Liquid Core Fibers based on Hollow Core Microstructured Fibers G. Vienne 1 , M. Yan 2 , Y. Luo 1 , T. K. Liang 1,3 , H. P.. Ho 1 , C. Lin 1 1 Center for Advanced Research in Photonics, The Chinese University of Hong Kong, China 2 Network Technology Research Centre, Nanyang Technological University, Singapore 3 Present address: National Institute of Information and Communications Technology, Japan gvienne@ee.cuhk.edu.hk Abstract We propose a new liquid core fiber capable of guiding in low refractive index liquids. The hollow core of a microstructured fiber is filled with water, in which visible light is efficiently guided. Introduction Liquid core fibers were extensively studied in the early days of fiber optics communications and shown to offer record-low transmission loss in the order of dB/km [1]. In these fibers the liquid material was chosen to have a higher refractive index than the surrounding solid material to guide by total internal reflection. In this paper we investigate the potential of microstructured fibers to guide in a liquid core even when the liquid material has a lower refractive index than the solid fiber material. Potentially such liquid core fiber could find useful applications in biosensing. Simulations & Experiments Progress in the fabrication of microstructured fibers now allows for control of structures of a few tens of nanometers over kilometer lengths. Fig. 1 Proposed fiber structure for guiding in arbitrary liquid core. The proposed fiber structure illustrated in Fig. 1 takes advantage of this ability. A layer of air holes separated by support bridges significantly thinner than the wavelength of light forms a region presenting an effective refractive index close to unity, which is used to optically decouple the liquid filled core from the outer cladding material. Although periodicity is not required in this design, photonic bandgap fibers offer structures suitable to demonstrate the principle. Here we chose to use a recently demonstrated air-silica Bragg fiber and selectively filled its core with deionized water. This fiber, illustrated in Fig. 2, is 90µm in diameter, with a core 20µm in diameter and support bridges tens of nanometers thick. Details can be found in [2] (fiber referred to as OD90). (a) (b) Fig. 2 (a) Cross sectional optical microscope picture of fiber with core filled with water; (b) Corresponding microstructure used for simulations. The 2D structure represented in Fig. 2(b) closely approximates the fabricated structure and is used as input to simulate the light propagation by the full-vector finite-difference frequency-domain (FDFD) method [3]. The complex refractive index of water at different wavelengths is interpolated from data in [4]. The solid material and air are assumed to be lossless and to have a refractive index of 1.45 and 1.00, respectively. Contour plots for the three lowest order modes are shown in Fig. 3(a-c) and the complex effective indices are shown in Table 1. The real part of the refractive index decreases with increasing mode order as expected from the increasing transverse component of the wavevector. For the first three low-order modes the field strongly overlaps with the water Liquid Air Solid CWM1-1 551