Jongki Kim

Compact all-fiber Bessel beam generator based on hollow optical fiber combined with a hybrid polymer fiber lens

We report a compact all-fiber Bessel beam generator using hollow optical fiber (HOF) and coreless silica fiber based on a self-assembled polymer lens. A nearly diffraction-free Bessel beam pattern was observed with its focused beam diameter of 20 microm maintained over a propagation distance of 550 microm. The generated Bessel beams were experimentally tested under various structural parameters such as the diameters of the HOF and operating wavelengths. A beam propagation method was applied to simulate the proposed device, which shows good agreement with the experimental observations.

Fabrication of Micro Fresnel Zone Plate Lens on a Mode-Expanded Hybrid Optical Fiber Using a Femtosecond Laser Ablation System

We present a compact all-fiber zone plate lens directly ablated on the surface of a mode-expanded hybrid fiber end by using a high precision femtosecond laser processing technique. To achieve a sufficiently large beam size and focusing efficiency, a segment of a coreless silica fiber having a 200-mum diameter was adapted. Focusing properties of the zone plate lens were experimentally investigated and compared with numerical simulations.

All-silica fiber Bessel-like beam generator and its applications in longitudinal optical trapping and transport of multiple dielectric particles

A Bessel-like beam was generated in a novel all-fiber integrated structure. A concentric ring intensity pattern was achieved by the multimode interference along the coreless silica fiber, which was then focused by the integrated micro-lens to result in a Bessel-like beam. The average beam diameter of 7.5 μm maintained over 500 μm axial length for a continuous wave Yb-doped fiber laser input oscillating at the wavelength of 1.08 μm. The generated beam was successfully applied to two-dimension optical trapping and longitudinal transport of multiple dielectric particles confirming its unique non-diffracting and self-reconstructing nature. Physical principle of operation, fabrication, and experimental results are discussed.

Subwavelength plasmonic lens patterned on a composite optical fiber facet for quasi-one-dimensional Bessel beam generation

We demonstrate a beam shaping method by engraving plasmonic lens consisted of subwavelength slit-metallic groove structure on the cleaved end facet of a composite optical fiber. The fiber consisted of a single mode optical fiber serially concatenated with a coreless silica fiber segment to expand the beam diameter. The subwavelength slit-groove structure on a gold film was fabricated using focused ion beam milling process. Plasmonic interaction in the film generated a quasi-one-dimensional Bessel-like beam with a beam width of 0.8 μm at the focal length of 2 μm. Detailed two- and three-dimensional finite-difference time-domain analyses showed beam characteristics consistent with experimental observations.

Objective-free optical-resolution photoacoustic microscopy

Abstract. Optical-resolution photoacoustic microscopy (OR-PAM) becomes a premier microscopic imaging tool in biomedicine because it provides agent-free optical absorption information in tissues. By tightly focusing light to a spot, a significantly improved lateral resolution can be achieved in OR-PAM. The focal spot size is typically determined by the numerical aperture of the used objective lens. Here, we demonstrate objective-free OR-PAM using a fiber optic Bessel beam generator. In this approach, no objective lens is required and, beneficially, the complexities of conventional OR-PAM systems can be greatly relieved. We have obtained photoacoustic images of a carbon fiber with a diameter of ∼6  μm, whose lateral resolution was measured to be better than 6 to 7 μm.

Micro Fresnel Zone Plate Lens Inscribed on a Hard Polymer Clad Fiber Using Femtosecond Pulsed Laser

We report a novel process to directly inscribe a Fresnel zone plate (FZP) lens on the cleaved facet of a hard clad polymer fiber using a femtosecond pulsed laser. The FZPs are designed by adopting modal analysis and free-space diffraction, which are experimentally implemented by locally ablating silica glass using femtosecond laser pulses. A good agreement between the design and fabricated FZP lens is obtained with an exceptionally long focal length over 600 m, which can have a strong potential in all-fiber beam shaping technology and optical interconnections.

Fourier optics along a hybrid optical fiber for Bessel-like beam generation and its applications in multiple-particle trapping

Highly efficient Bessel-like beam generation was achieved based on a new all-fiber method that implements Fourier transformation of a micro annular aperture along a concatenated composite optical fiber. The beam showed unique characteristics of tilted washboard optical potential in the transverse plane and sustained a nondiffracting length over 400 μm along the axial direction. Optical trapping of multiple dielectric particles and living Jurkat cells were successfully demonstrated along the axial direction of the beam in the water.

Crossed fiber optic Bessel beams for curvilinear optofluidic transport of dielectric particles

Due to its unique non-diffracting and self-reconstructing nature, Bessel beams have been successfully adopted to trap multiple particles along the beams axial direction. However, prior bulk-optic based Bessel beams have a fundamental form-factor limitation for in situ, in-vitro, and in-vivo applications. Here we present a novel implementation of Fourier optics along a single strand of hybrid optical fiber in a monolithic manner that can generate pseudo Bessel beam arrays in two-dimensional space. We successfully demonstrate unique optofluidic transport of the trapped dielectric particles along a curvilinear optical route by multiplexing the fiber optic pseudo Bessel beams. The proposed technique can form a new building block to realize reconfigurable optofluidic transportation of particulates that can break the limitations of both prior bulk-optic Bessel beam generation techniques and conventional microfluidic channels.

Simple and reliable light launch from a conventional single-mode fiber into a helical-core fiber through an adiabatically tapered splice

We propose a simple and efficient light launch scheme for a helical-core fiber (HCF) by using an adiabatically tapered splice technique, through which we overcome its inherent difficulty with light launch owing to the large lateral offset and angular tilt of its core. We experimentally demonstrate single-mode excitation in the HCF in this configuration, which yields the coupling efficiency of around -5.9 dB (26%) for a ~1.1-μm light input when the splice joint is tapered down to 30 μm in diameter. To our knowledge, this is the first proof-of-principle report on the fusion-splice coupling between an HCF and a conventional single-mode fiber.

A micro-structured aperture made of a hollow triangular-core fiber for novel beam shaping

We demonstrate a micro-structured aperture made of a unique hollow triangular-core fiber (HTCF) that consists of a central air hole, a high-index hollow triangular core, and silica cladding for all-fiber novel beam shaping. Detailed fabrication processes to embed a hollow triangular structure into a cylindrical optical fiber are described and unique diffraction patterns out of the HTCF for monochromatic light are analyzed both experimentally and theoretically. Fourier-optic analysis combined with guided mode calculation was pursued to interpret experimental patterns in terms of the beam propagation distance.