Humidity sensor based on small-diameter fiber bragg grating

Abstract

A humidity sensor based on a small-diameter fiber bragg grating is characterized by comprising a fiber bragg grating demodulator, a 2 * 1 fiber coupler, a transmission fiber, a temperature sensing fiber bragg grating and a humidity sensing fiber bragg grating, the fiber bragg grating demodulator comprises a broadband light source output part and a signal demodulation part. The humidity sensing fiber bragg grating is prepared on a small-diameter optical fiber, a gate region is coated with polyimide added with lithium bromide with the concentration of 0.5-2%, the thickness of a coating layer ranges from 10 micrometers to 100 micrometers, and the humidity sensing fiber bragg grating is used for sensing relative humidity. Compared with a traditional fiber bragg grating humidity sensor, the fiber bragg grating humidity sensor has higher sensitivity, better humidity linear response and shorter response time due to the fact that the small-diameter fiber bragg grating has higher deformation quantity compared with the traditional fiber bragg grating humidity sensor.

Description

Technical Field

[0001] The utility model belongs to the technical field of optical fiber sensing, and particularly relates to a humidity sensor based on a fine-diameter optical fiber grating. Background Art

[0002] Human life, industrial and agricultural production, and military fields are closely related to the humidity of the surrounding environment. Real-time humidity monitoring plays an important role in military and agricultural production fields such as arsenals, weapon moisture prevention, grain storage, food mildew prevention, and precision instrument manufacturing. There are many types of humidity measurement devices on the current market. The dry-wet bulb method has a simple structure and is easy to operate, but the number of uses in actual applications is limited. Electronic humidity sensors have high accuracy, but are easily affected by factors such as electromagnetic interference, and have large measurement errors when used in some special occasions. And because this type of sensor needs to be powered on site, it will bring additional risks when used in special places such as arsenals, granaries, and museums.

[0003] Fiber optic humidity sensor is a sensor that converts the humidity change of the external environment into the change of light wave characteristic quantity. Compared with electronic sensors, fiber optic humidity sensor has the advantages of high sensitivity, anti-electromagnetic interference, intrinsic safety, easy distribution and long-distance sensing measurement, and has attracted widespread attention. According to the different sensing mechanisms and selected optical parameters, fiber optic humidity sensors are mainly divided into interferometric fiber optic humidity sensors, evanescent wave fiber optic humidity sensors and fiber grating humidity sensors. Interferometric fiber optic humidity sensor is a humidity sensor designed based on fiber optic interferometer, which is usually composed of fiber optic interferometer and humidity sensitive materials. According to different structures, it can be divided into Mach-Zehnder interferometer, Michelson interferometer, Fabry-Perot interferometer, etc. Interferometric fiber optic humidity sensor realizes the sensing of relative humidity by comparing and analyzing the interference spectrum before and after measurement. Interferometric fiber optic humidity sensor has high sensitivity and wide dynamic measurement range, but because the fiber optic interferometer is used as the sensor structure, the relatively complex structure of the sensor requires a high manufacturing process and high cost. The evanescent wave type fiber optic humidity sensor belongs to the optical power type humidity sensor. It changes the local structure of the optical fiber through processing techniques such as fusion etching, and then coats the humidity sensitive material on the processed optical fiber structure to form a humidity sensitive film. When the relative humidity in the environment changes, the humidity sensitive film contacts the water molecules in the environment, causing its physical or chemical properties to change, changing the refractive index of the humidity sensitive film, thereby affecting the transmission of the evanescent wave and changing the light intensity. By demodulating the output light intensity, the relative humidity in the environment can be measured. The evanescent wave type fiber optic humidity sensor has the advantage of high sensitivity, but its stability and accuracy are low. The structure of the fiber grating humidity sensor is to coat a layer of humidity sensitive material near the grating area of ​​the fiber grating. When the relative humidity in the environment changes, the humidity sensitive coating will expand, causing the grating area to produce strain, thereby causing the central wavelength of the grating period to shift, thereby achieving the purpose of measuring relative humidity. In contrast, the fiber grating humidity sensor has a good linear response to the measurement of relative humidity, with fewer interference factors, and can be multiplexed over long distances, greatly saving the cost of measurement. To improve the resolution of humidity sensors, the most direct way is to increase the humidity sensitivity of the sensor, but the increase in sensitivity is often accompanied by an increase in linear error and a decrease in stability, thereby reducing the overall performance of the sensor.

[0004] In view of the above problems, the utility model proposes a humidity sensor based on fine-diameter fiber Bragg grating, which consists of two parts: a demodulation area and a sensing area. The demodulation area is a fiber Bragg grating demodulator; the sensing area can be divided into two parts: a temperature sensing fiber Bragg grating and a humidity sensing fiber Bragg grating, both of which use fine-diameter fiber Bragg gratings. The temperature sensing fiber Bragg grating part does not require special treatment; the humidity sensing fiber Bragg grating is prepared on a fine-diameter optical fiber, and polyimide with a concentration of 0.5 to 2% lithium bromide is added to the grating area, and the coating thickness is between 10 and 100 μm for relative humidity sensing. Since the fine-diameter fiber Bragg grating has a higher deformation than the traditional fiber Bragg grating, the fiber Bragg grating humidity sensor has higher sensitivity, better humidity linear response and faster response time than the traditional fiber Bragg grating humidity sensor. Utility Model Content

[0005] In order to improve the problem of increased linear error and reduced stability caused by increased sensitivity in traditional fiber Bragg grating humidity sensors, thereby reducing sensor performance, the utility model proposes a humidity sensor based on fine-diameter fiber Bragg gratings. Since fine-diameter fiber Bragg gratings have a higher deformation amount than traditional fiber Bragg gratings, the sensitivity of the sensor can be improved, and the sensor can have good linear response and repeatability stability.

[0006] The device adopted by the utility model to solve the technical problem:

[0007] A humidity sensor based on fine-diameter fiber Bragg grating, characterized in that it comprises a fiber Bragg grating demodulator, a 2×1 fiber coupler, a transmission fiber, a temperature sensing fiber Bragg grating, and a humidity sensing fiber Bragg grating; two ports on one side of the 2×1 coupler are respectively connected to the light source output end and the optical signal receiving end of the fiber Bragg grating demodulator, and a port on the other side of the 2×1 coupler is connected to the transmission fiber; the tail of the transmission fiber is connected to the temperature sensing fiber Bragg grating and the humidity sensing fiber Bragg grating in sequence; the temperature sensing fiber Bragg grating and the humidity sensing fiber Bragg grating are both prepared on the fine-diameter fiber; the temperature sensing fiber Bragg grating and the humidity sensing fiber Bragg grating are both The humidity sensing fiber grating is in a free state; the grating area of ​​the humidity sensing fiber grating is coated with a polyimide humidity sensitive coating with a concentration of 0.5-2% lithium bromide, and the coating thickness ranges from 10 to 100 μm; the temperature sensing fiber grating and the humidity sensing fiber grating have different central wavelengths, and maintain a certain wavelength spacing to ensure that the central wavelengths of the two fiber gratings do not overlap when an offset occurs; the fiber grating demodulator includes a light source output part and a signal demodulation part, wherein the fiber grating demodulator is the emission area and demodulation area of ​​the optical signal, and the temperature sensing fiber grating and the humidity sensing fiber grating together constitute the sensing area.

[0008] The beneficial effects of the utility model are:

[0009] The utility model uses a thin-diameter fiber Bragg grating, which has a higher deformation amount than a traditional fiber Bragg grating, improves the sensitivity of the sensor, and makes the sensor have good linear response and repeatability.

[0010] In the utility model, polyimide with lithium bromide added is combined with the optical fiber grating, and the water absorption of the polyimide is increased by the good water absorption of lithium bromide, thereby increasing the axial stress of the optical fiber grating and improving the sensitivity.

[0011] The utility model writes the temperature sensing fiber grating in front of the humidity sensing fiber grating, and the two are written on the same optical fiber. The error caused by temperature change during humidity sensing can be eliminated through the temperature measurement of the temperature sensing fiber grating. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 The utility model is a schematic diagram of a humidity sensor based on fine-diameter fiber grating. DETAILED DESCRIPTION

[0013] The utility model is further described below in conjunction with the accompanying drawings.

[0014] A humidity sensor based on fine-diameter fiber Bragg grating comprises: a fiber Bragg grating demodulator 1, a 2×1 fiber coupler 2, a transmission optical fiber 3, a temperature sensing fiber Bragg grating 4, and a humidity sensing fiber Bragg grating 5.

[0015] The light source output port of the fiber Bragg grating demodulator 1 is connected to the 2-1 port of the 2×1 coupler 2, the signal receiving port of the fiber Bragg grating demodulator 1 is connected to the 2-2 port of the 2×1 coupler 2, the 2-3 port of the 2×1 coupler 2 is connected to the transmission optical fiber 3, the transmission optical fiber 3 remains in a free state, the tail of the transmission optical fiber 3 is sequentially connected to the temperature sensing fiber Bragg grating 4 and the humidity sensing fiber Bragg grating 5, the temperature sensing fiber Bragg grating 4 and the humidity sensing fiber Bragg grating 5 are in a free state. The fiber Bragg grating demodulator 1 is the emission area and demodulation area of ​​the optical signal, and the temperature sensing fiber Bragg grating 4 and the humidity sensing fiber Bragg grating 5 together constitute the sensing area.

[0016] The working mode of the utility model is as follows: the light waves emitted by the light source of the fiber grating demodulator 1 enter the transmission optical fiber 3 through the 2-1 port and the 2-3 port of the 2×1 coupler 2, and the light is transmitted from the transmission optical fiber 3 to the temperature sensing fiber grating 4. At this time, the light waves that meet the Bragg reflection conditions of the temperature sensing fiber grating 4 are reflected, and the light of other wavelengths continues to propagate to the humidity sensing fiber grating 5. The light that meets the Bragg reflection conditions of the humidity sensing fiber grating 5 is reflected again, transmitted in the reverse direction along the transmission optical fiber 3, and enters the signal receiving end of the fiber grating demodulator 1 through the 2-2 port of the 2×1 coupler. When the relative humidity in the environment changes, the polyimide humidity sensitive film layer will deform due to the absorption or release of water molecules. The deformation of the humidity sensitive film layer is equivalent to applying axial stress to the humidity sensing fiber grating 5, and the relative humidity change has no effect on the temperature sensing fiber grating 4. The central wavelength of the humidity sensing fiber grating 5 is very sensitive to the axial stress response. The distribution of the spectrum is obtained by the signal demodulation part of the fiber grating demodulator. The relative humidity can be accurately measured by analyzing the spectra of the two fiber gratings.

[0017] The specific expression for realizing relative humidity measurement and temperature compensation in the utility model is:

[0018] Δλ1=BΔ T ,(1)

[0019] Δλ2=CΔ RH +DΔ T ,(2)

[0020] When the relative humidity in the environment changes, equation (1) expresses the change in the central wavelength of the temperature sensing fiber Bragg grating Δλ1 and the temperature change Δ T The relationship between the humidity sensing fiber Bragg grating center wavelength change Δλ2 and the relative humidity change Δ RH , Temperature change Δ T When the fiber Bragg grating is subjected to axial stress, the central wavelength of its reflection spectrum changes significantly.

[0021] The central wavelength of the fiber Bragg grating is related to the effective refractive index and grating period of the fiber Bragg grating. The specific expression is:

[0022] λ B =2n eff Λ,(3)

[0023]

[0024] Formula (3) represents the central wavelength λ of the fiber Bragg grating B The effective refractive index n of the fiber coreeff , the relationship between the fiber grating period Λ; where n eff and Λ will change with temperature and stress. Equation (4) represents the change in the central wavelength of the fiber Bragg grating Δλ B The relationship between temperature T and stress ε. α is the thermal expansion coefficient of the optical fiber material, p ij is the elastic-optic tensor component of the optical fiber material, ξ is the thermo-optic coefficient of the optical fiber material, Δε is the strain change, ΔT is the temperature change, and ν is the Poisson's ratio coefficient of the optical fiber material.

[0025] The key technologies that enable this device to realize fiber Bragg grating humidity sensing are:

[0026] 1. Compared with traditional fiber Bragg gratings, thin-diameter fiber Bragg gratings have higher deformation, improve the sensitivity of the sensor, and make the sensor have good linear response and repeat stability. The utility model uses a thin-diameter optical fiber with a diameter of 80μm, which has higher sensitivity than the common optical fiber with a diameter of 125μm on the market.

[0027] 2. The sensitivity of the fiber Bragg grating humidity sensor is related to the water absorption of the polyimide coating. The stronger its water absorption, the greater the deformation, the stronger the axial stress on the optical fiber, and the higher the sensitivity. In the utility model, 0.5-2% lithium bromide is added to the polyimide to increase the water absorption of the polyimide, so as to ensure that the polyimide coating has a larger deformation amount, so that the fiber Bragg grating has a larger axial stress and increases the sensitivity. The thickness of the polyimide coating is 10-100μm, and the thickness of the polyimide coating can be adjusted according to the actual application requirements so that the relative humidity response speed and measurement range meet the application requirements.

[0028] 3. The temperature sensing fiber Bragg grating and humidity sensing fiber Bragg grating used in the utility model are written on the same optical fiber, and the error caused by temperature change during humidity sensing can be eliminated by measuring the temperature of the temperature sensing fiber Bragg grating. The central wavelengths of the two fiber Bragg gratings are not equal, and a certain wavelength spacing is maintained to ensure that the central wavelengths of the two fiber Bragg gratings will not overlap when offset occurs.

[0029] An embodiment of a humidity sensor based on a fine-diameter fiber Bragg grating, such as Figure 1As shown. The wavelength range of the fiber Bragg grating demodulator 1 is 1520-1580nm, the demodulation accuracy is 0.001nm, and the scanning frequency is 1000Hz; the central wavelength of the temperature sensing fiber Bragg grating 4 is 1535.2nm, the reflectivity is 99.9%, the side mode suppression ratio is 22.5dB, and the 3dB bandwidth is 0.2nm; the central wavelength of the humidity sensing fiber Bragg grating 4 is 1540.3nm, the reflectivity is 99.7%, the side mode suppression ratio is 22.2dB, the 3dB bandwidth is 0.25nm, the thickness of the polyimide coating is 20μm, and the lithium bromide concentration is 1.5%. In the relative humidity range of 25-85%, the humidity sensitivity is 5.71pm / %RH; when the humidity changes rapidly from 39.7%RH to 60.1%RH, the response time of the humidity sensor is 237s. The standard deviation of 30%RH fluctuation is 3.37, the standard deviation of 50%RH fluctuation is 2.65, and the standard deviation of 70%RH fluctuation is 4.70.

Claims

1. A humidity sensor based on fine-diameter fiber Bragg grating, characterized in that It includes a fiber Bragg grating demodulator, a 2×1 fiber coupler, a transmission fiber, a temperature sensing fiber Bragg grating, and a humidity sensing fiber Bragg grating; two ports on one side of the 2×1 coupler are respectively connected to the light source output end and the optical signal receiving end of the fiber Bragg grating demodulator, and a port on the other side of the 2×1 coupler is connected to the transmission fiber; the tail of the transmission fiber is connected to the temperature sensing fiber Bragg grating and the humidity sensing fiber Bragg grating in sequence; the temperature sensing fiber Bragg grating and the humidity sensing fiber Bragg grating are both prepared on a thin-diameter optical fiber; the temperature sensing fiber Bragg grating and the humidity sensing fiber Bragg grating are both in a free state; the above-mentioned humidity The grating area of ​​the sensing fiber Bragg grating is coated with a polyimide humidity-sensitive coating with a concentration of 0.5-2% lithium bromide, and the coating thickness ranges from 10 to 100 μm; the temperature sensing fiber Bragg grating and the humidity sensing fiber Bragg grating have different central wavelengths and maintain a certain wavelength spacing to ensure that the central wavelengths of the two fiber Bragg gratings do not overlap when an offset occurs; the fiber Bragg grating demodulator includes a light source output part and a signal demodulation part, wherein the fiber Bragg grating demodulator is the emission area and demodulation area of ​​the optical signal, and the temperature sensing fiber Bragg grating and the humidity sensing fiber Bragg grating together constitute the sensing area.

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