Showing papers on "Interferometry published in 1987"

Atomic force microscope–force mapping and profiling on a sub 100‐Å scale

Abstract: A modified version of the atomic force microscope is introduced that enables a precise measurement of the force between a tip and a sample over a tip‐sample distance range of 30–150 A. As an application, the force signal is used to maintain the tip‐sample spacing constant, so that profiling can be achieved with a spatial resolution of 50 A. A second scheme allows the simultaneous measurement of force and surface profile; this scheme has been used to obtain material‐dependent information from surfaces of electronic materials.

New measurement system for fault location in optical waveguide devices based on an interferometric technique

Abstract: A new measurement system for fault location in optical waveguide devices is presented. The system consists of a fiber-optic Mach-Zehnder and a bulk-type Michelson interferometers. The spatial resolution of the scatter distribution is <380 microm, which is limited by the averaging time. The minimum detectable backscattered power is -116 dB relative to the light power propagating in the waveguides. Preliminary experimental results using single-mode fibers <10 cm long are demonstrated.

Interferometric radar measurement of ocean surface currents

Abstract: A new method of measuring surface currents using an interferometric synthetic aperture radar is presented. An airborne implementation has been tested over San Francisco Bay near the time of maximum tidal flow, resulting in a map of the east-west component of the current. Only the line-of-sight component of velocity is measured by this technique. Where the SNR ratio was strongest, statistical fluctuations of less than 4 cm/s were observed for ocean patches of 60 x 60 m.

Squeezed-light-enhanced polarization interferometer.

Abstract: Enhancement of the sensitivity of a polarization interferometer beyond the limit set by vacuum fluctuations of the electromagnetic field has been obtained with use of squeezed light generated by a ${\mathrm{KTiOPO}}_{4}$ optical parametric amplifier. The increase in signal-to-noise ratio relative to the shot-noise limit is 2 dB. The corresponding improvement in the response time of the interferometer is measured to be a factor of 1.6.

Step height measurement using two-wavelength phase-shifting interferometry.

Abstract: Two-wavelength phase-shifting interferometry is applied to an interference phase-measuring microscope enabling the measurement of step features. The surface is effectively tested at a synthesized equivalent wavelength lambda(eq) = lambda(a)lambda(b)/| lambda(a) - lambda(b)| by subtracting phase measurements made at visible wavelengths lambda(a) and lambda(b). The rms repeatability of the technique is lambda/1000 at the equivalent wavelength. To improve the precision of the data, the phase ambiguities in the single-wavelength data are removed using the equivalent wavelength results to determine fringe orders. When this correction is made, a measurement dynamic range (feature height/rms repeatability) of 10(4) is obtainable. Results using this technique are shown for the measurement of an optical waveguide and a deeply modulated grating.

Use of highly elliptical core fibers for two-mode fiber devices.

Abstract: The four complex, almost degenerate, second-order eigenmodes of a two-mode fiber having a circular core are reduced to two nondegenerate, linearly polarized second-order eigenmodes with stable-intensity lobe positions in a highly elliptical core fiber. Existing two-mode-fiber devices can be improved by this stabilization of the second-order modes. Practical sensors employing the two spatial modes as the two arms of an interferometer are described. The two arms of an interferometer of this type can have the same group delays, while the difference in phase delays is large.

Double phase-conjugate mirror: analysis, demonstration, and applications.

Abstract: We report on the operation of the double phase-conjugate mirror (DPCM). Two inputs to opposite sides of a photorefractive barium titanate crystal, which may carry different spatial images, are shown to pump the same four-wave mixing process mutually and are self-refracted without any external or internal crystal surface. This results in the phase-conjugate reproduction of the two images simultaneously. This device is analyzed theoretically, and applications in image processing, interferometry, and rotation sensing are discussed. We also demonstrate the operation of a ring laser, using the DPCM, as well as a photorefractive resonator with two facing DPCM’s that can support spatial information in its oscillations.

Guided-wave reflectometry with micrometer resolution.

Abstract: We describe a new type of optical reflectometry which is useful in testing single-mode lightguide systems. This technique uses a scanning Michelson interferometer in conjunction with a broadband illuminating source and cross-correlation detection. High resolution is achieved through the limited coherence of the backscattered radiation. With this approach it is possible to distinguish scattering centers separated by only a few micrometers. In some cases loss may be estimated for components in the transmission path of a test lightguide. The basic principles of this diagnostic technique, along with some performance characteristics, are illustrated for an all-fiber reflectometer. We also discuss several laboratory applications which serve to demonstrate the resolution capabilities of this measurement concept.

Extreme scattering events caused by compact structures in the interstellar medium

Abstract: Daily flux density measurements of 36 extragalactic radio sources over a seven year period reveal several unusual minima in the light curves that do not follow typical source variations. The observations were performed using the Green Bank interferometer operating at 2.7 and 8.1 GHz1. The most significant departure from typical source variability occurred at both frequencies in the quasar 0954 + 658 between 1980.95 and 1981.3. It is unlikely that this event can be explained by intrinsic variability. The most satisfactory explanation appears to involve refractive focusing by small-scale inhomogeneities in an ionized structure in the interstellar medium.

Microwave Passive Direction Finding

Abstract: Evolution and Uses of Passive Direction Finding DF Receiver Theory Antenna Elements for Microwave Passive Direction Finding DF Receiver Configurations DF Antenna Arrays Interferometer DF Techniques Methods for Signal Detection Probability of Detection Accuracy of DF Systems Signal Processing and Display Methods Future Trends Index.

Residual errors in laser interferometry from air turbulence and nonlinearity

Abstract: Laser displacement interferometry is used extensively in precision equipment for semiconductor manufacture. In these applications it is often necessary to introduce a high velocity airflow to the measurement environment to minimize the density of airborne particulate contaminants. The performance of the heterodyne interferometer is degraded by the resulting fluctuations in the index of refraction along the beam path. The magnitude, correlation length, and probability distribution of the optical path length (OPL) fluctuations are measured for several airflow conditions. The data are interpreted in terms of the path length errors for some common interferometric configurations. The OPL fluctuations are generally less significant than the systematic sources of measurement error. A more fundamental limit on the accuracy of the heterodyne Michelson interferometer is the periodic nonlinearity caused by leakage of the frequency components in the beamsplitter. The effect is discussed in detail. A direct observation of the nonlinearity is reported. The magnitude of the effect is about λ/64 for the beam splitters used in this experiment. A simple technique which indicates the presence and magnitude of the nonlinearity is described.

Digital phase-measuring interferometry with a tunable laser diode.

Abstract: A digital phase-measuring interferometer with a laser-diode source has been developed that is based on a fringe-scanning technique with a stepwise wavelength change by variation of the laser injection current. The phase is changed to produce a relative phase difference between the beams in the two arms of the interferometer. Calibrated phase shifts used for a phase-extraction algorithm are derived from one-dimensional least-squares fits to cosine fringe functions to achieve accurate results. Experimental results are presented.

Effect of piezoelectric transducer nonlinearity on phase shift interferometry.

Abstract: If the nonlinearity of the motion of a piezoelectric transducer (PZT) can be described as a quadratic function, the integrated intensity of one frame in phase shift interferometry can be calculated using the Fresnel integral. For a PZT with smaller nonlinearity, the rms phase error is almost linearly proportional to the quadratic coefficient. The effects of PZT nonlinearity on the three- and the four-bucket algorithms are compared.

Design Of A Long Trace Surface Profiler

Abstract: The design of a long-trace surface profiler for the non-contact measurement of surface profile, slope error and curvature on cylindrical synchrotron radiation (SR) mirror is pre-sented here. The optical system is based upon the concept of a pencil-beam interferometer with an inherent large depth-of-field. The key feature of the optical system is the zero-path-difference beam splitter, which separates the laser beam into two colinear, variable-separation probe beams. A linear array detector is used to record the interference fringe in the image, and analysis of the fringe location as a function of scan position allows one to reconstruct the surface profile. The optical head is mounted on an air bearing slide with the capability to measure 38" long aspheric optics, typical of those encountered in SR applications. A novel feature of the optical system is the use of a transverse "outrigger" beam which provides information on the relative alignment of the scan axis to the cylinder optic symmetry axis.

Laser diode feedback interferometer for stabilization and displacement measurements.

Abstract: Active laser diode interferometers in which the interference signal is fed back to the diode current are investigated for Twyman-Green and self-coupling interferometers. The Twyman-Green interferometer is stabilized with a stabilization factor of more than 100. By using the feedback signal of either type of interferometer, displacement is measured in a linear scale over a dynamic range of 8–9 μm with a precision of 10–60 nm. The feedback signal vs displacement shows hysteresis and multistable behavior, in accordance with theoretical results.

Wavelength-selective filters for single-mode fiber WDM systems using Fabry-Perot interferometers.

Abstract: The applications of the Fabry-Perot interferometer to high capacity wavelength division multiplexed optical systems are discussed. The performances of several practical embodiments, their anticipated role and ultimate performance limitations are described.

Construction and performance of a Brillouin scattering set-up using a triple-pass tandem Fabry-Perot interferometer

Abstract: A high performance Brillouin scattering set-up is described, which consists of a vernier-type tandem Fabry-Perot interferometer (TFPI) for frequency analysis. The optical system is designed in such a way that alignment of the whole set-up can be achieved and maintained very easily. The very good long-term stability allows sampling times of several days. Along with a detailed discussion of the construction the authors give specifications of the optical components and outline some fast and easy alignment procedures for the TFPI as well as for the peripheral optics. Measurements of light scattered from thermally excited bulk and surface waves in NaCl are presented to demonstrate the performance of the system.

Theoretical noise-limited sensitivity of classical interferometry

Abstract: The signal-to-noise ratios for the path-stabilized, the active-heterodyne, the Sag, and the Fabry-Perot interferometers are calculated for cases in which dynamic surface displacements are detected. Expressions for the minimum-detectable displacements are given for the sensitivity limit of each interferometer. It is found that the ultimate sensitivities of the interferometers considered are nearly equal for similar conditions of use. Sensitivity values are shown to be about 10−15 m Hz−1/2 for systems that use typical laboratory instruments.

Optical tracking novelty filter.

Abstract: We demonstrate an optical filter that displays the time-dependent features of a scene. The heart of the device is an interferometer that is sensitive not to the difference between two optical paths lengths but to changes in the path-length difference. The interferometer arms share a phase-conjugating mirror. The phase conjugator ensures that, at steady state, the output of the interferometer is dark. The response of the interferometer to a step differential change in the optical lengths is a decaying exponential having a time constant governed by the time response of the phase conjugator. The interferometer may be used to monitor time- and space-dependent optical phase changes that are due, for example, to transparent fluid motion. With a modified liquid-crystal television used as a spatial light phase modulator in the interferometer, we detect time-dependent features of an image viewed by a video camera.

Unsharp particle-wave duality in a photon split-beam experiment

Abstract: In a quantum mechanical two-slit experiment one can observe a single photon simultaneously as particle (measuring the path) and as wave (measuring the interference pattern) if the path and the interference pattern are measured in the sense of unsharp observables. These theoretical predictions are confirmed experimentally by a photon split-beam experiment using a modified Mach—Zehnder interferometer.

Frequency domain interferometry: A high resolution radar technique for studies of atmospheric turbulence

Abstract: A new radar technique suitable for high resolution studies of narrow scattering layers in the atmosphere is introduced. The positions of the layers within the radar scattering volume are deduced from the phase differences between radar returns at two distinct transmitter frequencies. Initial mesospheric observations demonstrate the applicability of the technique, and show that the scattering layers are often a few hundred meters thick and widely separated in the mesosphere. Their vertical motions are generally consistent with vertical Doppler velocity estimates.

Non-linearity in length measurement using heterodyne laser Michelson interferometry

Abstract: Heterodyne laser Michelson interferometry is widely used in precision length measurement but the accuracy is often limited by the unknown size of the periodic non-linearity due to imperfect separation of the two optical frequencies. A method for measuring this non-linearity with an accuracy of about 0.15 nm using pressure scanning of the interferometer is described and results are given for three configurations of a commercial measuring system.

Diode laser direct modulation heterodyne interferometer.

Abstract: A new type of heterodyne interferometer was made through direct modulation of a diode laser wavelength. A measurement accuracy of better than λ/50 and repeatability of λ/100 were obtained. This interferometer shows promise for use in testing wavefront aberrations, especially in optical disk systems.

Tensile behavior of concrete

Abstract: Under tensile stress, concrete fails at low average strain but a high local strain due to nonuniform strain distribution. It is demonstrated that the use of optical interferometry with laser light both provides adequate sensitivity and allows measurement of strain over a wide field. The measurement is continuous and capable of detecting the localization of strain in zones of very small width. It is shown also that while the use of strain gages would lead to unobjective constitutive stress‐strain relations, interferometric measurement on notched specimens allows art indirect determination of the local stress‐strain and stress‐separation relations. From the knowledge of these and of the strain path in the fracture zone, the value of the fracture energy of concrete can be calculated. It is shown that the energy dissipated in the microcracked zone is a small fraction of the energy dissipated in the final separation of concrete.

A fiber-optic interferometric seismometer

Abstract: A fiber-optic interferometric sensor has been developed which consists of a seismic mass of 520 gm supported by two rubber mandrels, each wound with a single layer of single-mode optical fiber 6.5 m long. One end of each fiber is cleaved to enhance reflection. The other ends are interconnected via a fiber-to-fiber 3-dB coupler, forming a Michelson interferometer. When the case of the sensor is displaced, the fiber around one mandrel extends in length while the other contracts. The resulting "push-pull" mechanical operation of the sensor allows both legs of the interferometer to be active, providing good common mode rejection of spurious effects, as a reference leg is not required. This, together with the fact that the light traverses each leg of a Michelson interferometer twice due to reflection, provides the sensor with four times the sensitivity of a conventionally constructed interferometric sensor. Sensitivities of 8500 rad of optical phase shift per micrometer of case displacement have been measured above the mass-spring resonance, where the sensor operates as a seismometer. Below resonance the sensor operates as an accelerometer with a measured sensitivity of 10 500 rad/g, the highest reported to date. Including both thermodynamic and demodulator noise sources ( \approx 10 \mu rad/ \sqrt{Hz} ), below resonance the sensor has a detection threshold of 1 ng/ \sqrt{Hz} , a 20- dB improvement over the best existing conventional low noise vibration sensors.

Time-domain addressing of remote fiber-optic interferometric sensor arrays

Abstract: This paper describes and analyzes a particular application of high duty-cycle time-division multiplexing to the separation and identification of signals from an interferometric sensor array. Using the method discussed here, the coherence length of the laser is no longer a severe design constraint. Also, the source phase-induced intensity noise which limits some other multiplexing methods may be overcome, leading to a higher sensitivity. The arrays of all-passive remote sensors exhibit minimal crosstalk between sensors, and have downlead insensitivity. A synthetic heterodyne demodulation technique prevents environmentally induced signal fading. Analysis includes coupling ratios for all directional couplers in the system, signal and noise spectra, minimum detectable phase shift, and the effect of ac coupling on noise and crosstalk. An experimental all-fiber implementation of a two sensor array has yielded a measured sensitivity of approximately 10 μrad/ \sqrt{Hz} over a range of signal frequencies, and a crosstalk level of better than 55 dB.

Interferometer for measuring displacement and distance

Abstract: A simple interferometer for measuring both relative displacement and absolute distance is fabricated that uses a laser diode. The sign of the displacement is detected by means of a λ/8 plate, and the distance is measured by a FM radar technique of modulating the laser-diode frequency. Measurement accuracies of 0.02 μm for displacement and 100 μm for distance are obtained over a range of a few meters.

Squeezed States for Interferometric Gravitational-wave Detectors

Abstract: The possibility of using squeezed states for improving the shot-noise limit of the strain sensitivity of Michelson interferometers is discussed. We find that the spectrum of squeezing required depends on the method of stabilization used in the experiment. Details are given for the widely used phase-modulation technique (which also allows for recycling of the field), where we find an important application for broadband (‘two-mode’) squeezing.

Distributed sensor array and method using a pulsed signal source

Abstract: A distributed sensor system using pulsed optical signals optionally produced by a short coherence length source to provide a phase difference output signal representative of conditions affecting a selected sensor. In one preferred embodiment, an array of fiber-optic sensors are organized in a ladder configuration, with the sensors positioned in spaced relation and defining the rungs of the ladder. Light pulses transmitted through the sensors are multiplexed onto a return arm of the ladder. The multiplexed signals are received by an optical fiber compensating interferometer which coherently couples portions of adjacent multiplexed light signals to produce a phase difference signal representing conditions influencing selected sensors. In other preferred embodiments, the system is configured to define a plurality of adjacent Mach-Zehnder interferometers which provide output signal pairs which coherently couple to yield a phase difference signal directly representing the environmental effects on a particular sensor. Functional equivalents of the Mach-Zehnder interferometer configurations comprise configurations including adjacent Michelson interferometers. A phase and amplitude modulation technique is disclosed for providing heterodyned output signals from the distributed sensor system.