CN100526821C - Thin film type optical fiber temperature sensor and its temperature sensing method - Google Patents

Abstract

The invention relates to the field of optical measuring instruments, and aims at overcoming the shortcomings in the prior art, and providing a thin-film optical fiber temperature sensor with sensitive temperature sensing and high reliability and a temperature sensing method thereof. Its structure includes a section of optical fiber, the optical fiber has an optical signal incident end face and an optical signal output end face, and an optical thin film whose reflectivity changes with temperature is plated on the optical signal output end face. The thin-film optical fiber temperature sensor is a new type of optical fiber temperature sensor that mainly utilizes the characteristics that the refractive index of the optical thin film medium changes with temperature, and then affects the reflectivity or transmittance of specific wavelength light waves.

Description

A thin-film optical fiber temperature sensor and temperature sensing method thereof

technical field

The invention relates to the field of optical measuring instruments, in particular to a film-type optical fiber temperature sensor and a temperature sensing method thereof.

technical background

In a strong electromagnetic field and high voltage system, temperature is one of the important physical quantities that need to be measured and controlled. Traditional electrical measurement techniques such as thermocouples cannot be used due to electromagnetic noise and dangers such as short circuits and explosions. The fiber optic temperature sensor has good electrical insulation performance, anti-strong electromagnetic interference, safe and reliable use, small size, and light weight. It is especially suitable for use in harsh environments such as strong electromagnetic fields, high voltage, toxic, harmful, flammable and explosive, and has a wide range of applications. prospect.

At present, the research on optical fiber temperature sensor at home and abroad is divided into two directions. The first type is the light-transmitting fiber optic temperature sensor. The light transmission type regards the intensity of light (absorption, thermal radiation, refractive index change, scattering), wavelength (fluorescence, photoluminescence), polarization plane (birefringence), time change (fluorescence), etc. as temperature signals, but only uses optical fiber For the channel function of transmitting light, another temperature-sensitive device is arranged on one end face of the optical fiber, and coupled with the optical fiber to form an optical fiber sensor. The second type is the sensor-type optical fiber temperature sensor, which uses phase (interference) and intensity (scattering) as temperature signals, and uses the characteristics of the physical parameters of the optical fiber itself to change with temperature to form an optical fiber sensor. Although the light transmission type sensor has poor sensitivity to temperature detection, it has high reliability. Among them, the optical fiber temperature sensor using fluorescence absorption and thermal radiation has reached the practical level. Sensing-type optical fiber temperature sensor has high sensitivity, but it is also very sensitive to changes in mechanical quantities other than temperature, such as pressure and vibration, so its reliability is an unresolved problem.

Contents of the invention

The purpose of the present invention is to overcome the shortcomings in the prior art, and provide a thin-film optical fiber temperature sensor with sensitive temperature sensing and high reliability and a temperature sensing method thereof.

First of all, the invention discloses a film-type optical fiber temperature sensor. The structure includes a section of optical fiber. The optical fiber has an optical signal incident end face and an optical signal output end face. film. The thin-film optical fiber temperature sensor is a new type of optical fiber temperature sensor that mainly utilizes the characteristics that the refractive index of the optical thin film medium changes with temperature, and then affects the reflectivity or transmittance of specific wavelength light waves. When the present invention is used, it will not be affected by the strong electromagnetic field or high voltage generated during the temperature measurement process, and has the sensitivity of the sensor-type optical fiber temperature sensor, and because the thickness of the optical film is very small, the present invention will not be subjected to external pressure. Influenced by mechanical quantities such as vibration, it has high stability.

The present invention further provides a temperature sensing method suitable for the film-type optical fiber temperature sensor, by obtaining the reflectance or transmittance of the optical film on the end face of the optical fiber, according to the curve of the reflectance or transmittance of the optical film with the external temperature Get the outside temperature. This is different from the existing sensor-type optical fiber temperature sensor, which uses phase (interference) and intensity (scattering) as temperature signals, and uses the properties of the optical fiber itself to change with temperature to obtain temperature, which also determines the advantages of the present invention. Stability advantage. The curve of the reflectance or transmittance of the optical film changing with the external temperature is obtained by measuring the change of the reflectance or transmittance of a specific wavelength at the calibration temperature and fitting multiple sets of corresponding data on the coordinate system calibration curve. It is the most direct way to obtain the calibration curve by calibrating the temperature, and it is also a way that is most in line with the characteristics of each specific temperature sensor, which can further reduce the error caused by the process.

The above method specifically includes the following steps:

① Place the film-type optical fiber temperature sensor in the temperature environment to be detected, and the optical signal is incident from the optical signal incident end face of the optical fiber;

②Detect the reflected light intensity or transmitted light intensity of the optical signal on the optical film plated on the output end surface of the optical fiber optical signal;

③ Calculate the reflectance or transmittance of the optical film;

④ Obtain the temperature of the detected environment according to the above-mentioned curve of reflectance or transmittance corresponding to the reflectance or transmittance of the optical film as a function of external temperature.

The method of the invention is simple and easy to realize, and is suitable for measuring and calculating with a computer, which is beneficial to improving the degree of automation of the measuring system, and makes temperature measurement easier and more humanized.

The sensing end of the above-mentioned film-type optical fiber temperature sensor is composed of an optical film plated on the end face of the optical fiber, so that the reflectance or transmittance curve has good linearity and is convenient for measurement. The present invention selects the cut-off filter film layer structure as the standard for coating the optical fiber end face.

或

，H表示光学厚度为四分之一膜系的中心波长的高折射率层，L表示光学厚度为四分之一中心波长的低折射率层。m为基本周期的重复次数，一般为5<m<10，可以根据具体采用的镀膜机而定，采用现有通用的镀膜机，其最佳数值为6或7。截止滤光膜层由高、低折射率的材料组成，环境温度对光学薄膜器件温度稳定性的影响主要是通过改变光学薄膜器件的光学常数(n，k，d)来实现的。在可见、近红外波段，所述低折射率介质层和高折射率层材料采用氧化物、氟化物、硫化物或碲化物。这类材料的折射率温度系数和其热膨胀系数相当，其温度漂移主要由折射率n和几何厚度d共同作用决定，影响主要由折射率n和几何厚度d共同作用决定。对折射率温度系数比较大的材料，光学薄膜器件的温度漂移主要由折射率n的变化引起的。所述低折射率层材料为SiO₂、NaF或MgF₂，高折射率层材料为TiO₂、PbTe或ZeS，截止滤光膜层的光谱性能由各层膜的折射率和厚度决定。环境温度的变化时，所述光学薄膜的光学特性会随着环境温度的变化而发生变化。这种变化主要表征为光学薄膜的峰值透过率发生变化，截止波长或中心波长的位置发生漂移，带宽改变等。The film system structure of described optical thin film is

or

, H represents the high refractive index layer whose optical thickness is a quarter of the central wavelength of the film system, and L represents the low refractive index layer whose optical thickness is a quarter of the central wavelength. m is the number of repetitions of the basic cycle, generally 5<m<10, which can be determined according to the specific coating machine used, and the best value is 6 or 7 if the existing general coating machine is used. The cut-off filter layer is composed of high and low refractive index materials. The influence of ambient temperature on the temperature stability of optical thin film devices is mainly realized by changing the optical constants (n, k, d) of optical thin film devices. In the visible and near-infrared bands, the low refractive index medium layer and the high refractive index layer are made of oxide, fluoride, sulfide or telluride. The temperature coefficient of the refractive index of this kind of material is equivalent to its thermal expansion coefficient, and its temperature drift is mainly determined by the joint action of the refractive index n and the geometric thickness d, and the influence is mainly determined by the joint action of the refractive index n and the geometric thickness d. For materials with a relatively large temperature coefficient of refractive index, the temperature drift of optical thin film devices is mainly caused by the change of refractive index n. The material of the low refractive index layer is SiO ₂ , NaF or MgF ₂ , the material of the high refractive index layer is TiO ₂ , PbTe or ZeS, and the spectral performance of the cut-off filter film layer is determined by the refractive index and thickness of each film. When the ambient temperature changes, the optical properties of the optical film will change with the change of the ambient temperature. This change is mainly characterized by a change in the peak transmittance of the optical film, a shift in the position of the cut-off wavelength or center wavelength, and a change in bandwidth.

Compared with the prior art, the present invention has the following outstanding substantive features and remarkable progress.

1. The sensor structure disclosed in the present invention has the advantages of small size and light weight, and is especially suitable for temperature measurement in harsh environments such as strong electromagnetic fields and high voltages;

2. High sensitivity and good repeatability and temperature stability;

3. The method of the present invention is simple and easy to implement, and can be applied to computer control to reduce errors.

Description of drawings

The film system reflectivity curve of Fig. 1 sensor of the present invention;

Fig. 2 is the reflectance-temperature relational curve of film;

Fig. 3 is a schematic structural diagram of the sensor of the present invention;

Figure 4 is a schematic diagram of the sensor measurement system;

Fig. 5 is the reflectivity-temperature relation curve of the optical fiber sensor.

Detailed ways

The present invention will be described in further detail below in conjunction with the accompanying drawings.

Principle Description

The cut-off wavelength of the band-pass cut-off filter film layer structure can be expressed as:

$\frac{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; \&PlusMinus;</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{\mathrm{<span\; class="notranslate">\; sin</span>}}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>\frac{{\mathrm{<span\; class="notranslate">\; \&eta;</span>}}_{\mathrm{<span\; class="notranslate">\; h</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&eta;</span>}}_{\mathrm{<span\; class="notranslate">\; L</span>}}}{{\mathrm{<span\; class="notranslate">\; \&eta;</span>}}_{\mathrm{<span\; class="notranslate">\; h</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&eta;</span>}}_{\mathrm{<span\; class="notranslate">\; L</span>}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

where λ ₀ is a reference wavelength.

Its cut-off band width is:

$\mathrm{<span\; class="notranslate">\; \&Delta;\; g</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{\mathrm{<span\; class="notranslate">\; sin</span>}}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>\frac{{\mathrm{<span\; class="notranslate">\; \&eta;</span>}}_{\mathrm{<span\; class="notranslate">\; h</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&eta;</span>}}_{\mathrm{<span\; class="notranslate">\; L</span>}}}{{\mathrm{<span\; class="notranslate">\; \&eta;</span>}}_{\mathrm{<span\; class="notranslate">\; h</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&eta;</span>}}_{\mathrm{<span\; class="notranslate">\; L</span>}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

The corresponding transmittance is:

$\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{<span\; class="notranslate">\; -</span>}}{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{<span\; class="notranslate">\; +</span>}}<span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; |</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; |</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; |</span>\frac{{\mathrm{<span\; class="notranslate">\; \&eta;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}\mathrm{<span\; class="notranslate">\; B</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; C</span>}}{{\mathrm{<span\; class="notranslate">\; \&eta;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}\mathrm{<span\; class="notranslate">\; B</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; |</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

It can be seen from the above that Δg is a function of η _H and η _L , and when it is normal incidence, η _H =n _H , η _L =n _L . And n _H and n _L are functions of temperature T, then the cut-off band width Δg is also a function of temperature T, which can be written as follows:

Δg＝f(T) (4)

Differentiate the above formula to get,

$\frac{\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&Delta;g</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; dT</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; df</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; dT</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; )</span>$

When the temperature changes, the cut-off band width changes, as does the reflectivity at a specific wavelength. Generally, the refractive index of a material has a linear relationship with the change of temperature, and the reflectivity at the edge of the transmittance curve also has a linear relationship with the change of temperature. Based on this change characteristic, the present invention designs a novel thin-film optical fiber sensor.

。通过模拟计算，得出不同波长下的反射率曲线，如图1所示。该膜系的反射率曲线在1310nm附近有着良好的线性度。选定这段曲线作为传感器的工作区域。当温度产生变化时候，由于材料折射率的变化，这段曲线会产生漂移，从而使得特定波长处的透过率也产生相应的变化，如图2所示，其中T₁<T₂<T₃且T₃-T₂＝T₂-T₁。通过标定温度与特定波长下透过率的变化关系，可以得到一条定标曲线。利用定标曲线，当测得透过率的值，便可找到相对应的温度，从而达到传感温度的作用。将前面设计的膜系结构镀制在单模光纤端面上，制备出设计的薄膜型光纤温度传感器。Considering that the center wavelength of the optical signal used by the generally designed sensor is 1310nm, in the coating on the end face of the bare optical fiber, the reflectivity curve near 1310nm should have good linearity. The present invention selects the film layer structure of the cut-off filter as the standard for coating the end face of the optical fiber. Choose TiO ₂ as the high refractive index material, SiO ₂ as the low refractive index material, and first select the central wavelength of the film system as 1080nm. The incident medium is glass and the transmitted medium is air. The film structure is

. Through simulation calculation, the reflectance curves at different wavelengths are obtained, as shown in Fig. 1 . The reflectance curve of this film system has good linearity around 1310nm. Select this curve as the working area of the sensor. When the temperature changes, due to the change of the refractive index of the material, this curve will drift, so that the transmittance at a specific wavelength will also change accordingly, as shown in Figure 2, where T ₁ <T ₂ <T ₃ And T ₃ -T ₂ =T ₂ -T ₁ . A calibration curve can be obtained by calibrating the relationship between temperature and transmittance at a specific wavelength. Using the calibration curve, when the value of the transmittance is measured, the corresponding temperature can be found, so as to achieve the function of sensing temperature. The previously designed film structure was plated on the end face of the single-mode optical fiber to prepare the designed film-type optical fiber temperature sensor.

The structure of the optical fiber temperature sensor is shown in Figure 3. The sensor is composed of high and low refractive index periodic films 2 plated on the end face of the bare optical fiber 1. The film structure is The high-refractive-index material is TiO ₂ , the low-refractive-index material is SiO ₂ , the repetition times of the basic period is 6, and the center wavelength of the selected film system is 1080nm. The reflectance curve of the film system is shown in FIG. 2 mentioned above, and the transmittance curve of the film system has good linearity between 1260nm and 1360nm. When the temperature changes, the spectral characteristics of the film system will change, and the change curve between the reflectivity and temperature between 1260nm and 1360nm has a relatively good linearity. By measuring the reflectivity of the sensor, the external temperature detected by the sensor will be obtained.

Utilize the sensor described in the present invention, set up the sensor simulation experiment measuring system according to Fig. 4. In this experimental system, the LD light source 3 with a center wavelength of 1310nm is selected as the signal source, the 3dB coupler 4 and the 1830-C photodetector 5 produced by Newport Company are selected. The light that LD light source 3 sends enters film-type optical fiber temperature sensor 6 of the present invention through 3dB coupler 4, and the reflected light of film-type optical fiber temperature sensor 6 will enter photodetector 5 by 3dB coupler 4 again, and reflected light signal passes through detected by photodetector 5. The other branch of the coupler 4 is inserted into the refractive index matching liquid 8 to eliminate end surface reflection. Put the film-type optical fiber temperature sensor 6 into the water bath 9, change the ambient temperature by changing the temperature of the water bath 9, and the temperature of the film-type optical fiber temperature sensor 6 can be read directly from the water temperature. Experiments have found that when the temperature of the film-type optical fiber temperature sensor 6 changes, the light intensity detected by the photodetector 5 also changes, and the change in reflectivity has a relatively good linear relationship with the temperature change of the film-type optical fiber temperature sensor 6 . The temperature value detected by the thin-film optical fiber temperature sensor 6 can be obtained by analyzing the obtained data through the computer 7 . In actual use, the temperature measurement and calibration of the designed film-type optical fiber temperature sensor 6 should be carried out first.

Based on the measured linear relationship between the optical fiber transmittance at a specific wavelength and the temperature, the experiment of the thin-film optical fiber temperature sensor 6 was carried out according to the experimental system diagram in FIG. 4 . The measured relationship curve between reflectivity and temperature of the thin-film optical fiber temperature sensor 6 at 1310 nm is shown in FIG. 5 . Experimental results show that the measurement accuracy of the sensor of the present invention can reach 1° C., and it also shows good stability and repeatability in multiple measurements. If materials with a higher temperature coefficient of refraction index are selected, such as lead telluride and zinc sulfide, the measurement accuracy of the present invention will be further improved.

Claims (5)

1. a thin film type optical fiber temperature sensor is characterized in that comprising one section optical fiber, and optical fiber has a light signal incident end face and light signal outgoing end face, is coated with on light signal outgoing end face that one deck reflectivity varies with temperature and the optical thin film that changes; The film structure of described optical thin film is the light cutoff filter film structure, and its film structure is

Or

H represents that optical thickness is the high refractive index layer of the centre wavelength of 1/4th films system, L represents that optical thickness is the low-index layer of 1/4th centre wavelengths, m is the multiplicity of basic cycle, and light signal center wavelength is 1310nm, and the centre wavelength of film system is 1080nm.

2. thin film type optical fiber temperature sensor according to claim 1 is characterized in that multiplicity 5＜m＜10 of described basic cycle.

3. thin film type optical fiber temperature sensor according to claim 1 is characterized in that described low-index layer and high index of refraction layer material are oxide, fluoride, sulfide or telluride.

4. thin film type optical fiber temperature sensor according to claim 3 is characterized in that described low-refraction layer material is SiO ₂, NaF or MgF ₂, described high index of refraction layer material is TiO ₂, PbTe or ZeS.

5. temperature sensing method that is applicable to the described thin film type optical fiber temperature sensor of claim 1, it is characterized in that obtaining reflectivity or transmitance at the optical thin film of fiber end face, obtain ambient temperature according to optical thin film reflectivity or transmitance with the curve that ambient temperature changes, the curve that described optical thin film reflectivity or transmitance change with ambient temperature is by demarcating the variation of measuring specific wavelength reflectivity or transmitance under the temperature, and will organize the corresponding data calibration curve that match obtains on coordinate system more, specifically may further comprise the steps:

1. thin film type optical fiber temperature sensor is placed the temperature environment that is detected, light signal is from the light signal incident end face incident of optical fiber;

2. survey light signal at the reflective light intensity or the transmitted light intensity that are coated on the optical thin film on the optical fiber light signal outgoing end face;

3. the reflectivity of calculating optical film or transmissivity;

4. the curve that changes with ambient temperature according to the corresponding optical thin film reflectivity of above-mentioned reflectivity or transmissivity or transmitance obtains to be detected the temperature of environment.

Images