阅读说明：本技术 一种基于保偏光纤及fp腔的双参量光纤传感器 (Double-parameter optical fiber sensor based on polarization maintaining optical fiber and FP (Fabry-Perot) cavity ) 是由 夏历 阮田甜 于 2019-09-17 设计创作，主要内容包括：本发明公开了一种基于保偏光纤及FP腔的双参量光纤传感器,包括宽带光源、环形器、起偏器和级联传感单元,环形器的第一端口与宽带光源连接,环形器的第二端口与起偏器的一端连接,起偏器的另一端与级联传感单元连接,环形器的第三端口输出干涉信号；起偏器将宽带光源提供的光转变为线偏振光,线偏振光经过级联传感单元分为不同偏振态的两束线偏振光,分别产生偏振相关干涉和FP干涉,实现同时对外界应力和温度的双参量监测,传感单元内部存在的偏振相关干涉对温度敏感、应力不敏感,FP干涉对应力敏感、温度不敏感,因此应用于双参量解调时,具有无交叉串扰的优点,可以实现双参量的同时解调。(The invention discloses a double-parameter optical fiber sensor based on a polarization maintaining optical fiber and an FP (Fabry-Perot) cavity, which comprises a broadband light source, a circulator, a polarizer and a cascade sensing unit, wherein a first port of the circulator is connected with the broadband light source, a second port of the circulator is connected with one end of the polarizer, the other end of the polarizer is connected with the cascade sensing unit, and a third port of the circulator outputs an interference signal; the polarizer converts light provided by the broadband light source into linearly polarized light, the linearly polarized light is divided into two beams of linearly polarized light in different polarization states through the cascade sensing unit, polarization-related interference and FP interference are respectively generated, double-parameter monitoring on external stress and temperature is achieved simultaneously, the polarization-related interference existing in the sensing unit is sensitive to temperature and insensitive to stress, and the FP interference is sensitive to stress and insensitive to temperature, so that the polarizer has the advantage of no cross crosstalk when being applied to double-parameter demodulation, and the double-parameter simultaneous demodulation can be achieved.)

1. A double-parameter optical fiber sensor based on a polarization maintaining optical fiber and an FP (Fabry-Perot) cavity is characterized by comprising a broadband light source, a circulator, a polarizer and a cascade sensing unit, wherein a first port of the circulator is connected with the broadband light source, a second port of the circulator is connected with one end of the polarizer, the other end of the polarizer is connected with the cascade sensing unit, and a third port of the circulator outputs an interference signal;

the polarizer converts light provided by the broadband light source into linearly polarized light, the linearly polarized light is divided into two beams of linearly polarized light in different polarization states through the cascade sensing unit, polarization-dependent interference and FP interference are respectively generated, and double-parameter monitoring on external stress and temperature is achieved simultaneously.

2. The dual-parameter optical fiber sensor according to claim 1, wherein the cascade sensing unit includes a polarization maintaining optical fiber, a first single-mode optical fiber, a hollow-core optical fiber and a second single-mode optical fiber which are sequentially welded, the hollow-core optical fiber serves as an FP cavity, an interface between the first single-mode optical fiber and the hollow-core optical fiber serves as a first reflecting surface of the FP cavity, and an interface between the hollow-core optical fiber and the second single-mode optical fiber serves as a second reflecting surface of the FP cavity.

3. The dual parameter fiber sensor of claim 2, wherein the polarization maintaining fiber has a high birefringence effect, and has two polarization axes inside, i.e. a fast axis and a slow axis, respectively, and the directions are perpendicular.

4. The dual-parameter optical fiber sensor as claimed in claim 2, wherein the linearly polarized light is divided into two beams after passing through the polarization maintaining optical fiber, a phase difference is generated by transmission along different axes, the two beams of linearly polarized light are partially reflected at a first reflecting surface, the rest beams are irradiated into the hollow optical fiber, then a second partial reflection is generated at a second reflecting surface, and the rest beams are irradiated into a second single mode optical fiber, then input into the external environment and attenuated; a phase difference exists between the reflected light of the first reflecting surface and the reflected light of the second reflecting surface, and the two reflected lights are reflected back to the polarization maintaining optical fiber after FP interference and then reflected back to the polarizer.

5. The dual parameter fiber sensor of claim 4, wherein the FP interference light intensity between the first and second reflecting surfaces is:

wherein I₁Is the first reflecting surface reflects light intensity, I₂The intensity of light reflected by the second reflecting surface, n is the refractive index of the medium in the FP cavity, and L_FPIs the length of the FP cavity.

6. The dual-parameter optical fiber sensor according to claim 5, wherein the FP interference spectrum is shifted due to the change of the length of the FP cavity along with the axial stress applied from the outside, and the outside stress is demodulated according to the FP interference spectrum.

7. The dual-parameter optical fiber sensor according to claim 4, wherein the phase difference between the two linearly polarized light beams is formulated as:

wherein B is the birefringence coefficient of the polarization maintaining fiber, and L and lambda are the length and the operating wavelength of the polarization maintaining fiber respectively;

the birefringence coefficient B of the polarization maintaining fiber changes along with the external temperature to cause the polarization-dependent interference spectrum to drift, and the external temperature is demodulated according to the polarization-dependent interference spectrum.

8. The dual-parameter fiber sensor of claim 4, wherein the polarizer is fused to the rotation axis of the polarization maintaining fiber such that the angle between the polarization axis of the polarizer output light and the polarization axes of the polarization maintaining fiber is 45 °.

9. A dual parameter fiber sensor according to claim 2, wherein the second single mode fiber tail end is roughened for reducing tail end reflected light intensity.

10. The dual-parameter optical fiber sensor according to claim 1, wherein the third port of the circulator is connected to a spectrometer, and the spectrometer performs wavelength demodulation on two superposed interference signals transmitted by the cascade sensing unit.

Technical Field

The invention belongs to the technical field of optical fiber sensing, and particularly relates to a double-parameter optical fiber sensor based on a polarization maintaining optical fiber and an FP (Fabry-Perot) cavity.

Background

In the industrial application fields of coal chemical industry, bridge health, geological exploration and the like, a sensing system capable of measuring parameters such as temperature, stress and the like simultaneously is urgently needed, and along with the increasing complexity of industrial systems, the traditional measurement mode mainly based on electricity cannot meet the actual requirements. Considering that the optical fiber sensor has multiple advantages of electromagnetic interference resistance, small volume, light weight, simple structure, convenience in integration, convenience in networking, corrosion resistance and the like, researchers begin to focus on the research of the optical fiber sensing technology of temperature and stress double-parameter demodulation.

However, the current fiber sensors for temperature and stress dual parameter demodulation mainly include a mode multiplexing fiber sensor based on dual peanut nodes, a polarization-maintaining fiber bragg grating, a photonic crystal fiber sensor, a sensor in which polarization maintaining and multimode fibers are cascaded, a ring-shaped Sagnac hybrid interferometer, and the like, but these sensors also need to consider many factors in practical application, such as: cost of manufacture, sensitivity, compactness, etc. of the sensor. The mode multiplexing optical fiber sensor based on the peanut knot and the polarization maintaining and multimode optical fiber cascade structure have the characteristics of low sensitivity and incapability of meeting high-precision temperature detection; the preparation process of the polarization maintaining fiber Bragg grating is complex, and has higher requirements on the inscribing technology; the photonic crystal fiber sensor has higher preparation cost; and the ring-shaped Sagnac interferometer is not compact enough in structure. In addition, all the optical fiber sensors mentioned above are of a transmission type, the input end and the exit end are located on different sides, which is not beneficial to the integration of two ends, and is not easy to realize the sensing test under the conditions of slits and the like.

Therefore, the temperature and stress double-parameter sensor which is low in cost, simple to prepare, high in sensitivity, compact in structure, rich in application environment and free of cross crosstalk still has higher research and application values at present.

Disclosure of Invention

Aiming at the defects of the prior art, the invention aims to provide a dual-parameter optical fiber sensor based on a polarization maintaining optical fiber and an FP (Fabry-Perot) cavity, and aims to solve the problem of cross talk in dual-parameter demodulation of the conventional optical fiber sensor.

In order to achieve the purpose, the invention provides a double-parameter optical fiber sensor based on a polarization maintaining optical fiber and an FP (Fabry-Perot) cavity, which comprises a broadband light source, a circulator, a polarizer and a cascade sensing unit, wherein a first port of the circulator is connected with the broadband light source, a second port of the circulator is connected with one end of the polarizer, the other end of the polarizer is connected with the cascade sensing unit, a third port of the circulator is connected with a spectrometer, and the spectrometer is used for carrying out wavelength demodulation on two superposed interference signals transmitted by the cascade sensing unit;

the polarizer converts light provided by the broadband light source into linearly polarized light, the linearly polarized light is divided into two beams of linearly polarized light in different polarization states through the cascade sensing unit, polarization-related interference and FP interference are respectively generated, and the two interference spectrums are output to the spectrometer for detection after being superposed. When stress is applied to the FP cavity, polarization dependent interference is sensitive to temperature and insensitive to stress, and FP interference is sensitive to stress and insensitive to temperature. Therefore, the wavelength demodulation of the superimposed interference spectrum can be realized without crosstalk, and the temperature and stress dual-parameter demodulation can be realized at the same time.

Preferably, the cascade sensing unit comprises a polarization maintaining fiber, a first single-mode fiber, a hollow fiber and a second single-mode fiber which are sequentially welded, the hollow fiber is used as an FP cavity, an interface between the first single-mode fiber and the hollow fiber is used as a first reflecting surface of the FP cavity, and an interface between the hollow fiber and the second single-mode fiber is used as a second reflecting surface of the FP cavity.

Preferably, the polarization maintaining fiber has high birefringence effect, and has two polarization axes inside, namely a fast axis and a slow axis, and the directions are vertical.

Preferably, the linearly polarized light from the polarizer is divided into two linearly polarized light beams with phase difference after being transmitted along different axes of the polarization maintaining optical fiber, the two linearly polarized light beams are partially reflected at the first reflecting surface, the rest light beams are emitted into the hollow optical fiber, then the second partial reflection is carried out at the second reflecting surface, and the rest light beams are emitted into the second single-mode optical fiber, then are input into the external environment and are attenuated; a phase difference exists between the reflected light of the first reflecting surface and the reflected light of the second reflecting surface, and the two reflected lights are reflected back to the polarization maintaining optical fiber after FP interference and then reflected back to the polarizer.

The FP interference light intensity between the first reflecting surface and the second reflecting surface is as follows:

wherein I₁Is the first reflecting surface reflects light intensity, I₂The intensity of light reflected by the second reflecting surface, n is the refractive index of the medium in the FP cavity, and L_FPIs the length of the FP cavity. The length of the FP cavity changes along with axial stress applied by the outside to cause the FP interference spectrum to drift, and the outside stress is demodulated according to the FP interference spectrum.

Two vertical linearly polarized light beams are transmitted twice along the polarization maintaining fiber, which is equivalent to doubling the length of the polarization maintaining fiber, so that the phase difference of the two polarized light beams reflected out of the polarization maintaining fiber is doubled. The phase difference between the two linear polarized light beams is expressed by a formula as follows:

wherein B is the birefringence coefficient of the polarization maintaining fiber, and L and λ are the length and the operating wavelength of the polarization maintaining fiber, respectively. Subsequently, the two reflected linearly polarized light beams with the phase difference meet at the polarizer, and polarization-dependent interference is formed between the two light beams. The birefringence coefficient B of the polarization maintaining fiber is sensitive to temperature T, so that the polarization dependent interference spectrum is sensitive to temperature, and the drift quantity delta lambda of the polarization dependent interference spectrum_pThe relationship with temperature change can be expressed as follows:

the birefringence coefficient of the polarization maintaining fiber changes along with the external temperature to cause the polarization-dependent interference spectrum to drift, and the external temperature is demodulated according to the polarization-dependent interference spectrum.

Preferably, the polarizer is welded with the rotating shaft of the polarization maintaining fiber, so that the angle between the polarization axis of the output light of the polarizer and the two polarization axes of the polarization maintaining fiber is about 45 degrees, and the polarization-dependent interference spectrum with a high extinction ratio is obtained.

Preferably, the second single-mode optical fiber tail end is roughened for reducing the intensity of the tail end reflected light.

Through the technical scheme, compared with the prior art, the invention has the following beneficial effects:

1. the double-parameter optical fiber sensor provided by the invention adopts a cascade structure of the polarization maintaining optical fiber and the FP cavity as a main sensing unit, polarization related interference existing in the sensing unit is sensitive to temperature and insensitive to stress, and FP interference is sensitive to stress and insensitive to temperature, so that when the double-parameter optical fiber sensor is applied to double-parameter demodulation, the double-parameter optical fiber sensor has the advantage of no cross crosstalk, and can realize simultaneous demodulation of double parameters;

2. the cascade sensing unit only consists of a section of hollow fiber with two ends welded with standard single mode fiber, does not need complex operations such as coating and the like, and has compact structure and simple preparation;

3. the invention adopts a reflective polarization interference structure based on the polarization maintaining optical fiber, and utilizes the characteristic of high sensitivity of the birefringence coefficient of the polarization maintaining optical fiber to temperature, so that the sensor has the advantage of high sensitivity;

4. all devices of the invention adopt an all-fiber coupling mode, have compact and stable structure and stronger anti-electromagnetic interference capability, and have higher application value in the industrial application fields of coal chemical industry, bridge health, geological exploration and the like.

Drawings

FIG. 1 is a schematic structural diagram of a dual-parameter fiber sensor based on a polarization maintaining fiber and an FP cavity according to the present invention;

FIG. 2 is a schematic structural diagram of a cascaded sensing unit provided by the present invention;

FIG. 3 is a schematic diagram of the drift of the superimposed interference spectrum detected by the spectrum analyzer with the temperature of the external environment;

FIG. 4 is a schematic representation of the superimposed interference spectrum detected by the spectral analyzer of the present invention red-shifted with increasing axial stress applied by the FP cavity.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

As shown in fig. 1, an embodiment of the present invention provides a dual-parameter optical fiber sensor based on a polarization maintaining optical fiber and an FP cavity, including a broadband light source 1, a circulator 2, a polarizer 3, and a cascade sensing unit 4, where a first port of the circulator 2 is connected to the broadband light source 1, a second port of the circulator 2 is connected to one end of the polarizer 3, another end of the polarizer 3 is connected to the cascade sensing unit 4, a third port of the circulator 2 is connected to a spectrometer 5, and the spectrometer performs wavelength demodulation on two superposed interference signals transmitted by the cascade sensing unit;

the polarizer 3 converts light provided by the broadband light source 1 into linearly polarized light, the linearly polarized light is divided into two beams of linearly polarized light in different polarization states through the cascade sensing unit 4, polarization-dependent interference and FP interference are respectively generated, and the two interference spectrums are output to the spectrometer for detection after being superposed. The polarization-dependent interference is sensitive to temperature and insensitive to stress, and the FP interference is sensitive to stress and insensitive to temperature. Therefore, the wavelength demodulation of the superimposed interference spectrum can be realized without crosstalk, and the temperature and stress dual-parameter demodulation can be realized at the same time.

As shown in fig. 2, the cascade sensing unit 4 includes a polarization maintaining fiber 41, a first single mode fiber 42, a hollow fiber 43, and a second single mode fiber 44, which are welded in sequence, the polarization maintaining fiber 41 has a high birefringence effect, and two stress regions exist inside the polarization maintaining fiber, which are a first stress region 411 and a second stress region 412, respectively, and due to the existence of the stress regions, two polarization axes, which are a fast axis and a slow axis, exist in the polarization maintaining fiber 41, and the directions are perpendicular. The hollow-core fiber 43 serves as an FP cavity, the interface between the first single-mode fiber 42 and the hollow-core fiber 43 serves as a first reflecting surface 431 of the FP cavity, and the interface between the hollow-core fiber 43 and the second single-mode fiber 44 serves as a second reflecting surface 432 of the FP cavity. The second single mode fiber 44 has a roughened end for reducing the intensity of the reflected light at the end.

The polarizer is welded with a rotating shaft of the polarization maintaining fiber, so that the angle between the polarization axis of the output light of the polarizer and the two polarization axes of the polarization maintaining fiber is close to 45 degrees, and the polarization-related interference spectrum with a high extinction ratio is obtained. The polarization maintaining fiber 41 receives the linearly polarized light from the polarizer 3 and divides the linearly polarized light into two linearly polarized light beams which have phase difference and are transmitted along two vertical polarization axes, the two linearly polarized light beams are partially reflected at the first reflecting surface 431, the rest light beams are irradiated into the hollow fiber, then the second partial reflection is carried out at the second reflecting surface 432, and the rest light beams are irradiated into the second single mode fiber, then input into the external environment and are attenuated; the reflected lights of the first and second reflection surfaces have a phase difference, and are reflected back to the polarization maintaining light 41 after FP interference and then reflected back to the polarizer 3.

The FP interference light intensity between the first and the second reflection surfaces is:

wherein I₁Is the light intensity reflected by the first reflecting surface 431, I₂The second reflecting surface 432 reflects light intensity, n is the refractive index of the medium in the FP cavity, L_FPIs the length of the FP cavity. The length of the FP cavity changes along with axial stress applied by the outside to cause the FP interference spectrum to drift, and the outside stress is demodulated according to the FP interference spectrum.

Two perpendicular linearly polarized light beams are transmitted twice along the polarization maintaining fiber 41, which is equivalent to doubling the length of the polarization maintaining fiber 41, thereby doubling the phase difference of the two polarized light beams reflected out of the polarization maintaining fiber 41. The phase difference between the two linear polarized light beams is expressed by a formula as follows:

where B is the birefringence of the polarization maintaining fiber and L and λ are the length and operating wavelength of the polarization maintaining fiber 41, respectively. Subsequently, the two reflected linearly polarized light beams with phase difference meet at the polarizer 3 and form polarization dependent interference between the two light beams. The birefringence coefficient B of the polarization maintaining fiber 41 is sensitive to temperature T, resulting in the polarization dependent interference spectrum being sensitive to temperature, and the relationship of the drift amount of the interference spectrum with the temperature can be expressed as follows:

the birefringence coefficient B of the polarization maintaining fiber 41 changes with the outside temperature, causing the polarization dependent interference spectrum to drift, and demodulating the outside temperature according to the polarization dependent interference spectrum.

Fig. 3 shows the superimposed spectra at different temperatures measured by the spectrum analyzer experiment, wherein the abscissa is the wavelength and the ordinate is the reflected light power. There is interference of two frequency components in the spectrum, the relatively low frequency interference forms an envelope corresponding to the polarization dependent interference spectrum, the relatively high frequency interference is amplitude modulated corresponding to the FP interference, so that the drift of dip1 and dip2 with temperature corresponds to the change of the polarization dependent interference and the FP interference with temperature, respectively. As can be seen from the figure, dip1 undergoes blue shift with increasing temperature, and has a temperature sensitivity of 1.82 nm/deg.c and a high sensitivity characteristic to temperature; dip2 has little drift, which indicates that FP interference has temperature insensitive characteristics, so temperature sensing has no cross talk.

Fig. 4 shows the variation of dip2 obtained from experimental tests with the stress applied to a single mode fiber at both ends of the FP cavity, where the abscissa is the wavelength and the ordinate is the reflected optical power. It can be seen that dip2 is red-shifted with increasing stress, and has a stress sensitivity of 1.1 pm/. mu.epsilon.and a stress sensitivity characteristic. And because the two ends of the polarization maintaining fiber are not loaded with stress, the sensitivity of polarization-dependent interference to the stress is almost zero, and cross crosstalk does not exist in stress sensing.

It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.