CN112432724B - Stress sensor based on vernier effect of optical fiber resonant cavity and stress measurement method - Google Patents
Stress sensor based on vernier effect of optical fiber resonant cavity and stress measurement method Download PDFInfo
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- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L1/00—Measuring force or stress, in general
- G01L1/24—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet
- G01L1/242—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet the material being an optical fibre
- G01L1/246—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet the material being an optical fibre using integrated gratings, e.g. Bragg gratings
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Abstract
The invention discloses a stress sensor based on a vernier effect of an optical fiber resonant cavity and a stress measuring method, wherein the sensor comprises a light source, an optical fiber coupler, an optical fiber ring, a long-period optical fiber grating, a spectrometer and a processing system; the optical output end of the light source is connected with the first optical input end of the optical fiber coupler, the second optical input end and the second optical output end of the optical fiber coupler are respectively connected with two ends of the optical fiber ring, the long-period fiber grating is connected with the middle part of the optical fiber ring, the first optical output end of the optical fiber coupler is connected with the optical input end of the spectrometer, the electrical output end of the spectrometer is connected with the electrical input end of the processing system, and the electrical output end of the processing system outputs a sensor output signal; the fiber coupler, the fiber ring and the long-period fiber grating form a fiber resonant cavity; the sensing element of the invention is a long-period fiber grating, the vernier effect of the fiber resonant cavity is the same as the measuring principle of a vernier caliper, and the invention has the advantages of simple structure, no electromagnetic field influence on the sensing element and high precision.
Description
Technical Field
The invention relates to the technical field of optical devices, in particular to a stress sensor and a stress measuring method based on a vernier effect of an optical fiber resonant cavity.
Background
The stress sensor is a device which converts stress into an available output signal by utilizing the rule that the property of a substance changes along with the stress, and the stress measurement and control can be used in actual production practice to maintain production safety, ensure product quality, improve production efficiency and the like, thereby playing an important role in promoting the development of national economy. Due to the universality of stress measurement, the number and the variety of stress sensors are various, and actually, the characteristics of a plurality of materials and elements change along with the change of stress, so the materials and the elements which can be used for the stress sensors are quite large, at present, the resistance stress sensors, potentiometer type stress sensors (see displacement stress sensors) and manganin piezoresistive stress sensors are most commonly used, but the sensors have complex structures and low precision, particularly, the response signals are electric signals, the response of the sensors is greatly influenced by electromagnetic fields, and the output of some stress sensors is extremely unstable or even cannot work under the strong electromagnetic field environment.
For example, when the fiber grating is subjected to an external field (such as a stress field, a temperature field, and the like), the grating period or the effective refractive index of the fiber grating changes, so that the drift of the reflection (or transmission) wavelength of the fiber grating is caused, and the state of the external field to be measured can be estimated by detecting the drift of the reflection (or transmission) wavelength of the fiber grating. However, the existing optical fiber stress sensor also has the problems of complex structure and low precision.
Disclosure of Invention
Based on the defects, the invention provides the stress sensor based on the vernier effect of the optical fiber resonant cavity and the stress measuring method, and solves the problems that the existing stress sensor is complex in structure, easy to be influenced by an electromagnetic field and low in sensing precision.
The purpose of the invention is realized as follows: a stress sensor based on a vernier effect of an optical fiber resonant cavity comprises a light source, an optical fiber coupler, an optical fiber ring, a long-period optical fiber grating, a spectrometer and a processing system, wherein the optical output end of the light source is connected with the first optical input end of the optical fiber coupler, the second optical input end and the second optical output end of the optical fiber coupler are respectively connected with two ends of the optical fiber ring, the long-period optical fiber grating is connected with the middle part of the optical fiber ring, the first optical output end of the optical fiber coupler is connected with the optical input end of the spectrometer, the electrical output end of the spectrometer is connected with the electrical input end of the processing system, and the electrical output end of the processing system outputs a sensor output signal;
the optical fiber coupler, the optical fiber ring and the long-period fiber grating form an optical fiber resonant cavity; the long-period fiber grating does not reflect light, the transmission spectrum of the long-period fiber grating comprises a transmission valley, the minimum transmissivity of the transmission valley is zero, and the bandwidth of the transmission valley is 10 times or more of the free spectral width of the fiber resonant cavity; the strain sensitivity of the long-period fiber grating is positive; the light source is a broadband light source, and the bandwidth of the broadband light source is 100 times or more of the transmission valley bandwidth of the long-period fiber bragg grating; the optical fiber coupler is a 2x 2 optical fiber coupler.
The invention also has the following technical characteristics:
1. the processing system comprises a sampling circuit, a comparison circuit and an output circuit; the electrical input end of the sampling circuit is the electrical input end of the processing system, and the electrical output end of the output circuit is the electrical output end of the processing system; the electric output end of the spectrometer is connected with the electric input end of the sampling circuit, the electric output end of the sampling circuit is connected with the electric input end of the comparison circuit, the electric output end of the comparison circuit is connected with the electric input end of the output circuit, and the electric output end of the output circuit outputs a sensor output signal.
2. The spectrometer collects the transmission spectrum of the optical fiber resonant cavity, converts the transmission spectrum into a voltage signal and marks the voltage signal as a spectrum voltage signal, then the spectrometer inputs the spectrum voltage signal into the sampling circuit, and the sampling circuit sends the spectrum voltage signal into the comparison circuit; the comparison circuit firstly obtains and records a transmission spectrum 1, a center frequency 1, a transmission valley center frequency 1, an origin 1 and an FSR1, then the comparison circuit obtains and records a transmission spectrum 2 and a center frequency 2 at intervals, and simultaneously compares the sizes of the center frequency 2 and the center frequency 1, and carries out the following two processes: if the central frequency 2 is greater than the central frequency 1, the comparison circuit sends an instruction to the output circuit to enable the output circuit to output a sensor output signal, and at the moment, the sensor output signal is zero; if the central frequency 2 is less than the central frequency 1, judging that the long-period fiber grating is subjected to tension, wherein the direction of the tension is as follows: the center of the long-period fiber grating points to the two ends of the long-period fiber grating along the axial direction of the long-period fiber grating; the comparison circuit obtains and records the transmission valley center frequency 2, the FSR2, the resonance frequency 2 and the resonance frequency 2x, takes the FSR1 as a scale, the FSR2 as a vernier scale, and takes the original point 1 as a coordinate original point, the frequency difference y is obtained at the resonance frequency 2x according to the reading method of the vernier caliper, then the comparison circuit obtains the variation of the tensile force borne by the long-period fiber grating through the frequency difference y, the variation is the tensile force borne by the long-period fiber grating, finally, the comparison circuit sends the tensile force information to the output circuit, the output circuit outputs a sensor output signal, at the moment, the sensor output signal is the tensile force borne by the long-period fiber grating,
recording a transmission spectrum of an optical fiber resonant cavity under the condition that a long-period fiber grating is not stressed, recording the transmission spectrum as a transmission spectrum 1, determining the central frequency of a transmission valley with zero minimum transmittance and maximum bandwidth in the transmission spectrum 1, recording the central frequency as a central frequency 1, recording the central frequencies of other transmission valleys in the transmission spectrum 1 as a transmission valley central frequency 1, selecting any one of the transmission valley central frequencies 1, recording the frequency as an origin 1, and recording the frequency interval of the transmission valley central frequency 1 as the free spectral width of the optical fiber resonant cavity at the moment and recording the free spectral width as FSR 1;
during measurement, at intervals, recording a transmission spectrum of the optical fiber resonant cavity, recording the transmission spectrum as a transmission spectrum 2, determining a center frequency of a transmission valley with zero minimum transmittance and maximum bandwidth in the transmission spectrum 2, and recording the center frequency as a center frequency 2, wherein the following two conditions are adopted according to the size of the center frequency 2:
if the center frequency 2 is in the high-frequency direction of the center frequency 1, namely the center frequency 2 is greater than the center frequency 1, the output signal of the sensor is zero;
if the central frequency 2 is in the low-frequency direction of the central frequency 1, namely the central frequency 2 is less than the central frequency 1, judging that the long-period fiber grating is subjected to a pulling force, wherein the direction of the pulling force is as follows: the center of the long-period fiber grating points to two ends of the long-period fiber grating along the axial direction, at this time, the center frequency of other transmission valleys in the transmission spectrum 2 is recorded as "transmission valley center frequency 2", the frequency interval of the transmission valley center frequency 2 is the free spectral width of the fiber resonator at this time, the free spectral width is recorded as "FSR 2", at this time, some frequencies are included in both the transmission valley center frequency 1 and the transmission valley center frequency 2, the frequencies are recorded as "resonance frequency 2", the frequency which is closest to the origin 1 and smaller than the origin 1 in the resonance frequency 2 is selected, and the frequency is recorded as "resonance frequency 2 x".
3. The stress sensor is used for measuring the tensile force applied to the long-period fiber grating, and the magnitude of the tensile force applied to the long-period fiber grating is obtained from the movement amount of the resonant frequency of the fiber resonant cavity, and the direction of the tensile force is as follows: the center of the long-period fiber grating points to the two ends of the long-period fiber grating along the axial direction of the long-period fiber grating.
4. When the output light of the light source is output through the optical fiber resonant cavity, the transmission spectrum of the optical fiber resonant cavity comprises a transmission valley with the minimum transmission rate of zero and a known bandwidth which is at least 10 times of the free spectrum width of the optical fiber resonant cavity; the remainder of the transmission spectrum of the fiber cavity is periodic, being a series of peaks and troughs spaced by the free spectral width of the fiber cavity.
5. And judging whether the long-period fiber grating is subjected to tension or not according to the moving direction of the center frequency of the transmission valley with zero minimum transmittance and maximum bandwidth in the transmission spectrum of the fiber resonant cavity.
6. The stress measurement method obtained by the stress sensor based on the vernier effect of the optical fiber resonant cavity comprises the following steps:
the method comprises the following steps: recording a transmission spectrum of an optical fiber resonant cavity under the condition that a long-period fiber grating is not stressed, recording the transmission spectrum as a transmission spectrum 1, determining the central frequency of a transmission valley with zero minimum transmittance and maximum bandwidth in the transmission spectrum 1, recording the central frequency as a central frequency 1, recording the central frequencies of other transmission valleys in the transmission spectrum 1 as a transmission valley central frequency 1, selecting any one of the transmission valley central frequencies 1, recording the frequency as an origin 1, and recording the frequency interval of the transmission valley central frequency 1 as the free spectral width of the optical fiber resonant cavity at the moment and recording the free spectral width as FSR 1;
step two: during measurement, at intervals, recording a transmission spectrum of the optical fiber resonant cavity, recording the transmission spectrum as a transmission spectrum 2, determining a center frequency of a transmission valley with zero minimum transmittance and maximum bandwidth in the transmission spectrum 2, and recording the center frequency as a center frequency 2, wherein the following two conditions are adopted according to the size of the center frequency 2:
if the center frequency 2 is in the high-frequency direction of the center frequency 1, namely the center frequency 2 is greater than the center frequency 1, the output signal of the sensor is zero;
if the central frequency 2 is in the low-frequency direction of the central frequency 1, namely the central frequency 2 is less than the central frequency 1, judging that the long-period fiber grating is subjected to a pulling force, wherein the direction of the pulling force is as follows: the center of the long-period fiber grating points to two ends of the long-period fiber grating along the axial direction of the long-period fiber grating, at the moment, the center frequency of other transmission valleys in the transmission spectrum 2 is recorded as transmission valley center frequency 2, the frequency interval of the transmission valley center frequency 2 is the free spectral width of the fiber resonant cavity at the moment, the free spectral width is recorded as FSR2, at the moment, some frequencies are contained in the transmission valley center frequency 1 and the transmission valley center frequency 2, the frequencies are recorded as resonance frequency 2, the frequency which is closest to the origin 1 and smaller than the origin 1 in the resonance frequency 2 is selected and recorded as resonance frequency 2x, the FSR1 is used as a scale, the FSR2 is used as a vernier scale, the origin 1 is used as a coordinate origin, and the frequency difference y is obtained at the resonance frequency 2x according to the reading method of the vernier scale;
step three: and obtaining the variation of the tension force applied to the long-period fiber grating according to the frequency difference y, wherein the variation is the magnitude of the tension force applied to the long-period fiber grating.
The invention has the advantages that: the sensing element of the invention is a long-period fiber grating, and combines the transmission spectrum of the long-period fiber grating with the vernier effect of the fiber resonant cavity to further measure the stress.
Drawings
Figure 1 is a schematic view of the overall structure of the present invention,
fig. 2 is a schematic circuit diagram of the processing system of fig. 1.
Detailed Description
The invention is further illustrated by way of example in the accompanying drawings of the specification:
example 1
Referring to fig. 1-2, a stress sensor based on a vernier effect of an optical fiber resonant cavity includes a light source 1, an optical fiber coupler 2, an optical fiber ring 3, a long-period optical fiber grating 4, a spectrometer 5, and a processing system 6;
the optical output end of the light source 1 is connected with the first optical input end of the optical fiber coupler 2, the second optical input end and the second optical output end of the optical fiber coupler 2 are respectively connected with two ends of the optical fiber ring 3, the long-period fiber grating 4 is welded in the middle of the optical fiber ring 3, the first optical output end of the optical fiber coupler 2 is connected with the optical input end of the spectrometer 5, the electrical output end of the spectrometer 5 is connected with the electrical input end of the processing system 6, and the electrical output end of the processing system 6 outputs a sensor output signal;
the optical fiber coupler 2, the optical fiber ring 3 and the long-period fiber grating 4 form an optical fiber resonant cavity;
the long-period fiber grating 4 has no reflected light, and its transmission spectrum includes a transmission valley, the minimum transmittance of the transmission valley is zero, and the bandwidth of the transmission valley is known and at least 10 times of the free spectrum width of the fiber resonator;
the strain sensitivity of the long-period fiber grating 4 is known and is positive;
the light source 1 is a broadband light source, and the bandwidth of the broadband light source is at least 100 times of the transmission valley bandwidth of the long-period fiber grating 4;
the optical fiber coupler 2 is a 2x 2 optical fiber coupler;
the processing system 6 consists of a sampling circuit 6-1, a comparison circuit 6-2 and an output circuit 6-3;
the electrical input end of the sampling circuit 6-1 is the electrical input end of the processing system 6, and the electrical output end of the output circuit 6-3 is the electrical output end of the processing system 6; the electric output end of the spectrometer 5 is connected with the electric input end of the sampling circuit 6-1, the electric output end of the sampling circuit 6-1 is connected with the electric input end of the comparison circuit 6-2, the electric output end of the comparison circuit 6-2 is connected with the electric input end of the output circuit 6-3, and the electric output end of the output circuit 6-3 outputs a sensor output signal.
The working principle is as follows: the optical fiber coupler 2, the optical fiber ring 3 and the long-period fiber grating 4 form an optical fiber resonant cavity; the output light of the light source 1 enters an optical fiber resonant cavity, is transmitted through a long-period optical fiber grating 4 in the optical fiber resonant cavity and resonates in the optical fiber resonant cavity, then is output by the optical fiber resonant cavity and enters a spectrometer 5, the spectrometer 5 collects a spectrum and converts the spectrum into a voltage signal, then the spectrometer 5 inputs the voltage signal into a processing system 6, and the processing system 6 performs electric signal processing and outputs a sensor output signal;
assuming that the long-period fiber grating 4 is removed and the fiber ring 3 is communicated with the long-period fiber grating 4, only the fiber coupler 2 and the fiber ring 3 can also form a resonant cavity, when light enters the resonant cavity, if the optical path of the light transmitted in the resonant cavity for one cycle is an integral multiple of the optical wavelength, the optical wavelength is called as the resonant wavelength of the resonant cavity, the optical frequency corresponding to the resonant wavelength of the resonant cavity is called as the resonant frequency of the resonant cavity, the frequency intervals of any two adjacent resonant frequencies of the resonant cavity are equal, the frequency intervals are called as the free spectral width of the resonant cavity, the light with the optical frequency being the resonant frequency of the resonant cavity can be resonated in the resonant cavity, and the transmittance of the light is minimum when the light is resonated, so that the transmission spectrum of the resonant cavity has periodicity and is a series of peaks and valleys with the intervals being the free spectral width of the resonant cavity;
because the long-period fiber grating 4 does not reflect light, the transmission spectrum thereof comprises a transmission valley, the minimum transmittance of the transmission valley is zero, the bandwidth of the transmission valley is known and is at least 10 times of the free spectral width of the fiber resonator, therefore, when the light passes through the long-period fiber grating 4, the light can only be transmitted by the long-period fiber grating 4, and a transmission valley with the minimum transmittance of zero, the bandwidth of which is known and is at least 10 times of the free spectral width of the fiber resonator is generated;
because the light source 1 is a broadband light source, the bandwidth of the light source is at least 100 times of the transmission valley bandwidth of the long-period fiber grating 4, and the transmission valley bandwidth of the long-period fiber grating 4 is at least 10 times of the free spectral width of the fiber resonator, the bandwidth of the light source 1 is far larger than the free spectral width of the fiber resonator, and the output light of the light source 1 contains a large number of resonant frequencies of the fiber resonator;
since the fiber resonator is a resonator and the resonator contains the long-period fiber grating 4, when the output light of the light source 1 is output through the fiber resonator, the transmission spectrum of the fiber resonator contains a transmission valley with a minimum transmittance of zero, a known bandwidth and at least 10 times of the free spectral width of the fiber resonator, and besides, the rest part of the transmission spectrum of the fiber resonator has periodicity, namely a series of peaks and valleys with intervals of the free spectral width of the fiber resonator;
the stress changes the center frequency of the transmission valley of the long-period fiber grating 4, and since the strain sensitivity of the long-period fiber grating 4 is positive, when the long-period fiber grating 4 is under tension (the tension is in a direction from the center of the long-period fiber grating 4 to both ends thereof along the axial direction), the center frequency of the transmission valley of the long-period fiber grating 4 moves to a low frequency, so that whether the long-period fiber grating 4 is under tension can be determined from the moving direction of the center frequency of the transmission valley of the long-period fiber grating 4, that is, whether the long-period fiber grating 4 is under tension can be determined from the moving direction of the center frequency of the transmission valley of the fiber resonator with the minimum transmittance of zero and the maximum bandwidth.
The stress can change the resonant frequency of the fiber resonant cavity, and because the strain sensitivity of the long-period fiber grating 4 is a positive number, when the long-period fiber grating 4 is subjected to a pulling force (the direction of the pulling force is that the light is transmitted for one circle in the fiber resonant cavity from the center of the long-period fiber grating 4 along the axial direction of the long-period fiber grating and points to the two ends of the long-period fiber grating), the optical path of the light is increased, so that the resonant frequency of the fiber resonant cavity moves to a low frequency;
since the strain sensitivity of the long-period fiber grating 4 is known, the magnitude of the tensile force applied to the long-period fiber grating 4 can be obtained from the shift amount of the resonant frequency of the fiber resonator, and the specific measurement process is as follows in combination with the vernier effect of the fiber resonator:
firstly, under the condition that the long-period fiber grating 4 is not stressed, recording a transmission spectrum of a fiber resonant cavity, recording the transmission spectrum as a transmission spectrum 1, then determining the center frequency of a transmission valley with zero minimum transmittance and maximum bandwidth in the transmission spectrum 1, recording the center frequency as a center frequency 1, recording the center frequencies of other transmission valleys in the transmission spectrum 1 as a transmission valley center frequency 1, selecting any one frequency in the transmission valley center frequencies 1, recording the frequency as an origin 1, and recording the frequency interval of the transmission valley center frequency 1 as the free spectral width of the fiber resonant cavity at the moment and recording the free spectral width as FSR 1;
during measurement, at intervals, recording a transmission spectrum of the optical fiber resonant cavity, recording the transmission spectrum as a transmission spectrum 2, determining a center frequency of a transmission valley with zero minimum transmittance and maximum bandwidth in the transmission spectrum 2, and recording the center frequency as a center frequency 2, wherein the following two conditions can be adopted according to the size of the center frequency 2: if center frequency 2 is in the high frequency direction of center frequency 1 (i.e., center frequency 2 is greater than center frequency 1), the sensor output signal is zero; if the center frequency 2 is in the low frequency direction of the center frequency 1 (i.e. the center frequency 2 is smaller than the center frequency 1), it can be determined that the long-period fiber grating 4 is under tension (the tension is in a direction from the center of the long-period fiber grating 4 to the two ends thereof along the axial direction), at this time, the center frequencies of other transmission valleys in the transmission spectrum 2 are denoted as "transmission valley center frequency 2", the frequency interval of the transmission valley center frequency 2 is the free spectral width of the fiber resonator at this time, and this free spectral width is denoted as "FSR 2", because the long-period fiber grating 4 is under tension (the tension is in a direction from the center of the long-period fiber grating 4 to the two ends thereof along the axial direction), the optical path of light transmitting a circle in the fiber resonator increases, which results in the decrease of the free spectral width of the fiber resonator, i.e. FSR2 is smaller than FSR1, and the resonant frequency of the fiber resonator moves to a low frequency, since FSR2 is not equal to FSR1, there are some frequencies included in both transmission valley center frequency 1 and transmission valley center frequency 2, these frequencies are denoted as "resonant frequency 2", then, the frequency closest to origin 1 and smaller than origin 1 in resonant frequency 2 is selected and denoted as "resonant frequency 2 x", FSR1 is used as a scale, FSR2 is used as a vernier scale, and origin 1 is used as a coordinate origin, and frequency difference y is obtained at resonant frequency 2x according to a vernier caliper reading method;
since the strain sensitivity of the long-period fiber grating 4 is known and the FSR1 and FSR2 can be obtained, the variation of the tension applied to the long-period fiber grating 4 can be obtained from the frequency difference y, and the variation is the magnitude of the tension applied to the long-period fiber grating 4;
the spectrometer 5 collects the transmission spectrum of the fiber resonant cavity, converts the transmission spectrum into a voltage signal and records the voltage signal as a spectrum voltage signal, and then the spectrometer 5 inputs the spectrum voltage signal into the processing system 6;
the processing system 6 firstly obtains and records a transmission spectrum 1, a central frequency 1, a transmission valley central frequency 1, an origin 1 and an FSR1 under the condition that the long-period fiber grating 4 is not stressed; then, the processing system 6 acquires and records the transmission spectrum 2 and the center frequency 2 at intervals, and compares the center frequency 2 with the center frequency 1, and performs the following two processes: if the center frequency 2 is greater than the center frequency 1, the processing system 6 outputs a sensor output signal, and at this time, the sensor output signal is zero; if the central frequency 2 is less than the central frequency 1, it can be determined that the long-period fiber grating 4 is under tension (the tension is in a direction from the center of the long-period fiber grating 4 to the two ends thereof along the axial direction), then the processing system 6 obtains and records the transmission valley central frequency 2, the FSR2, the resonant frequency 2 and the resonant frequency 2x, the FSR1 is used as a scale, the FSR2 is used as a vernier, the origin 1 is used as a coordinate origin, a frequency difference y is obtained at the resonant frequency 2x according to a reading method of a vernier caliper, then the processing system 6 obtains a variation of the tension on the long-period fiber grating 4 from the frequency difference y, the variation is the magnitude of the tension on the long-period fiber grating 4, and finally the processing system 6 outputs a sensor output signal, and at this time, the sensor output signal is the magnitude of the tension on the long-period fiber grating 4.
The working principle of the processing system 6 is as follows: the spectrometer 5 inputs the spectrum voltage signal into the sampling circuit 6-1, and the sampling circuit 6-1 sends the spectrum voltage signal into the comparison circuit 6-2; the comparison circuit 6-2 first obtains and records the transmission spectrum 1, the center frequency 1, the transmission valley center frequency 1, the origin 1, and the FSR1, then the comparison circuit 6-2 obtains and records the transmission spectrum 2 and the center frequency 2 at intervals, and at the same time, compares the sizes of the center frequency 2 and the center frequency 1, and performs the following two processes: if the central frequency 2 is greater than the central frequency 1, the comparison circuit 6-2 sends an instruction to the output circuit 6-3 to enable the output circuit 6-3 to output a sensor output signal, and at the moment, the sensor output signal is zero; if the central frequency 2 is less than the central frequency 1, the long-period fiber grating 4 can be judged to be under tension (the tension is in a direction that the center of the long-period fiber grating 4 points to two ends of the long-period fiber grating along the axial direction of the long-period fiber grating), then the comparison circuit 6-2 obtains and records the central frequency 2 of the transmission valley, the FSR2, the resonant frequency 2 and the resonant frequency 2x, the FSR1 is used as a ruler, the FSR2 is used as a vernier, the origin 1 is used as a coordinate origin, the frequency difference y is obtained at the resonant frequency 2x according to the reading method of a vernier caliper, then the comparison circuit 6-2 obtains the variation of the tension of the long-period fiber grating 4 according to the frequency difference y, the variation is the magnitude of the tension of the long-period fiber grating 4, finally, the comparison circuit 6-2 sends the tension information to the output circuit 6-3, the output circuit 6-3 outputs a sensor output signal, at this time, the sensor output signal is the magnitude of the tensile force applied to the long-period fiber grating 4.
Claims (7)
1. The utility model provides a stress sensor based on optical fiber resonant cavity vernier effect, includes light source (1), fiber coupler (2), optic fibre ring (3), long period fiber grating (4), spectrum appearance (5) and processing system (6), its characterized in that: the optical output end of the light source (1) is connected with the first optical input end of the optical fiber coupler (2), the second optical input end and the second optical output end of the optical fiber coupler (2) are respectively connected with two ends of the optical fiber ring (3), the long-period optical fiber grating (4) is connected with the middle part of the optical fiber ring (3), the first optical output end connected with the optical fiber coupler (2) is connected with the optical input end of the spectrometer (5), the electrical output end of the spectrometer (5) is connected with the electrical input end of the processing system (6), and the electrical output end of the processing system (6) outputs a sensor output signal;
the optical fiber coupler (2), the optical fiber ring (3) and the long-period optical fiber grating (4) form an optical fiber resonant cavity; the long-period fiber grating (4) does not reflect light, the transmission spectrum of the long-period fiber grating comprises a transmission valley, the minimum transmittance of the transmission valley is zero, the bandwidth of the transmission valley is known, and the bandwidth of the transmission valley is 10 times or more of the free spectral width of the fiber resonant cavity; the strain sensitivity of the long-period fiber grating (4) is known and is positive; the light source (1) is a broadband light source, and the bandwidth of the broadband light source is 100 times or more than the transmission valley bandwidth of the long-period fiber grating (4); the optical fiber coupler (2) is a 2x 2 optical fiber coupler.
2. The stress sensor based on vernier effect of fiber resonator as claimed in claim 1, wherein: the processing system (6) comprises a sampling circuit (6-1), a comparison circuit (6-2) and an output circuit (6-3); the electrical input end of the sampling circuit (6-1) is the electrical input end of the processing system (6), and the electrical output end of the output circuit (6-3) is the electrical output end of the processing system (6); the electric output end of the spectrometer (5) is connected with the electric input end of the sampling circuit (6-1), the electric output end of the sampling circuit (6-1) is connected with the electric input end of the comparison circuit (6-2), the electric output end of the comparison circuit (6-2) is connected with the electric input end of the output circuit (6-3), and the electric output end of the output circuit (6-3) outputs a sensor output signal.
3. The stress sensor based on vernier effect of fiber resonator as claimed in claim 2, wherein: the spectrometer (5) collects the transmission spectrum of the optical fiber resonant cavity, converts the transmission spectrum into a voltage signal and records the voltage signal as a spectrum voltage signal, then the spectrometer (5) inputs the spectrum voltage signal into the sampling circuit (6-1), and the sampling circuit (6-1) sends the spectrum voltage signal into the comparison circuit (6-2); the comparison circuit (6-2) firstly obtains and records the transmission spectrum 1, the central frequency 1, the transmission valley central frequency 1, the origin 1 and the FSR1, then the comparison circuit (6-2) obtains and records the transmission spectrum 2 and the central frequency 2 at intervals, and simultaneously compares the central frequency 2 with the central frequency 1, and carries out the following two processes: (1) if the central frequency 2 is greater than the central frequency 1, the comparison circuit (6-2) sends an instruction to the output circuit (6-3) to enable the output circuit (6-3) to output a sensor output signal, and at the moment, the sensor output signal is zero; (2) if the central frequency 2 is less than the central frequency 1, judging that the long-period fiber grating (4) is subjected to a tensile force, wherein the direction of the tensile force is as follows: the center of the long-period fiber grating (4) points to the two ends of the long-period fiber grating along the axial direction; the comparison circuit (6-2) acquires and records the transmission valley center frequency 2, the FSR2, the resonance frequency 2 and the resonance frequency 2x, the FSR1 is used as a ruler, the FSR2 is used as a vernier scale, the origin 1 is used as a coordinate origin, the frequency difference y is obtained at the resonance frequency 2x according to the reading method of a vernier caliper, then the comparison circuit (6-2) obtains the variation of the tension force borne by the long-period fiber grating (4) according to the frequency difference y, the variation is the magnitude of the tension force borne by the long-period fiber grating (4), finally, the comparison circuit (6-2) sends the tension force information to the output circuit (6-3), the output circuit (6-3) outputs a sensor output signal, and at the moment, the sensor output signal is the magnitude of the tension force borne by the long-period fiber grating (4),
recording a transmission spectrum of the fiber resonant cavity and marking the transmission spectrum as 'transmission spectrum 1' under the condition that the long-period fiber grating (4) is not stressed, then determining the center frequency of a transmission valley with zero minimum transmittance and maximum bandwidth in the transmission spectrum 1, marking the center frequency as 'center frequency 1', meanwhile, marking the center frequency of other transmission valleys in the transmission spectrum 1 as 'transmission valley center frequency 1', selecting any frequency in the transmission valley center frequency 1 and marking the frequency as 'origin 1', wherein the frequency interval of the transmission valley center frequency 1 is the free spectral width of the fiber resonant cavity at the moment and marking the free spectral width as 'FSR 1';
during measurement, at intervals, recording a transmission spectrum of the optical fiber resonant cavity, recording the transmission spectrum as a transmission spectrum 2, determining a center frequency of a transmission valley with zero minimum transmittance and maximum bandwidth in the transmission spectrum 2, and recording the center frequency as a center frequency 2, wherein the following two conditions are adopted according to the size of the center frequency 2:
(1) if the center frequency 2 is in the high-frequency direction of the center frequency 1, namely the center frequency 2 is greater than the center frequency 1, the output signal of the sensor is zero;
(2) if the central frequency 2 is in the low-frequency direction of the central frequency 1, namely the central frequency 2 is less than the central frequency 1, judging that the long-period fiber grating (4) is subjected to a pulling force, wherein the direction of the pulling force is as follows: the center of the long-period fiber grating (4) points to two ends of the long-period fiber grating along the axial direction, at this time, the center frequency of other transmission valleys in the transmission spectrum 2 is recorded as transmission valley center frequency 2, the frequency interval of the transmission valley center frequency 2 is the free spectral width of the fiber resonant cavity at this time, the free spectral width is recorded as FSR2, at this time, some frequencies are contained in the transmission valley center frequency 1 and the transmission valley center frequency 2, the frequencies are recorded as resonance frequencies 2, the frequency which is closest to the origin 1 and smaller than the origin 1 in the resonance frequencies 2 is selected, and the frequency is recorded as resonance frequency 2 x.
4. The stress sensor based on vernier effect of fiber resonator as claimed in claim 1, wherein: the stress sensor is used for measuring the tensile force applied to the long-period fiber grating (4), and the magnitude of the tensile force applied to the long-period fiber grating (4) is obtained by the movement amount of the resonant frequency 2x of the fiber resonant cavity, and the direction of the tensile force is as follows: the center of the long-period fiber grating (4) points to the two ends of the long-period fiber grating along the axial direction of the long-period fiber grating.
5. The stress sensor based on vernier effect of fiber resonator as claimed in claim 1, wherein: when the output light of the light source (1) is output through the optical fiber resonant cavity, the transmission spectrum of the optical fiber resonant cavity comprises a transmission valley with the minimum transmissivity of zero, known bandwidth and at least 10 times of the free spectrum width of the optical fiber resonant cavity; the remainder of the transmission spectrum of the fiber cavity is periodic, being a series of peaks and troughs spaced by the free spectral width of the fiber cavity.
6. The stress sensor based on vernier effect of fiber resonator as claimed in claim 1, wherein: and judging whether the long-period fiber grating (4) is subjected to tension or not according to the moving direction of the central frequency of the transmission valley with zero minimum transmittance and maximum bandwidth in the transmission spectrum of the fiber resonant cavity.
7. The stress measurement method obtained by using the stress sensor based on the vernier effect of the optical fiber resonant cavity as claimed in any one of claims 1 to 6, characterized by comprising the following steps:
the method comprises the following steps: recording a transmission spectrum of the fiber resonator under the condition that the long-period fiber grating (4) is not stressed, recording the transmission spectrum as a transmission spectrum 1, determining the center frequency of a transmission valley with zero minimum transmittance and maximum bandwidth in the transmission spectrum 1, recording the center frequency as a center frequency 1, recording the center frequencies of other transmission valleys in the transmission spectrum 1 as a transmission valley center frequency 1, selecting any one frequency in the transmission valley center frequency 1, recording the frequency as an origin 1, and recording the frequency interval of the transmission valley center frequency 1 as the free spectral width of the fiber resonator at the moment and recording the free spectral width as FSR 1;
step two: during measurement, at intervals, recording a transmission spectrum of the optical fiber resonant cavity, recording the transmission spectrum as a transmission spectrum 2, determining a center frequency of a transmission valley with zero minimum transmittance and maximum bandwidth in the transmission spectrum 2, and recording the center frequency as a center frequency 2, wherein the following two conditions are adopted according to the size of the center frequency 2:
(1) if the center frequency 2 is in the high-frequency direction of the center frequency 1, namely the center frequency 2 is greater than the center frequency 1, the output signal of the sensor is zero;
(2) if the central frequency 2 is in the low-frequency direction of the central frequency 1, namely the central frequency 2 is less than the central frequency 1, judging that the long-period fiber grating (4) is subjected to a pulling force, wherein the direction of the pulling force is as follows: the center of the long-period fiber grating (4) points to two ends of the long-period fiber grating along the axial direction, at the moment, the center frequency of other transmission valleys in the transmission spectrum 2 is marked as 'transmission valley center frequency 2', the frequency interval of the transmission valley center frequency 2 is the free spectral width of the fiber resonant cavity at the moment, the free spectral width is marked as 'FSR 2', at the moment, some frequencies exist in the transmission valley center frequency 1 and the transmission valley center frequency 2, the frequencies are marked as 'resonant frequency 2', the frequency which is closest to the origin 1 and smaller than the origin 1 in the resonant frequency 2 is selected and marked as 'resonant frequency 2 x', the FSR1 is used as a scale, the FSR2 is used as a vernier scale, the origin 1 is used as a coordinate origin, and the frequency difference y is obtained at the resonant frequency 2x according to the reading method of a vernier caliper;
step three: and obtaining the variation of the tension force applied to the long-period fiber grating (4) according to the frequency difference y, wherein the variation is the magnitude of the tension force applied to the long-period fiber grating (4).
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