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Abstract
We proposed an optical fiber salinity sensor with a composite Fabry-Perot (F-P) cavity structure for simultaneous measurement of temperature and salinity based on microelectromechanical system (MEMS) technology. The sensor contains two sensing cavities. The silicon cavity is used for temperature sensing, and the seawater cavity processed by the glass microstructure is sensitive to the refractive index of seawater for salinity sensing. At the same time, the influence of the salinity-temperature cross-sensitivity error of the seawater cavity is effectively compensated by using the temperature single parameter sensitivity characteristics of the silicon cavity. The structural design of the sensor seawater cavity includes a cross-shaped groove and a cylindrical fluid cavity. The surface hydrophilicity treatment was performed on the interior of the cavity to solve the effect of no water injection in the cavity caused by the miniaturization of the sensor. The optical path difference (OPD) demodulation method is used to demodulate the two F-P cavities with large dynamic range and high resolution. In the range of 5∼40°C and 5∼ 40 ‰, the temperature and salinity sensitivity of the sensor can reach 110.25 nm/°C and 178.75 nm/‰, respectively, and the resolution can reach 5.02 × 10−3°C and 0.0138‰. It has the advantages of mass production, high stability, and small size, which give it great potential for marine applications.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Seawater salinity is one of the most common physical parameters in marine hydrological elements. it is crucial to monitor the change of seawater salinity, which is closely related to climate change, marine ecology, fishery and industrial production [1]. Long-term and large-scale monitoring of sea surface salinity will contribute to the study of global climate change [2].
Currently, the most common methods of measuring seawater salinity include the conductivity method, microwave remote sensing, and optical measurement. The conductivity sensor obtains the salinity value by measuring the conductivity of seawater. Hyldgård et al. designed and fabricated a micro-CTD (conductivity-temperature-depth) system for seawater salinity measurement, which has an accuracy of 0.06S/m [3]. Although the CTD system has high measurement accuracy, it is susceptible to electromagnetic interference and expensive. Microwave remote sensing is based on the salinity sensitivity of sea surface brightness temperature (Tb) at microwave frequency [4]. This method can monitor the global ocean surface salinity change in real time and large area, but the accuracy is not high, and it is easily affected by other parameters of the ocean surface.
In recent years, optical fiber sensors have become a research hot-spot in the field of marine sensing due to their advantages of anti-electromagnetic interference, corrosion resistance, small size, easy multiplexing and long-distance transmission. At present, the research on optical fiber salinity sensors includes optical refraction method, fiber grating method [9–11], fiber coupler [12,13], surface plasmon resonance (SPR) [14,15], optical interference method (FPI, MZI, Sagnac) etc. The principle of optical refraction method to measure seawater salinity is based on the movement of the beam position caused by the change of refraction angle, which has high sensitivity but complex system [5–8]. Zhao et al. developed a long-period fiber grating (LPFG) seawater salinity sensor using bend-insensitive single mode fiber (SMF), which has a salinity sensitivity of 163.299pm/‰ [11]. Zhou et al. proposed an optical fiber salinity sensor based on an optic microfiber coupler interferometer (OMCI) with a salinity sensitivity of 303.7pm/‰ and a resolution of less than 0.03‰ [13]. Zhao et al. first proposed the use of integrated reflective optical fiber sensors to simultaneously detect salinity, temperature and pressure in seawater. The salinity, temperature, and pressure sensitivities of the sensor are 0.560 nm/g/kg, 1.802 nm/°C, and 2.838 nm/MPa, respectively [15]. Zheng et al. designed an optical fiber salinity sensor based on MZI, with a sensing structure consisting of an asymmetrically offset spliced fiber and a salinity sensitivity of -2.4473 nm/‰ [16]. In the next year, on the basis of the same structure, they plated a gold film on the end face of the fiber to enhance the reflected light, and used the FPI to monitor the temperature and salinity changes [17]. Aslam Mollah et al. designed a fiber salinity sensor based on Sagnac effect using photonic crystal fiber, and its sensitivity and resolution can reach 0.7 nm/% and 0.0133% [18]. However, the above sensors have high sensitivity. But, due to its low structural stability, the optical fiber is affected by processing technology, which makes the sensor have poor consistency and easy loss, and is still limited to laboratory measurement. Chen et al. proposed a Fabry-Perot (F-P) interferometric fiber optic cantilever sensor that can simultaneously measure acoustic pressure and temperature, and realized the dual-parameter measurement of the F-P structure [19]. Wang et al. designed an optical fiber composite F-P cavity refractive index based on MEMS technology, and measured the refractive index of gas and liquid respectively [20,21]. However, since seawater salinity is affected by many factors, its refractive index measurement standard and sensing structure are not suitable for seawater salinity measurement.
In this paper, we proposed an optical fiber salinity sensor with a composite F-P cavity structure. The sensor consists of three reflectors and measures the salinity of seawater by detecting the interference information of the reflection spectrum. The sensor includes a temperature cavity and a seawater cavity, which can realize simultaneous measurement of temperature and salinity. At the same time, in order to overcome the cross-sensitivity of seawater cavity caused by temperature change, the high temperature sensitivity of silicon cavity is used to compensate the temperature of seawater cavity. By using the OPD hybrid demodulation method, the two F-P cavities are demodulated with large dynamic range and high resolution. The experimental results show that the sensitivity of temperature and salinity can reach 110.25 nm/°C and 175.75 nm/‰, and the resolution of temperature and salinity can reach 5.02 × 10−3°C and 0.01386‰ within the range of 5∼40°C and 5∼40‰.
2. Sensor structure and its operating principles
Unlike conventional conductivity sensors, which measure the electrical conductivity of seawater to determine salinity, optical sensors primarily measure the refractive index of seawater to obtain salinity values. Based on the Lorentz-Lorenz relationship, the refractive index of seawater is directly related to its density, which varies with changes in salinity (S) and temperature (T). Additionally, the changes in wavelength (λ) also affect the refractive index of seawater. Therefore, Quan et al. proposed the relation between refractive index, temperature, salinity, and wavelength [22]. And the empirical equation for seawater refractive index is given below:
In this paper, the structure of the optical fiber seawater salinity sensor with MEMS composite F-P cavity structure proposed is shown in Fig. 1(a). The sensor consists of three parts: sensing chip, glass insert core and single-mode fiber. The sensor chip is connected by three layers in sequence, which are the bottom silicon layer, the open glass layer and the top silicon layer. The bottom silicon layer is polished on both sides, while the top silicon layer is polished only on the inner surface and rough on the outer surface. The single-mode fiber is fixed together with the center of the bottom silicon outer surface through the glass insert core for transmitting the optical signal.
In order to keep the seawater cavity of the sensor connected with the solution in the external environment. We have designed a variety of open cavity structures, including cross-shaped, X-shaped, fan-shaped and 1-shaped, and carried out water entry tests. The cross-shaped open structure is different from other structures. There are grooves in both longitudinal and transverse directions, and water can be in and out under different placement conditions. At the same time, the hydrophilic treatment of the material used in the seawater chamber can also enable the seawater to flow freely into the chamber when the sensor volume is small.
After the light is emitted from the single-mode fiber, it is reflected successively on the inner and outer surfaces of the bottom silicon and the inner surface of the top silicon. The outer surface of the top layer of silicon is so rough that light forms a diffuse reflection on the surface and its reflected light is very weak. The three reflection surfaces are recorded as reflection surfaces M1, M2 and M3, respectively. As shown in Fig. 1(b), the sensing structure consists of three cavities, one is a silicon cavity for temperature compensation (composed of M1 and M2, denoted as FP1, with cavity length L1); The other is an seawater cavity that can directly sense the refractive index of seawater (composed of M2 and M3, denoted as FP2, with cavity length L2); The long cavity is composed of the reflector M1 and M3, which is a combination of FP1 and FP2, and the cavity length is (L1 + L2). The light emitted by the single-mode fiber passes through the sensing chip to form a reflection spectrum, which carries the interference phase information of the silicon cavity and the seawater cavity and is collected by the single-mode fiber.
Due to the low reflectivity of the three reflectors, it can be regarded as a low-finesse F-P dual-beam interference. The reflection spectrum of the proposed structure is:
Among them, n1, n2, L1, L2 correspond to the refractive index and cavity length of silicon cavity and seawater cavity respectively. n10 is the initial refractive index of the silicon cavity, L10 and L20 are the initial cavity lengths of the silicon cavity and the seawater cavity, βsi is the thermo-optic coefficient of silicon, α1 and α2 are the thermal expansion coefficients of silicon and Pyrex glass, and T is the temperature. Therefore, the optical path difference between FP1 and FP2 can be expressed as:
Because the salinity of seawater has little effect on the refractive index n1 and length L1 of silicon. It is considered that the OPD1 of FP1 is only affected by temperature. And the thermo-optic effect and thermal expansion effect of silicon will affect the OPD1 of FP1. Therefore, ignoring the higher-order term, the temperature sensitivity of FP1 can be expressed as:
In the low temperature range, the thermo-optic coefficient of silicon can be considered as a constant. When the initial values of the relevant parameters are determined, the theoretical value of the temperature sensitivity of the sensor FP1 can be calculated. When T0 = 0°C, L10 = 300µm, βsi = 1.86 × 10−4K-1, α1 = 2.6 × 10−6K-1, n10 = 3.46456 is used, the calculated temperature sensitivity is Ssit =117.0047 nm/°C.
Similarly, the OPD2 of the seawater cavity (FP2) is affected by both temperature and seawater salinity. When the temperature changes, L2 will change due to the thermal expansion effect of the glass. And when the seawater is filled with FP2, the change of temperature and salinity will change the refractive index n2 of FP2. Therefore, the salinity and temperature sensitivity of the sensor FP2 after the high-order terms (T2 and T3) are removed can be expressed as:
Similarly, when T = 25°C, S = 30‰, substituting L10 = 500µm, α1 = 3.3 × 10−6K-1, the theoretical values of salinity sensitivity and temperature sensitivity of FP2 sensor can be calculated as follows: Ssa = 169.12 nm/‰, Ssat = -0.6045 nm/°C.
In addition, silicon is chosen as the temperature sensing material because of its high thermo-optical coefficient, which can obtain a high temperature measurement sensitivity. And silicon as a temperature sensitive element, there is no salinity cross-sensitivity phenomenon. Using FP1 for high-sensitivity temperature measurement can eliminate the cross-sensitivity of FP2 to temperature and salinity, which is helpful to achieve high-precision seawater salinity measurement.
3. Sensor processing and experiment
The machining method of the proposed optical fiber salinity sensor: Firstly, the glass microstructure of 500 µm thick Pyrex glass wafer (BOROFLOAT 33) was processed by spark-assisted chemical engraving (SACE) technology. The glass microstructure design of the sensor includes a cross-shaped groove (Width:1500 µm) and a cylindrical fluid cavity (Radius: 1250 µm). The advantages of SACE technology are small edge collapse, low stress, high precision and high verticality. It is very suitable for large-depth glass through-hole processing and can obtain a perfect open cavity structure. Next, the H2SO4:H2O2 solution was used to clean the surface of the double-sided polished silicon wafer, and then the surface was treated with hydrophilicity. The silicon wafer and the processed microstructured glass wafer (the surface is also treated with hydrophilicity) are placed in a bonding machine for anodic bonding. Then, the single-sided polished silicon is bonded to the top of the glass wafer in the same way. The thickness of both silicon wafers is 300 µm. Finally, the wafer after double-sided bonding is sliced according to the processing array interval by a dicing machine, and cut into multiple 5 mm × 5 mm sensing chips, as shown in Fig. 2(a). Figure 2(b) and (c) show the structure of the glass layer and the fabricated sensor chip, respectively. The center of the glass insert core is aligned with the center of the sensing chip, and then the optical fiber is inserted into the glass core and fixed with UV Light Adhesives to complete the encapsulation of the sensor, the fabricated sensor is shown in Fig. 2(d).
Fig. 2. (a) The wafer after slicing; (b) the structure of the glass layer; (c) the sensor chip; (d) the fabricated sensor.
In order to test the performance of the proposed optical fiber salinity sensor, a seawater salinity and temperature measurement system was built. The schematic diagram of the experimental system is shown in Fig. 3(a). The light is emitted from a broadband light source (SLD), and is incident into the optic fiber salinity sensor through a circulator. The reflected light returned by the sensor is received by the optical spectrum analyzer (OSA, AQ6370) through the circulator. Different concentrations of standard seawater produced by National Center of Ocean Standards and Metrology (NCOSM) were selected as the sensor test solution. At the same time, in order to obtain a stable experimental environment, a thermostatic water bath (SCIENTZ, GDH-2010) can be used to measure salinity at different stable temperatures, as shown in Fig. 3(b).
Fig. 3. (a) Schematic diagram of the experimental system; (b) salinity measurement environment system.
Figure 4(a) shows the reflection spectrum of the sensor. The demodulation method uses a hybrid demodulation method combining the fast Fourier transform (FFT) and single-peak tracing method, which can achieve large dynamic range and high-resolution demodulation of the OPD of the double F-P cavity. Figure 4(b) is the spectrum after performing FFT, from which it can be clearly seen that the OPD of different cavities corresponds to different peaks. The peaks of two different frequencies with abscissas 118 and 186 correspond to FP2 and FP1. Then, the independent interference spectra of FP1 and FP2 are separated by using different bandpass filters and IFFT. The insets in Fig. 4(b) represent the independent interference spectra of the two F-P cavities, respectively. Finally, by combining the single-peak tracking method, the accurate OPD of two F-P cavities are obtained respectively.
The test scheme of this experiment is as follows: five salinity points (5,20,30,35,40‰) and eight temperature points (5∼40°C, steps: 5°C) were selected for orthogonal temperature and salinity measurements. At the same time, the thermostatic water bath can ensure prolonged stability of seawater temperature.
The temperature and salinity responses of the seawater cavity (FP2) were obtained by demodulating and analyzing the collected spectra in the experiment. Figure 5(a) shows the salinity characteristics of FP2 at different temperatures. It can be seen that the salinity sensitivity of FP2 can reach 178.75 nm/‰, and the salinity sensitivity decreases as the temperature increases, which is consistent with Eq. (1). Figure 5(b) shows the temperature response curves of FP2 at different salinities. According to (1), the temperature response characteristic of FP2 is quadratic.
Combining Eq. (1) and Eq. (5), we proposed a calculation formula that reflects the relationship between the OPD2 of FP2 and temperature(T) and salinity(S). Eq. (8) is:
Then, the data points used in Fig. 5 are fitted by Eq. (8). After fitting, the sensitive characteristics of FP2 are shown in Fig. 6. It can be seen from the figure that the R2 of the fitted surface is 0.99985, indicating that Eq. (8) can well reflect the characteristics of FP2.
Fig. 5. Temperature and salinity characteristics of FP2. (a) Salinity characteristics at different temperatures. (b) Temperature response curves at different salinities.
Next, the temperature response characteristic of temperature cavity (FP1) is analyzed. Since silicon is not sensitive to salinity changes, FP1 will not respond to salinity changes. At the same time, because of the thermo-optical effect and thermal expansion effect of silicon, the temperature response characteristics of FP1 are quadratic. Similarly, we propose Eq. (9):
Figure 7(a) shows the temperature response curve of FP1. It can be seen from the figure that there are slight differences in the temperature response curves of heating and cooling. This is caused by the inevitable error in the experimental process. But the fitting curve is basically the same, and the difference of the fitting coefficient is very small. Thus, the temperature measurement data points are averaged and fitted to obtain the average temperature response curve of FP1 as shown in Fig. 7(b). Because the fitting curve is extremely close to linearity. Therefore, the temperature sensitivity of the proposed fiber optic sensor can be regarded as 110.25 nm/°C.
Fig. 7. Temperature response characteristic of FP1. (a) Temperature response of FP1 under heating and cooling. (b) Average temperature characteristics of the sensor.
In summary, the sensitivity of the sensor obtained by experiment is basically consistent with the theoretical value. From Eq. (8) and Eq. (9), the relationship between the OPD and temperature-salinity can be written as a matrix equation, expressed by Eq. (10). According to the Eq. (10), the measured OPD of two FP cavity can be used to derive the values of temperature and salinity. The measurement formula of temperature and salinity is expressed by Eq. (11):
To verify the performance of the sensor, the calibrated sensor measurement formula is verified. The temperature and salinity experiments were repeated for the sensor, and the sensor was placed in different temperature and salinity environments. The collected spectral demodulation results were substituted into the sensing equation to obtain the temperature and salinity measurements. The calculated temperature and salinity values are compared with the true values, and the results are shown in Table 1.
Table 1. Comparison of test results
From Table 1, we can see that the errors between the calculated values and the real values of temperature and salinity are less than 0.22°C and 0.43‰, respectively, and the average errors are 0.58% and 1.3%. The main reason for the salinity error is that when the sensor performs different salinity measurements, the solution of the last measurement may remain in the seawater cavity. And the residual solution is not sufficiently cleaned, thus affecting the measurement effect. The main reason for the temperature error is that the water bath may be not sufficient to make the temperature of the measured solution deviate from the temperature of the thermostatic water bath.
In order to verify the cause of the measurement deviation, the stability experiment of the sensor was carried out. The sensor was continuously tested at 25°C and 35‰ (Times: 200, Interval: 30s), the measurement results are shown in Fig. 8. Figure 8(a) shows the test stability of the sensor seawater cavity (FP2). The standard error of the continuous recording data of the sensor is less than 8.2348 × 10−4µm. At the same time, the demodulation value of the temperature cavity (FP1) is continuously recorded in Fig. 8(b), and the standard error is 1.84453 × 10−4µm. Considering the 3 times standard error as the resolution of the sensor, dividing the standard error by the sensor measurement sensitivity. The corresponding temperature resolution is 5.02 × 10−3°C, and the salinity resolution is 0.0138‰. This indicates that the sensor has high stability and resolution, and the above measurement deviation is caused by the test environment. And the performance of the proposed sensor can be further improved with the improvement of the test environment.
Fig. 8. Sensor stability test diagram. (a) Test stability of the seawater cavity (FP2). (b) Test stability of the temperature cavity (FP1).
Table 2 is a comparison of our proposed salinity sensor with other types of optical salinity sensors. Compared with other optical salinity sensors, our proposed sensor has higher sensitivity and resolution. The designed seawater cavity ensures that the sensor has sufficient measurement range and sensitivity while making the sensor have small size and strong enough reflected light. The use of silicon as a temperature sensitive material makes the sensor have high temperature sensitivity. The silicon cavity is introduced to compensate the temperature of the salinity measurement, and a more accurate salinity value can be obtained. Different from the direct use of optical fiber as the sensing material, the sensor FP cavity proposed in this paper can overcome the influence of insufficient free flow of seawater in the fiber cavity. And the external FP cavity has strong stability, corrosion-resistance and anti-environmental interference ability.
Table 2. Results compared with other sensors
4. Conclusion
In summary, we proposed a mass-produced fiber-optic reflective salinity sensor based on MEMS technology. The sensor is a double F-P structure with two sensing cavities. The silicon cavity is used for temperature sensing, and the seawater cavity is sensitive to the refractive index of seawater for salinity sensing. In the infrared band, the surface of the silicon layer used for reflection is polished, which can be used as an ideal reflective surface. At the same time, silicon is selected as a temperature-sensitive element to eliminate the cross-sensitivity of temperature-salinity, and a more accurate salinity value can be obtained. The OPD demodulation method realized the real-time measurement of large dynamic range and high resolution of temperature and salinity. The experimental results demonstrate that the salinity sensitivity can reach 178.75 nm/‰ in the range of 5∼40‰, and the temperature sensitivity can reach 110.25 nm/°C in the range of 5∼40°C. The resolution of the sensor is 5.02 × 10−3°C and 0.0138‰, respectively. The high salinity sensitivity and resolution of the proposed sensor make it have broad application prospects in ocean observation.
Funding
National Natural Science Foundation of China-Shandong Joint Fund (U2006216).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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