Recent Publications (selected)

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. 

H. An and Y. Jeong, "Numerical study of superradiant mixing by an unsynchronized superradiant state of multiple atomic ensembles." Optics Express 28.15, July 2020: 22276-22286. 

Abstract: We numerically analyze superradiant dynamics in atomic nsembles that have different transition frequencies using a numerical model that can take account of the transient behavior of an unsynchronized superradiant state. The numerical results unveil that the superradiant emission of a periodic pulse train can be induced by means of collective multiple frequency generation, which we call superradiant mixing. This is, in fact, due to the superradiant coupling of unsynchronized atomic ensembles. We numerically investigate the superradiant mixing in detail, varying the collective decay rate, repumping rate, and the number of the individual atomic ensembles with detuned frequencies. This work broadens our understanding of the collective atomic behavior in a detuned system, and it also suggests a novel method for frequency generation without relying on the conventional Kerr nonlinear effect. 

K. Park, J. Na, J. Kim and Y. Jeong, "Numerical Study on Supercontinuum Generation in an Active Highly Nonlinear Photonic Crystal Fiber With Anomalous Dispersion", vol. 56, no. 2, April 2020, IEEE Journal of quantum electronics. 

Abstract: We numerically study supercontinuum (SC) generation (SCG) in a rare-earth-doped highly nonlinear photonic crystal fiber (HNL-PCF) with anomalous dispersion (AD) in the sub-picosecond pulse regime. We develop a semi-classical numerical model based on the generalized Ginzburg-Landau equation in order to take account of ultrafast interactions between gain ions and ultra-broadband SC radiation encompassing sub-100-femtosecond solitons. Based on the numerical model, we analyze SCG characteristics of an active HNL-PCF in comparison with a passive-type counterpart, unveiling novel optical gain effects in a highly nonlinear optical fiber with AD. We rigorously investigate gain-induced soliton dynamics, such as soliton-cascade-like behaviors, soliton-quasi-soliton collisions, and phase-matched dispersive wave generation, which eventually contributes to enhancement of energy scaling of SC radiation without incurring considerable degradation of its spectral flatness. We also verify that such superior performance characteristics of an active HNL-PCF make it suitable for the use as a boost amplifier for SC radiation. We think that the findings from this study will incite other subsequent studies on unveiling detailed nonlinear pulse dynamics in various gain-embedded nonlinear optical media. 

H. Kim and Y. Jeong, "Metal-Coated Fiber-Optic Platforms for Surface Plasmon Polariton Generation and Interconnection", vol. 56, no. 2, April 2020, IEEE Journal of quantum electronics. 

Abstract: We analyze a novel metal-coated fiber-optic platform for surface plasmon polariton (SPP) generation and interconnection. It is based on a metal-coated angled fiber facet (MCAFF), which enables alignment-free and unidirectional SPP generation from a fiber-optic mode with high conversion efficiency. We verify its functionality by means of both numerical simulation and preliminary experiment. We implement a two-level-thickness (TLT) configuration into the MCAFF to maintain its high optic-to-plasmonic conversion efficiency: The thin metallic layer just above the core region efficiently generates SPPs whilst the thick metallic layer beyond the core region enables its low-loss propagation by diminishing decoupling possibility into the dielectric region. We moreover devise a fiber-in-fiber-out (FIFO) platform that consists of a pair of TLT-MCAFFs. We numerically verify that it can yield more than 60% of FIFO coupling efficiency. We further show that the transmission spectrum of the FIFO-MCAFF is highly correlated with the refractive index of the top layer put on the metallic layer, and that it can be exploited to sensing applications that is required to measure and identify delicate changes in the refractive index of the top-layer material. We expect that the proposed metal-coated fiber-optic platforms will provide an efficient way to SPP generation and interconnection, and also has great potential to be novel sensing platforms for gas- or liquid-phase volatile substance. 

H. Kim and Y. Jeong, "Theoretical and Numerical Study of Cylindrical-vector-mode Radiation Characteristics in Periodic Metallic Annular Slits and Their Applications," vol. 2, no. 5, pp. 482-487, 2018, Current Optics and Photonics. 

Abstract: We investigate the radiation characteristics of radially polarized light and azimuthally polarized light through plasmonic subwavelength-scale annular slit (PSAS) structures, by means of both theoretical and numerical methods. Effective-medium theory was utilized to analyze the characteristics of PSAS structures, and the corresponding results showed that PSAS structures can function as a metallic medium for azimuthally polarized light, or as a low-loss dielectric medium for radially polarized light. Numerical calculations based on the finite-element method were also performed, to verify the theoretical analyses. It turned out that the numerical results supported the theoretical results. Moreover, we exploited the PSAS structures in novel nanophotonic elements with dual functionalities that could selectively focus or pass/block incident light, depending on its polarization state. For example, if PSAS structures were implemented in the dielectric region of a metallic Fresnel zone plate, the modified zone plate could function as a blocking element to azimuthally polarized light, yet as a focusing element to radially polarized light. On the contrary, if PSAS structures were implemented in the metallic region of a metallic Fresnel zone plate (i.e. the inverted form of the former), it could function as a focusing element to azimuthally polarized light, yet as a simple transparent element to radially polarized light. 

S. Hong, H. Kim, K. Park, Y. Kwon, H. Chang, S. Piao, Y. Lee, S. Oh and Y. Jeong, "Spatiotemporal laser speckle flowmetry based on elastic-walled U-shaped tubing apparatus, optical method for urinary flow measurement," vol. 57, no.10, pp.104110, 2018, Optical Engineering. 

Abstract: We propose an optical method for uroflowmetry, exploiting the laser speckle contrast imaging (LSCI) technique onto an intermediate tubing apparatus having an elastic wall that can sensitively respond to flow-induced shedding vortices. Based on the method, we devised and fabricated an elastic-walled U-shaped tubing apparatus (EWUSTA), using the three-dimensional printing technique. We utilized the spatiotemporal contrast scheme for the LSCI as a fast and reliable computational algorithm. We investigated three different materials of flex-vinyl, ninja-flex, and natural rubber latex for the elastic wall of the EWUSTA in steady flow conditions, and verified that their optimal operational ranges could extend up to 7, 15, and 25 ml/s, respectively. We characterized the natural-rubber-latex-based EWUSTA in dynamic flow conditions in comparison with a commercial reservoir-weight-transducer-based gravimetric flowmeter, and verified its feasibility. We stress that the proposed method can offer precise and accurate information on flow dynamics. In addition, we found that the upper limit of the optimal operational range of the proposed apparatus had strong correlation with the tensile strength of the elastic-wall material. We reckon that the proposed and demonstrated method has great potential not only for uroflowmetry but also for other flow-related medical and industrial applications. 

S. Lee, K. Park, H. Kim, L. A. Vazquez-Zuniga, J. Kim and Y. Jeong, "Intermittent burst of a super rogue wave in the breathing multi-soliton regime of an anomalous fiber ring cavity, "Opt. Express, vol. 26, no. 9, pp. 11447-11457, 2018. 

Abstract: We report the intermittent burst of a super rogue wave in the multi-soliton (MS) regime of an anomalous-dispersion fiber ring cavity. We exploit the spatio-temporal measurement technique to log and capture the shot-to-shot wave dynamics of various pulse events in the cavity, and obtain the corresponding intensity probability density function, which eventually unveils the inherent nature of the extreme events encompassed therein. In the breathing MS regime, a specific MS regime with heavy soliton population, the natural probability of pulse interaction among solitons and dispersive waves exponentially increases owing to the extraordinarily high soliton population density. Combination of the probabilistically started soliton interactions and subsequently accompanying dispersive waves in their vicinity triggers an avalanche of extreme events with even higher intensities, culminating to a burst of a super rogue wave nearly ten times stronger than the average solitons observed in the cavity. Without any cavity modification or control, the process naturally and intermittently recurs within a time scale in the order of ten seconds. 

J. Kim, J. Kim, J. Na and Y. Jeong, "Numerical study of a novel bi-focal metallic Fresnel zone plate having shallow depth-of-field characteristics," Curr. Opt. Photon., vol. 2, no. 2, pp. 147-152, 2018. 

Abstract: We propose a novel bi-focal metallic Fresnel zone plate (MFZP) with shallow depth-of-field (DOF) characteristics. We design the specific annular slit patterns, exploiting the phase-selection-rule method along with the particle swarm optimization algorithm, which we have recently proposed. We numerically investigate the novel characteristics of the bi-focal MFZP in comparison with those of another bi-focal MFZP having equivalent functionality but designed by the conventional multi-zone method. We verify that whilst both bi-focal MFZPs can produce dual focal spots at 15 μm and 25 μm away from the MFZP plane, the former exhibits characteristics superior to those of the latter from the viewpoint of axial resolution, including the axial side lobe suppression and axial DOF shallowness. We expect the proposed bi-focal MFZP can readily be fabricated with electron-beam evaporation and focused-ion-beam processes and further be exploited for various applications, such as laser micro-machining, optical trapping, biochemical sensing, confocal sensing, etc. 

J. Kim, H. Kim, G. Lee, J. Kim, B. Lee and Y. Jeong, "Numerical and experimental study on multi-focal metallic Fresnel zone plates designed by the phase selection rule via virtual point sources," Appl. Sci., vol. 8, no.3, pp. 449-459, 2018. 

Abstract: We propose a novel design method for multi-focal metallic Fresnel zone plates (MFZPs), which exploits the phase selection rule by putting virtual point sources (VPSs) at the desired focal points distant to the MFZP plane. The phase distribution at the MFZP plane reciprocally formed by the VPSs was quantized in a binary manner based on the phase selection rule, thereby leading to a corresponding on-off amplitude pattern for the targeted MFZP. The resultant phase distribution was dependent on the complex amplitudes of the VPSs, so that they could be determined from the perspective of both multi-focal functionality and fabrication feasibility. As a typical example, we utilized the particle swarm optimization algorithm to determine them. Based on the proposed method, we designed and numerically analyzed two types of novel MFZPs—one for a monochromatic multi-focal application and the other for a multi-chromatic mono-focal application—verifying the effectiveness and validity of the proposed method. We also fabricated them onto Au-deposited glass substrates, using electron beam evaporation and a focused ion beam milling process. We experimentally characterized them and also verified that they successfully demonstrated their feasibilities. The former produced distinct hot spots at three different focal distances of 10, 15, and 20 μm for monochromatic incidence at 650 nm, and the latter produced a single hot spot at a focal distance of 15 μm for multi-chromatic incidence at 660, 532, and 473 nm. The experimental results were also in good agreement with their corresponding numerical results. We expect that both MFZPs will have various applications, such as laser micromachining, optical trapping, biomedical sensing, confocal collimation, achromatic optics, etc. 

H. Kim, J. Kim, H. An, Y. Lee, G. Lee, J. Na, K. Park, S. Lee, S. Lee, B. Lee, and Y. Jeong, "Metallic Fresnel zone plate implemented on an optical fiber facet for super-variable focusing of light," Opt. Express, vol. 25, no. 24, pp. 30290-30303, 2017. 

Abstract: We propose and investigate a metallic Fresnel zone plate (FZP/MFZP) implemented on a silver-coated optical fiber facet for super-variable focusing of light, the focal point of which can be drastically relocated by varying the wavelength of the incident light. We numerically show that when its nominal focal length is set to 20 μm at 550 nm, its effective focal length can be tuned by ~13.7 μm for 300-nm change in the visible wavelength range. This tuning sensitivity is over 20 times higher than that of a conventional silica-based spherical lens. Even with such high tuning sensitivity with respect to the incident wavelength change, the effective beam radius at the focal point is preserved nearly unchanged, irrespective of the incident wavelength. Then, we fabricate the proposed device, exploiting electron- and focused-ion-beam processes, and experimentally verify its super-variable focusing functionality at typical red, green, and blue wavelengths in the visible wavelength range, which is in good agreement with the numerical prediction. Moreover, we propose a novel MFZP structure that primarily exploits the surface-plasmon-polariton-mediated, extraordinary transmission effect. For this we make all the openings of an MFZP, which are determined by the fundamental FZP design formula, be partitioned by multi-rings of all-sub-wavelength annular slits, so that the transmission of azimuthally polarized light is inherently prohibited, thereby leading to super-variable and selective focusing of radially polarized light. We design and fabricate a proof-of-principle structure implemented on a gold-coated fused-silica substrate, and verify its novel characteristics both numerically and experimentally, which are mutually in good agreement. We stress that both the MFZP structures proposed here will be very useful for micro-machining, optical trapping, and biomedical sensing, in particular, which invariably seek compact, high-precision, and flexible focusing schemes. 

S. Lee, L. A. Vazquez-zuniga, H. Kim, Y. Kwon, K. Park, H. S. Kim, and Y. Jeong, "Experimental spatio-temporal analysis on the shot-to-shot coherence and wave-packet formation in quasi-mode-locked regimes in an anomalous dispersion fiber ring cavity," Opt. Express, vol. 25, no. 23, pp. 28385-28397, 2017. 

Abstract: We carry out systematic and dedicated experimental investigations on the shot-to-shot coherence and wave-packet formation in quasi-mode-locked (QML) regimes, including noise-like-pulse, symbiotic, and multi-soliton regimes in an anomalous-dispersion fiber ring cavity. To analyze the regimes in real-time, we take advantage of the spatio-temporal shot-to-shot measurement technique. We show that the individual regimes exhibit significantly different coherence characteristics, depending not only on the amount of nonlinear phase shift accumulated per roundtrip but also on the degree of soliton interaction, the latter of which crucially governs the bunching (i.e., the wave-packet formation) or anti-bunching mechanisms in the corresponding QML regimes. In fact, solitons with higher intensities tend to undergo higher nonlinear phase shift and stronger soliton interactions. Subsequently, the intensified soliton interactions among the individual solitons in the multi-soliton-regime cavity trigger them to form a bunched soliton-group, i.e., a wave packet, thereby resulting in QML pulses in the noise-like pulse or symbiotic regime. This complicated nonlinear process, in turn, causes a severe degradation in the shot-to-shot coherence of the resultant QML pulses. In addition, the shot-to-shot coherence trends observed in the experiment are in good agreement with our previous numerical predictions, so that the strong correlation between the shot-to-shot coherence of QML pulses and the corresponding nonlinear phase shift accumulated per roundtrip is confirmed by this experimental observation. 

Y. Kwon, K. Park, S. Hong, and Y. Jeong, "Numerical study on the supercontinuum generation in an active highly-nonlinear photonic crystal fiber with flattened all-normal dispersion," IEEE J. Quant. Electron., vol 53, no 5, 6100308, 2017. 

Abstract: We numerically study the dynamics of supercontinuum generation (SCG) in an ytterbium-doped highly nonlinear photonic crystal fiber (HNL-PCF) with flattened all-normal dispersion (FAND) in the sub-picosecond pulse regime. We discuss the enhancement of the energy spectral density and the recovery of the peak power depletion in the SCG process through the fiber in comparison with the SCG based on a passive-type counterpart. As a unique application of the novel characteristics of the active HNL-PCF with FAND, we also analyze the direct amplification of an SC pulse through it, showing that the incident SC pulse can be amplified by 10 dB without undergoing significant degradations in terms of spectral bandwidth and flatness. Our numerical investigations on the active HNL-PCF with FAND will be helpful for opening up new opportunities for fiber-based SCG technology in the sub-picosecond regime. 

M. Ferreira, E. Castro-Camus, D. Ottaway, J. López-Higuera, X. Feng, W. Jin, Y. Jeong, N. Picqué, L. Tong, B. Reinhard, P. Pellegrino, A. Méndez, M. Diem, F. Vollmer, and Q. Quan,"Roadmap on optical sensors," J. Opt., vol 19, no. 8, 083001, 2017. 

Abstract: Sensors are devices or systems able to detect, measure and convert magnitudes from any domain to an electrical one. Using light as a probe for optical sensing is one of the most efficient approaches for this purpose. The history of optical sensing using some methods based on absorbance, emissive and florescence properties date back to the 16th century. The field of optical sensors evolved during the following centuries, but it did not achieve maturity until the demonstration of the first laser in 1960. The unique properties of laser light become particularly important in the case of laser-based sensors, whose operation is entirely based upon the direct detection of laser light itself, without relying on any additional mediating device. However, compared with freely propagating light beams, artificially engineered optical fields are in increasing demand for probing samples with very small sizes and/or weak light−matter interaction. Optical fiber sensors constitute a subarea of optical sensors in which fiber technologies are employed. Different types of specialty and photonic crystal fibers provide improved performance and novel sensing concepts. Actually, structurization with wavelength or subwavelength feature size appears as the most efficient way to enhance sensor sensitivity and its detection limit. This leads to the area of micro- and nano-engineered optical sensors. It is expected that the combination of better fabrication techniques and new physical effects may open new and fascinating opportunities in this area. This roadmap on optical sensors addresses different technologies and application areas of the field. Fourteen contributions authored by experts from both industry and academia provide insights into the current state-of-the-art and the challenges faced by researchers currently. Two sections of this paper provide an overview of laser-based and frequency comb-based sensors. Three sections address the area of optical fiber sensors, encompassing both conventional, specialty and photonic crystal fibers. Several other sections are dedicated to micro- and nano-engineered sensors, including whispering-gallery mode and plasmonic sensors. The uses of optical sensors in chemical, biological and biomedical areas are described in other sections. Different approaches required to satisfy applications at visible, infrared and THz spectral regions are also discussed. Advances in science and technology required to meet challenges faced in each of these areas are addressed, together with suggestions on how the field could evolve in the near future. 

S. Piao, Y. Wang, Y. Lee, S. Hong, Y. Jeong, and S. Oh,"The in vivo effect of ytterbium-doped fiber laser on rat buccal mucosa as a simulation of its effect on the urinary tract: A preclinical histopathological evaluation," Inter.Neurourol. J., vol. 21.Suppl. 1, S17-23, 2017. 

Abstract: The aim of this study was to perform a histological analysis of the effect of a ytterbium-doped fiber (YDF) laser on oral buccal mucosa tissue in vivo to simulate its effect on the mucosa of the lower urinary tract. A total of 90 8-week-old Sprague-Dawley rats were anesthetized with urethrane (1.2 g/kg intraperitoneally). A prespecified inner buccal mucosal site was irradiated with a YDF master-oscillator power amplifier (MOPA) system for 60 seconds, with output power settings of 0.5, 1, and 2 W, respectively, in 3 treatment groups. Specimens of irradiated tissue were harvested at 2 hours, 24 hours, 2 weeks, and 4 weeks after irradiation. The tissue specimens were stained with hematoxylin and eosin for histological analysis. In the group treated with 0.5 W, basal cell elongation and vacuolization were observed at 2 hours and 24 hours after treatment, respectively. No evident injury was observed after 2 or 4 weeks. The group treated with 1 W presented partial basal layer separation, and even complete epidermal ablation, within 2 hours. At 24 hours after laser treatment, new capillaries on an edematous background of fibroblasts and myofibroblasts, as well as profuse infiltration of the neutrophils to the basal layer, were observed. Collagen deposition and reepithelization were observed in specimens taken 2 weeks and 4 weeks after treatment. The group treated with 2 W presented bigger and deeper injuries at 2 hours after irradiation. Meanwhile, subepidermal bullae with full-thickness epidermal necrosis and underlying inflammatory infiltrate were observed 24 hours after treatment. The presence of fibrous connective tissue and collagen deposition were observed 2 weeks and 4 weeks after the treatment. To our knowledge, this is the first report regarding the effect of a YDF laser on living tissue. Our study demonstrated that the typical histological findings of the tissue reaction to the YDF MOPA apparatus were very similar to those associated with thermal injuries. The extent and degree of tissue damage increased proportionally to the output power. 

H. Kim, H. An, J. Kim, S. S. Lee, K. Park, S. Lee, L. A. Vazquez-Zuniga, S.-Y. Lee, B. Lee, and Y. Jeong, “Corrugation-assisted metal-coated angled fiber-facet for wavelength-dependent off-axis directional beaming,” Opt. Express, vol. 25, no, 7, pp. 8366-8385, 2017. 

Abstract: We propose a fiber-optic-plasmonic hybrid device that is based on a corrugation-assisted metal-coated angled fiber facet (CA-MCAFF) for wavelength-dependent off-axis directional beaming (WODB). The device breaks into two key structures: One is the MCAFF structure, which is a modified Kretschmann configuration implemented onto a fiber platform, thereby being able to generate a unidirectional surface plasmon with dramatically enhanced properties in terms of non-confined diffracted radiation loss and operational bandwidth. The other is the periodic corrugation structure put on the MCAFF, thereby enabling WODB functionality out of the whole structures. The corrugated metal surface out-couples the surface plasmon mode to free-space optical radiation into a direction that varies with the wavelength of the optical radiation with excellent linearity. We perform extensive numerical investigations based on the finite-element-method and analyze the out-coupling efficiency (OCEout) and spectral bandwidth (SBout) of the proposed device for various designs and conditions. We determine the seven structural parameters of the device via taking sequential optimization steps. We deduce two optimal conditions particularly for the fiber-facet angle, in terms of the averaged OCEout or the SBout in the whole visible wavelength range (400 – 700 nm), which eventually leads to OCEout = 30.4% and SBout = 230 nm or to OCEout = 24.5% and SBout = 245 nm, respectively. These results suggest substantial enhancements in both OCEout and SBout, in comparison with the performance properties of a typical nano-slit-based device having a similar type of WODB functionality. The proposed CA-MCAFF is a simple, compact and efficient WODB device that is fully compatible with the state-of-the-art optical fiber technology. 

Y. Kwon, L. A. Vazquez-Zuniga, S. Lee, H. Kim, and Y. Jeong, “Numerical study on multi-pulse dynamics and shot-to-shot coherence property in quasi-mode-locked regimes of a highly-pumped anomalous dispersion fiber ring cavity,” Opt. Express, vol. 25, no. 4, pp. 4456-4469, 2017. 

Abstract: We numerically investigate quasi-mode-locked (QML) multi-pulse dynamics in a fiber ring laser cavity in the anomalous dispersion regime. We show that the laser cavity can operate in five constitutively different QML regimes, depending on the saturation power of the saturable absorber element and the length of the passive fiber section that parameterize the overall nonlinearity and dispersion characteristic of the laser cavity. We classify them into the incoherent noise-like-pulse, partially-coherent noise-like-pulse, symbiotic, partially-coherent multi-soliton, and coherent multi-soliton regimes, accounting for their coherence and multi-pulse formation features. In particular, we numerically clarify and confirm the symbiotic regime for the first time to the best of our knowledge, in which noise-like pulses and multi-solitons coexist stably in the cavity that has recently been observed experimentally. Furthermore, we analyze the shot-to-shot coherence characteristics of the individual QML regimes relative to the amount of the nonlinear-phase shift per roundtrip, and verify a strong correlation between them. We also show that the net-cavity dispersion plays a critical role in determining the multi-pulse dynamics out of the partially-coherent noise-like-pulse, symbiotic, and partially-coherent multi-soliton regimes, when the cavity bears moderate nonlinearity. We quantify and visualize all those characteristics onto contour maps, which will be very useful and helpful in discussing and clarifying the complex QML dynamics.