Incident Intensity Distribution Research Articles

All-Optical Switching Based on the Plasma Channel Induced by Laser Pulses

We display a theoretical and experimental study of all-optical switching for signal lasers based on the plasma channel induced by the control laser. Using the plasma channel generated in the carbon disulfide (CS2) solution, the signal light can be modulated as some spatial distributions including unchanging, ring-shaped beam, and other intensity profiles. The modulation on the signal light can be conveniently adjusted by changing the control light’s incident intensity distribution. We can infer the dark spot shape in the modulated signal laser through the intensity profile of control laser beam. These results provide the great potential of plasma channel induced by lasers as an all-optical switching for various optoelectronic applications.

Temporally weighting a time varying noise field to improve Green function retrieval.

The authors consider the retrieval of Green functions G from the correlations of non-stationary non-fully diffuse noise incident on an array of sensors. Multiple schemes are proposed for optimizing the time-varying weights with which correlations may be stacked. Using noise records created by direct numerical simulation of waves in a two-dimensional multiply scattering medium, cases are shown in which conventional stacking does a poor job and for which the proposed schemes substantially improve the recovered G, rendering it more causal and/or more symmetric, and more similar to the actual G. It is found that the schemes choose weights such that the effective incident intensity distribution is closer to isotropic.

High-precision laser alignment technique based on spiral phase plate

The laser alignment technique is widely used in the measurement of straightness due to its unique capabilities of non-contact detection, fast response and high accuracy. By coupling the laser beam into a single-mode fiber, traditional alignment method can be greatly improved, since the emitting beam from the fiber basically won’t be affected by the variation of the incident intensity distribution. However, such method still suffers from errors caused by the vibration of the emitting fiber end and air turbulence. In this paper, the traditional alignment reference, which is the centroid of the intensity distribution, is replaced by center of the dark spot in a diffraction pattern, by adding a spiral phase plate (SPP) into the optical path. The center of the dark spot is proved to be stationary when the laser beam drifts. The far-field diffraction model of SPP is theoretically analyzed in detail. In the calibration experiments, the detected position data of the scanned CCD is highly linear with a correlation coefficient of 0.99999. The system is guaranteed with outstanding stability by a series of experiments, in which the standard deviation of the x and y coordinates are respectively only 7.0% and 33.7% in short-term tests and 20.3% and 25.3% in long-term tests. With simple configuration, the proposed system demonstrates satisfying performance in laser alignment.

Accelerating ray convergence in incident heat flux calculations using Sobol sequences

In this paper an improved quadrature scheme based on the reverse Monte Carlo method implemented using Sobol sequences to generate ray orientations is presented. This has the property that a more uniform pattern of rays on the unit hemisphere is produced compared to the usual implementation of the reverse Monte Carlo method. The use of Sobol sequences gives a ray convergence rate for the incident heat flux that is asymptotically equivalent to O ( N Ray − 1 ) . The generation of ray directions using Sobol sequences means that the Central Limit Theorem no longer holds. In its place a Gaussian variable is formulated from the incident intensity distributions calculated using Sobol sequences. This makes it possible to calculate confidence limits for a prediction of incident heat flux and the confidence limits contract with ray number at a rate of O ( N Ray − 1 l n ( N Ray ) ) . An extension to the Monte Carlo method combined with Sobol sequences is also presented that exploits the shape of the incident intensity distribution to a receiver. The new methodology is relatively simple to implement and shows some promising improvements in computational efficiency.

Measurements of third-order nonlinearity of inhomogeneous samples using nonlinear-imaging technique with a phase object

Using electrostatic self-assembly of a [CuPc(COOH)4]/polydiallyldimethylammonium chloride (PDDA) film, we demonstrate that signal fluctuation induced by a random surface inhomogeneity of less than 70 nm can be approximately resolved by a nonlinear-imaging technique with a phase object (NIT-PO) as opposed to Z-scan. In our NIT-PO simulation the sample is regarded as perfectly parallel, while the incident intensity distribution is modified to mimic that of an extremely weak laser beam traversing the sample, which is detected by a charge-coupled-device (CCD) camera. However, the original phase in the simulations is maintained, since phase distortions caused by the sample are not measurable at the CCD plane.

MO‐D‐L100F‐04: Imaging With Distributed Source Arrays

While the vast majority of x‐ray imaging procedures use a single x‐ray source, a growing number require measurement of x‐ray transmission from different locations. These exams are generally performed by moving a single x‐ray source to many positions. Examples include computed tomography (CT) and tomosynthesis. At the same time, several technologies are now becoming more available that enable to construction of source arrays, i.e., a large number of sources that can be turned on in sequence. The purpose of this presentation is to explore the capabilities and benefits of imaging with these source arrays.In CT, small deflections of the x‐ray focal spot (on the order of a millimeter) have been used to improve data sampling. Multiple sources or much larger deflections (on the order of centimeters) in the axial direction can be quite useful for increasing the thickness of the imaging slab in a single scan while reducing cone beam artifacts. Distributing sources in the circumferential direction can dramatically reduce the mechanical rotation needed to obtain a complete data set and therefore can shorten the scan time. The recently introduced dual source CT system is the most recent incarnation in this direction. The limiting case is CT scanning with no mechanical rotation, as in electron beam CT. An array of sources separated in the circumferential direction can also be used to reduce the size of the detector array. This can lead to a reduction in detected x‐ray scatter and perhaps in the adoption of more advanced detector technologies. It also provides a reasonable method for optimization of the incident intensity distribution, thereby potentially reducing the dose to the patient while also tailoring the distribution of quantum noise across the image.In tomosynthesis, as in CT, measurements from different directions are used to produce tomographic information, albeit with imperfect spatial separation due to data insufficiency. Nonetheless, tomosynthesis can be powerful in many settings where obtaining complete CT data is difficult, and is gaining popularity. Data acquisition with mechanical motion of a single source is limiting both in the needed imaging time and in the number of projections that can be obtained. A small number of projections makes the blurring function (the “crosstalk” from one location to another from imperfect spatial separation) very discrete and potentially distracting to the viewer. Source arrays can be used to produce tomosynthesis images with very short imaging time and smooth blurring functions, both of which are preferred.Thus, there are significant benefits from the use of distributed sources. The author anticipates that these source arrays will become more widely adopted.Research sponsored by GE Healthcare.Educational Objectives:1. become familiar with the types of source arrays and their capabilities.2. understand the benefits of source arrays in computed tomography.3. understand the benefits of source arrays in tomosynthesis.

Time dependence of the polarization of short X-ray pulses after crystal reflection.

The short X-ray pulses coming out of a SASE FEL (self-amplified stimulated emission free-electron laser) have stimulated a closer inspection of the response of a crystal reflection to them. As the reflectivity of a crystal reflection depends on the polarization of the incident radiation, the time response does so as well. The response to a delta-pulse incident either under 45 degrees linearly polarized with respect to the reflection plane of a monochromator crystal or circularly polarized is investigated in more detail. In contrast to the purely linear polarization perpendicular and parallel to the reflection plane, these mixed states show a very pronounced time dependence. In addition, a simulated SASE FEL bunch is investigated as an incident intensity distribution.

Analysis and convergence of the iterative convolution/superposition dose reconstruction technique for multiple treatment beams and tomotherapy.

An iterative convolution/superposition (C/S) algorithm has been created to reconstruct dose distributions in patients from exit dose measurements during a radiotherapy treatment. The method is based on an extended phantom which includes the patient CT representation and an electronic portal imaging device (EPID). The patient CT is assumed to be a true and rigid representation of the patient at the time of treatment. The C/S method computes the dose throughout the extended phantom which allows the exit dose to be predicted in the EPID. The process is then reversed to take the exit dose measurement and infer what the dose distribution must have been to produce the measured exit dose. The dose distribution is modeled without knowledge of the incident intensity distribution, and includes the effects of scatter in the computation. The iterative method begins by assuming that the exit primary energy fluence (PEF) is equal to the exit dose, the PEF is then backprojected through the extended phantom and superposed with the dose deposition kernel to determine a new prediction of the exit dose. The ratio of the computed PEF to exit dose is then multiplied by the measured exit dose image to produce a better representation of the exit PEF. Successive iterations then converge to the exit PEF image that would produce the measured exit dose image. Once convergence is established, the dose distribution is determined by backprojecting the exit PEF followed by superposition with the dose deposition kernel. The method is used to reconstruct the dose from a stimulated dynamic wedge and verified with film. Convergence and termination of the algorithm is then investigated with no noise and in the presence of noise. The method is then expanded to handle multiple treatment beams by separating the representation of the EPID from the patient or phantom representation in the computation process. Investigation of the effects of noise during the process of iterative dose reconstruction is necessary to understand the capabilities of the algorithm using exit dose images that may contain significant amounts of noise. The capability of the algorithm is evaluated for multiple field treatments to a cube phantom and a prostate patient CT representation in the presence of noise. The method is then used to simulate the dose reconstruction process for tomotherapy using 72 intensity-modulated fan beams. Dose reconstruction is shown to be capable of verifying the dose distributions in patients including multiple beams and dynamic collimation, provided the patient CT is known at the time of treatment.

Modeling dose distributions from portal dose images using the convolution/superposition method.

Post-treatment dose verification refers to the process of reconstructing delivered dose distributions internal to a patient from information obtained during the treatment. The exit dose is commonly used to describe the dose beyond the exit surface of the patient from a megavoltage photon beam. Portal imaging provides a method of determining the dose in a plane distal to a patient from a megavoltage therapeutic beam. This exit dose enables reconstruction of the dose distribution from external beam radiation throughout the patient utilizing the convolution/superposition method and an extended phantom. An iterative convolution/superposition algorithm has been created to reconstruct dose distributions in patients from exit dose measurements during a radiotherapy treatment. The method is based on an extended phantom that includes the patient CT representation and an electronic portal imaging device (EPID). The convolution/superposition method computes the dose throughout the extended phantom, which allows the portal dose image to be predicted in the EPID. The process is then reversed to take the portal dose measurement and infer what the dose distribution must have been to produce the measured portal dose. The dose distribution is modeled without knowledge of the incident intensity distribution, and includes the effects of scatter in the computation. The iterative method begins by assuming that the primary energy fluence (PEF) at the portal image plane is equal to the portal dose image, the PEF is then back-projected through the extended phantom and convolved with the dose deposition kernel to determine a new prediction of the portal dose image. The image of the ratio of the computed PEF to the computed portal dose is then multiplied by the measured portal dose image to produce a better representation of the PEF. Successive iterations of this process then converge to the exiting PEF image that would produce the measured portal dose image. Once convergence is established, the dose distribution is determined by back-projecting the PEF and convolving with the dose deposition kernel. The method is accurate, provided the patient representation during treatment is known. The method was used on three phantoms with a photon energy of 6 MV to verify convergence and accuracy of the algorithm. The reconstructed dose volumes agree to within 3% of the forward computation dose volumes. Furthermore, this technique assumes no prior knowledge of the incident fluence and therefore may better represent the dose actually delivered.

Modelling of Temperature Distribution of Semiconductors During Rapid Thermal Processing

ABSTRACTThe modelling of temperature distribution on semiconductor wafers in common RTP-equipment is described. The incident intensity distribution on the wafer is calculated using raytracing. Based on this distribution the temperature distribution on the wafer is determined solving the two-dimensional heat conduction equation. If the dependence of a considered material property on the process temperature is known, the calculated temperature distribution can be convened to a distribution of this parameter.The distinctive feature of the described algorithms is the two-dimensional treatment of the distributions using a grid of ring segments, each with equal area. This grid is identical to the usual circular test patterns of multipoint measurement equipment. This is convenient since the evaluation of temperature uniformity in RTP equipment is done mostly by mapping an appropriate temperature dependent material property. All calculated distributions can be presented by contour plots as well as 3-D plots. This results in a very suitable method to compare simulated and experimental wafer maps.The agreement between simulated and experimental temperature distributions is shown.

前方散乱差動型ＬＤＶによる粒径，速度の測定

A method of in situ particle size and velocity measurement in parti-cle laden flow by forward-scatter dual-beam Laser Doppler Velocimeter (LDV) has been investigated both analytically and experimentally. The analysis is based on Mie's electromagnetic scattering theory for spherical particles to relate the collected scattered light power for the LDV optical arrangement to the particle diameter. The measured de-pendence of the pedestal height in the Doppler burst signal to the particle diameter agrees well with calculations from Mie's theory. It should be emphasized that the in-creasing dependence assumption of collected scattered power on the square of particle diameter is not satisfied for the LDV collection optics geometry and the calibration with numerical calculations based on Mie's theory is indespensable. Although the mask has to be attached on the collecting lens to block the unscattered beams, it may be noticed that the monotonic dependence of collected scattered power on the particle size can not be sustained for too small collection opening. The selective data sampling scheme to set the threshold of the certain Doppler cycle number for each burst signal is proved to re-duce significantly the error resulting from particle trajectry through nonuniform incident intensity distribution in the measurement volume, however, overall measurement error at most approximately 30% is evaluated.

Spatial light modulators and their applications

The function of an optically addressed spatial light modulator (SLM) is to convert the intensity distribution of an incident scene into a coherent image, or into a high brigthness incoherent image. The applications of these transducers are therefore very large since they extend, from the field of coherent optical processing and computing (Incoherent to coherent conversion) to the high resolution video image projection. A great interest is now devoted to the technological developments of these devices in several laboratories using different approaches. The basic principle of operation of an optically addressed light valve is the following. A photosensitive layer detects the incident intensity distribution and induces a charge distribution onto a material which modifies its optical properties as a function of the photoinduced voltage. Finally, the incoherent or coherent illumination beam transmitted (or reflected) by the SLM sees its amplitude or phase spatially modulated. In the last few years, the list of spatial light modulators under research and development expanded considerably.

Effect of Diffuseness of Incident Sound Field on the Measurement of Airborne Sound Transmission Loss

Much of the literature on airborne sound transmission loss (TL) indicates that TL of a panel at a given frequency is dependent on distribution of sound intensity with angle of incidence in the incident sound field. Observed TL in most laboratory tests employing a “diffuse” sound field yields data which may be described as lying between that to be anticipated if the energy in the sound field were concentrated at normal incidence, and that expected if the sound field were uniformly distributed over all angles of incidence (Random Incidence). An ad hoc approach to dealing with this phenomenon has involved the postulation of a “field incidence” sound field, which assumes that the distribution of incident intensity is uniform up to a limiting angle of 75° to 80° from the normal to the panel. This approach provides a means for calculating TL which agrees with measurements, but no satisfactory analytical model has evolved which provides justification for this approach. Some careful experimental evaluations involving convolution of the cross correlation of the incident pressure field with an analytic model of panel response have shown that the fundamental approach is reasonable, but still provides no basis for explaining this phenomenon. In this paper, the recent published and unpublished data of a number of authors are reviewed, which imply that TL is more sensitive to local perturbations from a diffuse incidence sound field than from gross perturbations in the source room. One hypothesis which seems to correlate with these observations is that the aperture in which a panel is tested should be viewed as a spatial filter which limits the incident sound field in addition to the effects of “diffusivity” present in the source room. It is postulated that the limitations produced by the aperture are more significant than those provided by the room for a “reasonably reverberant” source room.

Satellite shadows on Jupiter as probes of its upper atmosphere

The photometric profiles of the shadows cast on Jupiter by its large satellites are determined to some extent by the physical properties of the Jovian upper atmosphere. Profiles of Io's shadow are constructed on the assumption of exponentially decreasing mean free path and different path lengths. Variations from the incident intensity distribution occur, which may be useful in the atmospheric analysis.