CN105806414B - Optical fiber Temperature Humidity Sensor, temperature and humidity sensing system and humiture demodulation method - Google Patents

Abstract

The invention discloses a kind of optical fiber Temperature Humidity Sensor, temperature and humidity sensing system and humiture demodulation method, optical fiber Temperature Humidity Sensor, it is mainly made up of single-mode fiber, bunge bedstraw herb microstructured optical fibers, the part of ultra-violet curing glue-line three；Bunge bedstraw herb microstructured optical fibers one end and single-mode fiber welding, weld the first reflecting surface of formation；The bunge bedstraw herb microstructured optical fibers other end prepares ultra-violet curing glue-line dispensing and by way of solidifying, the second reflecting surface is formed between bunge bedstraw herb microstructured optical fibers and ultra-violet curing glue-line, the 3rd reflecting surface is formed between ultra-violet curing glue-line and outside air, three F P cavity configurations are formed by three reflectings surface；The present invention possesses that integrated level is high, and measurement is accurate, sensitivity, fast response time, the features such as electromagnetism interference, is not only adapted to single-point temperature and humidity measurement, can also realize the quasi-distributed temperature and humidity monitor of multiple spot.

Description

Optical fiber temperature and humidity sensor, temperature and humidity sensing system and temperature and humidity demodulation method

technical field

The invention belongs to the field of optical fiber sensing, in particular to an optical fiber temperature and humidity sensor based on microstructured optical fiber and ultraviolet curing glue and a demodulation method thereof.

Background technique

In recent years, the rapid development of optical fiber sensing technology has attracted more and more attention. Compared with traditional electrical sensors, the sensing parameters of optical fiber sensors, such as sensitivity, dynamic measurement range, and response time, have been greatly improved, and It is small in size and light in weight, resistant to electromagnetic interference, and easy to realize low-cost long-distance quasi-distributed sensing, so it is widely used in agricultural storage, machinery manufacturing, pharmaceutical production, aerospace and national defense technology and other fields. Among them, ambient humidity, as the most basic and important sensing parameter in production and life, its accurate demodulation has become one of the hotspots in the field of optical fiber sensing research.

Based on different structures, a variety of optical fiber humidity sensor schemes have been proposed, such as those proposed by Sens. , based on the optical fiber interferometer structure combined with the sensor design scheme of the humidity film material, the change of the refractive index of the humidity film material causes the drift of the interference wavelength of the interferometer, thereby demodulating the humidity information; another example is Appl.Opt.52(1), 90 -95(2012) and IEEE Photon.Tech.Lett.21(7), 441-443(2009) propose humidity sensors based on long-period grating and tilted grating structure modified by humidity film respectively, the refractive index of humidity film material The change causes the wavelength shift of the long-period grating or the energy attenuation of the resonant mode of the cladding of the tilted grating, so as to demodulate the humidity information. However, these schemes are all based on transmission spectra, and there is a large temperature or stress cross-sensitivity problem, which limits the practical application field, especially the feasibility of distributed sensing. On the other hand, since relative humidity is a function of temperature, changes in temperature will have a greater impact on the humidity value. In practical applications, it is meaningful to provide both temperature and humidity information. Therefore, it is particularly important to realize the simultaneous measurement of ambient temperature and humidity, and a fiber optic sensor solution that is easy to distribute and network.

Contents of the invention

The purpose of the present invention is to provide an optical fiber temperature sensor based on micro-structured optical fiber and ultraviolet curing glue in view of the lack of effective simultaneous temperature and humidity detection means in the existing optical fiber humidity sensing technology, and the lack of distributed networking capabilities. Humidity sensor and sensor demodulation method using fast Fourier transform analysis. The invention has the characteristics of high integration, accurate measurement, sensitivity, fast response speed, anti-electromagnetic interference, etc. It is not only suitable for single-point temperature and humidity measurement, but also can realize multi-point quasi-distributed temperature and humidity monitoring.

The purpose of the present invention is achieved through the following technical solutions: a fiber optic temperature and humidity sensor, which is mainly composed of a single-mode optical fiber, a four-leaf clover microstructure optical fiber, and an ultraviolet curing adhesive layer; one end of the four-leaf clover microstructure optical fiber is connected to The single-mode optical fiber is welded, and the first reflection surface is formed at the fusion joint; the other end of the four-leaf clover microstructure fiber is prepared by dispensing and curing the UV-curable adhesive layer, and the second reflection surface is formed between the four-leaf clover microstructure optical fiber and the UV-curable adhesive layer. Reflective surface, the third reflective surface is formed between the ultraviolet curing adhesive layer and the outside air, and three F-P cavity structures are formed by three reflective surfaces; the outer diameter of the four-leaf clover microstructure optical fiber is 125 microns, and the inner diameter of the optical fiber core is 11 microns, four fan-shaped air holes with a central angle of 85 degrees and a radius of 64 microns are symmetrically opened around the fiber core.

An optical fiber temperature and humidity sensing system containing the above optical fiber temperature and humidity sensor also includes a broadband light source, a spectrometer, an optical fiber coupler, a data acquisition processor, and a temperature and humidity control box; the two branch ends of the optical fiber coupler are respectively connected to the broadband The spectrum light source is connected to the spectrometer, the combined end of the fiber optic coupler is connected to the fiber optic temperature and humidity sensor, and the fiber optic temperature and humidity sensor is placed in the temperature and humidity control box for temperature and humidity testing; the data acquisition processor is connected to the spectrometer.

A temperature and humidity demodulation method based on the above-mentioned optical fiber temperature and humidity sensing system, the method comprises the following steps:

(1) Turn on the wide-spectrum light source, and the incident light enters the double FP cavity structure of the fiber optic temperature and humidity sensor through the fiber coupler, passes through three reflecting surfaces, and after being reflected in turn, enters the receiving end of the spectrometer through the fiber coupler again to generate an interference spectrum signal , is collected by the data line and enters the data acquisition processor, completes the FFT process, and obtains the final spectrum and phase information: the characteristic frequency of the first FP cavity formed by the first reflective surface and the second reflective surface is ξ ₁ , and the optical phase Item is The characteristic frequency of the second FP cavity formed by the second reflector and the third reflector is ξ ₂ , and the phase term is The characteristic frequency of the third FP cavity formed by the first reflecting surface and the third reflecting surface is ξ ₃ , and the phase term is

(2) Due to the water absorption of UV curing adhesive, the change of environmental humidity will cause the expansion or contraction of UV curing adhesive, and the refractive index will decrease or increase, so that with Changes; and changes in ambient temperature will simultaneously cause the expansion or contraction of the optical fiber and UV-curable adhesive, as well as changes in the refractive index, resulting in with Simultaneous change; detected by data acquisition processor with According to the following formula, the environmental humidity and temperature information can be obtained at the same time:

In the formula, k _RH1 and k _RH3 are respectively with The humidity sensitivity coefficient of k _T1 and k _T3 are respectively with The temperature sensitivity coefficients of , Δφ ₁ and Δφ ₃ are respectively with amount of change.

Compared with the prior art, the present invention has the following beneficial effects:

1. The temperature and humidity sensor based on microstructured optical fiber and ultraviolet curing glue proposed by the present invention is an optical fiber reflective structure, which has the characteristics of compact structure, small size, anti-electromagnetic interference, etc., and is easy to realize especially in large-scale, long-distance, and harsh environments Multi-point quasi-distributed optical fiber sensing.

2. The temperature and humidity demodulation method proposed by the present invention is based on frequency information, and the phase information is obtained by performing Fourier transform on the spectrum, so as to realize the demodulation of temperature and humidity sensing information. Compared with traditional demodulation methods such as spectral wavelength and intensity, it has higher sensitivity, and for interferometric quasi-distributed sensing, the spectral clutter formed by the superposition of reflection information of multiple sensor heads is difficult Extract wavelength and intensity change information, and when analyzing in the frequency domain, the characteristic frequencies corresponding to different cavity lengths can be easily separated and extracted, which facilitates the demodulation of sensing information and has obvious advantages.

3. Since relative humidity is a function of temperature, it is only meaningful to give humidity information at a certain temperature in practical applications. The optical fiber temperature and humidity sensor based on microstructure optical fiber and ultraviolet curing glue proposed by the present invention has a double F-P cavity structure, which can demodulate temperature and humidity information at the same time, so as to meet the precise sensing requirements for humidity at the practical application level.

Description of drawings

Fig. 1 is a schematic cross-sectional view of a four-leaf clover microstructure optical fiber used in the present invention;

Fig. 2 is the structural representation of the optical fiber temperature and humidity sensor based on four-leaf clover microstructure optical fiber and ultraviolet curing glue of the present invention;

Fig. 3 is a schematic diagram of the temperature and humidity sensing system of the present invention;

Fig. 4 is based on the sensor sample test spectrogram of the present invention;

Fig. 5 is the humidity response measurement result figure of the temperature and humidity sensing system based on the sensor sample used in Fig. 4;

Fig. 6 is a graph showing the temperature response measurement results of the temperature and humidity sensing system based on the sensor sample used in Fig. 4 .

detailed description

The invention is based on fusing a section of four-leaf clover microstructure optical fiber on the common single-mode optical fiber and then preparing an ultraviolet curing adhesive layer to form a composite F-P cavity structure. The principle is that different humidity and temperature will change the effective cavity length of the corresponding F-P cavity, resulting in the change of the light wave phase. By simultaneously detecting the change of the light wave phase at the characteristic frequency corresponding to multiple different cavity lengths, the simultaneous solution can demodulate the Temperature and humidity multi-parameter information.

Specifically, the UV-curable adhesive used in the present invention has water absorption, and the refractive index becomes smaller after absorbing water. Therefore, when the external humidity changes, the F-P cavity, UV-curable adhesive and four-leaf clover microstructure optical fiber formed by the UV-curable adhesive alone will The effective cavity length of the composite F-P cavity formed together will change. After the Fourier transform of the spectrum, the phase quantity of the corresponding characteristic frequency will change with the change of humidity; while the F-P cavity formed by the four-leaf clover microstructure fiber is not affected. Humidity changes, so the phase quantity corresponding to the characteristic frequency will remain constant after Fourier transform. For temperature changes, the effective cavity lengths of the three F-P cavities will be affected by the temperature due to the thermal expansion effect and thermo-optic effect of the material, so the phase quantities corresponding to the characteristic frequencies will change with the temperature after Fourier transform And change. Due to the differences in thermal expansion coefficient and thermo-optic coefficient of different materials, the temperature response of different F-P cavities is also quite different. Therefore, two F-P cavities are selected, and the simultaneous monitoring of ambient temperature and humidity can be realized by using the matrix method through the different responses of the phase to temperature and humidity at the corresponding two characteristic frequencies.

According to the multi-beam interference theory, the total light intensity reflected by the three reflective surfaces 1, 2 and 3 can be expressed as:

In formula (1), I ₁ , I ₂ , and I ₃ are the reflected light intensities of the three reflective surfaces, n ₁ , n ₂ are the effective refractive index of the core mode of the four-leaf clover microstructure optical fiber and the refractive index of the UV-curable adhesive, respectively , are the initial phases of the light waves at the positions of the three reflectors, respectively. The frequency domain information can be obtained by performing FFT transformation on the spectrogram. Since there are three reflecting surfaces, three FP cavities can be formed, corresponding to the three characteristic frequencies of the frequency domain spectrum: ξ ₁ =2n ₁ l, ξ ₂ =2n ₂ t, ξ ₃ =ξ ₁ +ξ ₂ =2(n ₁ l+n ₂ t). If wave number To perform substitution, the phase terms corresponding to the three eigenfrequencies can be expressed as:

Perform Fourier transform on formula (1), get F(ξ _i )=∑A _n (ξ _i )δ(ξ _i ), i=1,2,3. because It can be seen The range of is (-π, π), so when the phase reaches -π or π, a jump of 2π will occur, which can be passed (m is an integer) phase unwrapped (phase-unwrapped) is performed to ensure the monotonous change of the phase with the sensing quantity.

Since the length and refractive index of the UV-curable adhesive and the four-leaf clover microstructure fiber will change with temperature, when the temperature changes, the following relationship exists:

Therefore, the temperature sensitivities k _T1 , k _T2 , and k _T3 of Φ ₁ , Φ ₂ , and Φ ₃ can be obtained respectively.

Because the UV curing adhesive has water absorption and is sensitive to humidity, when the humidity of the external environment changes, the cavity length and refractive index of the UV curing adhesive will change; while the four-leaf clover optical fiber is not sensitive to humidity, and the cavity length and refractive index will not change with the humidity. ,Right now Then when the humidity changes, the following relationship exists:

Therefore, it can be seen from the above relationship that Φ ₁ is not sensitive to humidity, that is, k _RH1 ≈ 0, which can be used to demodulate the ambient temperature information; Φ ₂ and Φ ₃ are both sensitive to humidity, and the humidity sensitivity is the same, that is, k _RH2 = k _RH3 , either of them can be used to demodulate the ambient humidity information.

Taking Φ ₁ and Φ ₃ as examples, the two phase changes have the following matrix relationship with humidity and temperature changes:

The corresponding humidity and temperature changes can be obtained by solving the above matrix:

In this way, the simultaneous monitoring of the ambient humidity and temperature can be realized.

The present invention will be further described in detail below in conjunction with the embodiments and the accompanying drawings, but the embodiments of the present invention are not limited thereto.

Accompanying drawing 1 has provided the cross-section diagram of the four-leaf clover microstructure optical fiber used in the present invention. The outer diameter of this kind of fiber is the same as that of ordinary single-mode fiber, which is 125 microns, and the inner diameter of the fiber core is 11 microns. Since four fan-shaped air holes 5 with a central angle of 85 degrees and a radius of 64 microns are symmetrically distributed around the core 4, this special microstructured fiber is named as a four-leaf clover microstructured fiber.

Accompanying drawing 2 has provided the structural representation of the optical fiber temperature and humidity sensor based on four-leaf clover microstructure optical fiber and ultraviolet curing adhesive of the present invention, and it mainly consists of single-mode optical fiber 1, four-leaf clover microstructure optical fiber 2, ultraviolet curing adhesive layer 3 It consists of three parts. The single-mode optical fiber 1 and the four-leaf clover optical fiber 2 are fused by an optical fiber fusion splicer. Since the diameter of the four-leaf clover microstructure optical fiber core 4 is 11 microns, which does not match the diameter of the single-mode optical fiber core layer, a first reflective surface6. Under the microscope, a UV-curable adhesive layer 3 is prepared on the rear end of the four-leaf clover microstructured optical fiber 2 by dispensing, and cured under an ultraviolet curing lamp to form a gap between the four-leaf clover microstructured optical fiber 2 and the ultraviolet-cured adhesive layer 3 The second reflective surface 7, and the third reflective surface 8 between the UV-curable adhesive layer 3 and the air. The three F-P cavity structures formed by the three reflective surfaces 6, 7, 8 finally constitute the optical fiber F-P type temperature and humidity sensor of the present invention.

As shown in FIG. 3 , the temperature and humidity sensing system using the fiber optic temperature and humidity sensor 13 includes a broadband light source 10 , an optical fiber coupler 12 , a spectrometer 11 , a data acquisition processor 15 and a temperature and humidity control box 14 . Firstly connect the two shunt ends 12-1 and 12-2 of the fiber coupler 12 with the broadband light source 10 and the spectrometer 11 respectively, and then connect the combiner end 12-3 of the fiber coupler 12 with the fiber optic temperature and humidity sensor 13 Connected, finally put the optical fiber temperature and humidity sensor 13 into the temperature and humidity control box 14, carry out the temperature and humidity test, and then transmit the spectral information received by the spectrometer 11 to the data acquisition processor 15 for data analysis to obtain the temperature and humidity of each F-P cavity sensitivity.

According to the above-mentioned optical fiber temperature and humidity sensor, the length of the four-leaf clover microstructure optical fiber 2 is about 150 microns, and the thickness of the ultraviolet curing adhesive layer 3 is on the order of tens of microns, which can be determined according to the fast Fourier transform of the reflection spectrum of the formed composite F-P cavity. According to the actual requirements of the spatial frequency after transformation (FFT), the different cavity length ratios of the composite F-P cavity are designed by adjusting the length of the four-leaf clover micro-structured optical fiber 2 and the dispensing thickness of the UV-cured adhesive layer 3 .

Example:

Conduct temperature and humidity tests on a sensing sample, and obtain the frequency domain spectrum information shown in Figure 4 by Fourier transforming the obtained reflection spectrum. The three F-P cavities correspond to three characteristic frequencies. Since the magnitude of the characteristic frequencies is proportional to Because of the length of the F-P cavity, the three spectral peaks correspond to the UV-cured adhesive layer 3, the four-leaf clover microstructured optical fiber 2 and the composite F-P cavity from left to right (the length of the ultraviolet-cured adhesive layer 3 is less than that of the four-leaf clover microstructured optical fiber 2 length). The phase information of the two peaks is optional as the basis for temperature and humidity demodulation, here the last two peaks are taken as an example.

Use the temperature and humidity control box to keep the ambient temperature at 21°C, and gradually increase the relative humidity from 15% to 90% with 5% as the adjustment unit, record the spectral information at each humidity, and perform Fourier transform to obtain the attached The phase change of the two FP cavities shown in Figure 5, by fitting the drawn phase-temperature curve with a straight line, and calculating the slope of the line, it can be known that the humidity sensitivity k _RH1 ≈ 0 of the phase of the four-leaf clover microstructure fiber FP cavity, the compound FP The humidity sensitivity of the cavity phase k _RH3 = 0.025 rad/%RH.

Use the temperature and humidity control box to keep the ambient humidity at 30%, and gradually increase the temperature from 30°C to 55°C with 5°C as the adjustment unit, record the spectral information at each temperature respectively, and perform Fourier transform to obtain the attached drawing For the phase changes of the two FP cavities shown in 6, by fitting the drawn phase-temperature curve with a straight line, and calculating the slope of the line, it can be known that the temperature sensitivity of the phase of the four-leaf clover microstructure fiber FP cavity k _T1 = 0.018rad/℃, The temperature sensitivity of the composite FP cavity phase k _T3 =0.065rad/°C.

Finally, the four temperature and humidity sensitivities of the two F-P cavities are substituted into Equation (12), and the following matrix Equation (13) can be solved through phase change to realize simultaneous demodulation of ambient humidity and temperature:

Claims (1)

1. a kind of humiture demodulation method of optical fiber temperature humidity sensing system, the temperature and humidity sensing system includes wide spectrum light source (10), spectrometer (11), fiber coupler (12), data collection processor (15) and temperature and humidity control box (14)；Fiber coupling Two branch ends (12-1,12-2) of device (12) are attached with wide spectrum light source (10) and spectrometer (11) respectively, fiber coupling The combining end (12-3) of device (12) is connected with optical fiber Temperature Humidity Sensor (13), and optical fiber Temperature Humidity Sensor (13) is placed in humiture In control cabinet (14), temperature and humidity test is carried out；Data collection processor (15) is connected with spectrometer (11)；

Described optical fiber Temperature Humidity Sensor (13) is by single-mode fiber (1), bunge bedstraw herb microstructured optical fibers (2), ultra-violet curing glue-line (3) three parts are constituted；Bunge bedstraw herb microstructured optical fibers (2) one end and single-mode fiber (1) welding, weld the first reflecting surface of formation (6)；Bunge bedstraw herb microstructured optical fibers (2) other end prepares ultra-violet curing glue-line (3) dispensing and by way of solidifying, and bunge bedstraw herb is micro- Form the second reflecting surface (7) between structured optical fiber (2) and ultra-violet curing glue-line (3), ultra-violet curing glue-line (3) and outside air it Between form the 3rd reflecting surface (8), form three F-P cavity structures by three reflectings surface (6,7,8)；The bunge bedstraw herb microstructured optical fibers External diameter is 125 microns, and the internal diameter of fiber core is 11 microns, symmetrical around fiber core to be provided with four central angles for 85 degree, Radius is 64 microns of fan-shaped airport (5)；

Described humiture demodulation method comprises the following steps：

(1) wide spectrum light source (10) is opened, incident light enters the double of optical fiber Temperature Humidity Sensor (13) via fiber coupler (12) F-P cavity structure, after being reflected successively via three reflectings surface (6,7,8), again passes by fiber coupler (12) and enters spectrometer (11) receiving terminal, produces interference spectrum signal, and data collection processor (15) is collected into by data wire, completes FFT mistakes Journey, the final frequency spectrum of acquisition and phase information：The first F-P cavity formed by the first reflecting surface (6) and the second reflecting surface (7) Characteristic frequency is ξ₁, phase of light wave beThe second F-P cavity formed by the second reflecting surface (7) and the 3rd reflecting surface (8) Characteristic frequency is ξ₂, phase term beThe feature of the 3rd F-P cavity formed by the first reflecting surface (1) and the 3rd reflecting surface (8) Frequency is ξ₃, phase term be

(2) because uv-curable glue has water imbibition, the change of ambient humidity can cause the expansion or shrinkage of uv-curable glue, refraction Rate is decreased or increased, so thatWithChange；And the change of environment temperature can cause optical fiber and uv-curable glue simultaneously Expansion or shrinkage, and refractive index change, so as to causeWithChange simultaneously；Pass through data collection processor 1 DetectionWithChange, according to below equation, the humidity and temperature information of environment can be obtained simultaneously：

<mrow> <mfenced open = "[" close = "]"> <mtable> <mtr> <mtd> <mi>&amp;Delta;</mi> <mi>R</mi> <mi>H</mi> </mtd> </mtr> <mtr> <mtd> <mi>&amp;Delta;</mi> <mi>T</mi> </mtd> </mtr> </mtable> </mfenced> <mo>=</mo> <mfrac> <mn>1</mn> <mrow> <mo>|</mo> <msub> <mi>k</mi> <mrow> <mi>R</mi> <mi>H</mi> <mn>1</mn> </mrow> </msub> <msub> <mi>k</mi> <mrow> <mi>T</mi> <mn>3</mn> </mrow> </msub> <mo>-</mo> <msub> <mi>k</mi> <mrow> <mi>T</mi> <mn>1</mn> </mrow> </msub> <msub> <mi>k</mi> <mrow> <mi>R</mi> <mi>H</mi> <mn>3</mn> </mrow> </msub> <mo>|</mo> </mrow> </mfrac> <mo>&amp;times;</mo> <mfenced open = "[" close = "]"> <mtable> <mtr> <mtd> <msub> <mi>k</mi> <mrow> <mi>T</mi> <mn>3</mn> </mrow> </msub> </mtd> <mtd> <mrow> <mo>-</mo> <msub> <mi>k</mi> <mrow> <mi>T</mi> <mn>1</mn> </mrow> </msub> </mrow> </mtd> </mtr> <mtr> <mtd> <mrow> <mo>-</mo> <msub> <mi>k</mi> <mrow> <mi>R</mi> <mi>H</mi> <mn>3</mn> </mrow> </msub> </mrow> </mtd> <mtd> <msub> <mi>k</mi> <mrow> <mi>R</mi> <mi>H</mi> <mn>1</mn> </mrow> </msub> </mtd> </mtr> </mtable> </mfenced> <mfenced open = "[" close = "]"> <mtable> <mtr> <mtd> <mrow> <msub> <mi>&amp;Delta;&amp;phi;</mi> <mn>1</mn> </msub> </mrow> </mtd> </mtr> <mtr> <mtd> <mrow> <msub> <mi>&amp;Delta;&amp;phi;</mi> <mn>3</mn> </msub> </mrow> </mtd> </mtr> </mtable> </mfenced> </mrow>

In formula, k_RH1And k_RH3It is respectivelyWithHumidity sensitive coefficient, k_T1And k_T3It is respectivelyWithTemperature sensitive coefficient, Δφ₁With Δ φ₃It is respectivelyWithVariable quantity.