Inverse conversion algorithm for an all-optical depth coloring camera

Lim, Sungjin,Heo, Daerak,Kim, Mugeon,Choi, Geunseop,Hahn, Joonku OSA OPTICAL SOCIETY OF AMERICA 2017 Applied Optics Vol.56 No.34

<P>Three-dimensional (3D) metrology has received a lot of attention from academic and industrial communities due to its broad applications, such as 3D contents, 3D printing, and autonomous driving. The all-optical depth coloring (AODC) camera has some benefits in computation load since it extracts depth information of an object fully optically. The AODC camera represents the depth of the object as a variation of wavelength, and spectroscopy is generally required to measure the wavelength. However, in the AODC camera, the color vector in RGB color space is convertible inversely into the wavelength after projection on the normalized rgb plane because the detected spectrum through the gating part has a narrow bandwidth as a result of the width of the slit in the projection part. In this paper, we propose an inverse conversion algorithm from RGB color to depth without spectroscopy. Experimental results are presented to confirm its feasibility. Also, some practical limitations are discussed, resulting from the nonlinearity of the response of the image sensor and the widths of the slits in the projection part and the gating part. (C) 2017 Optical Society of America</P>

Experimental power spectral density analysis for mid- to high-spatial frequency surface error control

Del Hoyo, Javier,Choi, Heejoo,Burge, James H.,Kim, Geon-Hee,Kim, Dae Wook OSA OPTICAL SOCIETY OF AMERICA 2017 Applied Optics Vol.56 No.18

<P>The control of surface errors as a function of spatial frequency is critical during the fabrication of modern optical systems. A large-scale surface figure error is controlled by a guided removal process, such as computer-controlled optical surfacing. Smaller-scale surface errors are controlled by polishing process parameters. Surface errors of only a few millimeters may degrade the performance of an optical system, causing background noise from scattered light and reducing imaging contrast for large optical systems. Conventionally, the microsurface roughness is often given by the root mean square at a high spatial frequency range, with errors within a 0.5 x 0.5 mm local surface map with 500 x 500 pixels. This surface specification is not adequate to fully describe the characteristics for advanced optical systems. The process for controlling and minimizing mid- to high-spatial frequency surface errors with periods of up to similar to 2-3 mm was investigated for many optical fabrication conditions using the measured surface power spectral density (PSD) of a finished Zerodur optical surface. Then, the surface PSD was systematically related to various fabrication process parameters, such as the grinding methods, polishing interface materials, and polishing compounds. The retraceable experimental polishing conditions and processes used to produce an optimal optical surface PSD are presented. (C) 2017 Optical Society of America</P>

Multiple-3D-object decryption based on one interference using two phase-only functions

Li, Wei-Na,Lee, Sang-Min,Gil, Sang Keun,Kim, Nam OSA OPTICAL SOCIETY OF AMERICA 2017 Applied Optics Vol.56 No.22

<P>We propose a multiple-3D-object decryption scheme based on one interference using two beams that are from two different phase-only functions. It takes advantage of off-axis digital holography to extract the optical fields of multiple 3D objects, and respectively utilize single different decryption keys to decrypt multiple 3D objects in the decryption procedure. The advantages of the proposed scheme include the following: each 3D object can be decrypted discretionarily without decrypting a series of other objects earlier; no iterative algorithm is involved; and the decrypted image of each object can be successfully clearly distinguished. The feasibility of the proposed scheme is verified by the optical holograms of real 3D objects. (C) 2017 Optical Society of America</P>

Steel-surface defect detection using a switching-lighting scheme

Jeon, Yong-Ju,Choi, Doo-chul,Lee, Sang Jun,Yun, Jong Pil,Kim, Sang Woo OSA OPTICAL SOCIETY OF AMERICA 2016 Applied Optics Vol.55 No.1

<P>In this paper a novel filtering scheme combined with a lighting method is proposed for defect detection in steel surfaces. A steel surface has non-uniform brightness and various shaped defects, which cause difficulties in defect detection. To solve this problem we propose a sub-optimal filtering that is combined with a switching-lighting method. First, dual-light switching lighting (DLSL) is explained, which decreases the effect of non-uniformity of surface brightness and improves the detection accuracy. By using the DLSL method, defects are represented as alternated black and white patterns regardless of the size, shape, or orientation of defects. Therefore, defects can be detected by finding alternated black and white patterns. Second, we propose a scheme for detecting defects in steel-surface images acquired using the DLSL method. The presence of scales strongly affects the optical properties of the surface. Moreover, the textures of steel-plate images vary greatly because of the temperature and grade of steel. Therefore, conventional filter-design methods are not effective for different image textures. A sub-optimal scheme based on an optimized general-finite impulse-response filter is also proposed. Finally, experimental results conducted on steel-surface images from an actual steel-production line show the effectiveness of the proposed algorithm. (C) 2015 Optical Society of America</P>

Wavelength and fluence-dependent third-order optical nonlinearity of mono- and multi-layer graphene

Ahn, Kwang Jun,Gwak, Ji Yoon,Lee, Byung Jic,Choi, Sun Young,Kim, Mi Hye,Baek, In Hyung,Jeong, Young Uk,Rotermund, Fabian OSA OPTICAL SOCIETY OF AMERICA 2017 Applied Optics Vol.56 No.36

<P>The objective of this study is to investigate third-order optical nonlinearities of monolayer and randomly stacked bi- and four-layer graphene samples, depending on wavelength and fluence of femtosecond laser pulses in the near-infrared spectral region. Nonlinear refractive indices and absorption coefficients, and in consequence, third-order susceptibilities of graphene samples, are determined by z-scan measurements at four different wavelengths and three fluence regimes categorized by nonlinear transmission measurements. In addition, nonlinear refractive indices are independently investigated by optical Kerr gate experiments, showing good agreement with z-scan results. Our study on third-order nonlinearities of mono-and multi-layer graphene samples is significant for understanding nonlinear optical characteristics of graphene and developing graphene-based nonlinear photonic devices. (C) 2017 Optical Society of America</P>

Three-dimensional image acquisition and reconstruction system on a mobile device based on computer-generated integral imaging

Erdenebat, Munkh-Uchral,Kim, Byeong-Jun,Piao, Yan-Ling,Park, Seo-Yeon,Kwon, Ki-Chul,Piao, Mei-Lan,Yoo, Kwan-Hee,Kim, Nam OSA OPTICAL SOCIETY OF AMERICA 2017 Applied Optics Vol.56 No.28

<P>A mobile three-dimensional image acquisition and reconstruction system using a computer-generated integral imaging technique is proposed. A depth camera connected to the mobile device acquires the color and depth data of a real object simultaneously, and an elemental image array is generated based on the original three-dimensional information for the object, with lens array specifications input into the mobile device. The three-dimensional visualization of the real object is reconstructed on the mobile display through optical or digital reconstruction methods. The proposed system is implemented successfully and the experimental results certify that the system is an effective and interesting method of displaying real three-dimensional content on a mobile device.(C) 2017 Optical Society of America</P>

Visualization and label-free quantification of microfluidic mixing using quantitative phase imaging

Park, GwangSik,Han, Dongsik,Kim, GwangSu,Shin, Seungwoo,Kim, Kyoohyun,Park, Je-Kyun,Park, YongKeun OSA OPTICAL SOCIETY OF AMERICA 2017 Applied optics Vol.56 No.22

<P>Microfluidic mixing plays a key role in various fields, including biomedicine and chemical engineering. To date, although various approaches for imaging microfluidic mixing have been proposed, they provide only quantitative imaging capability and require exogenous labeling agents. Quantitative phase imaging techniques, however, circumvent these problems and offer label-free quantitative information about concentration maps of microfluidic mixing. We present the quantitative phase imaging of microfluidic mixing in various types of polydimethylsiloxane microfluidic channels with different geometries; the feasibility of the present method was validated by comparing it with the results obtained by theoretical calculation based on Fick's law. (C) 2017 Optical Society of America</P>

Complex object wave extraction using time-multiplexing in off-axis digital holography

Dashdavaa, Erkhembaatar,Erdenebat, Munkh-Uchral,Shin, Chang-Won,Kim, Nam OSA OPTICAL SOCIETY OF AMERICA 2018 Applied optics Vol.57 No.1

<P>In this paper, we present an algorithm for complex object wave extraction in off-axis digital holography using a time-multiplexing and frequency spectrum-shifting technique. The proposed approach utilizes the digital time-multiplexing technique, in which two 90-deg-rotated off-axis holograms are recorded in sequence, and corresponding spectra are subtracted in the computed Fourier domain to eliminate the DC term. Then, the two subtracted holograms are digitally multiplexed into one complex hologram in the same plane, and by shifting the spatial frequency spectra of the subtracted hologram in the spatial frequency domain, one of the two cross-correlations can be obtained in the center. This technique simply extracts the spectrum of the real image in the frequency domain than the spatial filtering method. (C) 2017 Optical Society of America</P>

Wavelength measurement by Fourier analysis of interference fringes through a plane parallel plate

Lee, Choonghwan,Choi, Heejoo,Kim, Jiung,Cha, Myoungsik,Jin, Jonghan OSA OPTICAL SOCIETY OF AMERICA 2017 Applied Optics Vol.56 No.35

<P>When a diverging laser beam passed through a plane parallel glass plate, interference fringes were observed; analysis of these fringes provided accurate estimation of the source wavelength. The fringes had a unique angular range of uniform fringe density. Fourier transform of the fringes in this range directly provided wavelength information. Reference lasers were used to establish a calibration between the fringe density and wavenumber, with which we estimated the wavelength of a test laser. An accuracy of 4.5 x 10(-5) was obtained, which is better than that of conventional grating spectrometers, while providing a much broader free spectral range. Our method has unique features, such as extreme simplicity of the setup, fast analysis, and low cost, which are great advantages in practical wavelength meter applications. (C) 2017 Optical Society of America</P>

Broadening the reflection bandwidth of polymer-stabilized cholesteric liquid crystal via a reactive surface coating layer

Kim, Jongyoon,Kim, Hyungmin,Kim, Seongil,Choi, Suseok,Jang, Wonbong,Kim, Jinwuk,Lee, Ji-Hoon OSA OPTICAL SOCIETY OF AMERICA 2017 Applied Optics Vol.56 No.20

<P>We report a method of broadening the reflection bandwidth of polymer-stabilized cholesteric liquid crystal (PSCLC). The top substrate was consecutively coated with a polyimide (PI) and a reactive mesogen (RM) layer, while the bottom substrate was coated with only PI. We exposed the top substrate with the RM coating to UV light. The reflection bandwidth of the PSCLC samples where the top substrate was over-coated with RM was significantly broader than the samples where both substrates were coated with PI. In addition to the effect of the UV intensity gradient, the RM-coated top substrate has a chemical affinity to bulk RM, promoting formation of the pitch gradient and broadening the reflection bandwidth in the sample. (C) 2017 Optical Society of America.</P>