CN106442340B - Device and method for detecting seawater salinity by long-period fiber gratings - Google Patents

Abstract

The invention discloses a long-period grating-based seawater salinity detection device and method, which connects the input optical fiber connector _P0 of the optical fiber coupler (2) to the light source (1); the output optical fiber connector P2 of the optical fiber coupler ( ₂ ) is connected to the The spectrometer (9) is connected, and the output fiber connector P ₁ is connected to one end face of the optical fiber engraved with the long-period grating (6); the optical fiber in the gate area of the long-period grating (6) is straightened and fixed in the concave liquid tank (5) On the bracket (3), the flat end surface of the other end of the optical fiber engraved with the long-period grating (6) is coated with a reflective film (8); the output port of the spectrometer (9) and the temperature sensor module (7) are respectively connected with the signal acquisition module ( 10) are connected, the signal acquisition module (10) is connected with the A/D conversion module (11), and the A/D conversion module (11) is connected with the single-chip microcomputer (12). The device of the invention has small volume, light weight, corrosion resistance, high sensitivity and anti-electromagnetic interference.

Description

A long-period fiber grating device and method for detecting seawater salinity

technical field

The invention relates to the technical field of seawater salinity detection, in particular to a device and method for long-period optical fiber gratings to detect seawater salinity.

Background technique

Salinity is an important parameter of seawater. Many phenomena and processes in the ocean are closely related to the distribution of salinity. Accurate detection of seawater salinity is of great significance and guidance for many marine activities and marine scientific research, such as the study of ocean circulation, marine environmental protection, marine climate prediction, marine resource exploration, marine resource development and utilization, mariculture, military channel survey, etc. , can provide very important services and guarantees for my country's maritime economic activities, especially maritime military activities.

With the continuous advancement of people's understanding of the ocean and technology, people have proposed a variety of measurement techniques for detecting seawater salinity, mainly including electrical conductivity method, surface plasmon resonance method, microwave space remote sensing method, Brillouin scattering method, and Raman spectroscopy. For example, "A New Type of Salinity Optical Sensor Based on the Principle of Surface Plasmon Resonance" co-authored by Wu Yingcai, Fu Yunliang, etc., "Application of Photoelectric Technology "2011.07(10) published "A New Type of Seawater Salinity Measurement Technology" co-authored by Diao Zhenwei and Liu Lanjun, which introduced the technical solution of conductivity method to measure seawater salinity, "Ocean Technology" 2006.25(3) published Lu Zhaoshi, Shi Jiuxin Co-authored "Experimental Research on Pools of Seawater Salinity Remotely Sensed by Microwave Radiometer". The conductivity method has high measurement accuracy and can be continuously detected on site, but the conductivity is obtained with salinity, temperature, and pressure as parameters, and there are errors caused by the asynchronous parameters of the three parameters, and the electrodes are easily damaged, easily affected by water pollution and electromagnetic waves. The interference affects the measurement accuracy; the microwave remote sensing method is convenient and fast, and can monitor seawater salinity in a large area, all day, continuously and dynamically in real time. However, due to the weak microwave penetration ability (a few millimeters), this method is only suitable for Seawater surface measurement limits its application range, and the method is easily affected by external factors, and the measurement accuracy is very low; the instruments used in surface plasmon resonance method, Brillouin scattering method, ultraviolet absorption spectroscopy, nuclear magnetic resonance technology, etc. The structure is complex and bulky, and it is difficult to be used for on-site real-time detection of seawater salinity.

In view of the importance of seawater salinity detection, many patents for seawater salinity detection have appeared in recent years.

Patent application number 201510412705.9 discloses a method for making rhodium electrodes on sapphire substrates for detecting seawater salinity; patent application number 201310561955.X discloses a method for a contact-type pt four-electrode salinity sensor based on MEMS technology; application Patent No. 201410612672.8 discloses a salinity detection method using two parallel plates as pole plates and two gold fingers with gold-plated brass surfaces arranged in parallel as probes; patent application No. CN201210244182.8 discloses a method including a conductivity cell , constant temperature bath, high-precision standard resistance, sine wave generator, voltage signal converter and other components of seawater salinity measurement device. The methods of these several patents are conducted in the form of electrodes, and the detection of seawater salinity is realized by collecting signals such as voltage, current, and conductance. This electrode salinity sensor is susceptible to seawater corrosion and electromagnetic interference, and the electrode is prone to polarization during long-term use, which affects the accuracy of measurement and service life.

Application No. 201310006396.6 discloses a method for measuring the salinity of seawater by using the refractive index effect produced by flow injection spectrophotometry, but this method, which only relies on spectrophotometry, is easily affected by substances with ultraviolet or fluorescence absorption characteristics. The accuracy of seawater measurement needs to be further improved. Application number 201010603445.0 discloses a seawater salinity detection device with multiple refraction of prism model, which utilizes the multiple refraction of light beams between optical glasses of a group of prism models to test seawater salinity, due to the multiple use of prism glass blocks, The structure appears relatively complex. Application number 02117422.9 discloses an optical fiber seawater salinity detection method and device. Its principle is to realize salinity measurement by detecting the change of the light spot position on the photosensitive surface of the CCD caused by the change of the refractive index of different salinities. However, the device uses The prism structure is added, which virtually expands the volume of the detection device, and does not consider the influence of temperature changes on the light shift. Application No. 201410425894.9 discloses a method for measuring seawater salinity using a micro-nano optical fiber ring cavity sensor. However, the tail end of the standard single-mode optical fiber needs to be thinned in the process of making the micro-nano optical fiber ring cavity, which reduces the optical fiber's resistance to mechanical interference. Ability. Application No. 201120543880.9 discloses a sensor for measuring seawater salinity based on FBG. The corroded Bragg grating is used in the sensor, which reduces the mechanical resistance of the sensor. Moreover, the sensitivity of the Bragg grating is much lower than that of the long-period grating.

The above-mentioned patents and methods all have some shortcomings and limitations that are difficult to overcome. These devices and systems for detecting seawater salinity cannot perform long-term reliable real-time monitoring in complex and changeable marine environments, and cannot meet the requirements of modern marine surveys. . Therefore, it is an urgent problem to develop a new type of salinity sensor that is strong, reliable, small in size, high in precision, capable of long-distance signal transmission, corrosion-resistant, and anti-interference.

Contents of the invention

The object of the present invention is to provide a device and method for detecting seawater salinity with long-period fiber gratings, so as to solve the above-mentioned problems in the prior art, and make the device for detecting seawater salinity small in size, light in weight, corrosion-resistant, high in sensitivity, Anti-electromagnetic interference.

To achieve the above object, the present invention provides the following scheme:

The invention provides a seawater salinity detection device based on a long-period grating, including a light source, an optical fiber coupler, a liquid tank, a long-period grating, a temperature sensor module, a spectrometer, a signal acquisition module, an A/D conversion module, a single-chip microcomputer and an output module The fiber optic connector _P0 at the input end of the fiber coupler is connected to the light source; the second output fiber connector _P2 of the fiber coupler is connected to the spectrometer, and the first output fiber connector _P1 is connected to the long-period grating One end face of the optical fiber is connected; the optical fiber in the grating area of the long-period grating is straightened and fixed on a support containing a concave liquid tank that can hold sea water, and the flat end face of the other end of the optical fiber engraved with the long-period grating is coated with reflective film; the output port of the spectrometer and the temperature sensor module are respectively connected to the signal acquisition module, the signal acquisition module is connected to the A/D conversion module, and the A/D conversion module is connected to the The single-chip microcomputer is connected, and the single-chip microcomputer is connected with the output module.

Optionally, the temperature sensor module is placed in the liquid tank of the bracket that fixes the long-period grating.

Optionally, the fiber coupler is a 2×2 single-mode fiber coupler.

Optionally, the long-period grating is a bare long-period grating that has not been modified with a nano-film, a long-period grating etched with hydrofluoric acid, or a long-period grating modified with various nano-film materials on the cladding surface of the gate region.

Optionally, the output module includes at least one of a liquid crystal display, a mobile phone or a computer.

The present invention also provides a seawater salinity detection method based on long-period gratings, comprising the following steps:

Step 1, first connect the optical fiber connector P ₀ at the input end of the fiber coupler to the light source, connect one optical fiber connector P ₂ at the output end of the fiber coupler to the spectrometer, and connect the other optical fiber connector P ₁ at the output end of the fiber coupler to the engraved long period One end face of the optical fiber of the grating is connected; the optical fiber in the grating area of the long-period grating is straightened and fixed on a bracket containing a concave liquid tank that can hold sea water, and the flat end face of the other end of the optical fiber engraved with the long-period grating is coated with reflective film; the output port of the spectrometer and the temperature sensor module are respectively connected with the signal acquisition module, and then the signal acquisition module is connected with the A/D conversion module, and the A/D conversion module is connected with the single-chip microcomputer, and the salinity is displayed through the liquid crystal display Values, or read and store salinity data on mobile phones or computers through Bluetooth, WIFI or USB ports;

Step 2: Prepare a series of standard salinity solutions, and then test the prepared standard solutions with a fixed long-period grating at a constant temperature, then perform algorithmic processing on the signals and data, and formulate salinity and resonance wavelength, A standard curve of the relationship between the resonance intensity values;

Step 3: Gradually increase or decrease the temperature of the long-period grating, and synchronously test the change value of the resonance wavelength and resonance intensity of the fixed long-period grating in pure water, and then perform algorithmic processing on the signal and data to obtain the long-period grating The standard curve of the corresponding relationship between the resonant wavelength, the change value of the resonant intensity and the temperature;

Step 4: Use the fixed long-period grating and temperature sensor module to test the seawater in the real seawater environment in real time, and use the resonant wavelength and resonance intensity value of the long-period grating measured in the real seawater to subtract the long-period grating caused by temperature. The resonant wavelength or resonant intensity change value of the periodic grating is compared with the standard curve of salinity to obtain the salinity value of seawater by comparing the resonant wavelength and resonant intensity value obtained after the calibration process;

Step five, displaying the value of seawater salinity on the LCD screen, or reading and storing the seawater salinity data on the mobile phone or computer through Bluetooth, WIFI or USB port.

Optionally, in the second step, the long-period grating senses the refractive index of the water to infer the salinity of the water.

Optionally, in the first step, the reflective film is coated on the flat end surface of the fiber end of the LPFG by sputtering, evaporation, atomic layer deposition or chemical wet method.

Compared with the prior art, the present invention has achieved the following technical effects:

①Anti-electromagnetic interference, which can reflect the salinity of seawater more accurately and truly; ②Corrosion resistance, very suitable for long-term use in seawater; ③Light weight, small size, easy networking, which is conducive to realizing online continuous monitoring of seawater salinity Automated long-distance telemetry; ④ Wide range of applications, not only can measure salinity near sea level, but especially suitable for salinity detection in deep ocean;

Compared with other seawater salinity optical sensor devices such as Bragg gratings and micro-nano optical fiber ring cavity, the long-period grating seawater salinity sensor has higher sensitivity and can more accurately distinguish small changes in seawater salinity;

Since LPFGs are easily affected by bending, temperature, and seawater salinity, in practical applications, the true salinity of seawater can only be accurately determined by deducting the influence of bending and temperature on the transmission spectrum of LPFGs in seawater. The invention straightens and fixes the long-period grating on the concave liquid tank support, avoids the cross influence of bending on the transmission spectrum of the long-period grating; and eliminates the cross influence of temperature on the transmission spectrum of the long-period grating by adding a temperature sensor module and algorithm processing Influence; By using fiber coupler and preparing reflective film on the end face of the fiber of the LPFG, not only the transmission spectrum display mode of the LPFG is retained, but also the LPFG is more beneficial in the long-distance monitoring of seawater salinity; By covering the outer ring layer of the fixed bracket of the long-period grating with a micro-nanofiltration membrane, solid biochemical substances in seawater are prevented from adhering to the surface of the long-period grating region, and the accuracy of seawater salinity measurement is ensured.

The seawater salinity sensor with single-end surface transmission spectrum long-period grating has the advantages of simple operation process, anti-electromagnetic interference, and high detection sensitivity, has broad commercial application prospects, and is expected to be popularized and applied in the marine field on a large scale.

Description of drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the accompanying drawings required in the embodiments. Obviously, the accompanying drawings in the following description are only some of the present invention. Embodiments, for those of ordinary skill in the art, other drawings can also be obtained based on these drawings without paying creative labor;

Fig. 1 is the device system schematic diagram of the long-period fiber grating detection seawater salinity of the present invention;

Fig. 2 is the correspondence relation diagram of the resonant wavelength and salinity of the long-period grating of the present invention;

Fig. 3 is the corresponding relationship diagram of the resonant wavelength and temperature of the long-period grating of the present invention;

Explanation of reference signs: 1 is a light source, 2 is a fiber coupler, 3 is a long-period grating fixing bracket, 4 is a micro-nanofiltration membrane, 5 is a liquid tank, 6 is a long-period fiber grating, 7 is a temperature reactor module, 8 9 is a spectrometer, 10 is a signal acquisition module, 11 is an A/D conversion module, 12 is a single-chip microcomputer, 13 is a liquid crystal display screen, and 14 is a mobile phone or a computer.

Detailed ways

The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some, not all, embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

The object of the present invention is to provide a device and method for detecting seawater salinity with long-period fiber gratings, so as to solve the above-mentioned problems in the prior art, and make the device for detecting seawater salinity small in size, light in weight, corrosion-resistant, high in sensitivity, Anti-electromagnetic interference.

In order to make the above objects, features and advantages of the present invention more comprehensible, the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

Embodiment one

This embodiment provides a long-period grating-based seawater salinity detection device, as shown in Figure 1, including a light source 1, an optical fiber coupler 2 (the optical fiber coupler 2 is a 2×2 single-mode optical fiber coupler), a liquid Groove 5, long-period grating 6, temperature sensor module 7, spectrometer 9, signal acquisition module 10, A/D conversion module 11, single-chip microcomputer 12 and output module; the optical fiber connector P0 of the input end of the optical fiber coupler 2 and the light source 1 connection; the second output fiber connector P2 of the fiber coupler 2 is connected to the spectrometer 9, and the first output fiber connector P1 is connected to an end face of an optical fiber engraved with a long-period grating 6, and the long-period grating 6 is Bare long-period gratings without nano-film modification, long-period gratings etched by hydrofluoric acid, or long-period gratings decorated with various nano-film materials on the cladding surface of the gate; It is straightened and fixed on the support 3 that contains the concave liquid tank 5 that can hold seawater, and the flat end surface of the other end of the optical fiber that is engraved with the long-period grating 6 is coated with a reflective film 8; the output port of the spectrometer 9 and the temperature The sensor module 7 is connected with the signal acquisition module 10 respectively, and the temperature sensor module 7 is placed in the liquid tank 5 of the support 3 of the fixed long-period grating 6; the signal acquisition module 10 is connected with the A/D conversion module 11, the A/D conversion module 11 is connected to the single-chip microcomputer 12, and the single-chip microcomputer 12 is connected to the output module including at least one of a liquid crystal display 13, a mobile phone or a computer 14.

This embodiment also provides a seawater salinity detection method based on long-period gratings, including the following steps:

Step 1, first connect the optical fiber connector P ₀ at the input end of the fiber coupler 2 to the light source 1, connect one optical fiber connector P ₂ at the output end of the fiber coupler 2 to the spectrometer 9, and connect the other optical fiber connector P ₁ at the output end of the fiber coupler 2 It is connected with one end face of an optical fiber engraved with a long-period grating 6; the optical fiber in the gate area of the long-period grating 6 is straightened and fixed on the support 3 containing a concave liquid tank 5 that can hold seawater, and the long-period grating 6 is engraved The flat end face of the other end of the optical fiber adopts sputtering method, evaporation method, atomic layer deposition method or chemical wet method to be coated with reflective film 8; The output port of spectrometer 9 and temperature sensor module 7 are connected with signal acquisition module 10 respectively, then signal Acquisition module 10 is connected with A/D conversion module 11, and A/D conversion module 11 is connected with single-chip microcomputer 12, shows the numerical value of salinity through liquid crystal display 13, or through bluetooth, WIFI or USB port in mobile phone or computer 14 read and store the salinity data;

Step 2: Prepare a series of standard salinity solutions, and then test the prepared standard solution with a fixed long-period grating 6 at a certain constant temperature. The long-period grating 6 can infer the salt of water by sensing the refractive index of water degree, then carry out algorithmic processing on the signal and data, and formulate a standard curve of the relationship between salinity and resonance wavelength and resonance intensity value;

Step 3: Gradually increase or decrease the temperature of the long-period grating 6, and synchronously test the change value of the resonance wavelength and resonance intensity of the fixed long-period grating 6 in pure water, and then perform algorithmic processing on the signal and data to obtain the long-period grating 6 The standard curve of the corresponding relationship between the resonance wavelength of the periodic grating 6, the change value of the resonance intensity and the temperature;

Step 4, use the fixed long-period grating 6 and the temperature sensor module 7 to test the seawater in the real seawater environment in real time respectively, and use the resonant wavelength and resonant intensity value of the long-period grating measured in the real seawater to subtract the temperature caused by The long-period grating resonance wavelength or resonance intensity change value, the resonance wavelength and resonance intensity value obtained after the correction process is compared with the standard curve of salinity, and the salinity value of seawater is obtained;

Step 5, displaying the value of seawater salinity through the liquid crystal display 13, or reading and storing the seawater salinity data on the mobile phone or computer 14 through Bluetooth, WIFI or USB port.

The installation process and detection process of the long-period grating-based seawater salinity detection device of the present invention are described in detail below through specific examples:

First connect the optical fiber connector P0 at the input end of the fiber coupler 2 to the light source 1, connect one optical fiber connector P2 at the output end of the fiber coupler ₂ to the spectrometer 9, and connect the other optical fiber connector _P1 at the output end of the fiber coupler 2 to the One end face of the optical fiber of the periodic grating 6 is connected; the optical fiber of the long-period grating 6 grid area is straightened and fixed on the support 3 that contains the concave liquid tank 5 that can hold seawater, and the other end of the optical fiber of the long-period grating 6 is engraved The flat end face of the spectrometer 9 is coated with a reflective film 8; the output port of the spectrometer 9 and the temperature sensor module 7 are respectively connected with the signal acquisition module 10, and then the signal acquisition module 10 is connected with the A/D conversion module 11, and the A/D conversion module 11 is connected with the A/D conversion module 11. The single-chip microcomputer 12 is connected, and the numerical value of salinity is displayed through the liquid crystal display 13, or the salinity data is read and stored on the mobile phone or computer 14 through Bluetooth, WIFI or USB port, as shown in Figure 1 .

Secondly, a series of sodium chloride standard solutions with concentrations of 0‰, 5‰, 10‰, 15‰, 20‰, 25‰, 30‰, 35‰, and 40‰ were prepared, and then the above-mentioned Add the standard solution into the liquid tank where the long-period grating has been fixed. The relationship between the degree and the resonance wavelength is shown in Figure 2 of the attached drawing of the specification.

Adjust the pure water temperature of the liquid tank to the temperature of the long-period grating at 5°C, 10°C, 15°C, 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, and 50°C, and fix the synchronous test The resonant wavelength values of the long-period grating are 1486.584nm, 1486.5064nm, 1486.428nm, 1486.351nm, 1486.273nm, 1486.196nm, 1486.118nm, 1486.041nm, 1485.963nm, 1485.885nm, the resonant wavelength of the long-period grating Correspondence with temperature The standard curve of the relationship is shown in Figure 3 of the accompanying drawing of the description.

Use the fixed long-period grating and temperature-sensitive module to test the seawater in a certain area in real time. The results show that the seawater temperature is 26°C and the resonance wavelength is 1486.862nm. After algorithm processing of the data, the long-period caused by temperature is subtracted. The resonant wavelength of the grating is compared with the standard curve of salinity, and the salinity value of seawater is 10.74‰.

The invention straightens and fixes the long-period grating on the concave liquid tank support, avoids the cross influence of bending on the transmission spectrum of the long-period grating; and eliminates the cross influence of temperature on the transmission spectrum of the long-period grating by adding a temperature sensor module and algorithm processing Influence; By using fiber coupler and preparing reflective film on the end face of the fiber of the LPFG, not only the transmission spectrum display mode of the LPFG is retained, but also the LPFG is more beneficial in the long-distance monitoring of seawater salinity; By covering the outer ring layer of the fixed bracket of the long-period grating with a micro-nanofiltration membrane, solid biochemical substances in seawater are prevented from adhering to the surface of the long-period grating region, and the accuracy of seawater salinity measurement is ensured.

The seawater salinity sensor with single-end surface transmission spectrum long-period grating has the advantages of simple operation process, anti-electromagnetic interference, and high detection sensitivity, has broad commercial application prospects, and is expected to be popularized and applied in the marine field on a large scale.

This description uses specific examples to illustrate the principle and implementation of the present invention. The description of the above embodiments is only used to help understand the method and core idea of the present invention; meanwhile, for those of ordinary skill in the art, according to the present invention Thoughts, there will be changes in specific implementation methods and application ranges. In summary, the contents of this specification should not be construed as limiting the present invention.

Claims (8)

1. A seawater salinity detection device based on a long-period grating, characterized in that: comprising a light source (1), an optical fiber coupler (2), a liquid tank (5), a long-period grating (6), a temperature sensor module (7 ), spectrometer (9), signal acquisition module (10), A/D conversion module (11), single-chip microcomputer (12) and output module; The optical fiber connector _P of described fiber optic coupler (2) input end and described light source ( 1) connection; the second output fiber connector P ₂ of the fiber coupler (2) is connected to the spectrometer (9), and the first output fiber connector P ₁ is connected to one end of an optical fiber engraved with a long-period grating (6) The surface is connected; the optical fiber in the gate area of the long-period grating (6) is straightened and fixed on the support (3) that contains the concave liquid tank (5) that can hold seawater, and the optical fiber of the long-period grating (6) is engraved The smooth end face of the other end is coated with reflective film (8); The output port of described spectrometer (9) and described temperature sensor module (7) are connected with described signal acquisition module (10) respectively, and described signal acquisition module ( 10) being connected with the A/D conversion module (11), the A/D conversion module (11) being connected with the single-chip microcomputer (12), and the single-chip microcomputer (12) being connected with the output module. 2. the seawater salinity detection device based on the long-period grating according to claim 1, is characterized in that: the temperature sensor module (7) is placed in the liquid tank (3) of the support (3) of the fixed long-period grating (6) 5) in. 3. The long-period grating-based seawater salinity detection device according to claim 1, characterized in that: the optical fiber coupler (2) is a 2×2 single-mode optical fiber coupler. 4. The seawater salinity detection device based on the long-period grating according to claim 1, characterized in that: the long-period grating (6) is a bare long-period grating that has not been modified by nano-films, etched by hydrofluoric acid The long-period grating or the long-period grating modified on the surface of the cladding layer of the gate region by the nano-thin film material. 5. The long-period grating-based seawater salinity detection device according to claim 1, characterized in that: the output module includes at least one of a liquid crystal display (13), a mobile phone or a computer (14). 6. A seawater salinity detection method based on long period grating, is characterized in that: comprise the steps: Step 1, at first the optical fiber connector _P0 of the input end of the fiber coupler (2) is connected with the light source (1), an optical fiber connector _P2 of the output end of the fiber coupler (2) is connected with the spectrometer (9), and the fiber coupler ( 2) Another optical fiber connector P ₁ at the output end is connected to one end face of the optical fiber engraved with the long-period grating (6); the optical fiber in the gate area of the long-period grating (6) is straightened and fixed in a concave shape that can contain seawater. On the bracket (3) of the liquid tank (5), the flat end surface of the other end of the optical fiber engraved with the long-period grating (6) is coated with a reflective film (8); the output port of the spectrometer (9) and the temperature sensor module (7) are respectively Be connected with the signal acquisition module (10), then the signal acquisition module (10) is connected with the A/D conversion module (11), and the A/D conversion module (11) is connected with the single-chip microcomputer (12), through the liquid crystal display ( 13) Display the value of salinity, or read and store the salinity data on the mobile phone or computer (14) through Bluetooth, WIFI or USB port; Step 2: Prepare a series of standard salinity solutions, then test the prepared standard solution with a fixed long-period grating (6) at a certain constant temperature, then perform algorithmic processing on the signal and data, and formulate the salinity and The standard curve of the relationship between the resonance wavelength and the resonance intensity value; Step 3: Gradually increase or decrease the temperature of the long-period grating (6), and synchronously test the change value of the resonance wavelength and resonance intensity of the fixed long-period grating (6) in pure water, and then perform algorithmic processing on the signal and data , draw the standard curve of the corresponding relationship between the resonance wavelength of the long-period grating (6), the change value of the resonance intensity and the temperature; Step 4, use the fixed long-period grating (6) and the temperature sensor module (7) to test the seawater in the real seawater environment in real time respectively, and use the resonance wavelength and resonance intensity value of the long-period grating measured in the real seawater to subtract The long-period grating resonance wavelength or resonance intensity change value caused by the drop temperature, the resonance wavelength and resonance intensity value obtained after correction processing are compared with the standard curve of salinity, and the salinity value of seawater is obtained; Step five, displaying the value of seawater salinity through the liquid crystal display (13), or reading and storing the seawater salinity data on the mobile phone or computer (14) through Bluetooth, WIFI or USB port. 7. The seawater salinity detection method based on the long-period grating according to claim 6, characterized in that: in the step 2, the long-period grating (6) deduces the salinity of the water by sensing the refractive index of the water. 8. The seawater salinity detection method based on long-period gratings according to claim 6, characterized in that: in the step one, the flat end surface of the optical fiber end of the long-period grating (6) is plated with a reflective film (8). The most common method is sputtering, evaporation, atomic layer deposition or chemical wet method.

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