Photon Tracing Method Research Articles

Transversely solar-pumped fiber laser with parabolic trough

We studied transversely solar-pumped Nd3+ doped fiber laser with parabolic trough via simulation models developed by Monte Carlo photon tracing method. Our analysis involved estimating losses at cavity walls and the absorption efficiency within the fiber core. Factoring in the absorption distribution of pumped solar radiation in the fiber, we determined an absorption efficiency of 8% for a 287 meter long fiber arranged in a semicircular configuration. Subsequently, our calculations found a laser output of 3.03 W for this fiber laser system, resulting in an overall conversion efficiency of 4.29% from solar to laser beam.

Волоконные лазеры с солнечной накачкой без первичного концентратора.

We studied solar pumped fiber lasers without primary concentrator. A model of solar sidepumped Nd3+ doped silica fiber lasers was developed using photon tracing method. Theabsorbed power distribution, possible losses and the pumping efficiency are estimated using the model. We analyzed the estimated paramters and drawed conclutions which have many suggests for improved design of solar pumped fiber lasers. The absorption efficiency of a fiber solar laser without a primary concentrator or sun tracker was calculated to be 0.2% for a 190 m fiber. The output power of the fiber laser is 40 mW, and the conversion efficiency from sunlight to laser light is calculated to be 0.06%. The developed model allows one to estimate the overall efficiency, output power and absorption distribution of Nd3+ fiber lasers without focusing optics, enabling various investigative comparisons.

Side-pumped efficient Ce:Nd:YAG solar laser in a multi-pass scheme

We studied possible techniques for increasing the power-to-power conversion efficiency of the Ce:Nd:YAG solar laser using a simulation model developed with the Monte-Carlo photon tracing method framework. It is suggested that the multi-passage of photons through a medium can enhance the absorption probability. The effectiveness of the multi-pass scheme is experimentally investigated. The application of the multi-pass pumping scheme for Ce:Nd:YAG solar lasers is demonstrated. It is shown that the pumping efficiency in the Ce:Nd:YAG solar laser with the multi-pass pumping cavity is increased by 20%, paving the way for higher power-to-power conversion efficiency.

Simulating optical processes: Monte Carlo photon tracing method

In search of high accuracy and convenient implementation, computational optics uses several modeling techniques such as Maxwell’s equations-based methods, ray-tracing method, Fourier optics methods and many others. Each with its advantages, disadvantages, and applicability caveats. In this paper, we describe Monte Carlo photon tracing as a simulation technique for computational optics. The validity of the main assumptions in the simulation method is demonstrated with examples, and limitations of method’s applicability are discussed.

Reflection-Assisted Non-Line-of-Sight Ultraviolet Communications

Considering high path loss (PL) of the non-line-of-sight (NLOS) ultraviolet communications (UVC) realized by the atmospheric scattering effect of ultraviolet light, we propose to use a reflective plane to improve the received optical power and further reduce the PL. More specifically, a channel model for NLOS UVC with not only scattering but also reflection considered is proposed via Monte-Carlo (MC) photon-tracing method. Both scattering and reflective events are called collision-induced events (CIEs). The proposed channel model can obtain the PL and channel impulse response (CIR) performances of the NLOS UVC system with multiple-order CIEs. It is shown that the reflective plane could reduce the PL and the time delay of CIR, which could be a benefit for designing high data-rate NLOS UVC systems. Finally, experimental results in two representative scenarios are also given to verify the effectiveness of the proposed channel model for reflection-assisted NLOS UVC systems.

Foveated Photon Mapping.

Virtual reality (VR) applications require high-performance rendering algorithms to efficiently render 3D scenes on the VR head-mounted display, to provide users with an immersive and interactive virtual environment. Foveated rendering provides a solution to improve the performance of rendering algorithms by allocating computing resources to different regions based on the human visual acuity, and renders images of different qualities in different regions. Rasterization-based methods and ray tracing methods can be directly applied to foveated rendering, but rasterization-based methods are difficult to estimate global illumination (GI), and ray tracing methods are inefficient for rendering scenes that contain paths with low probability. Photon mapping is an efficient GI rendering method for scenes with different materials. However, since photon mapping cannot dynamically adjust the rendering quality of GI according to the human acuity, it cannot be directly applied to foveated rendering. In this paper, we propose a foveated photon mapping method to render realistic GI effects in the foveal region. We use the foveated photon tracing method to generate photons with high density in the foveal region, and these photons are used to render high-quality images in the foveal region. We further propose a temporal photon management to select and update the valid foveated photons of the previous frame for improving our method's performance. Our method can render diffuse, specular, glossy and transparent materials to achieve effects specifically related to GI, such as color bleeding, specular reflection, glossy reflection and caustics. Our method supports dynamic scenes and renders high-quality GI in the foveal region at interactive rates.

AN EFFICIENT SOLAR-PUMPED Ce:Nd:YAG LASER WITH FRESNEL LENS

We studied the potential of Ce:Nd:YAG crystal in solar pumped lasers. In the study, recently proposed multi-pass pumping configuration and the Fresnel lens, 0.8 m2, have been considered using statistical Monte-Carlo photon-tracing method. Our results show that 60 W laser power can be achieved at 1000 W/m2 solar radiation while overall efficiency is 7.5% and the slope efficiency is around 8.5%.

FOCAL SPOT STRUCTURE OF FRESNEL LENS AND ITS HOMOGENIZATION

Using statistical Monte-Carlo photon-tracing method, we numerically studied focal spot structure of flat Fresnel lens. It has been shown that due to dispersion effect, focal spot structure changes considerably. In addition, we demonstrated homogenization of strongly focused photons. Rectangular prism can be applied to achieve the homogenization.

LESS: LargE-Scale remote sensing data and image simulation framework over heterogeneous 3D scenes

Three-dimensional (3D) radiative transfer modeling of the transport and interaction of radiation through earth surfaces is challenging due to the complexity of the landscapes as well as the intensive computational cost of 3D radiative transfer simulations. To reduce computation time, current models work with schematic landscapes or with small-scale realistic scenes. The computer graphics community provides the most accurate and efficient models (known as renderers) but they were not designed specifically for performing scientific radiative transfer simulations. In this study, we propose LESS, a new 3D radiative transfer modeling framework. LESS employs a weighted forward photon tracing method to simulate multispectral bidirectional reflectance factor (BRF) or flux-related data (e.g., downwelling radiation) and a backward path tracing method to generate sensor images (e.g., fisheye images) or large-scale (e.g. 1 km2) spectral images. The backward path tracing also has been extended to simulate thermal infrared radiation by using an on-the-fly computation of the sunlit and shaded scene components. This framework is achieved through the development of a user-friendly graphic user interface (GUI) and a set of tools to help construct the landscape and set parameters. The accuracy of LESS is evaluated with other models as well as field measurements in terms of directional BRFs and pixel-wise simulated image comparisons, which shows very good agreement. LESS has the potential in simulating datasets of realistically reconstructed landscapes. Such simulated datasets can be used as benchmarks for various applications in remote sensing, forestry investigation and photogrammetry.

A Venation-Like Groove Structure of Vertical GaN-Based Light-Emitting-Diode Analyzed by the Monte Carlo Photon-Tracing Method

A venation-like groove structure (VGS) is proposed to improve light extraction efficiency (LEE) of vertical GaN-based light-emitting-diode by reshaping the undoped GaN (U-GaN) layer. The light extraction characteristics of the venation-like groove structure are simulated and compared to the same electrode structure without grooves by the Monte Carlo photon-tracing method. It is found that VGS has a 20.3% enhancement on LEE at the chip size of 300 μm × 300 μm. The simulated LEE distribution pattern shows that VGS has advantages in brightness and uniformity compared to interdigitated-multi-fingered and square groove structures. These advantages are contributed to the partition and deformation in VGS.

Turbulence Effect on Cloud Radiation

The effect of turbulent clustering of water droplets on radiative transfer is investigated by means of both a three-dimensional direct numerical simulation of particle-laden homogeneous isotropic turbulence and a radiative transfer simulation based on a Monte Carlo photon tracing method. The results show that turbulent clustering causes the formation of void regions of droplets and hence increases the direct transmittance. This effect decreases as the turbulent Reynolds number increases and is estimated to be negligibly small under the conditions in real clouds.

Light absorption and scattering by aggregates: Application to black carbon and snow grains

A geometric-optics surface-wave approach has been developed for the computation of light absorption and scattering by nonspherical particles for application to aggregates and snow grains with external and internal mixing structures. Aggregates with closed- (internal mixing) and open-cell configurations are constructed by means of stochastic procedures using homogeneous and core-shell spheres with smooth or rough surfaces as building blocks. The complex aggregate shape and composition can be accounted for by using the hit-and-miss Monte Carlo geometric photon tracing method. We develop an integral expression for diffraction by randomly oriented aggregates based on Babinet's principle and a photon-number weighted geometric cross section. With reference to surface-wave contributions originally developed for spheres, we introduce a nonspherical correction factor using a non-dimensional volume parameter such that it is 1 for spheres and 0 for elongated particles. The extinction efficiency, single-scattering albedo, and asymmetry factor results for randomly oriented columns and plates compare reasonably well with those determined from the finite-difference time domain (FDTD) and the discrete dipole approximation (DDA) computer codes for size parameters up to about 20. The present theoretical approach covers all size ranges and is particularly attractive from the perspective of efficient light absorption and scattering calculations for complex particle shape and inhomogeneous composition. We show that under the condition of equal volume and mass, the closed-cell configuration has larger absorption than its open-cell counterpart for both ballistic and diffusion-limited aggregates. Because of stronger absorption in the closed-cell case, most of the scattered energy is confined to forward directions, leading to a larger asymmetry factor than the open-cell case. Additionally, light absorption for randomly oriented snowflakes is similar to that of their spherical counterparts under the condition of equal geometrical cross section area for both external and internal mixing states; however, nonspherical snowflakes scatter less light in forward directions than spheres, resulting in a substantial reduction of the asymmetry factor. We further demonstrate that small soot particles on the order of 1 μm internally mixed with snow grains could effectively reduce snow albedo by as much as 5–10%. Indeed, the depositions of black carbon would substantially reduce mountain-snow albedo, which would lead to surface warming and snowmelt, critical to regional climatic surface temperature amplification and feedback.

Comparison of Monte Carlo ray-tracing and photon-tracing methods for calculation of the impulse response on indoor wireless optical channels

We present a comparison between the modified Monte Carlo algorithm (MMCA) and a recently proposed ray-tracing algorithm named as photon-tracing algorithm. Both methods are compared exhaustively according to error analysis and computational costs. We show that the new photon-tracing method offers a solution with a slightly greater error but requiring from considerable less computing time. Moreover, from a practical point of view, the solutions obtained with both algorithms are approximately equivalent, demonstrating the goodness of the new photon-tracing method.

Emission Line Profiles and Images of Geometrically Thin and Optically Thick Accretion Disks

Under the assumption that the accretion disk around a Kerr (spinning) black hole is geometrically thin and optically thick, the trajectories of photons in Kerr metric are calculated by using the photon tracing method. And by numerical calculations, we have made a study on the relativistic iron line profiles and images of thin accretion disks. The result shows that viewing at large inclination angles, because of the contribution of the photons from the lower surface of the accretion disk, the line profile becomes double-peaked and the flux image is also significantly modified.

Analysis of flash lamp structure using Monte Carlo photon tracing method

By analyzing the flash lamp structure, better illumination distributions in the lamp’s field of view can be obtained. Instead of geometrical optical approaches, the Monte Carlo photon tracing method was used here to trace the photon tracks in a three-dimensional space. The models of elemental structures in a camera flash lamp, such as the flash tube, reflector and focus lens, were set up by introducing the cosinusoidal random number and other mathematical methods. Initially, the single photon was traced in the flash lamp by using the Monte Carlo method to simulate various photon tracks. A large sum of photons was then generated to simulate the real situation in the flash lamp. Finally, a group of structural parameters was applied to verify the simulative computer program. The output light intensity distributions at different angles of view in the orthogonal directions meet the ISO standards and are very close to the measured ones. Hence, the Monte Carlo photon tracing method in the design of flash lamps has been proven to be applicable and useful.

Far-field pattern simulation of flip-chip bonded power light-emitting diodes by a Monte Carlo photon-tracing method

The far-field pattern of light-emitting diodes (LEDs) is an important issue in practical applications. We used a Monte Carlo photon-tracing method for the package design of flip-chip bonded power LEDs. As a first-order approximation, we propose using a plane light source model to calculate the far-field pattern of encapsulated LEDs. The far-field pattern is also studied by use of a more detailed model, which takes the structure of all epitaxial layers of a flip-chip bonded power LED into consideration. By comparing the simulation results with the experimental data, we have concluded that the plane light source model is much less time-consuming and offers fairly good precision for package design.