Salinity sensor based on few-mode optical fiber mode interference and measuring and manufacturing method thereof

Abstract

The invention belongs to the field of optical fiber sensors, and particularly relates to a salinity sensor based on few-mode optical fiber mode interference and a measuring and manufacturing method thereof.The salinity sensor comprises a first single-mode optical fiber, a few-mode optical fiber and a second single-mode optical fiber which are coaxially connected in sequence; the first single-mode optical fiber receives optical signal input, and the second single-mode optical fiber is connected with a spectrograph; performing annular local removal on the cladding of the non-end region of the few-mode optical fiber to obtain a concave part of the few-mode optical fiber; anti-corrosion layers are arranged on the outer surfaces of the first single-mode optical fiber, the non-concave part of the few-mode optical fiber and the second single-mode optical fiber; the whole sensor is arranged in a solution to be detected, interference occurs among different modes when an optical signal passes through the concave part of the few-mode optical fiber, spectral response is received by the spectrograph, and the salinity of the solution to be detected is obtained based on the corresponding relation between the pre-calibrated spectral response and the salinity. Compared with the prior art, the sensor provided by the invention is higher in precision and smaller in size.

Description

Technical Field

[0001] The present invention belongs to the field of optical fiber sensors, and in particular relates to a salinity sensor based on few-mode optical fiber mode interference and a measurement and manufacturing method thereof. Background Art

[0002] With the global climate change and the intensification of human activities, marine environmental monitoring has become increasingly important. Ocean salinity is one of the important parameters reflecting the marine environment, and is closely related to seawater density, temperature, ocean current distribution and climate change. Accurate and real-time monitoring of seawater salinity has important scientific significance and application value for marine resource development, marine ecological protection and climate research. Fiber optic sensors have been widely used in the field of environmental monitoring due to their unique advantages, especially optical fibers have extremely high sensitivity to changes in the external environment (such as refractive index, temperature, strain, etc.). Fiber optic sensors have the advantages of being free from electromagnetic interference, small size and corrosion resistance.

[0003] At present, the commonly used methods for detecting ocean salinity include chemical titration, conductivity and refractive index. These methods have the following shortcomings: poor real-time performance, many traditional methods require sampling and analysis in the laboratory, which makes it difficult to achieve on-site and continuous monitoring; large equipment size, existing ocean monitoring equipment is often complex and bulky, and difficult to deploy in open sea and deep sea environments; limited sensitivity and accuracy, under low salinity change conditions, traditional methods have low detection accuracy and are difficult to capture small changes; poor adaptability, the marine environment is highly corrosive, and sensors need to have corrosion resistance and long-term stability. Summary of the invention

[0004] In order to overcome the defects of poor sensitivity and large size of the monitoring equipment described in the above-mentioned prior art, the present invention provides a salinity sensor based on few-mode fiber mode interference and a measurement and manufacturing method thereof.

[0005] In order to achieve the above technical effects, the technical solution of the present invention is as follows:

[0006] The present invention proposes a salinity sensor based on few-mode fiber mode interference, comprising: 1. A first single-mode fiber, a few-mode fiber, and a second single-mode fiber coaxially connected in sequence; the first single-mode fiber is used to receive an optical signal input, and the second single-mode fiber is used to connect to an external spectrometer; the cladding of the non-end region of the few-mode fiber is locally removed in an annular manner to obtain a concave portion of the few-mode fiber;

[0007] The sensor is placed in a solution to be tested as a whole. When the optical signal passes through the recessed portion of the few-mode optical fiber, interference occurs between different modes. The spectrometer receives the signal and obtains a spectral response. Based on the correspondence between the pre-calibrated spectral response and the salinity, the salinity of the solution to be tested is obtained.

[0008] The present invention proposes a salinity measurement method based on few-mode fiber mode interferometry, and a salinity sensor based on few-mode fiber mode interferometry performs measurement, comprising:

[0009] When the optical signal is coupled from the first single-mode optical fiber into the few-mode optical fiber, multiple modes are excited in the few-mode optical fiber;

[0010] The refractive index of the concave portion of the few-mode optical fiber changes due to the influence of salinity, so that interference occurs between different modes;

[0011] The optical signal is coupled from the few-mode fiber to the second single-mode fiber to form an output spectrum;

[0012] The output light intensity distribution of the output spectrum is measured by a spectrometer to obtain the spectral response;

[0013] The salinity of the solution to be tested is obtained based on the correspondence between the pre-calibrated spectral response and the salinity.

[0014] The present invention also proposes a method for measuring salinity based on few-mode fiber mode interference, comprising: processing a structural material with an anti-corrosion layer, designing two openings at the two ends of the outer diameter of the few-mode fiber, and keeping the center lines of the two openings at the same height;

[0015] A groove is made along a non-end region, and then the few-mode optical fiber is inserted into the structural material along the opening;

[0016] Corroding the cladding of the non-end region of the few-mode optical fiber by using a chemical solvent;

[0017] Connect two ends of the few-mode optical fiber to the first single-mode optical fiber and the second single-mode optical fiber respectively.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] The present invention utilizes the interference phenomenon between different modes in the few-mode optical fiber, and can respond highly sensitively to the tiny refractive index changes caused by different salinities in the solution, significantly improving the detection accuracy; secondly, the sensor has a simple structure and a small size, which is convenient for deployment in complex marine environments such as the open sea and deep sea, and has excellent miniaturization and portability. In addition, the optical fiber material of the present invention has excellent corrosion resistance, and can be directly deployed in harsh marine environments such as high salt and high pressure and maintain long-term stable operation, ensuring the reliability and stability of the sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic diagram of the structure of a salinity sensor based on few-mode fiber mode interference in Example 1.

[0021] Figure 2 This is an axonometric diagram of a salinity sensor based on few-mode fiber mode interference in Example 2.

[0022] Figure 3 This is a cross-sectional view of a salinity sensor based on few-mode fiber mode interference according to Example 2.

[0023] Figure 4 This is a schematic diagram of a method for manufacturing a salinity sensor based on few-mode fiber mode interference according to Example 4. DETAILED DESCRIPTION

[0024] The accompanying drawings are only used for illustrative purposes and are not to be construed as limiting the present invention;

[0025] It is understandable to those skilled in the art that some well-known descriptions may be omitted in the drawings.

[0026] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0027] Example 1

[0028] This embodiment proposes a salinity sensor based on few-mode fiber mode interference, such as Figure 1 , which is a structural diagram of the salinity sensor based on few-mode fiber mode interference of this embodiment.

[0029] The salinity sensor based on few-mode fiber mode interference proposed in this embodiment includes a first single-mode fiber, a few-mode fiber, and a second single-mode fiber coaxially connected in sequence; the first single-mode fiber is used to receive an optical signal input, and the second single-mode fiber is used to connect to an external spectrometer; the cladding of the non-end region of the few-mode fiber is locally removed in an annular manner to obtain a depressed portion of the few-mode fiber;

[0030] The sensor is placed in a solution to be tested as a whole. When the optical signal passes through the recessed portion of the few-mode optical fiber, interference occurs between different modes. The spectrometer receives the signal and obtains a spectral response. Based on the correspondence between the pre-calibrated spectral response and the salinity, the salinity of the solution to be tested is obtained.

[0031] In the present embodiment, the SMF-FMF-SMF structure is formed by the first single-mode fiber, the few-mode fiber and the second single-mode fiber that are coaxially connected in sequence, and the SMF-FMF-SMF structure can make the single-mode fiber and the few-mode fiber have better mode matching. Specifically, the single-mode fiber usually only supports the LP01 base mode, while the few-mode fiber supports multiple modes (such as LP01, LP11, etc.). Through the coupling of two single-mode fibers and the few-mode fiber, the optical signal can be effectively controlled and guided to ensure that the signal can be smoothly transmitted, so that the incident light signal forms mode interference in the few-mode fiber, and the solution of different salinity will affect the refractive index of the few-mode fiber in the recessed part. The phase change of the mode interference is obtained by collecting the spectral response through the spectrometer, and the sensor can monitor the change of the solution refractive index in real time, thereby realizing high-precision detection of ocean salinity. And the optical fiber sensor in the present embodiment is small in size and light in weight, easy to install and transport, and suitable for integration into various monitoring systems.

[0032] Illustratively, in the salinity sensor of this embodiment, the first single-mode optical fiber, the few-mode optical fiber and the second single-mode optical fiber are connected by fusion splicing.

[0033] In an optional embodiment, the spectral response includes transmittance, reflectance or an interference signal.

[0034] In this embodiment, the interference phase change is closely related to the solution salinity, and selecting an appropriate spectral response can accurately reflect the change in the interference trough. Transmittance, reflectance, and interference signal have different response characteristics in different salinity ranges and measurement conditions. According to application requirements, selecting the most appropriate spectral response method can effectively improve the signal strength and stability. For example, reflectance can provide a stronger response under high salinity conditions, while the interference signal shows a stronger sensitivity under low salinity conditions. Selectively using different response methods can optimize the measurement process for different solution salinity ranges.

[0035] In an optional embodiment, the core range of the few-mode optical fiber is 10-25 μm.

[0036] In this embodiment, compared with multimode optical fiber, few-mode optical fiber can provide better mode control and interference effect, thereby achieving high sensitivity and high precision salinity detection. Selecting a suitable few-mode optical fiber can better meet the design requirements of the salinity sensor and improve accuracy.

[0037] Example 2

[0038] This embodiment makes improvements on the salinity sensor based on few-mode fiber mode interference proposed in Embodiment 1.

[0039] A salinity sensor based on few-mode fiber mode interference comprises a first single-mode fiber 1, a few-mode fiber 2 and a second single-mode fiber 3 which are coaxially connected in sequence; the first single-mode fiber 1 receives an optical signal input, and the second single-mode fiber 3 is connected to a spectrometer; the cladding of the non-end region of the few-mode fiber 2 is locally and annularly removed to obtain a depressed portion 4 of the few-mode fiber.

[0040] In an optional embodiment, an anti-corrosion layer 5 is provided on the outer surfaces of the first single-mode optical fiber 1 , the non-depressed portion of the few-mode optical fiber, and the second single-mode optical fiber 3 .

[0041] like Figure 2 As shown, it is a structural diagram of a salinity sensor based on few-mode fiber mode interference of this embodiment.

[0042] like Figure 3 , which is a cross-sectional view of a salinity sensor based on few-mode fiber mode interference according to this embodiment.

[0043] In this embodiment, the non-depressed portion of the non-few-mode optical fiber and the single-mode optical fiber can be protected by the anti-corrosion layer 5 to prevent corrosion by the solution, thereby extending the service life of the sensor.

[0044] In an optional embodiment, the step of locally removing the cladding of the non-end region of the few-mode optical fiber in an annular manner includes: corroding the cladding of the non-end region of the few-mode optical fiber by using a chemical solvent.

[0045] As an exemplary illustration, the chemical solvent is a hydrofluoric acid etchant.

[0046] In this embodiment, the chemical etching method can flexibly control the depth and shape of the cladding removal by controlling the concentration of the solvent and the etching time. The process is simple and can easily achieve high-precision removal.

[0047] Further optionally, a container composed of an anti-corrosion baffle 6 is provided outside the non-end region of the few-mode optical fiber, and the chemical solvent is placed in the container to corrode the cladding of the non-end region of the few-mode optical fiber.

[0048] In this embodiment, the container formed by the anti-corrosion baffle 6 can carry chemical solvents, which is convenient for corroding the cladding of the few-mode optical fiber; in addition, it can also provide additional structural support for the sensor as a whole.

[0049] In an alternative embodiment, the anti-corrosion layer comprises a fluoropolymer.

[0050] As an exemplary illustration, the anti-corrosion layer is made of polytetrafluoroethylene material.

[0051] In this embodiment, polytetrafluoroethylene is highly corrosion-resistant, and polytetrafluoroethylene can also improve the overall performance of the sensor in many aspects. First, polytetrafluoroethylene provides stable support for the optical fiber, ensuring that the optical fiber will not be displaced or have poor contact when connected to external equipment, especially at the connection between the two ends of the optical fiber and the single-mode optical fiber; secondly, since polytetrafluoroethylene has a low light absorption rate and a high light transmittance, it can effectively optimize the optical properties and ensure the high efficiency of signal transmission when the sensor is working; in addition, the good electrical insulation properties of polytetrafluoroethylene shield external electromagnetic interference to a certain extent, further ensuring the accuracy and stability of the optical fiber sensor.

[0052] In an optional embodiment, the diameter of the depressed portion of the few-mode optical fiber is 40% to 80% of the diameter before cladding removal.

[0053] In this embodiment, different degrees of cladding removal can affect the interaction area between the optical fiber and the solution, thereby changing the sensitivity of the optical fiber sensor to salinity changes. By controlling the diameter of the depression, a suitable depression diameter can be selected in different application scenarios to achieve high-sensitivity detection of tiny environmental changes.

[0054] Example 3

[0055] This embodiment proposes a salinity measurement method based on few-mode fiber mode interferometry based on a salinity sensor based on few-mode fiber mode interferometry proposed in Embodiments 1 and 2.

[0056] A method for measuring salinity based on few-mode fiber mode interference, comprising: when an optical signal is coupled from the first single-mode fiber into the few-mode fiber, multiple modes are excited in the few-mode fiber;

[0057] The refractive index of the concave portion of the few-mode optical fiber changes due to the influence of salinity, so that interference occurs between different modes;

[0058] The optical signal is coupled from the few-mode fiber to the second single-mode fiber to form an output spectrum;

[0059] The output light intensity distribution of the output spectrum is measured by a spectrometer to obtain the spectral response;

[0060] The salinity of the solution to be tested is obtained based on the correspondence between the pre-calibrated spectral response and the salinity.

[0061] Specifically, the optical signal input field E in It can be expressed as the few-mode fiber mode field {E i} is a linear combination of:

[0062]

[0063] Where N is the number of modes, c i is the coupling coefficient.

[0064] Different modes will interfere with each other due to phase differences.

[0065]

[0066] Where Δφ is the phase difference, Δn eff is the effective refractive difference between the modes, L is the length of the optical fiber sensing area, and λ is the wavelength of the input light.

[0067] In an optional embodiment, the step of calibrating the correspondence between the spectral response and the salinity includes:

[0068] Select a standard solution with known salinity for measurement, and record the interference troughs formed by the spectral response obtained under solutions with different salinity concentrations;

[0069] Select any interference trough as a reference, calculate the drift between the remaining interference troughs and the reference interference trough, and construct the corresponding relationship between the interference trough drift and the solution salinity;

[0070] The steps of obtaining the salinity of the solution to be tested based on the corresponding relationship include:

[0071] The drift between the interference trough formed by the spectral response of the solution to be tested and the reference interference trough is calculated, and the salinity of the solution to be tested is obtained based on the corresponding relationship.

[0072] Specifically, interference will make the intensity of the light output different at each wavelength. When Δφ=(2m+1)π, the energy of the two light beams coupled has a minimum value, and a trough appears. The wavelength of the interference trough is λ dip for:

[0073]

[0074] When the salinity of the external solution changes, the refractive index of the solution also changes. Since the few-mode fiber is in direct contact with the external solution, the effective refractive index of each mode in the few-mode fiber will also change, and the effective refractive index difference will also change, which will cause the drift of the interference trough:

[0075]

[0076] Among them, Δn c is the change in the refractive index of the solution to be tested.

[0077] In this embodiment, based on the mode interference principle of few-mode optical fiber, the spectral response of the sensor is sensitive to salinity changes, and the drift of the interference trough position can capture tiny salinity changes. This measurement method based on wavelength drift is more stable and reliable than directly measuring transmittance or reflectivity. The amplification effect of mode interference makes the detection limit lower, which can meet the needs of high-precision ocean monitoring or laboratory analysis, and the entire measurement process is based on optical signals, without electrical contact, reducing potential electrical corrosion problems.

[0078] Example 4

[0079] This embodiment manufactures a salinity sensor based on few-mode fiber mode interference proposed in Embodiment 1 and Embodiment 2.

[0080] A method for manufacturing a salinity sensor based on few-mode optical fiber mode interference comprises the following steps:

[0081] Process the structural material with the anti-corrosion layer, design the holes at both ends based on the outer diameter of the few-mode optical fiber, and keep the centerline height of the holes at both ends consistent;

[0082] Grooving the structural material along a non-end region, and then inserting the few-mode optical fiber into the structural material along the opening;

[0083] Corroding the cladding of the non-end region of the few-mode optical fiber by using a chemical solvent;

[0084] Connect two ends of the few-mode optical fiber to the first single-mode optical fiber and the second single-mode optical fiber respectively.

[0085] like Figure 4 As shown, it is a schematic diagram of making a salinity sensor based on few-mode fiber mode interference.

[0086] As an example, a whole piece of polytetrafluoroethylene plate is obtained as the structural material, corresponding to Figure 4 (a).

[0087] The polytetrafluoroethylene plate is processed, and the holes at both ends are designed based on the outer diameter of the few-mode optical fiber, and the center line height of the holes at both ends is kept consistent. The holes are processed at both ends by a laser cutting machine, corresponding to Figure 4 (b).

[0088] The polytetrafluoroethylene plate is grooved along the non-end area, and the groove width is designed to be slightly larger than the diameter of the few-mode fiber cladding as the exposure area of ​​the few-mode fiber. Ensure that the groove width and depth can allow the few-mode fiber to be placed smoothly in the groove without being affected by stress. Clean and polish the processed groove to remove debris and irregular edges to prevent damage to the fiber. Then, the few-mode fiber is inserted into the polytetrafluoroethylene plate along the opening, corresponding to Figure 4 (c).

[0089] Add hydrofluoric acid etchant to the slotted area, observe the corrosion of the few-mode fiber cladding and control the corrosion time to ensure that the few-mode fiber core will not be over-corroded and the cladding will not be completely corroded. After the corrosion is completed, use a neutralizer to neutralize the etchant and clean the corroded fiber, then use dry nitrogen to gently blow dry the fiber. Figure 4 (d).

[0090] Finally, the two ends of the few-mode optical fiber are connected to the first single-mode optical fiber and the second single-mode optical fiber respectively to obtain the ocean salinity sensor based on the principle of few-mode optical fiber cladding corrosion and mode interference of the present invention, corresponding to Figure 4 (e).

[0091] The terms in the drawings are for illustrative purposes only and should not be construed as limiting this patent;

[0092] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A salinity sensor based on few-mode fiber mode interferometry, characterized in that: The invention comprises a first single-mode optical fiber, a few-mode optical fiber and a second single-mode optical fiber which are coaxially connected in sequence; the first single-mode optical fiber is used to receive an optical signal input, and the second single-mode optical fiber is used to connect to an external spectrometer; the cladding of the non-end region of the few-mode optical fiber is locally removed in an annular manner to obtain a depressed portion of the few-mode optical fiber; The sensor is placed in a solution to be tested as a whole. When the optical signal passes through the recessed portion of the few-mode optical fiber, interference occurs between different modes. The spectrometer receives the signal and obtains a spectral response. Based on the correspondence between the pre-calibrated spectral response and the salinity, the salinity of the solution to be tested is obtained.

2. The ocean salinity sensor based on few-mode fiber mode interferometry according to claim 1, characterized in that: The spectral response includes transmittance, reflectance or an interference signal.

3. The ocean salinity sensor based on few-mode fiber mode interferometry according to claim 1, characterized in that: The outer surfaces of the first single-mode optical fiber, the non-depressed portion of the few-mode optical fiber and the second single-mode optical fiber are provided with an anti-corrosion layer.

4. The ocean salinity sensor based on few-mode fiber mode interferometry according to claim 1, characterized in that: The step of locally removing the cladding of the non-end region of the few-mode optical fiber in an annular manner comprises: corroding the cladding of the non-end region of the few-mode optical fiber by using a chemical solvent.

5. The ocean salinity sensor based on few-mode fiber mode interferometry according to claim 4, characterized in that: A container composed of corrosion-resistant baffles is arranged outside the non-end region of the few-mode optical fiber, and the chemical solvent is placed in the container to corrode the cladding of the non-end region of the few-mode optical fiber.

6. The ocean salinity sensor based on few-mode fiber mode interferometry according to claim 1, characterized in that: The core range of the few-mode optical fiber is 10-25 μm.

7. The ocean salinity sensor based on few-mode fiber mode interferometry according to claim 1, characterized in that: The diameter of the depressed portion of the few-mode optical fiber is 40% to 80% of the diameter before the cladding is removed.

8. A method for measuring salinity based on few-mode fiber mode interferometry, characterized in that: The salinity sensor based on few-mode fiber mode interference according to any one of claims 1 to 7 is used for measurement, and the method comprises: When the optical signal is coupled from the first single-mode optical fiber into the few-mode optical fiber, multiple modes are excited in the few-mode optical fiber; The concave portion of the few-mode optical fiber changes its refractive index to the optical signal due to the influence of salinity, so that interference occurs between different modes; The optical signal is coupled from the few-mode fiber to the second single-mode fiber and transmitted to the spectrometer to form an output spectrum; The output light intensity distribution of the output spectrum is measured by a spectrometer to obtain the spectral response; The salinity of the solution to be tested is obtained based on the correspondence between the pre-calibrated spectral response and the salinity.

9. The method for measuring salinity based on few-mode fiber mode interferometry according to claim 8, characterized in that: The step of calibrating the corresponding relationship between the spectral response and the salinity comprises: Select a standard solution with known salinity for measurement, and record the interference troughs formed by the spectral response obtained under solutions with different salinity concentrations; Select any interference trough as a reference, calculate the drift between the remaining interference troughs and the reference interference trough, and construct the corresponding relationship between the interference trough drift and the solution salinity; The steps of obtaining the salinity of the solution to be tested based on the corresponding relationship include: The drift between the interference trough formed by the spectral response of the solution to be tested and the reference interference trough is calculated, and the salinity of the solution to be tested is obtained based on the corresponding relationship.

10. A method for manufacturing a salinity sensor based on few-mode fiber mode interference as claimed in any one of claims 1 to 7, characterized in that: The following steps are involved: Process the structural material with the anti-corrosion layer, design the holes at both ends based on the outer diameter of the few-mode optical fiber, and keep the centerline height of the holes at both ends consistent; Grooving the structural material along a non-end region, and then inserting the few-mode optical fiber into the structural material along the opening; Corroding the cladding of the non-end region of the few-mode optical fiber by using a chemical solvent; Connect two ends of the few-mode optical fiber to the first single-mode optical fiber and the second single-mode optical fiber respectively.

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