1. The utility model provides a temperature sensor based on twist reverse two-core fiber, its characterized in that includes two-core fiber, and its both ends are connected single mode fiber through drawing the awl coupling region respectively, and two-core fiber contains and is two spiral helicine fibre cores after twisting, and the center of two fibre cores is different with the distance in optic fibre axle center, and the helix pitch of formation is different, and the group optical path difference of signal is zero in two fibre cores.

2. The twisted dual-core fiber-based temperature sensor of claim 1, wherein the ends of the dual-core fiber are tapered into the coupling region by fusion splicing.

3. The twisted dual-core fiber-based temperature sensor of claim 1, wherein the pitch of the two cores is adjusted such that the group optical path difference of the signals in the two cores approaches zero.

4. The twisted dual-core fiber-based temperature sensor of claim 1, wherein the diameter of the two cores of the dual-core fiber is 8-10 microns and the distance between the two cores and the axis of the fiber is 30-50 microns.

5. The twisted dual-core fiber-based temperature sensor of claim 1, wherein the dispersion curves of the two core materials differ significantly.

6. The twisted dual-core fiber-based temperature sensor of claim 2, wherein the coupling region is biconical with a waist diameter of 50-80 microns and a length of 200-1000 microns.

7. The twisted dual-core fiber-based temperature sensor of claim 1, wherein the two cores are made of materials having different refractive indices with different wavelength-dependent fitted linear coefficients.

8. The twisted dual-core fiber-based temperature sensor of any of claims 1-7, wherein the dual-core fiber is externally clad with a cladding.

9. A preparation method of a temperature sensor based on a twisted twin-core optical fiber is characterized by comprising the following steps:

removing the coating layer of the double-core optical fiber, and cutting flat end faces of two ends of the double-core optical fiber by using a cutter;

respectively cutting the two single-mode optical fibers with the coating layers removed into flat end faces by using a cutting knife, and respectively welding the two single-mode optical fibers with two ends of a double-core optical fiber;

fixing the optical fiber on a horizontal line by using a clamp, aligning a flame brush head to the welding position of two ends of the double-core optical fiber and the single-mode optical fiber for melting and heating, and simultaneously moving the clamps at the two ends in opposite directions on the horizontal line to form two biconical coupling areas;

the two ends of the double-core optical fiber are fixed on a horizontal line by clamps, the flame brush head is aligned to the middle area of the double-core optical fiber for melting and heating, and simultaneously the clamps are twisted in different directions at the same speed to enable the double-core to be in a spiral shape.

Background

The optical fiber temperature sensor has the advantages of high sensitivity, small volume, light weight, corrosion resistance, electromagnetic interference resistance, electric insulation and the like, and has great market demand in special fields such as high-temperature and high-pressure, inflammable and explosive, strong electromagnetic interference and strong chemical corrosivity. Among the optical fiber temperature sensors with different structures, the interferometric optical fiber temperature sensor has received attention from researchers due to its advantages of high sensitivity, flexibility, variety, and wide range of measurement objects. However, conventional fiber optic interferometers are typically only a few tens of picometers per degree celsius (c) in temperature sensitivity due to the thermo-optic coefficient of quartz. The temperature sensitivity of the interference type optical fiber sensor based on the dispersion turning point which is proposed recently can reach 1.899 nm/DEG C within the range of 25-32 ℃.

The dispersion turning point of the interferometer is the working state of the sensor corresponding to the situation that the difference between the effective refractive indexes of the two groups of the two modes generating interference is zero. Since the denominator of the sensor sensitivity formula is the difference between the effective refractive indices of the two mode groups, as shown in formula (3), the sensor has an ultra-high sensitivity near the dispersion inflection point. The existing fiber sensor structure based on the dispersion turning point is based on a fiber micro-cone with the diameter less than 5 microns. Interference occurs between two different modes in the optical fiber micro-cone, and the wavelength corresponding to one trough of the interference spectrum can be expressed as:

the response characteristic of the micro-cone interferometer to temperature can be expressed as:

where T is ambient temperature. Combining the same kind terms of the formula (2) and substituting the formula (1) to obtain

From equation (3), it can be seen that when the group effective refractive indices of the two modes are the same, the sensitivity of the sensor is infinite, and n is_g1＝n_g2The corresponding operating wavelength is called the dispersion break point. For conventional optical fibers, the dispersion curves for different modes within the fiber do not differ muchIt is difficult to obtain a dispersion turning point in the near infrared band. To obtain the dispersion break point, a single mode fiber can only be tapered to a microtone with a diameter of less than 5 microns, and the HE12 and HE11 modes with sufficiently different dispersion curves are formed in the tapered region and produce intermodal interference. The method has the problems of low mechanical strength, unstable structure and the like caused by the undersize diameter of the optical fiber sensing area.

Disclosure of Invention

The invention aims to solve the problems that the existing temperature sensor has a single method for acquiring the dispersion turning point of an interferometer, the mechanical strength of the sensor is not high and the like, and provides a temperature sensor based on a twisted double-core optical fiber.

The technical scheme adopted by the invention is as follows:

the utility model provides a temperature sensor based on twist reverse two-core fiber, includes two-core fiber, and its both ends are respectively through drawing the cone coupling region and connecting single mode fiber, and two-core fiber contains and is two spiral helicine fibre cores after twisting, and the center of two fibre cores is different with the distance in optic fibre axle center, and the helix pitch of formation is different, and the group optical path difference of signal is zero in two fibre cores.

According to the technical scheme, the two ends of the double-core optical fiber are welded and then tapered to form a coupling area.

According to the technical scheme, the thread pitches of the two fiber cores are adjusted to enable the group optical path difference of signals in the two fiber cores to approach zero.

According to the technical scheme, the diameters of the two fiber cores of the double-core optical fiber are 8-10 microns, and the distance between the two fiber cores and the axis of the optical fiber is 30-50 microns.

According to the technical scheme, the dispersion curves of the two fiber core materials have large difference.

According to the technical scheme, the coupling area is in a double-cone shape, the diameter of the cone waist is 50-80 microns, and the length of the double-cone coupling area is 200-1000 microns.

According to the technical scheme, the fitting linear coefficients of the refractive indexes of the materials used by the two fiber cores, which change along with the wavelength, are different.

According to the technical scheme, the double-core optical fiber is coated with a cladding layer.

The invention also provides a preparation method of the temperature sensor based on the twisted twin-core optical fiber, which comprises the following steps:

(1) removing the coating layer of the double-core optical fiber, and cutting flat end faces of two ends of the double-core optical fiber by using a cutter;

(2) respectively cutting the two single-mode optical fibers with the coating layers removed into flat end faces by using a cutting knife, and respectively welding the two single-mode optical fibers with two ends of a double-core optical fiber;

(3) fixing the optical fiber on a horizontal line by using a clamp, aligning a flame brush head to the welding position of two ends of the double-core optical fiber and the single-mode optical fiber for melting and heating, and simultaneously moving the clamps at the two ends in opposite directions on the horizontal line to form two biconical coupling areas;

(4) the two ends of the double-core optical fiber are fixed on a horizontal line by clamps, the flame brush head is aligned to the middle area of the double-core optical fiber for melting and heating, and simultaneously the clamps are twisted in different directions at the same speed to enable the double-core to be in a spiral shape.

The invention has the following beneficial effects: according to the invention, two ends of a double-core optical fiber are coupled with a single-mode optical fiber, a single-mode optical fiber fundamental mode is simultaneously coupled into two fiber cores of the double-core optical fiber in a coupling area at one side for transmission, and two fiber core modules are coupled back into the fiber cores of the single-mode optical fiber in another coupling area for interference to form a Mach-Zehnder interferometer; and simultaneously, the double-core optical fiber is twisted, and the two fiber core molds are transmitted along different spiral fiber core paths in the spiral area. The introduction of the double-helix fiber core enables the physical lengths of the transmission paths of the two fiber core modules which generate interference to be different, so that the group optical path difference of two paths of signals can be zero by setting the pitch of the twisting area of the double-core fiber, and the high-sensitivity effect similar to a dispersion turning point is realized. The invention can adjust the thread pitch without changing the diameter of the optical fiber, and has better mechanical strength and reliability.

Drawings

The invention will be further described with reference to the accompanying drawings and examples, in which:

FIG. 1 is a schematic diagram of a twisted twin-core fiber-based temperature sensor according to an embodiment of the present invention;

FIG. 2 is a schematic horizontal cross-sectional view of a dual core optical fiber in an embodiment of the present invention;

FIG. 3 is a dispersion curve of fundamental modes of two cores of a twisted dual-core fiber according to an embodiment of the present invention;

FIG. 4 is a graph showing the variation of the group optical path difference G with wavelength for a twisted twin-core fiber according to an embodiment of the present invention;

in the figure: 1. the fiber comprises a single-mode fiber, 2 single-mode fibers, 3 a double-core fiber coupling area, 4 a double-core fiber coupling area, 5 a double-core fiber twisting area, 7 a fiber core, 8 a fiber core and 9 a cladding.

Detailed Description

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

According to the temperature sensor based on the twisted double-core optical fiber, the group optical path difference of the fundamental modes of the two fiber cores of the double-core optical fiber can be adjusted by twisting the double-core optical fiber, and the sensor can work in the state of the quasi-dispersion turning point of an interferometer.

Fig. 1 is a schematic structural diagram of a temperature sensor based on a twisted twin-core optical fiber according to an embodiment of the present invention. The temperature sensor based on the twisted double-core optical fiber comprises a single-mode optical fiber 1, a single-mode optical fiber 2 and a double-core optical fiber, wherein the double-core optical fiber comprises coupling regions (which can be formed by tapering) at two ends: the double-core optical fiber comprises a double-core optical fiber coupling area 3, a double-core optical fiber coupling area 4 and a double-core optical fiber twisting area 5 formed by twisting the middle part of the double-core optical fiber, wherein the double-core optical fiber comprises a fiber core 7 and a fiber core 8, the double core is in a spiral state after being twisted, and a cladding 9 can be further coated outside the twisted double core.

Fig. 2 is a schematic horizontal cross-sectional view of a dual-core optical fiber according to an embodiment of the present invention, in which the fiber cores 7 and 8 are on two sides of the optical fiber axis, and the distances between the centers of the fiber cores 7 and 8 and the optical fiber axis are different, so that the pitches of the formed spiral lines are different, and the group optical path difference of signals in the two fiber cores approaches zero.

The fiber core 7 and the fiber core 8 are made of different materials, and the dispersion curves of the two materials are greatly different. The diameters of the two fiber cores are 8-10 microns, and the distance between the centers of the two fiber cores and the axis of the optical fiber is 30-50 microns. The diameter of the cladding 9 is 125 microns.

Furthermore, the biconical coupling region formed by the two ends of the dual-core fiber and the single-mode fiber is manufactured by using a fusion tapering technology, the diameter of the taper waist is 50-80 microns, and the length of the biconical coupling region is 200-1000 microns.

In the invention, the two ends of the double-core optical fiber are welded with the single-mode optical fiber, and the welding positions at the two ends are tapered to form a coupling area. The two fiber core fundamental modes of the double-core optical fiber are simultaneously excited through the first coupling area, the two fiber core fundamental modes are respectively transmitted along different spiral fiber core paths, and the fiber cores of the single-mode optical fiber are coupled back in the second coupling area. By twisting the dual-core fiber, the dual-core is in a dual-helix state due to the twisting because the dual-core is on both sides of the axis of the fiber. Meanwhile, because the distance from the center of the double fiber core to the axis of the optical fiber is different, the optical path difference of the double fiber core mold is changed along with the different screw pitches of the spiral line.

The welding positions of two ends of the double-core optical fiber and the single-mode optical fiber are tapered, a single-mode optical fiber fundamental mode is simultaneously coupled into two fiber cores of the double-core optical fiber in a coupling area at one side for transmission, and two fiber core molds are coupled back into the fiber cores of the single-mode optical fiber in the other coupling area for interference to form a Mach-Zehnder interferometer; and simultaneously, the double-core optical fiber is twisted, and the two fiber core molds are transmitted along different spiral fiber core paths in the spiral area. The introduction of the double-helix fiber core enables the physical lengths of the transmission paths of the two fiber core modules which generate interference to be different, so that the group optical path difference of two paths of signals can be zero by adjusting the pitch of the twisting area of the double-core fiber, and the high-sensitivity effect similar to a dispersion turning point is realized. The invention can adjust the thread pitch without changing the diameter of the optical fiber, and has better mechanical strength and reliability.

In summary, the modulation of the optical path difference between the two fiber core modules can be realized by controlling the twist parameter. The quasi-dispersion turning point provided by the invention is a point that the difference of the products of the group effective refractive indexes of the two modes and the physical lengths of the corresponding transmission paths is zero, so that for the two fixed modes, the quasi-dispersion turning point state can be achieved by changing the torsion parameter, and the sensitivity of the optical fiber sensor is improved.

In a preferred embodiment of the present invention, the fiber cores of the single mode fiber 1 and the single mode fiber 2 have a diameter of 8.2 microns and an outer diameter of 125 microns; the diameters of the fiber cores 7 and 8 are both 9 microns, and the outer diameter of the double-core optical fiber is 125 microns; the distances between the center of the fiber core 7 and the center of the fiber core 8 and the axis of the double-core optical fiber are 40 micrometers and 37.5 micrometers respectively.

FIG. 3 is a dispersion curve of fundamental modes of two cores obtained by simulation of a twisted twin-core fiber according to an embodiment of the present invention. The difference of the dispersion curves of the fundamental modes is mainly influenced by the properties of the fiber core materials, and the fitting linear coefficient of the refractive index of the material used for the fiber core 1 along with the change of the wavelength is set to be-0.08; the fitting linear coefficient of the refractive index of the material used for the fiber core 2, which varies with wavelength, is-0.023. The fitting linear coefficient of the refractive index of the cladding material used by the double-core optical fiber along with the change of the wavelength is-0.024. The wavelength range set in the embodiment of the invention is 1.4-1.6 microns, and the variation step size is 0.01 micron. And (5) obtaining a dispersion curve of the fundamental mode of the double fiber cores through simulation. The solid line in the figure is the corresponding fundamental mode HE of the fiber core 8_{11_core1}The dotted line is the fundamental mode HE corresponding to the fiber core 7_{11_core2}The dispersion curve of (1).

FIG. 4 is a graph showing the variation of the group optical path difference G with wavelength for a twisted twin-core fiber according to an embodiment of the present invention. According to the change curve of G along with the diameter in the figure, the parameters of the twisted twin-core temperature sensor of the best example in the twisted twin-core optical fiber in the embodiment of the invention can be obtained, when the screw pitch of a twisting zone is 6000 microns, and the number of spiral turns is 5; when the physical lengths of the fiber core 7 and the fiber core 8 are 3.00231 cm and 3.00263 cm respectively, the twisted double-core optical fiber reaches the vicinity of the quasi-dispersion turning point under the wavelength of 1.53 microns, and the sensitivity is improved. The principle of the twisted twin-core fiber type dispersion turning point is as follows:

in the mode transmission of the twisted dual-core optical fiber in the embodiment of the invention, the fundamental modes of the fiber cores transmitted by the dual fiber cores in the dual-core optical fiber are HE respectively_{11_core1}And HE_{11_core2}。

The pitch of the twisted twin-core fiber in the embodiment of the invention is recorded as S, HE_{11_core1}The length of the single-turn spiral line corresponding to mode transmission is L₁,HE_{11_core2}The length of the single-turn spiral line corresponding to mode transmission is L₂And have the same N consecutive helix periods, i.e. the physical distance from the start of the first helix to the end of the last helix of the twin-core twist is L₁xN and L₂X N, i.e. two core fundamental modes HE_{11_core1}And HE_{11_core2}Respectively corresponding transmission path lengths.

Further, the length of the twisted spiral line of the fiber core of the twisted double-core optical fiber, namely the fundamental mode HE, can be obtained through a spiral line length formula_{11_core1}And HE_{11_core2}Is recorded as the physical length of the transmission path Andwherein d is₁For the fundamental mode HE of the core_{11_core1}Corresponding to the distance between the center of the fiber core and the axis of the dual-core optical fiber, d₂For the fundamental mode HE of the core_{11_core2}The distance between the center of the corresponding fiber core and the axis of the dual-core optical fiber. Because of d₁Is not equal to d₂Therefore L is_core1Is not equal to L_core2The physical lengths of the transmission paths of the two modes are different.

Further, in the present invention,indicating the mode HE_{11_core1}And mode HE_{11_core2}The phase difference after passing through the torsion region has the following value for the wave trough of the interference spectrum

Wherein n is_eff-co1And n_eff-co2Respectively corresponding to the patterns HE_{11_core1}And mode HE_{11_core2}M is an odd number.Corresponding to a wave trough having a wavelength of

Further, when the ambient temperature changes, the effective refractive indexes of the two fiber core fundamental modes change, and the physical lengths of the two fiber cores are changed under the influence of the temperature. The sensitivity of the sensor can therefore be expressed as:

where T is ambient temperature. So that the same items are arranged and combined in the pair (6)

Further, substituting equation (5) into equation (7) can result in

Finally, the sensitivity formula is obtained by arrangement

Wherein the content of the first and second substances,two core modes HE of the dual-core optical fiber respectively_{11_core1}And HE_{11_core2}G ═ n can be obtained_g-co1×L_core1-n_g-co2×L_core2. It is apparent that S is present when G is 0_TThe sensitivity tends to be infinite, and a point corresponding to G ═ 0 is referred to as a dispersion-like inflection point. Therefore, we can regulate and control by twisting the double-core optical fiberL_core1And L_core2The difference of (b) makes the value of G approach 0, thereby achieving an improvement in sensitivity.

Therefore, the invention provides a twisted twin-core optical fiber temperature sensor by improving a derivation formula of a dispersion turning point. The dispersion-like turning point state can be achieved without tapering the optical fiber, so that the sensitivity of the optical fiber temperature sensor is effectively improved, and the mechanical strength and stability of the sensor are enhanced.

The preparation method of the temperature sensor based on the twisted twin-core optical fiber comprises the following steps:

(1) removing the coating layer of the double-core optical fiber, and cutting flat end faces of two ends of the double-core optical fiber by using a cutter;

(2) respectively cutting the two single-mode optical fibers with the coating layers removed into flat end faces by using a cutting knife, and respectively welding the two single-mode optical fibers with two ends of a double-core optical fiber;

(3) fixing the optical fiber on a horizontal line by using a clamp, aligning a flame brush head to the welding position of two ends of the double-core optical fiber and the single-mode optical fiber for melting and heating, and simultaneously moving the clamps at the two ends in opposite directions on the horizontal line to form two biconical coupling areas;

(4) the two ends of the double-core optical fiber are fixed on a horizontal line by clamps, the flame brush head is aligned to the middle area of the double-core optical fiber for melting and heating, and simultaneously the clamps are twisted in different directions at the same speed to enable the double-core to be in a spiral shape.

It will be understood that modifications and variations can be made by persons skilled in the art in light of the above teachings and all such modifications and variations are intended to be included within the scope of the invention as defined in the appended claims.