several approaches to improving this situation are described, including multiwavelength interferometry. Recent measurements of the short- and long-term frequency stability of lasers are summarized. Optical feedback is a subtle, but important source of frequency destabilization, and methods of detection and isolation are reviewed. Calibration of phase measuring electronics used for subfringe interpolation is included. Progress in 'in situ' identification of error sources and methods of validating accuracy are emphasized.

The fiber-optic gyroscope

Abstract: Principle of the fibre-optic gyro reciprocity of a fibre ring interferometer backreflection and backscattering analysis of polarization non-reciprocities with broadband source and high-birefringence fibre transient related drift and noise truly non-reciprocal effects scale factor accuracy alternative approaches for the I-FOG applications and trends.

WINDII, the wind imaging interferometer on the Upper Atmosphere Research Satellite

Abstract: The WIND imaging interferometer (WINDII) was launched on the Upper Atmosphere Research Satellite (UARS) on September 12, 1991. This joint project, sponsored by the Canadian Space Agency and the French Centre National d'Etudes Spatiales, in collaboration with NASA, has the responsibility of measuring the global wind pattern at the top of the altitude range covered by UARS. WINDII measures wind, temperature, and emission rate over the altitude range 80 to 300 km by using the visible region airglow emission from these altitudes as a target and employing optical Doppler interferometry to measure the small wavelength shifts of the narrow atomic and molecular airglow emission lines induced by the bulk velocity of the atmosphere carrying the emitting species. The instrument used is an all-glass field-widened achromatically and thermally compensated phase-stepping Michelson interferometer, along with a bare CCD detector that images the airglow limb through the interferometer. A sequence of phase-stepped images is processed to derive the wind velocity for two orthogonal view directions, yielding the vector horizontal wind. The process of data analysis, including the inversion of apparent quantities to vertical profiles, is described.

Coherent frequency-domain reflectometry for characterization of single-mode integrated-optical waveguides

Abstract: Based on the principles of optical frequency domain reflectometry (OFDR), a highly resolving and sensitive technique suitable for detecting, localizing, and quantifying weakly reflecting irregularities in single-mode optical waveguides is developed. A distributed feedback (DFB)-laser diode at lambda /sub 0/ equivalent to 1.3 mu m tuned within a range of Delta lambda equivalent to 6 nm and Delta v equivalent to 1 THz, respectively, is used as a source in the experimental arrangement. An auxiliary interferometer is employed so that the tuning need not be linear in time, in contrast to early implementations. At present, with waveguide structures on InP under test, a spatial resolution of 50 mu m and a dynamic range of about 60 dB are obtained. These data surpass OFDR results published so far. Prospects of closing the gap to coherence-domain reflectometric results and specific advantages make OFDR a promising technique. >

Radar interferometry: limits and potential

Abstract: The contribution of radar interferometry to the field of digital terrain modeling is important because this technique offers specific features which optical instruments cannot attain. However, the complexity of the height restitution and the accuracy of the result strongly depend on the orbital geometry at the time of the data takes. The present study aims at assessing the potential of a given image pair with regard to interferometry and at automatically reducing the phase ambiguity intrinsic to such processing. Particular applications of differential interferometry are also discussed in order to estimate their requirements and prepare future experiments. >

Femtosecond transillumination optical coherence tomography.

Abstract: We describe a new technique, femtosecond transillumination optical coherence tomography, for time-gated imaging of objects embedded in scattering media. Time gating is performed with a fiber-optic interferometer with femtosecond pulses and coherent heterodyne detection to achieve a 130-dB dynamic range. A confocal imaging arrangement provides additional spatial discrimination against multiply scattered light. By time gating ballistic photons, we achieve 125-microm-resolution images of absorbing objects in media 27 scattering mean free paths thick. We derive a fundamental limit on ballistic imaging thickness based on quantum noise considerations.

Performance of a highly sensitive optical waveguide Mach-Zehnder interferometer immunosensor

Abstract: We describe a highly sensitive sensor which uses the evanescent field of a reusable planar optical waveguide as the sensing element. The waveguide used is optimized to obtain a steep dependence of the propagation velocity on the refractive-index profile near the surface. The adsorption of a layer of proteins thus results in a phase change, which is measured interferometrically using a Mach-Zehnder interferometer set-up. The stability of the interferometer is such that phase changes = (1 × 10-2)2pi per hour can be measured. Immunoreactions have been monitored down to concentrations of 5 × 10-11 M of a 40 kDa protein.

Three-dimensional imaging by sub-Nyquist sampling of white-light interferograms

Abstract: We demonstrate a simple way of increasing the data acquisition and processing speed in a scanning white-light interferometer for surface topography measurement. The method consists of undersampling interference data and processing the resultant sub-Nyquist interferograms in the frequency domain to create complete three-dimensional images. Experimental results on a 20-μm step height standard show a measurement repeatability of 10 nm.

Single photon interference in 10 km long optical fibre interferometer

Abstract: Single-photon interference fringes with greater than 90% visibility were measured using a 10 km long optical fibre interferometer. The experiment employed a pulsed semiconductor laser source operating at a wavelength of 1.3 mu m and a novel single-photon counting scheme using high-speed germanium avalanche photodiodes. Interferometers of this type could form the basis of future quantum cryptography systems.

Chemical and biochemical sensors based on interferometry at thin (multi-) layers

Abstract: Spectral interferometry is presented as a tool to monitor the swelling of polymers caused by organic gases or hydrocarbons in waste water as well as the adsorption and interaction of antigens and antibodies in immunoreactions. Modern diode-array technology allows the consequent observation of changes in optical pathlength on a fractional nanometer scale with subsecond repetition times. The theory of multiple-reflection principles in white-light interferometry determines the possibilities and limitations of this method. The optical set-up and some applications in gas sensing and label-free immunosensing are discussed with respect to the sensitivity, selectivity and limits of detection at present.

Efficient radially polarized laser beam generation with a double interferometer.

Abstract: Conversion of a linearly polarized CO2 laser beam into a radially polarized beam is demonstrated with a novel double-interferometer system. The first Mach–Zehnder interferometer converts the linearly polarized input beam into two beams with sin2 θ and cos2 θ intensity profiles, where θ is the azimuthal angle. This is accomplished by using two spiral-phase-delay plates with opposite handedness in the two legs of the interferometer to impart a one-wave phase delay azimuthally across the face of the beams. After these beams are interfered with, the resulting beams are sent directly into the second Mach–Zehnder interferometer, where the polarization direction of one beam is rotated by 90°. The beams are then recombined at the output of the second interferometer with a polarization-sensitive beam splitter to generate a radially polarized beam. The output beam is ≈ 92% radially polarized and contains ≈ 85% of the input power. This system will be used in upcoming laser particle acceleration experiments.

Self-mixing interference in a diode laser: experimental observations and theoretical analysis.

Abstract: The experimental results of an investigation of self-mixing effects or backscatter modulation in diode lasers coupled with a simple theoretical analysis are presented. The laser is used to send light, either in free space or through an optical fiber, to a movable target from which the optical backscatter is detected and fed back into the laser. In the experiment three significant conclusions are drawn: (1) self-mixing interference is not dependent on the coherence length of the laser, (2) the interference is not dependent on the use of a single-mode or multimode laser as the source, and (3) the interference is independent of the type of fiber employed, i.e., whether it is single mode or multimode. A comparison of this kind of interference with that in a conventional interferometer shows that self-mixing interference has the same phase sensitivity as that of the conventional arrangement, the modulation depth of the interference is comparable with that of a conventional interferometer, and the direction of the phase movement can be obtained from the interference signal. The above factors have implications for the optical sensing of a wide range of physical parameters. Several applications of the method are discussed that highlight the significant advantages of simplicity, compactness, and robustness as well as the self-aligning and self-detecting abilities of fiber-based self-mixing interferometry when compared with the use of conventional interference methods.

Method and apparatus for performing optical measurements

Abstract: A method and apparatus for performing various optical measurements is provided utilizing an optical coherence domain refrectometer (OCDR). A short coherence optical radiation source applies optical radiation through like optical paths to a sample and an optical reflector. The optical reflector is movable in accordance with a predetermined velocity profile to permit interferometric scanning of the sample, the resulting output having a Doppler shift frequency modulation. This output may be demodulated and detected to obtain desired measurements and other information. Additional information may be obtained by applying radiation from two or more sources at different wavelengths to the sample and reflector and by separately demodulating the resulting outputs before processing. Birefringent information may be obtained by polarizing the optical radiation used, by suitably modifying the polarization in the sample and reference paths and by dividing the output into orthogonal polarization outputs which are separately demodulated before processing.

White-light interferometric multimode fiber-optic strain sensor

Abstract: A white-light interferometric extrinsic Fabry-Perot strain sensor that uses a multimode fiber is demonstrated. The Fabry-Perot cavity length is measured with the help of a Fizeau interferometer. The sensor is described, and some results obtained at this time are given. The strain measurements are absolute and perfectly linear, with a sensitivity of 0.25 micrometers per meter (micro). the design of a thermally autocompensated strain sensor is also presented.

Unified approach to phase-strain-temperature models for smart structure interferometric optical fiber sensors: part 1, development

Abstract: Using the unified theory that relates optical phase changes to applied strain and temperature fields in structurally embedded interferometric optical fiber sensors of all types, as applied to Mach-Zehnder, Michelson, intrinsic and extrinsic Fabry-Perot, polarimetric, dual-mode, and Bragg grating sensors, with resistance strain gauge concepts and the theory of elasticity solutions, the response of optical fiber sensors that are embedded in transversely isotropic composite materials is theoretically explored. The concepts of transverse strain sensitivity and thermal apparent strain are carefully defined for embedded optical fiber sensors, and it is found that errors resulting from these effects completely dominate the desired sensor response for all sensors except the extrinsic Fabry-Perot. Conditions that minimize these errors are presented. The theory of elasticity solutions used in this analysis encompasses six different thermomechanical loading conditions. Comparisons to Buffer and Hocker's model are also presented.

all optical switching devices based on large nonlinear phase shifts from second harmonic generation

Abstract: We show that the large nonlinear phase shifts obtained from phase‐mismatched second harmonic generation can be used to implement all‐optical switching devices such as a nonlinear Mach–Zehnder interferometer and a nonlinear directional coupler.

Quantum limits in interferometric detection of gravitational radiation

Abstract: A spectral analysis is given of the quantum fluctuations in an optical interferometer to detect gravitational radiation. Two different methods of beating the standard quantum limit are examined: directing a squeezed state into the nonlaser input port of the interferometer and placing a Kerr medium into both arms of the interferometer. For both the Kerr medium and large squeezing cases the interferometer system is limited ultimately by the damping noise in the mirrors, not by noise in the light.

Sagnac experiment with electrons: Observation of the rotational phase shift of electron waves in vacuum

Abstract: A Sagnac experiment with electron waves in vacuum is reported. The phase shift caused by rotation of an electron biprism interferometer placed on a turntable has been measured. It was found to agree with prediction within error margins of about 30%. A compact ruggedized electron interferometer was used. It is based on a high-precision optical bench of 36-cm length. This interferometer is less sensitive by orders of magnitude to mechanical vibrations and electromagnetic stray fields than conventional electron interferometers. A beam of low-energy electrons (150\char21{}3000 eV) emitted by a field-emission electron source was used. For the most part, electrostatic electron optical components were employed. The magnified interference fringe pattern was intensified by a dual-stage multichannel-plate intensifier, recorded by a charge-coupled-device video camera, transmitted from the turntable to the laboratory system via a slip ring, and evaluated by an image-processing system. Both the rotation rate and the area enclosed between the two partial waves were varied (up to values of 0.5 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and 3.9 ${\mathrm{mm}}^{2}$, respectively). Fringe shifts on the order of 5% of a fringe period were attained. Some historical aspects of the Sagnac effect as well as some aspects of its interpretation are mentioned. A brief informal discussion is included of the interpretation of the Sagnac phase shift as a geometric phase (``Berry phase'') caused by the global anholonomy of the local phase factor that is produced by the gauge field induced by rotation.

New compensating four-phase algorithm for phase-shift interferometry

Abstract: Phase-shift interferometry suffers from periodic systematic errors caused by erroneous reference phase adjustments and instabilities of the interferometer. A new method is described that uses only four interferograms and eliminates the errors caused by linear adjustment deviations of the reference phase or the mean phase in the interferometer. Test results confirm the theoretical predictions.

Femtosecond transillumination tomography in thick tissues

Abstract: We describe diffuse-light time-gated imaging with transillumination optical coherence tomography. Submillimeter-resolution imaging of objects hidden in thick biological tissue is achieved in cases in which ballistic light is absent by selecting only the early-arriving coherent portion of the diffuse transmitted light. By using a femtosecond laser as a high-power low-coherence light source, we perform high-sensitivity (130-dB dynamic range) optical gating with interferometric heterodyne detection. The dependence of image resolution on coherent photon arrival time is investigated in model scattering media.

Synthetic aperture radar interferometry applied to ship-generated internal waves in the 1989 Loch Linnhe experiment

Abstract: Interferometer synthetic aperture radar images collected during the 1989 Loch Linnhe experiment showed mean Doppler variations across the phase of ship-generated internal waves that corresponded to “velocity” variations of the order of 50 to 100 cm/s. The in situ current data, however, showed surface currents associated with the internal wave features of the order of 5 to 10 cm/s and virtually ruled out the existence of surface currents as large as the interferometer-inferred values. In this paper we show how the pixel-to-pixel phase difference measured by the Jet Propulsion Laboratory interferometer is related to the mean Doppler frequency of the backscattered field. Model calculations are used to show how this frequency can sometimes change by a large amount, even when rather small surface currents are present. In particular, for winds blowing roughly across the internal wave features, as was the case for the interferometer runs in Loch Linnhe, computations based on our wave-current interaction and time dependent scattering models show that changes in the mean Doppler frequency corresponding to large velocities can, in fact, be produced from the much smaller measured surface currents. We show that the larger interferometer velocity estimates are essentially due to the different modulation strengths of the surface Bragg waves advancing toward and receding from the radar. Thus for these crosswind conditions, care must be taken in converting the phase differences measured by the interferometer to a surface current image. When the wind is aligned more nearly along the internal wave propagation direction, the mean Doppler shifts (and the phase differences) are dominated mostly by advection, and interferometer current estimates are more accurate. C band computations predict that if the antenna spacing is small enough so that the fields from the two antennas remain correlated, then the C band interferometer current estimates will be better than those at L band.

Repeat-pass interferometry with airborne synthetic aperture radar

Abstract: It is shown that interference can be observed by coherently combining pairs of either X- or C-band airborne synthetic aperture radar (SAR) images from separate passes over the same test site. Coherence between separate images is obtained only if the aircraft is flown, and the data are processed in such a way that each resolution cell in the two images is viewed with very nearly the same geometry. Successful repeat-pass interferometric results were obtained from those passes flown by the CCRS Convair 580 aircraft with flight-line offsets of less than a few tens of meters. A summary of the experiment, the phase correction of nonrectilinear aircraft motion, and the subsequent data processing is provided. >

Enhanced single photon fringe visibility in a 10 km-long prototype quantum cryptography channel

Abstract: High visibility (V = 0985) single photon fringe measurements in a 10 km long, optical fibre-based, time- and polarisation-division Mach-Zehnder interferometer are reported The ability of the system to transmit key data in a quantum cryptography scheme is assessed

Extended-range optical low-coherence reflectometry using a recirculating delay technique

Abstract: We propose a novel technique allowing for a substantial increase in the distance measurement range for optical low-coherence reflectometry. By introducing a recirculating delay into the interferometer reference path, we achieve a 100 times increase in range compared to previous optical low-coherence reflectometry techniques. At this extended range, a reflection sensitivity greater than -80 dB is demonstrated at a probe wavelength of 1.55 mu m. >