Ik-Bu Sohn

Second-harmonic generation in periodically poled lithium niobate waveguides fabricated by femtosecond laser pulses

The authors have demonstrated the second-harmonic generation (SHG) in optical channel waveguides which were fabricated in periodically poled lithium niobate (PPLN) by tightly focused femtosecond laser (781nm) pulses. The measured quasi-phase-matched wavelength and 3dB bandwidth for SHG in a 44.8-mm-long PPLN waveguide which has a domain period of 16.6μm were about 1563 and 0.25nm, respectively. The value of the index difference between the fundamental and harmonic wavelengths in laser written waveguide was inferred to be 0.047.

Terahertz near-field enhancement in narrow rectangular apertures on metal film

We report huge field accumulations in rectangular aperture arrays on thin metal film by using shape resonance in THz frequency region. A huge far-field transmission enhancement is observed in samples of various widths ranging from 10 mum to 1.8 mum which correspond to only an order of lambda/100. Theoretical calculations based on vector diffraction theory indicates 230 times near-field enhancement in case of the 1.8 mum wide rectangular aperture. Transmission measurement through the single rectangular aperture shows that the shape resonance, not the periodicity, is mainly responsible for the transmission enhancement and the corresponding field enhancement.

Ultracompact Intrinsic Micro Air-Cavity Fiber Mach–Zehnder Interferometer

A compact intrinsic fiber Mach-Zehnder interferometer (MZI) is proposed and experimentally demonstrated by incorporating a micro air-cavity ablated by femtosecond laser irradiation. A short cavity of length ~ 10 mum along the single-mode fiber core provided two optical paths: one propagating through the air and the other guided along the ring-shaped silica cladding. A spectral analysis confirmed MZI in a good agreement with experimental results. Temperature-dependent spectral shifts were measured and analyzed.

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.

Single-body lensed photonic crystal fibers as side-viewing probes for optical imaging systems

We report the fabrication and performance of a lensed photonic crystal fiber (PCF) designed as a compact but effective side-viewing optical imaging probe. The lensed-PCF probe was implemented in a single body without using any other fibers or additional optics. The beam expansion region and a focusing ball lens, necessary for a focuser, were simultaneously formed along a small piece of PCF by applying arc discharges. The side-viewing ability was provided by polishing the ball lens with a femtosecond laser to form a total internal reflection surface. The working distance and the transverse resolution of the fabricated single-body lensed-PCF were experimentally measured to be 570 and 6.8 microm, respectively. With the proposed lensed-PCF probe, optical coherence tomography images of an in vitro biological sample were successfully obtained.

Femtosecond laser and arc discharge induced microstructuring on optical fiber tip for the multidirectional firing

Most optical fibers are designed for forward firing i.e. the light is emitted at the distal end along the optical axis of the fiber. In some applications such as the laser surgery and laser scanners, side firing of the optical fiber is required. In this paper, we present the microstructuring of an optical fiber tip using the femtosecond laser and an arc discharging process for the multidirectional firing of the beam. The distal end of the optical fiber with diameter of 125 μm was machined into a conical structure using a femtosecond laser. The surface of the machined tip was exposed to the arc discharge using a fiber splicer. The arc discharge leads to the melting and re-solidification of the fiber tip. This results in a smoothing of laser-induced conical microstructure at the tip of the fiber. We were able to demonstrate the multidirectional (circumferential) emission of the light from the developed fiber tip.

Fabrication of photonic devices directly written within glass using a femtosecond laser

We present a novel method for three-dimensional optical splitter that have U-grooves, which are used for fiber alignment, within a fused silica glass using near-IR femtosecond laser pulses. The fiber aligned optical splitter has a low insertion loss, less than 4 dB, including an intrinsic splitting loss of 3 dB and excess loss due to the passive alignment of a single-mode fiber. The output field pattern is presented, demonstrating the splitting ratio of the optical splitter is approximately 1:1. Finally, we demonstrate the utility of the femtosecond laser writing of periodic patterns by fabricating the submicron line and dot patterns inside the silica glass, which is applicable to 3-D optical memory.

Fabrication of optical splitter and passive alignment technique with a femtosecond laser

A novel packaging process using a femtosecond laser has been developed for passive alignment of planar lightwave circuit devices. With only one step of micromachining, optical splitter and U-grooves for fiber aligning are simultaneously defined on fused silica glass. The fiber aligned optical splitter has a low insertion loss, less than 4 dB, including an intrinsic splitting loss of 3 dB and excess loss due to the passive alignment of a single-mode fiber. Finally, the output field pattern is presented, demonstrating the splitting ratio of the optical splitter is approximately 1 : 1.

Relationship between the order of rotation symmetry in perforated apertures and terahertz transmission characteristics

Abstract. We investigate the terahertz (THz) transmission characteristics of complex slot arrays with various order of rotation symmetry. Our experimental results reveal that an asterisk-shaped aperture with sixfold rotational symmetry is the most appropriate shape for realizing polarization-independent, highly efficient, and frequency-selective transmission of THz radiation. A systematic understanding of the THz transmission characteristics is useful for achieving versatile platforms and custom-designed metallic devices with specific electromagnetic responses.

Laser Ablation of Polypropylene Films using Nanosecond, Picosecond, and Femtosecond Laser

Precise micropatterning of polypropylene film, which is highly transparent in the wavelength range over 250 nm has been demonstrated by 355 nm nano/picosecond laser and 785 nm femtosecond laser. Increments of both the pulse energy and the shot number of pulses lead to cooccurrence of photochemical and thermal effects, demonstrated by the spatial expansion of rim on the surface of PP. The shapes of the laser-ablated polypropylene films were imaged by optical microscope and measured by a 3D optical measurement system. And, the ablation depth and width of polypropylene film ablated by femtosecond laser at various pulse energy and pulse number were characterized. Our results demonstrate that a femtosecond pulsed laser is an efficient tool for fabricating micropatterns of polypropylene films, where the micropatterns are specifically tailored in size, location and number easily controlled by laser processing conditions.