Showing papers on "Collimated light published in 2011"

Single-cycle terahertz pulses with amplitudes exceeding 1 MV/cm generated by optical rectification in LiNbO3

Abstract: Using the tilted-pump-pulse-front scheme, we generate single-cycle terahertz (THz) pulses by optical rectification of femtosecond laser pulses in LiNbO3. In our THz generation setup, the condition that the image of the grating coincides with the tilted-optical-pulse front is fulfilled to obtain optimal THz beam characteristics and pump-to-THz conversion efficiency. By using an uncooled microbolometer-array THz camera, it is found that the THz beam leaving the output face of the LN crystal can be regarded as a collimated rather than point source. The designed focusing geometry enables tight focus of the collimated THz beam with a spot size close to the diffraction limit, and the maximum THz electric field of 1.2 MV/cm is obtained.

Single-cycle THz pulses with amplitudes exceeding 1 MV/cm generated by optical rectification in LiNbO3

Abstract: Using the tilted-pulse-intensity-front scheme, we generate single-cycle terahertz (THz) pulses by optical rectification of femtosecond laser pulses in LiNbO3. In the THz generation setup, the condition that the image of the grating coincides with the tilted-optical-pulse front is fulfilled to obtain optimal THz beam characteristics and pump-to-THz conversion efficiency. The designed focusing geometry enables tight focus of the collimated THz beam with a spot size close to the diffraction limit, and the maximum THz electric field of 1.2 MV/cm is obtained.

X-ray transfocators: focusing devices based on compound refractive lenses

Abstract: This paper describes a tunable X-ray focusing apparatus, referred to as a transfocator, based on compound refractive lenses. By varying the number of lenses in the beam, the X-ray energy focused and the focal length can be varied continuously throughout a large range of energies and distances. The instrument can be used in both white and monochromatic beams to focus, pre-focus or collimate the beam. The transfocator can be used with other monochromators and/or other focusing elements, leading to significant increases in flux. Furthermore, the chromatic nature of the focusing means the transfocator suppresses harmonics and can also be used as an extremely high flux broad-band-pass monochromator. These devices have been installed in the first optics and second experimental hutches at the ID11 beamline at the ESRF.

Directional flat illuminators

Abstract: Disclosed is an optical valve or light valve for providing large area collimated illumination from localized light sources, and system and method thereof for 2D, 3D, and/or autosteroscopic displays An optical valve may include a stepped structure, in which the steps include separated extraction features which may be optically hidden to light propagating in a first direction Light propagating in a second direction may be refracted, diffracted, or reflected by the features to provide illumination beams exiting from the top surface of the optical valve Such controlled illumination may provide for efficient, multi-user autostereoscopic displays as well as improved 2D display functionality

Microphotonic parabolic light directors fabricated by two-photon lithography

Abstract: We have fabricated microphotonic parabolic light directors using two-photon lithography, thin-film processing, and aperture formation by focused ion beam lithography. Optical transmission measurements through upright parabolic directors 22 μm high and 10 μm in diameter exhibit strong beam directivity with a beam divergence of 5.6°, in reasonable agreement with ray-tracing and full-field electromagnetic simulations. The results indicate the suitability of microphotonic parabolic light directors for producing collimated beams for applications in advanced solar cell and light-emitting diode designs.

Optical antennas integrated with concentric ring gratings: electric field enhancement and directional radiation

Abstract: We describe a means for improving the coupling of illumination to, and the collection of scattered radiation from, an optical antenna. This is achieved by integrating optical antennas with concentric ring gratings. Electromagnetic simulations demonstrate that the ring grating improves the coupling to the antenna, even if the incident illumination is focused by an aplanatic lens such as a microscope objective. Dipole radiation from the center of the structure is well collimated. Various aspects of field enhancement and dipole radiation behavior are analyzed. We propose this device for Raman scattering enhancement.

Laser speckle reduction element

Abstract: Despeckle elements, laser beam homogenizers and methods for despeckling are provided. The despeckle element includes a transparent material having a first surface including a plural number of optical steps and a second surface having a plural number of microlenses. Each of the number of optical steps is in a one-to-one correspondence with at least one of the microlenses. One of the first surface and the second surface is configured to receive collimated light having a coherence length and a remaining one of the first surface and the second surface is configured to pass the collimated light separated into a plurality of beamlets corresponding to the number of microlenses. A height of each step of at least two of the optical steps is configured to produce an optical path difference of the collimated light longer than the coherence length.

Formation of collimated beams behind the woodpile photonic crystal

Abstract: We experimentally observe formation of narrow laser beams behind the woodpile photonic crystal, when the beam remains well collimated in free propagation behind the crystal. We show that the collimation depends on the input laser beam's focusing conditions, and we interpret theoretically the observed effect by calculating the spatial dispersion of propagation eigenmodes and by numerical simulation of paraxial propagation model.

Magnetically guided fast electrons in cylindrically compressed matter.

Abstract: Fast electrons produced by a 10 ps, 160 J laser pulse through laser-compressed plastic cylinders are studied experimentally and numerically in the context of fast ignition. K(alpha)-emission images reveal a collimated or scattered electron beam depending on the initial density and the compression timing. A numerical transport model shows that implosion-driven electrical resistivity gradients induce strong magnetic fields able to guide the electrons. The good agreement with measured beam sizes provides the first experimental evidence for fast-electron magnetic collimation in laser-compressed matter.

Mapping the x-ray emission region in a laser-plasma accelerator.

Abstract: The x-ray emission in laser-plasma accelerators can be a powerful tool to understand the physics of relativistic laser-plasma interaction. It is shown here that the mapping of betatron x-ray radiation can be obtained from the x-ray beam profile when an aperture mask is positioned just beyond the end of the emission region. The influence of the plasma density on the position and the longitudinal profile of the x-ray emission is investigated and compared to particle-in-cell simulations. The measurement of the x-ray emission position and length provides insight on the dynamics of the interaction, including the electron self-injection region, possible multiple injection, and the role of the electron beam driven wakefield. Remarkable advances in relativistic laser-plasma interaction using intense femtosecond lasers have led to the development of compact electron accelerators and x-ray sources with unique properties. These sources use the very high longitudinal electric field associated with plasma waves, excited in an under-dense plasma by a relativistic laser pulse, to trap and accelerate electrons to relativistic energies [1]. For laser and plasma parameters corresponding to the bubble or blowout regimes [2,3], the production of quasimonoenergetic electron beams was demonstrated [4]. During their acceleration, these electrons perform transverse (betatron) oscillations due to the transverse focusing force of the wakefields. This leads to the emission of bright and collimated femtosecond beams of x rays [5–7]. Such a compact and cost effective x-ray source could contribute to the development of emerging fields such as femtosecond x-ray imaging [8]. The x-ray emission can be exploited as well to provide information on the physics of laser-plasma interaction, such as electron tra-jectories in the bubble [9]. In this Letter, we demonstrate that by measuring the position and the longitudinal profile of the x-ray emission, one can determine important features of the interaction: laser pulse self-focusing, electron self-injection and possible multiple injection, as well as the role of the electron beam wakefield. The method relies on the observation, in the x-ray beam profile, of the shadow of an aperture mask adequately positioned just beyond the end of the emission region. The size of the shadow on the x-ray image permits us to determine the longitudinal position of the x-ray emission in the plasma, while the intensity gradient of the edge of the shadow yields the longitudinal profile of the emission. Because the x-ray emission position and length are closely connected to the electron injection position and the acceleration length, this measurement provides a unique insight into the interaction. Particle-in-cell (PIC) simulations are performed to analyze the experimental results. The experiment was conducted at Laboratoire d'Optique Appliquee with the ''Salle Jaune'' Ti:Sa laser system, which delivers 0.9 Joule on target with a full width at half maximum (FWHM) pulse duration of 35 fs and a linear polarization. The laser pulse was focused inside a capillary at 3:5 AE 1:5 mm from the entrance, with a f=18 spherical mirror. The FWHM focal spot size was 22 m, and using the measured intensity distribution in the focal plane we found a peak intensity of 3:2 Â 10 18 W A cm A2 , corresponding to a normalized amplitude of a 0 ¼ 1:2. The target was a capillary made of two Sapphire plates with half-cylindrical grooves of diameter d cap ¼ 210 m and a length of 15 mm, filled with hydrogen gas at pressures ranging from 50 to 500 mbar. The target acts as a steady-state-flow gas cell [10]. The capillary wall surface roughness is around 1 m, and therefore x rays cannot be reflected by the capillary wall. In addition, from our laser contrast of 10 8 , f number and capillary diameter, we estimate that the pedestal intensity on the capillary wall is smaller than 10 7 W A cm A2 , and thus no preplasma is formed before the x-ray pulse arrival. We measured either the x-ray beam profile using an x-ray CCD camera (2048 Â 2048 pixels, 13:5 Â 13:5 m 2), set at D ¼ 73:2 cm from the capillary exit and protected from the laser light by a 20 m Al filter, or the electron beam spectrum with a focusing-imaging spectrometer. In our experiment, the betatron emission had a divergence larger than the opening angle associated with the capillary exit, which acts as the aperture mask. The x-ray beam was thus clipped by the capillary [11]. Figures 1(a)–1(c) present a sample of shadows of diverse sizes corresponding to different longitudinal positions of the source, z X .

High luminance hybrid light guide plate for backlight module application

Abstract: We introduce a hybrid backlight module, which consists of a hybrid light guide plate (HLGP) and a brightness enhancement film (BEF). The HLGP comprises functions of a conventional light guide plate, a reflector, and a BEF. The HLGP allows one-dimensional rays to be collimated. We add a BEF above the HLGP, and let the crossed-dimension rays to be collimated. Comparing with the conventional edge-lit backlight module, the optical efficiency improves to 1.3-times and the on-axis luminance improves to 3.7-times by using the hybrid backlight module.

Grating coupler integrated photodiodes for plasmon resonance based sensing

Abstract: In this work, we demonstrate an integrated sensor combining a grating-coupled plasmon resonance surface with a planar photodiode. Plasmon enhanced transmission is employed as a sensitive refractive index (RI) sensing mechanism. Enhanced transmission of light is monitored via the integrated photodiode by tuning the angle of incidence of a collimated beam near the sharp plasmon resonance condition. Slight changes of the effective refractive index (RI) shift the resonance angle, resulting in a change in the photocurrent. Owing to the planar sensing mechanism, the design permits a high areal density of sensing spots. In the design, absence of holes that facilitate resonant transmission of light, allows an easy-to-implement fabrication procedure and relative insensitivity to fabrication errors. Theoretical and experimental results agree well. An equivalent long-term RI noise of 6.3 × 10−6 is obtained by using an 8 mW He–Ne laser, compared to a shot-noise limited theoretical sensitivity of 5.61 × 10−9. The device features full benefits of grating-coupled plasmon resonance, such as enhancement of sensitivity for non-zero azimuthal angle of incidence. Further sensitivity enhancement using balanced detection and optimal plasmon coupling conditions are discussed.

Compact head up display (HUD) for cockpits with constrained space envelopes

Abstract: A head up display includes an image source, collimating optics, and a combiner. The collimating optics are disposed between the combiner and the image source. The combiner receives collimated light from the collimating optics at an input and provides the collimated light to an output. The collimating light travels from the input to the output within the combiner by total internal refraction. An input diffraction grating is disposed in a first area and an output diffraction grating is disposed in a second area. The first area is smaller than the second area.

Beam combining light source

Abstract: The invention relates to sources of optical radiation wherein polarized radiation from first and second rows of light emitters is first collimated and combined into two combined beam using first and second rows of collimating and beam re-directing elements, respectively, and then polarization multiplexed to form a polarization-multiplexed output beam. In order to reduce the footprint, emitters of the first and second emitter rows are disposed in an interleaved, staggered arrangement, and the second row of collimating and beam re-directing elements is disposed in a space between the first emitter row and the first row of collimating and beam re-directing elements.

Limited-angle x-ray luminescence tomography: methodology and feasibility study.

Abstract: X-ray luminescence tomography (XLT) has recently been proposed as a new imaging modality for biological imaging applications. This modality utilizes phosphor nanoparticles which luminesce near-infrared light when excited by x-ray photons. The advantages of this modality are that it uniquely combines the high sensitivity of radioluminescent nanoparticles and the high spatial localization of collimated x-ray beams. Currently, XLT has been demonstrated using x-ray spatial encoding to resolve the imaging volume. However, there are applications where the x-ray excitation may be limited by geometry, where increased temporal resolution is desired, or where a lower dose is mandatory. This paper extends the utility of XLT to meet these requirements by incorporating a photon propagation model into the reconstruction algorithm in an x-ray limited-angle (LA) geometry. This enables such applications as image-guided surgery, where the ability to resolve lesions at depths of several centimeters can be the key to successful resection. The hybrid x-ray/diffuse optical model is first formulated and then demonstrated in a breast-sized phantom, simulating a breast lumpectomy geometry. Both numerical and experimental phantoms are tested, with lesion-simulating objects of various sizes and depths. Results show localization accuracy with median error of 2.2 mm, or 4% of object depth, for small 2–14 mm diameter lesions positioned from 1 to 4.5 cm in depth. This compares favorably with fluorescence optical imaging, which is not able to resolve such small objects at this depth. The recovered lesion size has lower size bias in the x-ray excitation direction than the optical direction, which is expected due to the increased optical scatter. However, the technique is shown to be quite invariant in recovered size with respect to depth, as the standard deviation is less than 2.5 mm. Sensitivity is a function of dose; radiological doses are found to provide sufficient recovery for µg ml−1 concentrations, while therapy dosages provide recovery for ng ml−1 concentrations. Experimental phantom results agree closely with the numerical results, with positional errors recovered within 8.6% of the effective depth for a 5 mm object, and within 5.2% of the depth for a 10 mm object. Object-size median error is within 2.3% and 2% for the 5 and 10 mm objects, respectively. For shallow-to-medium depth applications where optical and radio-emission imaging modalities are not ideal, such as in intra-operative procedures, LAXLT may be a useful tool to detect molecular signatures of disease.

Beam divergence and various collimators for holographic or stereoscopic displays

Abstract: The invention relates to a holographic display comprising a lighting device, a magnifying unit (VE) and a light modulator (SLM), wherein the lighting device comprises at least one light source and a collimation unit (LCU), wherein the collimation unit (LCU) is designed such that it collimates the light of the at least one light source and generates a light wave field of the light of the light source with a predeterminable plane wave spectrum, wherein the magnifying unit (VE) is arranged downstream of the collimation unit (LCU) in the light propagation direction, wherein the magnifying unit (VE) comprises a transmissive volume hologram (VH) that is arranged and designed such that, on the basis of a transmissive interaction of the light wave field with the volume hologram (VH), an anamorphic divergence of the light wave field can be achieved, and wherein the light modulator (SLM) is arranged in the light propagation direction either upstream or downstream of the anamorphic magnifying unit (VE).

A method to correct for stray light in telecentric optical-CT imaging of radiochromic dosimeters.

Abstract: Radiochromic plastic and gel materials have recently emerged which can yield 3D dose information over clinical volumes in high resolution. These dosimeters can provide a much more comprehensive verification of complex radiation therapy treatments than can be achieved by conventional planar and point dosimeters. To achieve full clinical potential, these dosimeters require a fast and accurate read-out technology. Broad-beam optical-computed tomography (optical-CT) systems have shown promise, but can be sensitive to stray light artifacts originating in the imaging chain. In this work we present and evaluate a method to correct for stray light artifacts by deconvolving a measured, spatially invariant, point spread function (PSF). The correction was developed for the DLOS (Duke large field-of-view optical-CT scanner) in conjunction with radiochromic PRESAGE® dosimeters. The PSF was constructed from a series of acquisitions of projection images of various sized apertures placed in the optical imaging chain. Images were acquired with a range of exposure times, and for a range of aperture sizes (0.2–11 mm). The PSF is investigated under a variety of conditions, and found to be robust and spatially invariant, key factors enabling the viability of the deconvolution approach. The spatial invariance and robustness of the PSF are facilitated by telecentric imaging, which produces a collimated light beam and removes stray light originating upstream of the imaging lens. The telecentric capability of the DLOS therefore represents a significant advantage, both in keeping stray light levels to a minimum and enabling viability of an accurate PSF deconvolution method to correct for the residual. The performance of the correction method was evaluated on projection images containing known optical-density variations, and also on known 3D dose distributions. The method is shown to accurately account for stray light on small field dosimetry with corrections up to 3% in magnitude shown here although corrections of >10% have been observed in extreme cases. The dominant source of stray light was found to be within the imaging lens. Correcting for stray light extended the dynamic range of the system from ~30 to ~60 dB. The correction should be used when measurements need to be accurate within 3%.

SPEAR — ToF neutron reflectometer at the Los Alamos Neutron Science Center

Abstract: This article discusses the Surface ProfilE Analysis Reflectometer (SPEAR), a vertical scattering geometry time-of-flight reflectometer, at the Los Alamos National Laboratory Lujan Neutron Scattering Center. SPEAR occupies flight path 9 and receives spallation neutrons from a polychromatic, pulsed (20Hz) source that pass through a liquid-hydrogen moderator at 20K coupled with a Be filter to shift their energy spectrum. The spallation neutrons are generated by bombarding a tungsten target with 800MeV protons obtained from an accelerator. The process produces an integrated neutron flux of ∼ 3.4×106 cm−2 s−1 at a proton current of 100μA. SPEAR employs choppers and frame overlap mirrors to obtain a neutron wavelength range of 4.5–16 A. SPEAR uses a single 200mm long 3He linear position-sensitive detector with ∼ 2 mm FWHM resolution for simultaneous studies of both specular and off-specular scattering. SPEAR’s moderated neutrons are collimated into a beam which impinges from above upon a level sample with an average angle of 0.9° to the horizontal, to facilitate air-liquid interface studies. In the vertical direction, the beam converges at the sample position. The neutrons can be further collimated to the desired divergence by finely slitting the beam using a set of two 10B4C slit packages. The instrument is ideally suited to study organic and inorganic thin films with total thicknesses between 5 and 3000 A in a variety of environments. Specifically designed sample chambers available at the instrument provide the opportunity to study biological systems at the solid-liquid interface. SPEAR’s unique experimental capabilities are demonstrated by specific examples in this article. Finally, an outlook for SPEAR and perspectives on future instrumentation are discussed.

Channel response measurements for diffuse non-line-of-sight (NLOS) optical communication links underwater

Abstract: Presented are experimental measurements comparing the frequency (i.e. - impulse) response (up to 1GHz) for an underwater optical communications channel as a function of water clarity using collimated or diffuse laser light. The measurement technique used is adapted from a previously reported method used for measuring the channel response of collimated laser beam [1,2]. These measurements establish the bandwidth limitations for non-line-of-sight (NLOS) optical links underwater as a function of their operating environment. Comparisons to direct line-of-sight links using collimated beams are also presented.

Lens design of LED searchlight of high brightness and distant spot

Abstract: The study related in this paper is the design of a ship-mounted LED of high brightness and distant spot. The freeform lens design obeying the edge ray principle and Snell’s law is presented first. Then, to fit the illumination requirement of the searchlight, we designed a freeform lens to collimate all the light rays coming from the LED. However, theoretical analysis proves that there is a critical angle for incident rays beyond which the rays cannot be collimated, and 55% is the light-efficiency limit for polymethyl methacrylate freeform lens. We then designed a combination of a freeform lens–coupled parabolic reflector that improved light efficiency to 70%. In this paper, the design of the freeform lens–coupled parabolic reflector is given in detail. In addition, tolerance analysis and the effect of manufacturing defect are presented.

Off-axis collimation optics

Abstract: A light funnel collimator has a central lens surface and a back reflecting surface, shaped to provide a wider background beam and a narrower hotspot beam within but off- center of the wider beam. One of the beams is on-axis of the collimator, and the other beam is off-axis. The reflector is at least partly asymmetrical relative to the axis, and provides or contributes to the off-axis beam.

Highly collimated electron beams from double-gate field emitter arrays with large collimation gate apertures

Abstract: Electron collimation in field emitter arrays with electron extraction gate and collimation gate electrodes is studied with the goal to develop a high-brightness high current cathode. Using metallic field emitter arrays prepared by the molding method, we fabricated a stacked double-gate device with the two gates differing in diameter by a process utilizing focused-ion beam milling. We measured the field-emission beam characteristics and demonstrated a reduction of the emission angle by a factor of 7.1±0.8 with minimal emission current decrease under collimating conditions, resulting in a current density increase by a factor of 13.9±1.0.

Active illumination scanning imager

Abstract: An active-illumination scanning imager (10) comprises a light source (14) for producing a light beam (16), an optical collimator (18) for collimating the light beam, a scanning mirror (20) for scanning the light beam through a scene (12) to be imaged, and a light detector (22) arranged with respect to the scanning mirror in such a way as to receive a fraction (24) of said light beam reflected from said scene, via the scanning mirror. The imager further includes an actuator (40) configured to position the light source and/or the optical collimator relative to each other and/or the light detector relative to the scanning mirror, and a controller (46) operatively connected to the actuator for controlling the positioning.

Two-dimensional, high-density optical connector

Abstract: An optical connector includes a two-dimensional array of lenses to couple optical signals between an optical integrated circuit and an optical fiber. The optical connector has a total-internal-reflection or mirror surface that redirects light between lenses at different surfaces of the optical connector. The lens arrays collimate light directed toward the reflection surface and focuses light received from the reflection surface. The two-dimensional array and prism allows for a low-profile, high-density optical connector based on free space optical light propagation.

Optical multiplexing apparatus and projector

Abstract: An object of the present invention is to provide an optical multiplexing apparatus and a projector that keep an optical system small, and easily adjust a number of light source units. The optical multiplexing apparatus has: a light source section that uses light source units to emit collimated beams; a first lens portion that converges the collimated beams which enter at different incident angles from each other to define converging positions; and a second lens portion that has focal points that correspond to the converging positions, respectively, wherein the first and second lens portions form an optical reduction system, and an optical axis of the second lens portion, which is directed to the corresponding focal point to one of the converging positions, extends along anther optical axis of the second lens position, which is directed to the corresponding focal point to another of the converging positions.

High-energy radiation monitoring based on radio-fluorogenic co-polymerization II: fixed fluorescent images of collimated x-ray beams using an RFCP gel.

Abstract: We have produced an optically clear, close to water-equivalent gel that is radio-fluorogenic, i.e. fluoresces in UV light after exposure to high-energy radiation. Its potential as a 2D and 3D dosimetric medium is demonstrated by fixed fluorescent images of the cross-section, track and intersection of collimated (10 × 10 or 5 × 5 mm2) 205 kVp x-ray beams. The images, produced by doses on the order of 10 Gy, are formed instantaneously and can be digitally recorded and scanned with a spatial resolution on the order of 0.1 mm. No loss of spatial resolution occurs on standing under ambient conditions for at least 3 days.

A high resolution Small Field Of View (SFOV) gamma camera: a columnar scintillator coated CCD imager for medical applications

Abstract: We describe a high resolution, small field of view (SFOV), Charge Coupled Device (CCD) based camera for imaging small volumes of radionuclide uptake in tissues. The Mini Gamma Ray Camera (MGRC) is a collimated, scintillator-coated, low cost, high performance imager using low noise CCDs. The prototype MGRC has a 600 μm thick layer of columnar CsI(Tl) and operates in photon counting mode using a thermoelectric cooler to achieve an operating temperature of - 10°C. Collimation was performed using a pin hole collimator. We have measured the spatial resolution, energy resolution and efficiency using a number of radioisotope sources including 140 keV gamma-rays from 99mTc in a specially designed phantom. We also describe our first imaging of a volunteer patient.