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Abstract
The tilted fiber Bragg grating(TFBG), chirped fiber Bragg grating(CFBG), Vernier effect and metal surface plasmon resonance(SPR) effect are effectively combined to form a probe type fiber sensor for simultaneous measurement of seawater salinity, temperature and depth(STD). The SPR effect excited by the TFBG is achieved by covering a gold layer around the TFBG, which is used to measure the refractive index (RI) of seawater. The core mode of TFBG is used to detect the change of seawater temperature and the measurement of TFBG reflection spectrum is realized by inscribing a CFBG after the TFBG, which makes the sensor have a probe type design and more beneficial to practical applications. The fusion of quartz micro-spheres on the end face of the sensing fiber and the parallel connection of an Fabry Perot(F-P) interference cavity enables the use of Vernier effect to detect the depth of the ocean. Femtosecond laser line-by-line method is used to the inscribing of TFBG, which allows the grating parameters to be changed flexibly depending on the desired spectrum. The experimental results show that the temperature sensitivity is 10.82pm/°C, the salinity sensitivity is 0.122nm/g/Kg, the depth sensitivity is 116.85 pm/m and the depth can be tested to 1000 m or even deeper.
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1. Introduction
It is well known that the ocean occupies about $71 \%$ of the earth’s total area. The utilization and exploitation of ocean resources and ocean military have been paid more and more attention by all countries in the world. Improving ocean cognitive ability, researching and developing sensor technology and detection equipment for ocean observation are important contents of building a ocean power and realizing the strategy of "transparent ocean". At the same time, ecological and global climate changes are increasingly related to the ocean. The STD of seawater are the most critical and basic physical elements necessary for all ocean disciplines. Using sensors to obtain the information of seawater STD profile has important theoretical value and practical significance for studying oceanography, monitoring ocean ecological environment, developing and utilizing ocean resources, and ensuring military security [1,2,3,4].
Traditional ocean STD detection methods are divided into three types: ship-based direct reading measurement, discarding measurement and buoy-based measurement, respectively [5]. Although the first two methods are characterized by high spatial resolution and simple usage, they also face the disadvantages of low measurement efficiency, short measurement timeliness and the inability to reuse. It is also difficult to achieve ease of use, reusability and simultaneous measurement of multiple parameters of STD. The third measurement method can achieve long-term and continuous monitoring due to the flexibility of the buoy, on which the sensor is mounted and the purpose of this paper is to realize the long-term synchronous detection of STD in ocean by combiningfiber sensor and buoy based on the offshore buoy platform. However, at present, many research institutions and scholars have studied the measurement of STD based onfiber sensors, but most of them cannot achieve the simultaneous measurement of three parameters, or even if the simultaneous measurement of three parameters is achieved, the range and sensitivity of depth measurement cannot be guaranteed in a large measurement range and high test sensitivity at the same time. In 2016, Ricardo Oliveira et al. developed a fiber sensor capable of measuring strain, temperature, and RI simultaneously based on a combination of multiple sensing structures. Although the sensor is simple to make, its taper part was prone to fracture, and it is difficult to achieve long-term measurement [6]. Simon et al. in 2017 made a fiber sensor with a length less than $3.4 mm$ and can be used to measure thermal conductivity, pressure, RI, and temperature simultaneously through a series of splicing, cutting, and etching operations. However, it is difficult to fabricate and requires rational structural design and different fabrication methods to form cavities of different sizes to avoid crosstalk [7]. Zhang et al. proposed a miniature fiber sensing structure based on M-Z interference in 2018, and the experimental results demonstrated a RI sensitivity of $131.93 nm/RIU$, a strain sensitivity of $0.0007 nm/\mu \varepsilon$, and a temperature sensitivity of $0.0878 nm/^{\circ }C$. However, the sensor fabrication process involves complex processes such as shearing, splicing and tapering and its strain sensitivity is low, which is not suitable for detection in the subsea high pressure environment [8].
In this work, we propose and experimentally demonstrate a reliable and efficient sensor for the simultaneous measurement of seawater STD, which combines microcavity-assisted Vernier effect, TFBG-excited SPR effect and CFBG for reflection spectrum detection. The quartz spherical cavity of the sensor is finely processed by an engineered $CO_{2}$ laser, TFBG is manufactured by a femtosecond laser grating inscribing system, and the sputtering of metal films is also sputtered by a mature magnetron sputtering system. These mature manufacturing technologies ensure that the sensors proposed in this work have the consistency of manufacturing and the ability of mass production. It is expected to be widely used in the field of seawater STD detection.
2. Fabrication and principle
The femtosecond laser with a peak power of $2.8 W$, pulse duration of $290 fs$, and second harmonic wavelength of $515 nm$ is used to inscribe TFBG with an angle of $8 ^\circ$ by the line by line method, which has higher flexibility in manufacturing gratings. As shown in Fig. 1 that the sensor is composed of three sensitive units, namely, a F-P cavity seawater depth sensor using the Vernier effect generated by micro-spheres, a TFBG seawater RI and temperature sensor based on SPR effect and a commercial CFBG for probe-based sensing functions.
For the realization of the Vernier effect used to the measurement of ocean pressure(equivalent to depth), the capillary tube fused to two fibers to form the F-P cavity is used as the reference arm, while the sensing arm is in the form of quartz micro-spheres fused to the end face of the fiber. Compared with the traditional F-P cavity, which is composed of two optical fiber ends and a glass tube, the microsphere cavity is more conducive to measuring the ocean pressure due to its spherical thin wall, so as to achieve accurate measurement of the ocean depth.
Since the spectrum of TFBG is usually required to detect the transmission spectrum, but to make the sensor easier to use, CFBG with large bandwidth and high reflection characteristics is used as a reflector to realize the sensor fabrication with probe type sensing function. Through the reasonable design of grating period, the spectrum (comb band) of TFBG and the reflection bandwidth of CFBG are matched to enable the detection of the reflection spectrum of the TFBG. And a 30 $nm$ thick gold film was uniformly coated on the outer surface of the TFBG portion of the fiber by magnetron-sputtering to successfully excite the SPR effect, which was used to detect the salinity of the seawater [9]. As can be seen in Fig. 2 that due to the introduction of CFBG, the F-P interference spectrum based on the Vernier effect and the spectrum based on the TFBG-excited SPR effect are both integrated in the reflection spectrum as mentioned in previous reports [10]. Through reasonable grating period design, the cladding mode of TFBG is located in the reflection band of the CFBG, so that the excited SPR spectrum used to detect the RI change of the seawater environment can be detected by the reflection spectrum, at the same time, the core mode of TFBG is used to detect temperature changes in the ocean environment.
The effective index of the $i$th cladding mode $n^{i}_{clad}$ of TFBG, can be calculated from the resonance position $\lambda ^{i}_{clad}$ by [11]:
where $n_{core}$ is the the effective index of the core mode at $\lambda ^{i} _{clad}$ and $\Lambda$ and $\theta$ are the period and the tilt angle of the grating fringes of the TFBG. The period of the TFBG used in this experiment is $1.07 um$, the length is $2 mm$, and the tilt angle is $8^\circ$.The cladding mode excited by TFBG will incident to the metal coated area in the form of evanescent wave with a specific incidence angle when the nanoscale gold film is sputtered on the entire circumference of TFBG. When the evanescent wave vector of the cladding matches the Surface Plasmons(SPs) wave vector formed by the electron oscillation on the metal surface, the phase matching condition of SPR effect is satisfied. The propagation constant $\beta _{SPP}$ of SPs expresses as:
3. Sensing performance of the sensor probe for seawater temperature, salinity and depth
For the measurement of ocean depth, it can be detected by the wavelength shift of the spectral envelope of the Vernier effect. The spectral envelope can be calculated according to the following equation [13]:
However, when the spherical sensing arm is used for pressure measurement, the spherical cavity sensing arm will create a new Vernier effect due to the reflection of the inner and outer surfaces of the spherical cavity caused by the wall thickness, which will affect the accuracy of the pressure measurement as shown in Fig. 3(above). In order to eliminate the additional Vernier effect, femtosecond laser was used to roughen(roughening depth is about 2 $\mu m$) the outer surface of the spherical cavity to effectively reduce its reflectivity, thereby eliminating the Vernier effect caused by the wall thickness of the spherical cavity as shown in Fig. 3(below). As can be seen from the spectrum in Fig. 3, after femtosecond laser roughening of the outer surface of the microsphere cavity, the Vernier effect caused by its wall thickness has been eliminated. As can be seen in Fig. 4 that the design of the microsphere cavity ensures that the sensor achieves a balance between the depth detection range and the depth detection sensitivity of up to 116.85$pm/m$ while maintaining the detection depth of $0-1km(0-10Mpa)$. In fact, the theoretical detection depth should be able to reach $2 km$, but for laboratory safety, the maximum pressure test was only pressurized to $10 MPa$. Compared to single F-P cavity sensors, dual F-P cavity sensors with Vernier effect increase pressure sensitivity by approximately $24.91$ times.
Fig. 3. Micrograph of quartz microsphere and interference spectra of single microcavity. The outer surface of the microsphere was not roughed (above) and was roughed by femtosecond laser (below).
Fig. 4. Reflectance spectrum of pressure detection based on Vernier effect of parallel integration of dual F-P cavities and fitting curve of pressure sensitivity.
Seawater salinity is an important parameter that plays a key role in ocean dynamics and ocean atmosphere interaction. The change of salinity has a strong internal relationship with the ocean environment and its subsequent changes. Therefore, the accurate measurement of seawater salinity is of great practical value to the research of oceanography, climate monitoring and military affairs. For the measurement of seawater salinity and temperature, a probe type design of the sensor was realized using TFBG and CFBG as shown in Fig. 5. Magnetron sputtering technology (DENTON Explorer, USA) was used to sputter a gold layer ($30nm$) around the TFBG to measure the seawater salinity using the SPR effect. At the same time, the temperature measurement is realized by using the reflection spectrum of the core mode of TFBG.
Fig. 5. Influence of the polarization state of the incident light on TFBG-SPR spectra and the reflectance spectrum of salinity detection based on TFBG-excited SPR effect.
It is well known that TFBG and SPR effect are sensitive to polarization, so it is very important to explore the influence of polarization state on spectrum. As one can see from Fig. 5(a) that TFBG-excited SPR effect shows a stronger sensitivity to P-polarized light than S-polarized light. Fig. 5(b) shows the TFBG transmission spectrum converted to reflection spectrum by CFBG. The solid black line represents the reflection spectrum based on CFBG, and within the reflection wavelength bandwidth of CFBG is the reflection spectrum of part of the comb band of TFBG. For visualization, the red dotted line represents the complete transmission spectrum of TFBG, which does not actually exist in the actual measured spectrum. And the core mode of the TFBG ($1550 nm$) used to measure the seawater temperature is not in the reflection band of the CFBG, but is reflected by the TFBG at the Bragg wavelength. According to the test of NaCl solution with different concentrations, the relationship between the RI and concentration of NaCl solution is as $R_{NaCl}=0.000176*C_{salinity}+1.332$, where $C_{salinity}$ is the concentration of NaCl solution and $R_{NaCl}$ is the RI of NaCl solution.
The sensing probe was sequentially immersed in solutions with different NaCl concentrations in normal temperature and pressure, which allowed the sensing region to detect different RI. As shown in Fig. 6(a), the SPR effect was excited by the TFBG due to the sputtering of the gold film and the resonance wavelength is red-shifted with the increase of the RI. It can be seen in Fig. 6(b) that when the RI increases from 1.332 to 1.341, the RI detection fitting curve of the sensing probe shows good linearity with $R^{2}=0.998$. The sensitivity of RI measurement can be as high as $692.81nm/RIU$ and the corresponding salinity sensitivity is $0.122nm/g/Kg$.
Fig. 6. Salinity detection reflection spectrum based on SPR effect excited by TFBG and fitting curve of salinity sensitivity.
For the measurement of seawater temperature, the core mode of TFBG is used for temperature sensing. By placing the proposed sensing probe in a water bath calibrated with a thermocouple thermometer and varying the temperature from $4^{\circ }$C to $40^{\circ }$C, the temperature response of the sensing probe can be seen as shown in Fig. 7(a). Also, the sensitivity fitting curve in Fig. 7(b) shows that the temperature sensitivity of the sensor exhibits good linearity with $R^{2}=0.99968$ and its sensitivity can be as high as $10.82pm /^{\circ }$C.
Fig. 7. Bragg wavelength variation of TFBG-based core mode at different seawater temperatures and the sensitivity fitting curve of temperature detection.
4. Robustness analysis of sensor to changes of STD
However, as Prof Shao mentioned, temperature changes will modify the RI of the solution in which the fiber is immersed and the permittivity of the gold coating, which will inevitably cause the influence of SPR resonance wavelength, thus weakening the robustness of the sensor [14]. Therefore, the corresponding experiments were also done for the effect of temperature on the salinity detection accuracy of SPR effect. As can be seen in Fig. 8 that as the temperature increases from $16.1^{\circ }C$ to $40.2^{\circ }C$, the resonant wavelength increases from $1491.16 nm$ to $1491.66 nm$ corresponds to a temperature sensitivity of $20.7pm/^{\circ }C$. Therefore, for salinity measurement of the proposed sensor, the temperature sensitivity of the core mode of TFBG should be used for self calibration of salinity measurement error caused by seawater temperature change.
At the same time, in order to illustrate that the sensor is robust to simultaneous measurement of STD, the influence of temperature changes on pressure measurement has also been studied. As can be seen in Fig. 9 that as the temperature increases from $4^{\circ }C$ to $40^{\circ }C$, the spectral envelope of Vernier effect increases from $1125.62 nm$ to $1128.04nm$ corresponds to a temperature sensitivity of $67.2pm/^{\circ }C$. Similarly, for the depth measurement of the proposed sensor, the temperature sensitivity of the core mode of TFBG can also be used for self calibration of the depth measurement error due to the seawater temperature variation. It is clear that changes in the ambient RI(salinity) do not affect the pressure measurement of the sensor. And in the future, the grating area of the sensor will be packaged in a stainless steel tube with seawater entry and exit microchannels, thus eliminating the influence of pressure or seawater flow on the sensor temperature and salinity detection.
Fig. 8. The loss peak of SPR at different temperatures and wavelength shift of SPR effect with temperature changes.
Fig. 9. Envelope of Vernier effect variation at different ambient temperatures and the sensitivity fitting of pressure measurement with temperature change.
By combining the Vernier effect formed by the microsphere cavity, the SPR effect excited by TFBG, and CFBG, a probe type fiber sensor is formed with a pressure sensitivity of $116.85pm/m$, a salinity sensitivity of $0.122nm/g/Kg$ and a temperature sensitivity of $10.82$ $pm$/$^{\circ }$C for simultaneous measurement of seawater STD parameters. Compared with other types of STD sensors, the proposed sensor has very good advantages in terms of sensitivity and simultaneous detection of the STD parameters.
Table 1 shows the performance comparison of the proposed sensors of the same type. As one can seen, the proposed sensor has good advantages in the detection of seawater STD. The sensor can detect these three parameters simultaneously and shows a high test sensitivity. The mutual crosstalk problem among the three parameters is also avoided. This shows that the fabricated sensor is expected to have potential applications in the field of ocean parameter detection.
Table 1. Comparison with other fiber-based conductivity-temperature-depth sensors.
5. Conclusion
An SPR fiber probe sensor based on microcavity assisted Vernier effect, TFBG-excited SPR effect and CFBG is proposed for simultaneous measurement of seawater temperature, salinity and depth. The sensor consists of a reference F-P cavity and a sensing probe consisting of an external gold-plated TFBG, CFBG and a spherical closed F-P cavity connected in series to simultaneously detect STD in real time. The spherical F-P cavity is connected in parallel with the reference F-P cavity to realize the Vernier effect and increase the depth sensitivity of the sensor. The experimental results show that the temperature sensitivity of the sensor is $10.82 pm/^{\circ }C$ in the range of $0-40^\circ$C, the salinity sensitivity is $0.122 nm/g/Kg$ in the range of $0-50 g/Kg$, and the depth sensitivity is $116.85 pm/m$ in the experimental measurement range of $0-1000 m$. However, its theoretical maximum depth measurement range can reach $0-2000 m$. The sensor has the advantages of wide pressure detection range, high sensitivity, low crosstalk and easy fabrication, and has broad application potential in deep sea monitoring.
Funding
National Natural Science Foundation of China (51935011, 52075505, 62075199, 62105304); Innovative Research Group Project of National Science Foundation of China (51821003); Shanxi Provincial Key Research and Development Project (202102150101006).
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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