CN109668518B - Cascade LPFG (Low pass filter) self-filtering sensing system - Google Patents
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/16—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge
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- G—PHYSICS
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- G01K—MEASURING TEMPERATURE; MEASURING QUANTITY OF HEAT; THERMALLY-SENSITIVE ELEMENTS NOT OTHERWISE PROVIDED FOR
- G01K11/00—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00
- G01K11/32—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00 using changes in transmittance, scattering or luminescence in optical fibres
- G01K11/3206—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00 using changes in transmittance, scattering or luminescence in optical fibres at discrete locations in the fibre, e.g. using Bragg scattering
Abstract
The invention discloses a cascade LPFG self-filtering sensing system, which comprises a narrow-band light source, and also comprises a circulator connected with the output end of the narrow-band light source, wherein a first port of the circulator is connected with the output end of the narrow-band light source, a second port of the circulator is connected with a first FBG and a second FBG which are connected in series, a third port of the circulator is connected with a Y-shaped coupler, the first port of the Y-shaped coupler is connected with the third port of the circulator, the second port of the Y-shaped coupler is connected with a first LPFG and a second LPFG which are connected in cascade, the third port of the Y-shaped coupler is connected with a third LPFG and a fourth LPFG which are connected in cascade, the first FBG, the second FBG, the third LPFG and the fourth LPFG are placed in a constant temperature box, and the first LPFG and the second LPFG are placed in an environment to be tested. The sensing system has the advantages of reduced cost, high sensitivity, good linearity, capability of dynamically realizing the measurement of strain and temperature, improved sensing measurement precision, convenience and adaptability to high-speed demodulation after spectrum demodulation or photoelectric conversion, improved external measurement stability of the system, and wide application prospect.
Description
Technical Field
The invention relates to a self-filtering sensing technology, in particular to a cascade LPFG self-filtering sensing system.
Background
The long period fiber grating (Long Period Fiber Grating, LPFG) is a new fiber grating appearing after fiber Bragg grating (Fiber Bragg Grating, FBG) and is characterized in that the grating period is larger than that of the Bragg grating, and projection type filtering is the main. The LPFG couples the matched optical wavelength of the fiber core waveguide mode with the cladding waveguide mode, and is lost in the cladding propagation process, so that the backward coupling of the fiber core waveguide mode is avoided, and the projected loss spectrum bandwidth is far greater than the FBG spectrum bandwidth. When the physical quantity of the temperature and the strain to be measured is applied to the LPFG, the physical properties of the optical fiber and the grating are directly changed, and then the transmission loss spectrum red and blue drift is caused, and the sensing and the measurement of the temperature and the strain can be realized by establishing the functional relation between the transmission loss spectrum drift quantity and the physical quantity to be measured. The LPFG has great potential value in detection and control of medicine preparation engineering, railway traffic and oil-gas engineering equipment by virtue of the advantages of wide projection bandwidth and high sensitivity.
Because the LPFG has a plurality of loss transmission peaks in a longer wavelength range and is influenced by the invalidity of superposition of transmission spectrums, at present, the operation research of taking the LPFG as a sensor is more single elements, the operation of cascading the LPFG with a plurality of identical parameters is carried out, the physical quantity measurement of an actual environment is severely limited by the cross sensitivity problem, and effective demodulation and measurement of a target physical quantity are difficult. For the LPFG sensor cascaded with Bragg gratings, although the problem of cross sensitivity is solved to a certain extent, the LPFG sensor is limited by a reflection type data acquisition mode, the LPFG sensor can only perform single-ended operation of an optical fiber communication system, and the unstable light source is easily caused by improper reflected light treatment, and a high-stability broadband light source is usually required to be matched, so that the system is complex, heavy, high in cost and unfavorable for popularization. In addition, the current research on the LPFG sensor focuses on the sensing principle, but the research and proposal of compatible high-precision and high-speed demodulation modes corresponding to the matching are omitted, and the mass production and the production of the sensing system are restricted.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a cascade LPFG self-filtering sensing system. The sensing system is wide in sensing bandwidth and measuring range, does not require the wide bandwidth of a light source, can select a narrow-band laser light source matched with the sensing bandwidth, reduces the cost of the system, is high in sensitivity and good in linearity, can dynamically measure strain and temperature at the same time, improves the sensing measurement precision, is convenient and fast to adapt to high-speed demodulation after spectrum demodulation or photoelectric conversion, improves the external measurement stability of the system, and the double-end output of a sensing end and a reference end is used as a convenient and universal interface, so that different demodulation systems can be conveniently connected, and the sensing system has wide application prospects and important reference values.
The technical scheme for realizing the aim of the invention is as follows:
the cascade LPFG self-filtering sensing system comprises a narrow-band light source, and further comprises a circulator connected with an output end of the narrow-band light source, wherein the circulator is provided with a first port, a second port and a third port, the first port of the circulator is connected with the output end of the narrow-band light source, the second port of the circulator is connected with the first FBG and the second FBG which are connected in series, the third port of the circulator is connected with a Y-type coupler, the Y-type coupler is provided with the first port, the second port of the Y-type coupler is connected with the third port of the circulator, the second port of the Y-type coupler is connected with the first LPFG and the second LPFG which are connected in cascade, the third port of the Y-type coupler is connected with the third LPFG and the fourth LPFG which are connected in cascade, the first LPFG, the second FBG, the third LPFG and the fourth LPFG are placed in a constant temperature box, the first LPFG and the second LPFG are placed in an environment to be tested, and the output end of the narrow-band light source, the circulator, the Y-type coupler, the first LPFG and the fourth LPFG are all connected by single mode fiber.
The first LPFG and the second LPFG are obtained by processing a single mode fiber between a second port of the Y-type coupler and the first LPFG by a femtosecond laser and a micropipette control platform, and the center wavelengths of the first LPFG and the second LPFG are different.
The third LPFG and the fourth LPFG are obtained by processing single-mode optical fibers between a third port of the Y-type coupler and the third LPFG by a femtosecond laser and a micropipette control platform, and the center wavelengths of the third LPFG and the fourth LPFG are different.
The first FBG and the second FBG are obtained by processing a single-mode fiber between the second port of the circulator and the first FBG by a frequency doubling argon ion laser and a micro-shift control platform, and the center wavelengths of the first FBG and the second FBG are different.
The reflection spectrum of the first FBG is matched and intersected with the transmission spectrum of the first LPFG and the transmission spectrum of the third LPFG, the intersection point of the central wavelength of the reflection spectrum of the first FBG and the transmission spectrum of the first LPFG is located in the linear wave band of the transmission spectrum edge of the first LPFG, and the intersection point of the central wavelength of the reflection spectrum of the first FBG and the transmission spectrum of the third LPFG is located in the linear wave band of the transmission spectrum edge of the third LPFG.
The reflection spectrum of the second FBG is matched and intersected with the transmission spectrum of the second LPFG and the transmission spectrum of the fourth LPFG, the intersection point of the central wavelength of the reflection spectrum of the second FBG and the transmission spectrum of the second LPFG is located in the linear wave band of the transmission spectrum edge of the second LPFG, and the intersection point of the central wavelength of the reflection spectrum of the second FBG and the transmission spectrum of the fourth LPFG is located in the linear wave band of the transmission spectrum edge of the fourth LPFG.
The narrow-band light source is a double-peak output laser light source, the peak of the double-peak output laser light source comprises a first wavelength and a second wavelength, the first wavelength center wavelength is matched with the first FBG reflection spectrum center wavelength, the first wavelength bandwidth width is equal to or larger than the first FBG reflection spectrum zero bandwidth width, the second wavelength center wavelength is matched with the second FBG reflection spectrum center wavelength, and the second wavelength bandwidth width is equal to or larger than the second FBG reflection spectrum zero bandwidth width.
The single mode optical fiber is SMF-28 type optical fiber.
The circulator is a single-mode fiber circulator, the wavelength range is 1520nm-1620nm, and the total optical power loss is required to be less than 10dB.
The Y-type coupler is a 1X 2 single-mode fiber coupler, the insertion loss of the double ports is 3dB, and the coupling ratio of the double ports is 50 percent to 50 percent.
The sensing system is wide in sensing bandwidth and measuring range, does not require the wide bandwidth of a light source, can select a narrow-band laser light source matched with the sensing bandwidth, reduces the cost of the system, is high in sensitivity and good in linearity, can dynamically measure strain and temperature at the same time, improves the sensing measurement precision, is convenient and fast to adapt to high-speed demodulation after spectrum demodulation or photoelectric conversion, improves the external measurement stability of the system, and the double-end output of a sensing end and a reference end is used as a convenient and universal interface, so that different demodulation systems can be conveniently connected, and the sensing system has wide application prospects and important reference values.
Drawings
FIG. 1 is a schematic diagram of the structure of the present invention;
FIG. 2 is a schematic diagram of a circulator port;
FIG. 3 is a schematic diagram of a Y-coupler port;
FIG. 4 is a diagram of a desired bimodal light source spectrum in accordance with the present invention;
FIG. 5 is a graph of the transmission spectrum of a bimodal light source of the present invention after edge filtering;
FIG. 6 is a graph of edge line of dual transmission spectra of a first LPFG and a second LPFG in accordance with the present invention as a function of temperature;
FIG. 7 is a graph showing the edge line of the dual transmission spectra of the first and second LPFG cascade according to the invention as a function of strain;
FIG. 8 is a graph showing the difference between the optical power of the dual transmission peaks and the standard value according to the present invention;
FIG. 9 is a graph showing the difference between the dual transmission peak optical power and the standard value of the system according to the present invention.
In the figure, 1 is a narrow-band light source 2, a single-mode optical fiber 3, a circulator 1#, a first port 2#, a second port 3#, a third port 4, a first FBG5, a second FBG 4-1, a first FBG reflected light wave peak 5-1, a second FBG reflected light wave peak 6.Y type coupler 1'. A first port 2'. A second port 3'. A third port 7, a first LPFG8, a second LPFG 7-1, transmission spectrums 8-1 of the first LPFG and the third LPFG, transmission spectrums 9 of the second LPFG and the fourth LPFG, a third LPFG10, a fourth LPFG 11, an incubator 12, and an environment to be measured.
Detailed Description
The present invention will be further described with reference to examples and drawings, but the present invention is not limited thereto.
Examples:
referring to fig. 1, the cascade LPFG self-filtering sensing system comprises a narrowband light source 1, and further comprises a circulator 3 connected with an output end of the narrowband light source 1, wherein In is a spectrum input, an arrow direction is a light wave transmission direction, the circulator 3 is provided with a first port 1#, a second port 2# and a third port 3#, the first port 1# of the circulator 3 is connected with the output end of the narrowband light source 1, the second port 2# of the circulator 3 is connected with a first FBG4 and a second FBG5 which are connected In series, the third port 3# of the circulator 3 is connected with a Y-type coupler 6, the In is a spectrum input, the arrow direction is a light wave transmission direction, the Y-type coupler 6 is provided with a first port 1', a second port 2' and a third port 3# of the circulator 3, the second port 2 'of the Y-type coupler 6 is connected with a first LPFG7 and a second LPFG8 which are connected In cascade, the third port 3' of the Y-type coupler 6 is connected with a third LPFG9 and a fourth LPFG10 which are connected In cascade, the first LPFG 4 and the fourth LPFG 4, the fourth LPFG10 and the fourth LPFG 5 are connected In the cascade, the first and the fourth LPFG 4 and the fourth LPFG8 are connected with the fourth LPFG8, the fourth optical fiber 7 and the fourth LPFG10 and the fourth LPFG8 are connected In the constant-phase, and the fourth optical fiber 7 and the fourth optical fiber 8, and the fourth LPFG 4 and the fourth optical fiber 8 are connected between the fourth and the fourth LPFG8 and the fourth optical fiber 7 and the fourth optical fiber 8, the optical fiber 7 and the optical fiber 4 and the optical fiber.
The first LPFG7 and the second LPFG8 are obtained by processing a single mode fiber 2 between a second port 2' of the Y-coupler 6 and the first LPFG7 by a femtosecond laser and a micro-motion control platform, and the center wavelengths of the first LPFG7 and the second LPFG8 are different.
The third LPFG9 and the fourth LPFG10 are obtained by processing a single mode fiber 2 between a third port 3' of the Y-coupler 6 and the third LPFG9 by a femtosecond laser and a micro-motion control platform, and the center wavelengths of the third LPFG9 and the fourth LPFG10 are different.
The first FBG4 and the second FBG5 are obtained by processing a single-mode fiber 2 between a second port 2# of the circulator 3 and the first FBG4 by a frequency multiplication argon ion laser and a micro-shift control platform, and the center wavelengths of the first FBG4 and the second FBG5 are different.
The reflection spectrum of the first FBG4 is matched and intersected with the transmission spectrum of the first LPFG7 and the transmission spectrum of the third LPFG9, and the intersection point of the central wavelength of the reflection spectrum of the first FBG4 and the transmission spectrum of the first LPFG7 is located in the transmission spectrum edge linear wave band of the first LPFG7, and the intersection point of the central wavelength of the reflection spectrum of the first FBG4 and the transmission spectrum of the third LPFG9 is located in the transmission spectrum edge linear wave band of the third LPFG 9.
The reflection spectrum of the second FBG5 is matched and intersected with the transmission spectrum of the second LPFG8 and the transmission spectrum of the fourth LPFG10, and the intersection point of the central wavelength of the reflection spectrum of the second FBG5 and the transmission spectrum of the second LPFG8 is located in the transmission spectrum edge linear wave band of the second LPFG8, and the intersection point of the central wavelength of the reflection spectrum of the second FBG5 and the transmission spectrum of the fourth LPFG10 is located in the transmission spectrum edge linear wave band of the fourth LPFG 10.
Using a narrow-band light source 1 as a double-peak output laser light source, wherein the peak of the double-peak output laser light source comprises a first wavelength and a second wavelength, the linewidth of the first wavelength is 1549-1551nm, the output power is 0dB, the linewidth of the second wavelength is 1584-1586nm, the output power is 0dB, and the total power fluctuation range of the narrow-band light source 1 is 0dB-1dB; the loss of the 1# -2# port of the circulator 3 and the loss of the 2# -3# port of the circulator 3 are both 3.5dB; the insertion loss of the 1'-2' port of the Y-shaped coupler 6 and the 1'-3' port of the Y-shaped coupler 6 are 3dB; the access loss of the single-mode fiber 2 is totally 5dB; under the condition of no strain at the room temperature of 25 ℃, the center wavelength of the first FBG4 is 1550nm, the center wavelength of the second FBG5 is 1585nm, the side-to-side rejection ratio of the first FBG4 and the second FBG5 is 20dB, and the 3dB bandwidths of the first FBG4 and the second FBG5 are 0.5nm; under the condition of no strain at 25 ℃ at room temperature, the center wavelength of the first LPFG7 and the third LPFG9 is 1546nm, the zero bandwidth of the first LPFG7 and the third LPFG9 is 38nm, the amplitude of the first LPFG7 and the third LPFG9 is-7.5 dB, the center wavelength of the second LPFG8 and the fourth LPFG10 is 1588nm, the zero bandwidth of the second LPFG8 and the fourth LPFG10 is 48nm, and the amplitude of the second LPFG8 and the fourth LPFG10 is-8.75 dB; the output temperature of the incubator 11 was 25 ℃.
The light wave output by the narrow-band light source 1 sequentially passes through a first port 1# of the circulator 3 and a second port 2# of the circulator 3, the light wave is reflected by the first FBG4 and the second FBG5, the reflection spectrum is shown in fig. 4, the reflected light enters a third port 3# of the circulator 3 through the second port 2# of the circulator 3, the reflected light is evenly divided into two paths of light through a first port 1' of the Y-type coupler 6, the first path of light is output through a second port 2' of the coupler 6, the second path of light is output through a third port 3' of the coupler 6, the output spectrum is shown in fig. 5 under the standard state without applying temperature and strain, and when the first LPFG7 and the second LPFG8 are subjected to temperature and strain, the transmission spectrum 7-1 of the first LPFG7 and the third LPFG9, and the transmission spectrum 8-1 of the second LPFG8 and the fourth LPFG10 in fig. 5 drift.
Since the cascaded first and second LPFG7, 8 optical parameters are different, when the temperature and strain or one of the physical quantities changes, the first and second LPFG7 transmission peak center wavelengths change with different sensitivities, respectively, so that the quantization mathematical matrices corresponding to the first and second LPFG7, 8 sensitivities can be applied to the dual-parameter measurement and calculation.
The first LPFG7 and the second LPFG8 which are cascaded and have different sensitivities are adopted for sensing measurement, the third LPFG9 which has the same physical attribute as the first LPFG7 and the fourth LPFG10 which has the same physical attribute as the second LPFG8 are adopted for cascading, and the first LPFG and the second LPFG8 are put into an incubator 11 to keep the physical state stable to be used as a reference of the system state.
The transmission spectrum power after the light source impact signal edge filtering matched with the first LPFG7, the second LPFG8, the third LPFG9 and the fourth LPFG10 is obtained, the transmission spectrum power value of the first LPFG7 is subtracted from the transmission spectrum power value of the third LPFG9, the transmission spectrum power value of the second LPFG8 is subtracted from the transmission spectrum power value of the fourth LPFG10, sensing measurement power value and system state reference power value difference value data are obtained, and then the sensing measurement power value and the system state reference power value difference value data are substituted into a sensing characteristic sensitivity matrix, so that temperature and strain double-parameter calculation measurement is achieved.
The cascade LPFG self-filtering sensing system can realize measurement of temperature and strain, and the measurement demodulation process is expressed as follows:
calibration and calibration of the cascade of LPFG self-filtering sensing systems at t=25 ℃ and epsilon=200 μepsilon, the transmission power values of the cascade of first and second LPFG7 and second LPFG8, respectively, are denoted P 1 And P 2 The transmission power values of the third and fourth LPFG9 and 10 cascade are respectively denoted as P 10 And P 20 The temperature change is denoted as DeltaT, the strain change is denoted as Deltaε, and when the temperature and strain on the cascaded first and second LPFG7, 8 change, the difference in output power of the first and third LPFG7, 9 is denoted as DeltaP 1 The difference in output power between the second LPFG8 and the fourth LPFG10 is noted as ΔP 2 The mathematical expression is:
ΔP 1 、ΔP 2 the relation between the delta T and delta epsilon is as follows:
wherein, when i=1, 2, K Ti And K εi The temperature sensitivity and the strain sensitivity of the sensing system are respectively. The relation between the change amount and the temperature amount of the optical power output by the cascade LPFG self-filtering sensing system and the relation between the strain amount and the change amount can be obtained, and the mathematical matrix is expressed as follows:
the matrix inversion can be obtained:
for the two LPFGs adopted by the cascade LPFG self-filtering sensing system, the sensitivity coefficient matrix in the formula (4) cannot be simplified because the two LPFGs have different sensitivities to temperature and strain, so that the cascade LPFG self-filtering sensing system can effectively measure the temperature and strain.
The temperature sensing characteristic and the strain sensing characteristic of the present sensing system are analyzed by numerical calculation as follows.
Temperature sensing characteristic data analysis
By numerical simulation, the strain variation is controlled to be 0, the total temperature variation is controlled to be 40 ℃, 5 data points are taken from 25 ℃ to 65 ℃ in an increment of 10 ℃ for simulation calculation, 5 groups of transmission spectrum edge line patterns are obtained through MATLAB calculation, as shown in figure 6, the transmission peak wavelengths of the first LPFG7 and the second LPFG8 are blue shifted, and the first LPFG7 and the secondThe attenuation amplitude of LPFG8 increases, so that the output first wavelength light power difference gradually decreases with temperature, the output second wavelength light power difference gradually increases with temperature, the output double power value at room temperature of 25 ℃ and strain of 200 mu epsilon is used as standard reference value, the relation graph of double power and temperature is shown in figure 8, the temperature sensitivity of the first LPFG7 is K T1 Linear fitness R = -0.103dB/°c 2 =0.9945, the second LPFG8 has a temperature sensitivity K T2 Linear fitness r=0.142 dB/°c 2 ＝0.9949。
Strain sensing characteristic data analysis
Through numerical simulation, the temperature variation is controlled to be 0, the total strain variation is controlled to be 800 mu epsilon, 5 data points are taken from 200 mu epsilon to 1000 mu epsilon with the increment of 200 mu epsilon, simulation calculation is carried out, 5 groups of transmission spectrum edge line patterns are obtained through MATLAB calculation, as shown in figure 7, the transmission peak wavelengths of the first LPFG7 and the second LPFG8 generate weak red shift, the attenuation amplitudes of the first LPFG7 and the second LPFG8 are greatly reduced, the difference value of the optical power of the output first wavelength and the optical power of the second wavelength is increased along with the increase of the strain, the output double power value at the room temperature of 25 ℃ and the strain of 200 mu epsilon is used as a standard reference value, the relation pattern of the double power and the strain is shown in figure 9, and the strain sensitivity of the first LPFG7 is K ε1 Linear fitness r=5.5 dB/epsilon 2 = 0.9886, the second LPFG8 has a strain sensitivity of K ε1 Linear fitness r=4.9 dB/epsilon 2 ＝0.9992。
The analysis result of the comprehensive temperature and strain sensing characteristic data can obtain the relation matrix expression of the dual-power variable quantity output by the sensing system and the temperature quantity to be measured, wherein the relation matrix expression is as follows:
the matrix inversion can be obtained:
in summary, the cascade LPFG self-filtering sensing system provided by the embodiment can effectively solve the problem of cross sensitivity and is used for measuring temperature and strain double parameters. The experimental results show that: the first LPFG7 temperature sensitivity is-0.103 dB/DEG C, the second LPFG8 temperature sensitivity is 0.142 dB/DEG C, the first LPFG7 strain sensitivity is 5.5 mdB/mu epsilon and the second LPFG8 strain sensitivity is 4.9 mdB/mu epsilon within the strain variation range of 200 mu epsilon-1000 mu epsilon within the temperature variation range of 25℃ -65 deg. Therefore, the cascade LPFG self-filtering sensing system provided by the invention has the advantages of good linearity, high sensitivity, wide measuring range, and wide application prospect and important reference value, and can be effectively suitable for high-speed demodulation measurement by adopting power output.
Claims (6)
1. The cascade LPFG self-filtering sensing system comprises a narrow-band light source, and is characterized by further comprising a circulator connected with an output end of the narrow-band light source, wherein the circulator is provided with a first port, a second port and a third port, the first port of the circulator is connected with the output end of the narrow-band light source, the second port of the circulator is connected with a first FBG and a second FBG which are connected in series, the third port of the circulator is connected with a Y-type coupler, the first port of the Y-type coupler is connected with the third port of the circulator, the second port of the Y-type coupler is connected with the first LPFG and the second LPFG which are connected in cascade, the third port of the Y-type coupler is connected with a third LPFG and a fourth LPFG which are connected in cascade, the first FG, the second FG, the third LPFG and the fourth LPFG are placed in a constant temperature box, the first LPFG and the second LPFG are placed in an environment to be tested, and the output end of the narrow-band light source, the circulator, the Y-type coupler, the first FG, the second LPFG and the fourth LPFG are all connected by single-mode fibers.
2. The cascaded LPFG self-filtering sensing system of claim 1, wherein the first and second LPFGs are obtained by processing a single mode fiber between the second port of the Y-coupler and the first LPFG by a femtosecond laser and a nudge control stage, the first and second LPFGs having different center wavelengths.
3. The cascaded LPFG self-filtering sensing system of claim 1, wherein the third and fourth LPFGs are obtained by processing a single mode fiber between a third port of the Y-coupler and the third LPFG by a femtosecond laser and a nudge control stage, the third and fourth LPFGs having center wavelengths different from each other.
4. The cascaded LPFG self-filtering sensing system of claim 1, wherein the first and second FBGs are obtained by processing a single mode fiber between the circulator second port and the first FBG by a frequency doubled argon ion laser and a micro-shift control platform, the first FBG and the second FBG being different in center wavelength.
5. The cascaded LPFG self-filtering sensing system of claim 1, wherein the reflection spectrum of the first FBG matches and intersects the transmission spectrum of the first LPFG and the transmission spectrum of the third LPFG, a point of intersection of the center wavelength of the reflection spectrum of the first FBG and the transmission spectrum of the first LPFG is located within a transmission spectrum edge linear band of the first LPFG, and a point of intersection of the center wavelength of the reflection spectrum of the first FBG and the transmission spectrum of the third LPFG is located within a transmission spectrum edge linear band of the third LPFG.
6. The cascaded LPFG self-filtering sensing system of claim 1, wherein a reflection spectrum of the second FBG matches and intersects a transmission spectrum of the second LPFG and a transmission spectrum of the fourth LPFG, a point of intersection of a center wavelength of the reflection spectrum of the second FBG and the transmission spectrum of the second LPFG is located within a transmission spectrum edge linear band of the second LPFG, and a point of intersection of a center wavelength of the reflection spectrum of the second FBG and the transmission spectrum of the fourth LPFG is located within a transmission spectrum edge linear band of the fourth LPFG.
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