[Editor's Pick] Current Optics and Photonics Vol. 6 no. 3 (2022 June)
Spectral Reconstruction for High Spectral Resolution in a Static Modulated Fourier-transform Spectrometer
Ju Yong Cho1, Seunghoon Lee2, Hyoungjin Kim1, and Won Kweon Jang1*
Current Optics and Photonics Vol. 6 No. 3 (2022 June) pp. 244-251
Fig. 1 The static modulated Fourier-transform spectrometer during the reconstruction process and a magnified optical layout for its optical-path difference. (a) Schematic view of the static modulated Fourier-transform spectrometer consisting of a modified Sagnac interferometer. S, source; SD, source driver; M1, fixed mirror; M2, displaced mirror; BS, beam splitter; FL, focusing lens; D, onedimensional detector; CI, computer interface; SI, sample interferogram; TM, transfer-function matrix of the spectrometer; RS, reconstructed spectrum. (b) Magnified optical layout near the one-dimensional detector array.
Keywords: Interferometer, Reconstruction, Spectrometry, Static modulation
OCIS codes: (000.4430) Numerical approximation and analysis; (070.4790) Spectrum analysis;(120.3180) Interferometry; (300.6300) Spectroscopy, Fourier transforms
We introduce a spectral reconstruction method to enhance the spectral resolution in a static modulated Fourier-transform spectrometer. The optical-path difference and the interferogram in the focal plane, as well as the relationship of the interferogram and the spectrum, are discussed. Additionally, for better spectral reconstruction, applications of phase-error correction and apodization are considered. As a result, the transfer function of the spectrometer is calculated, and then the spectrum is reconstructed based on the relationship between the transfer function and the interferogram. The spectrometer comprises a modified Sagnac interferometer. The spectral reconstruction is conducted with a source with central wave number of 6,451 cm−1 and spectral width of 337 cm−1. In a conventional Fourier-transform method the best spectral resolution is 27 cm−1, but by means of the spectral reconstruction method the spectral resolution improved to 8.7 cm−1, without changing the interferometric structure. Compared to a conventional Fourier-transform method, the spectral width in the reconstructed spectrum is narrower by 20 cm−1, and closer to the reference spectrum. The proposed method allows high performance for static modulated Fourier-transform spectrometers.
[하이라이트 논문] 한국광학회지 Vol. 33 No.3 (2022 June)
이중 링 Add/Drop 필터와 All-pass 지연 필터로 구성된 이차원 OCDMA 인코더/디코더
Two-dimensional OCDMA Encoder/Decoder Composed of Double Ring Add/Drop Filters and All-pass Delay Filters
한국광학회지 Vol. 33 No.3 (2022 June) pp. 106-112
Fig. 1 Configuration of a 2-D optical code division multiple access (OCDMA) encoder/decoder composed of four cascaded double-ring add/ drop filters and three all-pass filters for delay operation.
Keywords: 집적 광학 소자, 광 코드분할 다중접속, 광 네트워크 보안, 전달 행렬 기법, 링 공진기
OCIS codes: (060.2330) Fiber optics communications; (060.4785) Optical security and encryption; (130.3120) Integrated optics devices; (130.7408) Wavelength filtering devices
이중 링 add/drop 필터와 all-pass 지연 필터로 구성된 이차원 optical code division multiple access (OCDMA) 인코더/디코더를 제안하고, 설계 예시 및 수치해석을 통하여 실현 가능성을 확인하였다. 제안된 OCDMA 인코더/디코더의 칩 면적은 지연 도파로를 사용하는 기존 OCDMA 인코더/디코더에 비하여 1/3 정도로 줄어든다. 제안된 소자의 성능을 모델링하기 위하여 고속 푸리에 변환(fast Fourier transform, FFT) 및 전달 행렬 기법을 사용하였다. 정확한 코드로 디코딩된 펄스의 중심에서 자기상관 피크 값은 어긋난 파장 호핑 코드 및 스펙트럼 위상 코드로 디코딩된 경우의 최대 교차상관 레벨에 비하여 3배 이상으로 관측되었다. 이를 통하여 forward error correction (FEC) 한계에 해당하는 10−3이하의 비트 에러 오율을 얻을 수 있음을 알 수 있다.
A two-dimensional optical code division multiple access (OCDMA) encoder/decoder, which is composed of add/drop filters and all-pass filters for delay operation, is proposed. An example design is presented, and its feasibility is illustrated through numerical simulations. The chip area of the proposed OCDMA encoder/decoder could be about one-third that of a previous OCDMA device employing delay waveguides. Its performance is numerically investigated using the transfer-matrix method combined with the fast Fourier transform. The autocorrelation peak level over the maximum cross-correlation level for incorrect wavelength hopping and spectral phase code combinations is greater than 3 at the center of the correctly decoded pulse, which assures a bit error rate lower than 10−3, corresponding to the forward error-correction limit.
[하이라이트 논문] 한국광학회지 Vol. 33 No.2 (2022 April)
불완전한 보정기를 적용한 자유형 뮬러행렬타원계의 설계
Design of a Free-form Mueller Matrix Ellipsometer with Imperfect Compensators
한국광학회지 Vol. 33 No.2 (2022 April) pp. 59-66
Fig. 1 Generation of an elliptically polarized light.
Keywords: 불완전한 보정기, 뮬러행렬타원계, 광파장대역
OCIS codes: (000.2170) Equipment and techniques; (120.2130) Ellipsometry and polarimetry; (120.5050) Phase measurement
각 편광소자의 방위각을 자유롭게 제어하는 자유형 구동방식에 기반한 뮬러행렬타원계(Mueller matrix ellipsometer, MME)를 제시한다. 보정기의 위상지연각 분산특성과 전기장 투과율비의 파장의존성을 고려하여 시료에 입사하는 빛의 편광상태를 최적화하기 위한 편광자의 방위각과 편광자측 보정기의 방위각 조합들을 제시한다. 시료로부터 출사하는 빛의 스톡스상수들을 빠르고 정확하게 측정할 수 있는 검광자의 방위각과 검광자측 보정기의 방위각 조합들, 그리고 출사하는 빛의 스톡스상수들을 구하는 수학적 표현들을 제시한다. 이 MME는 이상적이지 않은 4분파장 위상지연자를 적용할 수 있도록 보정기 선택의 폭을 넓혀주므로 깊은 자외선(deep ultraviolet)부터 근적외선(near infrared)에 이르는 매우 넓은 파장 대역에 걸쳐 작동하는 뮬러행렬 분광타원계(Mueller matrix spectroscopic ellipsometer, MMSE)의 제작과 활용을 용이하게 해 줄 것이다.
A free-form Mueller matrix ellipsometer (MME) based on independent control of the azimuthal angle of each polarizing element is introduced. The azimuthal angles of the polarizer and the matching compensator which generate the optimum Stokes vectors of an incident beam are investigated for the polarization state generator, where the spectral responses of the retardation angle and transmittance ratio of a nonideal compensator are taken into account. Similarly, the azimuthal angles of the analyzer and the corresponding compensator are investigated for the polarization-state detector, to unambiguously determine the Stokes vector of the outcoming beam from the sample, and explicit expressions for the Stokes elements are derived. Since the suggested technique enables one to utilize a nonideal quarter-wave plate as the compensator for an MME, it will contribute to the construction and application of a Mueller matrix spectroscopic ellipsometer (MMSE) operating over a wide spectral range from deep ultra-violet to near infrared.
[Editor's Pick] Current Optics and Photonics Vol. 6 no. 1 (2022 February)
Design of an 8× Four-group Zoom System without a Moving Group by Considering the Overall Length
Sung Min Park, Jea-Woo Lee, and Sung-Chan Park*
Current Optics and Photonics Vol. 6 No.1 (2022 February) pp. 104-113
Fig. 1 Paraxial ray traces in an equivalent optical system: (a) Ray tracing in the forward direction, (b) ray tracing in the reverse direction.
Keywords: Aberrations, First orders, Tunable polymer lens, Zoom lens, Zoom position
OCIS codes: (080.2740) Geometric optical design; (120.4570) Optical design of instruments; (220.3620) Lens system design
We present a method to count the overall length of the zoom system in an initial design stage. In a zoom-lens design using the concept of the group, it has been very hard to precisely estimate the overall length at all zoom positions through the previous paraxial studies. To solve this difficulty, we introduce T eq as a measure of the total track length in an equivalent zoom system, which can be found from the first-order parameters obtained by solving the zoom equations. Among many solutions, the parameters that provide the smallest T eq are selected to construct a compact initial zoom system. Also, to obtain an 8× four-group zoom system without moving groups, tunable polymer lenses (TPLs) have been introduced as a variator and a compensator. The final designed zoom lens has a short overall length of 29.99 mm, even over a wide focal-length range of 4‒31 mm, and an f-number of F/3.5 at wide to F/4.5 at tele position, respectively.
[하이라이트 논문] 한국광학회지 Vol. 33 No.1 (2022 February)
다초점 기능을 갖는 그래핀 전극 기반 적외선 프레넬 렌즈
A Graphene-electrode-based Infrared Fresnel Lens with Multifocal Function
한국광학회지 Vol. 33 No.1 (2022 February) pp. 28-34
Fig. 1 A graphene-electrode-based infrared Fresnel zone plate (IR FZP) lens. (a) Schematic. (b) Process flow.
Keywords: 프레넬 존 플레이트, 그래핀, 적외선 대역, 다초점 기능
OCIS codes: (050.1965) Diffractive lenses; (130.1750) Components; (160.4670) Optical materials; (220.3630) Lenses
그래핀 전극 아래에 놓인 다층 그래핀 존 플레이트로 구성된 적외선 프레넬 렌즈의 초점 성능을 전산모사를 통해 조사한다. 여기서 패턴된 다층 그래핀의 페르미 에너지 준위(EF)는 그 위에 놓인 그래핀 전극에 의해 조절된다. 4 μm에서 30 μm까지의 광대역 파장에서 유리 기판 위에 놓인 8층 그래핀 존 플레이트와 그래핀 전극의 반사도 대비비에 따른 프레넬 렌즈 효과를 분석하였다. 반사도와 반사도 대비비를 고려한 최적 파장인 8 μm 입사파가 초점거리 240 μm인 프레넬 렌즈에 입사 시, 다층 그래핀의 EF가 0.4 eV에서 1.6 eV로 증가함에 따라 초점 세기가 4.3배, 그래핀 층수가 2층에서 8층으로 증가함에 따라 5.8배 강화되었다. 이를 통해 인가된 EF에 따라서 다중 초점(240 μm 및 360 μm) 성능을 보이는 그래핀만으로 구성된 IR 프레넬 렌즈 구조를 초박형 렌즈 플랫폼으로 제안한다.
We study through computational simulation the focal performance of an infrared (IR) Fresnel lens, composed of a multilayer-graphene zone plate formed under a graphene electrode. Here the Fermi level EF of the patterned multilayer graphene is adjusted by the overlying graphene electrode. The Fresnel lens effect, with respect to the reflectance contrast between the graphene electrode and the 8-layer graphene zone plate placed on a glass substrate, has been analyzed over a broad wavelength range from 4 to 30 μm. As the optimal wavelength of 8 μm (considering the reflectance and the reflectance-contrast ratio) is incident upon the Fresnel lens with a focal length of 240 μm, the focal intensity is enhanced by a factor of 4.3 as the EF of multilayer graphene increases from 0.4 eV to 1.6 eV, and is improved by a factor of 5.8 as the number of graphene layers increases from two to eight. As a result, an all-graphene-based IR Fresnel zone-plate lens, exhibiting multifocal function (240 μm and 360 μm) according to the selected EF, is proposed as an ultrathin lens platform.
[Editor's Pick] Current Optics and Photonics Vol. 5 no. 6 (2021 December)
Fourier Modal Method for Optical Dipole Radiation in Photonic Structures
Sungjae Park1ㆍJoonku Hahn2ㆍHwi Kim1*
Current Optics and Photonics Vol. 5 No.6 (2021 December) pp. 597-605
Fig. 1 Photonic multiblock structure with an optical-source-embedding block, and the optical field distributions for (a) a dipole with polarization P = (1, 0, 0) and (b) a dipole with polarization P = (0, 0, 1). The thicknesses of block #1, the cathode, block #2, the active block, block #3, and the anode are 70 nm, 8 nm, 31 nm, 38 nm, 157 nm, and 114 nm respectively, and their refractive indices (n + jk) are 1.88, 0.191 + 3.24j, 1.71, 1.84 + 0.00272j, 1.94, and 0.129 + 3.19j, respectively.
Keywords: Fourier modal method, Numerical modeling
OCIS codes: (000.3860) Mathematical methods in physics; (050.1755) Computational electromagnetic methods; (050.1960) Diffraction theory
An extended Fourier modal method (FMM) for optical dipole radiation in three-dimensional photonic structures is proposed. The core elements of the proposed FMM are the stable bidirectional scatteringmatrix algorithm for modeling internal optical emission, and a novel optical-dipole-source model that prevents numerical errors induced by the Gibbs phenomenon. Through the proposed scheme, the FMM is extended to model a wide range of source-embedded photonic structures.
[하이라이트 논문] 한국광학회지 Vol. 32 No.6 (2021 December)
Korsch 망원경의 열분석을 통한 광학계 성능 평가 방법 및 비열화 구조 연구
A Study on the Method of Evaluating Optical-system Performance and an Athermal Structure through Thermal Analysis of the Korsch Telescope
한국광학회지 Vol. 32 No.6 (2021 December) pp. 266-275
Fig. 1 The layout of the thermal analysis process.
Keywords: 비열화 설계, 인공위성, 구조체, 망원경, 열분석
OCIS codes: (080.2740) Geometric optical design; (350.4600) Optical engineering
본 논문은 Korsch 망원경의 열분석을 통한 광학계 성능 평가 방법 및 비열화 구조를 연구하였다. 비대칭성의 복잡한 구조를 가진 광학계의 경우, 광학 설계 소프트웨어에 열팽창 계수를 적용하여 인공위성 구조를 구현하는데 한계가 있어 온도 변화에 대한 광학계 성능 평가가 이루어지기 어렵다. 이러한 문제점을 해결하기 위해 기계 설계 소프트웨어를 이용하여 온도에 따라 광학계에 영향을 주는 모든 구조체에 길이 변화를 구현하였고, 온도 변화에 대한 광학 부품 사이의 거리 변화량을 정리하였다. 또한 광학 설계 소프트웨어를 이용하여 온도 변화에 대한 광학 부품의 형상 및 두께 변화량을 정리하였다. 두 소프트웨어에서 도출한 모든 변화량을 광학 소프트웨어에 적용하여 광학계의 성능 평가를 진행하 였다. 그 결과 공간 분해능 71.4 cycles/mm에 대한 변조전달함수(MTF)가 9 ℃에서 33 ℃까지의 범위에서 25% 이상 유지되는 것을 확인하였 다. 또한 광학계 성능 변화에 가장 영향을 많이 주는 구조체를 찾아, 영향을 줄이도록 비열화 구조를 도출하여 개선된 구조물을 적용한 광학계의 성능 평가를 진행하였다. 그 결과, 분해능 71.4 cycles/mm에 대한 변조전달함수가 9 ℃에서 33 ℃까지의 범위에서 67% 이상 유지되는 것을 확인하였다.
In this paper, a method for evaluating optical-system performance and an athermal structure through thermal analysis of the Korsch telescope was studied. In the case of an optical system having a complex asymmetrical structure, there is a limit to implementing the satellite structure by applying the coefficient of thermal expansion (CTE) in the optical-design software, so it is difficult to evaluate the performance of the optical system against temperature changes. To solve this problem, using mechanical design software all length changes were implemented in all structures that affect the optical system according to temperature, and the value of the change in distance between optical components due to temperature change was organized. Also, the values of changes in shape and thickness of the optical components against temperature changes are organized in the optical-design software. All changes derived from both software packages were applied in the optical software to evaluate the performance of the optical system. As a result, it was found that the MTF for a spatial resolution of 71.4 cycles/mm was maintained at more than 25% in the range from 9 ℃ to 33 ℃. In addition, the performance of the optical system applying the improved structure was evaluated, by finding the structure that had the most influence on the optical system’s performance change, and deriving an athermal structure to reduce the effect. As a result, it was found that the MTF for a resolution of 71.4 cycles/mm was maintained at over 67% in the range from 9 ℃ to 33 ℃.
[Editor's 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.