Showing papers on "Interference (wave propagation) published in 1989"

Method and apparatus for simultaneously measuring a plurality of spectral wavelengths present in electromagnetic radiation

Abstract: An apparatus and method simultaneously measures a plurality of spectral wavelengths present in electromagnetic radiation. A modulatable birefringent optical element is employed to divide a polarized light beam into two components, thereby producing a phase difference in two resulting light beams such that the two beams can be made to interfere with one another when recombined, the interference pattern providing the wavelength information required for the analysis of the incident light. The interferometer thus created performs in a similar manner to a Michelson interferometer, but with no moving parts, and with a resolution dependent on the degree of phase shift introduced by the modulator.

On-site communication system with rf shielding having pager identification capability

Abstract: An on-site communication system includes a system for identifying communication receivers including acknowledge back capability and operating within the on-site communication system, and for controlling the delivery of messages to the communication receivers. The identification system operates on the same frequency as the on-site communication system, and includes an RF shielded enclosure for momentarily shielding the communication receivers from messsages transmitted on the on-site communication system. The RF shielded enclosure provides interference free identification of the communication receivers as the communication receivers pass through the enclosure, and enables the determination of those communication receivers which are operating within the coverage area of the on-site communication system, and those communication receivers which have left the coverage area of the on-site communication system.

Analysis of Electrical Interference from Power Lines to Gas Pipelines Part I: Computation Methods

Abstract: Analysis of electrical interference effects of transmission lines upon nearby pipelines has been a topic of growing interest due to the proliferation of rights-of-way which must be shared by transmission lines and pipelines. This paper describes the results of a recent joint EPRI/A.G.A. research project whose objectives were to develop an effective tool for simulating complex realistic right-ofway problems accurately and to investigate the effects of various system parameters. The ECCAPP computer program, which resulted from the EPRI/A.G.A. research program, combines a powerful input data preprocessor with a computation algorithm which accurately evaluates the effects of both conductive and inductive interference for arbitrarily positioned above-ground and buried conductors which could occur in typical rights-of-way. This paper discusses, in a detailed manner, the computation methods used by ECCAPP. A companion paper summarizes some of the results of an extensive parametric analysis which was conducted using ECCAPP and which illustrates some of the capabilities of the computer program and provides insight on how to control both conductive and inductive interference effects. Computer Program ECCAPP analyzes the effects of power transmission lines on neighbouring gas pipelines and determines the influence of mitigation measures, if any, on interference levels. A distinguishing feature of this computer program is its ability to analyze the combined effects of inductive (electromagnetic) and conductive (galvanic, through earth) coupling. These effects may develop simultaneously during power faults at transmission lines structures which are near gas pipelines, or during normal conditions.

Elastomeric optical interference films

Abstract: The present invention provides improved elastomeric interference films which are readily coextruded, have improved transparency, and have greater mismatches of refractive indices than those of the prior art. An improved elastomeric optical interference film comprising at least 100 alternating layers of at least first and second divergent substantially transparent elastomeric polymeric materials is provided. Adjacent layers of the first and second polymeric materials differ from each other in refractive index by at least about 0.05, and the thickness of the layers is from about 0.05 to about 0.5 micrometers. Stretching the films results in different wavelengths of light being reflected.

Modulation interference in detection and discrimination of amplitude modulation

Abstract: Two experiments were conducted to assess the effect of the rate of sinusoidal amplitude modulation (SAM) of a masker tone on detection of SAM of a probe tone (experiment 1) or on SAM‐rate discrimination for the probe tone (experiment 2). When modulated at the same rate as the probe, the masker interfered with both the detection of probe modulation and the discrimination of the rate of probe modulation. The interference was obtained when the masker was either higher or lower in frequency than the probe (the probe and masker were separated by 2 oct). The amount of interference in detecting probe modulation (experiment 1) decreased as the common base rate of modulation was increased from 5 to 200 Hz. For rate discrimination (experiment 2), the amount of interference remained approximately the same for base rates of 2–40 Hz, the range over which rate discrimination was measured. In both experiments, the amount of interference was reduced when the masker was modulated at a different rate than the probe.

Theory of two-photon interference

Abstract: The interference effects that can be observed in the two output arms of a lossless beam splitter are calculated for incident light in the form of a photon-pair excitation in the two input arms. The output state that occurs when the photon pair is excited in a single input arm resembles that expected for independent classical particles, whereas quantum interference effects occur when the photon pair is divided between the two input arms. Detailed output photocount correlation functions are calculated for two-photon input states produced by a two-atom light source, a degenerate or nondegenerate parametric oscillator in a high-Q cavity, and an atomic cascade emission light source.

Blood constituent measuring device and method

Abstract: A method and device are disclosed for the detection of the relative percentage of at least one light absorber in the blood stream of living tissue. The method and device are based upon the employment of a narrow band width amplifier and pulses of different wavelength light sources wherein the frequency of the pulses of the light sources are selected so that a large component of the signal falls within the frequency of the narrow band amplifier, so that there is reduced likelyhood of interference from extraneous electromagnetic waves.

Femtosecond-resolution pulse-front distortion measurement by time-of-flight interferometry.

Abstract: The pulse-front distortion occurring in lenses and lens systems has been measured by a Michelson interferometer. In this technique the plane pulse front from one arm of the interferometer is used as a temporal reference level to map contour lines of equal propagation time on the pulse-front surface. The experimental arrangement is capable of detecting pulse-front distortion with a resolution of 20 fsec, and this can be improved to approximately 1 fsec. The measured value of pulse-front distortion in a telescope (1.1 psec) is in good agreement with the calculated data.

Vacuum effects on interference in two-photon down conversion.

Abstract: A proposed experiment is analyzed theoretically. In the proposed experiment two coherent pump waves fall on two identical nonlinear crystals, down-converted signal and idler beams from the two crystals are mixed by two beam splitters, and the coincidence counting rate for photons leaving the beam splitters is measured. We show that this counting rate depends on the phase difference between the two coherent pump waves, and results from the interference of the vacuum with the down-converted photons. The experiment could be used to look for locality violations along the lines recently proposed by Grangier, Potasek, and Yurke [Phys. Rev. A 38, 3132 (1988)], but without the need for a coherent reference beam for homodyning.

An FFT based technique for suppressing narrow-band interference in PN spread spectrum communications systems

Abstract: Analyses and performance predictions of an FFT-based narrowband interference suppression filter for use in PN (pseudonoise) spread-spectrum communications systems are presented. The received baseband signal is processed in fixed-length blocks, transformed to the frequency domain with an FFT, filtered there by using an appropriate weighting, and then transformed back to the time domain. A general expression for the symbol matched filter signal-to-noise ratio (SNR) is given in terms of signal parameters and filter weighting coefficients. An optimal (maximum SNR) filter scheme and several suboptimal thresholding algorithms are discussed. Performance predictions, based on a realistic high-frequency interference model, are presented for these algorithms and FFT-based architecture. >

Interferometer having a short coherence length light source and means for identifying interference fringes

Abstract: An interferometer utilises a short coherence length light source in such a way that the visibility of fringes produced varies as a function of the measurand. In order to provide a signal which represents the value of the measurand, the visibility of the fringes is calculated.

Measurement of velocity vectors with orthogonal and non-orthogonal triple-sensor probes

Abstract: A new method of interpreting the signals from triple-sensor thermal anemometer probes has been developed based on fast solution for all the roots of the non-linear Jorgensen (1971) equations describing the directional response of each cylindrical sensor. The sensors can be oriented at arbitrary angles to each other, but always within a range of probe geometries that keep prong interference and thermal wake interference below acceptable levels. The properties of a class of non-orthogonal symmetric tetrahedral probe geometries are studied in relation to the range of flow vector angles that can be measured, the sensitivity of the probe with respect to changes in flow angle, and the sensitivity of the computed velocity components due to angular errors associated with the construction of the probe. The solutions of Jorgensen's equations are inherently multiple-valued, but if the velocity vector is restricted to be within a cone of angles, they are unique. It is shown that measurements with non-orthogonal triple sensor signals are sensitive to angular deviations of a few degrees of the sensor angles from the nominally orthogonal probe geometry, indicating the need of a non-orthogonal algorithm. The mean, rms, Reynolds stress, and power spectrum of the velocity in fully developed turbulent pipe flow were measured using a specially designed triple sensor probe and the proposed algorithm.

Pattern synthesis based on adaptive array theory

Abstract: Pattern synthesis based on either optimization of signal-to-noise ratio or noise minimization is developed. The connection between adaptive arrays and adapted pattern shape is used in synthesizing linear array patterns with freedom to adjust both amplitude and phase. Phase-only pattern synthesis is described in connection with suppressing interference or clutter in proportions of space. A planar array example applied to a constant-amplitude solid-state phased array is discussed. >

Adaptive narrow-band interference rejection in a DS spread-spectrum intercept receiver using transform domain signal processing techniques

Abstract: An intercept receiver which uses a transform-domain-processing filter is described. This receiver detects direct-sequence binary-phase-shift-keyed (DS-BPSK) spread-spectrum signals in the presence of narrowband interference by employing adaptive narrowband interference rejection techniques. The improvement in the system performance over that of conventional detection techniques is shown by presenting the results of experimental measurements of probability of detection versus false alarm for an enhanced total power detector. Also presented are certain results corresponding to detection of the spectral lines generated at twice the carrier frequency, wherein the goal is often not just signal detection, but also carrier frequency estimation. The receiver uses one of two transform-domain-processing techniques for adaptive narrowband interference rejection. In the first technique, the narrowband interference is detected and excised in the transform domain by using an adaptive notch filter. In the second technique, the interference is suppressed using soft-limiting in the transform domain. >

Theory of photothermal-wave diffraction and interference in condensed media

Abstract: Thermal-wave field diffraction has been treated as the extreme near-field approximation of a three-dimensional superposition integral that includes the generating optical aperture function. This formalism is quite general and is convenient for applications with many experimental diffracting apertures. Specific examples of useful photothermal excitation apertures have been treated explicitly. These include the spatial impulse function, a Gaussian laser beam, a circular aperture, and an expression for the interference field generated by two Gaussian laser beams.

Optimization of frequency assignment

Abstract: A systematic method of determining carrier frequency assignment is proposed to minimize the cochannel interference in satellite communication systems. For the mathematical treatment of the problem, discrete positioning of carriers is introduced to avoid the nonlinear expression inherent in interference evaluation. The proposed method converts this nonlinear problem into the well-known assignment problem and enables the development of an algorithm based on the branch-and-bound method. Extended models of the proposed method are presented for practical application. To demonstrate the effectiveness of the method, computation results are shown for two typical problems. In both of the examples, cochannel interference is significantly reduced as the result of optimization of frequency assignment. >

Performance analysis of both hybrid and frequency-hopped phase-coherent spread-spectrum systems. I. A hybrid DS/FH system

Abstract: The authors present and analyze a model for both hybrid and frequency-hopped spread-spectrum systems in which a fully digital coherent receiver is used to demodulate the data. A receiver for a hybrid DS/FH (direct-sequence/frequency-hopped) system using a digital delay-lock loop is considered. In the absence of frequency uncertainty, it is shown that the tracking error can be modeled as an ergodic Markov chain with a finite-state set, and the probability density function of the steady-state tracking error is evaluated. When there is a frequency uncertainty, the dynamics of the resulting nonstationary phase error can be obtained, and an expression to evaluate the probability distribution of the first time at which the phase error hits predetermined boundary values is derived. Bit error rate performance is determined in the presence of both additive white Gaussian noise and various types of interference, and the performance is compared to that of noncoherent FSK systems. >

User access in portable radio systems in a co-channel interference environment

Abstract: A computer-simulation study of user access in a universal digital portable communications system is described. An access algorithm consisting of relative power channel-ranking followed by data error detection is evaluated and compared to an optimal reference in a co-channel interference environment. For typical propagation conditions and radio configurations, it was found that an access algorithm based on the ranking of system channels by relative power provides a viable means of channel selection for the portable radio. It was found that the multiple-block data error check improved transmission quality in a co-channel interference environment, approaching the optimum with respect to the 99% statistical reliability criterion. >

Coherent interference suppression by an antenna array of arbitrary geometry

Abstract: An approach is investigated that can be applied to an array of arbitrary geometry for coherent and incoherent interference suppression. This approach consists of constructing a weight vector which minimizes the array output power subject to null constraint in the directions of the coherent interferers. In this way, the coherent interference is suppressed by the null constraint and the incoherent interference is suppressed by minimizing the array output. The approach requires the estimation of the angles of arrival of the coherent interferers. A simple bearing estimation method is described which can be applied to the scenario of one coherent interferer. The effect of error in bearing estimation on the array performance is discussed. The use of a high-order null constraint to counteract that error is discussed. >

Apparatus and method for minimizing polarization-induced signal fading in an interferometric fiber-optic sensor using input-polarization modulation

Abstract: An apparatus for minimizing polarization-induced signal fading in an interferometric fiber-optic sensor is disclosed. The apparatus includes an optical source for providing an input light beam, a polarization modulator for modulating the state of polarization of the input light beam into at least three states represented by mutually perpendicular polarization vectors on a Poincare Sphere. The apparatus further includes means for conveying the input light beam with the at least three mutually perpendicular modulated states of polarization to the fiber-optic sensor, the interferometric fiber-optic sensor being responsive to the input light beam with modulated polarization states for developing an interference pattern output, wherein the interference pattern output has at least three visibilities corresponding to the input states of polarization. A photodetector is responsive to the interference pattern output for developing an electrical signal having at least three amplitudes corresponding to the at least three states of polarization. An output circuit is responsive to the electrical photodetector signal for time-sampling the photodetector signal having three amplitudes corresponding to the at least three mutually perpendicular input states of polarization and selecting the one having maximum visibility. A method for minimizing polarization-induced signal fading is also disclosed.

Integrated-optic interferometer position sensor

Abstract: A fully integrated interferometer position/displacement sensor with direction-discrimination capability is discussed. The device is constructed by integrating three kinds of grating components and a pair of photodiodes in a glass waveguide on a silicon substrate and by butt-coupling a single-mode laser diode. Interference signals corresponding to mirror movement were obtained experimentally over a 15-cm range with a 0.39- mu m modulation period. >

Radio interference suppression

Abstract: An arrangement for the suppression of radio interference in an electric motor (10) having at least two brushes (11,13,15), the arrangement comprising a high frequency interference suppression circuit and a low frequency interference suppression circuit, the high frequency interference suppression circuit comprising a ferrite bead (24,26) which surrounds a portion of a lead (36,38) connectable to one (11,13) of the brushes, and a capacitor (28,30) which is capable of self resonance in the high frequency band, and the low frequency interference suppression circuit comprising a capacitor (32,34). Also disclosed are an isolating element between the low frequency and high frequency suppression circuits and a separate grounding method for each circuit for reducing the interaction between the high frequency and the low frequency suppression circuits.

Clipped diversity combining for channels with partial-band interference. II. Ratio-statistic combining

Abstract: For pt.I see ibid., vol.COM-3, no.12, p.1320 (1987). Ratio-statistic combining is proposed for mitigating partial-band interference in systems with diversity transmission and frequency-hop signaling. Systems with noncoherent demodulation and binary orthogonal signaling are covered. The partial-band interference is Gaussian, and Gaussian quiescent noise is included in the analysis to account for wideband noise sources. The exact probability of error is found for a receiver using ratio-statistic combining, and this is compared to the exact error probabilities for receivers with optimum combining with perfect side information, clipped-linear combining, the ratio-threshold test with majority-logic decoding, and self-normalization diversity combining. Numerical results are also given for a frequency-hop system which uses ratio-statistic combining for channels with Rayleigh fading and partial-band interference. It is determined that ratio-statistic combining is an excellent diversity combining scheme for systems with partial-band interference and fading. >

Continuous wave interference rejection for reconstructed carrier receivers

Abstract: System and method are disclosed for determining position related data from spread spectrum signals using reconstructed carrier receivers with continuous wave interference rejection The spread spectrum signals are collected at a mark with a substantially omni-directional antenna and continuous wave interference is then rejected at or near a selected frequency The spread spectrum signals are then reconstructed to provide a continuous wave component at that frequency form which the position data may be derived The data is derived without interference because the potentially interfering signals, if present, have been rejected before the components were reconstructed

Interferential mode locking: Gaussian pulse analysis.

Abstract: A simple analysis of the interference between two Gaussian pulses is presented. It is shown that the resulting pulse may be shorter than the initial ones if the interference phase is properly adjusted. The compression effect results from the difference of the durations of the interfering pulses and from their phase modulation. This mechanism may be implemented in mode-locked lasers, either by adjunction of a coupled cavity or use of a pulse-shaping Michelson interferometer as the output coupler. Gain-bandwidth-limited pulses may then be obtained. The model provides analytical expressions for pulse duration.

Fast acquisition of burst and transient signals using a predictive adaptive beamformer

Abstract: The author presents a simple prediction algorithm for blindly adapting a narrowband beamformer to rapidly acquire burst or transient signals by exploiting only the abrupt ON/OFF transitions of the transient waveforms, that is without using knowledge of the frequency or DOA (direction of arrival) of the signal of interest or knowledge of the manifold, geometry, or noise covariance of the array. It is shown via analysis and computer simulation that the algorithm is able to detect burst and transient signals, and to capture those signals with maximum attainable output SINR (signal-to-interference and -noise ratio) in environments containing severe stationary cochannel interference. In particular, it is shown that the algorithm can adapt a four-element antenna to blindly detect and capture FSK (frequency shift keying) bursts with 10-20-ms burst durations in less than 2 ms, even at low power levels ( approximately 4-dB signal-to-white-noise ratio) and in the presence of strong ( approximately 50-dB SIR) stationary interference. >

Phase-conjugate Fizeau interferometer.

Abstract: We describe a phase-conjugate interferometer that consists of a partially transmitting conventional mirror placed in front of and in close proximity to a phase-conjugate mirror. The interferometer is self-referencing, compact, and insensitive to environmental disturbances, provides twice the sensitivity of conventional (nonphase-conjugate) interferometers, and produces a direct representation of an incident wave front. We have constructed such a device using internally self-pumped phase conjugation in barium titanate and have used the device to characterize the wave front produced by an aberrated optical system.

Marine vibrator tuneable array

Abstract: A method of reducing horizontally propagating pressure waves in marine seismic exploration improves the signal-to-noise ratio at associated receivers and reduces interference at unassociated receivers. Marine vibrators are arranged into an array with a given separation between each vibrator, so that horizontally propagating pressure waves from the vibrators which generally traverse the array are out-of-phase at most frequencies of vibration. The amplitude output from each vibrator varies with the output frequency of each vibrator to maximize the cancellation of out-of-phase pressure waves along the array. A receiver, which usually includes a plurality of hydrophones disposed in a streamer, is spaced from the marine vibrators at a distance sufficient to establish the receiver as being in the far-field. The output of each of the marine vibrators is locked in phase and frequency with a known pilot signal so that downwardly propagating pressure waves are substantially unaffected. The overall effect is a narrowing of the radiation pattern of the pressure waves. The resulting radiation pattern effectively directs pressure waves downwardly toward an underwater land formation, and reduces horizontally propagating pressure waves which normally result in interference with marine seismic receivers.

Temperature and pressure monitors utilizing interference filters

Abstract: An interference edge filter (11) is mounted on the end of an optical fiber (10) to provide a sensor for pressure or temperature. The filter is formed such that the transition slope (21) of the filter curve (20) will move to the left or right depending upon the changes in the parameter being monitored. The shift in the position of the slope (21) of the edge filter curve (20) is detected by passing the reflected light through an interference bandpass filter (38) having an edge slope (50) intersecting the transition slope (21) and measuring the intensity of the light passing the bandpass filter.

Optical recording/reproducing system using interference techniques

Abstract: Apparatus and method are disclosed for encoding information and recording same as sets of variably spaced related marks on the optical record. The encoded information is read by illuminating each set of such recorded marks with a coherent read light to cause an optical interference pattern consisting of a varying distribution of amplitude maxima separated by amplitude nulls (or minimas) that change in accordance with the encoded information. This read interference pattern is detected by photosensing diode arrays and after decoding, the system outputs an information signal representing the original information.