Dae Wook Kim

Bio: Dae Wook Kim is an academic researcher from University of Arizona. The author has contributed to research in topics: Polishing & Telescope. The author has an hindex of 16, co-authored 122 publications receiving 1043 citations. Previous affiliations of Dae Wook Kim include Yonsei University & Steward Health Care System.

Rigid conformal polishing tool using non-linear visco-elastic effect

Abstract: Computer controlled optical surfacing (CCOS) relies on a stable and predictable tool influence function (TIF), which is the shape of the wear function created by the machine. For a polishing lap, which is stroked on the surface, both the TIF stability and surface finish rely on the polishing interface maintaining intimate contact with the workpiece. Pitch tools serve this function for surfaces that are near spherical, where the curvature has small variation across the part. The rigidity of such tools provides natural smoothing of the surface, but limits the application for aspheric surfaces. Highly flexible tools, such as those created with an air bonnet or magnetorheological fluid, conform to the surface, but lack intrinsic stiffness, so they provide little natural smoothing. We present a rigid conformal polishing tool that uses a non-linear visco-elastic medium (i.e. non-Newtonian fluid) that conforms to the aspheric shape, yet maintains stability to provide natural smoothing. The analysis, design, and performance of such a polishing tool is presented, showing TIF stability of <10% and providing surface finish with <10A roughness.

Non-sequential optimization technique for a computer controlled optical surfacing process using multiple tool influence functions.

Abstract: Optical surfaces can be accurately figured by computer controlled optical surfacing (CCOS) that uses well characterized sub-diameter polishing tools driven by numerically controlled (NC) machines. The motion of the polishing tool is optimized to vary the dwell time of the polisher on the workpiece according to the desired removal and the calibrated tool influence function (TIF). Operating CCOS with small and very well characterized TIF achieves excellent performance, but it takes a long time. This overall polishing time can be reduced by performing sequential polishing runs that start with large tools and finish with smaller tools. In this paper we present a variation of this technique that uses a set of different size TIFs, but the optimization is performed globally – i.e. simultaneously optimizing the dwell times and tool shapes for the entire set of polishing runs. So the actual polishing runs will be sequential, but the optimization is comprehensive. As the optimization is modified from the classical method to the comprehensive non-sequential algorithm, the performance improvement is significant. For representative polishing runs we show figuring efficiency improvement from ~88% to ~98% in terms of residual RMS (root-mean-square) surface error and from ~47% to ~89% in terms of residual RMS slope error.

Parametric modeling of edge effects for polishing tool influence functions.

Abstract: Computer controlled polishing requires accurate knowledge of the tool influence function (TIF) for the polishing tool (i.e. lap). While a linear Preston's model for material removal allows the TIF to be determined for most cases, nonlinear removal behavior as the tool runs over the edge of the part introduces a difficulty in modeling the edge TIF. We provide a new parametric model that fits 5 parameters to measured data to accurately predict the edge TIF for cases of a polishing tool that is either spinning or orbiting over the edge of the workpiece.

Static tool influence function for fabrication simulation of hexagonal mirror segments for extremely large telescopes.

Abstract: We present a novel simulation technique that offers efficient mass fabrication strategies for 2m class hexagonal mirror segments of extremely large telescopes. As the first of two studies in series, we establish the theoretical basis of the tool influence function (TIF) for precessing tool polishing simulation for non-rotating workpieces. These theoretical TIFs were then used to confirm the reproducibility of the material removal foot-prints (measured TIFs) of the bulged precessing tooling reported elsewhere. This is followed by the reverse-computation technique that traces, employing the simplex search method, the real polishing pressure from the empirical TIF. The technical details, together with the results and implications described here, provide the theoretical tool for material removal essential to the successful polishing simulation which will be reported in the second study.

Parametric smoothing model for visco-elastic polishing tools

Abstract: A parametric smoothing model is developed to quantitatively describe the smoothing action of polishing tools that use visco-elastic materials. These materials flow to conform to the aspheric shape of the workpieces, yet behave as a rigid solid for short duration caused by tool motion over surface irregularities. The smoothing effect naturally corrects the mid-to-high frequency errors on the workpiece while a large polishing lap still removes large scale errors effectively in a short time. Quantifying the smoothing effect allows improvements in efficiency for finishing large precision optics. We define normalized smoothing factor SF which can be described with two parameters. A series of experiments using a conventional pitch tool and the rigid conformal (RC) lap was performed and compared to verify the parametric smoothing model. The linear trend of the SF function was clearly verified. Also, the limiting minimum ripple magnitude PVmin from the smoothing actions and SF function slope change due to the total compressive stiffness of the whole tool were measured. These data were successfully fit using the parametric smoothing model.

The Observation of Gravitational Waves from a Binary Black Hole Merger

Abstract: On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0×10(-21). It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203,000 years, equivalent to a significance greater than 5.1σ. The source lies at a luminosity distance of 410(-180)(+160) Mpc corresponding to a redshift z=0.09(-0.04)(+0.03). In the source frame, the initial black hole masses are 36(-4)(+5)M⊙ and 29(-4)(+4)M⊙, and the final black hole mass is 62(-4)(+4)M⊙, with 3.0(-0.5)(+0.5)M⊙c(2) radiated in gravitational waves. All uncertainties define 90% credible intervals. These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.

Sub-surface mechanical damage distributions during grinding of fused silica

Abstract: The distribution and characteristics of surface cracking (i.e. sub-surface damage or SSD) formed during standard grinding processes has been investigated on fused silica glass. The SSD distributions of the ground surfaces were determined by: (1) creating a shallow (18-108 {micro}m) wedge/taper on the surface by magneto-rheological finishing; (2) exposing the SSD by HF acid etching; and (3) performing image analysis of the observed cracks from optical micrographs taken along the surface taper. The observed surface cracks are characterized as near-surface lateral and deeper trailing indent type fractures (i.e., chatter marks). The SSD depth distributions are typically described by a single exponential distribution followed by an asymptotic cutoff in depth (c{sub max}). The length of the trailing indent is strongly correlated with a given process. Using established fracture indentation relationships, it is shown that only a small fraction of the abrasive particles are being mechanically loaded and causing fracture, and it is likely the larger particles in the abrasive particle size distribution that bear the higher loads. The SSD depth was observed to increase with load and with a small amount of larger contaminant particles. Using a simple brittle fracture model for grinding, the SSD depth distribution has been related tomore » the SSD length distribution to gain insight into ''effective'' size distribution of particles participating in the fracture. Both the average crack length and the surface roughness were found to scale linearly with the maximum SSD depth (c{sub max}). These relationships can serve as useful rules-of-thumb for nondestructively estimating SSD depth and to identify the process that caused the SSD. In certain applications such as high intensity lasers, SSD on the glass optics can serve as a reservoir for minute amounts of impurities that absorb the high intensity laser light and lead to subsequent laser-induced surface damage. Hence a more scientific understanding of SSD formation can provide a means to establish recipes to fabricate SSD-free, laser damage resistant optical surfaces.« less

Freeform optics for imaging

Abstract: In the last 10 years, freeform optics has enabled compact and high-performance imaging systems. This article begins with a brief history of freeform optics, focusing on imaging systems, including marketplace emergence. The development of this technology is motivated by the clear opportunity to enable science across a wide range of applications, spanning from extreme ultraviolet lithography to space optics. Next, we define freeform optics and discuss concurrent engineering that brings together design, fabrication, testing, and assembly into one process. We then lay out the foundations of the aberration theory for freeform optics and emerging design methodologies. We describe fabrication methods, emphasizing deterministic computer numerical control grinding, polishing, and diamond machining. Next, we consider mid-spatial frequency errors that inherently result from freeform fabrication techniques. We realize that metrologies of freeform optics are simultaneously sparse in their existence but diverse in their potential. Thus, we focus on metrology techniques demonstrated for the measurement of freeform optics. We conclude this review with an outlook on the future of freeform optics.