Showing papers on "Interference (wave propagation) published in 1990"

Optimum transmission ranges in a direct-sequence spread-spectrum multihop packet radio network

Abstract: The authors obtain the optimum transmission ranges to maximize throughput for a direct-sequence spread-spectrum multihop packet radio network. In the analysis, they model the network self-interference as a random variable which is equal to the sum of the interference power of all other terminals plus background noise. The model is applicable to other spread-spectrum schemes where the interference of one user appears as a noise source with constant power spectral density to the other users. The network terminals are modeled as a random Poisson field of interference power emitters. The statistics of the interference power at a receiving terminal are obtained and shown to be the stable distributions of a parameter that is dependent on the propagation power loss law. The optimum transmission range in such a network is of the form CK/sup alpha / where C is a constant, K is a function of the processing gain, the background noise power spectral density, and the degree of error-correction coding used, and alpha is related to the power loss law. The results obtained can be used in heuristics to determine optimum routing strategies in multihop networks. >

Electrical signal processing techniques in long-haul fiber-optic systems

Abstract: The potential for electrical signal processing to mitigate the effect of intersymbol interference in long-haul fiber-optic systems is discussed. Intersymbol interference can severely degrade performance and consequently limit both the maximum distance and data rate of the system. Several techniques for reducing intersymbol interference in single-mode fiber systems with single-frequency lasers are presented, and those techniques which are appropriate at high data rates in direct coherent detection systems are identified. The performances of linear equalization (tapped delay lines), nonlinear cancellation (variable threshold detection), maximum-likelihood detection, coding, and multilevel signaling are analyzed. The results for a simulated binary 8-Gb/s system show that simple techniques can be used to reduce intersymbol interference substantially, thereby increasing the system margin by several decibels. A six-tap linear equalizer increases the dispersion-limited distance (due to chromatic or polarization dispersion) by 20% (or reduces the optical power penalty by as much as a factor of two) in direct detection systems, even when the distortion is nonlinear. A nonlinear cancellation technique (adjusting the decision threshold in the detector based on previously detected bits) can more than double the dispersion-limited distance and/or data rate. >

Combinations of an adaptive array antenna and a canceller of interference for direct-sequence spread-spectrum multiple-access system

Abstract: In the realization of code-division multiple access based on a spread-spectrum communication system, i.e. spread-spectrum multiple access (SSMA), reduction of cochannel interference is an important problem. An adaptive array antenna system is proposed that includes a cancellor of cochannel interference, which can improve performance by a combination of temporal and spatial filtering. While the adaptive array suppresses interference sources with arrival angles different from those of the desired user, the adaptive digital filter-canceller rejects those whose arrival angles are the same as those of the desired user. The proposed system can achieve stable acquisition and low error rate of demodulated data even in a heavy-interference channel where a conventional array antenna system cannot achieve satisfactory acquisition. >

Two-photon interference in a Mach-Zehnder interferometer.

Abstract: Pairs of 826.8-nm correlated photons, generated by parametrically down-converting 413.4-nm krypton-ion laser light, are directed into a single Mach-Zehnder interferometer such that each photon of the pair enters a different input port. The rate of coincidence at the two output ports displays oscillations (as the path-length difference is swept) with a spatial period equal to the 413-nm wavelength of the pump photons, and with a visibility of 62% when the path-length difference exceeds the coherence length of the individual photon beams. This unequivocally demonstrates the nonclassical and entangled nature of the two-photon state.

An adaptive canceller of cochannel interference for spread-spectrum multiple-access communication networks in a power line

Abstract: The authors propose and investigate an adaptive canceller of intersymbol and cochannel interference due to channel distortion and cross-correlation among pseudonoise sequences assigned to individual users of a DS-SSMA (direct-sequence spread-spectrum multiple-access) system. In order to implement a local area network (LAN) by using a power line installed in a building wall as a transmission channel, the authors have investigated utilization of DS-SSMA which has advantages such as robustness against narrow-band interference and noise and realization of asynchronous code division multiple access. In a power line, however, restriction of transmission bandwidth for communications makes it difficult to suppress cochannel interference and the channel is also time-varying due to fluctuation of loads. Since the proposed canceller adaptively eliminates cochannel interference as well as intersymbol interference, it can facilitate synchronization and increase the number of the simultaneously accessing users on a power line with restricted processing gain. The error probability in the output of the canceller is theoretically calculated for the steady-state case by using a Markov model. Computer simulations illustrate stable convergence properties of the canceller. >

Squeezing of pulses in a nonlinear interferometer

Abstract: A symmetric nonlinear Mach–Zehnder interferometer containing a Kerr medium performs the operation of squeezing. The operation is performed by the interference of cotraveling waves, and the pump wave is removed. This mechanism is broadband. We analyze the operation of the interferometer and determine the degree of shot-noise reduction achieved in a balanced detector. A modification of the interferometer into a fiber ring reflector is described that accomplishes the squeezing and the pump separation.

Method and apparatus for autonomous adaptive frequency assignment in TDMA portable radio systems

Abstract: To assign and coordinate radio frequencies at the fixed ports (30, 40, 50, 70) of a frequency-reusing radio communications system (5), most existing methods require pre-engineering or impose high complexity in system controller and port hardware. A simple autonomous procedure performed by each port to determine its own transmitting frequency and corresponding receiving frequency is disclosed. This procedure consists of signal strength measurements and an algorithm which selects the frequency with minimum interference from other ports. In particular, a port turns off (202) its own transmitter (306) and scans (203) all the candidate transmitter frequencies and measures (312) signal powers form the other ports. The frequency channel with the lowest received power is tentatively assigned (204) for downlink transmission by that port. This procedure is repeated by all the ports either independently and asynchronously or with a coordinated schedule. This cycle is repeated for either a predetermined number of iterations that is sufficient to stabilize the frequency-reuse pattern, or until no port requests a change in its assigned frequency for two consecutive cycles thereby indicating the convergence of the algorithm. This algorithm stabilizes rapidly, while adapting to changing operational conditions, such as the installation of new ports, which changes the system configuration, or the addition of new buildings which creates different interference patterns in the signal propagation paths.

Experiment on nonclassical fourth-order interference.

Abstract: A new fourth-order interference experiment has been carried out and analyzed theoretically in classical and in quantum terms. Two photons produced in the process of parametric down-conversion provide the two inputs to a Mach-Zehnder type of interferometer, while two photodetectors coupled to a coincidence counter measure the output. The coincidence rate, after subtraction of accidentals, exhibits a cosine variation with the optical path difference, in agreement with quantum mechanics, but in disagreement with a classical analysis. By contrast, when two coherent light beams from a He:Ne laser are used as inputs to the interferometer, no fourth-order interference is observed.

Optical interference in subcarrier multiplexed systems with multiple optical carriers

Abstract: It is shown that, when multiple optical carriers are used in a subcarrier multiplexing system, optical interference noise limits the system bandwidth or the number of available channels. Results of a statistical analysis are presented for a system with multilongitudinal mode semiconductor lasers. The results of the statistical calculations indicate that, when one channel per optical source is transmitted, then the system will be limited to only 16 video-quality channels. However, this number can be increased carefully by choosing and maintaining separate wavelengths for the lasers, in order to minimize the interference noise. >

Sinusoidal phase modulating laser diode interferometer with a feedback control system to eliminate external disturbance

Abstract: We propose a sinusoidal phase modulating laser diode interferometer that is insensitive to vibrations of the optical components and fluctuations in the optical wavelength of the laser diode. These external disturbances cause fluctuations in the phase of the interference signal. After we analyze the sinusoidal phase modulation in a laser diode interferometer, we describe the method of the feedback control of the injection current of the laser diode to eliminate the phase fluctuations of the interference signal. We construct two sinusoidal phase modulating interferometers for movement measurements and surface profile measurements, respectively. The experimental results make it clear that the interferometers work well in mechanically noisy circumstances.

Fourth-order interference of joint single-photon wave packets in lossless optical systems.

Abstract: We examine the fourth-order interference properties of two photons at a lossless four-port optical device. The photon pair is represented by a joint single-photon wave-packet state with arbitrary spectral composition. We explicitly determine the coincidence probabilities at the output ports of a beam splitter and a Mach-Zehnder interferometer when the incident wave packet is described by a joint Gaussian spectral distribution and the two photons are incident one at each port of the device. For uncorrelated and distinguishable input photons, the fourth-order interference is readily understood in terms of the particle or wave behavior of each photon acting separately. On the other hand, highly correlated photon pairs, such as those generated from spontaneous parametric down-conversion, exhibit fourth-order interference effects at the output of the interferometer that depend expressly on the joint nature of the photon-pair wave packet, and cannot be described by the particle or wave behavior of either photon.

In-fiber Bragg grating tap

Abstract: In many network and sensor applications, small amounts of light need to be coupled out at many points from a transmission bus fiber. In-line devices with some wavelength selectivity are particularly desirable, especially if light radiated from the fiber can be efficiently coupled into another fiber. This paper describes a new method developed for the noninvasive production of wavelength selective taps in germanosilicate optical fiber. The technique is based on a recent discovery that Bragg gratings can be formed in the core of a fiber by exposing it through the side of the cladding to a coherent UV interference pattern at a wavelength which lies in the oxygen vacancy defect band of germania.1-3 The grating period and tilt of the Bragg planes are selected to couple coherently a guided mode into a narrow radiated beam which propagates through the side of the cladding. The same arrangement is also useful for coupling light into the fiber.

Low-contrast and low-counting-rate measurements in neutron interferometry.

Abstract: Here we discuss the limits of the recognizability of neutron interference patterns either observed in low-contrast measurements or when collecting very few neutrons only. Low contrast can be caused by a strong beam attenuation or by a large phase shift applied to one beam path. Stochastic and deterministic cases have different influences on the interference pattern, which can be interpreted as a different wavelike or particlelike behavior of the system. Measurements of interference patterns with very few neutrons only are related to the phase--particle-number uncertainty relation, which is discussed on the basis of stochastic and quantum theory arguments. Analogies between coherent-state behavior known in atomic physics and the behavior of neutrons in an interferometer are discussed also.

Interpretation of non-linear frequency response functions

Abstract: Analytical and graphical methods of interpreting generalised frequency response functions for non-linear systems are derived. It is shown that non-linear phenomena can be classified into intra-kernel and inter-kernel interference and that worst-case responses can be computed. The results are illustrated using several discrete- and continuous-time non-linear systems.

Beamforming in the presence of correlated arrivals using structured correlation matrix

Abstract: Expressions are derived for the SNR (signal-to-noise-ratio) at the output of two antenna array processors in the presence of correlated arrivals, and the effect on the output SNR of the magnitude and the phase of the correlation, the number of elements in the array, the direction of the level of the interference source, and the level of the uncorrelated noise is investigated. The two processes considered are the optimal element space processor (ESP) and the optimal postbeamformer interference canceller (PIC). The PIC processes the signal by forming two beams and adjusting the weights of one beam, in contrast to the ESP where the signals from all the elements are weighted and all the weights are adjusted. A method is proposed for decorrelating the correlated sources, and an analytical expression is derived to show the decorrelation effect of the proposed technique for a line array of equispaced elements. Numerical examples are included. >

History of the principle of interference of light

Abstract: As stated in 1.3, Young wanted to build a mathematical theory of periodical colors. Before examining this theory, let us see how this problem had been considered before Young.

Despeckling screen utilizing optical fibers and method of reducing interference using same

Abstract: The invention comprises a method and apparatus for decohering coherent light projected onto a screen by transmitting the coherent light through a series of light conducting optical fibers, preferably of varying length.

Recording/reproducing system using wavelength/depth selective optical storage medium

Abstract: Apparatus and method for encoding information and recording same as sets of variably spaced related marks stored in wavelength (color) and/or depth selective layers on the optical record wherein the encoded information is read by illuminating each set of such recorded marks selected by wavelength (color) or focal depth 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 pat-tern is detected by photosensing diode arrays and after decoding, the system outputs an information signal representing the original information.

A new technique for line interference monitoring and reduction in biopotential amplifiers

Abstract: Hardware developed to record the common mode line frequency signal on the body simultaneously with the ECG lead signals of a 15-channel computerized cardiograph is described. This interference reference signal and its quadrature, obtained by software, are linearly combined to be subtracted from any one of the channels to reduce the interference to below the quantization level of the 12 b A/D converter. Coefficients of the linear combination are estimated using linear regression, which is applied to the relatively isoelectric regions of the data, excluding the QRS complexes. Since the interference reference signal is available in real time, simultaneously with the ECG signals, another software approach is adopted in which an adaptive interference reduction algorithm is used to cope with varying interference. A recursive-least-squares algorithm with forgetting factor is used to update the coefficients. This updating mechanism is gated by the output of a software QRS detector. Results regarding the performance of both the offline and the adaptive algorithms are given, and the effects of nonisoelectric portions of the ECG lead signals on the estimation of the coefficients are quantified. >

Apparatus and method for performing electronic shearography

Abstract: Two laterally-displaced images of a test object interfere with each other to form a shearogram. Two shearograms, taken while the object is unstressed and stressed, respectively, are compared electronically to yield a composite interference pattern. According to the invention, each shearogram is formed by the interference of pairs of distinct rays of coherent light, reflected from different points on the object. The first ray of each pair is reflected from the object, strikes a mirror spaced from the object, and is reflected from the mirror. At the same time, the second ray of each pair is reflected from the object and strikes a beam splitter positioned beside the mirror. The beam splitter directs a portion of the second ray in the same direction as the reflected first ray, and a portion of the first ray passes through the beam splitter without being reflected. Since the two rays have the same polarization angle, and are mutually parallel, the rays interfere. A detector is positioned to receive light from the beam splitter. The detector observes the interference between pairs of rays over the entire field of view, and the result is an interference pattern, i.e. a shearogram, formed from the optical interference of the two laterally-displaced images of the object. The amount of shearing can be controlled by varying the angle of the mirror. Subsequent computer processing can be used to compare the shearograms electronically. Such processing can include the averaging of several shearograms, taken sequentially, with different phase shifts induced by linearly moving the mirror.

Position measuring apparatus utilizing two-beam interferences to create phase displaced signals

Abstract: A phase grating is provided in this length or angle measuring apparatus, which operates by interference. A beam striking the phase grating from a laser is diffracted into ±1st order beams at the phase grating. The diffracted ±1st order beams are reflected at retroreflecting elements and, diffracted once again at the phase grating, and made to interfere in pairs. The modulations in intensity of the two-beam interferences are converted by detectors into electrical signals that are phase-displaced from one another. The diffraction grating is configured such that at least one partial beam cluster of the zero order of diffraction is involved in the formation of at least one of the two-beam interferences.

Measurements and simulation of multipath interference for 1.7-Gb/s lightwave transmission systems using single- and multifrequency lasers

Abstract: Power penalties due to multipath interference (MPI) have been measured for 1.7-Gb/s lightwave systems that use single-frequency (SF) or multifrequency (MF) lasers. Systems that use SF lasers potentially exhibit worse degradation than those using MF lasers. Bit-error-rate (BER) floors occur only under the worse-case conditions of poor receiver margin and large multiple reflections. The use of optical isolation to reduce laser feedback is ineffective in reducing multipath interference, and in many cases may worsen the penalty. It is shown that for a typical transmission system, these degradations are reduced if optical interconnection reflections are maintained below -20.5 dB. The experimental study is in good agreement with theoretical predictions using an analytic expression of the MPI noise power spectral density and with computer simulations using multimode laser rate equations. >

System and method for minimizing input polarization-induced phase noise in an interferometric fiber-optic sensor depolarized input light

Abstract: A system and method for minimizing polarization-induced phase noise in an interferometric fiber sensor is disclosed. The system includes an optical source of light; a single mode input fiber; means coupled between the optical source and the input fiber for scrambling the polarization state of the light from the optical source at a first frequency to effectively produce and pass depolarized light to the input fiber; an interferometric fiber sensor responsive to light from the input fiber for producing at its output an interference pattern proportional to a phase shift produced by a field being sensed by the interferometric fiber sensor; detection means responsive to the interference pattern for developing a photocurrent signal averaged over a period of time longer than the period of the first frequency; and demodulation means responsive to the photocurrent signal for producing an electrical output signal substantially proportional to the phase shift produced by the field being sensed by the interferometric sensor.

Fourier-Transform Evaluation Of Interference Patterns: The Role Of Filtering In The Spatial Frequency Domain

Abstract: The Fourier-transform method pointwisely determines the interference phase distribution out of a single digitally recorded and stored interference pattern. By using suitable filter parameters in the spatial frequency domain, speckle noise and known as well as unknown background variations can be suppressed. The paper presents a description of the Fourier-transform method, a discussion of the effects of filter parameters like orientation and cutoff-frequencies, and experimental results.

Measurement of direct frequency modulation characteristics of laser diodes by Michelson interferometry

Abstract: This paper reports a modification of an interferometric technique [ H. Tsuchida , “ A Novel Technique for Measuring the Frequency Deviation of Semiconductor Lasers under Direct Modulation,” Jpn. J. Appl. Phys.22, L19– L21 ( 1983)] for measuring the frequency deviation and phase difference between the AM and FM modulation of semiconductor laser diodes. The frequency deviation can be determined by the fringe visibility of interference patterns provided the phase difference, AM index, and time delay in an unbalanced Michelson interferometer are given. It covers a wide range of modulation frequencies, i.e., 10 Hz to 10 MHz. This method has such advantages that a high speed photodetector is not necessary and the accuracy of the measurement is not reduced by the spectral linewidth of the lasers.

Rotary encoder using reflected light

Abstract: This invention discloses a linear/rotary encoder for detecting the state of movement of a grating. The encoder of this invention comprises a system for directing a coherent beam of light onto a predetermined position or positions of a diffraction grating formed on an object to be measured, a converging optical system having a reflection plane at a position on which light is converged, for again making incident on the respective different positions diffracted lights which exit from the position, an interference optical system for forming interference fringes by superposing beam diffracted from the diffraction grating at the predetermined position or positions, and light receiving elements for photoelectrically converting the interference fringes obtained through the interference optical system.

Characteristics of a multimode laser diode in a dual-interferometer configuration

Abstract: The results of an experimental investigation of an interferometric sensor scheme using a Michelson interferometer in conjunction with a Fabry-Perot reference cavity, illuminated by light from a multimode compact disk (CD)-type laser diode, are reported. The results are compared with the results of calculations using a simple theoretical model and good agreement is seen. Potential applications to interferometric-based sensors are discussed. >

A new CFAR sidelobe canceler algorithm for radar

Abstract: Signal or target detection is sometimes complicated by the presence of strong interference. When this interference occurs mainly in the sidelobes of the antenna pattern, a solution to this problem is realized through a sidelobe canceler (SLC) implementation. Since the false-alarm probability is a system parameter of special importance in radar, an interference-canceling technique for radar application should maintain the false-alarm probability constant over a wide range of incident interference power. With the requirements of sidelobe interference cancellation and constant false alarm rate (CFAR), a new algorithm for radar detection in the presence of sidelobe interference is developed from the generalized likelihood ratio test of Neyman-Pearson. In this development, the received interference is modeled as a nonstationary but slowly varying Gaussian random process. Cancellation of the sidelobe interference is based upon a 'synchronous' estimate of the spatial covariance of the interference for the range gate being tested. This algorithm provides a fixed false-alarm rate and a fixed threshold which depend only upon the parameters of the algorithm. >