Dr. In-Ung Song Profile

Dr. In-Ung Song

at KRISS

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Publications (4)

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Proceedings Article | 27 August 2024 Poster + Paper

Fabrication of the first GMT ASM reference body

In-Ung Song, Ho-Soon Yang, Andrew Lowman, Chang Jin Oh, Daniel Caywood, Jamison Noenickx, Roberto Biasi, Daniele Gallieni

Proceedings Volume 13097, 130975J (2024) https://doi.org/10.1117/12.3020900

KEYWORDS: Fabrication, Surface finishing, Optical surfaces, Optical fabrication, Zerodur

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In this study, we present the report about the first segment of the Giant Magellan Telescope (GMT) Adaptive Secondary Mirror (ASM) reference body. The GMT ASM comprises 7 segmented deformable mirrors, including a thin shell, actuators, and a reference body. In this formation, one critical role of the reference body is to provide the reference surface form for the thin shell mirror with supporting actuators. Thus, it is one of key components to succeed in development of GMT ASM. Through collaboration between the Korea Research Institute of Standards and Science (KRISS) and the University of Arizona (UArizona), the first segment of the off-axis reference body has been successfully developed. Produced from Zerodur, the reference body has a light-weighted pattern characterized by a set of non-parallel rib/wall structures. It has 675 cylindrical actuator holes along the surface’s normal vector according to the designed off-axis aspheric form. A 5-axis computer numerical control (CNC) machine fabricated this complex structure. The front surface was ground from a best-fit sphere to an off-axis asphere using a large polishing machine. A laser tracker was employed to meticulously measure the surface form errors of the front surface under the grinding step during the fabrication process. After polishing, the mechanical measurement on a large Coordinate Measuring Machine (CMM) and the optical measurement using Software Configurable Optical Test System (SCOTS) confirmed that the GMT ASM reference body adhered to the stringent surface form error requirement. This product has been shipped to AdOptica, Italy, for the final assembly of the ASM with actuators and the thin shell.

Proceedings Article | 4 October 2023 Presentation + Paper

Full-aperture wavefront reconstruction method in quadri-wave lateral shearing interferometry (QWLSI)

In-Ung Song, Ho-Soon Yang, Yun-Woo Lee

Proceedings Volume 12672, 126720E (2023) https://doi.org/10.1117/12.2677170

KEYWORDS: Wavefronts, Wavefront reconstruction, Reconstruction algorithms, Interferograms, Interferometry, Wavefront errors, Optical testing, Mirrors

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This study presents a method for reconstructing the full-aperture wavefront, particularly at the edge-side wavefront in Quadri-Wave Lateral Shearing Interferometry (QWLSI). While QWLSI is a powerful method for measuring wavefront error in an optical system, it faces the challenge of obtaining wavefront information on the non-interfering region on the edge side of the input beam by lateral shearing. This challenge becomes more significant for larger apertures. To address this, the study proposes a modified method for reconstructing the full-aperture wavefront using the basic principle of partial slope detection in lateral shearing interferometry (LSI). QWLSI provides four directional two-point slope functions by wavefront shearing. The inverse LSI slope can be deduced using the spatial relation between the reconstructed center wavefront and the measured slope information. The partial edge-side wavefront can then be reconstructed using this inverse equation, following the four-shearing direction. Finally, the partial edge-side wavefront maps can be combined using a phase stitching algorithm to obtain a full-aperture wavefront. The effectiveness of this method was demonstrated through numerical simulation and experimental results, which showed that the proposed method accurately reconstructed the noninterfering edge-side wavefront in QWLSI, improving the spatial range of wavefront measurement. This technique has potential applications in optical testing, particularly in improving the efficiency of large aperture mirror testing by reducing the margin diameter of the mirror blank preparation. The study introduces the reconstruction method in detail and presents some testing results for verification.

Proceedings Article | 1 August 2021 Presentation + Paper

A displacement probe using backscattering for profiling the ground surface form

In-Ung Song, Ho-Soon Yang, Jun-Gyu Park, Goeun Kim, Kyuman Cho, Sug-Whan Kim

Proceedings Volume 11817, 1181709 (2021) https://doi.org/10.1117/12.2594367

KEYWORDS: Backscatter, Interferometers, Demodulation, Profilometers, Profiling, Modulation

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We present a new displacement probe for a gantry-type profilometer to measure the ground surface form. The system is based on heterodyne interferometry with an acousto-optic modulator (AOM) and I/Q demodulation scheme. When the light from the single-frequency laser diode passes through the AOM following the Bragg condition angle, it is divided into the non-diffracted zero-order and a diffracted first-order beam with a frequency shift as the amount of the AOM driving frequency. One beam is used for the test and the other for the reference beam. Orthogonally reflected beams pass through the AOM one more time, and these beams make a heterodyne signal with a double modulation frequency. In this case, the only backscattered beam from the ground surface satisfies the Bragg condition, producing the modulated interference. Therefore, the AOM tends to reduce the stray light noises from the other incidence angles, acting as a transmission filter for backscattered light. The interference light arrives at the high-speed photodiode and it is then demodulated using the I/Q demodulation to extract the phase value. Although the backscattered ray from the rough glass surface has a very low intensity, the I/Q interferometer can detect the signal because the phase can be acquired regardless of the intensity change. Preliminary experiments confirmed that the system can measure from backscattered light of ground glass surface form.

Proceedings Article | 15 September 2020 Presentation + Paper

Development of a new image based BRDF measurement system using a semicircular ring

Seul Ki Yang, In-Ung Song, Eunsong Oh, Ho-Soon Yang, Sug-Whan Kim

Proceedings Volume 11485, 114850M (2020) https://doi.org/10.1117/12.2568347

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We developed a new measurement system for bidirectional reflectance distribution functions (BRDF). The system can obtain simultaneously isotropic BRDF of all scattering angles utilizing a semicircular ring and an image sensor. First, we predicted the performance of our measurement system using integrated ray tracing simulation. The light path is as follows: the light from the light source at 635 nm is reflected off the target material, and the light is reflected back at the semi-circular ring. The image sensor records the light reflected from the semicircular ring. The results show good agreement with original and simulation BRDF, but detailed analysis suggested. The system improves significantly measurement time and resolution of reflection angles. Furthermore, the system is not only more cost effective than other traditional measurement systems, but also eliminated the temporal fluctuation of the light source intensity.

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