Recent Publications (selected)

B. Kim, J. Na, C. Jun, and Y. Jeong, "Direct Phase-Locking Algorithm for Tiled-Aperture Coherent Beam Combining Systems," J. Light. Technol., Vol. 43, No. 18, Sep. 2025: 8575-8585

Abstract: We present a novel method for coherent beam combining (CBC) by exploiting the analytic modal decomposition (MD) method based on the Moore-Penrose inverse matrix. Emphatically, this method produces a highest intensity in the central lobe on the target plane in a non-iterative or direct manner. We verify the feasibility of the proposed method both numerically and experimentally: The related extensive numerical investigations are carried out for 3-, 7-, 19-, and 37-channel CBC configurations, including the power-in-the-bucket (PIB) efficiency, channel phase error, and phase acquisition speed with three different target image resolutions (64 × 64, 128 × 128, and 256 × 256 pixels) under both noise-free and noise-present conditions. The proposed method demonstrates near-perfect performance under noise-free conditions, achieving a PIB efficiency of over 0.999 particularly for 19-channel CBC with an image resolution of 64 × 64 although the performance degrades somewhat under noisy conditions. We also conduct a proof-of-concept experiment based on a 3-channel CBC system in a triangular tiled aperture format implemented with the proposed method. An average PIB efficiency of 0.912 is achieved with the estimated phase acquisition time of 38.7 ms without dynamic tip-tilt and alignment control. To the best of our knowledge, this proof-of-concept experimental result is the first demonstration of a CBC system realized by the analytic MD method. In addition, we provide a theoretical analysis of how fringe pattern correlation affects the noise susceptibility of the proposed method, and explain that introducing slight irregularity in beam arrangement can mitigate this effect. We expect the proposed method to pave the way to the advancement of the CBC technology as a novel phase control method. 

M. Yeo, H. Kim, and Y. Jeong, "Phase Control Algorithms for CBC Systems Based on Evolutionary Strategy for Implementation on FPGA Boards," J. Light. Technol., Vol. 43, No. 18, Sep. 2025: 8563-8574

Abstract: As efforts to increase the number of combining channels in coherent beam combining (CBC) systems have progressed, the limitations of gradient-based phase control algorithms, particularly with respect to the local extremum problem, have become more apparent. Although adaptive phase control techniques can be used in conjunction with them to alleviate the problem, these algorithms remain fundamentally limited. Recent studies have proposed alternatives, including deep learning and statistical algorithms; however, gradient-based methods continue to dominate due to the considerable difficulty of implementing such novel algorithms on field-programmable gate array (FPGA) boards for real-world CBC systems. In this paper, we propose a novel gradient-free statistical phase control algorithm, the correspondence lookup-table adaptation evolution strategy (CLA-ES), which can readily be implemented on FPGA boards. Leveraging CBC system characteristics, the CLA-ES algorithm constructs a correspondence curve to map the relationship between combined beam intensity and phase distribution, which circumvents the otherwise required complex matrix operations and drastically reduces the dimensionality of the problem. Simulations demonstrate that the CLA-ES algorithm achieves convergence performance comparable to the CMA-ES algorithm but with notably greater computational efficiency, achieving over two orders of magnitude improvement in speed for a 127-channel configuration, for example. This result highlights that, in the context of CBC systems, the ultimate design of phase control algorithms should aim to be gradient-free, aligning well with the direction pursued by the CLA-ES algorithm. 

J. Na, B. Kim, C. Jun, and Y. Jeong, "Analysis of Major Error Factors in Coherent Beam Combination: Phase, Tip Tilt, Polarization Angle, and Beam Quality," Curr. Opt. Photonics, Vol 8, No. 4, Aug 2024: 406-415

Abstract: The major error factors that degrade the efficiency of coherent beam combining (CBC) are numerically studied in a comprehensive manner, paying particular attention to phase, tip-tilt, polarization angle, and beam quality. The power in the bucket (PIB), normalized to the zero-error PIB, is used as a figure of merit to quantify the effect of each error factor. To maintain a normalized PIB greater than or equal to 95% in a 3-channel CBC configuration, the errors in phase, tip-tilt, and polarization angle should be less than 1.06 radians, 1.25 μm, and 1.06 radians respectively, when each of the three parameters is calculated independently with the other two set to zero. In a worst-case scenario of the composite errors within the parameter range for the independent-95%-normalized-PIB condition, the aggregate effect would reduce the normalized PIB to 83.8%. It is noteworthy that the PIB performances of a CBC system, depending on phase and polarization-angle errors, share the same characteristic feature. A statistical approach for each error factor is also introduced, to assess a CBC system with an extended number of channels. The impact of the laser’s beam-quality factor M2 on the combining efficiency is also analyzed, based on a super-Gaussian beam. When M2 increases from 1 to 1.3, the normalized PIB is reduced by 2.6%, 11.8%, 12.8%, and 13.2% for a single-channel configuration and 3-, 7-, and 19-channel CBC configurations respectively. This comprehensive numerical study is expected to pave the way for advances in the evaluation and design of multichannel CBC systems and other related applications. 

K. Kim, T. Choi, S. Choi, H. Kim, Y. Lee, S. Kim, B. Lee, and Y. Jeong, "Multi-Objective Optimization of Dynamic VO2 Metagrating: Broadband Polarization-Independent Transmission Modulator," Opt. Express, Vol. 32, No. 19, Sep. 2024: 32680-32690

Abstract: In this paper, a novel thermo-optic metagrating based on phase-change material (vanadium dioxide, VO2) is proposed for broadband, polarization-independent, and non-dispersive transmission modulation at the telecommunication wavelengths. In the pursuit of concurrent attainment of multiple performance objectives, nanostructured VO2 metagratings are optimized numerically using inverse design algorithms. Notably, adjoint optimization pertaining to both phases of VO2 facilitates better modulation capabilities within free-form shaped VO2 metagratings compared to shape-optimized methods with predetermined designs. It is verified that the free-form metagrating modulator achieves broad bandwidth (∼300 nm), large modulation depth (>0.8) and non-dispersive transmittance (∼0.2), and polarization independent operation. The design ensures polarization-independence, verified through both simulations and experiments. Experimental validation included fabricating VO2 metagratings and measuring their performance across varying temperatures. The results demonstrate agreement with theoretical predictions, highlighting the device's potential for applications in next-generation spatial light modulators, optical routers, and three-dimensional optical scanning sensors. This study underscores the promising capabilities of VO2-based metagratings in dynamic optical modulation and broadband telecommunication applications. 

H. Son, T. Choi, K. Kim, Y. Kim, J. Bang, S. Kim, B. Lee, and Y. Jeong, "Strong Coupling Induced Bound States in the Continuum in a Hybrid Metal-Dielectric Bilayer Nanograting Resonator," ACS Photonics, Vol. 11, No. 8, Jul. 2024: 3221-3231

Abstract: In the field of modern optics, the capability of localizing light at the nanoscale is crucial. Recently, the concept of the bound state in the continuum (BIC) has emerged, demonstrating highly resonant photonic modes within lossless dielectric nanostructures. On the contrary, implementing BICs with plasmonic resonators, despite its distinct advantages of near-field concentration, has been less preferred due to inherent material losses. This study proposes a novel BIC nanoresonator utilizing a hybrid metal–dielectric bilayer nanograting. In this structure, the metallic upper nanograting functions as a concentrator of the incident wave, whereas the dielectric lower nanograting serves as the main resonator for the concentrated field, exhibiting negligible material loss. This design facilitates strong near-field coupling between the three modes induced within the hybrid nanograting, leading to the emergence of BIC with exceptional quality factors. Our comprehensive analysis, including theoretical, numerical, and experimental investigations, reveals that the attainment of strong coupling is followed by the formation of BIC, while distinct mode hybridization and large Rabi splitting energy of about 436 meV are observed. As a result, an enhancement of more than 8-fold in electromagnetic energy is achieved within a silicon nanograting compared with the conventional single-layer resonator. It is worth noting that the trade-off between the intensity and storage lifetime of confined energies is addressed and the novel formation principle of the strong coupling induced BIC at the Γ-point is unveiled via temporal and spatial coupled mode theories for the first time, to the best of our knowledge. Our findings are expected to enhance the functionalities of resonant nanophotonic applications. 

H. Kim, M. Yeo, and Y. Jeong, "Combinatorial Study of Coherent Multi-Channel Phase-Locking Based on Covariance Matrix Adaptation Evolution Strategy," J. Korean. Phys. Soc., Vol. 85, Jun. 2024: 406-414.

Abstract: This study explores the performance and channel-scalability of the covariance matrix adaptation evolution strategy (CMA-ES)-based coherent optical phase control algorithm for coherent beam combining (CBC) systems. Leveraging the probabilistic nature of its optimization process, the CMA-ES algorithm emerges as a promising candidate for a next-generation phase control algorithm for CBC systems. To assess its functionality and channel-scalability, we conduct numerical investigations into the CMA-ES-based phase control algorithm applied to both 37- and 61-channel CBC systems with varying its algorithmic parameters. For the 37-channel configuration, consistent results demonstrate an average beam combining efficiency (BCE) exceeding 0.9, with a constrained standard deviation of 0.1 with phase sample numbers surpassing 30. The analysis of the time complexity reveals that the CMA-ES-based algorithm efficiently converges to a BCE value of 0.9 within a 10 MHz bandwidth within a 5 μs atmospheric time scale. In the case of the 61-channel configuration, a majority of phase samples exhibit an average BCE exceeding 0.95, with a small number of trials slightly falling below 0.85 yet still achieving a BCE of approximately 0.8. Similar to the 37-channel case, with the bandwidth < 12 MHz, the CMA-ES-based algorithm can give rise to the BCE level of 0.9 within a 5 μs atmospheric time duration. 

M. Yeo, B. Kim, H. Kim, and Y. Jeong, "Channel Phase Extraction for a Coherent Beam Combining System Using a 2D Target Intensity Image and the CMA-ES Algorithm," J. Korean. Phys. Soc., Vol. 85, Jun. 2024: 120-128.

Abstract: We investigate a novel covariant matrix adaption-evolution strategy (CMA-ES)-based method proposed for extracting channel phase information by measuring a two-dimensional (2D) target intensity image (2D-TII) of a coherent beam combining (CBC) system both numerically and experimentally for the first time to the best of our knowledge. The proposed method was first investigated on 1,000 samples of 2D-TIIs numerically generated by a virtual 3-channel CBC system. For all samples, the channel phase information was extracted almost perfectly, with the inter-image correlation coefficient reaching or exceeding 0.99 and the overall root-mean-square phase error of 0.0735 rad within 17 iterations of the algorithm, for example. Next, the investigation was extended onto another 1,000 samples of 2D-TIIs experimentally measured with a real-world 3-channel CBC setup via a charge-coupled device (CCD) camera at a rate of 16 fps with an exposure time of 10 ms. The channel phase information was extracted with the inter-image correlation coefficient reaching or exceeding 0.9 for 972 or 979 samples within 15 or 45 iterations of the algorithm, respectively, with the latter case of which its overall average was estimated at 0.947. The relatively low performance of the proposed method within 21 out of 1,000 samples, where the overall average of the inter-image correlation coefficient remained at 0.880 regardless of further increases in the number of iterations, was attributed to the lowered image contrasts of the measured 2D-TIIs caused by the uncontrolled intrusion of external noise components that could not be rectified by the CCD camera due to its limited exposure time. We expect the proposed method to be useful for research and analysis on a variety of real-world CBC systems as well as other related applications where phase information needs to be extracted. 

H. Chang, M. Yeo, S. Kim, and Y. Jeong, "Engineering Angle Selectivity of Extraordinary Optical Transmission and Nonlocal Spatial Filtering," J. Opt. Soc. Am. B, Vol. 40, No. 10, Oct. 2023: 2576-2585.

Abstract: Plasmonic metasurfaces can provide rich opportunities for miniaturized analog optical computing, such as ultrathin spatial filtering and free-space wavefront differentiation. In this work, the extraordinary optical transmission (EOT), one of the most fascinating phenomena of plasmonics, is revisited and rigorously studied in terms of the angle selectivity of the transmission resonance. In particular, the fundamental mechanisms of angle-selective transmission in 1D plasmonic nanoslit metasurfaces are investigated. Based on temporal coupled mode theory and Fano resonances, it is elucidated that the angle selectivity is due to strong coupling between multiple resonance channels. Using the theory that explains the origin of angle selectivity, the basic design rules for nonlocal lowpass and bandpass spatial filters based on infinite periodic metasurface structures were determined and verified numerically. Furthermore, the effect of the finite size of the metasurface structures on the nonlocality of the EOT was investigated and formulated by both a full numerical calculation and an analytical scalar wave approximation based on the convolution theorem together with the optical transfer function obtained for the infinite periodic structures. It is expected that the formulation of angle-selective EOT will be fruitfully exploited for the development of ultrathin designer spatial filters required for coherent imaging processes in compact Fourier optics applications. 

B. Kim, J. Na and Y. Jeong, "Convolutional Neural Network Combined With Stochastic Parallel Gradient Descent to Decompose Fiber Modes Based on Far-Field Measurements," Journal of Lightwave Technology, vol. 41, no. 18, pp. 5973-5982, 15 Sept.15, 2023. 

Abstract: Modal decomposition (MD) of fiber modes based on direct far-field measurement combining the convolutional neural network (CNN) with a stochastic parallel gradient descent (SPGD) algorithm is investigated both numerically and experimentally. For obtaining the modal coefficients of fiber modes guided in a large-mode-area fiber, the fiber modes are decomposed into a finite number of Hermite gaussian modes, the initial conditions of the modal coefficients are obtained through the CNN, and further optimization of them are carried out through the SPGD. The ambiguity problem that may happen in the CNN owing to the existence of the pair-beam field is resolved by properly labelling the phase differences with a single-valued parameter set in consideration of the mode-order indices. The feasibility and effectiveness of the proposed MD method is verified both numerical simulations and experimental demonstrations with both recorded image data and online real-time image data. The correlation error incurred by the proposed method is below 6.6 × 10^−4 and 8.7 × 10^−3 in the numerical simulations and the experimental demonstrations, respectively. The online real-time operation of the proposed method is also experimentally demonstrated at a decomposing rate of ∼2Hz. 

H. Kim, Y. Jeong, K. Lee, and Y. Jeong, "Noise robust Zernike phase retrieval via learning based algorithm only with 2-step phase shift measurements," Opt. Express  31, 30248-30266 (2023). 

Abstract: We present a noise robust deep learning based aberration analysis method using 2-step phase shift measurement data. We first propose a realistic aberration pattern generation method to synthesize a sufficient amount of real-world-like aberration patterns for training a deep neural network by exploiting the asymptotic statistical distribution parameters of the real-world Zernike coefficients extracted from a finite number of experimentally measured real-world aberration patterns. As a result, we generate a real-world-like synthetic dataset of 200,000 different aberrations from 15 sets of real-world aberration patterns obtained by a Michelson interferometer under a variety of measurement conditions using the 4-step derivative fitting method together with the exploitation of the Gaussian density estimation. We then train the deep neural network with the real-world-like synthetic dataset, using two types of network architectures, GoogLeNet and ResNet101. By applying the proposed learning based 2-step aberration analysis method to the analysis of numerically generated aberrations formed under 100 different conditions, we verify that the proposed 2-step method can clearly outperform the existing 4-step iterative methods based on 4-step measurements, including the derivative fitting, transport of intensity equation (TIE), and robust TIE methods, in terms of noise robustness, root mean square error (RMSE), and inference time. By applying the proposed 2-step method to the analysis of the real-world aberrations experimentally obtained under a variety of measurement conditions, we also verify that the proposed 2-step method achieves compatible performance in terms of the RMSE between the reconstructed and measured aberration patterns, and also exhibits qualitative superiority in terms of reconstructing more realistic fringe patterns and phase distributions compared to the existing 4-step iterative methods. Since the proposed 2-step method can be extended to an even more general analysis of aberrations of any higher order, we expect that it will be able to provide a practical way for comprehensive aberration analysis and that further studies will extend its usefulness and improve its operational performance in terms of algorithm compactness, noise robustness, and computational speed. 

S. Lee, Y. Lim, H. Kim, D. Seo, J. Na, H. Kim, K. Nam, and Y. Jeong, "Random Lasing with a High Degree of Circular Dichroism by Chiral Plasmonic Gold Nanoparticles," ACS Photonics, vol. 9, no. 2, Jan. 2022: 613–620. 

Abstract: Random lasers have distinct advantages to be the next-generation light sources owing to their simple fabrication process, high flexibility in shape and size, and unique optical characteristics, such as low spatial coherence, high intensity, and multi-directionality. In this paper, we discuss how to realize random lasing with a high degree of circular dichroism with the aid of chiral plasmonic gold nanoparticles. The extinction dissymmetry factor of the chemically synthesized chiral plasmonic gold nanoparticles is measured to be −0.11 at its peak wavelength of 575 nm. The lasing properties and luminescence dissymmetry factor of the emission of the random laser are measured and characterized. An optimal inclusion of the chiral plasmonic gold nanoparticles to an ethylene glycol solution of rhodamine 6G laser dye molecules mixed with dielectric titanium dioxide nanoparticles eventually results in the laser emission having a considerably high level of asymmetry between the right- and left-handed circularly polarized light, yielding a luminescence dissymmetry factor of 0.20−0.23. This study paves the way for the development of a random laser of a high degree of circular dichroism in a highly flexible compact form through a simple, mass-productive fabrication process, inviting numerous potential applications in nano-photonics. 

K. Park, Achar V. Harish, Johan Nilsson, and Y. Jeong, "Study on the asymptotic behavior of the interplay of stimulated Brillouin scattering and Brillouin‑enhanced four‑wave mixing in standard single‑mode fibers," J. Korean Phys. Soc., volume 80, issue 1, Jan. 2022: 37–52. 

Abstract: We theoretically study stimulated Brillouin scattering (SBS) in a standard single-mode fiber (SMF), taking Brillouin-enhanced four-wave-mixing (BEFWM) effects into account. In particular, we investigate the case when there is non-negligible back-reflection of the forward-pump field at the rear fiber end although such reflection is typically weak and undesired. We first justify that BEFWM can be treated as a steady-state process under an undepleted pump approximation as long as the nominal SBS gain remains as low as 40 dB unless the pump, Stokes, anti-Stokes fields interact under near-perfect phase-matching condition, which hardly happens in normal circumstances with a standard SMF. Under the steady-state and undepleted-pump condition, we find analytical solutions to the Stokes and anti-Stokes fields generated by the forward and backward-pump fields, and also derive their asymptotic formulae in both infinitesimal and infinite limits in terms of the phase-mismatch parameter of |ΔkL| , assuming that both seeding Stokes and anti-Stokes fields arise from white background noise components. When |ΔkL| ≪ 1, the acoustic fields driven by SBS and BEFWM tend to interfere destructively, and thus, SBS and BEFWM are anti-resonant to each other, thereby eventually resulting in both Stokes and anti-Stokes scatterings minimized at Δk = 0 . When |ΔkL| ≫ 1, all the asymptotic curves for the amplification ratios and extra gain factor obey the inverse square law with respect to |ΔkL| , irrespective of the level of the back-reflection at the rear fiber. In particular, when |ΔkL| is in the intermediate range where the FWM gain remains relatively large, SBS and BEFWM can be cooperative via the phase-pulling effect by the FWM gain, thereby leading to quasi-resonant growths of both Stokes and anti-Stokes fields. However, the extra gain by BEFWM reduces significantly if the level of the back-reflection remains below one percent, irrespective of the value of |ΔkL| . Since the interplay between SBS and BEFWM is inherently phase-dependent whilst it can still happen with white noise seeding with random phases, the related mechanism can further be exploited for all-optical switching functionality. We expect our theoretical modeling and formulation will be useful for designing and analyzing a variety of fiber systems that incorporate high-power narrow-linewidth light undergoing non-negligible back-reflection under various conditions.

S. Lee, H. Kim, and Y. Jeong, "Angular distribution of luminescence dissymmetry observed from a random laser built upon the exocuticle of the scarab beetle Chrysina gloriosa," Opt. Express, vol. 29, no. 23, Nov 2021: 37712. 

Abstract: We investigate the angular distribution of luminescence dissymmetry of random lasing in the mixture of rhodamine 6G and titanium dioxide nanoparticles upon a biocompatible natural material substrate, i.e., the elytron of the scarab beetle Chrysina gloriosa. We look into both green and gold-colored areas of the elytron that exhibit distinctly different circular dichroism properties. The fabricated sample asymmetrically emits both left- and right-handed circularly polarized light at 570 nm when pumped at 532 nm, depending on the direction of emission and the angle of the pump incidence. We characterize the light via measuring the angular distribution of its luminescence dissymmetry factor (glum), which reaches an unusually high maximal value of 0.90 or −0.50 at some specific angle depending on the handedness of its polarization. This random laser source can be used in numerous potential optoelectronic applications which require light emission of distributed luminescence dissymmetry or of high luminescence dissymmetry. 

B. Kim, J. Na, J. Kim, H. Kim and Y. Jeong, "Modal decomposition of fiber modes based on direct far-field measurements at two different distances with a multi-variable optimization algorithm," Opt. Express, vol. 29, no. 14, July 2021: 21502-21520. 

Abstract: We present a novel method for modal decomposition of a composite beam guided by a large-mode-area fiber by means of direct far-field pattern measurements with a multi-variable optimization algorithm. For reconstructing far-field patterns, we use finite-number bases of Hermite Gaussian modes that can be converted from all the guided modes in the given fiber and exploit a stochastic parallel gradient descent (SPGD)-based multi-variable optimization algorithm equipped with the D4σ technique in order for completing the modal decomposition with compensating the centroid mismatch between the measured and reconstructed beams. We measure the beam intensity profiles at two different distances, which justifies the uniqueness of the solution obtained by the SPGD algorithm. We verify the feasibility and effectiveness of the proposed method both numerically and experimentally. We have found that the fractional error tolerance in terms of the beam intensity overlap could be maintained below 1 × 10^−7 and 3.5 × 10^−3 in the numerical and experimental demonstrations, respectively. As the modal decomposition is made uniquely and reliably, such a level of the error tolerance could be maintained even for a beam intensity profile measured at a farther distance. 

H. Kim and Y. Jeong, "Covariance matrix adaptation evolution strategy based optical phase control." Electron. Lett., 57.13, June 2021: 517-519. 

Abstract: In this letter, an investigation of the use of a covariance matrix adaptation evolution strategy (CMA-ES) algorithm is conducted as the phase-locking method for multi-channel coherent beam combining (CBC) for the first time. A comprehensive numerical analysis is carried out on the operational performances of the CMA-ES based phase-locking algorithm implemented into 7- and 19-channel CBC systems in a filled-aperture format. Through numerical simulations it can be verified that the CMA-ES algorithm can readily lead to over 0.90 of normalised beam combining efficiency with appropriate algorithm parameter sets, which can also be optimised by a combinatorial study. The proposed CMA-ES based phase-locking algorithm is a feasible option for novel phase-locking technique for a CBC system particularly when the local extremum issue becomes severe such as in CBC under turbulent atmospheric conditions.