摘要:

In this paper we propose a simple optical encoder using slit-width-modulated gratings. The distortion of the encoder signal (harmonic noise) is reduced by using the slit-width-modulated gratings to obtain a sinusoidal curve. The intensity of the light transmitted through the superimposed gratings was investigated by the Fresnel diffraction theory. The Fourier coefficient of the gratings was analysed in order to reduce the distortion of the encoder signal. For practical use, the slit width of the gratings was designed to reduce the third and fifth harmonics of the encoder signal. In the experiment, the slit-width-modulated gratings were fabricated by optical lithography. It was confirmed that the signal distortion was reduced and virtually independent of the air gap between the two gratings. Since the interpolation error is considerably suppressed, the proposed technique will be useful in precision machining.

ISSN:0950-0340    

DOI:10.1080/09500349908231251

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

We investigate thermally induced partial decay of a two-level atom into a photonic bandgap reservoir in regimes when the atomic transition is tuned in the forbidden gap or at the band edge. The atom decays into a quasi-equilibrium state different from those given by the Boltzmann statistical distribution. We take values of physical parameters typical for Rydberg atoms and currently existing photonic crystals. We show that specific thermally induced transitions can be significant for such Rydberg atoms even at bandgap frequencies provided that there is a peak in the density of modes at the band edge. We point out a temperature sensitivity of microwave quantum optics devices based on photonic band structures due to temperature-dependent transitions and level shifts in regimes when important quantum transitions occur at energies near to band-edge frequencies.

ISSN:0950-0340    

DOI:10.1080/09500349908231252

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

This paper presents a numerical method for the calculation of the scattered field when a Gaussian beam meets a flat or slightly rough conducting surface at a grazing angle. When the beam is horizontally polarized, the surface acts like a perfect conductor whereas in the vertical polarization case the reflection coefficient depends heavily on the angle of incidence. In the first case we use the image medium method and in the second we introduce a modification in order to handle the changing reflection coefficient. The method is a solution to the parabolic approximation of the full wave equation and, in the case of the horizontal polarization, is tested against an integral equation solution to the full wave equation. The results of the two polarizations are compared for a flat and Pierson-Moscovicz sea surface.

ISSN:0950-0340    

DOI:10.1080/09500349908231253

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

In this paper we present two different optical configurations providing a white-light array generator based on a diffractive lenslet array (DLA). In both cases, starting from a white-light point source we achieve a regularly spaced set of sharp light spots by use of a single DLA and a small number of extra lenses (only one or two). The first optical system permits us to change the separation between the intensity peaks in a tunable way. The second is very compact and consists only of diffractive lens elements. The key question in both set-ups is the use of achromatic Fourier-transform methods. In this way, we achieve, in a first-order approximation, the superposition of the chromatically dispersed arrays of monochromatic focal points produced by the DLA in a single plane and with the same magnification for all the wavelengths of the incident light. The residual chromatic aberrations of both novel array illuminators are low, even when the temporal spectrum of the source spreads over the entire visible spectrum. Some experimental results are also shown.

ISSN:0950-0340    

DOI:10.1080/09500349908231254

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

We compare scalar and electromagnetic methods for computing fields in the focal regions of diffractive lenses (DLs). The DL is treated locally as a linear grating with slowly varying period and groove orientation. Both the standard complex-amplitude transmission method and rigorous electromagnetic diffraction theory are employed to obtain the field immediately behind the DL. The field in the focal region is then evaluated using the first Rayleigh-Sommerfeld diffraction formula. It is shown that local optimization of the diffraction efficiency by modification of the grating profile improves the intensity at the focal point significantly for DLs faster thanf:2. Improvements of 18% and 41% are obtained for four-levelf:1 andf:0.5 lenses of focal length 1 mm at λ = 633 nm.

ISSN:0950-0340    

DOI:10.1080/09500349908231255

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Self-fractional Fourier images (SFFIs), which are invariant under the two-dimensional fractional Fourier transform, are considered. Necessary and sufficient conditions for an image to be a SFFI for given angles are formulated. An optical procedure for synthesizing SFFIs from a generator image is developed. The physical meaning of SFFI synthesis is discussed. An image decomposition on the set of orthogonal SFFIs, which simplifies image processing in fractional FT optical systems, is proposed.

ISSN:0950-0340    

DOI:10.1080/09500349908231256

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Conventional topometry, applied to surfaces with locally changing materials can result in significant errors in the measured heights. A new theoretical approach is developed for the case of two-beam interferometry. Applied to some surface structures—gold on glass, oil on steel and SiO2on Si—important effects of contrast and phase variations are brought to attention. They are explained by a new theory using the complex contrast. Ways for surface correction procedures are outlined.

ISSN:0950-0340    

DOI:10.1080/09500349908231257

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Results of numerical simulations are presented for the problem of reflection of an incident plane wave from a randomly rough surface when the detector is at a finite distance. In particular, the effect of changes in the detector distance on the enhanced backscatter effect is investigated. It is found that the enhanced backscatter peak reduces in size and becomes wider as the detector distance is reduced in agreement with a simple expression for the phase difference between the rays multiply scattered from a valley on the surface.

ISSN:0950-0340    

DOI:10.1080/09500349908231258

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

We report on a general analytical procedure to analyse the axial focusing properties of uniform cylindrical waves truncated by a rectangular window. The resulting on-axis diffraction pattern explicitly depends on the square of the window height-to-width ratio. Depending on the value of this parameter, different kinds of axial behaviour are observed. In particular, it is found that for low values of this parameter and low Fresnel number, instead of the expected focal-shift effect, an inverse focal-shift phenomenon can appear, i.e. the maximum of the axial-irradiance distribution is displaced further away from the window.

ISSN:0950-0340    

DOI:10.1080/09500349908231259

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

We compare the entanglement of formation with a measure defined as the modulus of the negative eigenvalue of the partial transpose. In particular we investigate whether both measures give the same ordering of density operators with respect to the amount of entanglement

ISSN:0950-0340    

DOI:10.1080/09500349908231260

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor