OPTICAL SOCIETY OF KOREA

게시판 최근논문

[Editor's Pick] Current Optics and Photonics Vol. 7 no. 5 (2023 October) Volume-sharing Multi-aperture Imaging (VMAI): A Potential Approach for Volume Reduction for Space-borne Imagers Jun Ho Lee1,2 *, Seok Gi Han1, Do Hee Kim1, Seokyoung Ju1, Tae Kyung Lee3, Chang Hoon Song3, Myoungjoo Kang3, Seonghui Kim4, and Seohyun Seong4 Current Optics and Photonics Vol. 7 No. 5 (2023 October) pp. 545-556 DOI: https://doi.org/10.3807/COPP.2023.7.5.545 Fig. 2 Illustration of volume-sharing multi-aperture imaging (VMAI) with one wide-field and three narrow-field cameras: (a) Unfolded and (b) compactly folded. Keywords: Deep-learning, Earth observation, Image fusion, Volume-sharing multi-aperture imaging OCIS codes: (110.3010) Image reconstruction techniques; (120.3620) Lens system design;(120.4640) Optical instruments; (220.4830) Systems design; (220.4991) Passive remote sensing Abstract This paper introduces volume-sharing multi-aperture imaging (VMAI), a potential approach proposed for volume reduction in space-borne imagers, with the aim of achieving high-resolution ground spatial imagery using deep learning methods, with reduced volume compared to conventional approaches. As an intermediate step in the VMAI payload development, we present a phase-1 design targeting a 1-meter ground sampling distance (GSD) at 500 km altitude. Although its optical imaging capability does not surpass conventional approaches, it remains attractive for specific applications on small satellite platforms, particularly surveillance missions. The design integrates one wide-field and three narrowfield cameras with volume sharing and no optical interference. Capturing independent images from the four cameras, the payload emulates a large circular aperture to address diffraction and synthesizes highresolution images using deep learning. Computational simulations validated the VMAI approach, while addressing challenges like lower signal-to-noise (SNR) values resulting from aperture segmentation. Future work will focus on further reducing the reduction ratio and refining SNR management.
[하이라이트 논문] 한국광학회지 Vol. 34 No.5 (2023 October) BL-ASM에서 U-net 기반 위상 홀로그램의 스펙클 노이즈 감소와 이미지 품질 향상 Speckle Noise Reduction and Image Quality Improvement in U-net-based Phase Holograms in BL-ASM 남오승1ㆍ권기철1ㆍ정종래2ㆍ이권연3ㆍ김 남1† 한국광학회지 Vol. 34 No.5 (2023 October) pp. 192-201 DOI: https://doi.org/10.3807/KJOP.2023.34.5.192 Fig. 1 Definition of the coordinate system of sampling window in the angular spectrum method. (a) Coordinate system of sampling window. (b) Coordinate system of source field. Keywords: 디지털 홀로그래피, 전파 방법, 해상도, 스펙클 OCIS codes: (030.6140) Speckle; (080.1510) Propagation methods; (090.1995) Digital holography; (350.5730) Resolution 초록 Band-limited angular spectrum method (BL-ASM)는 공간주파수 제어의 문제로 aliasing 오류가 발생한다. 본 논문에서는 위상 홀로그 램에 대한 표본화 간격 조정 기법과 딥 러닝 기반의 U-net 모델을 사용한 스펙클 노이즈 감소 및 이미지 품질 향상 기법을 제안하였다. 제안한 기법에서는 넓은 전파 범위에서 aliasing 오류를 제거할 수 있도록 먼저 샘플링 팩터를 계산하여 표본화 간격 조절에 의한 공간주파수를 제어 함으로써 스펙클 노이즈를 감소시킨다. 그 후 딥 러닝 모델을 적용한 위상 홀로그램을 학습시켜 복원 이미지의 품질을 향상시킨다. 다양한 샘플 이미지에 대한 S/W 시뮬레이션에서 기존의 BL-ASM과의 peak signal-to-noise ratio (PSNR), structural similarity index measure (SSIM)을 비교할 때 각각 평균 5%, 0.14% 정도 비율이 향상됨을 확인하였다. Abstract The band-limited angular spectrum method (BL-ASM) causes aliasing errors due to spatial frequency control problems. In this paper, a sampling interval adjustment technique for phase holograms and a technique for reducing speckle noise and improving image quality using a deep-learning-based U-net model are proposed. With the proposed technique, speckle noise is reduced by first calculating the sampling factor and controlling the spatial frequency by adjusting the sampling interval so that aliasing errors can be removed in a wide range of propagation. The next step is to improve the quality of the reconstructed image by learning the phase hologram to which the deep learning model is applied. In the S/W simulation of various sample images, it was confirmed that the peak signal-to-noise ratio (PSNR) and structural similarity index measure (SSIM) were improved by 5% and 0.14% on average, compared with the existing BL-ASM.
[Editor's Pick] Current Optics and Photonics Vol. 7 no. 4 (2023 August) Shearing Interferometry: Recent Research Trends and Applications Ki-Nam Joo* and Hyo Mi Park Current Optics and Photonics Vol. 7 No. 4 (2023 August) pp. 325-336 DOI: https://doi.org/10.3807/COPP.2023.7.4.325 Fig. 1 Optical layout. (a) Shearing interferometry, and (b) typical interferometry. Keywords: Azimuthal shearing interferometer, Lateral shearing interferometer, Radial shearing interferometer, Shearing interferometer, Wavefront reconstruction OCIS codes: (120.3180) Interferometry; (120.3930) Metrological instrumentation; (120.6650) Surface measurements, figure Abstract We review recent research related to shearing interferometry, reported over the last two decades. Shearing interferometry is categorized as azimuthal, radial, or lateral shearing interferometers by its fundamental principle to generate interference. In this review the research trends for each technique are provided, with a summary of experimental results containing theoretical background, the optical configuration, analysis, and perspective on its application fields.
[하이라이트 논문] 한국광학회지 Vol. 34 No.4 (2023 August) 고성능 광각 3차원 스캐닝 라이다를 위한 스터드 기술 기반의 대면적 고속 단일 광 검출기 Large-area High-speed Single Photodetector Based on the Static Unitary Detector Technique for High-performance Wide-field-of-view 3D Scanning LiDAR 한문현ㆍ민봉기† 한국광학회지 Vol. 34 No.4 (2023 August) pp. 139-150 DOI: https://doi.org/10.3807/KJOP.2023.34.4.139 Fig. 1 Schematic of the large-area high-speed photodetector based on the static unitary detector (STUD) technique. Keywords: 대면적, 라이다, 고속, 광 검출기, 스터드 OCIS codes: (040.5160) Photodetectors; (200.4560) Optical data processing; (280.3640) Lidar; (280.4788) Optical sensing and sensors 초록 다양한 구조의 라이다(light detection and ranging, LiDAR)가 존재함에도 불구하고 넓은 화각을 유지하면서 장거리 측정과 수직, 수평 방향 모두에서 높은 해상도를 만족하는 LiDAR를 구현하는 것은 매우 어렵다. 스캐닝 구조는 장거리 탐지 및 수직, 수평 방향에 대한 높은 해상도를 만족하는 고성능 LiDAR를 구현하는 데 유리하지만, 넓은 화각을 확보하기 위해서는 검출 속도에 불리한 대면적 광 검출기(photodetector, PD) 가 필수적이다. 따라서 이러한 문제점을 해결하기 위해 다수의 소면적 PD를 고속의 단일 대면적 PD로 작동할 수 있는 static unitary detector (STUD) 기술 기반의 PD를 제안하였다. 본 논문에서 제안하는 InP/InGaAs STUD PIN-PD는 1,256 μm × 19 μm의 단위 면적을 가지는 32 개 소면적 PD를 활용하여 1,256 μm × 949 μm 이내에서 다양한 형태로 설계 및 제작하였다. 이후 다양한 형태로 제작된 STUD PD의 특성과 감도는 물론 이를 활용한 LiDAR 수신 보드의 잡음 및 신호 특성에 대해 측정 및 분석하였다. 마지막으로 STUD PD가 적용된 LiDAR 수신 보드를 1.5-μm master oscillator power amplifier 레이저를 광원으로 활용하는 3차원 스캐닝 LiDAR 시제품에 적용하였고, 이를 통해 대각 32.6도의 광각에서 50 m 이상의 장거리 물체를 정밀하게 탐지하면서 320 px × 240 px의 고해상도 3차원 영상을 동시에 확보하였다. Abstract Despite various light detection and ranging (LiDAR) architectures, it is very difficult to achieve long-range detection and high resolution in both vertical and horizontal directions with a wide field of view (FOV). The scanning architecture is advantageous for high-performance LiDAR that can attain long-range detection and high resolution for vertical and horizontal directions. However, a large-area photodetector (PD), which is disadvantageous for detection speed, is essentially required to secure the wide FOV. Thus we propose a PD based on the static unitary detector (STUD) technique that can operate multiple small-area PDs as a single large-area PD at a high speed. The InP/InGaAs STUD PIN-PD proposed in this paper is fabricated in various types, ranging from 1,256 μm × 949 μm using 32 small-area PDs of 1,256 μm × 19 μm. In addition, we measure and analyze the noise and signal characteristics of the LiDAR receiving board, as well as the performance and sensitivity of various types of STUD PDs. Finally, the LiDAR receiving board utilizing the STUD PD is applied to a 3D scanning LiDAR prototype that uses a 1.5-μm master oscillator power amplifier laser. This LiDAR precisely detects long-range objects over 50 m away, and acquires high-resolution 3D images of 320 pixels × 240 pixels with a diagonal FOV of 32.6 degrees simultaneously.
[하이라이트 논문] 한국광학회지 Vol. 34 No.3 (2023 June) 고해상도 표면 측정을 위한 회전 프리즘 정합 간섭계 Rotational Prism Stitching Interferometer for High-resolution Surface Testing 송인웅1ㆍ권우성1,3ㆍ김학용1,2ㆍ이윤우1ㆍ이종웅3ㆍ양호순1,2† 한국광학회지 Vol. 34 No.3 (2023 June) pp. 117-123 DOI: https://doi.org/10.3807/KJOP.2023.34.3.117 Fig. 1 System layout of the rotational prism stitching interferometer (RPSI). QWP,quarter-wave plate; PBS, polarizing beam splitter. Keywords: 간섭계, 회전 프리즘, 부분 구경 정합법, 표면 형상 측정 OCIS codes: (120.3180) Interferometry; (120.3940) Metrology; (120.6650) Surface measurements, figure 초록 반사경의 광학면이 넓어질수록 높은 공간 주파수 형상이 광학계의 성능에 큰 영향을 주므로, 대형 반사경 성능 평가에서 이를 측정할 수 있어야 한다. 따라서 높은 주파수의 형상을 샘플링할 수 있는 고해상도 형상 측정 시스템이 필요하다. 본 연구에서는 반사경의 고해상도 형상 측정을 위한 회전 프리즘 정합 간섭계(rotational prism stitching interferometer, RPSI)라는 새로운 방법을 제안한다. RPSI는 기존에 사용되고 있는 상용 간섭계에 빔 확장 렌즈, 이송 거울, 회전 프리즘 등을 추가하여 구성되었으며, 간섭계를 움직이지 않고 원통 좌표계를 기준으로 전체 구경을 아우르는 부분 구경 형상들을 측정한다. 측정된 부분 구경들은 최소자승법을 이용한 부분 구경 정합법을 이용하여 정합되며, 빔확장 렌즈 배율의 제곱만큼 높은 샘플링 밀도로 전체 구경 형상 정보를 복원 가능하다. 3배율을 가지는 RPSI를 이용하여 직경 40 mm의 구면 반사경 형상을 측정하였고, 간섭계 단독으로 측정한 전체 구경 형상과 비교 검증하였다. 약 1 nm RMS의 근소한 차이로 유사한 형상을 잘 복원할수 있었으며, 간섭계 단독으로 사용한 형상 측정 결과보다 렌즈의 배율의 제곱배만큼 향상된 샘플링 밀도로 형상 측정이 가능함을 확인하였다. Abstract The size of an optical surface can significantly affect the performance of an optical system, and high spatial frequency errors have a greater impact. Therefore, it is crucial to measure the surface figure error with high frequency. To address this, a new method called the a rotational prism stitching interferometer (RPSI) is proposed in this study. The RPSI is a type of stitching interferometer that enhances spatial resolution, but it differs from conventional stitching interferometers in that it does not require the movement of either the mirror tested or the interferometer itself to obtain sub-aperture interferograms. Instead, the RPSI uses a beam expander and a rotating Dove prism to select particular sub-apertures from the entire aperture.These sub-apertures are then stitched together to obtain a full-aperture result proportional to the square of the beam expander's magnification. The RPSI's effectiveness was demonstrated by measuring a 40 mm diameter spherical mirror using a three-magnification beam expander and comparing the results with those obtained from a commercial interferometer. The RPSI achieved surface testing results with nine times higher sampling density than the interferometer alone, with a small difference of approximately 1 nm RMS.
[Editor's Pick] Current Optics and Photonics Vol. 7 no. 3 (2023 June) 285 mJ Electro-optically Q-switched Er:YAG Master Oscillator Power Amplifier (MOPA) System with Adjustable Pumping Delay between Flashlamps at 2.94 μm Heesuk Jang1 *, Hajun Song1, Hae Seog Koh1, and Han Young Ryu2 Current Optics and Photonics Vol. 7 No. 3 (2023 June) pp. 297-303 DOI: https://doi.org/10.3807/COPP.2023.7.3.297 Fig. 1 Mid-IR electro-optically Q-switched Er:YAG MOPA system with an adjustable pumping delay between Xenon flashlamps; (a) Schematic of the system, (b) photograph of the constructed system. Keywords: Er:YAG laser, Electro-optic Q-switching, Flashlamp pumping, Mid-infrared OCIS codes: (140.3500) Lasers, erbium; (140.3538) Lasers, pulse; (140.3540) Lasers, Q-switched;(140.3580) Lasers, solid-state; (140.3852) Lasers, amplifiers Abstract In this paper, we demonstrated a high-energy (285 mJ) mid-infrared flashlamp-pumped electro-optically Q-switched Er:YAG master oscillator power amplifier (MOPA) system and comprehensively investigated its temporal, spectral, and spatial characteristics. To increase the output energy, we optimized the delay between the timings at which the flashlamps of the master oscillator and power amplifier were triggered. In addition, the output energy was improved while minimizing thermal effects by cooling the MOPA system to a temperature slightly above the dew point. Consequently, the MOPA structure boosted the output energy without damaging the lithium niobate Pockels cell, which is a crucial element in Q-switching. This design realized pulses with energies up to 0.285 J and pulse durations of approximately 140 ns at a wavelength of 2,936.7 nm. This high-energy mid-IR Er:YAG MOPA system can be used for various scientific, engineering, and military underwater applications.
[하이라이트 논문] 한국광학회지 Vol. 34 No.2 (2023 April) 적색 및 적외선 빛을 이용한 Photobiomodulation: 각막상피세포에 대한 효과와 상처 치유에 관한 연구 Photobiomodulation Mediated by Red and Infrared Light: A Study of Its Effectiveness on Corneal Epithelial Cells and Wound Healing 안선희1ㆍ안재성1ㆍ이병일2† 한국광학회지 Vol. 34 No.2 (2023 April) pp. 45-52 DOI: https://doi.org/10.3807/KJOP.2023.34.2.045 Fig. 1 Representative picture of the experimental setup and emission spectra of light-emitting diodes (LEDs). (a) LEDs were irradiated to the 24-well plate from the top and a single-well-shaped mask was fixed on the top surface of the well plate to prevent LED light from being irradiated to adjacent wells. The power of LED light transmitted through the mask was measured with a photodiode sensor to derive the amount of light energy irradiated to the sample. (b) Normalized spectra of LED light sources. Keywords: 각막상피세포, 저출력광치료, 광생물조절, 상처치료 OCIS codes: (170.1530) Cell analysis; (350.5130) Photochemistry 초록 본 연구에서는 다양한 파장의 저출력 light-emitting diode (LED)를 이용한 photobiomodulation (PBM)이 각막 상처 치유에 미치는 영향을 분석하였다. 각막상피세포에 623 nm에서 940 nm 범위의 파장의 LED를 조사한 결과, 유의미한 세포독성 영향을 미치지 않는 것을 확인하였다. PBM의 세포이동 촉진 효과를 세포 이동능 평가 시험을 통해 분석한 결과 623 nm 파장의 광조사에 의한 PBM이 세포이동을 크게 증가시키고 상처 치유를 촉진하는 것으로 나타났다. 또한, 세포이동 및 상처 치유와 관련된 유전자의 발현을 분석한 결과, 623 nm 파장의 광조사에 의한 PBM이 세포 증식과 세포 외 기질 분해를 촉진하는 것으로 알려진 FGF-1과 MMP2 유전자의 발현을 상향 조절한다는 사실을 발견했다. 이러한 연구 결과는 특정 파장, 특히 623 nm 파장의 저출력 빛을 이용한 PBM이 각막 손상 치료에 활용될 수 있는 가능성을 시사한다. Abstract In this study, we have investigated the effect of photobiomodulation (PBM) on corneal wound healing, using a low-power light-emitting diode (LED) at different wavelengths. We found that LEDs with wavelengths ranging from 623 to 940 nm had no significant cytotoxic effects on corneal epithelial cells. The effect of PBM on promoting cell migration was analyzed by scratch assay, and it was found that PBM at 623 nm significantly increased cell migration and promoted wound healing. Furthermore, the expression of genes related to cell migration and wound healing was analyzed, and it was found that PBM at 623 nm upregulated the expression of the genes FGF-1 and MMP2, which are known to promote cell proliferation and extracellular matrix degradation. These findings suggest that PBM with low-powered light at specific wavelengths, particularly 623 nm, could be utilized to treat corneal injury.
[Editor's Pick] Current Optics and Photonics Vol. 7 no. 2 (2023 April) A Wide-field-of-view Table-ornament Display Using Electronic Holography Daerak Heo, Hosung Jeon, Sungjin Lim, and Joonku Hahn* Current Optics and Photonics Vol. 7 No. 2 (2023 April) pp. 182-190 DOI: https://doi.org/10.3807/COPP.2023.7.2.183 Fig. 1 Schematic of the 3f parabolic imaging optics. Keywords: Electronic holography, High-speed scanner, Tabletop displays, Three-dimensional display, Viewing zone OCIS codes: (090.2870) Holographic display; (100.6890) Three-dimensional image processing; (120.5800) Scanners; (220.4830) Systems design; (330.1400) Vision-binocular and stereopsis Abstract Three-dimensional (3D) displays provide a significant advantage over traditional 2D displays by offering realistic images, and table-style displays in particular are ideal for generating 3D images that appear to float above a table. These systems are based on multiview displays, and are typically operated using temporal or spatial multiplexing methods to expand the viewing zone (VZ). The VZ is an expanded space that results from merging the sub-viewing zones (SVZs) from which an individual view is made. To increase the viewing angle, many SVZs are usually required. In this paper, we propose a tableornament electronic holographic display that utilizes 3f parabolic mirrors. In holography, the VZ is not simply expanded but synthesized from SVZs to implement continuous motion parallax. Our proposed system is small enough to be applied as a table ornament, in contrast to traditional tabletop displays that are large and not easily portable. By combining multiview and holographic technologies, our system achieves continuous motion parallax. Specifically, our system projects 340 views using a time-multiplexing method over a range of 240 degrees.
[하이라이트 논문] 한국광학회지 Vol. 34 No.1 (2023 February) 피조 간섭계를 이용한 단일 조각거울 광축방향 변위 오차 측정 Measurement of the Axial Displacement Error of a Segmented Mirror Using a Fizeau Interferometer 장하림1,2ㆍ최재혁2ㆍ송재봉2ㆍ김학용1,2 † 한국광학회지 Vol. 34 No.1 (2023 February) pp. 22-30 DOI: https://doi.org/10.3807/KJOP.2023.34.1.022 Fig. 1 Relationship between axial displacement error and defocus. RoC, radius of curvature. Keywords: 인공위성, 조각거울, 광축방향 변위 오차, 초점오차 OCIS codes: (120.3180) Interferometry; (120.3940) Metrology; (120.6085) Space instrumentation; (120.6650) Surface measurements, figure; (230.4040) Mirrors 초록 조각거울은 우주용 관측위성의 주반사경을 크게 제작하기 위한 방법 중 하나로서, 여러 개의 작은 거울들을 이어 하나의 큰 거울로 이용하는 방법이다. 여러 개의 거울들을 하나의 거울로 정렬하기 위해서는 인접한 거울들 간에 기울기 오차(tilt)와 광축방향 정렬오차(piston)가 없어야 한다. 기울기 오차와 광축방향 정렬오차를 해결해야 여러 개의 거울이 한 방향으로 빛을 모으고, 이를 통해 뚜렷한 이미지를 얻을 수 있기 때문에 조각거울의 정렬오차를 나노미터 수준으로 측정할 수 있는 파면 센서가 필요하다. 기울기 오차는 조각거울을 통해 얻은 이미지를 통해 어떤 거울의 기울기가 틀어졌는지 쉽게 확인할 수 있는 반면, 광축방향 정렬오차는 이미지의 질은 떨어뜨리지만 드러나는 뚜렷한 특징이 없기 때문에 같은 방법으로 감지하기 어려워 세밀한 측정이 매우 중요하다. 이를 위해 본 논문에서는 지상용 초기 정렬시 많은 이점을 갖는 광학계 평가용 간섭계 중 하나인 피조 간섭계를 이용한다. 피조 간섭계를 사용한 복수 조각거울의 광축방향 정렬오차 측정을 위한 기초 연구로서 단일 조각거 울의 광축방향 변위 오차를 측정하고, 측정불확도를 계산해서 피조 간섭계의 광축방향 변위 오차 측정 한계를 규명한다. 또한 수식을 통해 조각 거울 광축방향 변위 오차와 간섭계로 측정한 표면 초점오차(defocus)의 관계를 계산했고, 도출한 수식의 타당성을 실험으로 검증했다. Abstract A segmented mirror is one of the ways to make the primary mirror of a spaceborne satellite larger, and several small mirrors are used as a large monolithic mirror. To align multiple segmented mirrors as one large mirror, there must be no discontinuity in x, y-axis (tilt) and axial alignment error (piston) between adjacent mirrors. When the tilt and piston are removed, we can collect the light in one direction and get an expected clear image. Therefore, we need a precise wavefront sensor that can measure the alignment error of the segmented mirrors in nm scale. The tilt error can be easily detected by the point spread image of the segmented mirrors, while the piston error is hard to detect because of the absence in apparent features, but makes a downgraded image. In this paper we used an optical testing interferometer such as a Fizeau interferometer, which has various advantages when aligning the segmented mirror on the ground, and focused on measuring the axial displacement error of a segmented mirror as the basic research of measuring the piston errors between adjacent mirrors. First, we calculated the relationship between the axial displacement error of the segmented mirror and surface defocus error of the interferometer and verified the calculated formula through experiments. Using the experimental results, we analyzed the measurement uncertainty and obtained the limitation of the Fizeau interferometer in detecting axial displacement errors.
[Editor's Pick] Current Optics and Photonics Vol. 7 no. 1 (2023 February) Partial Spectrum Detection and Super-Gaussian Window Function for Ultrahigh-resolution Spectral-domain Optical Coherence Tomography with a Linear-k Spectrometere Hyun-Ji Lee1,2 and Sang-Won Lee1,2 * Current Optics and Photonics Vol. 7 No. 1 (2023 February) pp. 73-82 DOI: https://doi.org/10.3807/COPP.2023.7.1.73 Fig. 1 Schematic of the SD-OCT based on a linear k-domain spectrometer. (a) The sample arm to measure the system performance and obtain the skin tissue’s image. (b) The sample arm to get the image of the retina. SD-OCT, spectral domain optical coherence tomography; PC, polarization controller; CL, collimation lens; OL, objective lens; DCB, dispersion compensation block; RM, reference mirror; RC, reflective collimator; G, grating; DP, dispersive prism; PM, prism mirror; L1 to L4, lenses. Keywords: Spectral domain, Super-Gaussian window, Ultrahigh resolution optical coherence tomography OCIS codes: (110.4500) Optical coherence tomography; (170.3880) Medical and biological imaging;(170.4500) Optical coherence tomography Abstract In this study, we demonstrate ultrahigh-resolution spectral-domain optical coherence tomography with a 200-kHz line rate using a superluminescent diode with a −3-dB bandwidth of 100 nm at 849 nm. To increase the line rate, a subset of the total number of camera pixels is used. In addition, a partialspectrum detection method is used to obtain OCT images within an imaging depth of 2.1 mm while maintaining ultrahigh axial resolution. The partially detected spectrum has a flat-topped intensity profile, and side lobes occur after fast Fourier transformation. Consequently, we propose and apply the super-Gaussian window function as a new window function, to reduce the side lobes and obtain a result that is close to that of the axial-resolution condition with no window function applied. Upon application of the super-Gaussian window function, the result is close to the ultrahigh axial resolution of 4.2 μm in air, corresponding to 3.1 μm in tissue (n = 1.35).

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