CN117129443A - High-sensitivity sensing method and sensing system based on cavity long spectrum vernier effect - Google Patents
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Abstract
The invention discloses a high-sensitivity sensing method and a sensing system based on a cavity length spectrum sampling vernier effect, which are characterized in that detection is carried out through optical fiber FP cavities with different cavity lengths, an actual measurement cavity length spectrum is obtained through fitting a plurality of obtained actual measurement light intensity values, a plurality of sampling cavity lengths which are different from a sensing module are set according to the actual measurement cavity length spectrum, a plurality of simulated light intensity values are obtained through calculation, a simulated reference cavity length spectrum is obtained through fitting the plurality of simulated light intensity values, and an output spectrum is obtained through superposition of the simulated reference cavity length spectrum and the actual measurement cavity length spectrum. According to the sensing method of the optimal design, sensing is not performed by relying on a spectrometer and a broadband light source, vernier effect is realized by simulating and overlapping the cavity length spectrum based on the actually measured cavity length spectrum, processing and manufacturing difficulties are greatly reduced, sensitivity is controllable from software, application is more suitable for actual conditions, system cost is greatly reduced, and sensitivity of a system is improved.
Description
Technical Field
The invention relates to the technical field of sensing based on cavity length spectrum, in particular to a high-sensitivity sensing method and a sensing system based on cavity length spectrum sampling vernier effect.
Background
The optical fiber Fabry-Perot (F-P) cavity sensor has the advantages of small volume, simple structure, high sensitivity, electromagnetic interference resistance and the like, and is paid attention to in the modern industry. FP sensors play an increasingly important role in monitoring chemical production, geological exploration, health monitoring, aerospace monitoring, and the like. FP sensors can be classified into two types depending on whether the cavity length or cavity filler refractive index changes: cavity length type and refractive index type. Cavity lengths are commonly used to measure mechanical or thermodynamic physical quantities such as pressure, stress, fluid level tension, displacement, inclination, flow rate, acceleration, acoustic pressure, temperature, etc. On the other hand, the refractive index type can detect various physical quantities through an electro-optical effect, a magneto-optical effect, an elastic optical effect and a thermo-optical effect, so that the application field of the optical sensor is wider than that of a cavity length type. To date, refractive index type has been studied for microwave power sensing, magnetic field sensing, pressure sensing, ultrasonic sensing, temperature sensing.
The existing FP cavity sensing system requires a supercontinuum laser and a spectrometer as a light source and a detector, respectively, which makes the high-precision sensing system costly and hinders the popularization of the optical fiber FP cavity sensor. The wavelength variation of the wavelength spectrum scheme is typically on the order of nm under the same interference structure, while the cavity length spectrum scheme has a cavity length variation on the order of μm. The wavelength spectrum measurement sensitivity is far lower than the cavity length spectrum measurement sensitivity, the former is used for improving the optical vernier effect of the sensitivity. Wavelength spectrum sensing systems to achieve vernier effects, a reference interferometer is usually added to the system, the FSR of the reference interferometer is close to the FSR of the sensing interferometer, and a spectral envelope is generated after the reflection spectra of the reference interferometer and the sensing interferometer are superimposed. The vernier effect is to utilize spectrum envelope for sensing, so as to achieve the effect of enhancing sensitivity. However, the vernier effect requires an additional reference interferometer and the stability of the sensing interferometer is ensured during the measurement, which increases the complexity of the sensing system. And once the reference interferometer is connected with the spectrometer, the sensitivity of the sensing system is fixed, and the adjustment is difficult. Patent CN115684083a realizes an optical fiber refractive index sensor based on vernier effect, and the vernier effect is formed by cascading two FP cavities, so that the measurement sensitivity of refractive index is improved to a certain extent, but the structure is complex, and the processing and manufacturing difficulties are great. The sensor is expensive in whole sensing equipment, inconvenient to carry and severely limited in use condition.
Disclosure of Invention
In order to solve the technical problems in the background technology, the invention provides a high-sensitivity sensing method and a sensing system based on a cavity length spectrum sampling vernier effect.
The invention provides a high-sensitivity sensing method based on a cavity length spectrum sampling vernier effect, which comprises the following steps:
s1, placing the optical fiber FP cavity sensing units with gradually increased cavity lengths into liquid to be measured, and obtaining actual measurement light intensity values corresponding to the FP cavities respectively;
s2, fitting the measured light intensity values obtained in the S1 to obtain a measured cavity length spectrum;
s3, setting a plurality of sampling cavity lengths which are different from those in the S1 according to the actually measured cavity length spectrum obtained in the S2, and calculating to obtain a plurality of analog light intensity values;
s4, fitting the plurality of simulated light intensity values obtained in the S3 to obtain a simulated reference cavity length spectrum;
s5, obtaining an output spectrum by superposing the simulated reference cavity length spectrum obtained in the S4 and the actually measured cavity length spectrum obtained in the S2.
Preferably, in S3, the calculation results in a plurality of simulated light intensity values, in particular calculated by the following equation:
R m ＝A+B-2Ccos[(4πn/λ)L m ]；
wherein lambda is the wavelength of incident light, L m The m-th sampling point corresponds to the cavity length, and n is the refractive index in the cavity;
A＝R 1 (1-C 1 )；
B＝(1-R 1 ) 2 R 2 (1-C 2 )(1-A 1 ) 2 (1-α) 2 ；
A 1 ，C 1 respectively the transmission loss coefficient and the reflection loss coefficient of the incident surface of the FP cavity, alpha is the transmission loss coefficient in the FP cavity, C 2 Is the reflection loss coefficient of the reflection surface of the FP cavity, R 1 And R is 2 The reflectivity of the FP cavity entrance face and the reflective face, respectively.
Preferably, in S3, the plurality of sampling cavities are equally spaced in increments;
L m ＝L 0 +mSI；
wherein L is 0 For the initial cavity length, SI is the sampling interval.
Preferably, the sampling interval is determined according to the drift magnification of the output spectrum.
Preferably, the sampling interval is determined according to the drift magnification of the output spectrum, specifically by the following equation:
wherein F is drift amount amplification factor, FSR L The free spectrum range corresponding to the measured cavity length spectrum.
Preferably, in S3, the calculating results in a plurality of simulated light intensity values, in particular implemented by Matlab software.
Preferably, the method further comprises the following steps:
s6, judging whether the output spectrum obtained in the S3 needs sensitivity adjustment or not;
if yes, returning to S2, changing the sampling interval, and recalculating the analog light intensity value.
According to the high-sensitivity sensing method based on the cavity length spectrum sampling vernier effect, detection is carried out through optical fiber FP cavities with different cavity lengths, an actual measurement cavity length spectrum is obtained through fitting of a plurality of obtained actual measurement light intensity values, a plurality of sampling cavity lengths which are different from a sensing module are set according to the actual measurement cavity length spectrum, a plurality of simulated light intensity values are obtained through calculation, a simulated reference cavity length spectrum is obtained through fitting of the plurality of simulated light intensity values, and an output spectrum is obtained through superposition of the simulated reference cavity length spectrum and the actual measurement cavity length spectrum. According to the sensing method of the optimal design, sensing is not performed by relying on a spectrometer and a broadband light source, vernier effect is realized by simulating and overlapping the cavity length spectrum based on the actually measured cavity length spectrum, processing and manufacturing difficulties are greatly reduced, sensitivity is controllable from software, application is more suitable for actual conditions, system cost is greatly reduced, and sensitivity of a system is improved.
The invention also provides a sensing system for realizing the high-sensitivity sensing method based on the cavity length spectrum sampling vernier effect, which comprises the following steps:
the sensing module is used for acquiring actually measured light intensity values corresponding to the FP cavities respectively;
the processing module is used for fitting the measured light intensity values obtained by the sensing module to obtain a measured cavity length spectrum, setting a plurality of sampling cavity lengths unequal to the sensing module according to the measured cavity length spectrum, calculating to obtain a plurality of simulated light intensity values, fitting the simulated light intensity values to obtain a simulated reference cavity length spectrum, and obtaining an output spectrum by superposition of the simulated reference cavity length spectrum and the measured cavity length spectrum.
Preferably, the sensing module includes: the device comprises a monochromatic laser diode, a beam splitter, a circulator, an optical fiber FP cavity sensing device, a photoelectric detector and a processor;
the optical fiber FP cavity sensing device comprises a plurality of optical fiber FP cavity sensing units with different cavity lengths, and each optical fiber FP cavity sensing unit is respectively connected with a monochromatic laser diode and a photoelectric detector through an optical fiber circulator.
In the present invention, the technical effects of the proposed sensing system are similar to those of the above sensing method, and thus will not be described again.
Drawings
Fig. 1 is a flowchart of an embodiment of a high-sensitivity sensing method based on a cavity length spectrum sampling vernier effect.
Fig. 2 is a signal transmission diagram of an embodiment of a sensing system according to the present invention.
FIG. 3 is a schematic diagram of an FP cavity of a sensor module of an embodiment of a sensor system according to the present invention.
Fig. 4 is a schematic structural diagram of an optical fiber FP cavity sensing device according to an embodiment of a sensing system according to the present invention.
Fig. 5 is a schematic diagram of realizing spectrum sampling vernier in an implementation mode of a high-sensitivity sensing method based on cavity length spectrum sampling vernier effect.
Detailed Description
Fig. 1 to 5 show, fig. 1 is a flowchart of an embodiment of a high-sensitivity sensing method based on a cavity length spectrum sampling vernier effect according to the present invention, fig. 2 is a signal transmission diagram of an embodiment of a sensing system according to the present invention, fig. 3 is a FP cavity schematic diagram of a sensing module of an embodiment of a sensing system according to the present invention, fig. 4 is a schematic structural diagram of an optical fiber FP cavity sensing device of an embodiment of a sensing system according to the present invention, and fig. 5 is a schematic diagram of a realizing spectrum sampling vernier in an embodiment of a high-sensitivity sensing method based on a cavity length spectrum sampling vernier effect according to the present invention.
Referring to fig. 1, the high-sensitivity sensing method based on the cavity length spectrum sampling vernier effect provided by the invention comprises the following steps:
s1, placing the optical fiber FP cavity sensing units with gradually increased cavity lengths into liquid to be measured, and obtaining actual measurement light intensity values corresponding to the FP cavities respectively;
s2, fitting the measured light intensity values obtained in the S1 to obtain a measured cavity length spectrum;
s3, setting a plurality of sampling cavity lengths which are different from those in the S1 according to the actually measured cavity length spectrum obtained in the S2, and calculating to obtain a plurality of analog light intensity values;
specifically, the calculation results in a plurality of simulated light intensity values, specifically calculated by the following equation:
R m ＝A+B-2Ccos[(4πn/λ)L m ]；
wherein lambda is the wavelength of incident light, L m The m-th sampling point corresponds to the cavity length, and n is the refractive index in the cavity;
A＝R 1 (1-C 1 )；
B＝(1-R 1 ) 2 R 2 (1-C 2 )(1-A 1 ) 2 (1-α) 2 ；
A 1 ，C 1 respectively the transmission loss coefficient and the reflection loss coefficient of the incident surface of the FP cavity, alpha is the transmission loss coefficient in the FP cavity, C 2 Is the reflection loss coefficient of the reflection surface of the FP cavity, R 1 And R is 2 The reflectivity of the FP cavity entrance face and the reflective face, respectively.
In a specific sampling design, the plurality of sampling cavity lengths can be set to be increased at equal intervals;
L m ＝L 0 +mSI；
wherein L is 0 For the initial cavity length, SI is the sampling interval.
The plurality of analog light intensity values are obtained through calculation by changing sampling intervals, and the calculation can be realized through Matlab software in actual calculation.
S4, fitting the plurality of simulated light intensity values obtained in the S3 to obtain a simulated reference cavity length spectrum;
s5, obtaining an output spectrum by superposing the simulated reference cavity length spectrum obtained in the S4 and the actually measured cavity length spectrum obtained in the S2.
The sensitivity magnification of the final output spectrum is significantly increased due to vernier effect by simulating the reference cavity long spectrum. This increase in sensitivity is brought about by sampling interval selection of the analog reference cavity long spectrum. Thus, the sampling interval for the sampling cavity length can be determined from the desired output spectral magnification before the analog reference cavity length spectrum calculation. Specifically, the sampling interval is determined according to the amplification factor of the output spectrum, specifically by the following equation:
wherein F is the magnification, FSR L The free spectrum range corresponding to the measured cavity length spectrum.
In addition, in order to avoid mismatch between the magnification of the vernier effect and the refractive index of the detection liquid, the sensing method of the present embodiment further includes the following steps:
s6, judging whether the output spectrum obtained in the S3 needs sensitivity adjustment or not;
if yes, returning to S2, changing the sampling interval, and recalculating the analog light intensity value.
The embodiment also provides a sensing system for implementing the high-sensitivity sensing method based on the cavity length spectrum sampling vernier effect, which is characterized by comprising the following steps:
the sensing module is used for acquiring actually measured light intensity values corresponding to the FP cavities respectively;
the processing module is used for fitting the measured light intensity values obtained by the sensing module to obtain a measured cavity length spectrum, setting a plurality of sampling cavity lengths unequal to the sensing module according to the measured cavity length spectrum, calculating to obtain a plurality of simulated light intensity values, fitting the simulated light intensity values to obtain a simulated reference cavity length spectrum, and obtaining an output spectrum by superposition of the simulated reference cavity length spectrum and the measured cavity length spectrum.
Wherein, the sensing module includes: the device comprises a monochromatic laser diode, a beam splitter, a circulator, an optical fiber FP cavity sensing device, a photoelectric detector and a processor;
the optical fiber FP cavity sensing device comprises a plurality of optical fiber FP cavity sensing units with different cavity lengths, and each optical fiber FP cavity sensing unit is respectively connected with a monochromatic laser diode and a photoelectric detector through an optical fiber circulator.
In this embodiment, the high-sensitivity sensing method and sensing system based on the cavity length spectrum sampling vernier effect are provided, detection is performed through an optical fiber FP cavity with different cavity lengths, an actual measurement cavity length spectrum is obtained by fitting an obtained plurality of actual measurement light intensity values, a plurality of sampling cavity lengths different from a sensing module are set according to the actual measurement cavity length spectrum, a plurality of analog light intensity values are obtained by calculation, a plurality of analog light intensity values are fitted to obtain an analog reference cavity length spectrum, and an output spectrum is obtained by superposition of the analog reference cavity length spectrum and the actual measurement cavity length spectrum. According to the sensing method of the optimal design, sensing is not performed by relying on a spectrometer and a broadband light source, vernier effect is realized by simulating and overlapping the cavity length spectrum based on the actually measured cavity length spectrum, processing and manufacturing difficulties are greatly reduced, sensitivity is controllable from software, application is more suitable for actual conditions, system cost is greatly reduced, and sensitivity of a system is improved.
The following describes in detail a high-sensitivity sensing method and a sensing system based on a cavity length spectrum sampling vernier effect according to the present embodiment by a specific example.
Referring to fig. 2-4, the sensing system of the present embodiment includes a monochromatic laser diode light source 10, a circulator 20, a photodetector 30, and an optical fiber FP cavity sensing device;
the optical fiber FP cavity sensing device comprises: the optical fiber FP cavity sensing device comprises a plurality of optical fiber FP cavity sensing units with different cavity lengths, wherein each optical fiber FP cavity sensing unit is respectively connected with a monochromatic laser diode and a photoelectric detector through an optical fiber circulator;
the mounting base 1 top is equipped with solution groove 11, and optical glass 2 is vertical to be installed in solution groove 11 and its one side has the reflecting surface, solution groove 11 with the opposite lateral wall of reflecting surface is equipped with a plurality of optic fibre installation positions, be equipped with the through-hole that is used for holding optic fibre on the optic fibre installation position, single mode fiber 3 one end is passed the through-hole stretches into in the solution groove 11, form the FP chamber between the terminal surface of single mode fiber 3 with the reflecting surface, the chamber length in a plurality of FP chambeies is all inequality.
In a specific embodiment, the ferrule 4 is mounted on the optical fiber mounting position, the through hole is located in the middle of the ferrule 4, and each single-mode optical fiber 3 is mounted on the mounting base 1 through one ferrule 4. The optical fiber is installed through the ceramic ferrule, so that accurate positioning is facilitated.
In a specific working process of the cavity long spectrum optical fiber refractive index sensor of the embodiment, a plurality of single mode optical fibers are respectively connected with a light source and a photoelectric detector through a circulator. During detection, liquid to be detected is placed in a solution tank of the mounting base, so that the liquid to be detected enters a plurality of FP cavities with different cavity lengths, light emitted by the light source enters the FP cavities through the circulator respectively through single-mode fibers of the sensor, light intensity signals are generated after reflection of the reflecting surface of the optical glass, and the light intensity signals are collected by the photoelectric detector through the single-mode fibers and the circulator again, so that detection of the cavity length spectrum is realized.
When signals acquired by the FP cavities meet the Nyquist law, the acquired signals are discrete point sets, an accurate continuous light intensity curve can be directly obtained by only fitting a curve to the signal sets, and at the moment, a demodulation step is not needed, and a spectrum signal output by the FP cavity group of the sensor can be used as a reliable cavity length detection signal.
The specific working procedure of the sensing system of this embodiment is as follows:
s1, by connecting a monochromatic laser diode light source, a beam splitter and a circulator, incident light is transmitted into an optical fiber FP cavity group device, reflected light is coupled back into the circulator, and at the moment, a photoelectric detector can display a light intensity value corresponding to the FP cavity. The plurality of cavities may correspond to a plurality of different measured light intensity values.
S2, introducing a plurality of light intensity values into the processing module and performing fitting curve, so that a continuous cavity length spectrum I1 corresponding to the refractive index can be obtained.
S3, enhancing the sensitivity of the sensor by utilizing vernier effect through the obtained known cavity length spectrum I1.
The size of the sampling interval is changed by Matlab software based on the spectrum detected by the sensing module. And taking the long axis of the abscissa cavity as a reference to sample the cavity length spectrum at equal intervals to obtain the light intensity points required by the sampling vernier curve (shown as square points in fig. 5). The specific formula can be expressed as:
R m ＝A+B-2Ccos[(4πn/λ)L m ]；
wherein the mth sampling point corresponds to the cavity length L m ＝L 0 +mSI。L 0 Representing the initial cavity size. Amplifying sensitivity multiple, sampling interval and FSR of sensing module of sampling vernier envelope based on cavity length spectrum L The relationship of (2) is as follows:
where si=fsr L From the sensitivity amplification formula derived above, it can be seen that the magnitude of the Sampling Interval (SI) is similar to the Free Spectral Range (FSRL) of the corresponding sampled cavity length spectrum, and the value of the sampling interval parameter should be set around FSRL. The closer the sampling interval is to the FSR of the sensing interferometer, the larger the sensitivity magnification, the larger the relative envelope curve drift amount, and the larger the sensing sensitivity of the obtained vernier envelope.
As shown in fig. 5, a complete continuous cursor envelope curve is formed, and the cursor envelope curve can be applied to sensitivity amplification under the corresponding refractive index.
The closer the algorithm adjusts the sampling interval size to the FSR L The larger the cursor curve envelope, the greater the sensitivity. The farther the sampling interval size is from the FSR L The cursor envelope is reduced and the sensitivity is correspondingly reduced. The size of the sampling interval is controlled by an algorithm to realize detection under environments with different refractive indexes, so that flexible monitoring is realized.
The foregoing is only a preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art, who is within the scope of the present invention, should make equivalent substitutions or modifications according to the technical scheme of the present invention and the inventive concept thereof, and should be covered by the scope of the present invention.
Claims (9)
1. The high-sensitivity sensing method based on the cavity length spectrum sampling vernier effect is characterized by comprising the following steps of:
s1, placing the optical fiber FP cavity sensing units with gradually increased cavity lengths into liquid to be measured, and obtaining actual measurement light intensity values corresponding to the FP cavities respectively;
s2, fitting the measured light intensity values obtained in the S1 to obtain a measured cavity length spectrum;
s3, setting a plurality of sampling cavity lengths which are different from those in the S1 according to the actually measured cavity length spectrum obtained in the S2, and calculating to obtain a plurality of analog light intensity values;
s4, fitting the plurality of simulated light intensity values obtained in the S3 to obtain a simulated reference cavity length spectrum;
s5, obtaining an output spectrum by superposing the simulated reference cavity length spectrum obtained in the S4 and the actually measured cavity length spectrum obtained in the S2.
2. The method according to claim 1, wherein in S3, the calculating obtains a plurality of analog light intensity values, specifically by the following equation:
R m ＝A+B-2C cos[(4πn/λ)L m ]；
wherein lambda is the wavelength of incident light, L m The m-th sampling point corresponds to the cavity length, and n is the refractive index in the cavity;
A＝R 1 (1-C 1 )；
B＝(1-R 1 ) 2 R 2 (1-C 2 )(1-A 1 ) 2 (1-α) 2 ；
A 1 ，C 1 respectively the transmission loss coefficient and the reflection loss coefficient of the incident surface of the FP cavity, alpha is the transmission loss coefficient in the FP cavity, C 2 Is the reflection loss coefficient of the reflection surface of the FP cavity, R 1 And R is 2 The reflectivity of the FP cavity entrance face and the reflective face, respectively.
3. The high-sensitivity sensing method based on the cavity length spectrum sampling vernier effect according to claim 2, wherein in S3, the plurality of sampling cavity lengths are increased at equal intervals;
L m ＝L 0 +mSI；
wherein L is 0 For the initial cavity length, SI is the sampling interval.
4. A cavity length spectrum sampling vernier effect based high sensitivity sensing method according to claim 3, wherein the sampling interval is determined according to the drift amount amplification of the output spectrum.
5. The method according to claim 4, wherein the determining the sampling interval according to the drift magnification of the output spectrum is determined by the following equation:
wherein F is drift amount amplification factor, FSR L The free spectrum range corresponding to the measured cavity length spectrum.
6. The high-sensitivity sensing method based on the cavity length spectrum sampling vernier effect according to claim 2, wherein in S3, the calculating obtains a plurality of analog light intensity values, which is realized by Matlab software.
7. The method for highly sensitive sensing based on cavity length spectrum sampling vernier effect as claimed in claim 3, further comprising the steps of:
s6, judging whether the output spectrum obtained in the S3 needs sensitivity adjustment or not;
if yes, returning to S2, changing the sampling interval, and recalculating the analog light intensity value.
8. A sensing system for implementing a high sensitivity sensing method based on cavity length spectrum sampling vernier effect according to any one of claims 1-7, characterized by comprising:
the sensing module is used for acquiring actually measured light intensity values corresponding to the FP cavities respectively;
the processing module is used for fitting the measured light intensity values obtained by the sensing module to obtain a measured cavity length spectrum, setting a plurality of sampling cavity lengths unequal to the sensing module according to the measured cavity length spectrum, calculating to obtain a plurality of simulated light intensity values, fitting the simulated light intensity values to obtain a simulated reference cavity length spectrum, and obtaining an output spectrum by superposition of the simulated reference cavity length spectrum and the measured cavity length spectrum.
9. The sensing system of claim 8, wherein the sensing module comprises: the device comprises a monochromatic laser diode, a beam splitter, a circulator, an optical fiber FP cavity sensing device, a photoelectric detector and a processor;
the optical fiber FP cavity sensing device comprises a plurality of optical fiber FP cavity sensing units with different cavity lengths, and each optical fiber FP cavity sensing unit is respectively connected with a monochromatic laser diode and a photoelectric detector through an optical fiber circulator.
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