Showing papers on "Phase (waves) published in 1984"

Birefringence measurements of liquid crystals

Abstract: A convenient and accurate technique for measuring the birefringence of liquid crystals at discrete wavelengths or as a continuous function of wavelength in the ultraviolet, visible, or infrared spectral regions is described. The method is based on determination of the phase differences which occur when monochromatic polarized light propagates through a medium with an anisotropic refractive index. Birefringence measurements at 0.6328 μm for two liquid crystal materials, BDH-E7 and ZLI-1132, and a continuous birefringence spectrum of ZLI-1132 from 2 to 16 μm are reported. Additionally, a liquid crystal based phase retardation plate which can be voltage tuned and calibrated to provide any degree of phase shift from 0 to 2π over a wide wavelength range (0.4–16 μm) is discussed.

Two-wavelength phase shifting interferometry.

Abstract: This paper describes a technique that combines ideas of phase shifting interferometry (PSI) and two-wavelength interferometry (TWLI) to extend the phase measurement range of conventional single-wavelength PSI. To verify theoretical predictions, experiments have been performed using a solid-state linear detector array to measure 1-D surface heights. Problems associated with TWLPSI and the experimental setup are discussed. To test the capability of the TWLPSI, a very fine fringe pattern was used to illuminate a 1024 element detector array. Without temporal averaging, the repeatability of measuring a surface having a sag of ~100 μm is better than 25-A (0.0025%) rms.

Instantaneous Phase Measuring Interferometry

Abstract: Conventional phase measuring interferometry normally requires one-half to sixty seconds acquisition time, limiting measurement to stationary phenomena such as optical element wavefronts. We have developed an instantaneous phase measuring interferometer (PMI) that measures displacements at one point to a resolution of 0.003 um. We describe this instrument and an interferometer measuring phase to X/2000 with a measurement time aperture of less than 1 As. Also described is an attached system for analyzing and displaying wavefronts at up to 10 Hz.

Quantum phase noise and field correlation in single frequency semiconductor laser systems

Abstract: The influence of quantum phase fluctuations which affect single frequency semiconductor lasers in various coherent detection systems is discussed in terms of photocurrent autocorrelation and spectral density functions. The general treatment given in this paper can be applied in diverse practical cases and points out the problems of phase correlation and phase matching between the two mixed optical beams. In the more general case the photocurrent spectrum is found to be composed of discrete and quasi-Lorentzian parts whose energies and spectral spreads are discussed as a function of the laser line width, the phase matching and the phase correlation between the two coherently combined fields.

Modulated, or long-periodic, magnetic structures of crystals

Abstract: The experimental results on modulated magnetic structures and the basic regularities of phase transitions between them are reviewed and are analyzed on the basis of the phenomenological theory of phase transitions with the use of the Ginzburg-Landau functionals for inhomogeneous distributions of the order parameter. Lists of presently known crystals, in which modulated magnetic structures have been observed, are presented and for many of them the form of these functionals, taking into account the crystalline anisotropy and the interaction with a magnetic field, is established. For systems admitting a Lifshitz invariant which is linear with respect to the gradient, a soliton picture of the structure of the incommensurate phase is established and the phase transition into the commensurate phase under the action of temperature or a magnetic field is analyzed. It is shown that this transition is accompanied by a "locking" of the wave vector to the commensurate value. For systems without Lifshitz invariants, which include most crystals with modulated structures, nonlinear equations for the distribution of the order parameter are investigated by asymptotic methods, and these solutions permit describing the entire complex of observed phenomena: the temperature and field dependence of the wave vector, the appearance of higher-order satellites in the neutron duffraction pattern, and the sequence of magnetic phases. Thus a systematic and complete exposition of the present experimental and theoretical status of long-periodic magnetic structures of crystals, such as the spiral structure, the longitudinal and transverse spin-wave structures, the fan structure, and others, is given in this review. The review is written so as to be accessible and of interest to a wide range of readers who are interested in both the theoretical and experimental aspects of the study of magnetic phase transitions in crystals.

Synchronization preamble correlation detector and frequency estimator

Abstract: A device is disclosed which simultaneously performs sync preamble correlation detection and frequency offset estimation functions for a spread spectrum receiver. The device performs a parallel frequency search, executed by a plurality of frequency sensitive circuit channels connected in parallel. These parallel circuit channels first apply phase corrections to the subcorrelator outputs and then linearly reintegrate the corrected outputs. In each channel the phase corrections applied to each successive subcorrelator output vary with the frequency offset subregions to which the channel is designed to match. By applying approximated phase corrections of 0°, 90°, 180° or 270° in lieu of the exact phase corrections required to remove the phase shift, the need for multiplication operations in the reintegration process is eliminated, as well as the need for storing large numbers of weighting coefficients. As a result, the reintegration process involves only addition and subtraction operations and the non-linear integration process is eliminated. The detector also establishes an estimate of the input signal frequency offset at the time of detection of the preamble correlation.

Directional coupler for separation of signals in two frequency bands while preserving their polarization characteristics

Abstract: The coupler comprises a principal corrugated waveguide (10) and four identical secondary waveguides (11) regularly disposed on the perimeter of the principal waveguide and in such a way that the five axis are parallel. Coupling units (12) allow transfer of energy between principal and secondary waveguides. Due to reactance boundary conditions and size, only the HE11 mode in the higher frequency band and the EH11 mode in the lower frequency band can propagate in the principal waveguide. This perserves the polarization characteristics. Directional filtering is obtained by close agreement of phase constants in the different waveguides and 90o phase change between two successive coupling units in the higher frequency band, and evanescence or very low phase constant in the secondary waveguides in the low frequency band.

Velocity-matching techniques for integrated optic traveling wave switch/modulators

Abstract: We propose and analyze a new technique for achieving velocity match between the traveling wave electrical drive and guided optical signal for modulators in substrates for which there is an inherent mismatch. The traveling wave electrodes are laterally shifted periodically to reverse the direction of the applied electric field within the optical waveguide which exactly compensates for the polarity reversal caused by the microwave-optical walkoff. Consequently, the electrooptically induced phase shifts of each section add in phase and several sections can be used to reduce the required drive voltage at the design frequency. This artificial velocity-matching technique moves the mismatch-limited bandwidth to an arbitrarily high design frequency. In addition, we extend the new concept of phase reversal and the previously suggested technique of intermittent interaction by proposing electrode structures with large inactive to active aspect ratios. This generalization provides increased flexibility for manipulating the total available bandwidth to, for example, allow efficient modulation by a train of arbitrarily short electrical pulses. These techniques are ideally suited for several proposed integrated optic devices, including picosecond samplers and gates, which require strong overmodulation at a single high frequency.

Multifrequency cross‐correlation phase fluorometer using synchrotron radiation

Abstract: The construction and operation of a cross‐correlation phase and modulation fluorometer using the synchrotron radiation facility at the ADONE–Frascati electron storage ring is described. In the frequency domain the high repetition rate pulsed source gives a large series of equally spaced harmonic frequencies. Use of cross‐correlation techniques in conjunction with such a light source permits one to isolate one harmonic frequency from the adjacent frequencies with high precision. The cross‐correlation frequency required for the analysis of the phase delay and modulation ratio is obtained using two coupled frequency synthesizers, one of which drives the radio‐frequency cavity of the storage ring and the other which modulates the response of the photomultipliers used for the signal detection. The accuracy, reproducibility, and sensitivity of the instrumentation have been determined on a number of systems and are reported.

Phase-reading, all-fiber-optic gyroscope.

Abstract: An open-loop, all-fiber-optic gyroscope with wide dynamic range and linear scale factor is described. This novel approach converts the Sagnac phase shift into a phase shift in a low-frequency electronic signal by using optical phase modulation followed by amplitude modulation of the electronic signal. Preliminary experimental results verify the theoretical predictions.

Simultaneous phase and amplitude measurements on optical signals using a multiport junction

Abstract: ‘Six-port’ techniques developed at microwave frequencies have been used to measure the relative phase and amplitude of optical signals with approximately 130 photons energy at 633 nm. Applications include phase measurements in coherent optical communication systems.

Brushless motor control circuitry with optimum current vector control

Abstract: A drive circuit for a brushless motor utilizing both phase shift and current shaping to provide for efficient operation of a brushless motor in a servo control system. The circuit includes circuitry for altering the phase of the drive signal as a function of motor velocity to compensate for increased current phase lag due to inductance of the motor at high commutation frequencies corresponding to high motor velocities. In addition to altering the phase shift of the drive signal, the circuitry also alters the commutation signal waveform to compensate for a non-sinusoidal field distribution to produce a nearly sinusoidal torque from each of the brushless motor windings. The compensation of both phase shift and commutation signal waveform is accomplished using digital circuitry. The phase commutation is a non-linear function of motor velocity. The waveform compensation is selected to compensate for a trapezoidal magnetic field distribution in the motor air gap.

The dynamic response of cat alpha-motoneurones investigated by intracellular injection of sinusoidal currents.

Abstract: 1) Sine-wave currents intracellularly injected into spinal α-motoneurones were found to modulate sinusoidally the regular rhythmic firing (carrier frequency) evoked by a current step. Cycle histograms of the instantaneous frequency could be accurately fitted by sinusoidal functions. Those functions were treated as the cell output. 2) For a given modulation frequency between 2 and 14–18 Hz, the amplitude of the cell output was linearly related to the amplitude of the sine-wave current, all over a wide range of current intensities. 3) The sensitivity (gain) and the phase relationships were estimated by varying the modulation frequency of a given sine-wave. When modulation frequency varied from 1–2 Hz to 14–18 Hz, there was a progressive increase of the gain and a phase advance. The experimental gain curve closely conformed to the response of an ideal linear transducer sensitive to both the intensity and the velocity of the input. The phase advance was instead less than that predicted by the model. No “carrier dependent” variations of gain and phase were detected. 4) Differences among motoneurones regarded both the static gain and the “corner frequency” (a measure of the dynamic sensitivity). In 10 motoneurones, the corner frequency ranged between 5 and 10 Hz.

Image formation in terms of the transport equation

Abstract: A scheme for recovering phase using irradiance data alone, without interferometric techniques, is developed using the transport equations for phase and irradiance. For the case of one transverse dimension a general solution, for an arbitrary irradiance distribution, of the transport equation for the optical phase is already given by an application of the divergence theorem. Numerical simulation results are given that indicate that the phase-recovery scheme works well even in the presence of large pupil-plane aberrations if the signal-to-noise ratio is sufficiently high. In particular, pupil-plane phase aberrations may be determined from irradiance measurements in two planes that are near the image plane.

Dynamical Theory of X-Ray Diffraction

Abstract: The kinematical theory of X-ray diffraction follows closely the secondary ex-tinction theory, which can be applied to situations involving very weak or no interaction between the incident wave and the diffracted wave. The theory is therefore suited for diffractions from small and imperfect crystals. If the diffraction from a large perfect crystal is concerned, the primary extinction becomes important. This is because the atomic planes in a perfect crystal are well ordered. Once the atomic planes are set to the exact Bragg diffraction position for an incident wave, they are also perfectly aligned for the reflected wave to be diffracted back into the direction of the incident wave. In analogy with the optics of the visible spectrum, a phase lag of about π/2 of the reflected wave in the incident-wave direction relative to the incident wave results. Namely, the phase difference between the twice-reflected wave and the incident wave leads to the modification of the amplitude of the incident wave. Similar modification takes place for the diffracted wave. Thus, the interaction between the incident and the diffracted waves via multiple reflection dominates the diffraction process. If a relatively strong reflection is involved, this multiple reflection effect cannot be ignored. Moreover, the phase lag of the reflected wave occurs for each plane of atoms inside the crystal. The resultant phase lags of the incident and diffracted waves modify their phase velocities in the crystal, thus changing the index of refraction.

Acoustic spectrogram and complex‐frequency poles of a resonantly excited elastic tube

Abstract: We present a study of the resonance scattering undergone by an air‐filled hollow elastic cylinder excited by an incident plane acoustic wave. We construct the boundary value problem, obtain its classical solution, the solution based on the Resonance Scattering Theory (RST), and generate a variety of useful computed results, some of which are later compared to experimental observations recently performed in France. We present highly accurate expressions for the phase and group velocities and for the phase and group attenuations of the first few surface waves circumnavigating (the extreme cases) of rigid and soft cylinders, and display these dispersion plots in all instances. We analyze the modal backgrounds and modal resonances of the shell, display them in a wide spectral band, determine the SEM‐type pole‐position diagram in the complex k1a plane, and obtain and display the background‐suppressed cross section of the tube. This result serves to generate the acoustic spectrogram of the shell as well as to s...

Neutron Phase Shift in a Rotating Two-Crystal Interferometer

Abstract: The phase shift introduced by rotational motion of a two-crystal neutron interferometer has been measured and found to agree with prediction within 0.4%. This agreement is obtained without making the in-crystal phase corrections employed in a recent study of a linearly accelerated three-crystal interferometer.

An inverse finite-element technique to determine the change of phase interface location in one-dimensional melting problems

Abstract: A “front-tracking” finite-element method has been developed for the study of inverse one-dimensional conduction heat transfer problems with phase change. The method can be used to determine the transient position of the change of phase interface and the thermal history in the solid region from information concerning the temperature history at two arbitrary points in the solid region. This technique can be employed in the control and experimental evaluation of practical heat transfer problems with phase change, such as welding. The accuracy of the method is illustrated through numerical experimentation and found to depend on a typical dimensionless Fourier number.

Real-Time Nonlinear Parameter Tomography Using Impulsive Pumping Waves

Abstract: Absrruci-In this paper a novel real-time tomographic system for imaging the nonlinear parameter B/A of biological objects is proposed. This parameter is related closely to the detailed structure and property of tissues, and may well provide a new dimensional and powerful tool for ultrasonic tissue characterization. In this system an impulsive, relatively high-power pumping wave is applied from a direction perpendiculax to the continuous low-intensity high-frequency probing wave so that the phase of the probing wave is modulated instantly by the product of the parameter B/A of the object and pressure of the pumping wave. Then the resulting spatially-modulated probing wave is detected and demodulated to derive the distribution of B/A along the probing beam. An inverse-filtering operation is employed to compensate for the spread of the pumping wave. The processes are repeated by shifting the position of the probing beam and a two-dimensional image is obtained. A prototype of the imaging system is constructed and images related to the nonlinear parameter of phantoms and human tissues are obtained. The usefulness of this method is shown in these experimental results.

Intensity noise suppression and modulation characteristics of a laser diode coupled to an external cavity

Abstract: Static and dynamic properties of a GaAlAs laser diode with an external cavity (1.5-50 cm) have been studied experimentally, it is found that the optical feedback induced intensity noise is strikingly suppressed by 30 dB when a single frequency oscillation is realized with good phase matching. The threshold of modulation depth, over which intensity noise increases abruptly, has been measured as a function of the modulation frequency in a range of 10-400 MHz. It has the maximum at about 40-70 MHz and increases with the decrease of the external cavity length. On the basis of the compound cavity model the phase and gain conditions for the single frequency oscillation are discussed and the threshold modulation depth is calculated, which is shown to be in good agreement with the experimental results.

Interferometric phase velocity measurements

Abstract: Phase velocities of plasma waves near the lower hybrid frequency were measured with an interferometer composed of two spatially separated electron-density probes. The plasma waves were produced in the F-region ionosphere by an argon ion beam. By calculating the normalized cross spectrum of the plasma waves a coherency of .98 was estimated along with a maximum phase difference of pi/3 radians between the two probes. This implies that the wavelength was 6 meters compared to an O(+) gyroradius of 3.8 meters, and that the phase velocity was 45 km/sec compared to an ion-beam velocity of 12.4 km/sec. These numbers compare favorably with recent predictions of a nonresonant mode produced by a dense ion beam.

Optical 3-d signature device for detecting chemical agents

Abstract: An optical device for obtaining a 3-dimensional signature or a plurality of 2-dimensional signatures for a given sample in order to detect predetermined chemical agents. The device comprises a light channel with a first sensing optical fiber with a coating thereon which changes its absorptive and refractive properties in the presence of a predetermined chemical agent, and a second light channel with a reference optical fiber isolated from the sample. A scanning monochromator is utilized to apply light beams to the sensing and reference fibers, and an optical detector is used to cross-correlate the outputs from the sensing and reference fibers. A variable optical delay line is disposed in one of the fiber channels in order to scan from 0° to 360° in phase shift for each light frequency. A 2-dimensional cross-correlation is obtained for this scan at this frequency. The scanning monochromator is set to run through a series of light frequencies in a given bandwidth. A 2-dimensional cross-correlation phase scan is obtained at each different light frequency. A 3-dimensional cross-correlation may then be obtained (cross-correlation value vs frequency vs phase delay) to provide a signature for a given sample which can be compared to known samples. The device may utilize separate channel bandwidth filters to obtain squeezed states and high S/N ratios.

Diffraction Efficiency and Angular Selectivity of Volume Phase Holograms Recorded in Photorefractive Materials

Abstract: The recording and replay of volume phase gratings in photorefractive crystals is investigated for both transmission and reflection geometries. Differential equations are derived and solved for a range of parameters including the length of the crystal, the magnitude, spatial distribution and phase angle of the refractive index modulation, the beam ratio at recording, and the angular range at replay. The recording process is assumed to reach a steady-state limit before replay with a weak probe beam. Solution of the repaly equations is mainly by numerical integration, although analytic solutions are derived for special cases. It is found that in certain cases the diffraction efficiency can be greatly increased by replaying the hologram at an angle different from the recording angle.

Adaptive nulling in monopulse antennas

Abstract: Theoretical and experimental results are presented for phase-only nulling in low-sidelobe monopulse antennas. Both results are based on a gradient search algorithm that simultaneously searches for a minimum in the sum and difference channel output powers. The array's beam steering phase shifters double as the adaptive weights. Each element in the gradient is found by changing phase shifter setting by Delta Psi (the phase shifter stepsize) and measuring the change in output power. Then the phase shifter is restored to its original value, and the process repeated for all the remaining array phase shifters. The algorithm iterates as long as each new adaptive weight setting reduces the total output power. If the output does not go down, then Delta Psi is decremented by one setting and the iteration is started again. The algorithm stops when Delta Psi =0. The adaptive weights act as random perturbations to the phase taper of the array. Consequently, the sidelobe level is proportional to the size of the phase perturbations and inversely related to the number of elements. By keeping the adaptive phase shifts small, the average sidelobe level and the main beam gain do not drastically change. >

Measurement of small electrophoretic mobilities by light scattering and analysis of the amplitude weighted phase structure function

Abstract: A novel signal processing scheme for the analysis of laser light scattered by small particles allows the measurement of very small collective particle motion in the presence of large diffusive motion. The technique uses a symmetric real fringe Doppler system with frequency shift. Amplitude and phase of the high pass filtered Doppler intensity signal are measured simultaneously and processed into an amplitude weighted phase structure function. An analysis of this function yields diffusion coefficient and, in a sinusoidal electric field, electrophoretic mobility. If present, a constant velocity motion, e.g., due to thermal convection, is simultaneously determined. The technique overcomes a principal limit of conventional light scattering techniques and allows the use of small fields or the measurement of small zeta potentials. Experimental results on latex systems and macroemulsions demonstrate possible improvements.

16-QAM Modulator with PLL amplification devices

Abstract: A 16-quadrature amplitude modulation (QAM) modulator for radio links converts a train of data into signals having a phase selected from sixteen predetermined phase values and an amplitude selected from four values. The modulator divides the data train into two data sub-trains. The two data sub-trains are respectively applied to a pair of four-phase modulators that are fed by a carrier source. The 16-QAM modulator also comprises two phase locked loops each including a voltage controlled microwave oscillator and an associated differential phase detector. The differential phase detector responds to the output of the associated four-phase modulator and the output of the associated voltage controlled microwave oscillator. Predetermined parts of the output signals of the two phase locked loops are added.

Data processing device

Abstract: PURPOSE:To make the adjustment for every product unnecessary by detecting a phase error between observation points by a storage element, varying a delayed quantity of a variable delay element so that the error is corrected to an allowable value or below, and executing a clock phase adjustment. CONSTITUTION:A waveform observation point TP1 is connected to storage elements F0-F3 through a delay element DL3, and an observation point TP2 is connected to the storage elements F0-F3 through delay elements DL4-7. The delay elements DL3-7 have a delay quantity of 3alpha, 0, 2alpha, 4alpha, and 6alpha, respectively. A reference timing clock is generated from a clock of the observation point TP2, a clock of the observation point TP1 is sampled, and from the contents of the storage elements F0-F3, a phase correction controlling circuit 9 decides a phase error, outputs correcting signals 10, 11, changes a delayed value of variable delay elements DL1-2, corrects a phase, and sets the phase error to an allowable value or below. In this way, the phase adjustment is executed automatically, and the test adjusting time is reduced.