2100 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 24, NO. 5, MAY2006 Nanofilms on Hollow Core Fiber-Based Structures: An Optical Study Javier Bravo, Ignacio R. Matías, SeniorMember,IEEE, Ignacio Del Villar, Jesús M. Corres, and Francisco J. Arregui, Member,IEEE Abstract—The optical characteristics of one multimode fiber (MMF)—hollow core fiber (HCF)—structure when a nanofilm is deposited on it has been theoretically and experimentally studied. The electrostatic self-assembly method has been used as the depo- sition technique, and the polymers chosen are polydiallyldimethy- lammonium and Poly R-478. Two different types of HCF have been used for the fabrication of the devices: 10/150 and 50/150 μm inner and outer diameters, respectively. Depending on several design parameters, the transmitted optical-power characteristic of the device experiences important changes that could be interesting towards development of several practical optical devices. The length and thickness of the HCF segment, the refractive index of the material deposited, the angle of the light when it reach the HCF section, and the wavelength of the light source will be analyzed. Index Terms—Deposition and fabrication, hollow core fibers (HCFs), optical fiber devices. I. I NTRODUCTION I N THE past few years, many different new fibers have been developed with the purpose of getting lower attenuation and lower nonlinearity than standard optical fibers. One good ex- ample are hollow optical waveguides, which have one or more air holes in their structure [1]. There exist many sophisticat- ed hollow fibers: holey fibers, omniguide fibers, square hollow fibers [2], hexagonal hollow fibers [3], polymer-coated hollow fibers [4], and many others. However, in this paper, the sim- plest structure will be studied: the hollow core fiber (HCF). It consists of one silica ring as the cladding of the fiber and the central air hole as the core. HCF-based structures have been used previously for many applications [5], such as fiber-optic strain sensors [6], broad- band bandpass filters [7], [8], electrically controllable long- period liquid crystal fiber gratings [9], or mode converters [10]. The structure used in this paper consists of one short segment of HCF spliced between two standard multimode fibers (MMFs), designated MMF—HCF—MMF (MHM) through the text for the shake of simplicity. We have chosen this structure because it has good mechanical properties and because its fabrication process could be automated for industrial purposes. Manuscript received August 16, 2005; revised November 22, 2005. This work was supported by Spanish Commission Interministerial do Ciencia y Technologia (CICYT) TIC 2003-00909 and Gobierno de Navarra Research Grants. The authors are with the Electrical and Electronic Engineering De- partment, Public University of Navarra, Pamplona 31006, Spain (e-mail: javier.bravo@unavarra.es; natxo@unavarra.es; ignacio.delvillar@unavarra.es; jmcorres@unavarra.es; parregui@unavarra.es). Digital Object Identifier 10.1109/JLT.2006.872307 Fig. 1. Scheme of the MHM. Similar structures have been already used in previous works [6]–[10]. In this paper, it will be studied for the first time, theoretically and experimentally, how the transmitted power characteristics of the light guided through the MHM changes when a nanofilm is being deposited on it using the electrostatic self-assembly (ESA) technique and how it changes if some geometrical and optical parameters are modified. Finally, one practical application of the device as humidity sensor is presented. Arregui et al. [11] demonstrated that the materials chosen for this paper have interesting humidity sens- ing properties. This paper presents the response of this sensor to the human breath. The reminder of this paper is organized as follows. In Section II, the design and fabrication arts are explained. In Section III, the experimental setup developed to characterize this nanostructure is explicated. In Section IV, the technique used to control the thickness of the nanodeposition is detailed. In Section V, there is an analysis of theoretical and experi- mental results obtained. Section VI presents results of humidity sensing with the device developed in this paper. Finally, some concluding remarks are given in Section VII. II. FABRICATION OF THE MHM The MHM structure consists of one short segment of HCF spliced between two standard MMFs over which the nanofilm will be deposited. Its scheme can be seen in Fig. 1. In its fabrication process, first, the jacket of the HCF is removed and the HCF is cleaved. Later on, it is spliced to the MMF using appropriate electric arc conditions so that the HCF collapses and creates a tapered solid fiber in the interface between both fibers [see Fig. 2(a) and (b)]. In these devices, the light that is guided in the core of the lead-in MMF can be coupled to the cladding of the HCF due to the tapered region instead of being confined in the air core. Fig. 2(c)–(e) shows some pictures of the light projected by the MHM structure when it is cleaved by different sections. As can be seen, the light that first reaches the HCF section is guided in the core of the lead-in MMF [Fig. 2(c)]. Then, in the HCF section, the light becomes guided by the cladding, projecting one light ring [Fig. 2(d)]. 0733-8724/$20.00 © 2006 IEEE