Experimental observation of the effect of astigmatic aperture lens on the spectral switches of polychromatic Gaussian beam

Experimental results of a study, conducted to investigate the effect of an astigmatic aperture lens on the spectral switches observed with spatially coherent polychromatic light, are reported. It is found that the spectrum at the observation plane exhibits anomalous behavior after passing through the astigmatic aperture lens. It is shown that at a particular position of the aperture, the spectrum splits into two halves, while at other positions the spectrum either shifts towards lower frequencies (red shift) or shifts towards higher frequencies (blue shift). These spectral changes take place in the vicinity of the dark region of the diffraction pattern. Experimental observations show that the behavior of the spectral switch is affected by the astigmatism of the lens. It is found that the spectral minimum value and the transition height of the spectral switch change as the astigmatism of the lens changes. Moreover, the critical position of the aperture where spectral switch occurs also changes with the astigmatism of the lens. The results are consistent with the theoretical predictions [Pan, L.; Lü, B. Opt. Commun. 2004, 234, 13–22].     

Effect of surface plasmons on spectral switching of polychromatic light with Au-double-slit

It is shown that the generation of surface plasmons on a metal-dielectric interface, i.e., a Au-double-slit and air interface, appreciably affects the intensity of the diffracted light. With a specific example of Au-double-slit and electro-optic devices before the slits, the spectral switching in the diffracted field with the polychromatic light is shown for the first time, to our knowledge. It is found that at the observation point due to the surface plasmon effect the intensity of the diffracted light periodically increases and decreases with the separation of the double-slit.     

Effect of spectral correlations on spectral switches in the diffraction of partially coherent light

The subject is the spectral characteristics of partially coherent light whose spectral degree of coherence satisfies or violates the scaling law in diffraction by a circular aperture. Three kinds of spectral correlations of the incident light are considered. It is shown that no matter whether the partially coherent light satisfies or violates the scaling law, a spectral switch defined as a rapid transition of spectral shifts is always found in the diffraction field. Different spectral correlations of the incident field in the aperture result in different points at which the spectral switch occurs. With an increment in the correlations, the position at which the spectral switch takes place moves toward the point at which the phase of the center frequency component omega0 becomes singular for illumination by spatially fully coherent light. For light that satisfies the scaling law, the spectral switch is attributed to the diffraction-induced spectral changes; for partially coherent light that violates the scaling law, the spectral switch is attributed to both the diffraction-induced spectral changes and the correlation-induced spectral changes.     

Stable deep nulling in polychromatic unpolarized light with multiaxial beam combination

In the context of the space-based nulling mission ESA-Darwin, Thales Alenia Space has developed a nulling breadboard for the European Space Agency (ESA): the multiaperture imaging interferometer (MAII) to demonstrate deep and stable nulling and to investigate various subsystems of the ESA-Darwin interferometer. Recently, we have extended our investigations to the multiaxial beam combination. This combination scheme presents many advantages: simplicity, compactness, and a high coupling efficiency for a three-beam combination. The near-infrared (lambda approximately 1.55 microm) MAII breadboard has been upgraded to the multiaxial beam combination. Polarization and stability issues have been thoroughly investigated. We report on the recent results we have obtained with the multiaxial configuration of MAII in unpolarized light with a relative spectral bandwidth of 5%: nulling ratios of mean value N=1.7 x 10(-5), stable over 1 h with a standard deviation sigma( N )=5.7 x 10(-7). These results indicate that the multiaxial beam combination has the potential to meet Darwin requirements.     

Generating light with a specified spectral power distribution

A particular version of a spectral integrator has been designed. It consists of a xenon lamp whose light is dispersed into a color spectrum by dispersing prisms. Using a transmissive LCD panel controlled by a computer, certain fractions of the light in different parts of the spectrum are masked out. The remaining transmitted light is integrated and projected onto a translucent diffusing plate. A spectroradiometer that measures the generated light is also attached to the computer, thus making the spectral integrator a closed-loop system. An algorithm for generating the light of a specified spectral power distribution has been developed. The resulting measured spectra differ from the specified ones with relative rms errors in the range of 1%-20% depending on the shape of the spectral power distribution.     

Spectral changes and 1 X N spectral switches in the diffraction of partially coherent light by an aperture

The off-axis and on-axis spectra in the far zone of an aperture for the case in which a particular class of partially coherent light with a broad spectrum is diffracted by an aperture are studied. It is shown that the spectrum in the far zone is generally different from that at the aperture; i.e., the spectrum is split into two or more peaks. Moreover, the spectrum varies with the diffractive angle. For a fixed diffractive angle, the spectral shift, defined as the difference between the frequencies at which the observed spectrum and the spectrum at the aperture take their maximum, shows a gradual change with the change in the coherence at the aperture. However, as the coherence reaches some critical values, the spectral shift exhibits a rapid transition; i.e., spectral switch occurs. The coherence that causes the spectral switch to take place is different for different diffractive angles. Therefore we propose a new kind of 1 x N spectral switch, where N detectors (output ports) are placed at different diffractive angles in the far zone, and the spectral shifts at different detectors are measured. By adjusting the coherence of the aperture (input port) to the desired values, we obtain a rapid transition of the spectral shift in the desired output ports.     

A new light source in the vacuum ultra violet spectral region

Excited rare gases at moderate pressures (<200 torr) emit a continuous broad spectrum in the vacuum uv region.     

Spectral switches of partially coherent light focused by a filter-lens system with chromatic aberration

It is shown that when partially coherent polychromatic light is focused by a filter-lens system with chromatic aberration, a spectral shift exists in the focused field, and a spectral switch that is defined as a sharp transition of the spectral shift also takes place at some positions of the focused field. The influence of the chromatic aberration of the lens, the coherence of the partially coherent light in the filter (a circular aperture), the radius of the aperture, and the spectral width of the partially coherent light in the aperture on the spectral shift and the spectral switch are investigated in detail. The numerical results show that these parameters affect the spectral shift and the spectral switch significantly. Potential applications of the spectral shift and the spectral switch of the partially coherent light are discussed.     

Formulation of light focusing through a plano-convex spherical lens in wave optics

Focusing of an incident light wave through a plano-convex spherical lens is discussed by calculating the light intensity distribution on the lens's optical axis after the incident wave is multiply scattered inside the lens. It is found that the size and location of the region into which the incident wave is focused are determined by two conditions. It is also found that it is possible for the wave to be focused into two such regions.     

Geometric phase in optics and angular momentum of light

Abstract

The Physical mechanism of the geometric phase in terms of angular momentum exchange is elucidated. It is argued that the geometric phase arising out of the cyclic changes in the transverse mode space of Gaussian light beams is a manifestation of the cycles in the momentum space of the light. The apparent non-conservation of orbital angular momentum in the spontaneous parametric down conversion for the classical light beams is proposed to be related to the geometric phase.     

Aspect ratio of elongated polychromatic far-field speckles of continuous and discrete spectral distribution with respect to surface roughness characterization

Here polychromatic speckle patterns generated either by a polychromatic light source that emits at discrete frequencies or by a light source showing a continuous narrow-band spectral distribution are studied. The purpose here is the application of polychromatic speckle-pattern analysis to an in-process surface roughness characterization. To compare the coherence properties of the different polychromatic light sources, first a modified definition of the coherence length is introduced. Furthermore, the relevant optical phenomena, namely, the speckle elongation caused by the angular dispersion and the roughness-dependent speckle decorrelation, are summarized. It is shown that light sources with a continuous spectral distribution have essential advantages in comparison with discrete wavelength sources. The theoretical results are confirmed by experimental investigations based on a digital algorithm for the evaluation of CCD images of polychromatic speckle patterns, which are recorded in the Fourier plane of a Fourier-transforming lens.     

Metal-Insulator Transition in Doped Systems as a Percolation Phenomenon

The percolative picture for the metal-insulator transition in doped materials is discussed. We focus on various properties of manganites. The approach has now a strong experimental support. The percolative phenomenon implies the presence of intrinsic inhomogeneity, but, nevertheless, the picture is different from that of the electronic phase separation.     

Soft-x-ray polarimeter with multilayer optics: complete analysis of the polarization state of light

The design of a versatile high-precision eight-axis ultrahigh-vacuum-compatible polarimeter is presented. This multipurpose instrument can be used as a self-calibrating polarization detector for linearly and circularly polarized UV and soft-x-ray light. It can also be used for the characterization of reflection or transmission properties (reflectometer) or polarizing and phase-retarding properties (ellipsometer) of any optical element. The polarization properties of Mo/Si, Cr/C, Cr/Sc, and Ni/Ti multilayers used in this polarimeter as polarizers in transmission and as analyzers in reflection have been investigated theoretically and experimentally. In the soft-x-ray range, close to the p edges of Sc, Ti, and Cr, resonantly enhanced phase retardation of the transmission polarizers of as much as 18 degrees has been measured. With these newly developed optical elements the complete polarization analysis of soft-x-ray synchrotron radiation can be extended to the water-window range from 300 to 600 eV.     

Design considerations of high performance optical code division multiple access: a new spectral amplitude code based on laser and light emitting diode light source

A new code of optical code division multiple access (OCDMA) based on spectral amplitude coding (SAC) is described and analysed. The coding technique is called random diagonal (RD) code. One of the important properties of this code is that the cross-correlation at data segment is always zero, which means that phase intensity induced noise (PIIN) is reduced. From the construction of RD code sequence, the authors can see that the RD code is constructed using code segment and data segment. Using this code property, RD code is implemented using coherent source (multi-laser) and incoherent source (light emitting diode) for the code segment and data segment, respectively. RD code using two multi-sources (incoherent and coherent) can be considered as an effective way for maximising the capacity while minimising the cost of SAC-OCDMA. It is shown that the system using this new code matrices not only suppresses PIIN, but also allows a larger number of active users compared with other codes.     

Limitations and guidelines for measuring the spectral width of a single pulse of light with a Fabry-Perot interferometer

We present a method of analyzing the output of a single pulse of light from a Fabry-Perot interferometer (FPI). Together with an independent measurement of the pulse width and shape, the analysis enables the determination of the linewidth, TBP, and, consequently, the degree of coherence of the individual light pulses. The analysis presented builds on the method presented by Marzenell et al. [Appl. Phys. B 71, 185-191 (2000)] by analyzing the ring pattern of the FPI.     

Time-resolved spectral and spatial description of laser-induced breakdown in air as a pulsed, bright, and broadband ultraviolet-visible light source

The optical breakdown induced in air at atmospheric pressure by Nd:YAG Q-switched laser pulses is studied in terms of the spectral features of the emitted radiation in the wavelength range 180-850 nm during the first 200 ns after the laser pulse onset. During the plasma build up, radiation emission features intense, broadband, and structureless ultraviolet-visible spectra before the appearence of atomic lines on the microsecond scale. Also, the emitting plasma kernel, imaged during the buildup and decay stages in the early tens of nanoseconds, turns out to have a size of ~0.3 mm and a volume of ~0.02 mm(3). The coupling of direct emission data and broadband absorption measurements allowed us to retrieve peak values of electron temperature above 100,000 K and of an optical depth of the order of unity, under the assumptions of local thermodynamic equilibrium and a homogeneous kernel. The simultaneous occurrence of such temporal, spatial, and spectral features of the plasma kernel suggests its exploitation as a pulsed, bright, and broadband ultraviolet-visible light source.     

Multimodal spectral imaging of cells using a transmission diffraction grating on a light microscope

A multimodal methodology for spectral imaging of cells is presented. The spectral imaging setup uses a transmission diffraction grating on a light microscope to concurrently record spectral images of cells and cellular organelles by fluorescence, darkfield, brightfield, and differential interference contrast (DIC) spectral microscopy. Initially, the setup was applied for fluorescence spectral imaging of yeast and mammalian cells labeled with multiple fluorophores. Fluorescence signals originating from fluorescently labeled biomolecules in cells were collected through triple or single filter cubes, separated by the grating, and imaged using a charge-coupled device (CCD) camera. Cellular components such as nuclei, cytoskeleton, and mitochondria were spatially separated by the fluorescence spectra of the fluorophores present in them, providing detailed multi-colored spectral images of cells. Additionally, the grating-based spectral microscope enabled measurement of scattering and absorption spectra of unlabeled cells and stained tissue sections using darkfield and brightfield or DIC spectral microscopy, respectively. The presented spectral imaging methodology provides a readily affordable approach for multimodal spectral characterization of biological cells and other specimens.     

Phase retrieval of singular scalar light fields using a two-dimensional directional wavelet transform and a spatial carrier

We evaluate a method based on the two-dimensional directional wavelet transform and the introduction of a spatial carrier to retrieve optical phase distributions in singular scalar light fields. The performance of the proposed phase-retrieval method is compared with an approach based on Fourier transform. The advantages and limitations of the proposed method are discussed.