High-sensitivity hot-wire type wind speed sensor based on cobalt-doped optical fiber long-period grating and wind speed measuring method

Abstract

The invention discloses a high-sensitivity hot-wire type wind speed sensor based on a cobalt-doped optical fiber long-period grating and a wind speed measuring method. The sensor comprises a broadband light source, the cobalt-doped optical fiber long-period grating and a spectrograph, and the output end of the broadband light source is connected with the spectrograph through the cobalt-doped optical fiber long-period grating. The long-period fiber grating is inscribed on the cobalt-doped fiber by using the laser, then the cobalt-doped fiber is heated by using the broadband light source, and the cobalt-doped fiber long-period grating can absorb broadband light energy and convert the broadband light energy into heat, so that the temperature of the cobalt-doped fiber long-period grating is increased, and the transmission wavelength is subjected to red shift. Under the action of wind speed, the temperature of the cobalt-doped optical fiber long-period grating is reduced, the transmission wavelength of the cobalt-doped optical fiber long-period grating is subjected to blue shift, and the change of the transmission wavelength of the cobalt-doped optical fiber long-period grating is measured by using a spectrograph, so that the wind speed is measured, and the device has the characteristics of simple structure, low manufacturing difficulty, low cost, high sensitivity, high measurement speed, small size and the like.

Description

Technical Field

[0001] The present invention belongs to the field of wind speed sensors, and more specifically, relates to a high-sensitivity hot-wire wind speed sensor based on a cobalt-doped optical fiber long-period grating and a wind speed measurement method. Background Art

[0002] Wind speed measurement plays an important role in various fields such as aerospace, wind power generation, transportation and agricultural production. Hot wire wind speed sensor is an important wind speed measurement device with the advantages of small size, high accuracy, short response time and wide applicability. However, in some harsh environments such as high temperature, radioactivity and corrosion, traditional hot wire wind speed sensors cannot work or the measurement effect is poor. In recent years, hot wire wind speed sensors based on fiber gratings have attracted widespread attention due to their advantages such as passive operation and anti-electromagnetic interference.

[0003] The basic principle of the hot-wire wind speed sensor based on fiber Bragg grating is that when the fiber Bragg grating is heated, the wind speed around the fiber Bragg grating can be calculated by measuring the cooling rate or temperature change of the fiber Bragg grating with wind speed. Most traditional fiber Bragg grating hot-wire wind speed sensors use fiber Bragg gratings with metal film on the surface, and use pump laser to heat the fiber Bragg grating through a pump light coupling structure. The specific process is that the pump laser is coupled to the cladding of the fiber Bragg grating through the pump light coupling structure, and is absorbed by the metal film on the surface of the fiber Bragg grating to generate heat, thereby generating a heating effect. At present, the reported coupling structures include multimode fiber, coreless fiber, long period fiber Bragg grating (LPG), waist cone amplification structure and offset fusion structure. Although the performance of fiber Bragg grating thermal wind speed sensors is constantly improving, the pump light coupling structure and the metal film coating on the fiber Bragg grating surface are required at the same time, which complicates the manufacturing process and structure and increases the cost of the wind speed sensor.

[0004] The article "All-optical fiber anemometer based on laser heated fiber Bragg gratings" reports a hot-wire wind speed sensor based on cobalt-doped fiber Bragg gratings (FBG). This wind speed sensor uses cobalt-doped fiber, which can directly absorb laser light and convert it into heat, thereby converting the fiber Bragg grating into a hot wire. Therefore, there is no need to couple the pump laser to the cladding and plate a metal film on the surface of the fiber Bragg grating, showing excellent application potential. However, all FBG hot-wire wind speed sensors, including those based on cobalt-doped fiber Bragg gratings, have the following two shortcomings: one is that they require pump laser heating; the other is that the sensitivity of wind speed measurement needs to be improved.

[0005] Long-period fiber grating (LPG) is a grating device with a grating period significantly larger than that of fiber Bragg grating (FBG), and the grating period is generally above hundreds of microns. LPG uses the coupling between the core mode and the cladding mode of the same direction of transmission. The light coupled to the cladding will be lost, thereby realizing the modulation and filtering functions. Compared with FBG, LPG has higher temperature sensitivity. The article "Low-power-consumption Fiber-optic Anemometerbased on Long-period Gratingwith SWCNT Coating" reports a wind speed sensor based on LPG. The surface of LPG is coated with a layer of single-walled carbon nanotubes. When the optical signal is transmitted to LPG, part of the coupled light is absorbed by the single-walled carbon nanotubes and generates heat to heat the LPG, making it a hot line. However, since the coupled optical power changes with the wavelength, the temperature of the single-walled carbon nanotube coating will also change, so the measurement accuracy will be affected by the change of wind speed. At the same time, the single-walled carbon nanotube coating complicates the manufacturing process and structure of the wind speed sensor.

[0006] The invention patent with the prior art publication number CN110133320A proposes a plasma resonance fiber hot wire anemometer, a detection system and a method, including a pump light source, a broadband light source, a fiber coupler, a polarizer, a polarization controller, a detection instrument and a tilted fiber Bragg grating with a nanometer-scale metal film coated on the outer surface of the cladding; when the output light of the pump light source and the broadband light source is transmitted to the tilted fiber Bragg grating, part of the light is coupled to the tilted fiber Bragg grating cladding and absorbed by the metal film on the surface of the tilted fiber Bragg grating to generate heat to increase its own temperature, causing the plasma resonance wavelength to shift. Under the action of wind, the heat loss on the surface of the tilted fiber Bragg grating is accelerated, thereby generating a strong plasma resonance wavelength shift and intensity modulation. By measuring the wavelength shift and intensity change of the cladding mode at the anemometer, high-precision measurement of different wind speeds is achieved. This tilted grating wind speed sensor still needs to be plated with a metal film for operation and has a low sensitivity, which is not conducive to popularization and use.

[0007] The existing invention patent with publication number CN109580984A proposes a wind speed sensor based on long-period fiber grating, which is composed of a long-period fiber grating and a fiber reflector. The wind speed sensor uses the long-period fiber grating to couple the signal light to the fiber reflector. Under the action of wind force, the long-period fiber grating bends, and the light intensity coupled to the fiber reflector will change. The wind speed is measured by measuring the power change reflected back to the fiber core by the fiber reflector. The wind speed sensor measures the wind speed by bending the optical fiber, which has the disadvantage of being easily damaged. Summary of the invention

[0008] The present invention provides a high-sensitivity hot-wire wind speed sensor based on a cobalt-doped fiber long-period grating and a wind speed measurement method to overcome the problems of complex structure, the need for coating and pump light coupling structure, and insufficient sensitivity in the prior art.

[0009] The primary purpose of the present invention is to solve the above technical problems. The technical solution of the present invention is as follows:

[0010] A first aspect of the present invention provides a high-sensitivity hot-wire wind speed sensor based on a cobalt-doped fiber long-period grating, comprising a broadband light source, a cobalt-doped fiber long-period grating, and a spectrometer, wherein the broadband light source is connected to the spectrometer via the cobalt-doped fiber long-period grating.

[0011] Furthermore, the devices are connected via single-mode optical fibers and flanges.

[0012] Furthermore, the cobalt-doped fiber long period grating is made on the cobalt-doped fiber by using a specific refractive index modulation technology.

[0013] Furthermore, the cobalt-doped fiber long period grating is connected in the single mode optical fiber by fusion splicing.

[0014] Furthermore, the light absorption coefficient of the cobalt-doped fiber long period grating is 5-15 dB / cm.

[0015] Furthermore, the length of the cobalt-doped fiber long-period grating is 2-10 cm.

[0016] Furthermore, the period of the cobalt-doped fiber long-period grating is 200-600 μm.

[0017] Furthermore, the operating wavelength range of the broadband light source is 1250-1600 nm.

[0018] A second aspect of the present invention provides a method for measuring wind speed, comprising the following steps:

[0019] A broadband light source is used to generate broadband light and transmit it to a cobalt-doped fiber long-period grating;

[0020] The cobalt-doped fiber long-period grating absorbs part of the broadband light from the broadband light source and converts it into heat, thereby increasing its own temperature and red-shifting the transmission wavelength, while transmitting the remaining broadband light to the spectrometer as signal light;

[0021] Under the action of wind speed, the temperature of the cobalt-doped fiber long period grating decreases, so its transmission wavelength blue shifts. The transmission wavelength change of the cobalt-doped fiber long period grating is measured by a spectrometer, thereby realizing the measurement of wind speed change.

[0022] Furthermore, the heat generated by the cobalt-doped fiber long-period grating absorbing broadband light is proportional to the power P of the broadband light. When the length of the cobalt-doped fiber long-period grating is constant, the higher the broadband light power, the greater the heat H generated. gen The larger the value, the heat H generated by the cobalt-doped fiber long-period grating gen Proportional to the power P of broadband light, its expression is as follows:

[0023] H gen =K1Pl

[0024] Where K1 is the proportional constant and l is the length of the cobalt-doped fiber long-period grating.

[0025] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0026] A long-period fiber grating is written on a cobalt-doped fiber using a laser, and then heated with a broadband light source. The cobalt-doped fiber long-period fiber grating can convert itself into heat by absorbing broadband light energy and converting it into heat energy, eliminating the need for pump lasers, pump light coupling structures, and metal coatings. The structure of the sensor is greatly simplified, and the manufacturing difficulty and cost will be reduced. Thanks to the high-temperature sensitivity of the cobalt-doped fiber long-period fiber grating and the characteristic that its wavelength shift does not affect the absorption of optical power, the measurement sensitivity and accuracy have been significantly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to make the purpose and technical solution of the present invention clearer, the present invention provides the following drawings and descriptions:

[0028] Figure 1 A schematic diagram of the structure of a high-sensitivity hot-wire wind speed sensor based on a cobalt-doped fiber long-period grating provided in an embodiment of the present invention;

[0029] Figure 2 A flow chart of a wind speed measurement method provided by an embodiment of the present invention;

[0030] Figure 3 The transmission spectrum of the cobalt-doped fiber long period grating provided by the embodiment of the present invention at different broadband optical powers;

[0031] Figure 4 The wavelength of the cobalt-doped fiber long-period grating provided in the embodiment of the present invention varies with the broadband optical power;

[0032] Figure 5 The transmission spectrum of the cobalt-doped fiber long period grating provided by the embodiment of the present invention at different air flow speeds;

[0033] Figure 6 The wavelength and sensitivity of the hot-wire wind speed sensor based on the cobalt-doped fiber long-period grating provided in the embodiment of the present invention vary with wind speed. DETAILED DESCRIPTION

[0034] In order to more clearly understand the above-mentioned purpose, features and advantages of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that the embodiments of the present application and the features in the embodiments can be combined with each other without conflict.

[0035] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Therefore, the protection scope of the present invention is not limited to the specific embodiments disclosed below.

[0036] The first aspect of the present invention provides a high-sensitivity hot-wire wind speed sensor based on a cobalt-doped fiber long-period grating, such as Figure 1 The structure diagram of a high-sensitivity hot-wire wind speed sensor based on a cobalt-doped fiber long-period grating provided by the present invention is shown. The sensor includes a broadband light source, a cobalt-doped fiber long-period grating, and a spectrometer. The working wavelength range of the broadband light source is 1250-1600nm. The output end of the broadband light source is connected to the spectrometer through a cobalt-doped fiber long-period grating. The light absorption coefficient of the cobalt-doped fiber long-period grating used is 5-15dB / cm, the length value is 2-10cm, and the period is 200-600μm. It should be particularly noted that the devices are connected through single-mode optical fibers and flanges, and the device does not require a pump light source, a pump light coupling structure, and a metal coating operation. Figure 1 The part pointed by the arrow is a schematic diagram of the structure of a cobalt-doped fiber long-period grating probe, which is made by connecting the cobalt-doped fiber long-period grating between two single-mode optical fibers through fusion splicing.

[0037] In a specific embodiment, the core diameter of the cobalt-doped optical fiber is 8 μm, and the cladding diameter is 125 μm. The long-period fiber grating is made by writing point by point on the cobalt-doped optical fiber with a period of 540 μm using a laser.

[0038] The second aspect of the present invention provides a method for measuring wind speed, the flow chart of which is as follows: Figure 2 As shown, the following steps are included:

[0039] The broadband light output by the broadband light source is transmitted to the cobalt-doped fiber long-period grating through the flange;

[0040] When the broadband light is transmitted to the cobalt-doped fiber long period grating, the cobalt-doped fiber long period grating absorbs part of the broadband light energy and generates heat, thereby heating the cobalt-doped fiber long period grating to convert it into heat line, and at the same time transmits the remaining broadband light to the spectrometer as the signal light measured by the spectrometer, eliminating the need for a pump light source, a pump light coupling structure and a metal coating operation;

[0041] When affected by wind and the broadband optical power remains substantially unchanged, the fiber Bragg grating temperature decreases, thereby causing the transmission wavelength of the transmitted light to change;

[0042] The change of the transmission wavelength is measured by a spectrometer, thereby calculating the wind speed value.

[0043] When broadband light is transmitted to the Co-doped fiber long period grating, the Co-doped fiber long period grating absorbs the broadband light energy and generates heat H. gen , the heat generated by the cobalt-doped fiber long-period grating H gen Proportional to the power P of broadband light, its expression is as follows:

[0044] H gen =K1Pl

[0045] Where P is the broadband optical power, l is the length of the cobalt-doped fiber long-period grating, and K1 is the proportional constant.

[0046] Under a certain broadband optical power, the cobalt-doped fiber long-period grating obtains a higher initial temperature. When the wind blows over the heated cobalt-doped fiber long-period grating, the wind will take away some heat, causing the temperature of the cobalt-doped fiber long-period grating to decrease. As the air flow speed increases, the heat dissipation speed of the cobalt-doped fiber long-period grating will accelerate. Heat dissipation H dis The relationship with wind speed can be expressed as:

[0047] H dis =(A+Bv n )(T v -T a )

[0048] In the formula, A and B are empirical calibration constants, v is the air velocity, T v is the temperature of the cobalt-doped fiber long-period grating when the air flow velocity is v, T a is the air flow temperature, n is an index. In a specific embodiment, the value of A is 2.25, the value of B is 1, and the value of n is 0.3.

[0049] Therefore, the temperature of the Co-doped FBG is controlled by two dynamic processes: broadband optical heating and airflow cooling. Only when thermal equilibrium is established, i.e., H gen =H dis The temperature tends to be stable when the temperature of the cobalt-doped fiber long-period grating is T v The relationship between it and wind speed v can be expressed as:

[0050]

[0051] Since the wavelength shift of the cobalt-doped fiber long-period grating is proportional to the temperature change, Δλ is defined as the wavelength shift of the cobalt-doped fiber long-period grating relative to the initial wavelength in the absence of broadband optical heating, which can be expressed as:

[0052]

[0053] In the formula, K T is the temperature sensitivity of the cobalt-doped fiber long-period grating. In a specific embodiment, K T The value of is 0.05nm / ℃, ΔT=T v -T a is the temperature difference between the cobalt-doped fiber long-period grating and the airflow.

[0054] Therefore, the wavelength of the cobalt-doped fiber long-period grating can be described as a function related to the gas flow velocity:

[0055]

[0056] Where λ0 is the initial wavelength of the cobalt-doped fiber long-period grating when there is no pump laser heating.

[0057] In a specific embodiment, when the broadband optical power varies between 0-500 mW, the transmission spectrum of the cobalt-doped fiber long period grating is measured. During the measurement, the ambient air temperature is maintained at 20°C and the wind speed is zero. By measuring the transmission spectrum of the cobalt-doped fiber long period grating, as shown in FIG. Figure 3 As shown, the wavelength of the cobalt-doped fiber long-period grating increases with the increase of broadband optical power, and the maximum wavelength shift is 19.7nm. Figure 4 The measured wavelength of the cobalt-doped fiber long-period grating varies with the broadband optical power. The wavelength response to the broadband optical power can reach 0.038nm / mW, and the linearity is 0.998.

[0058] This is consistent with the previous theoretical analysis, indicating that in this case, the linear relationship between any two of the three parameters of the cobalt-doped fiber long-period grating, namely wavelength, temperature and broadband optical power, holds true.

[0059] Then, the laser power was set to 500 mW and the air flow temperature was maintained at 20 °C. Figure 5 The transmission spectrum of the cobalt-doped fiber long period grating measured at different airflow speeds. It can be seen that as the airflow speed increases, the transmission spectrum wavelength moves rapidly toward the short-wave direction. When the wind speed is 8.0m / s, the maximum wavelength shift is -8.75nm. The measured wavelength of the cobalt-doped fiber long period grating changes with the airflow speed as shown in the figure. Figure 6 After fitting the data in the form of formula (1), the fitting curve of the cobalt-doped fiber long-period grating is obtained.

[0060]

[0061] The fitting degree is 0.997, which is a high fitting degree. According to the fitting equation, the airflow velocity can be easily calculated according to the measured wavelength of the cobalt-doped fiber long-period grating. The formula is:

[0062]

[0063] The sensitivity of the wind speed sensor based on cobalt-doped fiber long-period grating is equal to the derivative of the wavelength with respect to the airflow velocity, which can be expressed as

[0064]

[0065] It can be seen from the above formula that when the wind speed is 0.1, 0.5, 3.0, and 8.0 m / s, the calculated sensitivity of the cobalt-doped fiber long-period grating hot wire wind speed sensor is -8.70, -2.29, -0.46, and -0.18 nm / (m / s), respectively.

[0066] At a wind speed of 0.5 m / s, the wind speed sensor of the present invention exhibits extremely high sensitivity, with a sensitivity value of -2.29 nm / (m / s), far exceeding the prior art. In addition, the sensor is designed to provide a wide measurement range to accommodate different detection needs, and the test range of the present invention is moderate, 0-8.0 m / s. It is worth noting that the sensitivity of the sensor is more than ten times that of the traditional hot-wire wind speed sensor based on cobalt-doped fiber Bragg gratings. This excellent performance is achieved through a simplified structure without the need for a pump light source, a pump light coupling structure, and metal coating operations.

[0067] 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 high-sensitivity hot-wire wind speed sensor based on cobalt-doped fiber long-period grating, characterized in that: The invention comprises a broadband light source, a cobalt-doped optical fiber long period grating and a spectrometer. The broadband light source is connected with the spectrometer through the cobalt-doped optical fiber long period grating.

2. A high-sensitivity hot-wire wind speed sensor based on cobalt-doped fiber long-period grating according to claim 1, characterized in that: The devices are connected with each other through single-mode optical fibers and flanges.

3. The high-sensitivity hot-wire wind speed sensor based on cobalt-doped fiber long-period grating according to claim 1, characterized in that: The cobalt-doped fiber long period grating is made on the cobalt-doped fiber by utilizing a specific refractive index modulation technology.

4. The high-sensitivity hot-wire wind speed sensor based on cobalt-doped fiber long-period grating according to claim 1, characterized in that: The cobalt-doped fiber long period grating is connected in a single mode optical fiber by fusion splicing.

5. The high-sensitivity hot-wire wind speed sensor based on cobalt-doped fiber long-period grating according to claim 1, characterized in that: The light absorption coefficient of the cobalt-doped optical fiber long period grating is 5-15 dB / cm.

6. The high-sensitivity hot-wire wind speed sensor based on cobalt-doped fiber long-period grating according to claim 1, characterized in that: The length of the cobalt-doped optical fiber long-period grating is 2-10 cm.

7. The high-sensitivity hot-wire wind speed sensor based on cobalt-doped fiber long-period grating according to claim 1, characterized in that: The period of the cobalt-doped optical fiber long-period grating is 200-600 μm.

8. The high-sensitivity hot-wire wind speed sensor based on cobalt-doped fiber long-period grating according to claim 1, characterized in that: The working wavelength range of the broadband light source is 1250-1600nm.

9. A method for measuring wind speed, the method using a high-sensitivity hot-wire wind speed sensor based on a cobalt-doped fiber long-period grating according to any one of claims 1 to 8, characterized in that: The steps include: A broadband light source is used to generate broadband light and transmit it to a cobalt-doped fiber long-period grating; The cobalt-doped fiber long-period grating absorbs part of the broadband light from the broadband light source and converts it into heat, thereby increasing its own temperature and red-shifting the transmission wavelength, while transmitting the remaining broadband light to the spectrometer as signal light; Under the action of wind speed, the temperature of the cobalt-doped fiber long-period grating decreases, so its transmission wavelength blue-shifts; The transmission wavelength change of the cobalt-doped optical fiber long-period grating is measured by a spectrometer, thereby realizing the measurement of wind speed change.

10. A wind speed measurement method according to claim 9, characterized in that: The heat generated by the cobalt-doped fiber long-period grating absorbing broadband light is proportional to the power P of the broadband light. When the length of the cobalt-doped fiber long-period grating is constant, the higher the broadband light power, the greater the heat H generated. gen The larger the value, the heat H generated by the cobalt-doped fiber long-period grating gen Proportional to the power P of broadband light, its expression is as follows: H gen =K1Pl Where K1 is the proportional constant and l is the length of the cobalt-doped fiber long-period grating.

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