Flow cell with hybrid LPG and FBG optical fibre sensor for refractometric measurements

Abstract

ABSTRACT The combination of a long period grating and a fiber Bragg grating written on the sa me fiber is described as method to reduce noticeably the interferences caused by strain and temperature in the measur ement of refractive index. The hybrid LPG and FBG optical fiber sensor is manufactured and located in a small volume flow cell. The whole system with its flow cell and the gratings fabrication are extensively descri bed as well as both the acquisition and data processing. The maximum sensor sensitivity and resolu tion are about 3120 nm/RIU and 2 x 10 -5 RIU, respectively. Keywords: long period grating, strain, temperature, refractive index 1. INTRODUCTION Thanks to their sensitivity to external refractive index and all the other advantages offered by the optical fiber sensors, long period gratings (LPGs) have been proposed for refractometric measurements [1]-[6]. However, because of the influence of temperature, strain and fiber bending in the LPG response, it is crucia l to control, correct and stabilize these parameters in order to measure accurate ly the refractive index changes. On th e other hand a fiber Bragg grating (FBG) [7] is insensitive to external refractive index and fiber be nding, but it has a well-known res ponse to both temperature and strain [8]. Differently from what already exists in literature [4 ]-[6], the challenge aspect is both the realization of a close d flow cell of relatively low volume where the optical fiber gr atings can be fixed securely, and a methodology to control the LPG cross-sensitivities towards other physical parameters. In this paper in fact a refractometer based on the use of a LPG, able to reduce all the cross-sensitivities, is presented. The whole system is constituted by a low volume flow cell containing an optical fiber on which a FBG and a LPG are written. The full characterization of both gratings in terms of strain and temperature allows us to perform the refractive index measurement in small volumes reducing noti ceably the effects of strain and temperature.