IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 8, APRIL 15, 2006 935 Nonadiabatic Tapered Single-Mode Fiber Coated With Humidity Sensitive Nanofilms Jesus M. Corres, Javier Bravo, Ignacio R. Matias, Senior Member, IEEE, and Francisco J. Arregui, Member, IEEE Abstract—A new humidity sensor has been developed by coating a tapered single-mode standard communications fiber with a hu- midity sensitive nanofilm using the electrostatic self-assembled (ESA) monolayer technique. Power changes up to 20 dB have been recorded during the coating process. In order to take advantage of both the potential sensitivity of the tapered fiber and the precise thickness control, which is possible to achieve using the ESA technique, the optimal layer thickness has been adjusted to the maximum slope point of the transmitted optical power curve as a function of the overlay thickness. An optimal working point sensor is compared to a nonoptimal one demonstrating the sensitivity difference between both. Index Terms—Electrostatic self-assembly (ESA), humidity sensor, optical fiber sensor, tapered fiber. I. INTRODUCTION T HE STUDY of the transmission properties of tapered op- tical fibers has given rise to numerous research studies both from the point of view of theoretical behavior as well as of the applications to the sensor and coupler design. Tapers can be designed to be highly sensitive structures because of the mode beating that takes place in the waist zone. Stretching and waist radius reduction created in the tapering process cause the core/cladding interface to be redefined in such a way that the single-mode fiber in the tapered region will act as a multimode fiber, leading to mode coupling in the high slope zone and cre- ating an oscillatory optical power output [1]. This beating of modes in the taper waist makes the taper very sensitive to the external medium refractive index changes, allowing its use in refractometry. A different application lies in using the surface plasmon resonance when depositing a thin film of metal onto the taper’s surface. Azimuthally asymmetrical palladium coat- ings have been recently employed to fabricate hydrogen sen- sors [2]. However, when using this technique, devices are ob- tained with more than a 10% of thickness difference between the maximum and minimum thickness of the asymmetrical coating. Also, polymer coatings have been employed to fabricate gas sensors with the spin coating deposition technique [3]. This method does not allow monitoring the transmitted optical power as a function of the overlay thickness (construction curve) si- multaneously with the deposition process. In addition, other Manuscript received October 17, 2005; revised February 2, 2006. This work was supported by Spanish 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, 31006 Pamplona, Spain (e-mail: jmcorres@unavarra.es; javier.bravo@unavarra.es; natxo@unavarra.es; parregui@unavarra.es). Digital Object Identifier 10.1109/LPT.2006.873568 techniques like spin coating or dip coating do not allow the overlay thickness to be tuned-in. In this work, the electrostatic self-assembled (ESA) technique is proposed to create a uniform sensitive thin layer of polymeric material, allowing the sensor behavior to be monitored while the deposition is in process and to stop at the optimal sensitivity layer thickness. Another advan- tage of using the ESA method is the low cost of the materials and instruments used, and that it can be carried out under stan- dard temperature and humidity room conditions. The letter is di- vided as follows: In Section II, the tapering process is described. Then, in Section III, the ESA technique and experimental setup employed are described. In Section IV, some experimental re- sults of the transmitted power along the coating process and the humidity sensitivity response of the taper-based sensor are dis- cussed. II. FABRICATION OF THE TAPERED FIBER The tapering of the fibers can be achieved by heat pulling using flame, electric arc, or laser. Also, chemical etching using hydrofluoric acid has been reported [4]. In this work, several samples were prepared by stretching the uncovered region using a Ericsson FSU-905 electric splicing unit. In the method pro- posed here, it is possible to get a waist length of only 1 mm. The insertion losses (caused by the nonadiabatic structure) have been analyzed and range from 0.3 to 4 dB, depending on the ta- pering. Typical dimensions for the waist diameter of the tapers employed in this study are 20 and 25 m. Tapers created this way are sufficiently robust to be handled without special tools. In addition, the -parameter is the normalized frequency of the fiber, a dimensionless parameter, which gives information about the number of modes propagating in a fiber [6]. If continues decreasing it can reach its cutoff value , when the taper can be explained by the classical modal domain interferometric structure (single-mode—multimode—single-mode) [6]. The ta- pers used in this work accomplish with the condition allowing the guided light to be expanded to the whole cladding. The transmitted optical power through the taper shows an oscil- latory behavior with respect to elongation, bend, wavelength, or external refractive index changes [1]. Larger amplitude oscilla- tions could be obtained by fabricating tapers where the power is equitatively divided between two propagating modes. The cou- pling between the fundamental mode HE and the following mode with the same symmetry HE is the main cause of the high oscillations that appear when the refractive index of the external medium is changed [1]. When a thin film is deposited onto a tapered optical fiber, the modes effective indexes change. Those physical parameters, which induce variations in the thick- ness or refractive index of the film will change the transmitted 1041-1135/$20.00 © 2006 IEEE