New Method for Previsional Design of Fabrication Process of LPG’s with a Given Sensitivity A. Quintela, M. A. Quintela, D. A. Gonzalez, J.M. Lázaro, P.B. García-Allende, J.M. López-Higuera Photonic Engineering Group –University of Cantabria Avda. Los Castros s/n – C.P. 39005 Santander, Spain Tel: ++ 34-42-201495; Fax: ++ 34-42-200877 Email: quintela@ unican.es Abstract: A theoretical-experimental based method able to design in advance the fabrication process for a given sensitivity (temperature, strain, surround index, ...) is proposed. Previous and proper characterization of the LPG specific fabrication technique is required. OCIS codes: (060.2370) Fiber Optics Sensor; (050.2770) Gratings. 1. INTRODUCTION The long period gratings (LPG) have been used in many photonics applications, both in optical communications and optic sensing systems. In optical communications, many devices based on LPG have been achieved and matured, even other were used for commercialization. LPG`s can be used in EDFA gain flattening and ASE equalization [1]. In the sensors field, LPG can be used for highly sensitive temperature, axial strain and refractive index sensors [2]- [4]. The wavelength in the resonance loss bands depend on the grating period and the effective refractive index difference between the fundamental mode and the cladding mode. External perturbations, such as strain or temperature, can modify both parameters of the LPG. In this manner, the induced variations of the resonance wavelengths can be used to implement a simple demodulation technique. Additionally, the multiple bands of the LPG can be utilized to measure simultaneously two parameters [5]. So, it possesses a highly degree of flexibility that can be used to implement various optical fiber-based sensing systems. Other advantages of the LPG used in sensor application are their easy fabrication method and that it doesn’t affect the mechanical properties of the material in which they are embedded. They also present low insertion loss and low background reflections. In the design of sensors, a custom sensitivity of the LPG to external parameters is desirable, for example to the communication applications is desirable a minimum sensitivity. So the obtaining of the sensitivity characteristics of the LPG is required to customize the design of a sensor. The dependence of the LPG characteristics on the grating period, fiber parameters and the order of the cladding mode has been theoretically studied [6] but not experimentally checked. The objective of this paper is to present a method able to define in advance the LPG specific fa brication process for given or custom sensitivity. In order to obtain this is necessary to characterize perfectly the specific writing process of the installation. With this experimental characterization, the curves of the variation of mode resonance wavelength with the grating period are obtained for a particular fabrication conditions (UV laser power, scan speed, etc). From these results, it is possible to obtain the factor γ for each mode and each resonant wavelength. With this factor and with the some mechanical and optical parameters of the fiber material is possible to determinate the sensitivities (temperature, strain, surround index, etc) of any resonant wavelength for a LPG written with this method. So given a sensitivity the characterization curves will give us the proper cladding mode to be used and required grating period of the LPG. The proposed method has been successfully validated using as external measured parameter the temperature. Several grating are designed fabricated and experimentally characterized showing the experimental results very good agreement with the pre-visional calculations. 2. THEORY FOR THE SENSITIVITIES OF LPG The phase matching between the guided mode and the forward propagating cladding modes for a long-period grating is given by (1) ( 29 ( 29 ( 29 Λ - = λ λ λ i cladd eff i res n n a283_1.pdf TuE15.pdf © 2006 OSA/OFS 2006