[Editors Pick] Current Optics and Photonics Vol. 5 no. 5 (2021 October)
Absolute Depth Estimation Based on a Sharpness-assessment Algorithm for a Camera with an Asymmetric Aperture
Beomjun Kim, Daerak Heo, Woonchan Moon, and Joonku Hahn *
School of Electronic and Electrical Engineering, Kyungpook National University, Daegu 41566, Korea
Current Optics and Photonics Vol. 5 No.5 (2021 October) pp. 514-523
Fig. 1 Defocus model. (a) Geometry of the camera, and (b) the radius of the CoC when half of the aperture size is 7 mm.
Keywords: Coded aperture, Depth estimation, Image reconstruction
OCIS codes: (100.2000) Digital image processing; (100.3008) Image recognition, algorithms and filters; (100.3020) Image reconstruction-restoration; (120.3940) Metrology; (170.1630) Coded aperture imaging
Methods for absolute depth estimation have received lots of interest, and most algorithms are concerned about how to minimize the difference between an input defocused image and an estimated defocused image. These approaches may increase the complexity of the algorithms to calculate the defocused image from the estimation of the focused image. In this paper, we present a new method to recover depth of scene based on a sharpness-assessment algorithm. The proposed algorithm estimates the depth of scene by calculating the sharpness of deconvolved images with a specific point-spread function (PSF). While most depth estimation studies evaluate depth of the scene only behind a focal plane, the proposed method evaluates a broad depth range both nearer and farther than the focal plane. This is accomplished using an asymmetric aperture, so the PSF at a position nearer than the focal plane is different from that at a position farther than the focal plane. From the image taken with a focal plane of 160 cm, the depth of object over the broad range from 60 to 350 cm is estimated at 10 cm resolution. With an asymmetric aperture, we demonstrate the feasibility of the sharpness-assessment algorithm to recover absolute depth of scene from a single defocused image.
[Editors Pick] Current Optics and Photonics Vol. 5 no. 4 (2021 August)
Full-color Non-hogel-based Computer-generated Hologram from Light Field
without Color Aberration
Dabin Min, Kyosik Min, and Jae-Hyeung Park *
Department of Electrical and Computer Engineering, Inha University, Incheon 22212, Korea
Current Optics and Photonics Vol. 5 No.4 (2021 August) pp. 409-420
Fig. 1 Proposed method: (a) original color light field data with the same FoV for all color channels, (b) resampling by zeropadding, and (c) interpolation. Vertical axis is represented in ray angle θx.
We propose a method to synthesize a color non-hogel-based computer-generated-hologram (CGH) from light field data of a three-dimensional scene with a hologram pixel pitch shared for all color channels. The non-hogel-based CGH technique generates a continuous wavefront with arbitrary carrier wave from given light field data by interpreting the ray angle in the light field to the spatial frequency of the plane wavefront. The relation between ray angle and spatial frequency is, however, dependent on the wavelength, which leads to different spatial frequency sampling grid in the light field data, resulting in color aberrations in the hologram reconstruction. The proposed method sets a hologram pixel pitch common to all color channels such that the smallest blue diffraction angle covers the field of view of the light field. Then a spatial frequency sampling grid common to all color channels is established by interpolating the light field with the spatial frequency range of the blue wavelength and the sampling interval of the red wavelength. The common hologram pixel pitch and light field spatial frequency sampling grid ensure the synthesis of a color hologram without any color aberrations in the hologram reconstructions, or any loss of information contained in the light field. The proposed method is successfully verified using color light field data of various test or natural 3D scenes.
[Editors Pick] Current Optics and Photonics Vol. 5 no. 3 (2021 June)
Binary-phase Complex Spatial Light Modulators Driven by Mirror Symmetry
Minho Choi and Jaewu Choi *
Department of Information Display, Kyung Hee University, Seoul 02447, Korea
Current Optics and Photonics Vol. 5 No.3 (2021 June) pp. 261-269
Fig. 1 Schematics of a BP-C-SLM. (a) A-SLM, (b) binary-phase spatial light modulators (BP-SLM), (c) physical combination of one A-SLM with one BP-SLM to achieve a BP-C-SLM, (d) an intrinsically fused BP-C-SLM, and (e) the information-doubling process, using the two physically combined SLMs or the intrinsically fused BP-C-SLM.
Binary-phase complex spatial light modulators (BP-C-SLMs) are proposed and simulated. This study shows that bottom-top mirror-symmetrical uniaxial systems between two orthogonal polarizers allow one to construct BP-C-SLMs. BP-C-SLMs double the information-handling capacity per pixel, compared to the conventional amplitude-only spatial light modulators (A-SLMs), as well as being simply implemented with a single spatial light modulator (SLM), rather than a combination of an A-SLM and a binary-phase SLMs. Under limited conditions, BP-C-SLMs can control only the amplitude in single-phase space, and act as A-SLMs.
[하이라이트 논문] 한국광학회지 Vol. 32 No.2 (2021 April)
모드 잠금된 펄스 레이저와 연속 발진하는 반도체 레이저를 이용한 합주파수 생성
Sum-frequency Generation Using a Mode-locked Pulsed Laser and a Continuous-wave Diode Laser
김현학 ㆍ박남훈ㆍ염동일ㆍ차명식ㆍ문한섭 1†
한국광학회지 Vol. 32 No.2 (2021 April) pp. 62-67
Fig. 1 Experimental setup for sum-frequency generation in a PPLN crystal. Pump: 1,560.7 nm (mode-locked fiber laser), 1,551 nm (continuous wave diode laser). FPC, fiber polarization controller; PBS, polarizing beamsplitter; L, lens (focal length: 400 mm); PPLN, periodically poled lithium niobate; DM, dichroic mirror (R: 780 nm, T: 1,550 nm); ω c , continuous-wave diode laser frequency; ω p , pulse laser frequency; ω s , sum frequency.
본 연구에서는 모드 잠금된 ps-펄스 광섬유 레이저와 연속 발진하는 좁은 선폭의 반도체 레이저를 이용하여 주기적 분극반전된 LiNbO3 (periodically poled lithium niobate; PPLN) 결정에서 합주파수 생성 연구를 수행하였다. 모드 잠금된 펄스 레이저는 중심 파장이 1560.7 nm이고 스펙트럼의 폭은 약 1.1 nm이며, 연속 발진 반도체 레이저는 중심 파장이 1551 nm이고 스펙트럼의 폭은 약 6 MHz로 동작한다. 합주파수 생성을 효과적으로 수행하기 위해서 하나의 단일 모드 광섬유를 이용하여 PPLN 결정 내부에서 두 펌프 광원을 공간적으로 완전히 중첩하였다. 모드 잠금된 펄스 레이저와 좁은 선폭의 연속발진 반도체 레이저에 의해서 모드 잠금된 펄스 형태의 778 nm인 합주파수 생성을 스펙 트럼과 시간적인 변화로 확인하였다. 본 연구 결과는 주파수 제어가 가능한 광주파수 빗(optical frequency comb)을 이용한 광주파수 측정및 고분해 레이저 분광 연구 등 다양하게 응용될 수 있을 것으로 기대된다.
[Editors Pick] Current Optics and Photonics Vol. 5 no. 2 (2021 April)
Optical Encryption Scheme for Cipher Feedback Block Mode Using Two-step Phase-shifting Interferometry
Seok Hee Jeon1 and Sang Keun Gil2 *
1 Department of Electronic Engineering, Incheon National University, Incheon 22012, Korea
2 Department of Electronic Engineering, The University of Suwon, Hwaseong, Suwon 18323, Korea
Current Optics and Photonics Vol. 5 No.2 (2021 April) pp. 155-163
Fig. 1 Block diagrams for encryption. (a) Conventional CFB mode, (b) proposed CFB method.
We propose a novel optical encryption scheme for cipher-feedback-block (CFB) mode, capable of encrypting two-dimensional (2D) page data with the use of two-step phase-shifting digital interferometry utilizing orthogonal polarization, in which the CFB algorithm is modified into an optical method to enhance security. The encryption is performed in the Fourier domain to record interferograms on charge-coupled devices (CCD)s with 256 quantized gray levels. A page of plaintext is encrypted into digital interferograms of ciphertexts, which are transmitted over a digital information network and then can be decrypted by digital computation according to the given CFB algorithm. The encryption key used in the decryption procedure and the plaintext are reconstructed by dual phase-shifting interferometry, providing high security in the cryptosystem. Also, each plaintext is sequentially encrypted using different encryption keys. The random-phase mask attached to the plaintext provides resistance against possible attacks. The feasibility and reliability of the proposed CFB method are verified and analyzed with numerical simulations.
[하이라이트 논문] 한국광학회지 Vol. 32 No.1 (2021 February)_2
목표물 신호 모니터링 및 SPGD 알고리즘 기반 3 채널 타일형 결맞음 빔결합 시스템 연구
3-channel Tiled-aperture Coherent-beam-combining System Based on Target-in-the-loop Monitoring and SPGD Algorithm
한국광학회지 Vol. 32 No.1 (2021 February) pp. 1-8
Fig. 1 (a) Schematic of 3-channel fiber laser systems for coherent beam combining. (b) FPGA board for phase control. LD, laser diode; LDD, laser diode driver; VCO, voltage controlled oscillator; PRBS, pseudo-random binary sequence; YDF, ytterbium-doped fiber; FPGA, field programmable gate array.
단일 레이저의 출력 한계를 뛰어넘기 위한 빔결합 방법으로서, 보강간섭 원리를 이용한 타일형 결맞음 빔결합 시스템에 대해 연구하였다. 와트급 출력의 3 채널 결맞음 광섬유 레이저 및 삼각형 배치의 팁-틸트(tip-tilt) 기능을 갖춘 3 채널 출력단을 자체제작 하였다. 모니터링 시스템, 위상제어기, 3 채널 위상변조기 간의 궤환 제어 시스템(closed-loop system)을 구성하고 SPGD 알고리즘을 적용하여, 위상잠금 속도 5~67 kHz, 이상적인 계산값 대비 중심부 광세기 효율 53.3%의 성공적인 3 채널 위상잠금을 구현하였다. 빔결합 소자가 필요 없고, 가장 고출력 가능성을 가진 타일형 결맞음 빔결합을 위한 요소기술 개발이 완료되어, 향후 다채널, 고출력, 고속 제어 연구로 이어질 수 있을 것으로 기대된다.