Kyoungyoon Park

Fabrication of a piezoresistive pressure sensor for enhancing sensitivity using silicon nanowire

This paper presents a piezoresistive pressure sensor to enhance sensitivity using silicon nanowire. According to published paper, silicon nanowire under 340 nm has good piezoresistive effect. Silicon nanowire of 140 × 200 nm2 size has seven times more piezoresistive effect than bulk silicon. This paper proposes the piezoresistive pressure sensor using the high piezoresistive effect of the silicon nanowire. The nanowire is fabricated to be connected like a bridge between the bossed silicon diaphragm and the edge of the silicon substrate. The fabricated piezoresistive pressure sensor has high sensitivity of 337.5 mV/V·MPa and dynamic range of 150 kPa ∼ 300 kPa. The pressure sensor size is less than 1mm2 using diaphragm of 200 × 200 µm2.

A quasi-mode interpretation of acoustic radiation modes for analyzing Brillouin gain spectra of acoustically antiguiding optical fibers

We propose a novel quasi-mode interpretation (QMI) method to represent acoustic radiation modes in acoustically antiguiding optical fibers (AAOFs) in terms of discrete quasi-modes. The QMI method readily enables one to obtain the full quasi-modal properties of AAOFs, including the complex propagation constants, mode center frequencies, and field distributions in an intuitive and much simplified way, compared to other previous methods. We apply the QMI method to analyze the Brillouin gain spectrum of an AAOF that has typically been used to mitigate stimulated Brillouin scattering of optical waves. The result based on the QMI method is in good agreement with the numerical and experimental results for the same fiber structure previously reported in the literature. Considering the effectiveness and simplicity of its numerical procedure, we expect the use of the QMI method can further be extended to even more complicated numerical analyses with acoustic radiation modes, which include the acoustically antiguiding, large-core optical fibers in multi-mode regimes.

Corrugation-assisted metal-coated angled fiber facet for wavelength-dependent off-axis directional beaming

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.

Theoretical study on the generation of a low-noise plasmonic hotspot by means of a trench-assisted circular nano-slit

We propose a novel trench-assisted circular metal nano-slit (CMNS) structure implementable on a fiber platform for the generation of a low-noise cylindrical surface plasmon (CSP) hotspot. We design trench structures based on a multi-pole cancellation method in order that a converging surface plasmon signal is well separated from co-propagating non-confined diffracted light (NCDL) at the hotspot location. In fact, the secondary radiation by the quasi-pole oscillation at the edge of the trench cancels the primary NCDL, thereby enhancing the signal-to-noise ratio (SNR) of the CSP hotspot. In particular, we investigate two types of trench structures: a rectangular-trench (RT) structure and an asymmetric-parabolic-trench (APT) structure, which are considered for the sake of the simplicity of fabrication and of the maximal enhancement of the SNR, respectively. In comparison with a conventional CMNS having no trenches, we highlight that the mean SNR of the CSP hotspot is enhanced by 6.97 and 11.89 dB in case of the optimized RT and APT CMNSs, respectively. The proposed schemes are expected to be useful for increasing the SNR of plasmonic devices that are interfered by NCDL, such as various types of nano-slits for generating high-resolution plasmonic signals, for example.

Combinatorial Study of Supercontinuum Generation Dynamics in Photonic Crystal Fibers Pumped by Ultrafast Fiber Lasers

We numerically study the dynamics of supercontinuum generation (SCG) for a variety of possible combinations of photonic crystal fibers (PCFs) and ultrafast fiber laser pulses that the current technologies offer. Three types of PCFs typically used in SCG and four representative types of ultrafast fiber laser pulses are considered for this combinatorial study. We numerically model and qualitatively discuss the nonlinear evolution of the pulses for their whole 12 combinatorial cases. We also quantitatively analyze their output spectra and organize a performance chart for them in terms of spectral bandwidth, flatness, and degree of spectral coherence. Finally, we suggest the most viable combinations among the given PCFs and ultrafast fiber laser pulses in order for generating a target supercontinuum spectrum for various specific cases.

Metallic Fresnel zone plate implemented on an optical fiber facet for super-variable focusing of light

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, extra-ordinary 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.

Subwavelength ring assisted Fresnel zone plate for radially polarized light focusing

We propose a Fresnel zone plate comprised with periodic subwavelength rings which generates radially polarized focused light. The novel monolayer metasurface will simplify the generation of radially polarized focused light.

Numerical Study on the Supercontinuum Generation in an Active Highly Nonlinear Photonic Crystal Fiber With Flattened All-Normal Dispersion

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.

Temporal dynamics and shot-to-shot stability characteristics of three distinctive partially-mode-locked operation regimes in a fiber ring cavity

We numerically and experimentally study three distinctive partially-mode-locked regimes in a fiber ring laser. We discuss their temporal dynamics and shot-to-shot stability characteristics, which offers a clue to the existence of the partial coherence in such regimes.

Current Status and Prospects of High-Power Fiber Laser Technology (Invited Paper)

Over the past two decades, fiber-based lasers have made remarkable progress, now having reached power levels exceeding kilowatts and drawing a huge amount of attention from academy and industry as a replacement technology for bulk lasers. In this paper we review the significant factors that have led to the progress of fiber lasers, such as gain-fiber regimes based on ytterbium-doped silica, optical pumping schemes through the combination of laser diodes and double-clad fiber geometries, and tandem schemes for minimizing quantum defects. Furthermore, we discuss various power-limitation issues that are expected to incur with respect to the ultimate power scaling of fiber lasers, such as efficiency degradation, thermal hazard, and system-instability growth in fiber lasers, and various relevant methods to alleviate the aforementioned issues. This discussion includes fiber nonlinear effects, fiber damage, and modal-instability issues, which become more significant as the power level is scaled up. In addition, we also review beam-combining techniques, which are currently receiving a lot of attention as an alternative solution to the power-scaling limitation of high-power fiber lasers. In particular, we focus more on the discussion of the schematics of a spectral beam-combining system and their individual requirements. Finally, we discuss prospects for the future development of fiber laser technologies, for them to leap forward from where they are now, and to continue to advance in terms of their power scalability.