Scalar Diffraction Method Research Articles

Design and Experiment of a Quasi-Optical Mode Converter

Quasi-optical mode converter is an important component to realize the high-efficiency output for a high-power gyrotron oscillator. In this paper, design and experiments of a quasi-optical mode converter, consisting of a Denisov-type launcher and three quasi-optical mirrors, are carried out for the development of 140 GHz/TE28,8 mode gyrotron oscillator. Based on scalar diffraction method, the field distribution at the radiation aperture of the Denisov-type launcher is optimized to make the vector correlation of the aperture field with the ideal Gaussian field reach 96.2%. Based on the geometric optics method and the Gaussian beam matching method, the focusing mirror and the beam shaping mirrors were designed. A 3D full-wave analysis software Surf3D was used to obtain the field distribution on each mirror surface and the output window, which shows that the output beam with Gaussian mode content of 96.67% is obtained at the output window. The total power conversion efficiency of the quasi-optical mode converter is 93.98%. The design and experiments of the quasi-optical mode converter are performed to use the output signal of the self-developed TE28,8 mode generator as its input one under the conditions of the simulated demonstration for its conversion characteristics and the strict control for machining precision as well as the process of the converter assemble and testing. The tested results indicate that the design is in good agreement with the experiment, which is helpful for engineering design and experimental demonstration of the quasi-optical mode converter development.

Sensitivity of diffraction efficiency to period width errors for multilayer diffractive optical elements.

Multilayer diffractive optical elements (MLDOEs) can achieve broadband high efficiency in the hybrid refractive-diffractive optical systems. The reliable performance of MLDOEs requires high precision during manufacture. The microstructure period width errors should be well defined. In this work, the relationship between diffraction efficiency and different period width errors was analyzed with the rigorous electromagnetic method. Compared to results predicted by the most used scalar diffraction method, the effect of period width errors on diffraction efficiency is underrated. The simulation results showed that diffraction efficiency is sensitive to the period width errors. This research provides a reliable method to control the fabrication errors for fabricating MLDOEs.

Numerical Simulation of In-Line Gratings for Differential Push-Pull Signals Using the Scalar Diffraction Method

This paper investigates the in-line differential push-pull signal simulation for the Blu-ray disk system using the scalar diffraction theory. A simple calculation scheme was proposed. With the simulation flow, six different types of in-line gratings were analyzed, compared, and discussed. The simulation results show that the pattern with three segmented central regions has the best signal performance.

Investigation of Bessel beam propagation in scattering media with scalar diffraction method

Bessel beam propagation in scattering media is simulated using the angular spectrum method combined with slice-by-slice propagation model. Generating Bessel beams with a spatial light modulator, which provides a means to adjust flexibly the parameters of the Bessel beam, allows us to validate the simulation results experimentally. The study reveals that the self-reconstructing length changes oppositely with the axicon angle (i.e., the larger the axicon angle, the shorter the self-reconstructing length). The radius of the incident beam has little influence on the self-reconstruction of the Bessel beam central lobe.

Fluorescence volume imaging with an axicon: simulation study based on scalar diffraction method

In a two-photon excitation fluorescence volume imaging (TPFVI) system, an axicon is used to generate a Bessel beam and at the same time to collect the generated fluorescence to achieve large depth of field. A slice-by-slice diffraction propagation model in the frame of the angular spectrum method is proposed to simulate the whole imaging process of TPFVI. The simulation reveals that the Bessel beam can penetrate deep in scattering media due to its self-reconstruction ability. The simulation also demonstrates that TPFVI can image a volume of interest in a single raster scan. Two-photon excitation is crucial to eliminate the signals that are generated by the side lobes of Bessel beams; the unwanted signals may be further suppressed by placing a spatial filter in the front of the detector. The simulation method will guide the system design in improving the performance of a TPFVI system.

A Highly Sensitive Optical Sensor Design by Integrating a Circular-Hole Defect With an Etched Diffraction Grating Spectrometer on an Amorphous-Silicon Photonic Chip

We present a sensitive optical sensor design by integrating a circular-hole defect with an etched diffraction grating (EDG) spectrometer based on amorphous-silicon photonic platforms. The circular-hole defect obtained from the outdiffusion of the hydrogen in the annealing process can induce strong resonant scattering loss at some special wavelengths. The positions of resonant peaks will linearly shift with the refractive index change of the detected sample (the sensitivity is 9200 nm/RIU). In addition, the influences of the defect on the sensing characteristics are numerically analyzed based on a scalar diffraction method and a Green tensor technology.

The effect of polarization light on optical imaging system

Normally, Strehl ratio, resolution, and focus depth, the parameters that are used to evaluate the performance of an optical imaging system, are always calculated by scalar diffraction method. Considering the vectorial properties of light, especially when the system numerical aperture (NA) is high, the accuracy of scalar method can no longer fulfill the requirement of analysis. The variations of properties of these parameters accompanied by using of vectorial method are discussed. In order to describe the effects of polarization clearly, assuming a system with identical polarization state within the entrance pupil, by formula deduction and numerical simulation, we conclude that due to the influence of the polarization, the maximum intensity of system diffraction spot decreases, the resolution between image plane and parallel polarized orientation of incident light drops down, then the diffraction spot loses its circular symmetry, the focus depth changes slightly, and these effects becomes more apparent with the increase of system NA. Finally, the method of calculating the off-axis images is discussed in the paper.

Analysis and design of transmittance for an antireflective surface microstructure

In order to easily analyze and design the transmittance characteristics of an antireflective surface called the 'moth-eye structure', the validity of both scalar diffraction theory and effective medium theory is quantitatively evaluated by a comparison of diffraction efficiencies predicted from both simplified theories to exact results calculated by a rigorous electromagnetic theory. The effect of surface microstructure parameters including the normalized period and the normalized depth has been determined at normal incidence. It is found that, in general, when the normalized period is more than four wavelengths of the incident light the scalar diffraction theory is useful within the error of 5%. Besides, the effective medium theory is accurate for evaluating the diffraction efficiency within the error of less than 1% when the higher order diffraction waves other than zero order wave is not to propagate. In addition, the limitation of scalar diffraction method and effective refractive index method is dependent on not only the normalized period of surface profile but also the normalized groove depth.

Hybrid finite-difference time-domain Fresnel modeling of microscopy imaging

Full-wave electromagnetic analysis by the finite-difference time-domain (FDTD) method is combined with the scalar diffraction method to obtain a numerical tool for optical lens imaging. The FDTD method is applied to the modeling of scattering phenomenon in the vicinity of a target. Afterward, obtained results are coupled with the Fresnel diffraction method to project an image through the lens onto the image plane. The FDTD algorithm is able to provide the solution of the electromagnetic analysis of the target with all its geometrical complexity. It is also relatively effective, since it allows the arbitrarily shaped geometry of the target to be analyzed in a specified spectrum range with only one simulation run. Such coupled FDTD-Fresnel modeling exploits the advantages of both methods, maintaining the required accuracy and speeding up the overall computation of the image.

A hybrid diffraction method for the design of etched diffraction grating demultiplexers

A novel hybrid diffraction method is proposed to simulate the demultiplexing of an etched diffraction grating (EDG). The scalar diffraction formula is adopted to simulate the propagation of the light field in the free propagation region (FPR), and a rigorous coupled-wave analysis (RCWA) is used to calculate the polarization-dependent diffraction near the grating. The hybrid diffraction method takes the advantages of both the conventional scalar diffraction method and the rigorous coupled-wave analysis and can simulate accurately the imaging property as well as the polarization-dependent efficiency of an EDG demultiplexer. Compared with the conventional scalar diffraction method, the proposed hybrid diffraction method reduces greatly the design error in, for example, the polarization-dependent insertion loss.

Rigorous Coupled-Wave Analysis of Multilayered Grating Structures

Using a rigorous coupled-wave analysis, a (2 /spl times/ 2) matrix method in homogenous layered media is generalized for a grating-embedded multilayer structure with (2M /spl times/ 2M) dynamic and propagation matrices in both transverse-electric and transverse-magnetic polarizations, where M is the number of harmonics retained in the field expansion. A numerically stable algorithm to implement the method is described. Numerical results are presented for a diffraction-based optoacoustic sensor that consists of a reflective movable membrane and fixed grating fingers on a transparent quartz substrate and are compared with the scalar diffraction method to illustrate the limited applicability of the scalar approach.

Dependence of Signals from Embossed Recording Fields and Grooves on Filling of Aperture of Objective Lens

The dependence of signals from embossed recording fields and grooves on the filling of the aperture of the objective lens was studied using a simulation based on a scalar diffraction method. The results indicated that this dependence is too large to ignore and consequently, the difference in the filling D/W between testers should be considered for the correlation between testers.