> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Abstract— Nanometric narrowband Long Period Gratings (LPG) are investigated for the implementation of improved fiber optic biosensors. The reduction of more than one order of magnitude in the linewidth of the LPG with respect standard LPG at 1500 nm, leads to the improvement of the resolution of the sensor. By selecting the proper fabrication parameters (high numerical aperture, relatively high order mode and large length), LPGs with a 3-dB bandwidth of 1.5 nm were fabricated. The sensitivity of the LPG as a refractometer was calculated, and experimentally characterized in detail. In particular, the LP 0,17 and LP 0,18 resonances were investigated, in order to select the most suitable one for the bioexperiments. The surface of the LPG was functionalized and the detection of the hybridization of DNA is demonstrated. When the biosensor was immersed in a 2 µM solution of the complementary DNA strand, the resonances of the LPG shifted in wavelength. When measuring the response of the sensor in terms of variation of its transmittance as the resonance shifted in wavelength, the sensitivity was ~10%/µM for both resonances. The detection limit was estimated in 10 nM. Index Terms—Fiber optics, Fiber optics sensors, Fiber Gratings, Biosensors. I. INTRODUCTION HE development of photonic biosensors has been a rapid growth research field in the last years. In particular, the development of fiber-optic, label-free biosensors is of special interest: they can be designed to be specific and selective, they show compact size and are capable to be point-of-care devices [1-3]. Within the applications of fiber-optic biosensors, the detection of DNA hybridization is of much interest, due to their specific capability to detect particular DNA sequences that might be of interest for environmental, biological or This work is funded by projects iPhotoBio (PIRSES-GA-2013-612267, EU) and UV-INV-AE16-485280 (Universitat de València). M. Delgado-Pinar, L. Poveda-Wong, J. L. Cruz and M. V. Andrés are with the Departamento de Física Aplicada y Electromagnetismo – ICMUV, Universidad de Valencia, 46100, Valencia, Spain (e-mail: martina.delgado@uv.es, luis.poveda@uv.es, cruz@uv.es, miguel.andres@uv.es ). Q. Shi, B. Xu and J.Zhao are with the Shanghai Institute of Microsystem and Information Technology (SIMIT), Chinese Academy of Sciences – Shanghai Tech. University, China (e-mail : shiqing@shanghaitech.edu.cn, xbj@mail.sim.ac.cn, jlzhao@mail.sim.ac.cn) E. Delgado-Pinar is with the ICMOL, Universitat de València, C/Catedrático José Beltrán 2, 46980 Paterna, Spain (e-mail: estefania.delgado@uv.es). health applications, epidemic controls, diagnosis, drug research, etc. Several approaches have been reported up to date to measure the DNA-hybridization based on different photonic devices, as for example micro-fiber gratings [4], gold-coated microtapers [5], surface plasmon resonances based sensors [6], microwave photonic filters [7] or dual-peak Long Period Gratings (LPGs) [8]. Other sensors based on waveguides with strong evanescent fields show also high potential as, for example, reverse symmetry waveguides [9, 10]. LPGs are fiber devices whose optical properties rely on the resonant coupling between the fundamental mode and the proper cladding modes. The coupling is induced by a periodic modulation inscribed in the core refractive index, whose parameters set the optical response of the LPG. It will present a series of attenuation notches, corresponding to the coupling to different cladding modes. The bandwidth of the attenuation notches can be tailored within some limitations. Typical bandwidths are in the range from several to tens of nanometers. In a previous work, we demonstrate that LPGs with a reduced bandwidth down to the order of 1 nm can be fabricated [11]. In this way, the detection limit of a biosensor based on these narrowband LPGs is improved, since the difficulty of determining with accuracy the wavelength of resonance in a broad attenuation notch is overcome. LPGs show an intrinsic advantage as sensors, compared to other similar fiber photonic devices as FBGs: the cladding modes coupled by the LPG have a significant evanescent tail in the surrounding medium, thus the sensitivity to external changes is higher. Any modification in the external refractive index will modify the optical response of the LPG, in particular, the spectral position of the resonances. In general, the sensitivity will change for LPGs fabricated with different parameters, and it will also be different for each resonance of the same LPG, depending on the dispersion curve of the cladding modes involved in the coupling. Refractometers, solution concentration sensors and E. Coli biosensors have been demonstrated based on this sensing principle [12-15]. In this work, we propose the use of a narrowband LPG to measure the DNA hybridization in an aqueous solution. LPGs are simple, robust photonic devices when compared with the other approaches we mentioned before, which are based on a few microns in diameter fibers [4-7]. The unusual narrow bandwidth of the LPGs we present here allows improving the detection limit of this biosensor when compared to dual-peak Oligonucleotide-Hybridization Fiber-Optic Biosensor using a Narrow Bandwidth Long Period Grating Martina Delgado-Pinar, Qing Shi, Luis Poveda-Wong, Estefanía Delgado-Pinar, Baojian Xu, Jianlog Zhao, Jose Luis Cruz, and Miguel V. Andrés T