M-Z interference sensor with small excitation group velocity dispersion difference mode structure

Abstract

The utility model relates to the field of optical fiber interference sensors, and discloses an M-Z interference sensor with a small excitation group velocity dispersion difference mode structure. Comprising a light injection single-mode optical fiber, a small-core-diameter step multimode optical fiber, a wide annular core optical fiber, an annular core optical fiber, a gradient refractive index multimode optical fiber, a large-core-diameter step multimode optical fiber and a light receiving single-mode optical fiber which are connected in sequence. The wide annular core optical fiber comprises a third quartz cladding and a third quartz fiber core, and the third quartz fiber core annularly wraps a third quartz layer; the annular core optical fiber comprises a fourth quartz inner wrapping layer, a fourth quartz fiber core and a fourth quartz outer wrapping layer, the fourth quartz inner wrapping layer is annularly wrapped by the fourth quartz fiber core, the fourth quartz fiber core is annularly wrapped by the fourth quartz outer wrapping layer, and the optical fiber radial refractive index distribution index of the gradient refractive index multimode optical fiber is 1.95-2. According to the scheme, the annular core optical fiber is used for injecting light to the gradient refractive index multimode optical fiber, two modes with smaller group velocity dispersion difference can be excited to interfere, and the sensitivity of the sensor is improved.

Description

Technical Field

[0001] This invention relates to the field of fiber optic interferometric sensors, specifically to an MZ interferometric sensor constructed with a mode having a small excitation group velocity dispersion difference. Background Technology

[0002] Fiber optic interferometric sensors are characterized by high stability and repeatability, making them valuable for applications. In recent years, enhancing the sensitivity of fiber optic interferometric sensors has been a hot topic, with studies on dispersion inflection point (DTP) and vernier effects receiving widespread attention. The vernier effect, in essence, does not increase the offset of the interference valley; rather, it transforms a small interference valley offset into a larger envelope valley offset by cascading two sensors with similar optical path differences. Therefore, more complex demodulation methods are required to detect the movement of the envelope valley in practical applications. In contrast, interferometric sensors operating near the DTP have a smaller group velocity dispersion difference between the two interference modes, which can fundamentally increase the offset of the interference valley. Spectral demodulation is easier, and the sensing structure is integrated into a single fiber, making it convenient and practical. Therefore, this method of enhancing the sensitivity of interferometric sensors has extremely high research value.

[0003] At a certain wavelength, the effective group refractive index difference, group velocity difference, or group velocity dispersion difference between two modes is equal to zero; this wavelength is called the group velocity dispersion difference (DTP). Near this wavelength, the group velocity dispersion difference between the two modes is small, and the interferometric sensor exhibits high sensitivity to various sensing parameters such as refractive index, strain, and temperature. In typical single-mode and multimode fibers, the group velocity dispersion difference changes with wavelength. When the group velocity dispersion difference between the two modes is zero, the corresponding wavelength is between 800nm ​​and 1000nm, far from the conventional operating band of the interferometric sensor. This results in a large group velocity dispersion difference between the two modes at a wavelength of 1550nm, thus limiting the sensitivity performance of ordinary interferometric sensors. Currently, to obtain two interference modes with a small group velocity dispersion difference near 1550nm and further improve the sensitivity of the interferometric sensor, there are two approaches: one is to move the DTP to near 1550nm; the other is to use fiber with a slow change in group velocity dispersion difference with wavelength, so that even if the operating band is far from the DTP, the group velocity dispersion difference between the two interference modes remains small.

[0004] In the first type of scheme, the DTP needs to be moved to around 1550nm. Group velocity dispersion consists of material dispersion and waveguide dispersion; either material or waveguide dispersion can be adjusted separately to make the total group velocity dispersion difference zero. Adjusting material dispersion through doping in optical fibers is quite difficult; there are many other schemes to reduce group velocity dispersion by adjusting the waveguide structure of the fiber, such as using special structure fibers or tapering the fiber. In the use of special optical fibers, Lu et al. proposed a few-mode fiber interferometric sensor with a special structure. The fine ring core on the outside of the fiber core modulates the group velocity of the LP02 mode, which rapidly reduces the group velocity dispersion difference with the LP01 mode, shifting the DTP to around 1550 nm. It has ultra-high sensitivity of 43 pm / με and 704 pm / ℃ when measuring strain and temperature (LU C, SU J, DONG X, et al. Studies on Temperature and Strain Sensitivity of a Few-Mode Critical Wavelength Fiber Optic Sensor [J]. IEEE Sensors Journal, 2019, 19(5): 1794-801). In the process of taperizing optical fibers, Luo et al. tapered the diameter of single-mode fiber to 4.6 μm, moving the DTP to around 1550 nm. Under the condition of an ambient refractive index range of 1.333~1.334, the refractive index sensing sensitivity reached 10777.8 nm / RIU (LUO H, SUN Q, LI X, et al. Refractive index sensing characteristics near the dispersion turning point of the multimode microfiber-based Mach-Zehnder interferometer [J]. Opt Lett, 2015,40(21): 5042-5). However, special optical fibers require special fiber preforms, which are expensive. The fiber tapering technique is complex to process, and in order to bring the DTP closer to 1550 nm, the diameter of the taper region needs to be reduced to 2 μm, and the length of the taper region needs to be increased as much as possible, which significantly reduces the sensor strength; moreover, the repeatability of fiber tapering is low, which will affect the accuracy of the DTP position.

[0005] In the second type of scheme, it is necessary to use optical fibers with slow group velocity dispersion difference as a function of wavelength. Since graded-index fibers are designed to reduce the maximum group delay difference, they have the characteristics of small group velocity dispersion difference between different modes and slow change with wavelength. Therefore, graded-index fibers are generally chosen to construct interferometric sensors. The sensing structure is simple and can achieve high sensitivity near 1550 nm without processing. Liu et al. first comprehensively measured the sensing characteristics of interferometric sensors constructed with graded-index fibers, and obtained a strain sensitivity of -18.5 pm / µε and a temperature sensitivity of -58.5 pm / ℃ (LIU Y, WEI L. Low-cost high-sensitivity strain and temperature sensing using graded-index multimode fibers [J]. Applied Optics, 2007, 46(13): 2516-9).

[0006] Currently, research on the second type of scheme is limited. Although this type of sensor is simple to fabricate, has good stability, and strong robustness, its sensitivity still lags significantly behind that of the first type of scheme, which uses a directly moving DTP interferometer. Clearly, adopting the second type of scheme and further improving the sensitivity of the graded-index fiber optic interferometer sensor would be of great significance. Utility Model Content

[0007] The present invention aims to provide an MZ interferometer sensor with a mode structure that has a small excitation group velocity dispersion difference, so as to improve the sensitivity of the graded refractive index fiber interferometer sensor without changing the structure of the graded refractive index fiber.

[0008] To achieve the above objectives, this utility model adopts the following technical solution: an MZ interferometer sensor with a mode structure having a small excitation group velocity dispersion difference, comprising: a single-mode fiber for beam emission, a small-core-diameter step-index multimode fiber, a wide-ring-core fiber, a ring-core fiber, a graded-index multimode fiber, a large-core-diameter step-index multimode fiber, and a single-mode fiber for beam reception, connected sequentially; the wide-ring-core fiber comprises a third quartz cladding and a third quartz core, the third quartz cladding being cylindrical, and the third quartz core annularly covering the third quartz cladding; the ring-core fiber comprises a fourth quartz inner cladding, a fourth quartz core, and a fourth quartz outer cladding; the fourth quartz inner cladding is cylindrical; the fourth quartz core annularly covers the fourth quartz inner cladding; the fourth quartz outer cladding annularly covers the fourth quartz core; the radial refractive index distribution index of the graded-index multimode fiber is 1.95~2.

[0009] The principle of this scheme is as follows: the light is injected from the left end of the single-mode fiber, the light field is split by the small core diameter step-index multimode fiber, and the middle cladding and outer core structure of the wide ring core fiber are used to reduce the light field energy in the core part of the fiber. At this time, the light field energy is mainly concentrated in the ring region core of the ring core fiber. Therefore, most of the beam can enter the fourth quartz core of the ring core fiber and propagate in different modes. When the light of different modes propagates to the end face of the graded refractive index multimode fiber, it propagates along different curved paths.

[0010] Because the graded-index optical fiber in actual production needs to meet the requirement of reducing the group delay difference between different modes in the field of optical fiber communication, the radial refractive index distribution index of the fiber is generally set to a number slightly less than 2. In this case, the group velocity dispersion of each mode in the graded-index fiber is a convex function of the mode order; that is, as the mode order increases, the group velocity dispersion first increases and then decreases, and the group velocity dispersion of several modes near the maximum value is approximately equal. The sensitivity of an interferometric sensor is related to the group velocity dispersion difference between two interfering modes. The smaller the group velocity dispersion difference, the higher the sensing sensitivity. By injecting light into a graded-index multimode fiber through a ring-core fiber, two modes with smaller group velocity dispersion differences can be excited to interfere, further improving the sensing sensitivity.

[0011] Due to the self-focusing effect in graded-index multimode fiber, the beam periodically disperses and converges, and is eventually gathered by the large core at the tail end through the step-index multimode fiber and coupled into the seventh quartz core of the receiving single-mode fiber, resulting in interference.

[0012] Advantages of this scheme: The small-diameter step-index multimode fiber, the wide-ring core fiber, and the ring core fiber are injected into the fourth quartz core of the ring core fiber using a simple face-to-face welding method, without the need for tapered or eccentric cascade structures, ensuring the structural strength of the sensor. By injecting light into the graded-index multimode fiber using the ring core fiber, interference can be excited between two modes with smaller group velocity dispersion differences without altering the structure of the graded-index multimode fiber, improving the sensor's sensitivity. The sensor is simple to fabricate and convenient for practical applications. By using the middle cladding and outer core structure of the wide-ring core fiber, the optical energy can be concentrated in the ring region of the fourth quartz core of the ring core fiber, allowing most of the beam to enter the fourth quartz core and propagate in different modes.

[0013] Further specifying, the light-emitting single-mode optical fiber includes a first quartz core and a first quartz cladding, wherein the first quartz cladding annularly covers the first quartz core.

[0014] Further specifying, one end of the single-mode fiber is connected to a broadband light source via a bare fiber adapter, and the other end of the single-mode fiber is coaxially connected to a small step-index multimode fiber.

[0015] Further defining the small-diameter step-index multimode fiber, the fiber comprises a second quartz core and a second quartz cladding, wherein the second quartz cladding annularly covers the two quartz cores.

[0016] Further specified, the refractive index of the third quartz fiber core is the same as that of the fourth annular quartz fiber core; the refractive index of the third quartz cladding, the refractive index of the fourth inner quartz cladding, and the refractive index of the fourth outer quartz cladding are the same.

[0017] Further specifying, the graded-index multimode fiber includes a fifth quartz core and a fifth quartz cladding, wherein the fifth quartz cladding annularly covers the fifth quartz core.

[0018] Further specifying, the light-gathering single-mode fiber, small-core-diameter step-index multimode fiber, wide-ring-core fiber, ring-core fiber, graded-index multimode fiber, large-core-diameter step-index multimode fiber, and light-receiving single-mode fiber are sequentially coaxially aligned and fused together.

[0019] Further defining, the large-core-diameter step-index multimode fiber includes a sixth quartz core and a sixth quartz cladding, wherein the sixth quartz cladding annularly covers the sixth quartz core.

[0020] Further defined, the receiving single-mode fiber includes a seventh quartz core and a seventh quartz cladding; the seventh quartz cladding annularly covers the seventh quartz core, and the end of the receiving single-mode fiber away from the large-core-diameter step-index multimode fiber is connected to a fiber optic spectrometer via a bare fiber adapter.

[0021] The beneficial effects of this utility model are as follows:

[0022] 1. This invention constructs an MZI by cascading a ring-core fiber with a graded-index multimode fiber 15. It selectively excites two modes in the graded-index multimode fiber where the group velocity dispersion difference approaches zero. Without changing the structure of the graded-index multimode fiber, it reduces the group velocity dispersion difference between the two modes in the graded-index multimode fiber, thereby achieving the purpose of further enhancing the sensitivity of the high-sensitivity graded-index fiber interferometric sensor.

[0023] 2. In this invention, small-diameter step-index multimode fiber, wide-ring core fiber, and ring core fiber can inject light into the fourth quartz core of the ring core fiber through ordinary face-to-face welding without the need for tapered or off-core cascade structures, thus ensuring the structural strength of the sensor.

[0024] 3. This invention can fundamentally increase the offset of the interference valley, reduce the difficulty of spectral demodulation, and integrate the sensing structure into a single optical fiber, making it convenient and practical.

[0025] 4. The sensor has high mechanical strength and good remanufacturability. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the cross-sectional structure of the sensor.

[0027] Figure 2 This is a schematic diagram of the beam transmission of the sensor.

[0028] Figure 3 This is a diagram of a sensor testing and calibration system.

[0029] Figure 4 Simulation results show the relationship between fiber mode order and group velocity dispersion for different radial refractive index distributions.

[0030] Figure 5 The image shows the results of the sensor strain sensing test.

[0031] Figure 6 This is a graph showing the test results of the sensor temperature sensing.

[0032] Figure 7 This is a linear fitting graph of the sensor strain and temperature sensing test results. Detailed Implementation

[0033] The following detailed description illustrates the specific implementation method:

[0034] The reference numerals in the accompanying drawings include: 11 for beam-emitting single-mode fiber, 111 for the first quartz fiber core, 112 for the first quartz cladding, 12 for the small-core-diameter step-index multimode fiber, 121 for the second quartz fiber core, 122 for the second quartz cladding, 13 for the wide-ring-core fiber, 131 for the third quartz fiber core, 132 for the third quartz cladding, 14 for the ring-core fiber, 141 for the fourth quartz fiber core, 142 for the fourth quartz inner cladding, 143 for the fourth quartz outer cladding, 15 for the graded-index multimode fiber, 151 for the fifth quartz fiber core, 152 for the fifth quartz cladding, 16 for the large-core-diameter step-index multimode fiber, 161 for the sixth quartz fiber core, 162 for the sixth quartz cladding, 17 for beam-receiving single-mode fiber, 171 for the seventh quartz fiber core, 172 for the seventh quartz cladding, 2 for the broadband light source, 3 for the strain-temperature measuring stage, and 4 for the spectrometer.

[0035] Example:

[0036] An MZ interferometer sensor constructed using modes with small excitation group velocity dispersion difference is shown in the attached figure. Figure 1 and attached Figure 2As shown, the structure specifically includes, in sequence, a single-mode fiber 11 for light injection, a small-core-diameter step-index multimode fiber 12, a wide-ring-core fiber 13, a ring-core fiber, a graded-index multimode fiber 15, a large-core-diameter step-index multimode fiber 16, and a single-mode fiber 17 for light collection. In this embodiment, the small-core-diameter step-index multimode fiber 12 has a length of 300 μm, the wide-ring-core fiber 13 has a length of 300 μm, the ring-core fiber 14 has a length of 3 mm, the graded-index multimode fiber 15 has a length of 15 cm, and the large-core-diameter step-index multimode fiber 16 has a length of 1 mm.

[0037] The basic principle of intermodal interference sensors is: when transmitting the same length When two modes with different propagation constants exist, a phase difference exists, resulting in interference. When external variables change, the propagation constants of the two modes change differently, the propagation constant difference changes, leading to a change in the phase difference and a shift in the interference valley. The sensor sensitivity formula can be expressed as:

[0038] (1)

[0039] in, It is an external variable. It is the wavelength of light. It is the difference in propagation constants between the two modes that are interfering. The group velocity dispersion difference between the two modes is represented by equation (1). The smaller the value, the greater the sensing sensitivity.

[0040] Radial refractive index distribution function of graded-index optical fiber It can be represented as:

[0041] (2)

[0042] in, The refractive index at the axis of the fiber core. The core radius is... Where is the radial distance, and g is the radial refractive index distribution index of the optical fiber. The relative refractive index difference, i.e. , denoted as cladding refractive index.

[0043] Only when the refractive index distribution of the optical fiber is a step or parabolic distribution can the exact solution of the propagation constant be obtained. For other refractive index distributions, only approximate analyses can be performed. For example, the characteristic equation of the propagation constant of the optical fiber can be derived using the WKBJ method (Jacobsen G, Ramskovhansen JJ. Propagation constants and group delays of guided modes in graded-index fibers: a comparison of three theories [J]. Applied Optics, 1979, 18(16): 2837-42.), which can be expressed as:

[0044] (3)

[0045] in, In graded refractive index optical fibers The propagation constant of the pattern, It is the angular module. It is the radial module. For wave number, and These are the two turning points of the light field.

[0046] From formula (3), the eigenvalues ​​of the propagation constant can be expressed as (Gu Wanyi. Optical Fiber Communication Systems. 3rd Edition [M]. Beijing University of Posts and Telecommunications Press, 2013):

[0047] (4)

[0048] in, Let the order of the degenerate pattern group be given for different species. Combined linear polarization modes, as long as If we set the value to a constant, we can obtain approximately the same propagation constant, which can be used uniformly. To express. The maximum mode group order that can be transmitted in an optical fiber can be expressed as:

[0049] (5)

[0050] in, Let be the normalized frequency of the optical fiber. This allows for simulation and derivation of the effect of mode order on group velocity dispersion in the optical fiber. Let be... μm, , , nm, as shown in formula (4), the maximum number of modes that the optical fiber can transmit varies with different values ​​of g. At that time, it should meet the following requirements. Simulation analysis was performed on formula (5) within the propagation range allowed by the model, and the results are shown in the appendix. Figure 4 As shown.

[0051] Because the graded-index optical fiber in actual production needs to meet the requirement of reducing the group delay difference between different modes in the field of optical fiber communication, the radial refractive index distribution index of the fiber is generally set to a number slightly less than 2. In this case, the group velocity dispersion of each mode in the graded-index fiber is a convex function of the mode order; that is, as the mode order increases, the group velocity dispersion first increases and then decreases, and the group velocity dispersion of several modes near the maximum value is approximately equal. The sensitivity of an interferometric sensor is related to the group velocity dispersion difference between two interfering modes; the smaller the group velocity dispersion difference, the higher the sensing sensitivity. Therefore, selecting two modes whose group velocity dispersion difference is close to zero near their maximum value for excitation can further enhance the sensitivity of the graded-index fiber interferometric sensor.

[0052] This scheme uses a ring-core fiber 14 to inject light into a graded-index multimode fiber 15, selectively exciting two modes with smaller group velocity dispersion differences to interfere, thereby improving the performance of the fiber optic interferometer sensor.

[0053] The single-mode fiber 11 includes a cylindrical first quartz fiber core 111 and a ring-shaped first quartz cladding 112 covering the first quartz fiber core 111; the left end of the single-mode fiber 11 is connected to the broadband light source 2 through a bare fiber adapter, and the right end of the single-mode fiber 11 is coaxially fused with the small-core-diameter step-index multimode fiber 12.

[0054] The small-core-diameter step-index multimode fiber 12 includes a cylindrical second quartz fiber core 121 and a ring-shaped second quartz cladding 122 covering the second quartz fiber core 121. The second quartz fiber core 121 has a diameter of 50 μm and a numerical aperture of 0.22. The left end of the small-core-diameter step-index multimode fiber 12 is coaxially and directly fused with the light-emitting single-mode fiber 11, and the right end of the small-core-diameter step-index multimode fiber 12 is coaxially and directly fused with the wide ring-core fiber 13.

[0055] The wide-ring core fiber 13 includes a cylindrical third quartz cladding 132 and a ring-shaped third quartz core 131 covering the third quartz cladding 132. The third quartz cladding 132 has a diameter of 12 μm and a refractive index of 1.455. The third quartz core 131 has an outer diameter of 125 μm and a refractive index of 1.464. The left end of the wide-ring core fiber 13 is coaxially fused with the small-core-diameter step-index multimode fiber 12, and the right end of the wide-ring core fiber 13 is coaxially fused with the ring core fiber 14.

[0056] The annular core fiber 14 includes a cylindrical fourth quartz inner cladding 142, an annular fourth quartz fiber core 141 annularly covering the fourth quartz inner cladding 142, and an annular fourth quartz outer cladding 143 annularly covering the fourth quartz fiber core 141; the fourth annular quartz fiber core has an inner diameter of 57 μm, an outer diameter of 75 μm, and a refractive index of 1.464; the refractive indices of the fourth quartz inner cladding 142 and the fourth quartz outer cladding 143 are 1.455, and the outer diameter of the fourth quartz outer cladding 143 is 125 μm; the left end of the annular core fiber 14 is coaxially fused with the wide annular core fiber 13, and the right end of the annular core fiber 14 is coaxially fused with the graded refractive index multimode fiber 15.

[0057] The graded-index multimode fiber 15 includes a cylindrical fifth quartz core 151 and a ring-shaped fifth quartz cladding 152 covering the fifth quartz core 151. The fifth quartz core 151 has a diameter of 105 μm, a numerical aperture of 0.3, a relative refractive index difference of 0.2, and a radial refractive index distribution index of 1.96. The left end of the graded-index multimode fiber 15 is coaxially fused with the ring-core fiber 14, and the right end of the graded-index multimode fiber 15 is coaxially fused with the large-core step multimode fiber 16.

[0058] The large-core-diameter step-index multimode fiber 16 includes a cylindrical sixth quartz core 161 and a ring-shaped sixth quartz cladding 162 covering the sixth quartz core 161. The sixth quartz core 161 has a diameter of 105 μm and a numerical aperture of 0.22. The left end of the large-core-diameter step-index multimode fiber 16 is coaxially and directly fused with the graded refractive index multimode fiber 15, and the right end of the large-core-diameter step-index multimode fiber 16 is coaxially and directly fused with the light-receiving single-mode fiber 17.

[0059] The receiving single-mode fiber 17 includes a cylindrical seventh quartz fiber core 171 and a ring-shaped seventh quartz cladding 172 covering the seventh quartz fiber core 171; the left end of the receiving single-mode fiber 17 is coaxially fused with a large core diameter step-index multimode fiber 16, and the right end of the receiving single-mode fiber 17 is connected to a fiber optic spectrometer through a bare fiber adapter.

[0060] The sensor proposed in this scheme uses a ring-core fiber 14 to excite mode A (P=26) and mode B (P=29) in a graded-index multimode fiber 15. At this time... Since the group velocity dispersion difference approaches zero, the interference between mode A and mode B will have extremely high sensing sensitivity.

[0061] Specific implementation process:

[0062] Assemble the sensor. The sensor's sensing fiber structure, from left to right, consists of a single-mode fiber 11, a small-core-diameter step-index multimode fiber 12, a wide-ring-core fiber 13, a ring-core fiber 14, a graded-index multimode fiber 15, a large-core-diameter step-index multimode fiber 16, and a single-mode fiber 17. After assembly, light is injected into the single-mode fiber 11 from the left end. The small-core-diameter step-index multimode fiber 12 expands the light in the single-mode fiber 11, transferring energy away from the fiber axis. The wide-ring-core fiber 13 has a "middle cladding, outer core" structure, which reduces the light energy at the fiber axis and allows the remaining light energy to be injected into the core 141 of the ring-core fiber 14. Light in the ring-core fiber 14 propagates in multiple modes. Different modes of light are injected into the graded-index multimode fiber 15 at different incident angles, thus allowing them to continue propagating in different modes within the graded-index multimode fiber 15. Because the differences between different incident angles are small, the different modes generated in the graded-index multimode fiber 15 are similar modes. Since the inner diameter of the ring-core fiber core 141 is 57 μm and the outer diameter is 75 μm, the radial distance from the incident position of the light injected into the graded-index multimode fiber 15 from the axis is 33 μm. Calculations show that the group velocity dispersion of the modes in the graded-index multimode fiber 15 is near its maximum. In summary, the group velocity dispersion difference between these modes approaches zero. The large core is then bundled by the step-index multimode fiber 16 and injected into the single-mode fiber 17, forming an MZ interferometer sensor.

[0063] This scheme selectively excites two modes with smaller group velocity dispersion differences in the fiber without changing the structure of the graded-index fiber, thereby further enhancing the sensitivity of the graded-index fiber interferometric sensor. Simultaneously, it gives the sensor high mechanical strength and good reproducibility. The small-diameter step-index multimode fiber 12, the wide-ring-core fiber 13, and the ring-core fiber 14 are injected into the fourth quartz core 141 of the ring-core fiber 14 through ordinary face-to-face welding, without the need for tapered or eccentric cascade structures, ensuring the structural strength of the sensor. The sensor is simple to manufacture and convenient for practical applications.

[0064] This solution also provides a method for fabricating an MZ interferometer sensor with a small excitation group velocity dispersion difference, as detailed below:

[0065] In this embodiment, the single-mode fiber 11 for light injection is a single-mode fiber (SMF-28e, Corning), the small-core-diameter step-index multimode fiber 12 is a small-core-diameter step-index multimode fiber (SI 50 / 125-22 / 250, YOFC), the wide-ring-core fiber 13 is a wide-ring-core fiber (12 / 125-250, XYAT), the ring-core fiber 14 is a ring-core fiber (57 / 75-250, XYAT), the graded-index multimode fiber 15 is a graded-index multimode fiber (GI 105 / 125-30 / 250, YOFC), the large-core-diameter step-index multimode fiber 16 is a large-core-diameter step-index multimode fiber (SI 105 / 125-22 / 250, YOFC), and the single-mode fiber 17 for light reception is a single-mode fiber (SMF-28e, Corning).

[0066] When fabricating the sensor, firstly, strip the coating layers from both ends of a single-mode fiber (SMF-28e, Corning) and a small-core-diameter step-index multimode fiber (SI 50 / 125-22 / 250, YOFC) with a core diameter of 50μm and cut the rear end face flat. Then, fusion splice the right end of the single-mode fiber and the left end of the small-core-diameter step-index multimode fiber together axially aligned. Using a fiber optic cleaver (FL-500, Fiberlink), cut the small-core-diameter step-index multimode fiber 300μm to the right of the splice point. Next, fusion splice a wide-ring-core fiber (12 / 125-250, XYAT) with the right end of the small-core-diameter step-index multimode fiber axially aligned, with a fixed-length cut of 300μm. Finally, fusion splice a ring-core fiber (57 / 75-250, XYAT) with the wide-ring-core fiber axially aligned, with a fixed-length cut of 3mm. Finally, fusion splice a graded-index multimode fiber (GI...) with the other end of the single-mode fiber... A section of SI 105 / 125-30 / 250 (YOFC) multimode fiber is fused to a ring-core fiber with a cut length of 15cm. A section of SI 105 / 125-22 / 250 (YOFC) multimode fiber with a core diameter of 105μm is fused to the right-end axis of a graded-index multimode fiber with a cut length of 1mm. Finally, a section of single-mode fiber is fused to the right end, and the sensor fiber fabrication is completed.

[0067] This solution also provides a test system for the MZ interferometer sensor constructed with a mode of small excitation group velocity dispersion difference, specifically including: a broadband light source 2 (HY-ASE-CL-17-NB-FP, Haoyuan Optoelectronics), a strain temperature measuring stage 3, and a spectrometer 4 (AQ6370D, Yokogawa).

[0068] The sensing fiber was placed and fixed on the clamps on both sides of the strain measurement stage. The isothermal stage was placed below the sensing area, with its surface close to the sensing fiber. A broadband light source (HY-ASE-CL-17-NB-FP, HOYATEK) was used to illuminate the sensor, and the transmission spectrum was received using a spectrometer (AQ6370D, YOKAGAWA), as shown in the attached figure. Figure 3 As shown in the figure. First, the constant temperature stage was set to 26℃ to eliminate the influence of ambient temperature changes on the test results, and strain sensing tests were performed. The test range was 0με to 100με, with intervals of 10με. The strain sensing transmission spectrum is shown in the attached figure. Figure 5 As shown. The strain gauge parameters were set to 10 με to eliminate the influence of sensor thermal expansion on the test results. Temperature sensing tests were conducted by adjusting the temperature of the constant temperature stage from 35℃ to 45℃ in 1℃ increments. The temperature transmission spectrum is shown in the attached figure. Figure 6 As shown in the attached figures. The strain sensitivity fitting curves and temperature sensitivity fitting curves are also shown in the attached figures. Figure 7 As shown. From the appendix Figure 7 As can be seen from this, the sensor has a strain sensitivity of 176.4 pm / με and a temperature sensitivity of 1174.6 pm / ℃.

[0069] Experimental results show that the MZ interferometer sensor constructed from a ring-core fiber cascaded graded-index fiber proposed in this invention selectively excites two modes with near-zero group velocity dispersion difference in the graded-index fiber. Without altering the structure of the graded-index fiber, it reduces the group velocity dispersion difference between the two modes, achieving further enhancement of the high-sensitivity graded-index fiber interferometer sensor. The sensor presented in this design exhibits a strain sensitivity of 176.4 pm / με and a temperature sensitivity of 1174.6 pm / ℃. It demonstrates strong robustness and sensing performance far exceeding other interferometer sensors based on graded-index fibers, surpassing even tapered and special fiber-based direct-transfer (DTP) interferometer sensors. This provides a new approach for enhancing the sensitivity of interferometer sensors, and the novel sensor has application value in the field of high-resolution strain and temperature measurement.

[0070] The above descriptions are merely embodiments of this utility model, and common technical solutions and / or characteristics known in the scheme are not described in detail here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the technical solution of this utility model. In this utility model, unless otherwise explicitly specified and limited, the terms "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this utility model according to the specific circumstances. The scope of protection claimed in this application shall be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. An MZ interferometer sensor constructed using modes with small excitation group velocity dispersion differences, characterized in that, include: The fiber comprises, in sequence, a single-mode fiber for light injection, a small-core-diameter step-index multimode fiber, a wide-ring-core fiber, a ring-core fiber, a graded-index multimode fiber, a large-core-diameter step-index multimode fiber, and a single-mode fiber for light collection. The wide-ring-core fiber includes a third quartz cladding and a third quartz core. The third quartz cladding is cylindrical, and the third quartz core annularly covers the third quartz cladding. The ring-core fiber includes a fourth quartz inner cladding, a fourth quartz core, and a fourth quartz outer cladding. The fourth quartz inner cladding is cylindrical. The fourth quartz core annularly covers the fourth quartz inner cladding. The fourth quartz outer cladding annularly covers the fourth quartz core. The radial refractive index distribution index of the graded-index multimode fiber is between 1.95 and 2.

2. The MZ interferometer sensor constructed with modes exhibiting small excitation group velocity dispersion difference according to claim 1, characterized in that: The single-mode fiber includes a first quartz core and a first quartz cladding, wherein the first quartz cladding annularly covers the first quartz core.

3. The MZ interferometer sensor constructed with modes exhibiting small excitation group velocity dispersion difference according to claim 1, characterized in that: One end of the single-mode fiber is connected to a broadband light source via a bare fiber adapter, and the other end of the single-mode fiber is coaxially connected to a small step-index multimode fiber.

4. The MZ interferometer sensor constructed with modes exhibiting small excitation group velocity dispersion difference according to claim 1, characterized in that: The small-diameter step-index multimode fiber includes a second quartz core and a second quartz cladding, with the second quartz cladding annularly covering the two quartz cores.

5. The MZ interferometer sensor constructed with modes exhibiting small excitation group velocity dispersion difference according to claim 1, characterized in that: The refractive index of the third quartz core is the same as that of the fourth annular quartz core; the refractive index of the third quartz cladding, the refractive index of the fourth inner quartz cladding, and the refractive index of the fourth outer quartz cladding are the same.

6. The MZ interferometer sensor constructed with modes exhibiting small excitation group velocity dispersion difference according to claim 1, characterized in that: The graded-index multimode fiber includes a fifth quartz core and a fifth quartz cladding, wherein the fifth quartz cladding annularly covers the fifth quartz core.

7. The MZ interferometer sensor constructed with modes exhibiting small excitation group velocity dispersion difference according to claim 1, characterized in that: The light-emitting single-mode fiber, small-core-diameter step-index multimode fiber, wide-ring-core fiber, ring-core fiber, graded-index multimode fiber, large-core-diameter step-index multimode fiber, and light-receiving single-mode fiber are sequentially coaxially aligned and fused together.

8. The MZ interferometer sensor constructed with modes exhibiting small excitation group velocity dispersion difference according to claim 1, characterized in that: The large-core-diameter step-index multimode fiber includes a sixth quartz core and a sixth quartz cladding, wherein the sixth quartz cladding annularly covers the sixth quartz core.

9. The MZ interferometer sensor constructed with modes exhibiting small excitation group velocity dispersion difference according to claim 1, characterized in that: The receiving single-mode fiber includes a seventh quartz core and a seventh quartz cladding; the seventh quartz cladding annularly covers the seventh quartz core, and the end of the receiving single-mode fiber away from the large-core-diameter step-index multimode fiber is connected to a fiber optic spectrometer via a bare fiber adapter.

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