Design and Experiments for Diffractive Optical Elements in Beam Shaping and Superresolution Imaging
|School||University of Science and Technology of China|
|Keywords||Diffractive optical element Beam shaping Phase compensator Subwavelength grating Super-resolution imaging Rigorous Coupled Wave Analysis|
Content of this paper is divided into two parts, the first part of the design and experiment for beam shaping diffractive optical element (DOE: Diffractive Optical Elements), the second part is for super-resolution imaging of subwavelength diffraction grating theory Analysis. The first part of the research work around the diffractive optical element for beam shaping to expand. Diffractive optical element is a very wide range of applications, different system environment facing specific engineering applications, the aim of our study is to optimize the design of diffractive optical elements to make to adapt to a different system, the beam shaping effect be able to meet the engineering needs. Since the developed optical element the diffractive sub-unit in the order of hundreds or tens of microns, so using the design method based on the scalar diffraction theory. Its main contents and results are as follows: DOE reflected from the surface of the high-power laser systems and its solutions in inertial confinement fusion (ICF: Inertial Confinement Fusion) environment, intense laser special requirements for optical components , in the bright light applications, must pay attention to the the intensity modulation problems caused by the DOE surface reflection, to severe intensity modulated laser beam quality will deteriorate, or even damage to the optical components. Fresnel diffraction theory, we simulate the distribution of the of 320mm caliber DOE reflection light field, reflected field by the DOE phase modulation, there will be some of the larger extreme points of light intensity, and energy on these points should not be overlooked. Commonly used method to solve the optical surface processing and small-angle tilted AR AR, but DOE multi-step surface, there is no proper process, so we use reflected light to avoid direct exposure to the small angle tilted to the previous element. This paper analyzes the DOE tilted phase change, computational simulation of the optical field distribution tilted focal spot, found the focal spot smoothing will be a certain degree of damage, the center there will be a strong highlight. In order to verify the simulation results, our existing 70mm diameter test piece experimental test results show that the energy gathered in the focal spot center does happen, and the simulation results are basically the same. Subsequently, for the change of the the DOE phase modulation tilted phase compensation method that the phase difference caused by a tilt angle, the same as the non-tilted phase corresponding phase compensator, the phase distribution of the adjusted . The phase compensation method of simulation results show that the beam shaping effect after the inclined position is consistent with the original design results, i.e. tilted DOE's guarantee focal spot quality unchanged. , Beam shaping element of the dual-lens system design and experimental DOE have a lot of areas for beam shaping, such as laser direct writing equipment. DOEs the front end environment in different systems, have different, sometimes is a single lens, and sometimes is a lens group, a lens group has its own advantages, such as increased design freedom, and since the more consideration in the transmission of a sample plane, sampling the relationship between the input and output window is determined by the element distance, making it easy to reduce the ratio of the focal spot of the output window, in order to achieve better suppression of high order diffraction spots, reduce energy loss. In the two-lens system, the transmission beam is not a direct Fourier transform to represent conflans Wo fee diffraction process, but may be divided into two Fresnel diffraction to describe the process based on this beam transmission, this article will \The output algorithm improved to make it applicable to the design of the dual-lens system, and gives an iterative process. DOE design process produced it, and take a different test methods to evaluate their effectiveness. First pulsed laser as a light source with a photo paper to accept the focal spot the acquisition results showing significantly the shape of the contour; in order to more precisely understand the shaping results, we use the focal spot after the shaping of the weak continuous light source with the CCD receiving test results focal spot Octavia edge, which is consistent with the laser direct writing technology needs; Finally, we use the energy meter diffraction efficiency measurement, the measurement results in 70-80%, according to the DOE production experience, which is a normal range, and it is able to meet certain engineering requirements. The second part of the work around for super-resolution imaging of subwavelength metal grating expand. Since the idea of ??negative refraction materials to achieve the \Wherein the superset of the far-field lens (FSL: Far-field Superlens) the resolution of the experiments will increase to the level of several tens of nanometers, it imaging process consists of two important parts: First a thin layer of the surface plasmon resonance on the use of silver effects to enhance the evanescent field, and then take advantage of the evanescent field propagation field subwavelength grating will be enhanced. In the second process, the appropriate design subwavelength grating to meet a transmission rate far greater than 0 negative transmittance is a key issue. Currently, FSL only in experiments to achieve a one-dimensional imaging, because the ordinary subwavelength grating broadening the spectrum of the light field in one direction only. Able to design appropriate ring grating its diffraction characteristics of the evanescent field into the requirements of the communication field, the the FSL of two-dimensional imaging may be more feasible for the study of this part of the motivation in this. Subwavelength diffractive optical element vector diffraction theory to analyze such grating optical elements of the periodic structure we first introduced the ordinary grating rigorous coupled-wave analysis (RCWA: Rigorous Coupled Wave Analysis) method, and then given annular grating in cylindrical coordinates the RCWA, of annular grating diffraction characteristics. The analysis showed that the the annular grating can be within a certain range of angles of incidence satisfy the requirements of the FSL is.