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In this study, we propose a novel approach to enhance the sensitivity of salinity measurement using photonic crystal fiber. Our method involves filling the first ring of holes in the fiber with seawater, which serves as the analyte, and embedding it in a single-material silica substrate. This design allows for precise tuning of sensitivity to even minimal changes in salinity concentration. Through numerical simulations, we explore the influence of different geometrical parameters on the photonic crystal fiber's (PCF) characteristics. We calculate the sensitivity for two wavelengths, 1.3µm and 1.55µm, and achieve a highest salinity sensitivity of 0.017966 ps/(nm·km)/PSU and 0.021818 ps/(nm·km)/PSU, respectively. Our proposed sensor not only demonstrates excellent performance in salinity sensing but also holds promise for applications in the field of communication, thanks to its satisfactory and impressive sensing capabilities. [ABSTRACT FROM AUTHOR]

This paper proposes a highly sensitive, compact, and low-cost optical fiber salinity sensor based on the Mach-Zehnder interferometer. The sensor is constructed using a single mode fiber (SMF) - no-core fiber - double-C fiber (DCF) - NCF-SMF structure, with the DCF prepared by etching the dual side-hole fiber with HF acid. The DCF's large-size exposed microfluidic channels solve the previous microstructured optical fiber's challenging liquid filling and replacement problems. Theoretical simulations and experiments demonstrate that the sensor is suitable for high-sensitivity salinity measurement. The sensor exhibits a high salinity sensitivity of -2.26 nm/‰ in the salinity range of 10‰-50‰, as demonstrated by the experimental results. Additionally, the sensor exhibits some fascinating characteristics, including high repeatability, hysteresis, reversibility, and stability.

This paper presented a new kind of salinity and temperature dual-parameter sensor based on a fiber ring laser (FRL) with tapered side-hole fiber (SHF) embedded in a Sagnac interferometer. The sensing structure is majorly composed of tapered SHF located in the middle of SHF inside the Sagnac interferometer loop structure. The influences of the SHF's diameters of different tapered in the Sagnac interferometer loop on the FRL sensing system are studied. The presence of air holes in the SHF makes the cladding mode easier to excite, and the interaction between the cladding mode with its surroundings is enhanced, thus having higher salinity sensitivity. Besides, the unique advantages of high resolution, narrower linewidth, and high signal-to-noise ratio (SNR) of fiber laser make the measurement results more accurate. In this experiment, the SHF with different taper diameters was made, and it was found that reducing the diameter of the taper waist diameter could further improve the salinity sensitivity. When the waist diameter was 9.70 μm, the maximum salinity sensitivity of 0.2867 nm/‰ was achieved. Temperature sensing experiments were also carried out. The maximum temperature sensitivity of the FRL sensing system was −0.3041 nm/°C at the temperature range from 20 to 30 °C. The sensor has the characteristics of easy manufacture, good selectivity, and high sensitivity, proving the feasibility of simultaneous measurement of seawater salinity and temperature. [ABSTRACT FROM AUTHOR]

In this paper, a fiber optic salinity sensor based on Fabry–Perot interference (FPI) with a harmonic-Vernier effect is suggested. An open cavity and a resin solid cavity are grown at the end of the fiber by a unique fiber-end photopolymerization and integration (FEPI) technique. When seawater fills the open cavity, it changes the optical path length (OPL) of the interferometer directly. While the OPL of the reference interferometer is hardly impacted by the seawater. The insertion of the harmonic envelope shifts greatly with salinity. Tunable sensing performances are achieved by adjusting the cavity length ratio(CLR) according to a theoretical model. The maximum sensitivity of 6.53 nm/‰ is attained in the range of 25 ‰ to 45 ‰ when its CLR approaches 1.18. In addition, the measuring accuracy, consistency, and temperature compensation method are also experimentally investigated. The suggested sensor is a prospective competitor in the field of measuring the salinity of seawater due to the advantages of high sensitivity, compact size, large dynamic range, and adjustable capabilities. [Display omitted] • A salinity sensor based on laser-induced cavity with Vernier effect is proposed. • Fiber-end photopolymerization and integration technique is applied to develop FPIs. • The relationship between the performances and CLR is analyzed and characterized. • The maximum sensitivity is 10 times that of traditional FPI salinity sensors. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate a novel salinity sensor based on Tamm-plasmon-polariton (TPP), comprising different shapes of Bragg reflector (ordinary, texturing, and sawtooth) and metallic layer. The finite element method is used to study the considered structure and sensing performance by using the COMSOL multiphysics simulation procedure. Here, we study the effect of surface morphology on the sensitivity; firstly, in the case of one-dimensional photonic crystal-centered defect, it harms the sensitivity; secondly, texturing and sawtooth in the case of Tamm resonance increases the sensitivity, as for texturing the surface, the sensitivity quality factor (Q) = 236 and figure of merit (FOM) = 170. For sawtooth surfaces, Q = 272.4, and FOM = 199. The consequences of structural parameters on the efficiency of sensing are studied, and new procedures are proposed to enhance TPP-based sensors. A simple and functional alternative to conventional salinity sensors may be the proposed solution. [ABSTRACT FROM AUTHOR]

In this paper, a Tamm plasmon resonance-based sensor is theoretically studied to calculate the salinity of seawater as well as a temperature sensor based on photonic crystals. The transfer matrix method (TMM) is used to systematically study and analyze the reflected s-polarized electromagnetic waves from the sensing structure. The proposed structure composes of prism/Au/water/ (S i / S i O 2) N /Si. The sensitivity, figure-of-merit, quality factor, and detection limit of the sensors are improved by optimizing the thickness of the seawater layer, incident angle, salinity concentration, and temperature. The proposed salinity sensor records a very high sensitivity of 8.5 × 10 4 nm/RIU and quality factor of 3 × 10 3 , and a very low detection limit of 10 - 7 nm. Besides, the suggested temperature sensor achieves high sensitivity (from 2.8 to 10.8 nm/°C), high-quality factor of 3.5 × 10 3 , and a very low detection limit of 3 × 10 - 7 nm. These results indicate that the proposed sensor is a strong candidate for salinity and temperature measurements. [ABSTRACT FROM AUTHOR]

Encapsulation of microfiber Mach-Zehnder interferometer (MMZI) sensor for temperature and salinity sensing in seawater is developed. It includes a C-shape metal tube with a slit on the bottom and polymer adhesive with large elasticity modulus, by which a more robust sensor with relatively fast response can be obtained. To evaluate the performance of sensor after encapsulation, effects of different polymer adhesives on tension withstanding, temperature and salinity sensitivities are evaluated, and dependence of response time on shape of metal tube is investigated. Results show that sensitivities of the sensor in temperature and salinity sensing can reach to be about −1.77 nm/°C and 1.18 nm/ ‰, and response times are about 219.0 ms and 12.5 ms, respectively. By comparing temperatures and salinities measured by sensor with those measured by commercial device, accuracy of the sensor after encapsulation is proved. In addition, cross-sensitivities of strain/pressure and short/long-term stability after encapsulation are also evaluated. Finally, based on the basic structure, further improvements are attempted, such as double encapsulation with stainless steel mesh and complex encapsulation with two kinds of polymer. Encapsulations of MMZI demonstrated here show advantages of low cost, improved robustness, relatively fast response, long-term stability and easy expandability, which will not only afford a critical support for optical sensors operated in the ocean, but also offer some useful references for microfiber sensors used in other sensing applications. [ABSTRACT FROM AUTHOR]

The amount of salt dissolved in a body of water is an important factor in determining physical characteristics like the heat capacity of the water. Here, we report a theoretical and numerical investigation on a seawater salinity sensor using a miniaturized fiber-optic probe based on a dual-core photonic crystal fiber (PCF). The sensing mechanism is based on interplay between fiber fundamental mode in the central core and a secondary mode in the water-filled hole of the PCF. Using finite element method analysis with Comsol Multiphysics Software, we show that a spectral sensitivity larger than 5500 nm/RIU can be achieved while varying the salt concentration from 0% to 100%. Furthermore, we demonstrate that the sensitivity for highly saline water bodies can be further improved by applying the permanent twist on the cladding air holes of the fiber. We studied the role of the twist directions and twist rates on the performance of the sensor including sensitivity and the absolute loss value. While the twist does not have any major impact on sensitivity of the sensor for low salinities, our results indicate that twisted PCF has an average higher sensitivity for salt concentrations over 50% and a maximum sensitivity of nearly 7000 nm/RIU is reported around salt concentration of 80%. [ABSTRACT FROM AUTHOR]

A new high-sensitivity optical reflective probe is reported for the simultaneous measurement of seawater salinity and temperature. A concise interference structure was used for exciting the cladding mode. A nanoscale gold film served as the sensitive detection area to induce surface plasmon resonance (SPR). A photosensitive material (polydimethylsiloxane) was partially coated on the sensitive layer to produce double SPR peaks. The SPR effect model of the optical fiber structure was modified and a complete detection system was constructed. The preparation and structural parameters were optimized to obtain high-quality spectral properties. Through the calibration experiments, the response to salinity at its characteristic wavelength had a sensitivity of 0.0558 nm/%, while the temperature response at its characteristic wavelength was −4.418 nm/°C. The reflective probe was shown to have low crossover sensitivity. Compared with standard instrumentation, this device was shown to be stable and precise and is expected to have significant practical applications. [ABSTRACT FROM AUTHOR]

We present a microfluidic U-shaped micro-cavity sensor by splicing a segment of a microfiber of a few hundred micrometers in length tapered from a single-mode fiber (SMF) between two SMFs with a predesigned lateral offset for highly sensitive salinity measurement. The proposed sensing probe serves as an in-line microfiber-assisted Mach–Zehnder interferometer (MAMZI) with an ultra-high refractive index sensitivity of $10^{4}$ nm/RIU. Three Mach–Zehnder interferometer structures with different cavity lengths of 351.82, 242.56, and 181.31 $\mu \text{m}$ are fabricated, by which microfluidic sensing systems are established for in-line measurement of sodium chloride (NaCl) solution. Experimental results indicate that the detection limit of NaCl solution is as low as $4\times 10^{-3}$ wt% and the response time is less than 15 s, which would make the MAMZI-based microfluidic measuring system play an important role in label-free biological and chemical detection applications. [ABSTRACT FROM AUTHOR]

A salinity sensor employing photonic crystal fiber is designed for measuring the concentration of salt in sea water. The sea water sample is infiltrated into one of the air holes in cladding, which offers high confinement loss and act as an analyte core mode and background acts as silica mode. In order to satisfy the phase matching condition, the power transferred from silica core to the liquid filled analyte core, which is investigated using finite element method. An enhanced sensitivity of salinity in sea water reports as 5405 nm/RIU for x -polarization and 5675 nm/RIU for y -polarization with a detection limit of 0.0037 RIU has been reported. The proposed PCF also promises to yield the large birefringence of order 10 −3 along with the enhanced sensitivity. [ABSTRACT FROM AUTHOR]

For the hydrodynamic characterisation of reactors handling aqueous liquids, especially in conjunction with the validation of numerical simulations, data with high temporal resolution is essential. This paper describes the design of a rapid, flat, and low-cost sensor array for parallel salinity measurements, which allows to perform the required measurements in nearly any reactor type with minimal impact on the flow characteristics due to its very thin (125 μm) and flexible structure. Sample measurements for the sensor array system are shown using a step change in aqueous salinity and the resulting response of a thin-layer cascade algae reactor unit as well as performing measurements of a pulse in salinity in the circularly operated algae reactor containing a feedback loop. The system’s response, mixing, and cycle time can be easily extracted from the generated data sets. The sensors provide measurements with an accuracy of about 2% for a linear range of 0–10 g l −1 NaCl solution. In its default configuration one device consists of up to eight sensors, which are distributed on two stripes (220 × 25 mm, 125 μm thick) each containing four sensor spots, that are positioned 5 cm apart from each other. The modular set up of the sensor array facilitates simple modification depending on individual requirements. [ABSTRACT FROM AUTHOR]

This paper demonstrates the operation of a smartphone based platform salinity sensor for accurate and reliable monitoring of salinity level in oceanic water environment. To measure water salinity level using the smartphone, two different approaches have been proposed and their sensing performances have been compared. The first approach is based on Beer-Lambert principle where collimated light beam from an optical source while passing through the medium gets attenuated due to absorption by the medium which can be detected and analyzed by the smartphone. The second approach is based on evanescent field absorption from an uncladed U-bent sensing region of an optical fiber. Variation in salinity level of the surrounding medium of the fiber sensing region affects the absorption of evanescent field and this can be monitored by the smartphone. Two freely available android applications have been used for detection and analysis of salinity level. The designed smartphone sensor has an ability to measure salinity level variation as low as 0.1 parts per thousand (ppt) with high accuracy and repeatability. We envision that owing to its compact size, low-cost and truly user-friendly in nature, the sensing techniques could emerge as potential alternative to existing salinity sensors that would be useful for different in-field applications. [ABSTRACT FROM AUTHOR]

This study develops general equations to extend the applications of a GS3 salinity sensor to a wide range of soils including critical saline-alkali reclaimed tidal lands. The GS3 sensor measures volumetric water content, temperature, and electrical conductivity independently. As a result of laboratory-based sensor calibration testing, it was determined that the electrical conductivity of soil pore-water solution (EC p ) with slight to moderate salinity can be computed from the bulk EC (EC b ) without calibration. The EC b data obtained from the sensors were reconstructed via the calibration equation. We compared these results to EC p observed from suction cup samples. A good correlation was observed between the calibrated EC p and the observed EC p , in low-to high-salinity soils. However, the calibrated EC p at very high salinity underestimates the observed EC p . Based on these results, we conclude that these sensors offer clear additional value in ensuring control of the root-zone salinity, and in compensating for weather influences. [ABSTRACT FROM AUTHOR]

• The sensor is simply fabricated by welding SMF and TCF. • The sensor has a reflective sensing head, which can directly interact with seawater. • The sensor can reach a high sensitivity of 2.50883 nm/‰ due to the introduction of vernier effect. A new reflective fiber-optic Fabry-Perot (F-P) salinity sensor is proposed with a section of open liquid cavity, which ensures that the seawater flows in and out freely, and the seawater can directly participate in optical transmission. Variation of seawater salinity will cause change of the effective refractive index in the optical path, which in turn affects the output spectrum. By controlling the length of the spliced fiber to form a reflection spectrum with Vernier effect, the sensitivity of the sensor to salinity changes is greatly improved. The results show that the sensor can reach a salinity sensitivity of 2.50883 nm/‰ (11511.07 nm/RIU) in the salinity range of 0 ‰–26.15 ‰, which is approximately 13.17 times higher than that of the sensor without combining Vernier effect. The sensor has good robustness, high sensitivity, and is easy to fabricate to package, which can be applied in the marine monitoring field. [ABSTRACT FROM AUTHOR]

The aim of the paper is to assess the accuracy for physical oceanography purposes of some commercial sensors (Star-Oddi and NKE) installed on fishing boats in the Adriatic Sea. When mounted on fishing gears, they can retrieve huge amounts of daily datasets (temperature, depth and salinity), spanning a very large spatial region. The possibility to establish their accuracy would be of extreme importance for physical oceanography studies since it would be almost impossible to obtain the same amount of data by means of cruises onboard Research Vessels. Comparison tests against a calibrated CTD were performed during several surveys. Summarizing, the data collected by Star-Oddi sensors are useful only considering the data portion where a dwell time at a fixed depth permanence is longer than 50 s, while those collected by NKE sensors are much more accurate for both depth and temperature and could be usefully considered for broader oceanographic purposes. The weak point of the NKE sensors is the salinity measurement. The evaluation carried out in the present study underlined the optimal conditions for the usage of the considered sensors and produced a series of offsets that might be used to enhance the accuracy of the recorded datasets. [ABSTRACT FROM AUTHOR]

• Highly sensitive reflection based no-core optical fiber probe is developed for salinity measurement. • Well arrayed ZnO nanorods have been grown over silica fiber surface by hydrothermal route. • Band gap modification and evanescent wave absorption are responsible for sensing mechanism. • Proposed sensor shows excellent sensing characteristics and can be used in chemical and biological fields. This work reports an intensity-modulated and reflection based NCF salinity sensor using ZnO nanorods as outer layer coating. As the accurate measurement of salt concentration is highly essential to monitor and control the negative effects of saline present in aqueous resources. The sensing principle is based on change in refractive index (RI) of ZnO layer being in contact with various saline solution leading to different EWA. This absorption of evanescent field by external medium is responsible for change in intensity of light detected at the output end. The ZnO nanorods deposited fiber sensor exhibits good sensitivity as well as excellent reproducibility, repeatability and durability. Proposed sensor contains simple, low cost, and reliable probe to detect the ambient RI changes utilizing a laser source, and resultant output power was modulated as saline concentration is varied. The highest average sensitivity of 0.574 dB/% is obtained in the salinity range of 0–25 % with a good stability and fast response time at room temperature. [ABSTRACT FROM AUTHOR]

• Exploring mode coupling between few-mode fiber. • The sensor can measure salinity in the range of 0 % to 20 % and temperature in the range of 5 °C to 30 °C. • The sensor can be used in a wide range of applications such as marine environmental science and aquaculture. • Simple construction, good stability, and low cost. In this manuscript, a simple and innovative Mach-Zehnder interferometer (MZI) based on few-mode fiber (FMF) cascaded with a core-offset structure is presented to measure seawater salinity. The sensor is fused by the core-offset fusion method, which makes it easier for the sensor to excite higher-order modes and more sensitive to the changes in the external environment. Through the finite element analysis method, the interference spectrum is obtained by the interference of the fundamental mode LP 01 and the higher-order modes LP 21 and LP 02 , respectively. The experimental analysis reveals that the salinity sensitivity of the sensor is −0.27 nm/% from 0 % to 20 %. The temperature sensitivity is 0.15 nm/°C from 5 °C to 30 °C. The salinity sensor has a simple construction, good stability, and low cost, and has great application prospects in marine environment monitoring, aquaculture, and other fields. [ABSTRACT FROM AUTHOR]

A salinity sensor based on the principle of Mach-Zehnder interferometer (MZI) is presented and verified in this letter, which has certain advantages in the sensitivity and cost of the salinity sensor. This structure consists of a lead-in single-mode fiber (SMF), a small section of C-type micro-structured optical fiber (CMOF), a multi-mode fiber (MMF), and a lead-out single-mode fiber (SMF), which can be referred to as an SCMS structure for short. According to the experiment results, the salinity sensitivity of the sensing probe can reach − 3.25 nm/‰ with linearity of 0.999 when the CMOF is about 500 µm of the length. In addition, its refractive index (RI) sensitivity can be more than 10000 nm/RIU. Moreover, the sensing probe has a relatively good detection limit, namely 0.1‰. At the same time, it also provides a low-cost fabrication way to realize the direct contact with and exchange of the fluid being measured, and the sensor proposed has the prospect of monitoring ocean salinity. [Display omitted] • A salinity sensor based on a small offset splicing with a novel C-type microstructure optical fiber is proposed. • It achieves high sensitivity, good linearity and better detection limit, namely − 3.25 nm/‰, 0.999 and 0.1‰, respectively. [ABSTRACT FROM AUTHOR]

Abstract: Optical salinity sensors described here measure directly the seawater refractive index and thus enable a measurement of the seawater density and composition variation. We detail the measurement dependence to environmental parameters (in particular temperature and pressure) compared to conductivity sensors, and demonstrate that it may be advantageous to directly measure refractive index rather than electrical conductivity and so obtain a more direct route to density and absolute salinity. [Copyright &y& Elsevier]

• A compact multifunctional underwater optical fiber sensor based on SPNPS-PMFBG structure is proposed and experimentally demonstrated. • By monitoring the wavelength and output power change of PMFBG at two resonance peaks, the synchronous measurement of salinity, temperature and strain are achieved. • The proposed sensor has high stability with the wavelength fluctuations of less than 0.02 nm and the intensity variation of less than 0.1 dB during 3 h, and the fast response time with less than or equal to 3 s. • In the experiment under water, the measured salinity sensitivity based on the sum of two resonance peaks in the range of 0–7 wt% is −0.58 dB/wt% with good linear response and repeatability. In this paper, a compact multifunctional optical fiber sensor based on single mode fiber-photonic crystal fiber-no core fiber-photonic crystal fiber-single mode fiber-polarization maintaining fiber Bragg grating (SPNPS-PMFBG) has been proposed. By monitoring the wavelength and output power of the two resonance peaks of PMFBG, simultaneous measurement of salinity, temperature and strain can be realized. In our experiment under water, the sum output power of the two resonance peaks in 0–7 wt% salinity shows a good linear relationship and repeatability, and the sensitivity is −0.58 dB/wt% with 3 s response time. In addition, the measured temperature wavelength sensitivities at two resonance peaks are 0.042 nm/°C and 0.040 pm/°C, and the output power sensitivities are approximate to 0.079 dB/°C and 0.081 dB/°C, respectively. Further, the strain wavelength sensitivities are 0.535 nm/1000 μ ε , and the output power sensitivities are 0.65 dB/1000 μ ε and 0.70 dB/1000 μ ε. [ABSTRACT FROM AUTHOR]

In this paper, defective microwave photonic crystals (MPCs) are designed to sense the salinity of aqueous solutions. The defective MPC sensors are constructed by two kinds of microwave dielectric layers and one defective salt solution layer. Transfer matrix method (TMM) for lossy medium is developed to calculate the transmittance spectra of the sensors. It is found that the peak transmittance of both the defective resonance within the microwave band gap (MBG) and transmitting modes outside the MBG monotonously decrease with the increase of salinity, while the resonant and transmitting mode frequencies remain unchanged. By comparing the four MPC sensor structures, the first transmitting mode in the upper frequency band outside the MBG of the 15-layer MPC sensor has the largest salinity sensing range from 0 to 40‰ with relative stable detecting sensitivity. The sensing principle is based on the fact that the dielectric loss factor of saline solution is much more sensitive to salinity than the dielectric constant in the microwave frequency band. The sensitivity, quality factor, and salinity detection range of the MPC sensors are calculated and compared. The reported defective MPC sensors are suitable to be used for non-contact salinity detection. [ABSTRACT FROM AUTHOR]

This paper aims to theoretically study the concept of a photonic salinity and temperature sensor according to a deformed one-dimensional photonic structure. The fundamental capability of the proposed sensor is studied. Simultaneously we search to optimize the thickness of the structure and to get the maximum salinity and temperature sensitivity. The structure is constructed by alternating layers of TiO2 and fused-silica P times. In the middle of the structure, a cavity containing seawater is inserted to measure its salinity and temperature. The transfer matrix method (TMM) is used to simulate the wave-transmittance spectra. It is shown that the quality factor (Q-factor) of the resonance peaks depends on the number (P) of layers. After that, the thickness of the layers is deformed by changing the deformation degree (h). The parameters P and h are optimized to get the maximal Q-factor with the minimal number of layers and structure thickness. The best sensitivity SS of the proposed salinity sensor is 558.82 nm/RFIU with a detection limit of 0.0034 RFIU. In addition, the best sensitivity ST of the designed temperature sensor is 600 nm/RFIU with a detection limit of 0.0005 RFIU. [ABSTRACT FROM AUTHOR]

New results are presented for a sensitivity-tunable, inductive fluid conductivity sensor based on RF phase detection. An electronically controlled RF phase shifter allows the sensor to function in a wide range of conductivities from 2-70 mS/cm and helps tune the sensitivity of the response in a selected conductivity range. The noncontact nature of the sensor makes it suitable for corrosive fluids. Furthermore, the small size of the sensing element (1 inch. Sq X 6 mm thick) makes it suitable for compact in-line and hand held monitoring systems. [ABSTRACT FROM PUBLISHER]