Showing papers on "Wavelength published in 1982"

Phase Retrieval And Diversity In Adaptive Optics

Abstract: Wavefront sensing by phase retrieval implies extraction of the Fourier transform of a complex signal based on observation of the modulus of the signal Only the image intensity from a system's focal plane array is required to estimate the phase aberrations These estimates are used to derive control signals to align (or to maintain alignment of) the optical system The concept can be used in both a predetection and postdetection mode In the former, the control system labors to keep the optics in a diffraction-limited mode all the time In the latter, the control system induces a phase or wavelength diversity that allows successive images to be restored to nearly diffraction-limited quality by postprocessing of the image This second mode is particularly interesting because it will reduce the design effort for both the optical system and the control system How the phase or wavelength diversity is achieved is not clear at this time If the method has utility, it provides an interesting challenge to designers of optical devices In this paper we describe the mathematics of the technique and show some computer simulations which involve both point sources and extended objects

Parametric amplification and frequency conversion in optical fibers

Abstract: We find that the parametric four-photon gain for light pulses decreases for fibers longer than a characteristic length. This length is related to the common experimental observation that stimulated parametric emission is usually prominent only in short fibers while in long fibers stimulated Raman scattering dominates. Despite the fact that the actual process involves an intensity dependent bandwidth and broadening of the pump linewidth from self-phase modulation, it is possible to develop a simple expression for the characteristic length which requires only the initial pump linewidth and the low-power parametric bandwidth. This bandwidth can often be estimated from the pump wavelength and the measured frequency shift between the pump and the generated waves. Expressions for gain and amplification are derived from coupled wave equations and in the Appendixes it is shown that these are of the same form as the planewave equations, but modified by coupling coefficients called overlap integrals.

The limits of resolution of zero-phase wavelets

Abstract: This investigation deals with resolving reflections from thin beds rather than the detection of events that may or may not be resolved. Resolution is approached by considering a thinning bed and how accurately measured times on a seismic trace represent actual, vertical two-way traveltimes through the bed. Theoretical developments are in terms of frequency and time rather than wavelength and thickness because the latter two variables require knowledge of interval velocities. These results are compared with similar studies by Rayleigh, Ricker (19.53), and Widess (1973, 1980). We show that the temporal resolution of a broadband wavelet with a white spectrum is controlled by its highest terminal frequency f,,, and the resolution limit approximates I/ ( I .5 fU), provided the wavelet’s band ratio exceeds two octaves. The practical limit of resolution, however, occurs at a one-quarter wavelength condition and approximates I /( 1.4 fJ. The resolving power of zero-phase wavelets can be compared quantitatively once a wavelet is known in the time domain.

Energy saturation and phase speeds measured on a natural beach

Abstract: Field measurements of wave height and speed from 7-m depth shoreward are described. The experiment plan consisted of a shore-normal transect of closely spaced (compared to a dominant wave length) velocity, pressure, and elevation sensors on an almost plane profile having an inshore slope of 1:50. As the waves shoal and begin to break, the dominant dissipative mechanism is due to turbulence generated at the crest, and wave heights become increasingly depth controlled as they progress across the surf zone. Wave heights in the inner surf zone are strongly depth independent: the envelope of the wave heights is described by H/sub rms/ = 0.42 h. The depth dependence of the breaking wave height is shown to be related to the kinematic instability criterion. Celerity spectra were measured by using phase spectra calculated between pairs of adjacent sensors. Inshore of 4-m depth, the celerity was found distant over the energetic region of the spectrum. A 'mean' celerity was compared with linear theory and was within +20% and -10%, showing good agreement for such a nonlinear, dissipative region.

Theoretical model of absorption of laser light by a plasma

Abstract: A simple model of laser light absorption is described. The absorption mechanism is mainly inverse bremsstrahlung, but a crude description of resonance absorption is also included. The intensity and the wavelength dependence are emphasized, but the model takes into account the target material composition, the laser pulse length, and the focal spot radius. Plane and spherical expansion are treated. Results range from short‐wavelength, low‐intensity regime, where inverse bremsstrahlung absorption is total, to long‐wavelength high‐intensity regime, where inverse bremsstrahlung absorption is negligible. Scaling laws concerning absorption, electron temperature, and electron kinetic pressure are given in the two limiting regimes. A characteristic flux Φ*, or, alternatively, a characteristic wavelength λ* is defined which separates the two regimes, the other parameters being held constant.

The observation of ocean surface phenomena using imagery from the SEASAT synthetic aperture radar: An assessment

Abstract: The principles governing synthetic aperture radar (SAR) and its use on the Seasat spacecraft are reviewed. The way in which wind stress, surface currents, long gravity waves, and surface films modulate the scattering properties of resonant (approximately 30-cm-wavelength) waves is discussed, with particular emphasis placed on the mechanisms that could produce images of long gravity waves. Doppler effects by ocean motion are also described. Measurements of long (wavelength more than about 100 m) gravity waves made using Seasat SAR imagery are compared with surface measurements during several experiments. Combining these results, it is found that dominant wavelength and direction are measured by Seasat SAR within + or - 12% and + or - 15 deg, respectively. It is noted, however, that ocean waves are not always visible in SAR images, and detection criteria are discussed in terms of wave height, length, and direction.

Laser‐induced excited state and ultrasonic wave gratings: Amplitude and phase grating contributions to diffraction

Abstract: A detailed analysis of diffraction from laser‐induced gratings is presented. The changes which occur in both the real and imaginary parts of the index of refraction are accounted for when excited states are created. These lead to phase and amplitude grating contributions, respectively, to the diffraction from a laser‐induced excited state grating. Experimental confirmation of the predicted wavelength dependence of these contributions is presented. Diffraction from laser‐induced excited state gratings, ultrasonic wave gratings, and mixed excited state and acoustic gratings is analyzed with the phase and amplitude contributions to each accounted for. The results permit the interpretation of mixed grating data and predict conditions under which density‐dependent absorption spectral shifts and excited state‐phonon interactions can be measured.

Seabed-wave resonance and sand bar growth

Abstract: The interaction of surface water waves with undulating sea-bed topography is of fundamental importance to coastal engineersand sedimentologists. For example, wave reflection from submerged bars on beaches may provide a mechanism for protecting the beach from further wave attack. Additionally the interaction of surface water waves with other tidally or wave generated bedforms, such as sandwaves on offshore banks, may modify wave climates on adjacent coastlines. While there have been numerous studies of the interaction of sea waves with engineering structures, such as breakwaters, piers, and jetties, there have been comparatively few investigations of wave interactions with naturally occurring bedforms. Predictions have recently been made of the amount of wave energy reflected as a result of resonant interactions between surface water waves and undulating sea-bed topography but with no supporting experimental proof. I describe here preliminary results from what are believed to be the first published laboratory measurements of resonant interactions between surface water waves and submerged bars which show that significant and large amounts of wave energy may be reflected and that these reflections are brought about by resonant interactions between surface-water waves and the bedforms. In particular, at resonance, incident surface water wavelengths are approximately twice the bedform wavelengths. These results have implications not only in terms of wave reflection from naturally occurring bedforms, say bars on beaches, but also for sediment transport processes in general.

STARE and GEOS 2 observations of a storm time Pc 5 ULF pulsation

Abstract: A study is presented of a storm time Pc 5 pulsation, observed by GEOS 2 particle detectors and magnetometers at geocentric orbit and by the STARE auroral radar in the ionosphere. These measurements allow the comparison of phase relationships between the magnetic field components, energetic particle flux, and electric field in the ionosphere over a substantial region. They also permit measurements of the wavelength of the disturbance in the ionosphere. A theory of the coupling between a drift mirror wave and a standing Alfven wave is developed. The results are compared with this theory and are found to support it. In particular, it is found that the disturbance is of wavelike nature, propagating westward in the equatorial plane at 6.6 RE with a wavelength of about 1 RE. Its period is initially 213 s, lengthening to 300 s later in the event. In the equatorial plane the disturbance in the particle flux and the compressional magnetic field are out of phase as is consistent with a drift mirror wave. There is a large transverse magnetic field oscillation in the meridian plane as is consistent with a guided poloidal oscillation. In the ionosphere the oscillation is consistent with the existence of a guided poloidal Alfven wave and is entirely different from toroidal oscillations that have been observed at Pc 5 frequencies on other occasions. The observations are consistent with a particle driven drift mirror wave, couplied to a standing Alfven wave. The azimuthal wavelength is consistent with a drift mirror wave whose Doppler shifted frequency matches the natural frequency of the standing Alfven wave.

Laboratory experiments on fronts

Abstract: We describe a laboratory model of an upwelling front in a two-layer stratification. In the model the interface between the two layers slopes upwards toward a vertical boundary (or coastline) and can intersect the free surface to produce a front. Fluid motion in each layer is density driven and, in the undisturbed state, is in quasi-geostrophic balance. The front is observed to be unstable to (ageostrophic) disturbances with an along-front wavelength proportional to the Rossby radius of deformation. At very large amplitudes these unstable waves form closed circulations. However, in contrast to the behaviour of fronts far from vertical boundaries, where cyclone-anticyclone vortex pairs are formed, the presence of the coastline inhibits formation of anticyclonic eddies in the upper layer and enhances cyclonic rings of upper layer fluid which lie above cyclonic eddies in the lower layer. The cyclones move away from the vertical boundary and (as is also the case when no vertical boundary is present) t...

Wavelength variation of 1.6 µm wavelength buried heterostructure GaInAsP/InP lasers due to direct modulation

Abstract: Wavelength shift during the period of direct modulation (dynamic wavelength shift) for injection lasers having a BH structure has been investigated both experimentally and theoretically. A GaInAsP/InP BH laser emitting a nominal wavelength of 1.61 μm was modulated by a sinusoidal current at frequencies in the range of 0.2-2 GHz. The full width of the dynamic wavelength shift was 0.35 nm at a modulation frequency of 1.8 GHz, and a modulation depth of 63 percent at a bias current 1.14 times the threshold current. It was found that the width of the dynamic wavelength shift increases with proportion to the modulation depth, and with inverse proportion to the bias current at a frequency below 1 GHZ. The differential coefficient dn/dN of refractive index n for carrier density N in the active region was measured for the purpose of the analysis. The value obtained is -1.2 \times 10^{-20} cm3. The dynamic shift of the lasing wavelength was found to be characteristic of the change of the refractive index induced by the oscillation of carrier density in the active region during intensity modulation. The theoretical shift shows maximum value at a resonance-like modulation frequency. The peak height of the resonance wavelength shift is strongly affected by carrier diffusion in the transverse direction, and has a minimum value when stripe width is nearly equal to carrier diffusion length.

Wavelength Selection in Systems Far from Equilibrium

Abstract: It is shown that for systems developing stationary periodic patterns there exists at most one stable wavelength state if a supercritical region is connected to a subcritical one by the imposition of a slow spatial variation of the external parameters. In nonpotential systems the selected wavelength depends on the particular combination of parameters that vary but not on the (slow) rate of spatial variation. Suitable parameter variations force the system into a dynamic state.

Simultaneous rocket probe and radar measurements of equatorial spread F—Transitional and short wavelength results

Abstract: During the PLUMEX I rocket flight from Kwajalein Island, plasma density and electric field fluctuations were measured in situ, simultaneous with ground-based radar backscatter measurements at 0.96-m and 0.36-m wavelengths. The rocket penetrated an extremely turbulent topside region which had associated intense backscatter. As measured by the radar the backscatter power was decaying with time during and after the flight. The intermediate wavelength (0.1–10 km) in situ electron density measurements are described in a companion paper, while here we report the transitional and short wavelength results (λ < 100 m). These data include the first in situ equatorial spread F measurements of the electric field component of electrostatic fluctuations with wavelengths less than 1 m. At all altitudes above about 280 km, a repeatable form for the wave-number spectrum was found for the electron density and electric field fluctuations at wavelengths less than about 100 m. The density spectrum varies approximately as k−5 and the electric field spectrum as k−3. The steepness of the density spectrum corresponds to an absence of steep edges in the density waveform on the scale of 100 m and less. These two spectral forms are shown to be consistent with an explanation involving low-frequency waves with finite wave numbers parallel to the magnetic field (k∥). Both theory and laboratory experiments show a power law density fluctuation spectrum for gradient-driven drift waves with negative index in the range 4.5–6.0. Since such waves do have finite k∥, and since sharp gradients exist in the spread F environment, we conclude that at sufficiently high altitudes, drift waves act on the steep gradients caused by a primary longer-wavelength instability to create the observed spectral form. These waves may then create an anomalous diffusion as discussed by Huba and Ossakow (1981b). At lower altitudes a shallower spectral index was observed in the tens of meters range, which may be related to a collisional damping regime. This suggests an altitude threshold for the drift waves that is probably related to ion neutral collisions. The power law spectra show no marked change near k⊥ri ≈ 1 where ri is the ion gyroradius. Since low-frequency drift waves are linearly stable for k⊥ri ≳ 1, it seems that a wave-wave interaction (cascade) operates to deposit energy in a range where waves are linearly damped. There is a slight suggestion of a spectral change (smaller negative index) for k⊥re ≳ 0.2 which may be due to excitation of a lower-hybrid drift wave and which may be related to the observed enhanced backscatter at wavelengths on the order of 1 m. A stability analysis shows that the plasma is near but on the stable side of the marginal stability boundary for the lower-hybrid drift wave in the most intense region of backscatter. In regions devoid of drift waves, evidence is found for an exponential inner-scale cutoff at a wavelength of 200 m.

Long wavelength irregularities in the equatorial electrojet

Abstract: We have used the radar interferometer technique at Jicamarca to study in detail irregularities with wavelengths of a few kilometers generated in the unstable equatorial electrojet plasma during strong type 1 conditions. In-situ rocket observations of the same instability process are discussed in a companion paper. These large scale primary waves travel essentially horizontally and have large amplitudes. The vertical electron drift velocities driven by the horizontal wave electric fields reach or exceed the ion-acoustic velocity even though the horizontal phase velocity of the wave is considerably smaller. A straightforward extension to the long wavelength regime of the usual linear theory of the electrojet instability explains this and several other observed features of these dominant primary waves.

Background and temperature dependent current‐voltage characteristics of HgCdTe photodiodes

Abstract: The zero bias resistance‐area products and current‐voltage (I‐V) characteristics as a function of temperature and infrared background radiation have been measured for HgCdTe photodiode samples with cutoff wavelengths near 9 μm at 80 K. A model is presented to account for the background and temperature dependence of the data. It is found that the reverse I‐V curve shapes and magnitudes may be estimated as a function of both background and temperature by superposition of the total current from optical generation, thermal diffusion, and depletion region generation‐recombination centers. The influence of background radiation was found to create an exceptionally linear reverse I‐V characteristic that can be easily distinguished from other current generation mechanisms, and can be modeled by application of existing theory to HgCdTe photodiodes.

Characteristics of the ULF waves associated with upstream ion beams

Abstract: A new class of upstream wave is reported with relatively high frequencies of about 1 Hz and small amplitudes compared to the more common larger amplitude, low-frequency (0.03 Hz) upstream wave. The waves were first noted in association with beams of ions reflected back upstream at the bowshock, and although beam presence appears to be a necessary condition for the observation of the waves, it is not a sufficient condition for the existence of the waves. Magnetometer measurements are used to determine intrinsic properties of the waves, and simultaneous two point measurements are used to calculate and eliminate Doppler shifting effects. Results indicate that the waves are right-hand elliptically polarized whistler mode waves with plasma rest frame frequencies of about 20-100 times the proton gyrofrequency and wavelengths of about 100 km.

Low-frequency waves in the Ligurian Sea during December 1977

Abstract: Observations of low-frequency waves in the Ligurian Sea in December 1977 are presented. From time series of thermal infrared images obtained by satellite NOAA 5, the mean wavelength and phase velocity are estimated. They are, respectively, 38 km and 18 cm s−1. These waves are analyzed as large-amplitude baroclinic waves. Fairly good agreement is found with a two-layer model.

The Rietveld method in neutron and X-ray powder diffraction

Abstract: In the Rietveld method of analysing powder diffraction data, the crystal structure is refined by fitting the entire profile of the diffraction pattern to a calculated profile. There is no intermediate step of extracting structure factors. The method was applied first to diffraction patterns recorded with neutrons at a fixed wavelength. It has now been used successfully for the treatment of results from the four categories of experimental technique, with neutrons or X-rays as the primary radiation and with the scattered intensity measured at a fixed wavelength or at a fixed scattering angle. In this article we discuss the application of the Rietveld method to each of these techniques.

Depolarization of electromagnetic waves scattered from slightly rough random surfaces: a study by means of the extinction theorem

Abstract: The scattering and depolarization of electromagnetic waves from perfectly conductive slightly rough random surfaces is studied using the small perturbation method through the Ewald–Oseen extinction theorem. This permits predictions in those cases in which the physical optics, or the Kirchhoff approximation, fails, namely, at grazing incidence and when the wavelength of the incident radiation is comparable with the correlation length of the random heights. In this way it is seen, for example, that, as with the Rayleigh–Fano method, the depolarization of the fields is obtained from the second order of the expansion and exists even in the backscattering and specular directions. Also, unlike the predictions of the Kirchhoff approximation, this depolarization depends on the surface shape. However, this approach yields in the specular direction the same result as the Kirchhoff approximation for those cases in which the latter is known to be valid, i.e., for large correlation lengths and non-grazing-incidence directions and establishes precise conditions under which the Kirchhoff approximation is retrieved in the backscattering direction.

Submicroscopic contact imaging with visible light by energy transfer

Abstract: The resolution of contact imaging with light is limited by the distance between object and image and not by the wavelength. Contact imaging with visible light at submicroscopic resolution (100 nm) is demonstrated using the concept of imaging by energy transfer. The bleaching of a dye is inhibited by energy transfer to a metal in close proximity. This mechanism is exploited for imaging a planar metal pattern onto a film of a cyanine dye.

Amplitude attenuation of impulsive waves in random media based on travel time corrected mean wave formalism

Abstract: Waves gradually collapse with propagation through media with random velocity fluctuation; however, impulsive waves propagate without large attenuation when the wavelength is shorter than the correlation distance. The Q−1 value predicted from the usual mean wave formalism monotonously increases with frequency even in the high‐frequency limit, due to taking a mean over waves with large travel time fluctuations caused by the long scale velocity fluctuation compared with the wavelength studied. We propose a new statistical averaging method appropriate for the amplitude attenuation measurement of impulsive waves, in which the mean wave is defined after the correction of travel time fluctuations. We investigate impulsive scalar waves propagation in three‐dimensional media with homogeneous and isotropic random fractional velocity fluctuation, based on the binary interaction approximation in this improved mean wave formalism. We successfully derive the Q−1 value that has a peak of the order of the mean square fra...

Attenuation of S waves in the lithosphere due to scattering by its random velocity structure

Abstract: Recent seismological studies suggest that the amplitude attenuation of S waves in the lithosphere may be primarily due to scattering by its random inhomogeneity. There is, however, a disturbing discrepancy between statistical scattering theory and observation with regard to the asymptotic frequency dependence of Q−1 in the high-frequency limit. Although the observed Q−1 decreases with frequency above 1 Hz, the usual mean wave formalism predicts Q−1 of media with random velocity fluctuation increases with frequency; we take a mean over waves with large travel time fluctuations that are caused by long-scale velocity fluctuations compared with the wavelength studied. We propose a new statistical averaging method; mean wave is defined after the correction of travel time fluctuations. The random media with a mean square fractional velocity fluctuation of 5.0×10−3 and a correlation distance of 4.0 km will explain the observed and partially conjectured Q−1 for frequencies from 0.05 to 30 Hz.

Structure of air flow separation over wind wave crests

Abstract: Air flow over wind waves generated in a wind-wave tunnel was visualized by numerous tiny suspended particles (zinc stearate), and instantaneous air flow fields over about one wavelength of wind waves were obtained. Air flow separation was detected over the wave crest in about a half of the samples. In such cases, the separation started near the crest about half of the time, with a vortex trapped over the convergence point of the surface flow which appeared at the leeward face of the crest. This structure was much different from a previously imagined picture in which the separation started at the convergence point. The high frequency of its occurrence suggested the stability of this structure. However, even when this structure was clearly seen, the structure behind the vortex to the next wave crest had various patterns. This variety seems to be related to an instability of the high-shear layer accompanied by separation. Other varieties were also seen, such as the occurrence of separation without the above mentioned structure, as well as the existence of non-separated air flow structures. These varieties seem to be related to the variability of individual wind wave crests. An analysis of correlation between the wave form and the air flow structure over it shows that there is a critical value of local gradient of wave form, above which the air flow always separates. This fact suggests a strong coupling between the air and the water, i.e., the local stress exerted on the water surface changes the nature of a wave crest, especially its form, and as a result, the air flow structure over it changes drastically.