Exit Face Research Articles

General analysis of slab lasers using geometrical optics

A thorough and general geometrical optics analysis of a slab-shaped laser gain medium is presented. The length and thickness ratio is critical if one is to achieve the maximum utilization of absorbed pump power by the laser light in such a medium; e.g., the fill factor inside the slab is to be maximized. We point out that the conditions for a fill factor equal to 1, laser light entering and exiting parallel to the length of the slab, and Brewster angle incidence on the entrance and exit faces cannot all be satisfied at the same time. Deformed slabs are also studied. Deformation along the width direction of the largest surfaces is shown to significantly reduce the fill factor that is possible.

Assessment method for photo-induced waveguides

A method to probe the guiding characteristics of waveguides formed in real-time is proposed and evaluated. It is based on the analysis of the time dependent light distribution observed at the exit face of the waveguide while progressively altering its index profile and probed by a large diameter optical beam. A beam propagation method is used to model the observed dynamics. The technique is applied to retrieve the properties of soliton-induced waveguides.

Infrared broadband 50%-50% beam splitters for s-polarized light

Prisms and slabs made of high-refractive-index materials such as ZnSe, Ge, and Si can be designed as broadband, 50%-50%, beam splitters (BSs) for obliquely incident s- polarized light in the near- and mid-IR. The beam-splitting face of the prism or slab is uncoated, while the exit face is antireflection coated. The split beams travel in orthogonal directions when light is incident at the Brewster angle. A novel design is also described that uses Brewster-angle reflection at the SiO2-Si interface to achieve a 50%-50%s-polarization BS over the 1.2-3.5 microm spectral range. Such s-polarization BSs are particularly suited for interferometry and holography.

Metamaterial Covers Over a Small Aperture

Recently, there has been an increased interest in the problem of wave transmission through sub-wavelength apertures, following successful experimental demonstration by several groups for enhancing optical power transmission through nano-scale holes in metallic screens due to properly designed periodic corrugation. Oliner, Jackson, and their co-workers explained and justified this phenomenon as the result of the excitation of the leaky waves supported by the corrugated screen. Here we discuss in detail the mechanism and analysis for another setup we have recently proposed, in which metamaterial layers with special parameters may be utilized as covers over a single sub-wavelength aperture in a perfectly electric conducting (PEC) flat screen in order to increase the wave transmission through this aperture, and we provide a detailed physical insights and analytical explanation for this aperture setup that may lead to similar, potentially even more pronounced effects when the proper metamaterial layers are used in the entrance and the exit face of the hole in the flat PEC screen with no corrugation. Some numerical results confirming this theory are presented and discussed. We also investigate the sensitivity of the transmission enhancement to the geometrical and electromagnetic parameters of this structure.

Monte Carlo and Lambertian light guide models of the light output from scintillation crystals at megavoltage energies

A new model of the light output from single-crystal scintillators in megavoltage energy x-ray beams has been developed, based on the concept of a Lambertian light guide model (LLG). This was evaluated in comparison with a Monte Carlo (MC) model of optical photon transport, previously developed and reported in the literature, which was used as a gold standard. The LLG model was developed to enable optimization of scintillator detector design. In both models the dose deposition and light propagation were decoupled, the scintillators were cuboids, split into a series of cells as a function of depth, with Lambertian side and entrance faces, and a specular exit face. The signal in a sensor placed 1 and 1000 mm beyond the exit face was calculated. Cesium iodide (CSI) crystals of 1.5 and 3 mm square cross section and 1, 5, and 10 mm depth were modeled. Both models were also used to determine detector signal and optical gain factor as a function of CsI scintillator thickness, from 2 to 10 mm. Results showed a variation in light output with position of dose deposition of a factor of up to approximately 5, for long, thin scintillators (such as 10 X 1.5 x 1.5 mm3). For short, fat scintillators (such as 1 X 3 X 3 mm3) the light output was more uniform with depth. MC and LLG generally agreed to within 5%. Results for a sensor distance of 1 mm showed an increase in light output the closer the light originates to the exit face, while a distance of 1000 mm showed a decrease in light output the closer the light originates to the exit face. For a sensor distance of 1 mm, the ratio of signal for a 10 mm scintillator to that for a 2 mm scintillator was 1.98, whereas for the 1000 mm distance the ratio was 3.00. The ratio of quantum efficiency (QE) between 10 and 2 mm thicknesses was 4.62. We conclude that these models may be used for detector optimization, with the light guide model suitable for parametric study.

Projected potential profiles across interfaces obtained by reconstructing the exit face wave function from through focal series

An iterative method for reconstructing the exit face wave function from a through focal series of transmission electron microscopy image line profiles across an interface is presented. Apart from high-resolution images recorded with small changes in defocus, this method works also well for a large defocus range as used for Fresnel imaging. Using the phase-object approximation the projected electrostatic as well as the absorptive potential profiles across an interface are determined from this exit face wave function. A new experimental image alignment procedure was developed in order to align images with large relative defocus shift. The performance of this procedure is shown to be superior to other image alignment procedures existing in the literature. The reconstruction method is applied to both simulated and experimental images.

Projected Potential Profiles across Intergranular Glassy Films

Projected electrostatic and absorptive potential profiles across intergranular glassy films (IGFs) and interfaces between grains and glassy pockets in silicon nitride ceramics were obtained by reconstructing the electron exit face wave function from a series of defocused TEM images. The phase-object approximation (POA) was used for extracting the projected potential from the complex-valued exit-face wave function. The electrostatic as well as the absorptive potentials were scaled with respect to vacuum. For La2O3-MgO doped Si3N4, the potential profiles across the grain/glassy pocket interface and the IGF were observed to be very similar and give very strong evidence for the existence of a space charge layer at the interface.

Modeling porosity reductions caused by mineral fouling in continuous-wall permeable reactive barriers

A study was conducted to assess key factors to include when modeling porosity reductions caused by mineral fouling in permeable reactive barriers (PRBs) containing granular zero valent iron. The public domain codes MODFLOW and RT3D were used and a geochemical algorithm was developed for RT3D to simulate geochemical reactions occurring in PRBs. Results of simulations conducted with the model show that the largest porosity reductions occur between the entrance and mid-plane of the PRB as a result of precipitation of carbonate minerals and that smaller porosity reductions occur between the mid-plane and exit face due to precipitation of ferrous hydroxide. These findings are consistent with field and laboratory observations, as well as modeling predictions made by others. Parametric studies were conducted to identify the most important variables to include in a model evaluating porosity reduction. These studies showed that three minerals (CaCO 3, FeCO 3, and Fe(OH) 2 (am)) account for more than 99% of the porosity reductions that were predicted. The porosity reduction is sensitive to influent concentrations of HCO 3 −, Ca 2+, CO 3 2−, and dissolved oxygen, the anaerobic iron corrosion rate, and the rates of CaCO 3 and FeCO 3 formation. The predictions also show that porosity reductions in PRBs can be spatially variable and mineral forming ions penetrate deeper into the PRB as a result of flow heterogeneities, which reflects the balance between the rate of mass transport and geochemical reaction rates. Level of aquifer heterogeneity and the contrast in hydraulic conductivity between the aquifer and PRB are the most important hydraulic variables affecting porosity reduction. Spatial continuity of aquifer hydraulic conductivity is less significant.

Enhanced transmission of light through a circularly structured aperture

Using the differential theory of light diffraction by finite cylindrical objects, we study light transmission through a small circular aperture in a metallic screen with concentric corrugation around the nanohole. Poynting vector maps in the region below the screen show that the field enhancement compared with an unstructured aperture is obtained with corrugation lying on the entrance face of the screen. Corrugation on the exit face leads to a more directional radiation close to the normal to the screen. The spectral dependence of the transmission shows a sharp maximum linked with surface plasmon excitation.

High-precision measurement of azimuthal rotation of liquid crystals on solid substrates

A high-accuracy optical method for the determination of the rotation of the liquid-crystal director on solid substrates is reported. The measurements involve the use of a photoelastic modulator in conjunction with a white light source. The detection of the first and second harmonics of the light signal allows for the determination both of the director position and the twist deformation at the exit face of the cell. The method is illustrated by experiments on the liquid crystal penthylcyanobiphenyl aligned by polyimide layers. The importance of director gliding is pointed out as well.

Single-layer-coated beam splitters for the division-of-amplitude photopolarimeter

A design procedure is presented for a near-optimal, single-layer-coated prism beam splitter that serves as the key optical element of the division-of-amplitude photopolarimeter (DOAP). For given film and substrate refractive indices, the angle of incidence and film thickness are selected such that the ellipsometric differential phase shifts in reflection and transmission deltar, and deltat, differ by +/- pi/2, and the normalized determinant of the instrument matrix is maximized. The best results are obtained by using high-index films on low-index substrates. This is illustrated by examples of ZnS and GaP films on silica prisms in the visible and Si, Ge, and PbTe films on Irtran 1 substrates in the infrared. A 16 degrees Si-prism DOAP beam splitter at the 1.55-microm lightwave-communications wavelength is also presented. It uses a 163-nm SiO2 coating on the entrance face to satisfy the optimum delta condition at 73 degrees incidence, and the determinant of the instrument matrix is 78.23% of its theoretical maximum. The exit face of the Si prism is antireflection coated with a 208-nm Si3N4 film.

Redoubled effect of a neutron spin rotation in deformed noncentrosymmetric crystal for the Bragg diffraction scheme

A doubled effect of a neutron spin rotation in a noncentrosymmetric quartz crystal for the Bragg reflected neutrons from the deformed exit crystal side is first observed. The effect arises due to a neutron Schwinger interaction with the crystal and depends on the value of the crystal deformation near its back exit face. The electric field acting on a neutron in the quartz crystal is about ∼108 V/cm. This field affects the neutron during the whole time of its passage through the crystal, both there and back. This time is limited only by the available size of the crystal (14 and 27 cm in our case) or the neutron absorption length. Observation of such effects gives a real perspective on the essential improvement of the scheme and sensitivity of the experiment in the search for a neutron electric dipole moment (EDM) by the crystal-diffraction technique. Moreover, the presented experimental scheme can be applied for a neutron with an energy close to the P wave resonance to search for the T-odd part of a neutron-nuclei interaction, for example, because of relatively low requirements of crystal quality.

Consistent unconfined contaminated disposal facilities dike tidal flow and transport model

A two-dimensional time dependent coupled free-boundary numerical model for tidally-induced flow and transport through the dike of a confined disposal facility (CDF) for contaminated sediments is presented in this paper. The model simulations demonstrate the major effect that tidal fluctuations have on the dynamics of the discharge flux. Sensitivity analyses demonstrate that tidal fluctuation hastens the rate of plume migration near the exit face because of the relatively high advective and dispersive fluxes induced by tides. With or without tides, the rate of contaminant migration increases with higher CDF area hydraulic gradient.

Residual Stress Creep Relaxation Around Cold Expanded Holes in an Aluminium Alloy

Residual stress distributions were measured in aluminum-alloy 2650 disks after cold expansion and after exposure to creep at 150°C for various times. Strains measured as a result of boring out the hole after cold expansion were found to exhibit significant angular variations around the hole. However, after exposure to 150°C residual strains and stresses showed less angular variation. To take account of the angular variation, the Garcia-Sachs technique for measuring residual stresses was employed. Surface residual stresses on the entrance and exit faces of the disks were measured using the x-ray method. The most significant relaxation of the residual stresses occurred within the first 100 h. Two simple models were used to predict residual stress relaxation, one using a uniaxial creep relaxation model and a second using tensile and isochronous stress-strain curves. A good comparison between predictions and experiments was observed.

Large Raman gain and nonlinear phase shifts in high-purity As_2Se_3 chalcogenide fibers

Third-order Kerr nonlinearities and Raman gain are studied experimentally in high-purity As2Se3 optical fibers for wavelengths near 1.55 μm. Kerr nonlinear coefficients are measured to be nearly 1000 times higher than those for silica fibers. In pulsed mode, nonlinear phase shifts near 1.2-π rad are measured in fibers only 85 cm long with peak pulse powers near 3 W. However, there are nonlinear losses near 20% for nonlinear phase shifts near π. By use of a cw optical pump, large Raman gains nearly 800 times that of silica were measured. In the cw case there were losses in the form of index gratings formed from standing waves at the exit face of the fiber. Discrete Raman amplifiers and optical regenerators are discussed as possible applications.

A novel method of cold expansion which creates near‐uniform compressive tangential residual stress around a fastener hole

ABSTRACTA recognized way of improving the fatigue resistance of a fastener hole is to introduce compressive tangential residual stress around it. This can be achieved by using a cold expansion method in which an oversized pin or ball is forced through the hole to produce a local plastic region surrounded by an elastic one. Once the pin or ball is removed allowing the elastic region to spring back it results in compressive tangential residual stress around the hole. In practise, however, it is found that such a cold expansion method creates a non‐uniform residual stress distribution through the plate thickness and even tensile residual stress can be created at the entrance and exit faces. In this paper a new method of cold expansion is proposed. It uses a tapered pin with a mating tapered split sleeve and creates an almost uniform compressive residual stress around the hole as shown by FE method. Also, fatigue tests were carried out to verify that the method does significantly improve fatigue life. Finally the tangential residual stress distribution and fatigue life improvement of this new method were compared with those of a well‐established cold expansion method and it was shown that the new method is more efficient in improving fatigue life.

Measurement of changes in atmospheric Ar/N2 ratio using a rapid-switching, single-capillary mass spectrometer system

The atmospheric Ar/N2 ratio is expected to undergo very slight variations due to exchanges of Ar and N2 across the air—sea interface, driven by ocean solubility changes. Observations of these variations may provide useful constraints on large-scale fluxes of heat across the air—sea interface. A mass spectrometer system is described that incorporates a magnet with a wide exit face, allowing a large mass spread, and incorporates an inlet with rapid (5 s) switching of sources gases through a single capillary, thus achieving high precision in the comparison of sample and reference gases. The system allows simultaneous measurement of Ar/N2, O2/N2 and CO2/N2 ratios. The system achieves a short-term precision in Ar/N2 of 10 per meg for a 10 s integration, which can be averaged to achieve an internal precision of a few per meg in the comparison of reference gases. Results for Ar/N2 are reported from flasks samples collected from nine stations in a north-to-south global network over about a 1 yr period. The imprecision on an individual flask, as estimated from replicate agreement, is ±11 per meg. This imprecision is dominated by real variability between samples at the time of analysis. Seasonal cycles are marginally resolved at the extra-tropical stations with amplitudes of 5 to 15 per meg. Annual-mean values are constant between stations to within ±5 per meg. The results are compared with a numerical simulation of the cycles and gradients in Ar/N2 based on the TM2 tracer transport model in combination with air—sea Ar and N2 fluxes derived from climatological air—sea heat fluxes. The possibility is suggested that Ar/N2 ratios may be detectably enriched near the ground by gravimetric or thermal fractionation under conditions of strong surface inversions.

Optical metrology for analysis of lobster-eye x-ray optics.

A new method that uses optical microscopy to determine the physical structure of lobster-eye x-ray optics is described. This approach offers the ability to predict x-ray performance without having to take an x-ray measurement. An overlapping series of images of the entrance and exit faces of an optic are obtained and examined by purpose-built software. A 24-parameter description of each channel is obtained from which a quantitative analysis of all the major optic defects, except surface roughness, is performed. Results for a planar lobster-eye optic are used to illustrate this technique and discuss its abilities as well as directions for future enhancements.

Triangular-shaped bulk-optic glass sensor for simultaneous measurement of three ac currents

We propose a compact triangular-shaped bulk-optic glass sensor for simultaneous measurement of three ac currents using only one laser source. The sensor consists of three glass rods arranged as the three arms of a triangle, each of which is mounted with a coil of conducting wire that carries a current to be measured. Laser light is guided through the glass rods in a closed loop by reflecting twice at the critical angle of the two corners of the triangle. On the reflection surface of each corner, light is tapped with a right-angle prism that is pressed against the surface. The three unknown currents are determined by pro- cessing the light beams tapped from the two corners and that emerge from the exit face of the sensor. To demonstrate the principle, we con- struct an experimental sensor with SF6 glass and tested it with three- phase currents. © 2003 Society of Photo-Optical Instrumentation Engineers.

Foundations for low-loss fiber gradient-index lens pair coupling with the self-imaging mechanism.

A fiber-optic collimator that emits a Gaussian beam with its beam waist at a certain distance after the exit face of the lens is labeled a self-imaging collimator. For such a collimator, the waist of the emitted Gaussian beam and its location are partly dependent on the properties of the gradient-index (GRIN) lens. Parameters for the self-imaging collimator are formulated in terms of the parameters of a GRIN lens (e.g., pitch, core refractive index, gradient index, length) and the optical wavelength. Next, by use of the Gaussian beam approximation, a general expression for the coupling power loss between two self-imaging-type single-mode fiber (SMF) collimators is, for the first time to our knowledge, derived as a function of three types of misalignment, namely, separation, lateral offset, and angular tilt misalignment. A coupling experiment between two self-imaging collimators with changing separation distance is successfully performed and matches the proposed self-imaging mechanism coupling loss theory. In addition, using a prism, lateral offset, as well as angular tilt, misalignments are experimentally simulated for a two self-imaging collimator coupling condition by a single collimator reflective test geometry. Experimental results agree well with the proposed loss formulas for self-imaging GRIN lenses. Hence, for the first time to our knowledge, the mathematical foundations are laid for employing self-imaging-type fiber collimators in SMF-based free-space systems allowing optimal design for ultra-low-loss coupling.