Simple integrated fiber-optic sensor for measuring seawater temperature and salinity and method of use thereof

Abstract

The application provides a simple integrated optical fiber sensor for measuring seawater temperature and salinity and a use method thereof, and relates to the technical field of optical fiber sensing.The application is composed of a small section of hollow core fiber embedded in a pair of multimode optical fibers, and is connected with a demodulator through a single mode optical fiber.The hollow core fiber is opened and etched by using femtosecond laser micromachining technology, so that the MZI and the FPI coexist in a single compact structure.The application can simultaneously obtain the interference spectrum of the MZ and the FP, and can realize the simultaneous measurement of temperature and salinity.Meanwhile, the application ensures the direct contact between the MZ sensing light beam and the measured substance, and greatly improves the measurement sensitivity.The application has advantages in structural size, sensitivity and multi-parameter sensing, and is an effective way to realize seawater environment measurement.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing, and more particularly to a simple integrated fiber optic sensor for measuring seawater temperature and salinity and its usage method. Background Technology

[0002] Salinity affects marine biological characteristics, water movement, and human production and life, making it a crucial parameter for ocean monitoring. Currently, equipment for seawater salinity detection primarily relies on electronic sensors, such as temperature, salinity, and depth (CTD) meters. While electronic meters offer high measurement accuracy, they suffer from drawbacks including expensive probes, large size, difficult deployment, need for continuous power supply, susceptibility to damage and leakage, and unsuitability for long-term in-situ measurements. Compared to electronic sensors, fiber optic sensing technology offers unique advantages in ocean exploration. It is sensitive to multiple parameters, small in size, has a fast response time, low cost, long lifespan, and low transmission loss, enabling long-distance, multi-point, and high-capacity distributed measurements. It has garnered significant attention from researchers in recent years.

[0003] With the rapid development of fiber optic sensing technology in recent years, an increasing number of fiber optic salinity sensors have been proposed, such as fiber Bragg gratings (FBGs), Fabry-Perot interferometers (FPIs), Mach-Zehnder interferometers (MZIs), quantum sensors, and surface plasmon resonance (SPR) sensors. These salinity sensors are primarily based on measuring the refractive index (RI), but they are typically temperature-sensitive, thus exhibiting temperature crosstalk problems. To address this issue, a highly stable temperature environment is required, or temperature must be measured simultaneously during salinity measurement to compensate for temperature effects. Therefore, researchers both domestically and internationally have proposed many methods for simultaneously measuring temperature and salinity.

[0004] Hybrid interferometers, combining different sensing structures, are a common method for simultaneously measuring salinity and temperature. In 2014, Tong et al. proposed a cascade of two Mach-Zehnder (MZ) interferometers, and Tao Hu et al. proposed a sensing probe combining a Sagnac interferometer with a multimode interferometer (MMI). Typically, hybrid interferometers are quite large, leading to positional shifts in the temperature and salinity sensitive regions and causing measurement errors. Furthermore, MMIs have been shown to simultaneously acquire temperature and salinity responses. However, these sensing probes unfortunately exhibit low salinity sensitivity. To improve sensitivity, these sensing structures require tapering, but these structures are fragile. Additionally, optical microfiber couplers (OMCs) are another method for simultaneously acquiring temperature and salinity information, but they are bulky and difficult to package.

[0005] Interference structures based on multiple sensitive films are also a common approach to addressing temperature crosstalk. Lu Ping's team proposed a method of coating different sensitive films onto etched FBGs to achieve regional responses to temperature and salinity, but low sensitivity remains a concern. Furthermore, dual-channel SPR sensors based on different sensitive films have been demonstrated for two-parameter measurements. Zhang proposed a dual-channel sensing probe based on thin films of different materials (such as Au, Ag, and PDMS), achieving a temperature sensitivity of -0.956 nm / ℃ and a salinity sensitivity of 0.3769 nm / ‰. However, the characteristics of SPR sensors are highly dependent on the metal film, which can lead to instability in saline environments. Additionally, some sensors are designed with unique optical fibers, but fabrication is challenging. In summary, these fiber optic sensors for simultaneously measuring temperature and salinity have limitations in terms of measurement sensitivity, mechanical strength, manufacturing difficulty, and size.

[0006] Chinese patent "CN214372544U" proposes a highly sensitive dual-parameter seawater temperature and salinity sensor based on the interferometric vernier effect. Although the sensitivity is high, the structure is discrete and requires parallel connection of two FP interferometers, making the structure complex. Chinese patent "CN114414504A" proposes a seawater salinity and temperature detection device and its usage method based on a double-bending long-period fiber optic grating, which realizes the simultaneous measurement of seawater temperature and salinity, but the sensitivity is relatively low and the system is complex. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a simple integrated fiber optic sensor for measuring seawater temperature and salinity, along with its usage method. Solving the aforementioned problems in the simultaneous measurement of seawater temperature and salinity, this paper proposes a compact fiber optic sensing structure with an open cavity, which possesses high sensitivity, small size, and the potential to simultaneously measure seawater temperature and salinity.

[0008] A simple integrated fiber optic sensor for measuring seawater temperature and salinity, specifically comprising: single-mode fiber, multimode fiber, and hollow-core fiber;

[0009] The single-mode fiber includes an input single-mode fiber and an output single-mode fiber.

[0010] The multimode fiber includes an input multimode fiber and an output multimode fiber;

[0011] The input single-mode fiber, input multimode fiber, hollow fiber, output multimode fiber, and output single-mode fiber are sequentially aligned and fused together using a fusion splicer.

[0012] The connection end faces between the two ends of the hollow-core optical fiber and the input and output multimode optical fibers serve as the two reflecting end faces of the Fabry-Perot interferometer (FPI), namely mirror 1 and mirror 2, denoted as M1 and M2 respectively; the input multimode optical fiber acts as a beam splitter, and the output multimode optical fiber acts as a coupler, thus forming the Fabry-Perot interferometer (FPI) and the Mach-Zehnder interferometer (MZI).

[0013] The hollow fiber has a rectangular open cavity at its center, which is fabricated using femtosecond laser micromachining technology and runs through the entire hollow fiber.

[0014] On the other hand, a method for using a simple integrated fiber optic sensor for measuring seawater temperature and salinity, based on the aforementioned simple integrated fiber optic sensor for measuring seawater temperature and salinity, specifically includes the following steps:

[0015] Step 1: Package the simple integrated fiber optic sensor using a π-type tube;

[0016] The π-shaped tube includes ports 1 to 4. A simple integrated fiber optic sensor enters from port 2 of the π-shaped tube and exits from port 1. External fluid flows in and out through ports 3 and 4.

[0017] Step 2: Connect the input end of the simple integrated fiber optic sensor, i.e., the input single-mode fiber end, directly to the fiber Bragg grating demodulator, and connect the output end, i.e., the output single-mode fiber end, to the isolator and then to the fiber Bragg grating demodulator.

[0018] Step 3: Based on the interference principles of the Mach-Zehnder and Fabry-Perot interferometers, obtain the relationship between their interference troughs and the refractive index within the open cavity, as shown in Equations 1 and 2.

[0019]

[0020]

[0021] Where n cavity n cladding These are the effective refractive indices for light transmitted within the cavity and cladding, respectively; λ dip-MZ and λ dip-FP λ and L are the trough wavelengths of the interferograms of MZI and FPI, respectively, m is a constant, and L is the length of the hollow fiber.

[0022] When the refractive index within the cavity changes, the interference spectra of the Mach-Zehnder interferometer and the Fabry-Perot interferometer will shift, thus enabling the monitoring of salinity and temperature. The encapsulated structure is placed in a constant temperature chamber, and seawater of different salinities is injected into the π-shaped tube to measure salinity. Temperature is measured by changing the temperature of the constant temperature chamber. Simultaneously, the reflection and transmission spectra on the fiber optic grating demodulator are monitored, and its sensitivity matrix is ​​constructed to achieve simultaneous measurement of two parameters.

[0023] The beneficial effects of adopting the above technical solution are as follows:

[0024] This invention provides a simple integrated fiber optic sensor for measuring seawater temperature and salinity and its usage method. It can simultaneously achieve high-sensitivity sensing of seawater temperature and salinity, and has the advantages of in-fiber integration, compact structure, and simple fabrication. It is suitable for measuring seawater temperature and salinity in complex environments. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of a simple open-cavity integrated fiber optic sensor in an embodiment of the present invention;

[0026] Figure 2 These are microscope images of a simple integrated fiber optic sensor with an open cavity, as described in an embodiment of the present invention.

[0027] Figure 3 The images show the transmission and reflection spectra of the open-cavity simple integrated fiber optic sensor in this embodiment of the invention.

[0028] Figure (a) shows the transmission spectrum, and Figure (b) shows the reflection spectrum.

[0029] Figure 4 This is a diagram of the experimental system in an embodiment of the present invention;

[0030] Figure 5 This is a diagram showing the salinity response characteristics of MZI and FPI in an open-cavity simple integrated fiber optic sensor according to an embodiment of the present invention.

[0031] Figure 6 The temperature response characteristics of MZI and FPI in the open cavity simple integrated fiber optic sensor in this embodiment of the invention are shown. Detailed Implementation

[0032] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The following examples are for illustrative purposes only and are not intended to limit the scope of the invention.

[0033] A simple integrated fiber optic sensor for measuring seawater temperature and salinity, such as Figure 1As shown, it specifically includes: single-mode fiber, multimode fiber, hollow-core fiber HCF. The presence of the open cavity ensures the coexistence of Mach-Zehnder interferometer and Fabry-Perot interferometer, and can simultaneously acquire the interference spectra of Mach-Zehnder (MZ) and Fabry-Perot (FP). By detecting the spectral shifts caused by seawater salinity and temperature, simultaneous measurement of temperature and salinity can be achieved.

[0034] As shown in the structural diagram.

[0035] The single-mode fiber includes an input single-mode fiber and an output single-mode fiber.

[0036] The multimode fiber includes an input multimode fiber (MMF) and an output multimode fiber.

[0037] The input single-mode fiber, input multimode fiber, hollow-core fiber (HCF), output multimode fiber, and output single-mode fiber are sequentially aligned and fused together using a fusion splicer.

[0038] The connection ends of the hollow-core fiber (HCF) and the input and output multimode fibers serve as the two reflecting ends of the Fabry-Perot interferometer (FPI), namely mirror 1 and mirror 2, denoted as M1 and M2 respectively. Light arriving at mirrors M1 and M2 is split into transmitted and reflected light for propagation. The input multimode fiber acts as a beam splitter, and the output multimode fiber acts as a coupler. At M1, the reflected light I3 returns to the core of the input multimode fiber. Furthermore, the transmitted light is split into two parts for propagation: one part (I1) propagates along the cladding, while the remaining light (I2) propagates along the open cavity. At M2, the reflected light (I4) of light (I2) is reflected and propagates through M1, interfering with I3 to form the Fabry-Perot interferometer (FPI). Additionally, the transmitted light (I2) interferes with I1 to form a Mach-Zehnder interferometer (MZI). In summary, MZI and FPI with different sensitivity responses coexist in this sensing structure. The input multimode fiber ensures that part of the input light can enter the cladding of the hollow fiber and part can enter the air hole of the hollow fiber, realizing direct contact with the external measured environment and ensuring the high sensitivity measurement of the MZ interferometer.

[0039] A rectangular open cavity is provided in the center of the hollow fiber. The rectangular open cavity is prepared using femtosecond laser micromachining technology and runs through the entire hollow fiber to ensure the flow of the external measured substance.

[0040] The Mach-Zehnder interferometer (MZI) and the Fabry-Perot interferometer (FPI) are integrated into a compact and simple fiber optic sensing structure.

[0041] Changes in seawater temperature and salinity cause shifts in both the MZ and FP interferometric spectra. Simultaneous monitoring of both spectra allows for the simultaneous detection of seawater temperature and salinity. For example... Figure 2 , Figure 3 As shown.

[0042] On the other hand, a method for using a simple integrated fiber optic sensor for measuring seawater temperature and salinity, based on the aforementioned simple integrated fiber optic sensor for measuring seawater temperature and salinity, specifically includes the following steps:

[0043] Step 1: Package the simple integrated fiber optic sensor using a π-type tube;

[0044] The π-shaped tube includes ports 1 to 4. A simple integrated fiber optic sensor enters from port 2 of the π-shaped tube and exits from port 1. External fluid flows in and out through ports 3 and 4.

[0045] Step 2: Connect the input end of the simple integrated fiber optic sensor, i.e., the input single-mode fiber end, directly to the fiber Bragg grating demodulator, and connect the output end, i.e., the output single-mode fiber end, to the isolator and then to the fiber Bragg grating demodulator.

[0046] Step 3: Based on the interference principles of the Mach-Zehnder interferometer and the Fabry-Perot interferometer, obtain the relationship between their interference troughs and the refractive index inside the cavity;

[0047]

[0048]

[0049] Where n cavity n cladding These are the effective refractive indices for light transmitted within the cavity and cladding, respectively; λ dip-MZ and λ dip-FP λ and L are the trough wavelengths of the interferograms of MZI and FPI, respectively, m is a constant, and L is the length of the hollow fiber.

[0050] When the refractive index within the cavity changes, the interference spectra of the Mach-Zehnder interferometer and the Fabry-Perot interferometer will shift, thus enabling the monitoring of salinity and temperature. The encapsulated structure is placed in a constant temperature chamber, and seawater of different salinities is injected into the π-shaped tube to measure salinity. Temperature is measured by changing the temperature of the constant temperature chamber. Simultaneously, the reflection and transmission spectra on the fiber optic grating demodulator are monitored, and its sensitivity matrix is ​​constructed to achieve simultaneous measurement of two parameters.

[0051] The sensor microscope image designed in the embodiments of the present invention is shown below. Figure 2 As shown. Multimode fiber 1 and multimode fiber 2 are 1000 micrometers long, hollow fiber is 100 micrometers long, and the width of the cavity is 70 micrometers and 40 micrometers respectively, located in the center of the hollow fiber.

[0052] An overall view of the experimental equipment is shown below. Figure 4As shown. The system consists of a fiber Bragg grating demodulator, a thermostat, and a fiber optic isolator. The sensing probe is encapsulated in a π-tube with four ports. The sensing structure enters from port 1 and exits from port 2. Ports 1 and 2 are sealed with UV adhesive, while ports 3 and 4 serve as channels for liquid inflow and outflow. The thermostat provides a constant temperature environment. The reflectance and transmission spectra are collected by the fiber Bragg grating demodulator (Micron Optics, Si155), which acts as both a light source and a spectral analyzer. The transmission and reflectance spectra are shown below. Figure 3 As shown in (a) and 3(b).

[0053] First, the salinity characteristics of the sensing structure were tested. Under constant temperature conditions, seawater with different salinities was injected into the π-shaped tube using a syringe. Furthermore, the seawater concentrations used in this experiment were 0‰, 14‰, 21‰, 30‰, and 41‰. Figure 5 The salinity characteristics of MZI and FPI are shown. For both MZI and FPI, the wavelength of the interference tilt angle changes with external salinity, and their responses differ. Furthermore, the salinity sensitivities of MZI and FPI are -3.2 nm / ‰ and 0.2 nm / ‰, respectively.

[0054] Due to temperature crosstalk issues during salinity measurement, the temperature test of the sensing structure was conducted in a water environment with constant salinity. The temperature response curve is shown below. Figure 6 As shown, the wavelength of the interference tilt angle between MZI and FPI exhibits a quadratic change with external temperature. Sensitivity can be obtained by deriving the quadratic curve. Furthermore, the temperature sensitivity of MZI and FPI reaches a maximum of 1.64 nm / ℃, and down to -0.1 nm / ℃ in the range of 5-30℃. Calculations show that the temperature cross-sensitivity of MZI and FPI to salinity is 0.35‰ / ℃ and 0.39‰ / ℃, respectively, within the 5-30℃ range.

[0055] The present invention discloses a simple integrated fiber optic sensor for measuring seawater temperature and salinity, which can simultaneously achieve high-sensitivity sensing of seawater temperature and salinity. It has the advantages of in-fiber integration, compact structure, and simple fabrication, and is suitable for measuring seawater temperature and salinity in complex environments.

[0056] The above description is merely a preferred embodiment of this disclosure and an explanation of the technical principles employed. Those skilled in the art should understand that the scope of the invention involved in the embodiments of this disclosure is not limited to technical solutions formed by specific combinations of the above-described technical features, but should also cover other technical solutions formed by arbitrary combinations of the above-described technical features or their equivalents without departing from the above-described inventive concept. For example, technical solutions formed by substituting the above-described features with (but not limited to) technical features with similar functions disclosed in the embodiments of this disclosure.

Claims

1. A simple integrated fiber optic sensor for measuring seawater temperature and salinity, characterized in that, include: Including single-mode fiber, multimode fiber, and hollow-core fiber; The single-mode fiber includes an input single-mode fiber and an output single-mode fiber. The multimode fiber includes an input multimode fiber and an output multimode fiber; The input single-mode fiber, input multimode fiber, hollow fiber, output multimode fiber, and output single-mode fiber are sequentially aligned and fused together using a fusion splicer. The hollow fiber has a rectangular open cavity at its center, which is fabricated using femtosecond laser micromachining technology and runs through the entire hollow fiber. The connection end faces between the two ends of the hollow fiber and the input multimode fiber and the output multimode fiber serve as the two reflection end faces of the Fabry-Perot interferometer (FPI), namely mirror 1 and mirror 2, denoted as M1 and M2 respectively; the input multimode fiber serves as a beam splitter and the output multimode fiber serves as a coupler, thus forming the Fabry-Perot interferometer (FPI) and the Mach-Zehnder interferometer (MZI). The aforementioned simple integrated fiber optic sensor for measuring seawater temperature and salinity is used to implement the following method, which includes the following steps: Step 1: Package the simple integrated fiber optic sensor using a π-type tube; The π-shaped tube includes ports 1 to 4. A simple integrated fiber optic sensor enters from port 2 of the π-shaped tube and exits from port 1. External fluid flows in and out through ports 3 and 4. Step 2: Connect the input end of the simple integrated fiber optic sensor, i.e. the input single-mode fiber end, directly to the fiber Bragg grating demodulator, and connect the output end, i.e. the output single-mode fiber end, to the isolator and then to the fiber Bragg grating demodulator. Step 3: Based on the interference principles of the Mach-Zehnder and Fabry-Perot interferometers, the relationship between their interference troughs and the refractive index within the cavity is obtained, as shown in Equations 1 and 2: （1）； （2）； in , These are the effective refractive indices for light transmitted within the cavity and cladding, respectively. and These are the trough wavelengths of the interferograms of MZI and FPI, respectively. m It is a constant. L It is the length of the hollow fiber; When the refractive index within the cavity changes, the interference spectra of the Mach-Zehnder interferometer and the Fabry-Perot interferometer will shift, thus enabling the monitoring of salinity and temperature. The encapsulated structure is placed in a constant temperature chamber, and seawater of different salinities is injected into the π-shaped tube to measure salinity. Temperature is measured by changing the temperature of the constant temperature chamber. Simultaneously, the reflection and transmission spectra on the fiber optic grating demodulator are monitored, and its sensitivity matrix is ​​constructed to achieve simultaneous measurement of two parameters.

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