Zhaofeng Cen

Simulation of the inhomogeneous medium with a self-adapting grid

It is difficult to solve the problem of tracing rays through an inhomogeneous medium such as the atmosphere or a flowing liquid with the existing methods whose refractive distribution cannot be characterized by any combination of formulas. A new method is developed to efficiently describe such a medium. The method essentially consists of characterizing the inhomogeneous medium with a self-adapting grid, the cell size of which can be automatically varied according to the gradient of the refractive index at each position within the medium. The advantage of this method is its capability to trace rays through a medium with random refractive distribution in an accurate way. To test its correctness, the ray trace through a regular gradient medium has also been made with this method, and the results are compared with those from other commercial lens design codes. It is shown that for appropriate parameter settings this method can obtain great accuracy.

Depth estimation algorithm for light field data by epipolar image analysis and region interpolation

In this paper, we propose an optimized algorithm to estimate the depth information in the 4D light field data. Our scheme has the advantage of conciseness compared to the traditional epipolar-plane image analysis method. First, we have analyzed the depth resolution properties of light field data not mentioned by the previous researchers. In the depth estimation process, epipolar analysis is confined in a small range to reduce the running time, combining with a regression test to reduce estimation error. Occlusion condition is especially dealt with by recognizing object margin. To test the accuracy of our algorithm, we use a benchmark dataset to evaluate the output depth result. We get a competitive result in the estimation error evaluation and prevailing runtime result compared to that of baseline algorithms. Owing to the high performance, this algorithm can be used in real-time depth recognition with the aid of parallel computing.

Paraxial analysis of stray light caused by multi-order diffraction and multireflection

Diffraction components are applied in high power laser systems for beam shaping and harmonic separation. Because of the multi-order diffraction and multi-reflection to high power laser, the distributions of stray light energy and ghosts are much more completed in the systems than in conventional optical systems. In this paper a data structure of tree is presented for describing the stray light caused by multi-order diffraction and multi-reflection. All the nodes of the tree can be dynamically saved and be deleted, and the intermediate results those are useful for the next calculation step can be reserved in RAM. Using this method the multiple repeated calculations in conventional stray light analysis methods such as Monte Carlo technique are avoided and the analysis time is reduced. According to the paraxial tracing, the software which can be used for analyzing the stray light caused by multi-order diffraction and multi-reflection in high power laser systems is developed and the stray light tree of a laser system based on paraxial tracing is built. As shown by the example that this algorithm is available for quickly analyzing stray light in the systems including diffraction components, and the ghost positions with energy descriptions can be given by the software. The ghosts those are harmful to the important components will be picked.

Numerical simulation and comparison of nonlinear self-focusing based on iteration and ray tracing

Self-focusing is observed in nonlinear materials owing to the interaction between laser and matter when laser beam propagates. Some of numerical simulation strategies such as the beam propagation method (BPM) based on nonlinear Schrödinger equation and ray tracing method based on Fermat’s principle have applied to simulate the self-focusing process. In this paper we present an iteration nonlinear ray tracing method in that the nonlinear material is also cut into massive slices just like the existing approaches, but instead of paraxial approximation and split-step Fourier transform, a large quantity of sampled real rays are traced step by step through the system with changing refractive index and laser intensity by iteration. In this process a smooth treatment is employed to generate a laser density distribution at each slice to decrease the error caused by the under-sampling. The characteristics of this method is that the nonlinear refractive indices of the points on current slice are calculated by iteration so as to solve the problem of unknown parameters in the material caused by the causal relationship between laser intensity and nonlinear refractive index. Compared with the beam propagation method, this algorithm is more suitable for engineering application with lower time complexity, and has the calculation capacity for numerical simulation of self-focusing process in the systems including both of linear and nonlinear optical media. If the sampled rays are traced with their complex amplitudes and light paths or phases, it will be possible to simulate the superposition effects of different beam. At the end of the paper, the advantages and disadvantages of this algorithm are discussed.

Assessment by psychophysical methods for design courses of optical discipline

In the teaching of design courses, the process of students to complete the designs is considered more important than the final results, and usually there are no standard answers for these designs. Thus, the research on the process evaluation method in design courses is of great significance. Taking the assessments of two design courses in optical discipline as examples, the psychophysical experimental methods are introduced into the process evaluation of the design courses, i.e. the process evaluation given by each student to others in forms of classification, sorting or grading are adopted as one trial of a psychophysical experiment, which generate many experimental data of mutual evaluation. Based on these data, evaluation results are measured scientifically using the statistical method. Furthermore, through correlation analysis and regression analysis of these data, the relationship among various aspects of different process can be studied. It is easier for students to understand and accept their assessment results, and more helpful for teachers to analyze the influencing factors in their teaching process.

Chinese National Optical Education Small Private Online Course system

In order to realize the sharing of high quality course resources and promote the deep integration of ‘Internet+’ higher education and talent training, a new on-line to off-line specialized courses teaching mode was explored in Chinese colleges and universities, which emphasized different teaching places, being organized asynchronously and localized. The latest progress of the Chinese National Optical Education Small Private On-line Course (CNOESPOC) system set up by Zhejiang University and other colleges and universities having disciplines in the field of optics and photonics under the guidance of the Chinese National Steering Committee of Optics and Photonics (CNSCOP) was introduced in this paper. The On-line to Off-line (O2O) optical education teaching resource sharing practice offers a new good example for higher education in China under the background of Internet +.

Feedback modification strategy for designing a free-form optical surface with given illumination pattern

In order to achieve a uniform illumination distribution confined to a given rectangular region, a feedback modification strategy is introduced in the design of free-form optical surface. The whole process of this strategy includes two steps: solving the initial structure and feedback process. To solve the initial surface, we first divide the source space along its longitude and latitude direction with uniform angel. Because the energy of each longitude and latitude micro-belt of free-form surface and each corresponding micro-belt on the target plane is equal, the target plane can be divided into grids, and then the mapping relationship of source space and target space is constructed. By using Snell’s Law and tangent plane iterative method, we obtain the coordinates of all grid nodes and an initial free-form surface can be constructed. Afterwards we use specific feedback algorithm to improve the illuminating performance. This method is proved to be efficient, the shape of illumination spot is dramatically improved and the design result is satisfactory.

Analysis of nonlinear self-focusing phenomenon in high-power laser system based on ray-tracing

In high power laser systems, nonlinear effect, one of the key factors of beam wavefront aberration and even irreversible damage to system, has always been one of the top considerations of researchers for decades. A hybrid ray-tracing method for both linear media and nonlinear media based on geometric optics is presented in this paper and realized by programming. In a simple optic system with KDP crystal, an obvious decline of beam quality is observed in high laser power density conditions and a method taking component intervals as compensation of beam quality is proved feasible. Considering the complexity of traditional modeling method based on surfaces, a modeling method based on components is established. Hopefully, the conclusions and flaws of this paper can shed light on relevant work and further research.

A novel optimization strategy for LED street lamp designing

In light-emitting diode (LED) street lamp design, it has always been an obsession that how to achieve an expected illumination distribution confined to a given region. To solve this problem, a new optimization strategy is proposed. In view of the practicability of actual production, we select the relatively mature approach that is based on both the Snell law and the energy conservation law which would obtain coordinate relations between the spaces the light source and target plane owned respectively and a set of equations to establish the initial construction of free-form lens. In addition, all the processes of simulation, analysis and optimization as well are accomplished in software. Generally speaking, for construction method which is in the light of the set of equations, the major improvement ideology focus on finding out the most suitable mapping relationship between the two coordinate systems. So does our work. In order to get better performance, the grid of control points must be modified. Then the core problem lies in determining the direction and distance of every point’s movement contained in the grid. The rule of changing direction has a bearing on energy relations while migration length is gained by direct search algorithm.We apply the method in uniform illumination and get some effect. In short, the optimization strategy provides a practical and simple way for street lamp design of LED illumination.

Description and implementation studies on field dependent wavefront aberration

Zernike polynomials have been widely used to fit lens surface figure error and the wavefront aberration of optical systems, for its orthogonality in the unit circle and its corresponding relationships with optical aberrations1. Because the current extensively used Zernike polynomials are just functions of the aperture, without consideration of the field factor, it can only represent single field wavefront aberration. This is incomplete for the description of the wavefront aberration, especially for lithographic lens with a large field and high imaging quality2. Thus, considering the field factor in the description of wavefront aberration becomes very necessary. This paper presents a convenient and practical method to describe the full field wavefront aberration. A rotationally symmetric optical system has been taken as an example, in the scope of normalized full field, taking the Chebyshev zero points as nodes, and applying the Chebyshev polynomials to the fitting of Zernike coefficients of different fields. Meanwhile, the influence of the degree of Chebyshev polynomials and the number of fitting nodes on the fitting accuracy is taken into account. The results show that the fitting method used in this paper is of high accuracy, and this fitting method is very significant for the analysis of full field wavefront aberration.