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Abstract
The monitoring of hydrological elements in the polar region is the basis for the study of the dynamic environment under the ice. The traditional cross-season subglacial hydrological environment monitoring mainly relies on tether-type vertical profile measurement ice-based buoys, which have the advantages such as high reliability, high measurement accuracy, and real-time communication, while also has disadvantages of high-cost, large volume and weight, high power consumption, and complex layout. Therefore, it is urgent to develop a new type of ice-based profile buoy with low-cost, miniaturization, low power consumption, convenient deployment, and high reliability. In this paper, a novel optical fiber sensing scheme for ice-based buoy monitoring is proposed, which uses arrayed fiber grating to measure seawater temperature and depth profile and uses a dual-conduction mode resonance mechanism to measure seawater salinity. The temperature, depth, and salinity of seawater can be detected by an all-optical fiber technology in real-time. Preliminary experiments show that the temperature accuracy is ±0.1 °C in the range of -5∼35 °C, the salinity accuracy is ±0.03‰ in the range of 30‰∼40‰, and the vertical spatial resolution of depth can be adjusted in the range of 0∼1000 m, which can better meet the requirements of polar hydrological multi-layer profile observation. It can provide an innovative technology and equipment support for studying the spatiotemporal change process of the polar subglacial ocean.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The north and south poles of the Earth are the cold sources of the Earth system, and their changes involve the interactions of ice caps/ice shelves, sea ice, oceans, and atmosphere, and so on [1–3]. In-depth research on the polar environment is of great significance for solving the polar environmental crisis caused by global warming [4,5]. It also plays a vital role in monitoring the impact of polar regions on global climate change. The monitoring of hydrological elements (such as seawater temperature, depth, and salinity) in the polar region is the basis for the study of the dynamic environment under the ice [6,7].
At present, the observation methods of the polar ocean are mainly divided into remote sensing methods [8,9] and in-situ observation methods [10,11]. The remote sensing methods can mainly provide surface information by super-width visual images obtained by satellite radar. Compared with remote sensing methods, the in-situ observation methods can obtain vertical profile data which is mixed-layer and multi-faceted. The north pole environmental observatory (NPEO) is one typical in-situ observation campaign, which was established in 2000 as a means of tracking change in the central Arctic Basin [10]. Due to the need for large-scale and long-term seawater monitoring, in-situ observation methods have certain limitations and require a high level of staff and logistical support. Using ice-based buoys as the monitoring carrier, the buoy monitoring method does not require on-site personnel and can achieve long-term and uninterrupted observation, which has the advantages of low cost, long observation time, and more flexible observation location [12–14]. The first buoy to measure the sea under the ice was developed by Burke and Morison in 1986 [15]. Then, various types of polar ocean buoys have been developed and successfully applied to polar ocean observation. CALIB (Compact Air Launched Ice Beacon) buoys [16] can be dropped from aircraft and are equipped with sensors such as ice temperature detection sensors and barometric pressure detection sensors. The ice cable profiler ITP (Ice-Tethered Profiler) [17] developed by the Woods Hole Oceanographic Institution in the United States and the polar ocean profiling system POPS (Polar Ocean Profiling System) [18] made in Japan are effective loading platforms for polar sea ice, which can realize the collection of polar air temperature, sea ice temperature, atmospheric pressure, and upper marine environment data. The Ice Mass Balance Buoy (IMB) [19], developed by METOCEAN and CRREL (Cold Regions Research and Engineering Laboratory), enables the measurement of sea ice thickness and temperature. The other low-cost salinity sensors such as smartphone-based salinity sensors are based on Beer- Lambert principle or evanescent field absorption [20]. Those salinity sensors have high transmission losses which is not suitable for long-distance detection. More importantly, it is difficult to integrate those salinity sensors with the existing temperature and depth sensors. Therefore, they are not useful in hydrological applications.
The subglacial profile refers to the vertical distribution of different depths from tens of meters to several kilometers below sea ice. The polar subglacial profile observation of the temperature, salinity, and depth can provide an important basis for the construction of polar scientific research stations and the exploitation of subglacial lake resources in Antarctica. And the reliable and continuous subglacial temperature salt depth parameters and their change information can be obtained. It has become a hot research issue. Compared with traditional electrochemical sensors, optical fiber sensors are passive components and they have some advantages, such as multiplexing capability, anti-electromagnetic interference, resistance to corrosion, miniaturized probe structure [21–25]. Many optical fiber sensors have been used for polar sea ice monitoring. R. Law et al [26] from the University of Cambridge used optical fiber distributed temperature sensing (DTS) [27] on the Greenland ice sheet to obtain a detailed temperature profile, which can measure the temperature of the ice sheet for nearly one kilometer with no need for multiple sensors. A. D. Booth et al. [28] installed optical fiber distributed acoustic sensing (DAS) and DTS in glacier boreholes to monitor the rapid flow of the Greenland ice sheet and the seismic characteristics of subglacial conditions.
To overcome the shortcomings of traditional ice-based buoys, the ice-based observation buoy scheme based on optical fiber sensing technology is proposed, which uses arrayed fiber grating to measure seawater temperature and depth profile and uses a dual-conduction mode resonance mechanism to measure seawater salinity. The fiber grating sensor for seawater temperature and depth and the fiber sensor for seawater salinity have been fabricated and calibrated. The nearshore test data of the fiber-arranged sensor proved the feasibility of the scheme. We can observe the changes in temperature, depth, and salinity of the polar subglacial ocean using the proposed fiber observation system in vertical direction at different time. All those results can provide equipment and data support for studying the spatiotemporal change process of the polar subglacial ocean.
2. Sensing principle and characteristic experiment of fiber grating sensor for seawater temperature and depth
2.1 Fiber grating sensing principle and structure design
Fiber Bragg grating (FBG) is a structure that periodically varies along the refractive index of a fiber core. When light is incident into the FBG, the grating exhibits its wavelength-selective reflective function and reflects light of a specific wavelength. Based on the coupling mode theory, FBG can couple one of the transmitted guide modes to another guide mode transmitted in the opposite direction to form a narrowband reflection, and the Bragg reflection wavelength is λB [22],
where neff is the effective refractive index of the fiber core, and Λ is the period of the grating. When the temperature or stress of FBG changes, it will lead to changes in the refractive index of the fiber core and grating period, so that the FBG center wavelength will shift. The wavelength shift is related to temperature variation and strain variation [21],The designed fiber grating sensor for seawater temperature and depth is shown in Fig. 1, including two fiber gratings (FBG1 and FBG2), an elastic diaphragm, a pressure shell, two metal tubes, and a metal tab. FBG1 is used to measure the depth, and FBG2 is used to monitor the temperature. FBG1 is linked to a pressure shell and an elastic diaphragm via two metal tubes, which is affected by both depth and temperature. FBG2 is fixed in a metal tab outside the pressure shell, which is immune to pressure. FBG1 is pre-stretched before packing. The elastic diaphragm is welded with the surface of the pressure shell, and the external pressure directly acts on the metal diaphragm. The deformation of the center of the metal diaphragm presses the FBG1 and causes the center wavelength of FBG1 to shift. The water molecules have a great influence on long-period FBGs according to the Ref. [29]. The water-repellent pastes are used in the fabrication of temperature and salinity sensors to isolate water molecules.
Fig. 1. Structure design of fiber grating sensor for seawater temperature and depth. (a) Schematic diagram of fiber grating sensor (b) Elastic diaphragm (c) Pressure shell (d) Metal tab (e) Three-dimensional model
2.2 Fiber grating seawater temperature sensing characteristic experiments
The experimental setup of the fiber grating temperature and depth sensor for temperature sensing is shown in Fig. 2. The light emitted from the fiber grating interrogator enters the sensor structure along the single-mode optical fiber and then is reflected to the interrogator via FBGs in the sensor, and the reflection spectrum of the sensor is collected and recorded by the computer. The grade I standard platinum resistance thermometer is fixed together with the fiber grating seawater sensor and placed in the constant temperature sink. The temperature of the constant temperature sink is reduced from 35 °C to -5 °C. In order to ensure the accuracy of the temperature measurement data, the spectra were recorded after maintaining a constant temperature for at least 10 min at every temperature setpoint.
The center wavelengths of FBG1 and FBG2 shift linearly with the decrease in temperature. The temperature sensitivity of FBG1 and FBG2 can be obtained from the slopes of the fitting curve in Fig. 3, KT1 = 28.62 pm/°C and KT2 = 16.45 pm/°C with linearity of 0.9998 in the temperature range of -5∼35 °C.
The temperature errors of fiber grating temperature and depth sensor are tested and the results of FBG2 are shown in Table 1. To further analyze the above measurement data, the measured data are plotted in Fig. 4. Comparing the measurement results obtained by the FBG2 with those obtained by the standard platinum resistance, it can be seen that the maximum measurement error between FBG2 and the standard platinum resistance measurements in the range of -5 to 35°C is only 0.1 °C. There is a very good linearity between the measured temperature and the set temperature in the range of -5∼35 °C, and the temperature linearity reaches 99.998%, so the optical fiber sensor can achieve high-precision temperature measurement in the range of -5∼35 °C. The tested errors of FBG1 are close to those of FBG2.
Table 1. Temperature test results of the fiber sensor
2.3 Fiber grating seawater depth (pressure) sensing characteristic experiments
The experimental setup of the fiber grating temperature and depth sensor for pressure sensing is shown in Fig. 5. The sensor was placed on the quick connector of the piston manometer and fixed, and the pressure of the manometer was adjusted with intervals of 2 MPa reducing from 10 MPa to 0 MPa. In order to ensure the accuracy of the pressure measurement data, the spectra are recorded after staying at each pressure setpoint for at least 10 minutes. The pressure sensitivity of FBG1 KP = -5.735 nm/MPa is obtained by linearly fitting, as shown in Fig. 6. The pressure of the ocean can be converted into marine depth based on the empirical formula [30], and the equivalent depth sensitivity of FBG1 is KP = -5.735 × 10−2 nm/m.
Fig. 5. Experimental device diagram of fiber Bragg grating seawater pressure sensing characteristics
The pressure errors of fiber grating temperature and depth sensor are tested and the tested results are shown in Table 2. Comparing the pressure tank display value and the optical fiber pressure sensor measurement value, the maximum between the optical fiber pressure sensor measurement value and the pressure tank display value is only 0.1 MPa in the range of 0∼10 MPa. In order to further analyze the above measurement data, the pressure tank display value and the optical fiber pressure sensor measurement value in the pressure sensing characteristic experiment are plotted in Fig. 7. In the range of 0 to 10 MPa, the optical fiber pressure sensor has very good linearity between the pressure measurement and the pressure setpoint, and the linearity of the pressure test value reaches 99.945%, so the optical fiber pressure sensor can achieve high-precision pressure measurement in the range of 0 ∼1000 m and the vertical spatial resolution of depth can be adjusted.
Fig. 7. The relationship between the measurement pressure values of the optical fiber pressure sensor (FBG1) and pressure tank and the set pressure
Table 2. Pressure test results of the fiber sensor
2.4 Extremely low temperature environment adaptability test
The fiber grating temperature and depth sensor was placed in a temperature and humidity test chamber and connected to an interrogator for sub-zero temperature testing, and the experimental setup is shown in Fig. 8(a). The temperature range is -50∼0 °C with an interval of 10 °C. In order to ensure that the experimental data were real and effective, the data was recorded after the temperature was stable for 10 min each time. The center wavelength of the reflection spectrum was recorded by computer, and the test data obtained are shown in Fig. 8(b). The tested data were linearly fitted, and the sub-zero temperature sensitivity of FBG1 and FBG2 are 28.33 pm/°C and 16.29 pm/°C, respectively. The measurement errors of FBG1 and FBG2 in extremely low temperature are similar to those obtained in section 2.2.
Fig. 8. Extremely low temperature experiment test for fiber grating temperature and depth sensor (a) experimental setup (b) test results
3. Sensing principle and characteristic experiment of fiber sensor for seawater salinity
3.1 Dual-conduction mode seawater salinity sensing principle and fabrication
The interaction between local electric fields in microstructures and sensing media is a highly sensitive detection method. Although local surface plasmon resonance sensors based on metal nanostructures have been widely used in refractive index sensing, the inherent losses of these sensors can reduce their performance. In dielectric material microstructure sensors, all-dielectric sensors based on the guided mode resonance effect have attracted great interest due to their simple structure and no need for separate coupling units. Using the dual-conductive mode resonance, the relationship between the resonance wavelength drift and the change of salinity and temperature is expressed as [31,32],
The fiber seawater salinity sensor is fabricated at the end face of the fiber using electron beam lithography (EBL) and the fabrication process diagram is shown in Fig. 9. At first, the premise of fabrication is polishing the fiber facet to a flat and smooth surface. Then, a 400-nm-thick SiO2 film together with a 200-nm-thick negative photoresist is placed on the fiber surface by vapor deposition and spin coating. Nanodisks are written into the photoresist by EBL. Subsequently, the remaining SiO2 (except for the nanodisks) is removed by ion-beam etching. The photoresist residues are chemically removed. At last, an additional TiO2 film is deposited on the surface by Magnetron Sputtering deposition. After this step, the salinity sensor will be obtained.
3.2 Fiber seawater salinity sensing characteristic experiments
The salinity sensitivity can be measured by immersing the sensor into a prepared salt solution sample and carefully changing the salinity. The seawater adopted in this experiment is produced by the National Center of Ocean Standards and Metrology. The laboratory standard saltwater calibration test results are shown in Fig. 10. As shown in Fig. 10(a), the electrical SBE37SI CTD indicates about 19.5‰ while the optical salinity sensor indicates 20‰. The two results are close to each other in the standard saltwater environment. The reflection peaks measured with different salinities are shown in Fig. 10(b). It can be seen that the reflection peak moves towards the longer wavelengths linearly as the salinity increases. The optical fiber salinity sensor has a very good linearity between the salinity reflection peak and the saltwater, with a linearity of 99.95%, so the optical fiber salinity sensor can achieve high-precision salinity measurement in the range of 30‰∼40‰ with an accuracy of ±0.03‰.
Fig. 10. Standard saltwater test in the laboratory (a) experiment picture, (b) test results for standard saltwater
After the laboratory calibration was completed, the external field test was carried out at the barge in 120°32′E and 36°32′N. Firstly, the fiber salinity sensor was immersed in the seawater by fixing it through a stainless basket. Then the electrical SBE37SI CTD (Sea-bird Scientific) was put into the same position for comparison, as shown in Fig. 11(a). The measurement results of the optical fiber salinity sensor and the electrical SBE37SI CTD are shown in Fig. 11(b). The overall change trend is consistent, though the time period corresponding to the change trend of the two is slightly different. The salinity measurement deviation of the optical fiber sensor and electrical SBE37SI CTD can be explained as follows. On the one hand, the wider spectral salinity reflection peaks lead to jitter when the interrogator captures the resonance peaks. This can be further reduced by optimizing the structural parameters to reduce the width of the reflection peaks. On the other hand, the front end of the sensor swings seriously with seawater after the fiber salinity sensor is immersed in the seawater. The impact of oscillation can be reduced by optimizing the packaging process and applying the fixed structure. In addition, the complexity of nearshore water quality can affect the accuracy of optical salinity measurements.
Fig. 11. Comparison between optical fiber salinity sensor and electrical SBE37SI CTD (a) external field test picture, (b) result comparison
4. Fiber sensing scheme for multilayer hydrographic profile observations and preliminary sea trials
The multilayer hydrological profile observation system based on optical fiber sensing technology mainly includes three parts, fiber grating temperature and depth sensing unit, data acquisition unit, and display control unit, as shown in Fig. 12. The fiber grating temperature and depth sensing unit is the core of the whole system, which consists of multiple temperature and depth sensors uniformly arranged in the depth profile (multiple numbers can be arranged according to the depth resolution).
Fig. 12. Schematic diagram of optical fiber sensors for multilayer hydrological profile observation and the picture of seawater measurement experiment. The blue arrow is the incident light, and the red arrow is the reflected light.
As shown in Fig. 13, four temperature and depth sensors (TD1∼TD4) were uniformly arranged in 8 m water depth at the shore experiment station for the preliminary sea trial, and every sensor was arranged at an interval of 2 m in the depth range of 2 to 8 m on the depth profile, as shown in Fig. 8. The 6-day temperature and depth data were monitored and compared with the data acquired by the electrical SBE37SI CTD sensor. The measurement results at different positions of the arranged sensors are shown in Fig. 9. The temperature and depth measurement parameters of the sensors at the positions of 2 m and 8 m are close to the actual position. The depth information of the two sensors at the 2 m and the 8 m is basically consistent with the actual position, and the temperature information at the two positions is less than 2 °C with the electrical SBE37SI CTD. The temperature and depth parameters of the sensor at the 4 m and 6 m are seriously drifted, mainly because the sensor space here is limited and the bending radius of some fibers is small during the measurement process, which results in the sensor deviating from the laboratory calibration value. Therefore, the subsequent optimization of the packaging method to avoid this situation. Although some position measurements of this arranged sensor have a large deviation from the actual situation, the temperature and depth information of the two sensors at the 2 m and the 8 m are basically consistent with the actual situation, which proves the feasibility of the multilayer hydrological profile observation scheme based on optical fiber sensing technology. It is expected that the error will be further reduced by improving the process and optimizing the structure, and the high-precision measurement of the multilayer hydrological temperature and depth profile can be realized.
5. Integration design of optical fiber sensors
The temperature and depth sensor based on fiber grating and the salinity sensor based on dual-conduction mode resonance mechanism are integrated into one system, as shown in Fig. 14(a). A 3 dB fiber coupler is used to connect temperature and depth sensor and salinity sensor. The light emitted by the light source is divided into two beams after passing through the fiber coupler. One beam is reflected after passing through two optical fiber gratings, and the other beam is reflected after passing through the TiO2 film. After passing through the fiber coupler again, the two beams are combined into one and transmitted to the signal interrogator system. The reflectance spectrum of the integrated system is shown in Fig. 14(b). The three reflectance peaks from left to right come from the salinity sensor, the temperature sensor, and the depth sensor. The salinity, temperature, and depth of seawater can be detected by the shift of the three reflectance peaks.
Fig. 14. Integration of three optical fiber sensors (a) schematic diagram of the integrated design (b) reflectance spectrum. The blue arrow is the incident light, and the red arrow is the reflected light.
6. Conclusion and outlook
This paper proposes the hydrological profile observation scheme based on optical fiber sensing for polar ice-based buoy monitoring. The observation scheme is based on the seawater temperature and depth profile measurements by cascaded fiber grating temperature and pressure sensors and seawater salinity measurements by integrated all-dielectric nanostructure using dual-conduction mode resonance. The fiber grating sensor for seawater temperature and depth, and the fiber sensor for seawater salinity have been designed, and good linearity has been achieved by calibration. The nearshore test results of the fiber-arranged sensor proved the feasibility of the scheme. It is expected to further reduce the error by improving the fabrication process and optimizing the structure to achieve high-precision measurement. We can observe the changes in temperature, depth, and salinity of the polar subglacial ocean using the proposed fiber observation system in the vertical direction at different times. All those results can provide an innovative technology and equipment support for studying the spatiotemporal change process of the polar subglacial ocean.
Nevertheless, we must recognize that there are still some challenges to be addressed before putting the proposed technology into the application system, as discussed below. Firstly, the preliminary integration between the temperature and depth sensor and the salinity sensor is accomplished just by a fiber optic welded connection at present. A special integration design is required to meet the needs of polar sea ice testing in further research. Secondly, the proposed seawater salinity sensor is more suitable for the sea with better water quality. There are more impurities in the offshore sea, which may have a certain impact on the optical signal. Thirdly, both the water molecules and low-temperature environment may have an influence on the FBG sensor, so capsulation material pastes and capsulation material need to be waterproof and low-temperature resistant. The hydrological profile observation system based on optical fiber sensing with the advantages of low cost, miniaturization, and low power consumption can be developed and applied in the future, which may be an alternative or supplementary equipment for traditional polar ice-based hydrological profile buoy.
Funding
Key R&D Program of Shandong Province, China (2023ZLYS01); National Key Research and Development Program of China (2022YFC3104203); Science and Technology Innovation Project of Laoshan Laboratory (Qingdao) (LSKJ202204703); National Natural Science Foundation of China (61933004); Taishan Scholars Project Special Fund (ts20190951); Natural Science Foundation of Shandong Province (ZR2022QF086); Major Innovation Special Project of Qilu University of Technology (Shandong Academy of Sciences) Science Education Industry Integration Pilot Project (2023HYZX01); Postdoctoral Funded Project Qingdao City, Shandong Province (QDBSH20230102005).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this study are not publicly available at this time but may be obtained from the authors upon a reasonable request.
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