Opaque Disk Research Articles

Beam waist measurement for terahertz time-domain spectroscopy experiments

Classical masking aperture methods are found to be mostly inaccurate to determine the terahertz beam size in terahertz time-domain spectroscopy (TDS) experiments, owing to complex diffraction effects. Here, we present a simple and reliable method for measuring beam waists in terahertz TDS. It is based on the successive diffraction by an opaque disk followed by a small circular aperture.

GAS ABSORPTION IN THE KH 15D SYSTEM: FURTHER EVIDENCE FOR DUST SETTLING IN THE CIRCUMBINARY DISK

Na I D lines in the spectrum of the young binary KH 15D have been analyzed in detail. We find an excess absorption component that may be attributed to foreground interstellar absorption, and to gas possibly associated with the solids in the circumbinary disk. The derived column density is log N_NaI = 12.5 cm^-2, centered on a radial velocity that is consistent with the systemic velocity. Subtracting the likely contribution of the ISM leaves log N_NaI ~ 12.3 cm^-2. There is no detectable change in the gas column density across the "knife edge" formed by the opaque grain disk, indicating that the gas and solids have very different scale heights, with the solids being highly settled. Our data support a picture of this circumbinary disk as being composed of a very thin particulate grain layer composed of millimeter-sized or larger objects that are settled within whatever remaining gas may be present. This phase of disk evolution has been hypothesized to exist as a prelude to the formation of planetesimals through gravitational fragmentation, and is expected to be short-lived if much gas were still present in such a disk. Our analysis also reveals the presence of excess Na I emission relative to the comparison spectrum at the radial velocity of the currently visible star that plausibly arises within the magnetosphere of this still-accreting young star.

General formulation of digital in-line holography from correlation with a chirplet function

Digital in-line holography is revisited to propose a mathematical model that describes the recording-reconstruction process as a linear shift-invariant system with a pseudo-point spread function even when the images are out of the optimal plane in the sense of signal processing. A particular case is treated to show that the optimal plane is the best focus plane in the sense of optics. Next, an exact solution of the holographic reconstruction by correlation is given. By means of the previous results, we study the behavior of the result of the correlation function between the diffraction pattern function produced by an opaque disk and a chirplet function and between the diffraction pattern produced by a phase disk and the same chirplet function.

Directly recording diffraction phenomena in the time domain

The wave-field produced by a ~30 fs duration Ti:sapphire oscillator pulse behind a circular aperture and circular opaque disk is measured using the ultrashort-laser-pulse measurement technique, scanning SEA TADPOLE. The high spatial and temporal resolution of the measuring technique enables us to fully image the diffracted field behind the apertures and record the interference pattern produced by the so-called boundary diffraction wave pulses. Keywords: diffraction, interference

Drop size distribution measured by imaging: determination of the measurement volume by the calibration of the point spread function

Measurement of drop size distributions in a spray depends on the definition of the control volume for drop counting. For image-based techniques, this implies the definition of a depth-of-field (DOF) criterion. A sizing procedure based on an imaging model and associated with a calibration procedure is presented. Relations between image parameters and object properties are used to provide a measure of the size of the droplets, whatever the distance from the in-focus plane. A DOF criterion independent of the size of the drops and based on the determination of the width of the point spread function (PSF) is proposed. It allows to extend the measurement volume to defocused droplets and, due to the calibration of the PSF, to clearly define the depth of the measurement volume. Calibrated opaque discs, calibrated pinholes and an optical edge are used for this calibration. A comparison of the technique with a phase Doppler particle analyser and a laser diffraction granulometer is performed on an application to an industrial spray. Good agreement is found between the techniques when particular care is given to the sampling of droplets. The determination of the measurement volume is used to determine the drop concentration in the spray and the maximum drop concentration that imaging can support.

THE EVOLUTIONARY STATE OF THE MASSIVE INTERACTING BINARY BD+36°4063

We present a spectroscopic and photometric analysis of the remarkable massive binary system, BD +36 4063. We argue that the visible ON star is undergoing a rapid mass transfer episode that results in a thick and opaque disk that surrounds and renders invisible its massive companion. A comparison of the projected rotational velocity and the orbital semiamplitude of the visible star indicates a mass ratio near unity. Models for conservative mass transfer show that the equal mass state occurs at the point of minimum separation, and we argue that BD +36 4063 may represent the first system identified at this rapid and rare stage of massive binary evolution.

Algorithmic Aspects of Proportional Symbol Maps

Proportional symbol maps visualize numerical data associated with point locations by placing a scaled symbol—typically an opaque disk or square—at the corresponding point on a map. The area of each symbol is proportional to the numerical value associated with its location. Every visually meaningful proportional symbol map will contain at least some overlapping symbols. These need to be drawn in such a way that the user can still judge their relative sizes accurately. We identify two types of suitable drawings: physically realizable drawings and stacking drawings. For these we study the following two problems: Max-Min—maximize the minimum visible boundary length of each symbol—and Max-Total—maximize the total visible boundary length over all symbols. We show that both problems are NP-hard for physically realizable drawings. Max-Min can be solved in O(n 2log n) time for stacking drawings, which can be improved to O(nlog n) time when the input has certain properties. We also implemented several methods to compute stacking drawings: our solution to the Max-Min problem performs best on the data sets considered.

Rigorous description of holograms of particles illuminated by an astigmatic elliptical Gaussian beam

The digital holography is a non-intrusive optical metrology and well adapted for the measurement of the size and velocity field of particles in the spray of a fluid. The simplified model of an opaque disk is often used in the treatment of the diagrams and therefore the refraction and the third dimension diffraction of the particle are not taken into account. We present in this paper a rigorous description of the holographic diagrams and evaluate the effects of the refraction and the third dimension diffraction by comparison to the opaque disk model. It is found that the effects are important when the real part of the refractive index is near unity or the imaginary part is non zero but small.

Generation of femtosecond diffraction-compensated beam through an opaque disk

The generation of a femtosecond diffraction-compensated beam through an opaque disk is presented: 100 fs pulses are focused in the vicinity of an opaque disk. A bright beam surrounded by concentric rings is generated. The beam can be collimated by a lens. A non-diffracting femtosecond beam which conserves a 500microm large central spot over more than 1.7m is then observed, without any nonlinear mechanism. Our theoretical analysis predicts well the results.

Generation of Diffraction-Compensated Beams With a Pigtailed Laser Diode

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A new method to generate a diffraction-compensated beam at the output of a single-mode pigtailed laser diode is presented. It consists of positioning an opaque disk and a collimating lens at the output of the fiber. We could generate some beams of 900 <emphasis><formula formulatype="inline"><tex>$\mu$</tex> </formula></emphasis>m of diameter which conserves its dimension over more than 4 m. </para>

Improving Visibility of Diffraction Pattern with Pseudo-Thermal Light

We report an experimental observation of Poisson's spot with pseudo-thermal light. The experimental results show that the diffraction pattern disappears in the intensity distribution behind the opaque disc but emerges through both auto-correlation and cross-correlation intensity measurements. The auto-correlation scheme can take care of both better visibility and higher resolution of the diffraction pattern under the condition that the thermal light source has a larger spectral bandwidth.

Observation of superluminal wave-front propagation at the shadow area behind an opaque disk: erratum

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text M. Vasnetsov, "Observation of superluminal wave-front propagation at the shadow area behind an opaque disk: erratum," Opt. Lett. 33, 70-70 (2008) Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

Generation of nondiffracting beams through an opaque disk

A new method of generating nondiffracting beams is presented. It consists of focusing a Gaussian beam in the vicinity of an opaque disk. A beam is generated whose central peak is surrounded by a wide number of bright rings (approximately 250). After collimation, the beam propagates without changing the rings' radii, similar to a diffraction-free beam. The central peak can conserve its dimension over more than 5 m. The diameter of the central peak is adjusted by choosing the focal length of the collimating lens. Experimental results are well predicted by our theoretical developments that simulate exactly the paraxial diffraction.

Calibration of the Cloud Particle Imager Probes Using Calibration Beads and Ice Crystal Analogs: The Depth of Field

Abstract This paper explains and develops a correction algorithm for measurement of cloud particle size distributions with the Stratton Park Engineering Company, Inc., Cloud Particle Imager (CPI). Cloud particle sizes, when inferred from images taken with the CPI, will be oversized relative to their “true” size. Furthermore, particles will cease to be “accepted” in the image frame if they lie a distance greater than the depth of field from the object plane. By considering elements of the scalar theory for diffraction of light by an opaque circular disc, a calibration method is devised to overcome these two problems. The method reduces the error in inferring particle size from the CPI data and also enables the determination of the particles distance from the object plane and hence their depth of field. These two quantities are vital to enable quantitative measurements of cloud particle size distributions (histograms of particle size that are scaled to the total number concentration of particles) in the atmosphere with the CPI. By using both glass calibration beads and novel ice crystal analogs, these two problems for liquid drops and ice particles can be quantified. Analysis of the calibration method shows that 1) it reduces the oversizing of 15-μm beads (from 24.3 to 14.9 μm for the sample mean), 40-μm beads (from 50.0 to 41.4 μm for the sample mean), and 99.4-μm beads (from 103.7 to 99.8 μm for the sample mean); and 2) it accurately predicts the particles distance from the object plane (the relationship between measured and predicted distance shows strong positive correlation and gives an almost one-to-one relationship). Realistic ice crystal analogs were also used to assess the errors in sampling ice clouds and found that size and distance from the object plane could be accurately predicted for ice crystals by use of the particle roundness parameter (defined as the ratio of the projected area of the particle to the area of a circle with the same maximum length). While the results here are not directly applicable to every CPI, the methods are, as data taken from three separate CPIs fit the calibration model well (not shown).

Diffraction properties of opaque disks outside and inside a laser cavity

Diffraction of symmetrical Laguerre–Gauss TEMp0 beams incident on an opaque disk known as a stop is considered. The near- and far-field patterns are studied. Thanks to zero-field occluding, conversion from TEM10 beam to dark hollow beam can be achieved with better efficiency than from a TEM00 beam. It is shown that the fundamental mode of a laser cavity including a diaphragm and a stop can be TEM00- or TEM10-like in shape depending on their size. This result is interpreted from the new divergence hierarchy, which characterises the diffracted TEMp0 beams emerging from the stop.

Observation of superluminal wave-front propagation at the shadow area behind an opaque disk

We demonstrate in theory and experiment superluminal properties of optical field propagation in the shadow area behind an opaque disk (Poisson's spot). The wave front of the field in the shadow zone is initially delayed with respect to the unperturbed field. This phase shift gradually diminishes along the optical path and therefore ensures variation of the phase velocity of the field at the axis. A resulting excess of about cx10(-5) was detected.

Enhanced Longitudinal Magnetooptic Kerr Effect Contrast in Nanomagnetic Structures

We report on enhanced longitudinal magnetooptic Kerr effect signal contrast in thin-film nanomagnetic disks with in-plane magnetization when combined with dielectric layers that provide impedance matching to the structure and the underlying substrate. Kerr signals can increase by a factor of three, while substrate reflectance is almost completely suppressed. This leads to an increase in Kerr ellipticity relative to the background intensity and a subsequent improvement in the measured signal-to-noise ratio. Measurements using a beam focused on opaque 400-nm Ni disks yield contrast improvements of a factor of 8. Arrays of nanodisks demonstrate more complex behavior due to diffraction effects

General Relativistic Radiative Transfer: Emission from Accreting Black Holes in AGN

We present two covariant radiative transfer formulations, and apply them to calculate emission from relativistic accretion flows around black holes. The first formulation is for situations with only line-of-sight absorption and emission, while the second formulation is for situations where scattering is important. We use the first formulation to calculate emissions from opaque accretion disks and tori around black holes absorbed by high-velocity cloudlets. Our calculations show the importance of effects due to space-time curvature, relativistic motions of the emitters and absorbers, and external line-of-sight absorption. We find that external absorption tends to take away line fluxes at energies red-ward of the line-centre energy. In its presence, emission lines from geometrically thin opaque relativistic accretion disks may not have broad asymmetric double-peak profiles: in some situations the lines may appear to be narrow, sharp and blue-shifted. We also find that geometric effects are important for thick accretion disks (accretion tori). When viewed at high inclination angles, the inner surface of an accretion torus can be self-occulted. As the most highly red-shifted and blue-shifted emissions are blocked, emission lines from an opaque accretion torus would suffer less broadening and the line intensities are less boosted. These lines may have single-peak profiles, but their line centres are slightly red-shifted. We apply the radiative transfer formulation to calculate emission lines from optically thin and semi-opaque accretion tori, and generalise it to calculate continua emission, such as the reflection spectra, of accretion disks. The second convariant radiative transfer formulation, which is based on a moment method, is used to calculate the emissions from accretion tori with the opacity being dominated by electron scattering. We demonstrate that the formulation is applicable for a wide range of optical depths appropriate for accretion tori around black holes.

A Dusty Disk around GD 362, a White Dwarf with a Uniquely High Photospheric Metal Abundance

Eighteen years after an infrared excess was discovered associated with the white dwarf G29-38, we report ground-based measurements (JHKsKL'N') with millijansky-level sensitivity of GD 362 that show it to be a second single white dwarf with an infrared excess. As a first approximation, the excess around GD 362, which amounts to ~3% of the total stellar luminosity, can be explained by emission from a passive, flat, opaque dust disk that lies within the Roche radius of the white dwarf. The dust may have been produced by the tidal disruption of a large parent body such as an asteroid. Accretion from this circumstellar disk could account for the remarkably high abundance of metals in the star's photosphere.

Low‐Order Aberrations in Band‐limited Lyot Coronagraphs

We study the way Lyot coronagraphs with unapodized entrance pupils respond to small, low-order phase aberrations. This study is applicable to ground-based adaptive optics coronagraphs operating at 90% and higher Strehl ratios, as well as to some space-based coronagraphs with intrinsically higher Strehl ratio imaging. We utilize a second-order expansion of the monochromatic point-spread function (written as a power spectrum of a power series in the phase aberration over clear aperture) to derive analytical expressions for the response of a Lyot coronagraph (BLC) to small, low-order, phase aberrations. The BLC possesses a focal plane mask with an occulting spot whose opacity profile is a spatially band-limited function rather than a hard-edged, opaque disk. The BLC is, to first order, insensitive to tilt and astigmatism. Undersizing the stop in the reimaged pupil plane (the Lyot plane) following the focal plane mask can alleviate second-order effects of astigmatism, at the expense of system throughput and angular resolution. The optimal degree of such undersizing depends on individual instrument designs and goals. Our analytical work engenders physical insight and complements existing numerical work on this subject. Our methods can be extended to treat the passage of higher order aberrations through band-limited Lyot coronagraphs by using our polynomial decomposition or an analogous Fourier approach.