A temperature-compensated chirped fiber Bragg grating sensor for measuring solution concentration

Abstract

The present invention discloses a temperature-compensated chirped fiber grating sensor for measuring solution concentration, which is composed of a broadband light source, a filter, a sensing unit, a liquid to be measured, a liquid container, a spectrometer, and a single-mode optical fiber used for connection. The broadband laser output by the broadband light source is input into the sensing unit after being limited to a specific wavelength through a filter. When the solution concentration changes, the solution refractive index changes at the same time, thereby causing the effective refractive index of the sensing unit to change, causing the spectral wavelength to drift. At the same time, the graphene oxide film layer coated on the sensing unit will improve the sensing performance of the sensor. In addition, because the dual parameters of temperature and solution concentration can be measured, the sensor has the ability of temperature compensation at high sensitivity, and has a good application prospect.

Description

Technical Field

[0001] The invention provides a temperature-compensated chirped fiber grating sensor for measuring solution concentration, belonging to the technical field of optical fiber sensing. Background Art

[0002] Chirped fiber grating is a type of fiber grating, a special fiber grating whose period changes continuously along the axial direction of the optical fiber. Because its transmission spectrum has a very wide bandwidth, it can achieve band stop at many wavelengths in the transmission spectrum, which is suitable for multi-wavelength filtering and has special advantages in optical communications.

[0003] Usually, current solution concentration sensors can only measure a single parameter, which is the solution concentration. The measurement of the solution concentration is obtained by measuring the refractive index and then substituting it into the empirical formula. Therefore, in the final analysis, the measurement of solution concentration is actually the measurement of refractive index. The refractive index usually does not change with other environmental changes, but temperature changes will cause changes in the refractive index. Therefore, in order to accurately measure the refractive index change, it is very necessary to make a temperature-compensated refractive index sensor. Summary of the invention

[0004] In view of the shortcomings of the prior art, the present invention aims to provide a temperature-compensated chirped fiber grating sensor for measuring solution concentration. The broadband laser output by a broadband light source is limited to a specific wavelength through a filter and then input into a sensing unit. When the solution concentration changes, the refractive index around the sensing unit will change. The corroded end of the sensing unit is more sensitive to the refractive index change, and the temperature change can be detected at the uncorroded end. At the same time, after the grating surface is coated with a graphene oxide film, the liquid small molecules in the refractive index matching liquid can penetrate into the small gaps between the graphene oxide film layers and be absorbed by the graphene oxide. Later, since the change of carrier concentration will affect the complex refractive index of graphene itself, the existence of graphene film makes the sensing unit more sensitive to changes in the external environment. Therefore, when the sample coated with graphene is in an external environment with different refractive indices, its spectrum will change significantly. After the refractive index change is measured, the solution concentration can be obtained by substituting the empirical formula of solution concentration-solution refractive index. The sensing unit is very sensitive to changes in the surrounding refractive index and can detect temperature changes in the temperature sensitive wavelength region, achieving the effect of temperature compensation, improving the stability of the sensor, having good sensitivity, and having good application prospects.

[0005] 1. A temperature-compensated chirped fiber Bragg grating sensor for measuring solution concentration, characterized in that it is composed of a broadband light source (1), a filter (2), a sensing unit (3), a liquid to be measured (4), a liquid container (5), a spectrometer (6), and a single-mode optical fiber (7) for connection; characterized in that the sensing unit (3) is composed of a corroded chirped fiber Bragg grating refractive index sensing area (31), a non-corroded chirped fiber Bragg grating temperature compensation area (32), and a graphene oxide film layer (33); the output light of the broadband light source (1) is transmitted to the filter (2) so that the spectrum is limited to 1530nm-1585nm, and then the optical signal is input into the sensing unit (3); the liquid to be measured (4) is located in the liquid container (5), and the sensing unit (3) is connected to the liquid container (5). The unit (3) is placed in the liquid to be tested (4). The change of the environmental refractive index will cause the spectral peak to move, and the change of the environmental temperature will cause the overall drift in the detection wavelength range. The graphene oxide film layer (33) can improve the sensitivity of the sensor, so that the corrosion chirped fiber grating refractive index sensing area (31) can detect the change of the environmental refractive index, and the non-corrosion chirped fiber grating temperature compensation area (32) can play a role in temperature compensation. After that, the spectrum change is detected by the spectrometer (6), and the solution concentration change can be obtained by substituting it into the empirical formula of the solution concentration. The various parts of the sensor are connected by a single-mode optical fiber (7). The sensor has high sensitivity, a large detection range, and can be temperature compensated, and has a good application prospect.

[0006] Wherein, the wavelength range of the broadband light source (1) is 1530nm~1585nm.

[0007] Wherein, the filter (2) is a bandpass filter.

[0008] The sensing unit (3) is a linear chirped fiber grating with a diameter of 100 μm, a length of 14 mm, a core refractive index of 1.45, and a refractive index modulation amplitude of 5×10 -5 , the chirp coefficient is 2.5nm / mm.

[0009] The etched chirped fiber grating refractive index sensing area (31) in the sensing unit (3) is 7 mm long and has an etching degree of 85%.

[0010] Wherein, the non-corroded chirped fiber grating temperature compensation zone (32) in the sensing unit (3) is 7 mm long.

[0011] The graphene oxide film layer (33) in the sensing unit (3) has a thickness of 272 nm, a single layer thickness of 6.8 nm, and is coated with 40 layers.

[0012] The working principle of the present invention is: the corroded chirped grating combines the wavelength selection characteristics of the Bragg grating and the transmission characteristics of the corroded optical fiber potential field. When placed in different environments, the remaining grating after corrosion can be regarded as the core, and the surrounding medium of the optical fiber can be regarded as the cladding. The drift of the Bragg wavelength caused by the difference in the core diameter is studied below. When the period Λ of the grating remains unchanged, it can be known from the Bragg condition that the wavelength λ B With the effective refractive index n eff The phase factor β of the single-mode fiber is closely related to the diameter of the fiber, so the effective refractive index n of the grating eff =β / k0 (where k0 = 2π / λ) also changes with the change of fiber diameter;

[0013] Introducing fiber normalized frequency into calculations and the normalized lateral propagation constant Therefore, the effective refractive index n eff It is expressed as:

[0014]

[0015] Where a is the radius of the optical fiber, n1 is the refractive index of the optical fiber core, and n2 is the refractive index of the cladding. In addition, the lateral propagation attempt U and the normalized frequency V satisfy the optical fiber eigenvalue equation:

[0016] U 2 +W 2 =V 2 (2)

[0017] The Gaussian field approximation is used to simplify this equation, and the relationship between U and V can be expressed as follows:

[0018] We can get:

[0019]

[0020]

[0021] In fact, n1, k0 are constants, so the effective refractive index of the fiber Bragg grating n eff It is a function of the fiber radius a and the cladding refractive index n2. Combining the above formulas, when the grating radius a changes, the fiber Bragg wavelength will change accordingly; when the cladding refractive index n2 changes, the fiber Bragg wavelength will also change accordingly.

[0022] The beneficial effects of the present invention are as follows: a chirped fiber grating in the corroded part coated with graphene oxide and a chirped fiber grating in the non-corroded part are respectively used to detect the refractive index and the temperature, so that the sensor is very sensitive to the change of the environmental refractive index. At the same time, the temperature compensation can significantly improve the sensitivity of the sensor, providing a new method for solution concentration detection that is relatively simple, easy to build, reliable and highly sensitive. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of a temperature-compensated chirped fiber grating sensor characteristic device for measuring solution concentration of the present invention.

[0024] Figure 2 It is a schematic diagram of the structure of the sensor unit of the present invention. DETAILED DESCRIPTION

[0025] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0026] 1. A temperature-compensated chirped fiber Bragg grating sensor for measuring solution concentration, characterized in that it is composed of a broadband light source (1), a filter (2), a sensing unit (3), a liquid to be measured (4), a liquid container (5), a spectrometer (6), and a single-mode optical fiber (7) for connection; characterized in that the sensing unit (3) is composed of a corroded chirped fiber Bragg grating refractive index sensing area (31), a non-corroded chirped fiber Bragg grating temperature compensation area (32), and a graphene oxide film layer (33); the broadband light source (1) outputs light that is transmitted to the filter (2) so that the spectrum is limited to 1530nm-1585nm, and then the optical signal is input into the sensing unit (3); the sensing unit (3) is placed in the liquid to be measured (4), and the dielectric constant of the graphene oxide film layer (33) will change with the change of the environment, which will cause the overall temperature of the fiber Bragg grating to change. The increase in effective refractive index causes the drift of the optical fiber Bragg wavelength and increases the evanescent field intensity on the grating surface, which can improve the sensitivity of the sensor. The change in environmental refractive index will cause the spectral peak to move, and the change in environmental temperature will cause the entire detection wavelength range to drift, so that the corrosion chirped optical fiber grating refractive index sensing area (31) can detect the change in environmental refractive index, and the non-corrosion chirped optical fiber grating temperature compensation area (32) can play a role in temperature compensation. In the refractive index sensing area, the change in environmental refractive index is detected by a spectrometer (6), and the change in solution concentration can be obtained by substituting the detected refractive index change into the empirical formula of solution concentration. The change in environmental temperature is detected in the temperature compensation area, thereby achieving the effect of temperature self-compensation while detecting the refractive index. All parts of the sensor are connected by a single-mode optical fiber (7).

Claims

1. A temperature-compensated chirped fiber Bragg grating sensor for measuring solution concentration, characterized in that: The invention comprises a broadband light source (1), a filter (2), a sensing unit (3), a liquid to be measured (4), a liquid container (5), a spectrometer (6), and a single-mode optical fiber (7) for connection; the sensing unit (3) comprises a corrosion chirped optical fiber grating refractive index sensing area (31), a non-corroded chirped optical fiber grating temperature compensation area (32), and a graphene oxide film layer (33); the optical fiber grating surfaces of the corrosion chirped optical fiber grating refractive index sensing area (31) and the non-corroded chirped optical fiber grating temperature compensation area (32) are coated with a graphene oxide film layer (33); The broadband light source (1) outputs light which is transmitted to the filter (2) so that the spectrum is limited to 1530nm-1585nm, and then the light signal is input into the sensing unit (3). The liquid to be tested (4) is located in the liquid container (5). The sensing unit (3) is placed in the liquid to be tested (4). The corrosion chirped fiber grating refractive index sensing area (31) can detect the change of the environmental refractive index, and the non-corrosion chirped fiber grating temperature compensation area (32) can play a role of temperature compensation. Then, the spectrum change is detected by the spectrometer (6), and the solution concentration change is obtained by bringing it into the empirical formula of solution concentration. Each part of the temperature-compensated chirped fiber grating sensor for measuring solution concentration is connected by a single-mode optical fiber (7); The working principle of the temperature-compensated chirped fiber grating sensor for measuring solution concentration is as follows: after the grating surface is coated with the graphene oxide film (33), small liquid molecules in the refractive index matching liquid can penetrate into the small gaps between the graphene oxide film layers and be absorbed by the graphene oxide. After absorption, the change in carrier concentration will affect the complex refractive index of the graphene itself, making the sensing unit (3) more sensitive to changes in the external environment, resulting in obvious changes in the spectrum; When the concentration of the solution changes, the refractive index of the environment around the sensing unit will change, thereby causing the spectral peak to move. At the corroded end of the sensing unit (3), the grating remaining after corrosion is regarded as the fiber core, and the environmental medium around the optical fiber is regarded as the cladding. When the refractive index of the cladding changes, the optical fiber Bragg wavelength will also change accordingly, thereby making the sensing unit (3) more sensitive to the refractive index change; at the uncorroded end, the change in environmental temperature will cause the entire detection wavelength range to drift, thereby being able to detect temperature changes; The change in the ambient refractive index detected by the corrosion chirped fiber grating refractive index sensing area (31) is introduced into the empirical formula of solution concentration to obtain the change in solution concentration, and temperature compensation is performed by the change in ambient temperature detected in the non-corroded chirped fiber grating temperature compensation area (32) to measure accurate refractive index changes.

2. The temperature-compensated chirped fiber Bragg grating sensor for measuring solution concentration according to claim 1, characterized in that: The wavelength range of the broadband light source (1) is 1530nm-1585nm.

3. The temperature-compensated chirped fiber Bragg grating sensor for measuring solution concentration according to claim 1, characterized in that: The filter (2) is a bandpass filter.

4. The temperature-compensated chirped fiber grating sensor for measuring solution concentration according to claim 1, characterized in that: The sensing unit (3) is a linear chirped fiber grating with a diameter of 100 μm, a length of 14 mm, a core refractive index of 1.45, a refractive index modulation amplitude of 5×10-5, and a chirp coefficient of 2.5 nm / mm.

5. The temperature-compensated chirped fiber Bragg grating sensor for measuring solution concentration according to claim 1, characterized in that: The corrosion chirped fiber grating refractive index sensing area (31) in the sensing unit (3) is 7 mm long and has a corrosion degree of 85%.

6. The temperature-compensated chirped fiber Bragg grating sensor for measuring solution concentration according to claim 1, characterized in that: The non-corroded chirped fiber grating temperature compensation zone (32) in the sensing unit (3) is 7 mm long.

7. The temperature-compensated chirped fiber Bragg grating sensor for measuring solution concentration according to claim 1, characterized in that: The graphene oxide film layer (33) in the sensing unit (3) has a thickness of 272 nm, a single layer thickness of 6.8 nm, and is coated with 40 layers.

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