Large Beam Size Research Articles

Spatial effects in two-photon excitation transient absorption spectroscopy

We investigate the influence of the transverse mode of the pump beam and the geometry of the pump–probe configuration on the femtosecond two-photon excitation transient absorption (TA) spectroscopy. Strong modulation of the transient TA dynamics may result from the multi-peak distribution of the transverse mode of the pump beam, the large pump beam size, and the large separation angle between the pump and the probe beams, which is sensitive to the pulse length. Quantitative characterization of this kind of spatial effect is of general importance for the correct interpretation of the photophysical mechanisms that can be revealed by the two-photon TA measurements.

다중모드 광섬유 테이퍼를 이용한 모드 크기 변환기

다중모드 광섬유 테이퍼(taper)를 이용하여 레이저와 광섬유 혹은 두 개의 서로 다른 광섬유 사이의 효과적인 광결합을 위한 모드 크기 변환기를 제안하고 구현하였다. 이 소자의 입력 단은 다중 모드 구조이고 출력단은 단일 모드 구조이다. 소자의 구조 및 광원과 소자사이의 결합 조건이 결합 효율에 미치는 영향을 이론적으로 분석하였다. 이론적 결과는 제안된 소자가 가우시안 빔을 단일모드 광섬유로 거의 손실 없이 결합할 수 있는 것을 보였다. 제안된 소자를 두 개의 마이크로 토치가 부착된 장비를 이용해 제작하였다. 실험 결과 제작된 다중모드광섬유 테이퍼를 통해 <TEX>$50\;{\mu}m$</TEX> 빔 크기를 가지는 가우시안 빔을 단일모드 광섬유로 효과적으로 결합할 수 있는 것을 보였다. 소자의 삽입 손실은 1.3 dB였다. Based on the multimode fiber taper, a mode size converter for effective optical beam coupling between laser and optical fiber or between the two different optical fibers has been proposed and demonstrated. The device has a multimode input end and a single mode output end. The influence of various parameters, including device structure and launching conditions, on the coupling efficiency has been theoretically analyzed. The theoretical results revealed that the gaussian beam can be coupled into a single mode fiber without considerable insertion loss. The proposed multimode fiber taper has been fabricated using heating and pulling equipment incorporating two micro-torches. Experimental results showed that an optical beam with <TEX>$50\;{\mu}m$</TEX> of large beam size was effectively coupled into single mode fiber through the multimode fiber taper. The insertion loss of the device was 1.3 dB.

Analysis of C, N and O in aerosol collected on an organic backing using internal blank measurements and variable beam size

A method with low MDL (minimum detection limit) was developed for analysis of carbon (C), nitrogen (N) and oxygen (O) in aerosol samples collected on an organic backing. An accelerator-produced beam of protons interacting with the sample and the detection of elastically scattered protons (PESA; particle elastic scattering analysis) and emitted X-rays (PIXE; particle-induced X-ray emission) were the basic components of the setup. The method is based on measurement of internal blank concentrations, i.e. measurement of the blank concentration outside the aerosol deposit in each sample, and the use of two sizes of the beam in order to improve the MDL of the analysis. Large beam size covering the entire aerosol deposit was used to obtain quantitative analysis with PIXE. Small beam was used to obtain relative elemental concentrations with PESA and PIXE, which were transformed to absolute values by the aid of large-beam analysis. The small-size-beam served two purposes: to make internal blank measurements feasible and to improve the signal ratio of aerosol-deposit to backing. Compared with the traditional way of analysis, using a beam that is larger than the deposit and specially prepared blank samples, the new method reduced the MDL of C, N and O by a factor of 130, 70 and 90, respectively. The new method was applied to aerosol samples collected in the upper troposphere and the lowermost stratosphere from the CARIBIC platform. As far as the authors know, these measurements are the first quantitative determinations of C, N and O in the aerosol of this part of the atmosphere. The results show that these elements together with sulfur are major components of the aerosol.

Effect of Co2 Laser Radiation on Surface and Dyeing Properties of Synthetic Fibres

Chemical treatment methods are most often used nowadays for polymer surface modification; however, new technologies are now considered, especially in physical treatment methods, as laser technologies. Infra-red lasers, like CO2, appear to be advantageous due to their large beam size, high efficiency, easy operation, use of non-toxic gases and low cost. However, they have been less used than others for surface treatment of polymers, possibly due to the thermal damage effects caused by infra-red radiation. This shortcoming can be overcome by the use of pulsed lasers. CO2 pulsed laser can be considered as a non-contact and environmental-friendly treatment technique for the modification of the surface of polymers. Surface morphological modifications can be produced, resulting in changes in the physical and chemical properties of the materials, such as water absorption and dyeing. In this work, polyester and polyamide fabrics were treated with a CO2 pulsed laser and then dyed with different commercial dyes. Dyeing properties were investigated and changes in the dye ability of laser treated fibres are expected. Laser treatment created certain roughness on the fibre surface, resulting in an increase of the overall surface area and a subsequent enhancement of dye adsorption.

The effect of beam size on real-time determination of powder blend homogeneity by an online near infrared sensor

Online blend uniformity study was conducted with a near infrared (NIR) sensor and a simulated formulation consisting of acetaminophen and four excipients. Quantitative calibration models were developed and validated for the sensor and assay results were obtained in real time for acetaminophen and three excipients. Mechanical thief samples were also collected during the study. The samples were analyzed offline by a bench-top near infrared spectrometer and used as reference. Comparison of the online and offline data shows a significant difference in standard deviation for acetaminophen and excipients. R.S.D. data calculated from the real-time assay values for acetaminophen was 3.5–13.2-fold lower than those from the offline results. The cause for the discrepancy is believed to be the large beam size of the online sensor. A simple complete-random-mixture model was used to explain the discrepancy. It is concluded that beam size is an important factor in quantitative online blending uniformity studies.

Far-infrared detection limits - I. Sky confusion due to Galactic cirrus

Fluctuations in the brightness of the background radiation can lead to confusion with real point sources. This type of confusion with background emission is relevant when making infrared (IR) observations with relatively large beam sizes, since the amount of fluctuation tends to increase with the angular scale. To quantitively assess the effect of the background emission on the detection of point sources for current and future far-IR observations by space-borne missions such as Spitzer, ASTRO-F, Herschel and Space Infrared Telescope for Cosmology and Astrophysics (SPICA), we have extended the Galactic emission map to a higher level of angular resolution than that of the currently available data. Using this high-resolution map, we estimate the sky confusion noise owing to the emission from interstellar dust clouds or cirrus, based on fluctuation analysis and detailed photometry over realistically simulated images. We find that the confusion noise derived by simple fluctuation analysis agrees well with the results from realistic simulations. Although sky confusion noise becomes dominant in long wavelength bands (>100 µm) with 60‐90 cm aperture missions, it is expected to be two orders of magnitude lower for the next generation of space missions (with larger aperture sizes) such as Herschel and SPICA.

First polarimetry results of two candidate high-mass protostellar objects

We present m imaging polarimetry of two high-mass star forming regions – W48 and S152. Within these regions we have identified two candidate high-mass protostellar objects – W48W and S152SE. The submillimetre continuum emission from the candidate HMPOs is bright in comparison to the nearby regions. W48W is a cold dense source, with no radio or mid-infrared emission. S152SE has an IRAS source IRAS 22566 +5828 in the Southwestern part of the region, which appears in the mid-infrared m emission, but there is no radio emission. The m data shows another core within the region, in the Northeast. The polarimetry is ordered and the degree of polarisation is high over the candidate HMPOs – for W48W and for S152SE. Polarimetry results of this nature indicate a strong, ordered magnetic field threading the candidate HMPOs. The magnetic field direction in both S152SE and W48W is perpendicular to the direction of elongation of the cloud which would imply collapse along the field lines. Estimates of the magnetic field strength are derived using the Chandrasekhar & Fermi method. We calculate plane of the sky field strengths of ~0.7 mG for W48W and ~0.2 mG for S152SE. We discuss the drawbacks of using the Chandrasekhar & Fermi method with a large beam size. Mass-to-flux ratios have been calculated and both clouds are found to be roughly critical.

Muon beam ionization cooling in a linear quadrupole channel

In a scenario for either a Neutrino Factory or Muon Collider, the anticipated transverse beam emittance subsequent to capture and phase rotation is so large that it permits a relaxation of the requirements on beam spot size in the early stages relative to the final stages of ionization cooling. Staging the cooling process according to initial emittances, coupled with modest cooling factors, permits more optimal and efficient cooling channel designs and avoids much of the difficulty encountered with channels which attempt to maintain strong focusing (large, 300–500 mrad, divergences) across ultra-large momentum ranges (⩾±20% δ p/ p). Relaxation of spot size at the absorber, especially in the “precooling” stage, allowed development of an efficient transverse cooling channel based simply on a quadrupole FODO cell. This work describes the design of such a cooling channel and its application as an upstream stage of beam cooling. Being a linear channel with no bends, it serves to reduce the large transverse beam size delivered from muon-beam capture and bunching before entering more restricted optical structures such as transverse plus longitudinal cooling channels or accelerators.

Improvement in crystalline quality of Cd 1− xZn xTe( x=4%) crystals grown in graphite crucible

In this paper, we report improvement in the crystalline quality of CdZnTe (Zn=4%) crystals, with respect to etch pit density and full-width half-maximum (FWHM) of the X-ray rocking curve, grown in graphite crucible as compared to crystals grown in carbon-coated quartz ampoule. Uniform dislocation density distribution of the order of 2–3×10 4 cm −2 with no cellular structure and improvement in FWHM value (18–30 arcsec) with reduced splitting of peaks in X-ray rocking curves have been observed in the complete cadmium zinc telluride (CZT) wafers (30 mm×50 mm). We have used a larger X-ray beam (10 mm×1 mm) to study the crystalline quality of the complete CZT ingot. The effect of variation of beam size on X-ray rocking curve and its FWHM value indicates that the use of large beam size gives the true assessment of the spatial crystalline perfection of the grown CZT crystals. Although improvement in terms of crystalline perfection has been achieved in CZT material, the fall in optical transmission in the longer wave length spectral region and measured electrical properties in the annealed CZT samples point towards the incorporation of impurities in CZT during growth from the graphite crucible. Our results clearly indicate that overall improvement in the quality of CZT crystal is achievable with the use of ultra-high-purity graphite as the crucible.

A stationary target for the CERN-neutrino-factory

As production target for Neutrino Factories, free mercury jets with high axial velocity of about 20 m/ s are being studied. For the CERN-Neutrino-Factory proposal with a 4 MW beam power, but with a relatively large beam size at 2.2 GeV/c and pulsed at 50 Hz , maximum energy deposition densities of below 20 J/ g and average power densities of about 1 kW/ g are expected in the target. Therefore, a study has been made which discusses the feasibility and limits of a stationary target. It is proposed to use densely packed solid spheres of heavy material and with diameters in the millimeter range. These spheres are confined inside a Titanium container and cooled by an efficient water circuit or possibly by He-gas. Dynamic response, as pressure pulses and vibrations are greatly reduced in the small target granules due to relatively long beam bursts, each with a duration of 3.3 μs . The encouraging results achieved in this assessment justify to pursue further experimental tests, in particular of the cooling of the target, its lifetime under repetitive thermal cycles (4.3 Mio/ day) and of radiation damage, to validate this proposal. It is expected that the lifetime of this target will be adequate for the operation of the CERN-Neutrino-Factory, at least over its initial phase below 4 MW beam power.

The submillimeter wave astronomy satellite: instrument & mission description

The Submillimeter Wave Astronomy Satellite ( SWAS), launched in December 1998, is a NASA mission dedicated to the study of star formation through direct measurements of: (1) molecular cloud composition and chemistry; (2) the cooling mechanisms that facilitate cloud collapse; and, (3) the large-scale structure of the UV-illuminated cloud surfaces. To achieve these goals, SWAS is conducting pointed observations of dense ( n(H 2)>10 3 cm −3) molecular clouds throughout our Galaxy in either the ground-state or a low-lying transition of five astrophysically important species: H 2O, H 2 18O, O 2, CI, and 13CO. By observing these lines SWAS is: (1) testing long-standing theories that predict that these species are the dominant coolants of molecular clouds during the early stages of their collapse to form stars and planets; and (2) supplying previously missing information about the abundance of key species central to the chemical models of dense interstellar gas. SWAS carries two independent Schottky barrier diode mixers — passively cooled to ∼ 175 K — coupled to a 54 × 68-cm off-axis Cassegrain antenna. During its baseline three-year mission, SWAS is observing giant and dark cloud cores with the objective of detecting or setting a 3σ upper limit on the water and molecular oxygen abundance of 3 × 10 −6 (relative to H 2). In addition, advantage is being taken of SWAS's relatively large beamsize of 3.3 × 4.5 arcminutes at 553 GHz and 3.5 × 5.0 arcminutes at 490 GHz to obtain large-area (∼1° × 1°) maps of giant and dark clouds in the 13CO and CI lines. With the use of a 1.4 GHz bandwidth acousto-optical spectrometer, SWAS has the ability to simultaneously observe either the H 2O, O 2, CI, and 13CO lines or the H 2 18O, O 2, and CI lines with a velocity resolution ∼- 1 km s −1.

Quantitative Electron Diffraction from Small Volumes

Heterogeneous catalysts are extremely important to the chemical industry. The goal of much current research is to develop predictive models for real world catalysts in order to optimize and exploit new processes. The first step in any predictive modeling is to understand the systems that are currently in use. A key question regarding these materials is the distribution of atoms within the catalyst particle, i.e., are the materials equally distributed within the particle or is there clustering or shelling of the constituents. This is a more general question than just catalytic systems and has applicability to many small cluster problems where normally immiscible systems can be forced to mix or order when confined to small volumes.EXAFS analysis has provided useful information for these systems; due to the small scattering cross-section and large beam sizes of x-rays, the systems have only been studied as ensemble averages.

Direct measurement of emission current distribution of Spindt-type field emitters

A specially designed Spindt-type emitter array is used to study field emission characteristics of the Spindt-type field emitter. Each pixel in the array contains only one tip, is independently addressable, and is spaced 110 μm from the adjacent pixels. The pixels are electrically and optically distinguishable from each other. The emission current distribution of 2560 field emitters in an array is directly measured as a function of gate voltage. The ratio of maximum current to minimum current is as high as 100:1. There are two peaks observed in the current distribution. The position of the high current peak is strongly dependent on the gate voltage, while the position of the low current peak is less sensitive to the gate voltage. The variations in beam size and beam position of individual field emitters are also measured. These variations within individual emitters are partly responsible for large beam size observed in field emission displays with pixels containing hundreds of emitters. Change in the current distribution of individual emitters was recorded as the emitters were aging. Statistically, the emitters operating at high current age faster than those at low current.

Propagation of Laguerre-Bessel-Gaussian beams.

New exact solutions to the paraxial wave equation are obtained in the form of a product of Laguerre polynomials, Bessel functions, and Gaussian functions. In the limit of large Laguerre-Gaussian beam size, the Bessel factor dominates and the solution sets reduce to the modes of closed resonators, hollow metal waveguides, and dielectric waveguides. In the opposite limit the solutions reduce to Laguerre-Gaussian modes of open resonators and graded-index waveguides. These solutions are valid for electromagnetic waves traveling through free space, and they are valid for propagation through circularly symmetric optical systems representable by ABCD matrices as well. An interesting feature of the new solution set is the existence of three mode indices, where only two are required for an orthogonal expansion. As an example, Laguerre-Gaussian beam propagation through an optical system that contains a Bessel-like amplitude filter is discussed.

Correction of high gradient quadrupole harmonics with magnetic shims

Superconducting quadrupole magnets with 70 mm aperture and nominal field gradient of 215 T/m are being developed by the US-LHC Accelerator Project for the Interaction Regions of the Large Hadron Collider. Due to large beam size and orbit displacement in the final focusing triplet, these magnets are subject to stringent field quality requirements. For this reason, a correction scheme based on design calculations, fabrication issues and tests results involving magnetic shims.

Probabilistic Nonlocal Theory for Quasibrittle Fracture Initiation and Size Effect. II: Application

The nonlocal probabilistic theory developed in Part I is applied in numerical studies of plain concrete beams and is compared to the existing test data on the modulus of rupture. For normal size test beams, the deterministic theory is found to dominate and give adequate predictions for the mean. But the present probabilistic theory can further provide the standard deviation and the entire probability distribution (calculated via Latin hypercube sampling). For very large beam sizes, the statistical size effect dominates and the mean prediction approaches asymptotically the classical Weibull size effect. This is contrary to structures failing only after the formation of a large crack, for which the classical Weibull size effect is asymptotically approached for very small structure sizes. Comparison to the existing test data on the modulus of rupture demonstrates good agreement with both the measured means and the scatter breadth.

Production and propagation of Hermite–sinusoidal-Gaussian laser beams

Hermite-sinusoidal-Gaussian solutions to the wave equation have recently been obtained. In the limit of large Hermite-Gaussian beam size, the sinusoidal factors are dominant and reduce to the conventional modes of a rectangular waveguide. In the opposite limit the beams reduce to the familiar Hermite-Gaussian form. The propagation of these beams is examined in detail, and resonators are designed that will produce them. As an example, a special resonator is designed to produce hyperbolic-sine-Gaussian beams. This ring resonator contains a hyperbolic-cosine-Gaussian apodized aperture. The beam mode has finite energy and is perturbation stable.

Improvements to the polarised-neutron reflectometer CRISP

The reflectometery CRISP at ISIS is being upgraded to expand its capabilities in the polarised-beam mode. The polarisers are being replaced by FeCoV: TiN x supermirrors to give a higher incident flux and a two-times larger dynamic range. The flippers have also been redesigned to give higher flipping efficiencies over larger beam sizes and wavelength ranges. The improvements are described and compared to the current instrument performance.

Sinusoidal-Gaussian beams in complex optical systems

Sinusoidal-Gaussian beam solutions are derived for the propagation of electromagnetic waves in free space and in media having at most quadratic transverse variations of the index of refraction and the gain or loss. The resulting expressions are also valid for propagation through other real and complex lens elements and systems that can be represented in terms of complex beam matrices. The solutions are in the form of sinusoidal functions of complex argument times a conventional Gaussian beam factor. In the limit of large Gaussian beam size, the sine and cosine factors of the beams are dominant and reduce to the conventional modes of a rectangular waveguide. In the opposite limit the beams reduce to the familiar fundamental Gaussian form. Alternate hyperbolic-sinusoidal-Gaussian beam solutions are also found.

A Search for Millimetric Emission from Gamma‐Ray Bursts

We have used the 2- year Differential Microwave Radiometer data from the COsmic Background Explorer (COBE) satellite to systematically search for millimetric (31 - 90 GHz) emission from the Gamma Ray Bursts (GRBs) in the Burst And Transient Source Experiment (BATSE) GRB 3B catalog. The large beamsize of the COBE instrument (7 degs FWHM) allows for an efficient search of the large GRB positional error boxes, although it also means that fluxes from (point source) GRB objects will be somewhat diluted. A likelihood analysis has been used to look for a change in the level of millimetric emission from the locations of 81 GRB events during the first two years (1990 & 1991) of the COBE mission. The likelihood analysis determined that we did not find any significant millimetric signal before or after the occurance of the GRB. We find 95% confidence level upper limits of 175, 192 and 645 Jy or, in terms of fluxes, of 9.6, 16.3 and 54.8 10^{-13} erg/cm^2/s, respectively at 31, 53 and 90 GHz. We also look separately at different classes of GRBs, including a study of the top ten (in peak flux) GRBs, the short burst and long burst subsets, finding similar upper limits. While these limits may be somewhat higher than one would like, we estimate that using this technique with future planned missions could push these limits down to \sim 1 mJy.