CN109580035B - Sapphire fiber high temperature sensor with high fringe visibility and its temperature measurement method - Google Patents

Abstract

The invention discloses a sapphire fiber Faber high temperature sensor with high fringe contrast, comprising a sapphire wafer (1), a sapphire ferrule (2), a sapphire-quartz fiber constituting an optical signal transmission waveguide in a high temperature region and a normal temperature region, an LED light source ( 9) The emitted light enters the high temperature sensor (8), exits from the end face of the sapphire optical fiber (15), and the diffused light irradiates the first reflection surface (16) of the sapphire wafer (1) for the first reflection; the rest of the light transmits The second reflection occurs on the second reflection surface (17) of the wafer; the first reflected light (11) and the first reflected light (12) generated on the two surfaces of the Fa-Per interference cavity are coupled to the sapphire fiber (4 ), the Fa-Pert double fiber interference signal is formed, and the Fa-Per optical path difference is obtained by demodulating the interference signal by spectroscopic method, and then the temperature is reversed. The invention greatly improves the fringe visibility of the interference signal of the Faber sensor, and simultaneously improves the temperature sensitivity and temperature measurement resolution of the sensor.

Description

Sapphire fiber high temperature sensor with high fringe visibility and its temperature measurement method

technical field

The invention relates to the field of optical fiber sensing, in particular to a sapphire optical fiber high-temperature sensor with a fiber-coupled beam splitting design and high stripe visibility, which can realize extreme temperature monitoring in a complex test environment.

Background technique

With the rapid development of aerospace and internal combustion engine industries, higher requirements have been placed on high temperature monitoring technology under extreme conditions. Traditional electrical sensors cannot meet the measurement requirements in the harsh environment of conductive, flammable, explosive and corrosive. High-temperature sensing technology based on sapphire fiber plays an important role in the field of high-temperature monitoring due to its oxidation resistance, high precision, and resistance to electromagnetic interference.

In recent years, various types of sapphire fiber sensors have been proposed to achieve extremely high temperature (above 1000°C) measurement, such as sapphire fiber grating type, blackbody radiation type, and Faber-Perth type sensors. However, the sapphire fiber grating type sapphire fiber sensor needs to be engraved with an expensive femtosecond laser, which is expensive, and is limited by the large numerical aperture of the sapphire fiber, severe mode interference and lower measurement accuracy than other methods. The black body radiation type sapphire fiber sensor is based on Planck's law of black body radiation, and has good temperature measurement accuracy in the high temperature region (600-1600 ℃); however, due to the significant reduction of radiation power in the low temperature section, the signal-to-noise ratio decays rapidly below 600 ℃ , the temperature measurement range is limited, and it can only be used for temperature monitoring in the high temperature section. Faber-type sapphire optical fiber sensor has an extremely wide measurement range, can be flexibly designed according to requirements, and is produced by traditional grinding process, which can be mass-produced and low cost, so it has a wide range of applications. However, because sapphire fiber is made by crystal growth, its length is limited. Internationally, long-distance sensing is generally realized by splicing sapphire fiber and silica fiber, that is, sapphire fiber is used in high temperature area, and quartz fiber is used in normal temperature area. Transmission distance. In the process of hetero-fiber coupling, in order to achieve the highest possible coupling efficiency, the end faces of the sapphire fiber and the silica fiber need to be polished to reduce the scattering loss at the splice point. This is beneficial to improve the optical energy coupling rate, but the precisely polished optical fiber section will introduce a background reflected light into the transmission optical path, which will be superimposed on the output signal of the sensor, reducing the visibility of the interference fringes of the sensor, and thus affecting the demodulation accuracy. At the same time, in order to ensure high fringe visibility, the sensor manufacturing process is very demanding, and the wafer and the fiber end face must be strictly parallel, which puts forward high requirements for the accuracy of clamping and fixing the original.

SUMMARY OF THE INVENTION

Aiming at the shortcomings of the traditional sapphire optical fiber sensor that the fringe contrast and coupling quality cannot be achieved simultaneously, the present invention proposes a sapphire optical fiber high temperature sensor with high fringe visibility and a temperature measurement method thereof. The direct current background amount of the filter is filtered out, which solves the contradiction between the coupling quality of the heterogeneous fiber and the high fringe visibility, and improves the effective coupling strength of the sensor and the visibility of the interference fringe.

The high fringe contrast sapphire fiber Faber high temperature sensor of the present invention comprises a sapphire wafer 1, a sapphire ferrule 2, and a sapphire-quartz fiber constituting an optical signal transmission waveguide in a high temperature region and a normal temperature region; wherein, the sapphire wafer 1 and the sapphire ferrule 2 The circular section of the core 2 is close and fixed with a high-temperature ceramic glue 3; the sapphire-quartz optical fiber is formed by fusion of the sapphire optical fiber 4 and the cut-flat described quartz optical fiber 5 through the fiber fusion point 6 between the end faces; the sapphire-quartz fiber The quartz optical fiber is inserted into the middle hole of the sapphire ferrule 2 from one end of the sapphire optical fiber 4, and the high-temperature ceramic glue 3 is used to fix the optimal sensing signal between the sapphire optical fiber 4 and the sapphire wafer 1; the sapphire-quartz optical fiber is fixed by the sapphire optical fiber. 4 and the cut-flat said silica optical fiber 5 are formed by splicing the fiber fusion point 6 between the end faces, and form an input waveguide 21 and an output waveguide 22; the sapphire-quartz fiber is inserted into the sapphire ferrule from one end of the sapphire fiber 4 The middle hole of 2 is fixed using high-temperature ceramic glue 3 at the optimal place for sensing signals between the sapphire optical fiber 4 and the sapphire wafer 1; the sapphire-quartz optical fiber is respectively connected to the LED light sources 9, The spectrometer 10 realizes the split transmission between the input waveguide 21 and the output waveguide 22, and finally transmits the interference signal to the spectrometer; the two reflecting surfaces of the sapphire wafer 1 constitute a Fa-Per interference cavity, and the divergent light is irradiated to the spectrometer 10. The first reflection occurs on the first reflection surface 16 of the sapphire wafer 1 to form a first beam of reflected light 11 ; the rest of the light is transmitted to the second reflection surface 17 of the sapphire wafer 1 for the second reflection to form a carrying optical path difference The second beam of reflected light 12 of the information, the input waveguide 21 projects the original optical signal emitted by the LED light source 9 onto the sapphire wafer 1, and the output waveguide 22 receives the interference signal reflected back from the two reflective surfaces of the sapphire wafer 1, realizing dual optical paths Separate input and output signal light

The temperature measurement method realized by the sapphire fiber Faber high temperature sensor with high fringe contrast of the present invention, the method comprises the following steps:

The high temperature sensor 8 in working state is connected with the LED light source 9 and the spectrometer 10 through the optical fiber jumper 7; the light emitted by the LED light source 9 enters the high temperature sensor 8 through the optical fiber jumper 7, and passes through the heterogeneous optical fiber fusion point 6 from the sapphire fiber end face. 15, the diffused light is irradiated on the first reflective surface 16 of the sapphire wafer 1 for the first reflection to form the first beam of reflected light 11; the rest of the light is transmitted to the second reflective surface 17 of the wafer for the second reflection, A second beam of reflected light 12 carrying optical path difference information is formed; the first beam of reflected light 11 and the first beam of reflected light 11 generated on the two surfaces of the Fa-Per interference cavity are coupled to the sapphire fiber 4 and output to form a Fa-Per double The optical fiber interference signal 18, that is, the optical path difference between the first reflected light 11 and the second reflected light 12 changes accordingly, resulting in the change of the interference signal; the interference signal 18 passes through the sapphire fiber 4 , Silica fiber 5, fiber jumper 7 and then back to the spectrometer 10;

The interference spectrum signal collected from the spectrometer is expressed as:

表示初始光程差，L、n表示蓝宝石晶片的厚度和折射率；Among them, k=2π/λ; I _B (k) represents the DC background amount in the interference spectral signal, S ₁ (k), S ₂ (k) represent the two beams of reflected light received by the fiber, and Δ represents the two beams of coherent light The optical path difference between,

represents the initial optical path difference, L and n represent the thickness and refractive index of the sapphire wafer;

When the ambient temperature changes, the thickness of the sapphire wafer and the refractive index of the material change:

The formula for the change of refractive index of a sapphire wafer with temperature is expressed as:

n(T) _850nm =a ₀ +a ₁ T+a ₂ T ²

Wherein, T is the temperature in degrees Celsius, and n(T) _850nm is the refractive index of the sapphire wafer material at 850nm;

The thermal expansion function of sapphire material along the C axis is expressed as:

L(T)=[b ₀ +b ₁ T+b ₂ T ² +b ₃ T ³ ]×L ₀

where T represents the Kelvin temperature, and L(T) represents the initial length under the condition of temperature T and initial length L ₀ ;

From the above, it can be seen that the optical path difference Δ=2n(T)L(T) is expressed as the fifth-order polynomial relationship of the temperature T, and the Fa-Per optical path difference is obtained by demodulating the interference signal by spectroscopic method, and then back to the position where the sapphire wafer is located. temperature.

其中F_V表示在背景光信号中干涉条纹的可见度。In the step of obtaining the information of the Faber-Perspective optical path difference by demodulating the interference signal by the spectroscopic method, the demodulation accuracy depends on the acquisition resolution of the interference spectrum and the accurate finding of the fringe peak position: the spectrum acquisition resolution is determined by the resolution of the spectrometer. , the accurate finding of the peak position is closely related to the visibility of the interference spectral fringes. Therefore, in the actual measurement, the visibility of the interference spectral fringes is further expressed as:

where F _V represents the visibility of interference fringes in the background light signal.

The present invention has the following positive effects:

1. Through the application of the fiber-coupled beam splitting model, the reflected background light introduced by the input fiber end face and the fusion point of the heterogeneous fiber is filtered from the output signal, so that the input optical signal and the output interference signal are fully stripped, and the DC background is eliminated. The effect of scattered light on the interference signal at the fusion splicing point and the hetero-optical fiber greatly improves the fringe visibility of the interference signal of the Fa-Per sensor, overcomes the mutual restriction between the coupling quality of the fusion splicing point and the high fringe visibility, and improves the temperature sensitivity of the sensor. temperature measurement resolution;

2. By optimizing the optical path structure of the Faber sensor, the DC background light and scattered interference light in the output signal are fundamentally filtered, and the stability and resolution of the sensor in harsh environments are improved. High temperature monitoring provides an effective means;

3. When the measurement temperature increases and the ambient stray light has a greater impact on the sensor signal, the higher fringe visibility can improve the noise tolerance for accurate peak identification. It is of great significance to improve the accuracy and resolution of the sensor in the complex measurement environment.

Description of drawings

1 is a schematic structural diagram of a sapphire fiber Faber high temperature sensor with high fringe visibility of the present invention;

Fig. 2 is the optical path transmission schematic diagram of the sapphire fiber Faber high temperature sensor of high fringe visibility of the present invention;

Fig. 3 is the beam expansion (a) of the partial space optical path of the sapphire fiber Faber high temperature sensor head with high fringe visibility of the present invention, and the schematic diagram of the fiber coupling model (b);

Fig. 4 is the laboratory test system diagram of the high fringe visibility sapphire fiber Faber high temperature sensor of the present invention

Fig. 5 is the sapphire fiber Faber high temperature sensor of high fringe visibility of the present invention and traditional single fiber Faber high temperature sensor contrast laboratory test result, wherein (a) is temperature measurement resolution, (b) is measurement error;

6 is a graph showing the results of a comparison test of the temperature measurement stability of the sapphire fiber Faber high temperature sensor with high fringe visibility of the present invention and the traditional single fiber sensor.

In the picture: 1. Sapphire wafer, 2. Sapphire ferrule, 3. High temperature ceramic glue, 4. Sapphire fiber, 5. Silica fiber, 6. Heterogeneous fiber splicing point, 7. Fiber patch cord, 8. High temperature sensor, 9 , LED light source, 10, Spectrometer, 11, First reflected light, 12, Second reflected light, 13, Input beam, 14, Splice scattered light, 15, Sapphire fiber end face, 16, First reflective surface, 17 , the second reflecting surface, 18, the interference signal, 19, the DC background light, 20, the high temperature muffle furnace, 21, the input waveguide, 22, the output waveguide.

Detailed ways

The technical solutions of the present invention will be described in further detail below with reference to examples.

As shown in FIG. 1 , the structure of the sensor includes a sapphire wafer 1, a sapphire ferrule 2, a sapphire optical fiber 4 and a silica fiber 5; wherein, the circular cross-sections of the sapphire wafer 1 and the sapphire ferrule 2 are in close contact, and are fixed by a high-temperature ceramic glue 3 , use an optical fiber grinder to grind the two end faces of the sapphire optical fiber 3 to achieve a certain smoothness. Then, it is spliced with the end face of the cut silica fiber 5 to construct an optical signal transmission waveguide in a high temperature region and a normal temperature region. Align the end faces of the two spliced sapphire-quartz fibers, and insert the sapphire fiber 4 into the hole in the sapphire ferrule 2. The end faces of the two silica fibers 5 are respectively connected to the LED light source 9 and the spectrometer 10 through the fiber jumper 7 to realize the input waveguide. Shunt transmission with the output waveguide. The relative position between the two sapphire optical fibers 4 and the sapphire wafer 1 is realized through a precise displacement console, and the optimal position for sensing the signal is found, and fixed with high-temperature ceramic glue 3 . The two reflective surfaces of the sapphire wafer 1 constitute a Fa-Per interference cavity, which is used as a temperature sensitive element to realize sensing;

When the sensor is working, the high temperature sensor 8 is connected to the LED light source 9 and the spectrometer 10 through the fiber jumper 7 . The light emitted by the LED light source 9 enters the sensor through the fiber jumper 7, passes through the heterogeneous fiber fusion point 6, and exits from the sapphire fiber end face 15. The diffused light is irradiated on the first reflection surface 16 of the sapphire wafer 1 and the first reflection occurs. The first beam of reflected light 11 is formed; the remaining part of the light is transmitted to the second reflection surface 17 of the wafer for a second reflection to form a second beam of reflected light 12 carrying optical path difference information; The beam and the second beam of reflected light 11 and 12 are coupled into the output sapphire fiber to form a double-fiber Faradic-Perspective interference. The interference signal 18 is transmitted back to the spectrometer 10 through the sapphire fiber 4 , the silica fiber 5 , and the fiber jumper 7 . When the ambient temperature where the sensor is located changes, the thickness of the temperature sensing wafer and the refractive index of the material change, and the optical path difference between the two reflected lights changes, resulting in changes in the interference signal. By demodulating the interference signal, the information of the Far-Pert optical path difference can be obtained. Then push back the temperature information of the sapphire wafer;

The interference spectrum signal collected from the spectrometer is expressed as:

表示初始光程差。其中，由于法珀晶片的厚度L和折射率n都是温度的函数，所以Δ表示温度的函数。Among them, k=2π/λ; I _B (k) represents the DC background amount in the signal, which is mainly composed of splice point scattering and sapphire fiber end-face background reflection; S ₁ (k), S ₂ (k) represent the received Two beams of reflected light; Δ represents the optical path difference between the two beams of coherent light, that is, 2nL;

Indicates the initial optical path difference. Among them, since the thickness L and the refractive index n of the Fa-Per wafer are both functions of temperature, Δ represents a function of temperature.

The formula for the change of sapphire Faber wafer with temperature is expressed as:

n(T) _850nm =a ₀ +a ₁ T+a ₂ T ²

Among them, T represents the temperature in Celsius, and n(T) _850nm represents the refractive index of the sapphire wafer material at 850nm. The thermal expansion function of the sapphire material along the C-axis can be expressed as:

L(T)=[b ₀ +b ₁ T+b ₂ T ² +b ₃ T ³ ]×L ₀

where T represents the temperature in Kelvin, and L(T) represents the initial length at the temperature T and the initial length L ₀ . It can be seen from the above that the optical path difference Δ=2n(T)L(T) can be expressed as a fifth-order polynomial relationship of the temperature T. Therefore, the target temperature can be deduced by measuring the optical path difference.

The precision of the spectroscopic method to demodulate the interference optical path difference depends on the acquisition resolution of the interference spectrum and the accurate finding of the fringe peak position. The spectral acquisition resolution is determined by the spectrometer resolution, and the accurate finding of the peak position is closely related to the visibility of the interference spectral fringes. The fringe visibility F _V is commonly used to represent the visibility of interference fringes in the background light signal and is defined as follows:

In practical measurements, the fringe visibility can be further expressed as:

Because the changes of S ₁ (k) and S ₂ (k) are relatively small, the DC background light I _B (k) in the interference signal at the receiving end can be effectively filtered out through the optical fiber branch transmission and the reasonable application of coupling technology. , significantly improving the visibility of interference fringes. According to the demodulation principle of white light Faroese optical path difference, the high fringe visibility helps to improve the peak-finding accuracy, thereby improving the temperature measurement accuracy and temperature measurement resolution.

Example 1:

As shown in FIG. 4, the broad-spectrum light output by the LED broadband light source 9 is introduced into the high-temperature sensor 8 through the fiber jumper 7, the multi-mode silica fiber 5, the heterogeneous fiber fusion point 6, and the sapphire fiber 4, and the reflected signal light passes through the sapphire fiber in turn. Optical fiber 4, heterogeneous optical fiber fusion point 6, quartz optical fiber 5, and optical fiber jumper 7 are received by the spectrometer. The high temperature sensor 8 is placed in the tubular cavity of the high temperature muffle furnace 20, and a temperature variable is applied to the sensor by adjusting the temperature in the muffle furnace cavity, and the measurement range is 100-1080°C. The change of temperature causes the optical refractive index and material expansion and contraction of the sapphire wafer 1, resulting in the change of the Faroese optical path difference. By calculating the interference spectrum information received by the spectrometer 10, the sensor optical path difference at the ambient temperature can be obtained. Since the optical path difference of the sensor has a fixed relationship with the refractive index of the sapphire wafer and the thermal expansion length of the wafer, Δ=2n(T)L(T), the real-time sensing temperature can be obtained by inversion.

Figure 5 shows the test results in the laboratory environment. Figure 5(a) shows the high fringe visibility sensor and the traditional sapphire fiber Faber sensor with a step of 100 °C, and 100 frames of data were collected at each temperature, and the standard deviation was obtained. The fluctuation amount of the optical path difference at each temperature is also called the temperature measurement resolution of the temperature sensor. Visible temperature sensors with high fringe visibility have higher temperature measurement resolution due to higher signal quality. Figure 5(b) shows the difference between the temperature measurement results of the high fringe visibility sensor and the traditional sapphire fiber Faber sensor at various temperatures and the temperature set in the high temperature muffle furnace, which is also the temperature measurement error of the sensor. The temperature measurement accuracy of the high stripe visibility sensor is ±1°C, which is higher than the traditional sensor.

Example 2:

Set the high-temperature muffle furnace to 1000℃, place the high-stripe visibility sensor and the traditional sapphire fiber Faber sensor in the same position in the high-temperature furnace chamber in turn, collect data continuously for 1 hour after the ambient temperature is stable, and analyze the temperature measurement stability of the sensor. The experimental results are shown in Figure 6. It can be seen from the figure that the high fringe visibility sapphire fiber Fa-Per sensor has better temperature stability than the traditional single fiber sensor.

Claims (3)

1. A sapphire optical fiber Fabry-Perot high-temperature sensor with high stripe contrast is characterized by comprising a sapphire wafer (1), a sapphire ferrule (2) and sapphire-quartz optical fibers forming optical signal transmission waveguides in a high-temperature region and a normal-temperature region; the sapphire wafer (1) and the sapphire ferrule (2) are tightly attached to each other in circular section and are fixed by high-temperature ceramic cement (3); the sapphire-quartz optical fiber is formed by welding a sapphire optical fiber (4) and a flattened quartz optical fiber (5) through an optical fiber welding point (6) between end faces, and an input waveguide 21 and an output waveguide 22 are formed; the sapphire-quartz optical fiber is inserted into the middle hole of the sapphire ferrule (2) from one end of the sapphire optical fiber (4), and the optimal position of a sensing signal between the sapphire optical fiber (4) and the sapphire wafer (1) is fixed by using high-temperature ceramic glue (3); the sapphire-quartz optical fiber is respectively connected with an LED light source (9) and a spectrometer (10) from one end of a quartz optical fiber (5) through an optical fiber jumper (7), so that the shunt transmission of an input waveguide (21) and an output waveguide (22) is realized, and an interference signal is finally transmitted to the spectrometer; the two reflecting surfaces of the sapphire wafer (1) form a Fabry-Perot interference cavity, and the diffused light irradiates a first reflecting surface (16) of the sapphire wafer (1) to be reflected for the first time to form a first beam of reflected light (11); the rest part of light is transmitted to a second reflection surface (17) of the sapphire wafer (1) to be reflected for the second time, a second beam of reflected light (12) carrying optical path difference information is formed, an input waveguide (21) projects an original light signal emitted by an LED light source (9) onto the sapphire wafer (1), and an output waveguide (22) receives interference signals reflected from the two reflection surfaces of the sapphire wafer (1), so that the input signal light and the output signal light are separated by a double optical path.

2. The temperature measurement method implemented by the high fringe contrast sapphire fiber Fabry-Perot high temperature sensor according to claim 1, wherein the method comprises the following steps:

connecting a high-temperature sensor (8) in a working state with an LED light source (9) and a spectrometer (10) through an optical fiber jumper (7); light emitted by an LED light source (9) enters a high-temperature sensor (8) through an optical fiber jumper (7), is emitted from a sapphire optical fiber end face (15) through a heterogeneous optical fiber fusion point (6), and is radiated to a first reflecting surface (16) of a sapphire wafer (1) to be reflected for the first time to form a first beam of reflected light (11); the rest part of the light is transmitted to a second reflection surface (17) of the wafer to be reflected for the second time, and a second beam of reflection light (12) carrying optical path difference information is formed; the first beam of reflected light (11) and the first beam of reflected light (12) generated on the two surfaces of the Fabry-Perot interference cavity are coupled into the sapphire optical fiber (4) to be output, so that a Fabry-Perot double-optical-fiber interference signal (18) is formed, namely the optical path difference between the two beams of reflected light of the first beam of reflected light (11) and the second beam of reflected light (12) is changed along with the first beam of reflected light and the second beam of reflected light, so that the interference signal is changed; the interference signal (18) is transmitted back to the spectrometer (10) through the sapphire optical fiber (4), the quartz optical fiber (5) and the optical fiber jumper (7);

the interference spectrum signal collected from the spectrometer is represented as:

wherein k is 2 pi/λ; i is_B(k) Representing the amount of DC background, S, in the interference spectrum signal₁(k),S₂(k) Representing two reflected lights received by the optical fiber, delta representing the optical path difference between the two coherent lights,

representing the initial optical path difference, L, n representing the thickness and refractive index of the sapphire wafer;

when the temperature of the environment changes, the thickness and the material refractive index of the sapphire wafer change:

the formula of the change of the refractive index of the sapphire wafer with the temperature is expressed as follows:

n(T)_850nm＝a₀+a₁T+a₂T²

wherein T is temperature in centigrade, n (T)_850nmThe refractive index of the sapphire wafer material is below 850 nm;

the thermal expansion function of the sapphire material along the C-axis is expressed as:

L(T)＝[b₀+b₁T+b₂T²+b₃T³]×L₀

wherein T represents a Kelvin temperature, L (T) represents a temperature T and an initial length L₀An initial length under conditions;

as can be seen from the above, the optical path difference Δ is 2n (T) l (T) and is expressed by a fifth-order polynomial relationship of the temperature T, and the fabry-perot optical path difference is obtained by demodulating the interference signal by the spectroscopic method, and the temperature at which the sapphire wafer is located is further reversely returned.

3. The temperature measuring method according to claim 2, wherein in the step of obtaining the information of the fabry-perot optical path difference by spectroscopically demodulating the interference signal, the demodulation accuracy depends on the acquisition resolution of the interference spectrum and the accurate finding of the fringe peak position: spectral acquisition resolution is determined from spectraThe resolution of the instrument determines that the peak position is accurately found and closely related to the visibility of the interference spectral fringes, so in practical measurement, the visibility of the interference spectral fringes is further expressed as:

wherein F_VIndicating the visibility of the interference fringes in the background light signal.

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