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In this work, a comparative study of the behaviour of an in-fibre Mach-Zehnder interferometer for salinity measurement in a water solution is presented. The fibre transducer is composed of two nearly identical long period gratings forming an inseries 7.38 cm long device written in the same fibre optic. Two inorganic and one organic salts (NaCl, KCl, NaCOOH) were characterized within the concentration range from 0 to 150 g L −1. For the long period grating interferometer, the average obtained sensitivities were −6.61, −5.58 and −3.83 pm/(g L −1) for the above salts, respectively, or equivalently −40.8, −46.5 and −39.1 nm RIU −1. Salinity measured by means of fibre refractometry is compared with measurements obtained using an Abbe refractometer as well as via electrical conductivity. For the long period grating refractometer, the best resolutions attained were 1.30, 1.54 and 2.03 g of salt per litre for NaCl, KCl and NaCOOH, respectively, about two times better than the resolutions obtained by the Abbe refractometer. An average thermal sensitivity of 53 pm °C −1 was measured for the grating transducer immersed in water, indicating the need for the thermal correction of the sensor. Resolutions for the same ionic constituent in different salts are also analysed.

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Abstract-In this work is evaluated the performance of etched fiber Bragg gratings, assembled in different configurations to operate as a refractometric sensor, in the analysis of water-ethanol mixtures. Two fiber Bragg gratings operating close to 1300 nm and 1500 nm were wet-etched and employed in the sensor design. Four configurations for the refractometric sensor were studied, in dualwavelength and single-wavelength operation modes, and at two sample temperatures. Calibration curves were determined for the range between 0.0 and 100.0 % v/v of water in ethanol, and the sensor performance for each configuration was analyzed by comparing its sensitivity, conformity, repeatability and combined uncertainty. The best results showed that the sensor can be used to measure the ethanol-water concentration with combined uncertainty of 2. I. INTRODUCTION The ethanol is a substance miscible in both non-polar substances as hydrocarbons and polar substances as water. This characteristic makes the ethanol a versatile solvent used in several industrial sectors such as chemical, pharmaceutical, beverage and fuel. Particularly in the fuel sector, due to the inevitable depletion of the world's petroleum supply, there is an increasing worldwide interest in alternative, non-petroleum-based sources of energy.The ethanol production process is characterized by sequential procedures, and in several cases for which water is a constituent of the final product, the water-ethanol proportion must be periodically monitored and compared to standardized conditions to preserve the product quality. In Brazil, the ethanol used as fuel is obtained by the fermentation of sucrose. In the production process, sulfuric acid is employed to hydrolyze the carbohydrate and after the hydrolysis, the acid is separated from the sugar which is fermented to obtain the ethanol [1]. A careful monitoring of water content in ethanol is fundamental for the product commercialization as the product cost and destination are specified as a with spectral parameters of λ c1500 = 1539.87 nm and R 1500 = 22 dB, was written by using a phase mask with pitch of 1062.5 nm. B. FBG EtchingFBGs have been extensively used as temperature and strain transducers, with typical sensitivities of 13.7 pm/ºC and 12.0 pm/µε, respectively [11]. However, FBGs are intrinsically insensitive to surrounding medium refractive index once the light coupling takes place only between well-bound core modes, which are shielded from the influence of the surrounding medium refractive index by the fiber cladding. To make the FBG sensitive to changes in the surrounding refractive index, the cladding radius around the grating region must be reduced, allowing the effective refractive index of the fiber core to be significantly affected by the refractive index of external medium [12].The change in the Bragg wavelength δλ B associated with the chemical etching is given by [13]:(1) where Λ is the grating period, η po is the fraction of the total power of the unperturbed mode that flows in the etched region an...