Spatial and temporal characterization of phase fluctuations in non-Kolmogorov atmospheric turbulence

Abstract

Atmospheric turbulence severely limits the performance of ground-based imaging and laser propagation systems. Some observational results, showing atmospheric turbulence which does not obey Kolmogorov's theory, have prompted the study of optical propagation through non-Kolmogorov turbulence. This paper presents a theoretical approach to analyse the spatial and temporal characterizations of phase fluctuations in non-Kolmogorov turbulence. The spatial structure function, the temporal structure function and the temporal power spectrum of phase fluctuations are derived. The generalized coherence length ρ₀, the characteristic frequency f_R and the characteristic time T_R are expressed as functions of the index structure constant along the propagation path and the wind velocity. The long exposure MTF, the short exposure MTF and the imaging Strehl ratio are computed.     

Experimental study on two-lens slit filtering

ABSTRACT

Two-lens slit filtering has great prospects for high-power lasers due to the excellent properties of inhibition of pinhole closure and compactness of the laser facility. With analysis of the beam profile, power spectrum density and near-field parameters, two-lens slit filtering and its image relay characteristic are experimentally demonstrated, showing a great improvement in beam quality and a good image relay property.     

Effects of chromatic dispersion on microwave and millimetre wave carrier transmission in optical fibres

Abstract

A new formalism for the analysis of optical (phase) noise in fibre-optic communication links is presented. The formalism is applied to an amplitude modulated millimetre-wave (or microwave) link with chromatic dispersion. A realistic link model is described and rigorously solved. In general, the power spectrum of the output signal consists of a ‘comb’ of spectral components with centre separation equal to the modulation frequency. Each spectral component contains a smooth ‘noise’ curve and a narrow spike which represents a modulated field component (carrier). With the new method it is possible to compute each spectral component and, consequently, the full power spectrum. From the power spectrum one can compute quantities relevant to the link design, in particular the carrier-to-noise ratio. Several graphs are presented, showing the power spectrum and the carrier-to-noise ratio for different values of the modulation frequency, link dispersion and modulation index. Several conclusions regarding the millimetre-wave fibre-optic link are dis-cussed.     

Cavity field spectrum for multiphoton Jaynes-Cummings model

Abstract

The field spectra in an ideal cavity for the multiphoton Jaynes-Cummings model are studied. The analytical expression for the spectrum is obtained from the finite double-Fourier transform of the two-time field correlation function. The spectral differences between the initial coherent and initial thermal states are discussed, and the comparisons between the field spectra and atomic emission spectra for k = 2, 3 and 4 are presented. It is shown that the kth power of the photon annihilation operator and atomic lowering operator are subjected to identical forms of a second-order differential equation, in which the coefficients consist of the constants of motion. The field spectrum in the cavity and the emission spectrum of the atom for the initial vacuum state are compared.     

Dual nonlinear correlator based on computer controlled joint transform processor: Digital analysis and optical results

Abstract

The hybrid optoelectronic processor, presented in this paper, realizes the dual nonlinear correlation (DNC) in a set-up based on a two-step nonlinear joint transform correlator architecture. In the first step three power spectrum distributions (input scene power spectrum, reference target power spectrum, and the joint power spectrum) necessary for the nonlinear processing are captured with a CCD camera. Nonlinear modification of the joint power spectrum, which does not have to be symmetrical in the input and reference channels, is introduced digitally. In the second step, the modified joint power spectrum is Fourier transformed optically. Numerical analysis of this processor shows a crucial influence of the dynamic range and the limited number of grey levels of the CCD camera during image acquisition in the first step, on the output signal parameters and the discrimination capability of the set-up. Optical results of recognition obtained for noise-free segmented input scenes show that the set-up enables the realization of various correlation operations as CMF, POF, IF, PPC, etc. without any filters and that the discrimination capability is easy to control by a proper selection of the type of nonlinear processing.     

Range-dependent Optical Feedback Effects on the Multimode Spectrum of Laser Diodes

Abstract

Weak optical feedback from diffusely-reflecting objects can dramatically affect the optical spectrum of homogeneously-broadened laser diodes operated near the lasing threshold. Switching between two or more oscillation modes can be controlled by as little as 1 nW of optical feedback power. The modulation effects on the optical spectrum and total power output depend on the phase of the returned light, and the strength of the effect has a range dependency that is useful for metrology applications. In the present work, experimental and theoretical analyses of these phenomena are presented, and a simple metrology tool for position sensing and remote surface profiling is proposed.     

The Fabry-Perot Interferometer. History,Theory, Practice and Applications

Abstract

A simple algebraic algorithm is proposed as a computational tool for the thin-lens design of a triplet which consists of a singlet and a cemented doublet. The triplet is required to yield specified amounts of lens power, primary spherical aberration, central coma, longitudinal chromatic aberration and secondary spectrum. The three element powers of the triplet are first obtained by solving the simultaneous linear equations of the total lens power, longitudinal chromatic aberration and secondary spectrum. A quadratic equation is obtained by combining the equations of spherical aberration and central coma, and the lens shapes are then obtained by solving this quadratic equation. The solving process is purely algebraic and is therefore easy to calculate and guarantees that all the solutions can be found.     

Domain nucleation in thermomagnetic memory systems near the Curie point

The approximation of Landau second-order phase transitions for ferromagnetics near the Curie point is used for a numerical determination of the two-dimensional microstructure which forms the basis of a data-storage domain. This structure and its lower level of the spectrum of small oscillations are strongly delocalized. Depending on the recording regimes, this may be responsible for the severe irregularity of the domain wall shape sometimes observed or the formation of ring domains. Pis’ma Zh. Tekh. Fiz. 24, 13–17 (August 26, 1998)     

Self-reconstruction of wave packets due to spatio-temporal couplings

Abstract

The spatio-temporal self-reconstruction of the wave function fulfilling the wave equation is examined. The effect is based on the sampling of the spatial spectrum of the wave function in the cylindrical coordinates and on its coupling to the temporal frequency spectrum. The predetermined spatio-temporal profile of the wave function is approximately reconstructed at the periodic spatial intervals along the propagation direction for the continuous temporal spectrum. The periodicity appearing in both the spatial and temporal evolutions are examined for wavefields with the discrete temporal spectrum. The reconstructed field then simultaneously exhibits propagation properties known as the self-imaging and the mode-locking.     

Analysis of beam properties in the neighbourhood of a double-heterostructure laser source

Abstract

The power density distribution and beam characteristics of double-heterostructure lasers with very thin active layers in the neighbourhood of the light source are evaluated by using a plane wave spectrum approach and a second-moment order theory for diffracted scalar light beams. It is shown that the evanescent waves contribute to shape the power density when the distance is smaller than the emission wavelength λ or of the order of λ. It should be noted that the optical fields deeply penetrate into the cladding layers when the thickness of the active layer is much smaller than the emission wavelength λ.     

An Extended Model of Double Optical Resonance

Abstract

A ladder-type model of double optical resonance, extended by allowing some internal structure for the middle level, is considered and its classical analogue is given. Ionization and photoelectron energy spectra from such a model with singlet, doublet and triplet middle-level structures are evaluated numerically for a wide range of laser pulse intensities and durations. The topics of most interest are peak narrowing by extremely strong upper-level ionization damping, reversal of the peak-height asymmetry by increasing lower-resonance strength at non-zero upper-resonance detuning, and number-of-peak reduction and flattening of the spectrum by shortening of the atom-photon interaction time. When applied to natural barium, this model gives results in good agreement with experiment.     

Spatial coherence wavelets

Abstract

It is shown that the cross-spectral density at a plane in the Fresnel—Fraunhofer domain can be expressed as a certain diffraction pattern, which is generated by the superposition of second-order spatial coherence wavelets that emerge from the aperture. The amplitude of each coherence wavelet exhibits units of power density (average energy) and the power spectrum at the far zone plane will be the summation of the amplitudes of such wavelets. Thus, the spatial coherence wavelet constitutes a vehicle for both correlation and energy transport in free space. Some simulation results are discussed to illustrate these ideas.     

Scattering measurements on curved rough surfaces

Abstract

A theoretical model is developed to examine light scattered from a curved optical surface with a defined surface form. The contribution of the surface form to scattering patterns is separated from that of the surface roughness. Experiments are carried out on a curved surface. Measurements are analysed and compared with theoretical calculations. The power spectrum density function and the autocorrelation function of the surface roughness are recovered from the measured scattering pattern.     

ON THE ABSORPTION SPECTRUM OF C60 IN THE VISIBLE SPECTRAL REGION

ABSTRACT

A complete (720–380?nm), strong visible absorption spectrum (OD₅₉₈=1.6) for C₆₀ in a decalin/methylcyclohexane inert matrix at 77?K is reported for the first time. The structure of this spectrum allows one to construct an acceptable approximation to the gas-phase spectrum for this substance at a low temperature. The 0–0 component of the S₀→S₁ transition in the gas phase at a low temperature was estimated to be at about 635?nm.     

The asymptotic behaviour of the spectrum of resonance fluorescence

Abstract

The spectrum of resonance fluorescence from laser-driven multi-level systems is investigated and especially its asymptotic frequency dependence in the wings. Analytical results are presented and the transitions within multilevel systems are classified into three categories according to the asymptotic frequency dependence of their spectra. Under certain conditions an arbitrarily fast asymptotic fall-off behaviour of the spectrum is found which, for intermediately strong driving fields, leads to subnatural linewidths. Numerical examples are presented to illustrate the analytical results.     

Optical pattern recognition in motion acquired scenes using a binary joint transform correlation

Abstract

The problem of detecting patterns in moved scenes is analysed. The correlation when the input scene presents a relative motion with respect to the recording system image is studied analytically and an original method for detecting patterns by means of a binary joint transform correlator is presented. The method is based on the compensation of the sign errors introduced in the joint power spectrum by the transfer function of the degradation. Two alternatives to determine the sign of the transfer function are demonstrated: the first is based on an algorithm to extract information from the Fourier spectrum of the blurred image and the second method determines the sign errors by post-processing the correlation. Digital simulations and optical results are provided.     

Determination of the uniformity of the carrier lifetime in a material from the profile of the amplitude spectrum of an ion detector

An analysis is made of charge transport in the neutral base of a p ⁺-n structure as a result of its diffusion to the boundary of the p-n junction under conditions where nonequilibrium carriers are generated by single α-particles. It is assumed that nonuniformity of the carrier lifetime (τ), described by a Gaussian distribution, exists over the area of the structure. The profile of the transported charge spectrum is calculated for these conditions and its correlation with the measure of nonuniformity τ is obtained. Since the tracks of the diffusing α-particles occupy an extremely small volume, recording them is equivalent to local probing of the material for τ. It is suggested that the calculated function should be used as a calibration function to determine the spread of τ values in materials. The method is tested on Si for a surface-barrier structure by recording 8.78 MeV π-particles. Pis’ma Zh. Tekh. Fiz. 24, 44–50 (March 12, 1998)     

Deposition process,microstructure and mechanical behaviours of RF magnetron sputtered (Ti,Al)N thin films

Abstract

Thin films of (Ti,Al)N with different Al contents were co-deposited using one Ti and one Al targets by radio frequency (RF) pulsed magnetron sputtering. Their composition, microstructure, nanohardness, surface morphology and deposition process were investigated by energy dispersive spectrometer system, X-ray diffraction, nanoindentation, atomic force microscopy and optical emission spectrum. A face cubic centred (fcc) TiN (B1) structure was found in the thin films when Al target power was low. When Al target power was increased, an additional hexagonal AlN (B4) structure appeared. With increasing Al content, the resulting films gradually changed from B1 structure to that of B4, accompanying with decrease of the lattice constant of B1 structure. Simultaneously, the preferred orientation of B4 structural thin films gradually transformed from (111) to (200). The mode of thin films transformed from island to fibre, subsequently to column with increasing Al target power. Optical emission spectrum analysis indicated that Al target surface reached non-metal sputtering mode earlier than that of Ti target under the same deposition parameters, which resulted in a lower sputtering rate of Al target than Ti target and loss of Al content in (Ti,Al)N thin films.     

Cyclic and Linear Structures of C13: A Computational Study

Abstract

Recently, a linear structure of C₁₃ was observed and its infrared spectrum recorded. Computations of the linear and cyclic structures of C₁₃ are thus presented. The computations are performed at ab initio level using the standard 6-31G? basis set. Electron correlation is treated by density functional theory (Becke3LYP) and perturbation treatment (MP2=FC). With the density-functional approach the standard 6-311G? and 6-311G(2d,p) basis sets are applied, too. In all the treatments the cyclic structure (C _2v symmetry) is located about 90 kJ/mol below the linear form (D _∞h symmetry), while a tricyclic structure is considerably higher in energy (about 280 kJ/mol above the monocyclic species). Even at high temperatures the linear form represents less than 10% of the equilibrium isomeric mixture. Computed IR spectra are also reported. The stability evaluation corresponds to the findings of ion chromatography.     

Emission Spectra of a Two-level Atom in a Kerr-like Medium

Abstract

We have investigated the spectrum of light emitted by a single atom interacting with a single mode of the radiation field in an ideal cavity filled with a Kerr-like medium. It is shown that owing to the Kerr-like nonlinearity in the system the spectrum of the emitted light exhibits a single-peaked structure for sufficiently high intensities of the initial coherent field instead of the triplet structure in the case of the standard Jaynes-Cummings model.