Liquid core fibers based on hollow core microstructured fibers

CWM1-1 Liquid Core Fibers based on Hollow Core Microstructured Fibers 1Center G. Vienne1, M. Yan2, Y. Luo1, T. K. Liang1,3, H. P.. Ho1, C. Lin1 for Advanced Research in Photonics, The Chinese University of Hong Kong, China Technology Research Centre, Nanyang Technological University, Singapore 3Present address: National Institute of Information and Communications Technology, Japan [email protected] 2Network 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). 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. (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 Liq uid Air Solid 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 551 leading in all cases to more than 99% water dominated loss values of 0.18dB/m and 1.32dB/m at 532nm and 633nm, respectively. The thin support bridges very efficiently prevent penetration of the field into the cladding but the overlap with the glass nodes of the inner glass ring, which are up to 0.4µm thick, increases with increasing mode order. This fact is also reflected by the decreasing contribution of water to the imaginary part of the effective index with increasing mode order. (a) laser at 633nm and the results were very similar. We tested three cases: dry fiber (without liquid); core and cladding holes filled with water; and core filled with water only. The speed of water-filling by capillary action was observed to be about 1.5mm/sec for the core, and at least four times lower for the cladding holes. For the dry fiber the output power was more than 50dB below the input power (the fiber supported no bandgap at the wavelengths tested). When the fiber (core and cladding) was entirely filled with water the transmitted power increased but was still about 30dB below the input power. On the other hand, more than 50% of the power could be transmitted (<3dB loss) when the core only was filled with water. As expected the transmitted light appeared multimode with some overlap with the glass nodes of the first silica ring, see Fig. 3 (d). The locations of the air-water interfaces were seen to be critical with best coupling achieved when the interfaces were close to the fiber-end facets. This prompted to the need for fiber-end sealing to avoid evaporation. The lens formed by the curvature of an air-water interface caused by surface tension is also expected to influence inand out-coupling of light but this effect can be mitigated by surface treatment [5]. (b) Benefits & Applications Optical interaction with an aqueous solution is particularly attractive for biosensing. Jensen et al. recently demonstrated the use of a microstructured fiber for interaction with an aqueous solution through the evanescent field [6], but the interaction was much weaker than when guiding in an aqueous core. Guiding in an aqueous core was demonstrated in planar geometry using a nanoporous polymer cladding [7]. The approach presented here is similar but should lead to lower loss and longer interaction length. The combination of high intensity in an aqueous core with long interaction length is ideal for sensing based on stimulated and non-linear effects such as Raman scattering, a powerful tool for molecule “fingerprinting”. Moreover, the high numerical aperture expected from the proposed design should yield high collection efficiency in fluorescence sensing. (c) (d) Fig 3 Mode distributions at 532nm (a) LP01-like mode; (b) LP11-like mode; (c) LP21-like mode; (d) Example of measured output mode distribution. Water LP01-like mode LP11-like mode LP21-like mode Table 1 Complex effective indices of at 532 and 633nm. 532nm 633nm 1.3371 + 1.3315 + 1.8111E-9i 1.5334E-8i 1.3369 + 1.3313 + 1.810E-9i 1.532E-8i 1.3367 + 1.3309 + 1.804E-9i 1.531E-8i 1.3364 + 1.3304 + 1.800E-9i 1.527E-8i index of water & complex the three lowest order modes References [1] W. A. Gambling et al., Opt. Comm. 6, 317-22 (1972) [2] G. Vienne et al, Opt. Express 12, 3500-8 (2004) [3] S. Guo et al., Opt. Express 12, 3341-52 (2004) [4] http://philiplaven.com/p20.html [5] C. Grillet et al, Opt. Express 12, 5400-7 (2004) [6] J. B. Jensen et al, Opt. Lett. 29, 1974-6 (2004) [7] W. P Risk et al, Opt. Express 12, 6446-57 (2004) We checked the transmission of 10cm pieces of fiber under excitation by a frequency-doubled Nd:YAG laser at 532nm. The beam was launched in the center of the core. We also used a He:Ne 552 View publication stats