CN110057307B - Method for improving strain sensitivity of optical fiber interferometer and optical fiber interferometer - Google Patents
Method for improving strain sensitivity of optical fiber interferometer and optical fiber interferometer Download PDFInfo
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Abstract
The invention discloses a method for improving strain sensitivity of an optical fiber interferometer and the optical fiber interferometer. When the length difference of the two sections of polarization maintaining optical fibers meets the preset condition, the spectrum of the structure contains different frequency components, so that vernier effect is generated, the spectrum of interference light consists of an envelope spectrum and a fine spectrum, and the strain sensitivity of the envelope spectrum is increased compared with that of an interferometer consisting of a single section of polarization maintaining optical fibers, so that the sensing sensitivity is greatly improved. According to the invention, two sections of polarization maintaining optical fibers are cascaded, and the sensitivity of the sensor is determined by the length and the inherent sensitivity of the polarization maintaining optical fibers through calculation, so that compared with an interferometer based on a single polarization maintaining optical fiber, the envelope and the fine spectrum react differently when the strain and the temperature change, and the interferometer has the potential of realizing synchronous measurement of the strain and the temperature, and has the advantages of simple structure, convenience in manufacturing and low cost.
Description
Technical Field
The invention relates to the technical field of optical fiber sensing, in particular to a method for improving strain sensitivity of an optical fiber interferometer and the optical fiber interferometer.
Background
Fiber optic strain sensors are important candidates for structural health monitoring and distributed sensing applications. Compared with the traditional strain sensing method, the optical fiber sensing has the characteristics of small volume, simple manufacture, electromagnetic interference resistance, high resolution and the like. To date, many different types of fiber optic sensing devices have been proposed, such as fiber optic gratings and fiber optic interferometers. In fiber grating strain sensors, fiber bragg gratings (Fiber Bragg Grating, FBG) and long period gratings (Long Period Fiber Grating, LPG) are potential sensing elements and have been widely studied. These sensors are easy to manufacture, but have low strain sensitivity, typical fiber grating sensors have strain sensitivity of only 1.2 pm/. Mu.epsilon.and sensors based on long period fiber gratings have strain sensitivity of only 7.6 pm/. Mu.epsilon.. Interferometric fiber optic sensors have high sensitivity, but the manufacturing process is often complex. In addition, the vernier effect is a method capable of greatly improving the sensitivity, but a sensor based on the vernier effect generally needs two independent interferometers, and has a complex structure.
A common problem with the above-described sensors is that the strain sensing is disturbed by temperature. To solve this problem, simultaneous measurement of multiple parameters based on the matrix method becomes an attractive solution. One possible approach is to design a multimode interferometer based method that uses the differences in multiple modes to achieve simultaneous sensing of multiple parameters, but such sensors are less repeatable. Another approach is to cascade two separate fiber devices with different sensitivities, but this approach still faces the challenges of complex structure and high cost.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to solve the problems that the existing optical fiber strain sensor has high sensitivity, low cost and simple structure and the strain sensor is easy to be interfered by temperature.
To achieve the above object, in a first aspect, the present invention provides an optical fiber interferometer comprising: the device comprises a coupler, a polarization controller, a first section of polarization-maintaining optical fiber and a second section of polarization-maintaining optical fiber;
the first end of the coupler receives input light, and the coupler divides the input light into two beams;
the second end of the coupler is connected with one end of a first section of polarization-maintaining optical fiber, the third end of the coupler is connected with one end of a polarization controller, the other end of the polarization controller is connected with one end of a second section of polarization-maintaining optical fiber, the other end of the first section of polarization-maintaining optical fiber is connected with the other end of the second section of polarization-maintaining optical fiber, and the included angle between the fast axis of the first section of polarization-maintaining optical fiber and the fast axis of the second section of polarization-maintaining optical fiber is a preset angle; the coupler, the polarization controller, the first section of polarization maintaining optical fiber and the second section of polarization maintaining optical fiber form an optical fiber ring, and the polarization controller is used for controlling the polarization rotation angle introduced in the optical fiber ring;
the coupler divides input light into two beams, the two beams respectively pass through the optical fiber ring clockwise and anticlockwise, one beam passes through the first section of polarization maintaining optical fiber to the second section of polarization maintaining optical fiber to return to the coupler, the other beam passes through the second section of polarization maintaining optical fiber to the first section of polarization maintaining optical fiber to return to the coupler, a phase difference is introduced when the two beams of input light pass through the two sections of polarization maintaining optical fibers, and the two beams of input light returned to the coupler interfere to obtain interference light;
the fourth end of the coupler outputs interference light, when the length difference of the first section of polarization maintaining optical fiber and the second section of polarization maintaining optical fiber meets the preset condition, the spectrum of the interference light consists of an envelope spectrum and a fine spectrum, and the strain sensitivity of the envelope spectrum is increased compared with that of an interferometer consisting of single section of polarization maintaining optical fiber.
Optionally, the transmittance of the interference light output by the sagnac optical fiber interferometer is:
wherein T represents transmissivity, alpha and beta respectively represent polarization rotation angles introduced from a first end of the coupler to a splicing position of a first section of polarization maintaining optical fiber, beta represents a polarization rotation angle introduced from a second section of polarization maintaining optical fiber to a third end of the coupler, theta represents the preset angle, B represents double refractive indexes of the two sections of polarization maintaining optical fibers, lambda represents wavelength of input light and L 1 Indicating the length of the first polarization-maintaining fiber, L 2 Representing the length of the second length of polarization maintaining fiber;
when L 1 And L 2 When the difference value of the fine spectrum is smaller than a preset threshold value, the fine spectrum is formed by a high-frequency periodic functionDeciding that the envelope spectrum is defined by a low frequency periodic function +.>Determining, forming a cascading vernier effect.
Optionally, the strain sensitivity K of the envelope spectrum ε,E Strain sensitivity K of interferometer composed of single-section polarization maintaining fiber ε The relation is satisfied:L 0 indicating the length of the portion of the length of the polarization maintaining fiber that is subject to strain.
Optionally, the relation between the wavelength drift and the parameter variation of the sagnac optical fiber interferometer is expressed as follows:
wherein Deltalambda E And Deltalambda F The displacement of the envelope spectrum and the fine spectrum is represented respectively, delta epsilon represents the strain variation, delta T represents the temperature variation, K ε,E Representing the strain sensitivity, K of the envelope spectrum ε,F Represents the strain sensitivity, K of the fine spectrum T,E Representing the temperature sensitivity, K of the envelope spectrum T,F Indicating the temperature sensitivity of the fine spectrum.
Optionally, the first section of polarization-maintaining fiber and the second section of polarization-maintaining fiber are panda-type polarization-maintaining fibers.
In a second aspect, the present invention provides a method of increasing strain sensitivity of an optical fiber interferometer, comprising the steps of:
replacing a single-section polarization maintaining optical fiber in the interferometer with two sections of polarization maintaining optical fibers, wherein the two sections of polarization maintaining optical fibers are connected, and the included angle of the fast axes of the two sections of polarization maintaining optical fibers is a preset angle;
when the length difference of the two sections of polarization maintaining optical fibers meets the preset condition, the spectrum of the interference light obtained after the action of the two sections of polarization maintaining optical fibers is composed of an envelope spectrum and a fine spectrum, a vernier effect is generated, and the strain sensitivity of the envelope spectrum is increased compared with that of an interferometer composed of the single section of polarization maintaining optical fibers.
Optionally, the length difference of the two sections of polarization-maintaining optical fibers meets a preset condition, which specifically comprises: the length difference of the two sections of polarization maintaining optical fibers is smaller than a preset threshold value;
the transmittance of the interference light output by the optical fiber interferometer is as follows:
wherein T represents transmissivity, alpha represents a polarization rotation angle introduced from a first end of the coupler to a splicing position of a first section of polarization maintaining optical fiber, beta represents a polarization rotation angle introduced from a second section of polarization maintaining optical fiber to a third end of the coupler, theta represents the preset angle, B represents double refractive indexes of the two sections of polarization maintaining optical fibers, lambda represents wavelength of input light and L 1 Indicating the length of the first polarization-maintaining fiber, L 2 Representing the length of the second length of polarization maintaining fiber;
when L 1 And L 2 When the difference value of the fine spectrum is smaller than a preset threshold value, the fine spectrum is formed by a high-frequency periodic functionDeciding that the envelope spectrum is defined by a low frequency periodic function +.>Determining, forming a cascading vernier effect.
Optionally, the strain sensitivity K of the envelope spectrum ε,E Strain sensitivity K of interferometer composed of single-section polarization maintaining fiber ε The relation is satisfied:L 0 indicating the length of the portion of the length of the polarization maintaining fiber that is subject to strain.
Optionally, the wavelength drift versus parametric variation of the 8-fiber interferometer is expressed as follows:
wherein Deltalambda E And Deltalambda F The displacement of the envelope spectrum and the fine spectrum is represented respectively, delta epsilon represents the strain variation, delta T represents the temperature variation, K ε,E Representing the strain sensitivity, K of the envelope spectrum ε,F Represents the strain sensitivity, K of the fine spectrum T,E Representing the temperature sensitivity, K of the envelope spectrum T,F Indicating the temperature sensitivity of the fine spectrum.
Optionally, the two polarization-maintaining optical fibers are panda-type polarization-maintaining optical fibers.
In general, the above technical solutions conceived by the present invention have the following beneficial effects compared with the prior art:
according to the invention, two sections of polarization maintaining optical fibers with similar lengths are adopted for cascading, the included angle of the fast axis is a preset angle, and the strain measurement is carried out on the preset length of one section, so that the interferometer with the structure generates a vernier effect, and the sensitivity of sensing is greatly improved.
According to the invention, the transmission spectrum of the structure is composed of the envelope and the fine spectrum by calculating the transfer function of the cascade polarization-maintaining optical fiber and analyzing and setting the proper parameters, so that different reactions are respectively generated on the strain and the temperature change, the simultaneous measurement of the strain and the temperature is realized, the expression of the sensitivity is obtained, and the influence of the temperature on the strain sensitivity is eliminated.
The invention has simple structure, convenient manufacture, good repeatability and low cost, and can be used as a good choice for practical engineering.
Drawings
FIG. 1 is a schematic block diagram of an optical fiber interferometer according to an embodiment of the present invention;
FIG. 2 (a) is a schematic diagram of a strain sensing experiment of an optical fiber interferometer according to an embodiment of the present invention;
FIG. 2 (b) is a schematic diagram of a temperature sensing experiment of an optical fiber interferometer according to an embodiment of the present invention;
FIG. 3 is a schematic diagram of the relationship between the envelope of the strain sensing experiment of the Sagnac fiber interferometer based on two polarization maintaining fibers and the wavelength drift and strain of the fine spectrum according to the embodiment of the invention;
fig. 4 is a schematic diagram of the relationship between the wavelength drift of the envelope and the fine spectrum of the temperature sensing experiment of the sagnac optical fiber interferometer based on two polarization maintaining fibers and the temperature according to the embodiment of the present invention.
Detailed Description
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention. In addition, the technical features of the embodiments of the present invention described below may be combined with each other as long as they do not collide with each other.
Aiming at the problems existing in the prior art, the invention aims to solve the problems that the existing optical fiber strain sensor has high sensitivity, low cost and simple structure and is not compatible with the existing optical fiber strain sensor and the strain sensor is easy to be interfered by temperature.
In order to achieve the above purpose, the invention provides a Sagnac (Sagnac) optical fiber interferometer based on vernier effect, which structurally comprises a 3dB coupler, a common single-mode fiber, a polarization controller and two sections of polarization-maintaining optical fibers. The Sagnac is a classification of optical fiber interferometers, and specifically refers to a ring interferometer which divides one beam of light into two beams of light which are transmitted in opposite directions and finally generates interference.
Specifically, the lengths of the two polarization-maintaining optical fibers are 28cm and 23cm respectively.
Specifically, the included angle of the fast axes of the two sections of polarization maintaining optical fibers is 45 degrees.
Specifically, strain was measured using a portion of one of the polarization maintaining fibers having a length of 12.5 cm.
In particular, the structure allows for simultaneous measurement of strain and temperature.
Specifically, the structure can generate vernier effect, greatly improve sensitivity, and experimentally measure that the strain sensitivity of envelope and fine spectrum is 58.0 pm/mu epsilon and 5.9 pm/mu epsilon respectively, and the temperature sensitivity is-1.05 nm/DEG C and-1.36 nm/DEG C respectively.
Specifically, the polarization-maintaining fiber can be panda-type polarization-maintaining fiber.
Fig. 1 is a schematic diagram of an optical fiber interferometer according to an embodiment of the present invention, as shown in fig. 1, which includes a conventional single-mode fiber, a 3dB coupler OC, a Polarization controller PC, and two Polarization-maintaining fibers (PMF). In this structure, light is input from one end of the coupler, after passing through the coupler, the light is split into two beams, and passes through the optical fiber ring clockwise and anticlockwise, a phase difference is introduced through the two sections of polarization maintaining optical fibers on the way, and after passing through the ring, the light returns to the coupler to interfere, so that an interference spectrum is obtained. For convenience of study, only the case where the coupling ratio of the coupler is 0.5 will be discussed.
Wherein the polarization controller PC is used to control the rotation angle of polarization introduced in the optical fiber loop.
Compared with a general Sagnac interferometer, the structure welds two sections of polarization-maintaining optical fibers together, and the polarization-maintaining optical fibers are inserted into an optical fiber ring to replace a single polarization-maintaining optical fiber. Then it is readily available for this segment of cascaded polarization maintaining fiber, whose Jones matrices are respectively:
wherein M is CW And M CCW Jones matrix of the cascade polarization-maintaining optical fiber under the clockwise and anticlockwise conditions respectively, theta is the included angle between the fast axes of the two polarization-maintaining optical fibers,and->The phase difference introduced by the two sections of polarization maintaining optical fibers respectively meets the following requirementsWherein λ is the wavelength, L 1 、L 2 Is the length of the two PMFs and B is the birefringence of the polarization maintaining fiber.
Specifically, the two sections of polarization maintaining optical fibers have the same parameters except for different lengths.
In the case of a coupling ratio of 0.5, the relationship of the input light to the output light can be simplified as:
wherein E is inx ,E iny ,E outx And E is outy The components of the incident light field and the emergent light field on the x axis and the y axis respectively, alpha represents the polarization rotation angle introduced from the first end of the coupler to the splicing position of the first section of polarization maintaining optical fiber, and beta represents the polarization rotation angle introduced from the splicing position of the second section of polarization maintaining optical fiber to the third end of the coupler.
The transmissivity is as follows:
the transmittance of the structure can be obtained by combining the components (1) - (5):
as can be seen from equation (6), the spectrum of this structure contains two frequency components, the weights of which can be determined by θ, α+β, where θ is determined when the two polarization maintaining fibers are fused and then no longer changed, α+β can be adjusted by a polarization controller, and the polarization controller can adjust the sum of α+β. Specifically, alpha+beta can be controlled to be pi/4 in experiment.
If the optical path difference of the two polarization maintaining fibers is similar, cos [ pi B (L 1 +L 2 )/λ]Can be regarded as a high-frequency periodic function, cos [ pi B (L 1 -L 2 )/λ]Can be seen as a low frequency periodic function. It can be seen that the final spectrum contains a high frequency fine spectrum, consisting of cos [ pi B (L 1 +L 2 )/λ]And (5) determining. In addition, a low-frequency envelope is included, consisting essentially of cos [ pi B (L 1 -L 2 )/λ]And (5) determining. The structure can form a cascading vernier effect and has an amplifying effect on sensitivity and dynamic range.
In one example, to create a vernier effect, the PMF 1 Should be set at the length of the PMF 2 The parameters set up in the experiments as mentioned above are: l (L) 1 ＝28cm，L 2 =23 cm, b=0.0006, θ=45°, the structure is as shown in fig. 1.
A sensor based on cascaded PMFs, wherein during sensing, strain is applied to one PMF and temperature change is applied to both PMFs. With the change of temperature or strain, the envelope and the fine spectrum are displaced, and the displacement of the spectrum can be calculated by analyzing a formula for determining the peak of the envelope and the fine spectrum.
For a single PMF-based Sagnac interferometer, the spectrum shifts when the strain or temperature changes. The calculation formula of strain sensitivity and temperature sensitivity:
wherein B represents the birefringence of PMF, L represents the length of PMF, Δε is the strain variation, Δλ ε Indicating the wavelength shift, delta, corresponding to this strain change ε (BL) represents the optical path length corresponding to the polarization maintaining fiberVariation of difference after variation of applied strain, K ε Indicating the strain sensitivity of the PMF. Similarly, ΔT is the temperature variation, Δλ T Indicating the wavelength shift, delta, corresponding to this temperature change T (BL) represents the change of the optical path difference corresponding to the polarization maintaining fiber after the external temperature is changed, K T Indicating the temperature sensitivity of the PMF.
For cascaded PMFs, a length L in one of the polarization maintaining fibers is used 0 To detect strain. The strain-induced envelope displacement can be expressed as:
Δλ ε,E represents the wavelength shift amount, K, of the envelope under strain change ε,E Representing the strain sensitivity of the envelope, L 1 ，L 2 Respectively represent the lengths of two PMFs, delta ε (BL 0 ) Representing the section L 0 The PMF of the length corresponds to the change in optical path difference after an external strain change. Comparing equations (7) and (9) can obtain the relation between the envelope sensitivity of the cascaded PMF and the sensitivity of the PMF:
when L 1 And L 2 When in closer proximity, the sensitivity is amplified. The amplification effect brought by the vernier effect of the cascade PMF is reflected, and the amplification coefficient M is:
while the fine spectrum is defined by cos [ pi B (L) 1 +L 2 )/λ]Determining, by comparing the same with the equation (7), the strain sensitivity K of the fine spectrum can be calculated ε,F The method comprises the following steps:
in temperature measurement, two lengths of optical fiber are used to sense temperature changes, and optical path difference changes are generated in both lengths of optical fiber. In contrast to equation (8), the envelope spectrum temperature sensitivity and the fine spectrum temperature sensitivity thereof can be expressed as:
wherein K is T,E Represents the temperature sensitivity, K of the envelope spectrum T,F Representing the temperature sensitivity, delta of the fine spectrum T [B(L 1 -L 2 )]，Δ T [B(L 1 +L 2 )]The changes in the optical path difference introduced by PMFs of corresponding lengths in the envelope spectrum and the fine spectrum, respectively, after a temperature change.
From equations (10), (12), (13) and (14), it can be seen that the envelope spectrum of the cascaded PMF has a vernier effect amplified strain sensitivity and the fine spectrum has a reduced strain sensitivity on strain measurement. In temperature measurement, both the envelope spectrum and the fine spectrum of the cascaded PMF have the same temperature sensitivity as the single PMF. This obvious difference in sensitivity gives it the potential to measure strain and temperature simultaneously, and the wavelength shift versus parametric variation can be given by the following crossover matrix:
wherein Deltalambda E And Deltalambda F Is the displacement of the measured envelope and fine line.
To verify the actual performance of the sensor, strain and temperature measurement experiments were performed. The strain sensing experimental apparatus is schematically shown in fig. 2 (a). In order to observe the real-time spectrum of the fiber structure, the device comprises a spectrometer (OSA, yokogawa AQ6370 c) and a broad spectrum light source (BBS), two fiber optic conditioning frames were used in the experiment to fix the fiber and make small displacements. In the experiment, the optical fiber bracket fixes the sensing PMF on the platform, and the original interval between the two stages is L 0 =12.5 cm, i.e. the length to receive strain is L 0 =12.5 cm, the minimum step size that can be adjusted is 10 μm, with a corresponding stress applied to 80 με. Fig. 2 (b) is an experimental apparatus for measuring temperature, and the spectrum is observed in real time using the spectrometer OSA and the broad spectrum light source BBS. To apply the temperature change, a thermo-electric refrigerator (Thermal Electric Cooler, TEC) is used, the minimum temperature adjustment step of which is 0.1 ℃. First, temperature and strain sensing experiments were performed on a single PMF-based Sagnac interferometer using a linear fit, with strain and temperature sensitivity measurements of 20 pm/. Mu.epsilon and-1.38 nm/. Degree.c in one experiment.
Experiments were then performed on a two PMF based Sagnac interferometer. The strain change versus wavelength shift is shown in fig. 3. After linear fitting, the strain sensitivity of the envelope spectrum and the fine spectrum are 58.0 pm/. Mu.epsilon.and 5.9 pm/. Mu.epsilon.respectively, and compared with the Sagnac interferometer of a single PMF, the sensitivity of the envelope spectrum is greatly improved, and the sensitivity of the fine spectrum is slightly reduced, which is consistent with theoretical analysis. The sensitivity amplification factor m=2.9 can be calculated, the amplification factor predicted by equation (11) is 2.5, the theoretical value substantially coincides with the experimental measurement value, and the error may be caused by the measurement inaccuracy of the optical fiber length.
Experiments also verify the temperature performance of the structure. Both PMFs were placed on TEC and heated from 20℃to 80℃in 5℃steps. After obtaining spectra at different temperatures, the envelope and fine spectrum drift can be obtained in the same way as in strain measurement experiments, and the relationship between temperature and peak displacement is shown in fig. 4. It is apparent that as the temperature increases, both the envelope and the fine spectrum shift toward shorter wavelengths. By linear fitting, the temperature sensitivity of the envelope and the fine spectrum were-1.05 nm/DEG C and-1.36 nm/DEG C, respectively. As shown in the previous experiments, the temperature sensitivity of the single PMF is-1.38 nm/DEG C, basically not quite different, and is consistent with the theoretical analysis, which shows that the temperature sensitivity of the envelope spectrum and the fine spectrum of the cascaded PMF is approximately the same as that of the single PMF. According to the strain and temperature sensitivity obtained by experiments, the synchronous measurement of the strain and the temperature can be realized.
The invention discloses a Sagnac optical fiber interferometer based on vernier effect, which structurally comprises a 3dB coupler, a common single mode fiber, a polarization controller and two sections of polarization maintaining optical fibers with lengths of 28cm and 23cm respectively. The invention adopts two polarization maintaining optical fibers to introduce different optical path differences, so that the spectrum of the structure contains different frequency components, thereby generating vernier effect and greatly improving the sensing sensitivity. According to the invention, two sections of polarization maintaining optical fibers are cascaded, and the sensitivity of the sensor is determined by the length and the inherent sensitivity of the polarization maintaining optical fibers through calculation, so that compared with an interferometer based on a single polarization maintaining optical fiber, the envelope and the fine spectrum react differently when the strain and the temperature change, and therefore, the interferometer has the potential of realizing synchronous measurement of the strain and the temperature, and has the advantages of simple structure, convenience in manufacturing and low cost.
It will be readily appreciated by those skilled in the art that the foregoing description is merely a preferred embodiment of the invention and is not intended to limit the invention, but any modifications, equivalents, improvements or alternatives falling within the spirit and principles of the invention are intended to be included within the scope of the invention.
Claims (8)
1. A fiber optic interferometer, comprising: the device comprises a coupler, a polarization controller, a first section of polarization-maintaining optical fiber and a second section of polarization-maintaining optical fiber;
the first end of the coupler receives input light, and the coupler divides the input light into two beams;
the second end of the coupler is connected with one end of a first section of polarization maintaining optical fiber, the third end of the coupler is connected with one end of a polarization controller, the other end of the polarization controller is connected with one end of a second section of polarization maintaining optical fiber, the other end of the first section of polarization maintaining optical fiber is connected with the other end of the second section of polarization maintaining optical fiber, and the included angle between the fast axis of the first section of polarization maintaining optical fiber and the fast axis of the second section of polarization maintaining optical fiber is a preset angle; the coupler, the polarization controller, the first section of polarization maintaining optical fiber and the second section of polarization maintaining optical fiber form an optical fiber ring, and the polarization controller is used for controlling the polarization rotation angle introduced in the optical fiber ring;
the coupler divides input light into two beams, the two beams respectively pass through the optical fiber ring clockwise and anticlockwise, one beam passes through the first section of polarization maintaining optical fiber to the second section of polarization maintaining optical fiber to return to the coupler, the other beam passes through the second section of polarization maintaining optical fiber to the first section of polarization maintaining optical fiber to return to the coupler, a phase difference is introduced when the two beams of input light pass through the two sections of polarization maintaining optical fibers, and the two beams of input light returned to the coupler interfere to obtain interference light;
the fourth end of the coupler outputs interference light, when the length difference of the first section of polarization maintaining optical fiber and the second section of polarization maintaining optical fiber meets the preset condition, the spectrum of the interference light consists of an envelope spectrum and a fine spectrum, and the strain sensitivity of the envelope spectrum is increased compared with that of an interferometer consisting of single section of polarization maintaining optical fiber;
the optical fiber interferometer is a Sagnac optical fiber interferometer, and the transmittance of the interference light output by the optical fiber interferometer is as follows:
wherein T represents transmissivity, alpha represents a polarization rotation angle introduced from a first end of the coupler to a splicing position of a first section of polarization maintaining optical fiber, beta represents a polarization rotation angle introduced from a second section of polarization maintaining optical fiber to a third end of the coupler, theta represents the preset angle, B represents double refractive indexes of the two sections of polarization maintaining optical fibers, lambda represents wavelength of input light and L 1 Indicating the length of the first polarization-maintaining fiber, L 2 Representing the length of the second length of polarization maintaining fiber;
when L 1 And L 2 When the difference value of the fine spectrum is smaller than a preset threshold value, the fine spectrum is formed by a high-frequency periodic functionDeciding that the envelope spectrum is defined by a low frequency periodic function +.>Determining, forming a cascading vernier effect.
2. The fiber optic interferometer of claim 1, wherein the envelope spectrum has a strain sensitivity K ε，E Strain sensitivity K of interferometer composed of single-section polarization maintaining fiber ε The relation is satisfied:L 0 indicating the length of the portion of the length of the polarization maintaining fiber that is subject to strain.
3. The fiber optic interferometer of claim 2, wherein the wavelength drift versus parametric variation of the sagnac fiber optic interferometer is expressed as follows:
wherein Deltalambda E And Deltalambda F The displacement of the envelope spectrum and the fine spectrum is represented respectively, delta epsilon represents the strain variation, delta T represents the temperature variation, K ε，E Representing the strain sensitivity, K of the envelope spectrum ε，F Represents the strain sensitivity, K of the fine spectrum T，E Representing the temperature sensitivity, K of the envelope spectrum T，F Indicating the temperature sensitivity of the fine spectrum.
4. A fiber optic interferometer according to any of claims 1 to 3 wherein both lengths of polarization maintaining fiber are panda type polarization maintaining fibers.
5. A method for improving strain sensitivity of an optical fiber interferometer, comprising the steps of:
replacing a single-section polarization maintaining optical fiber in the interferometer with two sections of polarization maintaining optical fibers, wherein the two sections of polarization maintaining optical fibers are connected, and the included angle of the fast axes of the two sections of polarization maintaining optical fibers is a preset angle;
when the lengths of the two sections of polarization maintaining optical fibers are different, the spectrum of interference light obtained after the action of the two sections of polarization maintaining optical fibers consists of an envelope spectrum and a fine spectrum, and the strain sensitivity of the envelope spectrum is increased compared with that of an interferometer formed by the single section of polarization maintaining optical fibers;
the length difference of the two sections of polarization maintaining optical fibers meets the preset condition, and specifically comprises the following steps: the length difference of the two sections of polarization maintaining optical fibers is smaller than a preset threshold value;
the transmittance of the interference light output by the optical fiber interferometer is as follows:
wherein T represents transmissivity, alpha represents a polarization rotation angle introduced from a first end of the coupler to a splicing position of a first section of polarization maintaining optical fiber, beta represents a polarization rotation angle introduced from a second section of polarization maintaining optical fiber to a third end of the coupler, theta represents the preset angle, B represents double refractive indexes of the two sections of polarization maintaining optical fibers, lambda represents wavelength of input light and L 1 Indicating the length of the first polarization-maintaining fiber, L 2 Representing the length of the second length of polarization maintaining fiber;
when L 1 And L 2 When the difference value of the fine spectrum is smaller than a preset threshold value, the fine spectrum is formed by a high-frequency periodic functionDeciding that the envelope spectrum is defined by a low frequency periodic function +.>Determining, forming a cascading vernier effect.
6. The method of increasing strain sensitivity of a fiber optic interferometer of claim 5, wherein the envelope spectrum has a strain sensitivity K ε，E Strain sensitivity K of interferometer composed of single-section polarization maintaining fiber ε The relation is satisfied:L 0 indicating the length of the portion of the length of the polarization maintaining fiber that is subject to strain.
7. The method of increasing strain sensitivity of a fiber optic interferometer of claim 6, wherein the wavelength drift versus parametric variation of the fiber optic interferometer is expressed as follows:
wherein Deltalambda E And Deltalambda F The displacement of the envelope spectrum and the fine spectrum is represented respectively, delta epsilon represents the strain variation, delta T represents the temperature variation, K ε，E Representing the strain sensitivity, K of the envelope spectrum ε，F Represents the strain sensitivity, K of the fine spectrum T，E Representing the temperature sensitivity, K of the envelope spectrum T，F Indicating the temperature sensitivity of the fine spectrum.
8. The method of increasing strain sensitivity of an optical fiber interferometer according to any of claims 5 to 7, wherein the two polarization maintaining fibers are panda type polarization maintaining fibers.
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