Spiral Spectrum Research Articles

Partial angular coherence and the angular Schmidt spectrum of entangled two-photon fields

We study partially coherent fields that have a coherent-mode representation in the orbital-angular-momentum-mode basis. For such fields, we introduce the concepts of the angular coherence function and the coherence angle. Such fields are naturally produced by the process of parametric down-conversion---a second-order nonlinear optical process in which a pump photon breaks up into two entangled photons, known as the signal and idler photons. We show that the angular coherence functions of the signal and idler fields are directly related to the angular Schmidt (spiral) spectrum of the down-converted two-photon field and thus that the angular Schmidt spectrum can be measured directly by measuring the angular coherence function of either the signal or the idler field, without requiring coincidence detection.

Spiral spectrum of Laguerre-Gaussian beams propagating in turbulent atmosphere

An analysis of the propagation of Laguerre-Gaussian beam in fluctuation turbulent atmosphere is performed. Under the Rytov approximation, the integral expression of the spiral spectrum of the beam at the receiver aperture is obtained. It is found that the atmospheric turbulence will induce the spread of the spiral spectrum. With increasing topological charge, propagation distance, refractive index structure constant and radius of receiver aperture, the spectrum spread becomes more serious. It is also shown that the dimensionless variance describing the degree of the spread is a quadratic function of the refractive index structure constant and the radius of receiver aperture respectively, while it is an 11th order function of the topological charge and a 6th order function of the propagation distance. The analytical expression of the spiral spectrum is derived by taking account of the simulation results that both radial index and waist radius have little effect on the spiral spectrum.

拉盖尔高斯光束经单缝后的光强分布和螺旋谱

拉盖尔高斯光束经单缝后的光强分布和螺旋谱

Angular momentum decomposition of nonparaxial light beams

The total angular momentum per unit length of a general non-paraxial beam is decomposed into an orbital component associated with the spiral spectrum at the far field and a component concerning the balance between right- and left-handed circular-polarization content of the angular spectrum. Expressions for the linear momentum and energy per unit length are also provided. The well-known division into orbital and spin components is shown to be recovered in the paraxial limit.

Measurement of the Spiral Spectrum of Entangled Two-Photon States

We implement an interferometric method to measure the orbital angular momentum (OAM) spectrum of photon pairs generated by spontaneous parametric down-conversion. In contrast with previous experiments, which were all limited by the modal capacity of the detection system, our method operates on the entire down-conversion cone and reveals the complete distribution of the generated OAM. In this geometry, new features can be studied. We show that the phase-matching conditions can be used as a tool to enhance the azimuthal Schmidt number and to flatten the spectral profile, allowing the efficient production of high-quality multidimensional entangled states.

Effect of Coma Aberration on Orbital Angular Momentum Spectrum of Vortex Beams

Spiral spectra of vortex beams with coma aberration are studied. It is shown that the orbital angular momentum (OAM) states of vortex beams with coma aberration are different from those aberration-free vortex beams. Spiral spectra of beams with coma aberration are spreading. It is found that in the presence of coma aberration, the vortex beams contain not only the original OAM component but also other components. A larger coma aberration coefficient and/or a larger beam waist will lead to a wider spreading of the spiral spectrum. The results may have potential applications in information encoding and transmittance.

Influences of misaligned optical beam carrying orbital angular momentum on the information transfer

An analysis of the orbital angular momentum of an optical beam misaligned with respect to a receiver during propagation in the information transfer is investigated. The analytical expressions of Laguerre-Gaussian beams laterally displaced， angularly deflected and simultaneously misaligned in both ways are derived, and the features of the spiral spectrum in these cases are studied. It is shown that both parallel displacement and angular tilt will lead to the spread of the spiral spectrum, which is independent of the misalign direction. The larger the misalign, the more serious the spectrum spread. However, with the increasing of the propagation distance, the degree of the spread for the laterally displacement is decreasing gradually, while that for the angular tilt is increasing. In the angular tilt case, the spread of the spiral spectrum always keeps symmetric during propagation, while in the case of lateral displacement, the spiral spectrum is asymmetric, and goes to a stable state. The spiral spectrum is also investigated for the case with simulataneous lateral misalignment and angular tilt, and we conclude that the effects of both misaligns on the spectrum are related with one another.

Overall second-order parametric characterization of light beams propagating through spiral phase elements

In terms of the spiral spectrum of the light beams, second-order irradiance moments of partially polarized, partially coherent beams are considered. We investigate the effects generated by spiral phase elements on the beam size, divergence and waist-plane position of the field. Attention is also focused on the beam quality parameter. These results are applied to several types of depleted-center (doughnut-shape) beams.

Realizing high density optical data storage by using orbital angular momentum of light beam

The orbital angular momentum (OAM) states can be used to construct a high-dimensional Hilbert space, which implies higher data density when applied in data storage. This work proposes a method to realize high density optical data storage by using OAM of light beam, with the encoded area illuminated by a light beam with certain OAM resulting in distribution of power in the OAM states (the spiral spectrum or OAM spectrum), and detection of the power distribution conducing to its decoding. A quadrant staircase structure (QSS) used as an encoded element can realize 57 different states, and it is expected to enhance the data density 6 times as high as the conventional method.

Curvature effects on spiral spectra: Generation of point eigenvalues near branch points

It is shown that spiral waves may possess many isolated point eigenvalues that appear near branch points of the linear dispersion relation. These eigenvalues are created by the same mechanism that leads to infinitely many bound states for selfadjoint Schrödinger operators with sufficiently weakly decaying long-range potentials. For spirals, the weak decay of the potential is due to the curvature effects on the profile of the spiral in an intermediate spatial range that separates the spiral core from the far field.

The spatial shape of entangled photon states generated in non-collinear, walking parametric downconversion

We address the spatial shape of entangled photon states generated in spontaneous parametric downconversion in non-collinear geometries, when the interacting waves exhibit Poynting vector walk-off. We discuss how such shape depends on the interplay between the state ellipticity caused by the non-collinear geometry, and the Poynting vector walk-off. In particular, we study the impact of the new features exhibited by walking entangled photons on the implementation of quantum protocols based on the spiral spectra of the entangled photons.

Digital spiral imaging

A major application of optics is imaging all types of structural, physical, chemical and biological features of matter. Techniques based on most known properties of light have been developed over the years to remotely acquire information about such features. They include the spin angular momentum, encoded in the polarization, but not yet the orbital angular momentum encoded in its spiral spectrum. Here we put forward the potential of such spiral spectra. In particular, we use several canonical examples to show how the orbital angular momentum spectra of a light beam can be used to image a variety of intrinsic and extrinsic properties encoded, e.g., in phase and amplitude gradients, dislocations or delays.

Star formation in early-type galaxies in the Coma cluster

view Abstract Citations (189) References (38) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Star Formation in Early-Type Galaxies in the Coma Cluster Caldwell, Nelson ; Rose, James A. ; Sharples, Ray M. ; Ellis, Richard S. ; Bower, Richard G. Abstract We have obtained multifiber spectra of 125 early-type galaxies in the Coma cluster of galaxies, using the Hydra bench spectrograph on the 4 m telescope at Kitt Peak. The sample selection was based on both color and morphology, care being taken to exclude intermediate and late-type spirals. Surprisingly, a large fraction (about 1/3) of all galaxies in a field located approximately 40' SW of the center of the cluster exhibits enhanced Balmer absorption lines or emission lines, indicative of recent star formation or nuclear activity. In contrast, very few such galaxies are found in the central field of the cluster Those galaxies with abnormal spectra cover a range in luminosity down to B ~ 19 (M_B_ ~ -15). Furthermore, the bulk of the abnormal-spectrum galaxies are located near a secondary peak in the cluster x-ray emission found by Briel et al. [A&A, 259, L31 (1992)] and by Watt et al. [MNRAS, 258,738(1992)1. We argue by detailed comparisons with Kennicutt's [ApJS,79,255(1992a)] sequence of normal spiral galaxy spectra that the anomalous spectral features cannot arise from morphological misindentifications. Specifically, the abnormal spectra are fundamentally different, with [O II] emission lines too weak for the enhanced Balmer absorption. The spectra are, however, remarkably similar to the red Hδ-strong galaxies studied by Couch & Sharples [MNRAS, 229,423(1987)] in clusters at redshift z ~ 0.3 Hence it appears that an environmental process similar to that frequently found in clusters at moderate redshift is occurring at the present epoch in the outskirts of the Coma cluster. Publication: The Astronomical Journal Pub Date: August 1993 DOI: 10.1086/116656 Bibcode: 1993AJ....106..473C Keywords: Absorption Spectra; Astronomical Spectroscopy; Spatial Distribution; Spiral Galaxies; Star Clusters; Star Formation; Balmer Series; Coma; Red Shift; Astrophysics; GALAXIES: CLUSTERS: INDIVIDUAL: COMA CLUSTER; GALAXIES: STELLAR CONTENT full text sources ADS | data products SIMBAD (130) NED (127)

Spiral audio spectrum display system

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Spiral structures and spectra in two-dimensional turbulence

Saffman argues that in decaying two-dimensional turbulence approximate dis-continuities of vorticity will form, and the energy spectrum will fall off as k−4 Saffman assumes that these discontinuities are well separated; in this paper, we examine how accumulation points of such discontinuities may give an energy spectrum of between k−4 and k−3. In particular we examine the energy spectra of spiral structures which form round the coherent vortices that are observed in numerical simulations of decaying two-dimensional turbulence. If the filaments of the spiral are assumed to be passively advected, the instantaneous energy spectrum has a range. Thus we come some way to reconciling the argument of Saffman and the k−3 energy spectrum predicted by models of quasi-equilibrium two-dimensional turbulence based on a cascade of enstrophy in Fourier space.