• Title, Summary, Keyword: diffraction

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Millimeter-wave diffraction-loss model based on over-rooftop propagation measurements

  • Kim, Kyung-Won;Kim, Myung-Don;Lee, Juyul;Park, Jae-Joon;Yoon, Young Keun;Chong, Young Jun
    • ETRI Journal
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    • v.42 no.6
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    • pp.827-836
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    • 2020
  • Measuring the diffraction loss for high frequencies, long distances, and large diffraction angles is difficult because of the high path loss. Securing a well-controlled environment to avoid reflected waves also makes long-range diffraction measurements challenging. Thus, the prediction of diffraction loss at millimeter-wave frequency bands relies on theoretical models, such as the knife-edge diffraction (KED) and geometrical theory of diffraction (GTD) models; however, these models produce different diffraction losses even under the same environment. Our observations revealed that the KED model underestimated the diffraction loss in a large Fresnel-Kirchhoff diffraction parameter environment. We collected power-delay profiles when millimeter waves propagated over a building rooftop at millimeter-wave frequency bands and calculated the diffraction losses from the measurements while eliminating the multipath effects. Comparisons between the measurements and the KED and GTD diffraction-loss models are shown. Based on the measurements, an approximation model is also proposed that provides a simple method for calculating the diffraction loss using geometrical parameters.

Fresnel Diffraction and Fraunhoffer Diffraction (TEM 관련 이론해설 (1): 프레넬 회절과 프라운호퍼 회절)

  • Lee, Hwack-Joo
    • Applied Microscopy
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    • v.32 no.2
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    • pp.81-90
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    • 2002
  • In this review, the author discussed how the Fresnel and Fraunhoffer Diffraction can be deduced from the Huygens-Fresnel principle and Kirchhoff Diffraction Theory. Fresnel diffraction became the basic theory of the CTEM image theory, and Fraunhoffer diffraction became the base for electron diffraction and HRTEM image theory by Fourier transformation. The author also discussed the diffraction based on Born series.

Fabrication and improvement of diffraction grating with femtosecond and $CO_2$ laser (레이저를 이용한 회절격자 제작 및 효율 향상 연구)

  • Choi, Hun-Kook;Sohn, Ik-Bu;Noh, Young-Chul;Kim, Jin-Tae
    • Laser Solutions
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    • v.15 no.2
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    • pp.6-10
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    • 2012
  • We fabricated the diffraction grating on the surface of fused silica glass using a femtosecond laser. The grooves of diffraction grating has a lot of micro crack and debris result in reduced diffraction efficiency. So, we polished the diffraction grating with $CO_2$ laser beam. With different scan number of $CO_2$ laser beam, we observed the image of diffraction grating and measured the diffraction efficiency.

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Spectral Analysis of Arrayed Waveguide Grating (Arrayed Waveguide Grating의 스펙트럼해석)

  • Jung, Jae-Hoon
    • Journal of IKEEE
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    • v.8 no.1
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    • pp.121-127
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    • 2004
  • We performed the spectrum analysis of arrayed waveguide grating using Fresnel Kirchhoff diffraction formula and its approximated Fraunhofer diffraction equation and applied both methods to 16 channel and 40 channel models. We presented the spectra and found out the limitations of Fraunhofer diffraction in analysis of arrayed waveguide grating and compared the errors coming from Fraunhofer diffraction approximation and due to imperfection during the fabrication process.

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The Inverse Modeling of Diffraction Phenomena under Plane Wave Incidence using Neural Network (평면파 입사시 신경회로망을 이용한 회절현상의 역모델링)

  • Na, Hui-Seung
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.24 no.5
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    • pp.1175-1182
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    • 2000
  • Diffraction systematically causes error in acoustic measurements. Most probes are designed to reduce this phenomenon. On the contrary, this paper proposes a spherical probe a] lowing acoustic inten sity measurements in three dimensions to be made, which creates a diffracted field that is well-defined, thanks to analytic solution of diffraction phenomena. Six microphones are distributed on the surface of the sphere along three rectangular axes. Its measurement technique is not based on finite difference approximation, as is the case for the ID probe but on the analytic solution of diffraction phenomena. In fact, the success of sound source identification depends on the inverse models used to estimate inverse diffraction phenomena, which has nonlinear properties. In this paper, we propose the concept of nonlinear inverse diffraction modeling using a neural network and the idea of 3 dimensional sound source identification with better performances. A number of computer simulations are carried out in order to demonstrate the diffraction phenomena under various angles. Simulations for the inverse modeling of diffraction phenomena have been successfully conducted in showing the superiority of the neural network.

Analysis on Design and Fabrication of High-diffraction-efficiency Multilayer Dielectric Gratings

  • Cho, Hyun-Ju;Lee, Kwang-Hyun;Kim, Sang-In;Lee, Jung-Hwan;Kim, Hyun-Tae;Kim, Won-Sik;Kim, Dong Hwan;Lee, Yong-Soo;Kim, Seoyoung;Kim, Tae Young;Hwangbo, Chang Kwon
    • Current Optics and Photonics
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    • v.2 no.2
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    • pp.125-133
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    • 2018
  • We report an in-depth analysis of the design and fabrication of multilayer dielectric (MLD) diffraction gratings for spectral beam combining at a wavelength of 1055 nm. The design involves a near-Littrow grating and a modal analysis for high diffraction efficiency. A range of wavelengths, grating periods, and angles of incidence were examined for the near-Littrow grating, for the $0^{th}$ and $-1^{st}$ diffraction orders only. A modal method was then used to investigate the effect of the duty cycle on the effective indices of the grating modes, and the depth of the grating was determined for only the $-1^{st}$-order diffraction. The design parameters of the grating and the matching layer thickness between grating and MLD reflector were refined for high diffraction efficiency, using the finite-difference time-domain (FDTD) method. A high reflector was deposited by electron-beam evaporation, and a grating structure was fabricated by photolithography and reactive-ion etching. The diffraction efficiency and laser-induced damage threshold of the fabricated MLD diffraction gratings were measured, and the diffraction efficiency was compared with the design's value.

Electromagnetic scattering characteristics of a hyperbolic reflector antenna accounting for the UTD higher order diffraction (UTD 고차회절을 고려한 쌍곡면 반사판 아테나의 전자파 산란 특성)

  • 최재훈;이병우;이상설
    • Journal of the Korean Institute of Telematics and Electronics A
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    • v.33A no.5
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    • pp.85-93
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    • 1996
  • The far-zone scattered field patterns of a hyperbolic reflector antenna are analyzed by using uniform geometrical theory of diffraction(UTD). The main objective of this paper is to obtain the higher order diffraction contributions which provide the continuity over the shadow boundaries of the first order solution. to obtain the scattered magnetic field characteristics, the scattered field components of the secodn-order diffraction, diffraction-reflection, diffraction-reflection-diffraction terms are added to the result of the previous research. The results of the present research are compared to those of the first order solution and the method of moments. One can observe the improvemtn of the current approach over the first order solution. also, the results of the present method agree very well with those of the moment methods especially in the transition regions near the first order diffraction shadow boundaries.

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Optical Analysis of Diffraction Grating and Fresnel Zone Plate Fabricated on Fused Silica Glass by a Femtosecond Laser (펨토초 레이저를 이용한 회절격자와 Fresnel Zone Plate 제작 및 광학적 분석)

  • Ryu, Jin-Chang;Kim, Jin-Tae;Sohn, Ik-Bu
    • Journal of the Korean Society for Precision Engineering
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    • v.27 no.3
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    • pp.18-26
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    • 2010
  • Diffraction gratings with precise spatial periods of 2 ${\mu}m$ and 5 ${\mu}m$ have been fabricated by using a femtosecond laser which does not have limits on materials of micromachining and small thermal effects due to high peak power. Diffraction angle and diffraction efficiency of those were measured. Simulation results of diffraction angle and diffraction efficiency of the diffraction grating calculated with the parameters such as line width, depth, and spatial period of the fabricated gratings were compared with experimental results measured with a He-Ne laser. Besides these, Fresnel Zone Plates (FZPs) with focal distances of 50 mm and 25 mm were fabricated and focal distances of fabricated FZP were measured. Those experimental results for diffraction gratings and FZPs match well with experimental results.

Residual Stress Measurement on Welded Specimen by Neutron Diffraction (중성자 회절을 이용한 용접부위의 잔류응력 측정)

  • 박만진;장동영;최희동
    • Journal of Welding and Joining
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    • v.20 no.2
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    • pp.50-58
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    • 2002
  • Residual stress is generated in the structures as a result of irregular elastic-plastic deformation during fabrication processes such as welding, heat treatment, and mechanical processing. There are several factors attributed to the origin of residual stresses, tensile or compressive. The stresses can be determined by destructive ways or nondestructive ways using X-ray or neutron diffraction. Although X-ray diffraction is a well established technique, it is practically limited to near-surface stresses. Neutrons penetrate easily into most materials and neutron diffraction permits non-destructive evaluation of lattice strain within the bulk of large specimens because the radiation is more deeply penetrating for metallic engineering components. This paper presented application of neutron diffraction technique to the residual stress measurement using 20 mm thick welded stainless steel plate($100{\times}100 \textrm{mm}^2$)

Frequency domain analysis of Froude-Krylov and diffraction forces on TLP

  • Malayjerdi, Ebrahim;Tabeshpour, Mohammad Reza
    • Ocean Systems Engineering
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    • v.6 no.3
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    • pp.233-244
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    • 2016
  • Tension Leg Platform (TLP) is a floating structure that consists of four columns with large diameter. The diffraction theory is used to calculate the wave force of floating structures with large dimensions (TLP). In this study, the diffraction and Froude-Krylov wave forces of TLP for surge, sway and heave motions and wave force moment for roll, pitch degrees of freedom in different wave periods and three wave approach angles have been investigated. From the numerical results, it can be concluded that the wave force for different wave approach angle is different. There are some humps and hollows in the curve of wave forces and moment in different wave periods (different wavelengths). When wave incidents with angle 0 degree, the moment of diffraction force for pitch in high wave periods (low frequencies) is dominant. The diffraction force for heave in low wave periods (high wave frequencies) is dominant. The phase difference between Froude-Krylov and diffraction forces is important to obtain total wave force.