Inscription of zone plate pattern on the optical fiber end surface using femtosecond laser pulses Jun Ki Kim a , Hae Young Choi a , Yongmin Jung a , Byeongha Lee a and Kyunghwan Oh c Ik-Bu Sohn b , Young-Chul Noh b , Jongmin Lee b a Department of Information and Communications, Gwangju Institute of Science and Technology (GIST) b Advanced Photonics Research Institute (APRI), Precision Optics Lab, 1 Oryong-dong, Buk-gu, Gwangju 500-712, South Korea Tel: +82-62-970-2288, Fax: +82-62-970-2237, E-mail: koh@yonsei.ac.kr c Institute of Physics and Applied Physics, Yonsei University 134 Shinchon-dong Sudaemoon-ku, Seoul, 120-749, South Korea. Abstract We fabricated a compact fiber based zone plate on the coreless-silica-fiber (CSF) segments of 200μm diameter by femtosecond laser. We investigated the focusing properties by launching a He-Ne laser beam into the optical fiber. 1. Introduction Demands of Fresnel zone plates which is attractive devices for micro optics are increasing as the manufacturing technique using laser has been developed [1-4]. Although the zone plates are thin and compact and have the focusing abilities, it cannot be compatible with fiber optic system in the existing fabrication process using laser because of the limitation of resolution and thermal problem [5]. To overcome these problems, the femtoscond laser is strongly recommended due to the advantages that enble to apply to light, dense and integrated system such as the optical fiber [6-7]. The process offers precise manufacturing compared with current laser system as well as 3-dimensional fabrication. In this paper, we formed a zone plate by femtosecond laser on the coreless silica fiber (CSF) end. To expand the mode field, a CSF segment of 200 ㎛ diameter and the length range of 1000 μm were spliced on single mode fiber(SMF). The zone plate structure is designed to have maximum focal length at 632.8 ㎛. The focusing properties of zone plate is investigated by launching a He-Ne laser beam into the optical fiber. 2. Experiments The experimental setup and proposed zone plates structure on the CSF end surface are schematically illustrated in Fig.1. Firstly, we prepared the sample fiber which has the CSF with large diameter. As the light exits from the SMF core, it passes through CSF expanding the beam diameter, which can be approximated by Gaussian beam propagation. Beam propagation after SMF can be controlled in the CSF segment by changing its diameter and length. The enough mode field expansion will be induced at optimized CSF length prohibiting the generation of the multiple path interference. The lengh of 1000μm CSF with 200μm diameter was used in this work. An amplified Ti:Sapphire laser operating at 785.5nm wavelength with 184-fs pulse duration and 1-kHz repetition rate was utilized to engrave zone plate pattern. The writing pulse energy is about 0.45 μ J. The beam passing through a shutter and dichroic mirror was focussed to sample fiber mounted upon three dimensional translation stage via 50X (N.A. 0.42) objective. X Y Z 3D stage Sample fiber 50X Objective Dichroic mirror Amplified Ti:Sapphire laser Shutter Filter CCD camera White light source D=200μm L=1000μm Fig. 1 The experimental setup to form a zone plate pattern and schematic diagram of the light propagation after engraving zone plate patern. The microscope images and blow up images of the fabricated zone plate pattern are shown in Fig. 2. We