Salinity measurement in water environment with a long period grating based interferometer

Abstract

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.

Optical fiber laser salinity sensor based on multimode interference effect

“…Measurement of salinity by using optical fiber technology has attracted lots of attentions in recent years due to the well known advantages of fiber-optic sensors such as high sensitivity, immunity to electromagnetic interference, and in situ measurements. Various fiber optic salinity sensors have been reported that are mainly based on fiber Bragg gratings (FBGs) [1][2][3], long-period fiber gratings (LPGs) [4,5], and various optical fiber interferometers [6,8]. As we know the fabrication process of optical fiber gratings is relatively complex.…”

Salinity sensor based on polyimide-coated photonic crystal fiber

“…In addition to the unique features reported in [1][2][3], PCFs also possess the advantages of conventional optical fibers in sensing applications such as EMI immunity, compact size, corrosion resistance, light weight, multiplexing capability, etc. Optical fiber sensors are also attractive for in situ monitoring of different physical, chemical, and biological parameters, e.g., strain [4,5], temperature [6], refractive index [7], pH [8], relative humidity [9,10], and water salinity [11][12][13][14][15][16][17][18]. Salinity sensors play an important role in manufacturing process control and protection of ecosystems.…”

“…Salinity sensors play an important role in manufacturing process control and protection of ecosystems. Fiber optic salinity sensors have been reported based on salinity sensitive hydrogel/polyimide-coated fiber Bragg gratings (FBG) [11,12], surface Plasmon resonance effect [13,14], and refractive index detection with long-period fiber gratings [15,16], etched FBG [17], and femtosecond laser micro-machined Fabry-Pérot interferometer [18] to derive salinity response. However, these salinity sensors have relatively low sensitivity and/or exhibit large temperature cross sensitivity which degrades the sensor performance.…”

Pd-Ag film coated cascaded long period gratings for hydrogen gas sensing

“…Such cascaded LPGs show finer-scale features in the envelope and offer higher sensitivity and resolution for monitoring many environmental parameters changes than single LPGs [5]. Cascaded LPGs have been applied to measure the external refractive index [6], [7], liquid concentration [8], temperature [9], bending [5], and transverse load [10]. They also offer the ability to multiplex several sensors together [11].…”