Publications by authors named "Ki-Nam Joo"

24 Publications

  • Page 1 of 1

Motionless Polarizing Structured Illumination Microscopy.

Sensors (Basel) 2021 Apr 17;21(8). Epub 2021 Apr 17.

3D Optical Metrology Laboratory, Department of Photonic Engineering, Chosun University, 309 Pilmun-Daero, Dong-gu, Gwangju 61452, Korea.

In this investigation, we propose a motionless polarizing structured illumination microscopy as an axially sectioning and reflective-type device to measure the 3D surface profiles of specimens. Based on the spatial phase-shifting technique to obtain the visibility of the illumination pattern. Instead of using a grid, a Wollaston prism is used to generate the light pattern by the stable interference of two beams. As the polarization states of two beams are orthogonal with each other, a polarization pixelated CMOS camera can simultaneously obtain four phase-shifted patterns with the beams after passing through a quarter wave plate based on the spatial phase-shifting technique with polarization. In addition, a focus tunable lens is used to eliminate a mechanical moving part for the axial scanning of the specimen. In the experimental result, a step height sample and a concave mirror were measured with 0.05 µm and 0.2 mm repeatabilities of step height and the radius of curvature, respectively.
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http://dx.doi.org/10.3390/s21082837DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8073734PMC
April 2021

Vision chromatic confocal sensor based on a geometrical phase lens.

Appl Opt 2021 Apr;60(10):2898-2901

A vison chromatic confocal sensor used to monitor the location of a measured point is proposed and experimentally verified. To induce chromatic aberration of the sensor, a geometrical phase lens is adopted and is also used as a beam splitter. Near the geometrical phase lens, a focused beam is used for the chromatic confocal sensor, and a diverging beam is used for imaging of the specimen. In the experiment, the performance of the proposed system was verified with regard to distance sensing and the capability of monitoring the measured points. The measuring range was approximately 10 mm, and the repeatability was 0.4 µm when a geometrical phase lens with a 75 mm focal length was used.
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http://dx.doi.org/10.1364/AO.423339DOI Listing
April 2021

Spectrally resolved polarizing interferometer for single-shot line profiling.

Appl Opt 2020 Nov;59(32):10107-10112

In this investigation, we describe a spectrally resolved polarizing interferometer to modify the typical spectrally resolved interferometer based on the Fourier method, which has a dead zone caused by the minimum measurable range. By use of a polarization pixelated CMOS camera, which enables us to obtain spatially phase-shifted spectral interferograms, the spectral phase can be extracted with a single image, and the proposed interferometer can obtain a line profile of a specimen at once. By the nature of phase-shifting technique, moreover, the directions of the measured distances can be identified, and the measuring range is extended to twice that of the typical spectrally resolved interferometer. In the experimental results, the measuring range was 104 µm, and the capability to obtain a line profile of a specimen with a single image was confirmed. For 3D surface profiling, lateral scanning was adopted, and the measurement results were compared with the result of the Fourier method.
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http://dx.doi.org/10.1364/AO.408594DOI Listing
November 2020

Diverging cyclic radial shearing interferometry for single-shot wavefront sensing.

Appl Opt 2020 Oct;59(28):9067-9074

In this investigation, we describe a simple cyclic radial shearing interferometer for single-shot wavefront sensing. Instead of using the telescope lens system used in typical radial shearing interferometry, a single lens is used to generate two diverging radial shearing beams. This simple modification leads to the advantages of conveniently adjusting the radial shearing ratio, compactness of the system, and practical ease of alignment. With the aid of a polarization pixelated CMOS camera, the spatial phase-shifting technique is used to extract the phase with a single image. The most important feature is the fringe contrast enhancement by reducing the aberrations caused by the complicated optical system even though an incoherent light is used. The experimental results show the fringe contrast enhancement is at least 0.1 better than that of the conventional method, and the wavefronts are properly reconstructed with less than 0.071 root-mean-squared wavefront error regardless of the coherence of the light.
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http://dx.doi.org/10.1364/AO.402903DOI Listing
October 2020

Advances in optical metrology and instrumentation: introduction.

J Opt Soc Am A Opt Image Sci Vis 2020 Sep;37(9):OMI1-OMI2

Optical measurement and characterization are two of the pillars of metrology. The ability to measure precisely with high dynamic range and accuracy betters our understanding of nature and the universe. In this feature issue, we present a collection of articles that delves into the fundamental techniques used to advance the field.
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http://dx.doi.org/10.1364/JOSAA.405559DOI Listing
September 2020

A compact high-precision periodic-error-free heterodyne interferometer.

J Opt Soc Am A Opt Image Sci Vis 2020 Sep;37(9):B11-B18

We present the design, bench-top setup, and experimental results of a compact heterodyne interferometer that achieves picometer-level displacement sensitivities in air over frequencies above 100 MHz. The optical configuration with spatially separated beams prevents frequency and polarization mixing, and therefore eliminates periodic errors. The interferometer is designed to maximize common-mode optical laser beam paths to obtain high rejection of environmental disturbances, such as temperature fluctuations and acoustics. The results of our experiments demonstrate the short- and long-term stabilities of the system during stationary and dynamic measurements. In addition, we provide measurements that compare our interferometer prototype with a commercial system, verifying our higher sensitivity of 3 pm, higher thermal stability by a factor of two, and periodic-error-free performance.
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http://dx.doi.org/10.1364/JOSAA.396298DOI Listing
September 2020

Single-shot freeform surface profiler.

Opt Express 2020 Feb;28(3):3401-3409

We propose a novel and simple method of single-shot freeform surface profiler based on spatially phase-shifted lateral shearing interferometry. By the adoption of birefringent materials, the laterally shearing waves are simply generated without any bulky and complicated optical components. Moreover, the phase maps that lead to the 3D profile of the freeform surface can be instantly obtained by the spatial phase-shifting technique using a pixelated polarizing camera. The proposed method was theoretically described and verified by measuring several samples in comparison to the measurement results with a well-established stylus probe.
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http://dx.doi.org/10.1364/OE.380305DOI Listing
February 2020

Single-Shot Imaging of Two-Wavelength Spatial Phase-Shifting Interferometry.

Sensors (Basel) 2019 Nov 21;19(23). Epub 2019 Nov 21.

3D Optical Metrology Laboratory, Department of Photonic Engineering, Chosun University, 309 Pilmun-daero, Dong-gu, Gwangju 61452, Korea.

In this investigation, we propose an effective method to measure 3D surface profiles of specimens with single-shot imaging. Based on the two-wavelength interferometric principle and spatial phase-shifting technique using a polarization pixelated camera, the proposed system can not only rapidly measure the phase, but also overcome the 2π-ambiguity problem of typical phase-shifting interferometry. The rough surface profile can be calculated by the visibility of the interference fringe and can compensate for the height discontinuity by phase jumps occurring in a fine height map. An inclined plane mirror and a step height specimen with 9 μm were used for the validation of capability of measuring continuously smooth surface and large step heights. The measurement results were in good agreement with the results of typical two-wavelength interferometry.
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http://dx.doi.org/10.3390/s19235094DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6929118PMC
November 2019

High-speed polarized low coherence scanning interferometry based on spatial phase shifting.

Appl Opt 2019 Jul;58(20):5360-5365

In this investigation, we describe polarized low coherence scanning interferometry (PLCSI) to enhance the measurement speed based on the spatial phase shifting technique by using a polarized CMOS camera. In every scanning step, the visibility of the correlogram can be directly extracted by spatial phase shifting. PLCSI does not need any scanning conditions such as a scanning step size smaller than that determined by the Nyquist sampling limit and equidistant scanning step, which restrict the measurement speed of the typical low coherence scanning interferometry (LCSI). The measurement data can also be significantly reduced due to the larger scanning step size. PLCSI can be comparable to confocal scanning microscopy in the view of monitoring visibilities. In the experiments, three types of specimens such as a plane mirror, a concave mirror, and a step height specimen were measured by PLCSI with various scanning step sizes, and it was confirmed that the surface profiles were successfully reconstructed. Moreover, the compensation technique of the surface profile, precisely determined by the phase information, was also discussed.
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http://dx.doi.org/10.1364/AO.58.005360DOI Listing
July 2019

Single-shot characterization of multi-film structures based on combined spectral interferometry and spatially recorded spectroscopic ellipsometry.

Appl Opt 2019 Jul;58(21):5637-5643

In this investigation, we propose an improved combined scheme with spectral interferometry and spatially recorded spectroscopic ellipsometry to measure surface height and film thicknesses at once in real time. Instead of rotating polarizing optical components, a spatial phase retarder, which consists of spatially rotating liquid crystal arrays, is used to obtain the ellipsometric spectral data from a single image. In addition, interferometric configuration is combined to collect the surface height information in the same image. The spatial phase retarder can be characterized by the phase retardation and the rotation angle of the liquid crystal during the calibration procedure. In the experiments, single-layered and multi-layered film specimens were measured to verify the measurement capability of the proposed system, and it was confirmed that the measurement results were in good agreement with the provided reference values.
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http://dx.doi.org/10.1364/AO.58.005637DOI Listing
July 2019

Rigorous single pulse imaging for ultrafast interferometric observation.

Opt Express 2019 Jul;27(14):19758-19767

We discuss how to realize rigorous single pulse imaging using a fiber mode-locked laser for the purpose of ultrafast interferometric observation of fast varying dynamic objects. Sub-picosecond pulses are readily picked up in synchronization with the camera operation, allocating one pulse per frame, but rigorous ultrashort single pulse imaging is disturbed by the accumulation of amplified spontaneous emission (ASE) over the exposure time of the camera. Here, we propose four distinct methods to eliminate the ASE-accumulated disruption in the ultrashort optical gating by pulse interferometry and then evaluate their merits and limitations individually by experiments. The proposed four methods are referred to respectively as the time averaged phase modulation, unbalanced pulse overlapping, tandem pulse picking, and second harmonic generation.
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http://dx.doi.org/10.1364/OE.27.019758DOI Listing
July 2019

Novel combined measurement system to characterize film structures by spectral interferometry and ellipsometry.

Opt Express 2018 Dec;26(26):34396-34411

We propose a novel measurement system to simultaneously measure surface and thickness profiles of thin film structures, which cannot be realized in typical measurement techniques. This measurement is accomplished by combining spectral interferometry and ellipsometry. These two distinct measurement techniques are involved in a single system by the abnormal optical configuration. Further, the measurement results are complementary in order to characterize film structures. Film thickness profiles are measured by spectroscopic imaging ellipsometry and surface profiles are obtained from the spectral phase by spectrally resolved interferometry. This method eliminates the theoretical spectral phase by film thicknesses. The proposed system can determine the dimensional film structures at once, even though they have multi-layered films, substrate textures, and even thin film layers. In the experiments, each measurement principle was fundamentally verified with standard specimens. Further, a 4-layered film specimen was measured in order to reconstruct its 3D film structure. As the result, the repeatability of spectroscopic imaging ellipsometry was less than 1 nm and that of spectrally resolved interferometry was a few nanometers, which dominantly affected the performance of the whole system. Several issues for improving accuracy and precision of the proposed system are also discussed in this paper.
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http://dx.doi.org/10.1364/OE.26.034396DOI Listing
December 2018

High-speed combined NIR low-coherence interferometry for wafer metrology.

Appl Opt 2017 Nov;56(31):8592-8597

In this investigation, we describe a combined low-coherence interferometric technique to measure the surface and thickness profiles of wafers at once with high speed. The measurement system consists of a spectrally resolved interferometer to provide and monitor the optical path difference between two incident beams of the optical source part and a low-coherence scanning interferometer to measure the dimensions of wafers with significantly shortened scanning length. In the experiments, a silicon wafer and a sapphire wafer, of which both sides are polished, were used as targets of the measurement system for verification of the proposed system. As a result, the scanning length of the low-coherence scanning interferometer was reduced from a few millimeters to a few hundreds of micrometers approximately 10 times. In addition, surface profiles of both sides and thickness profiles were simultaneously measured to reconstruct 3D shapes of wafers.
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http://dx.doi.org/10.1364/AO.56.008592DOI Listing
November 2017

SPARSE (spatially phase-retarded spectroscopic ellipsometry) for real-time film analysis.

Opt Lett 2017 Aug;42(16):3189-3192

In this Letter, we propose a novel type of spectroscopic ellipsometer, named spatially phase-retarded spectroscopic ellipsometry (SPARSE), based on the spatial polarization distribution opposed to the temporal polarization changes. SPARSE can collect all information necessary to characterize film structures with a single image acquisition, and it has the benefit of real-time measurements. For the verification, feasible experiments with single film-layered certificated reference materials and multi-layered film specimens were carried out.
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http://dx.doi.org/10.1364/OL.42.003189DOI Listing
August 2017

Dual low coherence scanning interferometry for rapid large step height and thickness measurements.

Opt Express 2016 Dec;24(25):28625-28632

In this investigation, we propose a dual low coherence scanning interferometer as the novel concept to measure large height steps on the topographic surface of a specimen and the thickness profile of a transparent optical plate. Dual low coherence characteristics by the tandem interferometric configuration can generate several discrete correlograms for the measured surfaces, which provide the possibility to reduce the scanning length of typical low coherence scanning interferometry significantly. Also, the spectrally-resolved interferometric method is combined to monitor the distance gaps between correlograms caused by the dual low coherence. To verify the proposed interferometry, a large height step specimen and a silicon wafer were used and the 3D surface and thickness profiles were rapidly and successfully measured. In addition, the technique which can identify each correlogram by the insertion of dispersive plates are suggested in this paper.
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http://dx.doi.org/10.1364/OE.24.028625DOI Listing
December 2016

Sub-sampling low coherence scanning interferometry and its application: refractive index measurements of a silicon wafer.

Authors:
Ki-Nam Joo

Appl Opt 2013 Dec;52(36):8644-9

In this investigation, refractive indices of a silicon (Si) wafer were measured by low-coherence scanning interferometry adopting the sub-sampling technique to reduce measurement time. Based on Fourier domain analysis method, the sub-sampled correlogram was analyzed and the refractive indices were calculated by the simple refractive index model and curve fitting of the phase extracted from the sub-sampled correlogram. In the experiment to verify the proposed technique, near-infrared light emitted by a super-luminescent diode with 1050 nm center wavelength was used as an optical source because it is partially transparent to an undoped Si wafer. As the result of measuring an undoped double-side polished Si wafer, group and phase refractive indices were successfully obtained with the sub-sampled correlogram, and the deviations from the reference value were within 0.001.
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http://dx.doi.org/10.1364/AO.52.008644DOI Listing
December 2013

Low cost wafer metrology using a NIR low coherence interferometry.

Opt Express 2013 Jun;21(11):13648-55

Department of Photonics Engineering, Chosun University, Dong-gu, Gwanju, Republic of Korea.

In this investigation, a low cost Si wafer metrology system based on low coherence interferometry using NIR light is proposed and verified. The whole system consists of two low coherence interferometric principles: low coherence scanning interferometry (LCSI) for measuring surface profiles and spectrally-resolved interferometry (SRI) to obtain the nominal optical thickness of the double-sided polished Si wafer. The combination of two techniques can reduce the measurement time and give adequate dimensional information of the Si wafer. The wavelength of the optical source is around 1 μm, for which transmission is non-zero for undoped silicon and can be also detected by a typical CCD camera. Because of the typical CCD camera, the whole system can be constructed inexpensively.
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http://dx.doi.org/10.1364/OE.21.013648DOI Listing
June 2013

Frequency stabilized three mode HeNe laser using nonlinear optical phenomena.

Opt Express 2010 Jan;18(2):1373-9

Mechatronic System Design, Delft University of Technology, Delft, The Netherland.

Accurate and traceable length metrology is employed by laser frequency stabilization. This paper describes a laser frequency stabilization technique as a secondary standard with a fractional frequency stability of 5.2x10(-10) with 2 mW of power, suitable for practical applications. The feedback stabilization is driven by an intrinsic mixed mode signal, caused by nonlinear optical phenomena with adjacent modes. The mixed mode signals are described theoretically and experimentally verified.
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http://dx.doi.org/10.1364/OE.18.001373DOI Listing
January 2010

High resolution heterodyne interferometer without detectable periodic nonlinearity.

Opt Express 2010 Jan;18(2):1159-65

Mechatronic System Design, Department of Precision and Microsystems Engineering, Delft University of Technology, Delft, The Netherlands.

A high resolution heterodyne laser interferometer without periodic nonlinearity for linear displacement measurements is described. It uses two spatially separated beams with an offset frequency and an interferometer configuration which has no mixed states to prevent polarization mixing. In this research, a simple interferometer configuration for both retroreflector and plane mirror targets which are both applicable to industrial applications was developed. Experimental results show there is no detectable periodic nonlinearity for both of the retro-reflector interferometer and plane mirror interferometer to the noise level of 20 pm. Additionally, the optical configuration has the benefit of doubling the measurement resolution when compared to its respective traditional counterparts. Because of non-symmetry in the plane mirror interferometer, a differential plane mirror interferometer to reduce the thermal error is also discussed.
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http://dx.doi.org/10.1364/OE.18.001159DOI Listing
January 2010

Balanced interferometric system for stability measurements.

Appl Opt 2009 Mar;48(9):1733-40

PME: Mechatronic System Design, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands.

We describe two different, double-sided interferometer designs for measuring material stability. Both designs are balanced interferometers where the only optical path difference is the sample and the reference beams are located within the interferometer. One interferometer is a double-pass design, whereas the other is a single-pass system. Based on a tolerancing analysis, the single-pass system is less susceptible to initial component misalignment and motions during experiments. This single-pass interferometer was tested with an 86 nm thin-film silver sample for both short-term repeatability and long-term stability. In 66 repeatability tests of 30 min each, the mean measured drift rate was less than 1 pm/h rms. In two long-term tests (>9 h), the mean drift rate was less than 1.1 pm/h, which shows good agreement between the short- and long-term measurements. In these experiments, the mean measured length change was 2 nm rms.
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http://dx.doi.org/10.1364/ao.48.001733DOI Listing
March 2009

Simple heterodyne laser interferometer with subnanometer periodic errors.

Opt Lett 2009 Feb;34(3):386-8

Mechatronic System Design, Department of Precision and Microsystems Engineering, Delft University of Technology, Delft, The Netherlands.

We describe a simple heterodyne laser interferometer that has subnanometer periodic errors and is applicable to industrial fields. Two spatially separated beams can reduce the periodic errors, and the use of a right-angle prism makes the optical configuration much simpler than previous interferometers. Moreover, the optical resolution can be enhanced by a factor of 2, because the phase change direction is opposite between reference and measurement signals. Experiments have demonstrated the periodic errors are less than 0.15 nm owing to the frequency mixing of the optical source. The improvements for reducing the frequency mixing of the optical system are also discussed.
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http://dx.doi.org/10.1364/ol.34.000386DOI Listing
February 2009

Distance measurements by combined method based on a femtosecond pulse laser.

Opt Express 2008 Nov;16(24):19799-806

Mechatronic System Design, Department of Precision and Microsystems Engineering, Delft University of Technology, Delft, The Netherlands.

We describe a combined interferometric scheme that enables absolute distance measurements using a femtosecond pulse laser. This method is combined with synthetic wavelength interferometry (SWI), time of flight (TOF) and spectrally-resolved interferometry (SRI) using the optical comb of femtosecond laser. Each technique provides distinct measuring resolutions and ambiguity ranges which are complementary to each other. These separate measurement principles are incorporated and implemented simultaneously and the unified output can enhance the dynamic range of the measuring system. Our experimental results demonstrate an example of absolute distance measurement with the proposed technique and we discuss the possibility of the combined method to measure long distances and the important factors for the implementation.
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http://dx.doi.org/10.1364/oe.16.019799DOI Listing
November 2008

Refractive index measurement by spectrally resolved interferometry using a femtosecond pulse laser.

Opt Lett 2007 Mar;32(6):647-9

Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, Daejeon, South Korea.

We describe a measurement method of refractive indices by way of spectrally resolved interferometry using a femtosecond pulse laser. The method is dispersion insensitive and requires no prior precise knowledge of the geometrical thickness of the specimen. Not only the group but also the phase refractive index can be determined over the wide spectral range covered by the optical comb of the femtosecond pulse laser in use.
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http://dx.doi.org/10.1364/ol.32.000647DOI Listing
March 2007

Absolute distance measurement by dispersive interferometry using a femtosecond pulse laser.

Opt Express 2006 Jun;14(13):5954-60

We describe an interferometric method that enables to measure the optical path delay between two consecutive femtosecond laser pulses by way of dispersive interferometry. This method allows a femtosecond laser to be utilized as a source of performing absolute distance measurements to unprecedented precision over extensive ranges. Our test result demonstrates a non-ambiguity range of ~1.46 mm with a resolution of 7 nm over a maximum distance reaching ~0.89 m.
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http://dx.doi.org/10.1364/oe.14.005954DOI Listing
June 2006