M-Z interference sensor of seven-core optical fiber cascade gradient refractive index multimode optical fiber

Abstract

The utility model relates to the field of optical fiber interference sensors, and discloses an M-Z interference sensor of a seven-core optical fiber cascaded graded-index multimode optical fiber, which comprises a light injection single-mode optical fiber, a step multimode optical fiber, a seven-core optical fiber, a graded-index multimode optical fiber and a light receiving single-mode optical fiber which are connected in sequence, the axis of the joint of the seven-core optical fiber and the gradient refractive index multimode optical fiber deviates by a preset offset; the outer side of a fiber core of the seven-core optical fiber is coated with a stress layer, and the radius of the fiber core of the step multimode optical fiber is larger than the radius of the stress layer of a central fiber core of the seven-core optical fiber and smaller than the minimum distance from the central fiber core of the seven-core optical fiber to the stress layer of the fiber core on the outer side of the seven-core optical fiber. The length of the seven-core optical fiber is far smaller than that of the gradient multimode optical fiber; and the refractive index of the stress layer material outside the fiber core of the seven-core optical fiber is smaller than that of the cladding material of the seven-core optical fiber. According to the scheme, the problems that the interference valley is too wide and the extinction ratio is too low in a gradient refractive index multimode optical fiber interference sensor can be solved while the high-sensitivity sensing characteristic is maintained.

Description

TECHNICAL FIELD

[0001] The utility model relates to the field of optical fiber interference sensor, concretely relates to a kind of seven-core optical fiber cascade gradually varied refractive index multimode optical fiber's M-Z interference sensor. BACKGROUND

[0002] The M-Z interference sensor made of gradually varied refractive index multimode optical fiber has high sensitivity, but the gradually varied refractive index multimode optical fiber interferometer (i.e., the M-Z interference sensor made of gradually varied refractive index multimode optical fiber) has very wide interference valley and low extinction ratio. The bandwidth of coherent light source and spectrometer is limited. The width of interference valley of the gradually varied refractive index multimode optical fiber interferometer and the movement amount of interference valley during sensing must be within the bandwidth of light source and spectrometer, so that the interference valley can be observed and sensing can be performed. This results in great difficulty in signal demodulation in actual application.

[0003] In order to reduce the width of interference valley to within the bandwidth of light source and spectrometer, the length of sensing region optical fiber needs to be lengthened to meter level. The long sensing structure is easily affected by external environment, resulting in unstable factors. Therefore, it is a key problem for the development of gradually varied refractive index multimode optical fiber M-Z sensor to maintain high sensitivity of the gradually varied refractive index multimode optical fiber interference sensor while eliminating the problems of wide interference valley, long sensing region length and low extinction ratio.

[0004] The principle of optical fiber interference sensing is that two beams of light with the same frequency have different optical paths in the optical fiber, resulting in optical path difference when converging, and thus interference phenomenon occurs. When the external conditions change, the responses of the two beams of light to the external conditions are different, the optical path difference changes, and the interference valley shifts. The wavelength change of the interference valley can be used to sense the external conditions.

[0005] The principle of high sensitivity sensing of optical fiber interference sensor is as follows: first, the refractive index of the transmission path of the two interfering beams is modulated by the physical quantity to be measured (such as photoelastic effect, thermo-optic effect, etc.), and the difference in refractive index change is large, so the change amount of effective refractive index difference is large, the change amount of optical path difference is large, the movement amount of interference valley is large, and the sensitivity is high. Second, the group velocity difference of the two interfering beams is small, which will result in larger displacement of the interference valley under the influence of the same external change. The gradually varied refractive index multimode optical fiber is an optical fiber that reduces the group velocity difference of each mode, so the interference sensor made of the gradually varied refractive index multimode optical fiber has high sensitivity.

[0006] The width of the interference valley of the fiber optic interferometric sensor is related to the optical path difference of the two light modes that interfere, and the principle is that the larger the optical path difference, the narrower the interference valley. In the graded-index multimode fiber, the transmission constants of each mode are approximately equal, it is difficult to produce a large optical path difference, resulting in a too wide interference valley. In order to effectively observe the interference valley within the spectral width of the light source in order to carry out sensing, it is necessary to produce a larger optical path difference and thus reduce the width of the interference valley. Usually, the length of the graded-index multimode fiber is increased, and the sensing structure becomes long.

[0007] The extinction ratio of the fiber optic interferometric sensor is related to the relative size of the energy of the two light modes that interfere, and the principle is that the closer the energy size, the higher the extinction ratio. When a single-mode fiber is used to inject light into a graded-index multimode fiber, the light energy at the wavelength in the interference band is mainly in the fundamental mode, and the energy of other modes is too low, and the energy difference between the two interference modes is too large, so there is a problem of low extinction ratio.

[0008] Therefore, the key problem faced by the development of the graded-index multimode fiber interferometric sensor is: how to maintain high sensitivity of the interference sensing characteristics while increasing the optical path difference between the two interference modes to solve the problem of wide interference valley and the need for long sensing fiber; and to average the energy of the two modes to increase the extinction ratio. Zhang et al. processed a hollow cavity under the graded-index multimode fiber, which increased the difference in refractive index between the upper and lower sides of the fiber. The light on the upper and lower sides travels through different media, resulting in a large optical path difference and reducing the width of the interference valley. The length of the sensing fiber is in the order of microns. Because the energy of the light passing through the hollow cavity is roughly equivalent to the energy of the light not passing through, the energy of the two interference modes is approximately evenly distributed, and the extinction ratio is high. However, the hollow structure destroys the characteristic that the group velocities of the light beams on the upper and lower sides of the graded-index multimode fiber are close, and by injecting a solution sensitive to temperature changes in the hollow cavity, a relatively high temperature sensing sensitivity can be maintained. Felipe et al. tapered the graded-index multimode fiber, which caused the evanescent field to leak. The two light modes that interfere have different degrees of evanescent field leakage, the difference in transmission constants increases, the optical path difference increases, and the width of the interference valley decreases, but the length of the sensor is still in the order of meters. Because the taper changes the radial refractive index distribution of the fiber, the proportion of the energy of the fundamental mode is reduced, and the extinction ratio is improved. However, the taper structure destroys the refractive index distribution of the graded-index multimode fiber, and the sensitivity is relatively low.

[0009] Although the above two schemes solve the problem of increasing the optical path difference and evenly distributing the interference mode energy, and achieve the purpose of reducing the interference valley width and increasing the extinction ratio, they produce new problems. The method of processing a hole in the optical fiber must use a high-cost femtosecond laser processing device, and a solution is injected into the small air cavity, and the processing conditions are harsh; the processing method of fiber taper needs to reduce the fiber diameter to below 20μm to effectively increase the optical path difference, and the sensor has poor robustness. Moreover, both of them destroy the refractive index distribution of the graded-index multimode fiber, and if a sensitization means is not used, the sensitivity will be greatly reduced. Therefore, a new way should be developed to overcome the defects of the graded-index multimode fiber interferometer (i.e. the M-Z interference sensor made of graded-index multimode fiber) and avoid the above problems. Content of the utility model

[0010] The utility model intends to provide a kind of seven-core fiber cascaded graded-index multimode fiber's M-Z interference sensor, to solve the problem of interference valley being too wide and extinction ratio being too low in graded-index multimode fiber interference sensor while maintaining high sensitivity sensing characteristics.

[0011] To achieve the above object, the utility model adopts the following technical scheme: a kind of seven-core fiber cascaded graded-index multimode fiber's M-Z interference sensor, comprising: sequentially connected light injection single-mode fiber, step multimode fiber, seven-core fiber, graded-index multimode fiber and light receiving single-mode fiber;The axis of the seven-core fiber and the graded-index multimode fiber connection place is offset by a preset offset amount;The seven-core fiber includes seven identical cores, and one of the cores is located at the center of the seven-core fiber, and the remaining six cores are symmetrically arranged outside the central core;The stress layer is coated on the outside of the core of the seven-core fiber;The core radius of the step multimode fiber is greater than the stress layer radius of the central core of the seven-core fiber, and at the same time, it is less than the minimum distance from the central core of the seven-core fiber to the stress layer of the outer core;The length of the seven-core fiber is much smaller than the length of the graded multimode fiber;The refractive index of the stress layer material outside each core of the seven-core fiber is less than the refractive index of the cladding material of the seven-core fiber.

[0012] The principle of the scheme is as follows: the left end entrance of the single-mode optical fiber of the sensor with an angular offset of 0° is put into a bare fiber adapter, then a broadband light source is connected, the seven-core optical fiber has a central symmetric structure and contains seven identical optical fibers, the stress layer with a lower refractive index than the cladding material is coated outside the fiber core, so that the energy in the cladding is divided into six parts and is concentrated in the transmission of the partial area surrounded by every three fiber cores (the central fiber core and any two adjacent outer fiber cores), when the seven-core optical fiber is injected with light, the light will not enter the six outer fiber cores, but only enter the central fiber core and the cladding outside the central fiber core, and when the light is transmitted to the graded-index multimode optical fiber, different light modes are excited, received by the receiving single-mode optical fiber, and sent for interference, after the light is transmitted in the seven-core optical fiber, a large optical path difference is generated, which can reduce the width of the interference valley, due to the fact that the capacity of each light in the fiber core and the cladding of the seven-core optical fiber is close to each other, the energy of the two interference modes is evenly distributed, and due to the offset cascade mechanism formed by the seven-core optical fiber and the graded-index multimode optical fiber, the two interference modes are close to each other, the capacity received by the receiving single-mode optical fiber is close to each other, the extinction ratio can be increased, the length of the seven-core optical fiber and the length of the graded-index multimode optical fiber are quite different, and the group velocity of the two light modes can be obtained by weighting average according to the length of the optical fiber, due to the fact that the graded-index multimode optical fiber is not processed and the refractive index is not damaged, the group velocity of each propagation mode in the graded-index multimode optical fiber is still close to each other, and the short-distance seven-core optical fiber does not affect the overall group velocity, so that the sensor maintains high sensitivity characteristics, and the offset cascade mechanism can excite modes close to each other in the graded-index multimode optical fiber, further reducing the group velocity difference and improving the sensitivity of the sensor.

[0013] The advantages of the scheme are as follows: after the light is transmitted in the fiber core and the cladding of the seven-core optical fiber, a large optical path difference is generated, which can reduce the width of the interference valley; the capacity of each light in the fiber core and the cladding of the seven-core optical fiber is close to each other, so that the two interference modes are evenly distributed, and the offset cascade mechanism formed by the seven-core optical fiber and the graded-index multimode optical fiber makes the two interference modes close to each other, the capacity received by the receiving single-mode optical fiber is close to each other, and the extinction ratio can be increased; the length of the seven-core optical fiber and the length of the graded-index multimode optical fiber are quite different, the refractive index distribution of the graded-index multimode optical fiber is not changed, so that the group velocity of each propagation mode in the graded-index multimode optical fiber is still close to each other, the short-distance seven-core optical fiber does not affect the overall group velocity, and the sensor maintains high sensitivity characteristics, and the offset cascade mechanism can excite modes close to each other in the graded-index multimode optical fiber, further reducing the group velocity difference and improving the sensitivity of the sensor.

[0014] Further, the injection light single-mode fiber includes a first quartz core, a first quartz cladding, and a first coating layer; the first quartz cladding annularly covers the first quartz core; the first coating layer annularly covers the first quartz cladding, and the first coating layer is located outside a preset surface area of the first quartz cladding away from the step multimode fiber end.

[0015] Further, the step multimode fiber includes a second quartz core and a second quartz cladding; the second quartz cladding annularly covers the second quartz core; and the step multimode fiber has a central symmetry structure.

[0016] Further, the second quartz cladding has a diameter greater than 2 times the diameter of the second quartz core.

[0017] Further, the seven-core fiber includes a third quartz core, a fourth quartz core, a fifth quartz core, a sixth quartz core, a seventh quartz core, an eighth quartz core, a ninth quartz core, a first quartz stress layer, a second quartz stress layer, a third quartz stress layer, a fourth quartz stress layer, a fifth quartz stress layer, a sixth quartz stress layer, a seventh quartz stress layer, and a third quartz cladding; the fourth quartz core, the fifth quartz core, the sixth quartz core, the seventh quartz core, the eighth quartz core, and the ninth quartz core are annularly arranged around the third quartz core; the first quartz stress layer, the second quartz stress layer, the third quartz stress layer, the fourth quartz stress layer, the fifth quartz stress layer, the sixth quartz stress layer, and the seventh quartz stress layer annularly cover the third quartz core, the fourth quartz core, the fifth quartz core, the sixth quartz core, the seventh quartz core, the eighth quartz core, and the ninth quartz core, respectively; and the third quartz cladding annularly covers the outer surface of the first quartz stress layer, the second quartz stress layer, the third quartz stress layer, the fourth quartz stress layer, the fifth quartz stress layer, the sixth quartz stress layer, and the seventh quartz stress layer.

[0018] Further, the angular offset amount of the seven-core fiber when connected with the graded-index multimode fiber includes 0° or 30°.

[0019] Further, the graded-index multimode fiber includes a tenth quartz core and a fourth quartz cladding; the fourth quartz cladding annularly covers the tenth quartz core; the graded-index multimode fiber has a central symmetry structure; and a radial refractive index function of the graded-index multimode fiber has a square law distribution.

[0020] Further, the light collection single-mode fiber includes an eleventh quartz core, a fifth quartz cladding, and a second coating layer; the fifth quartz cladding annularly covers the eleventh quartz core; the second coating layer annularly covers the fifth quartz cladding, and the second coating layer is located outside a preset surface area of the fifth quartz cladding away from the graded-index multimode fiber end.

[0021] Further limited, the first quartz core of the injection light single-mode optical fiber is axially aligned with the second quartz core of the step multimode optical fiber when connected, the second quartz core of the step multimode optical fiber is axially aligned with the third quartz core of the seven-core optical fiber when connected; the axis of the third quartz core of the seven-core optical fiber is axially offset from the tenth quartz core of the graded-index multimode optical fiber by a preset offset amount when connected; the tenth quartz core of the graded-index multimode optical fiber is axially aligned with the eleventh quartz core of the light receiving single-mode optical fiber when connected.

[0022] The utility model has the advantages that:

[0023] 1. The utility model provides a kind of M-Z interference sensor of seven-core optical fiber offset cascade graded-index multimode optical fiber structure, and the characteristic that the group velocity difference between different modes is small is not influenced by short-distance seven-core optical fiber, so that sensor can be high sensitivity Sensing strain and temperature.

[0024] 2. There is a large refractive index difference between the cladding and core material of seven-core optical fiber, which can increase the optical path difference, reduce the width of interference valley, reduce the length of required sensing area, and make the sensor have good stability.

[0025] 3. Offset cascade seven-core optical fiber can make the energy of two beams of light that interfere close, increase the depth of interference valley, and increase interference extinction ratio.

[0026] 4. Changing the angular bias of seven-core optical fiber can adjust the sensitivity of sensor, even change the moving direction of interference valley, provide more selectivity for sensor design, and add potential practical scenarios of sensor.

[0027] 5. Sensing optical fiber is not treated by microprocessing means, and the connection method is simple, the structural strength is high, and the sensor has practical value. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 It is a structural schematic diagram of sensor.

[0029] Figure 2 It is a sensor optical fiber cross-sectional view.

[0030] Figure 3 It is a seven-core optical fiber angular bias schematic diagram.

[0031] Figure 4 It is a sensor test system diagram.

[0032] Figure 5 -(a) is a sensor strain sensing test result diagram when angular bias is 0°.

[0033] Figure 5 -(b) is a sensor temperature sensing test result diagram when angular bias is 0°.

[0034] Figure 6 -(a) is the sensor strain sensing test result diagram when the angular offset is 30°.

[0035] Figure 6 -(b) is the sensor temperature sensing test result diagram when the angular offset is 30°. DETAILED DESCRIPTION

[0036] Further details are described below through specific embodiments:

[0037] The reference signs in the attached drawings of the specification include: injection light single-mode optical fiber 1, first quartz core 11, first quartz cladding 12 and first coating layer 13, step multimode optical fiber 2, second quartz core 21, second quartz cladding 22, seven-core optical fiber 3, third quartz core 31, fourth quartz core 32, fifth quartz core 33, sixth quartz core 34, seventh quartz core 35, eighth quartz core 36, ninth quartz core 37, first quartz stress layer 38, second quartz stress layer 39, third quartz stress layer 310, fourth quartz stress layer 311, fifth quartz stress layer 312, sixth quartz stress layer 313, seventh quartz stress layer 314, third quartz cladding 315, graded-index multimode optical fiber 4, tenth quartz core 41, fourth quartz cladding 42, light receiving single-mode optical fiber 5, eleventh quartz core 51, fifth quartz cladding 52, second coating layer 53, broadband light source 6, strain measurement platform 7, temperature measurement platform 8, optical spectrum analyzer 9.

[0038] Embodiment:

[0039] A seven-core optical fiber 3 offset cascade graded-index multimode optical fiber 4 configuration M-Z interference sensor, as shown in the attached Figure 1 and attached Figure 2 , specifically includes injection light single-mode optical fiber 1, step multimode optical fiber 2, seven-core optical fiber 3, graded-index multimode optical fiber 4, light receiving single-mode optical fiber 5; the injection light single-mode optical fiber 1, step multimode optical fiber 2, seven-core optical fiber 3, graded-index multimode optical fiber 4 and light receiving single-mode optical fiber 5 are connected in order from left to right, the first quartz core 11 of the injection light single-mode optical fiber 1 and the second quartz core 21 of the step multimode optical fiber 2 are aligned in axis when connected, the second quartz core 21 of the step multimode optical fiber 2 and the third quartz core 31 of the seven-core optical fiber 3 are aligned in axis when connected, the axis of the third quartz core 31 of the seven-core optical fiber 3 and the tenth quartz core 41 of the graded-index multimode optical fiber 4 exist an offset of 12.5 microns in axis when connected, because there is an offset of 12.5 microns between the axis of the seven-core optical fiber 3 and the graded-index multimode optical fiber 4, therefore multiple injection light beams will excite multiple transmission modes in the graded-index multimode optical fiber 4. Changing the injection position of the point light source can adjust the modes excited in the graded-index multimode optical fiber 4.

[0040] The tenth silica core 41 of the graded-index multimode optical fiber 4 is axially aligned with the eleventh silica core 51 of the light-receiving single-mode optical fiber 5 when connected.

[0041] The light-injecting single-mode optical fiber 1 comprises a first silica core 11, a first silica cladding 12 annularly covering the first silica core 11, and a first coating layer 13 annularly covering the first silica cladding 12, the first coating layer 13 being located outside a predetermined surface area on the left end of the first silica cladding 12.

[0042] The step multimode optical fiber 2 comprises a second silica core 21 and a second silica cladding 22 annularly covering the second silica core 21; the step multimode optical fiber 2 has a central symmetry structure and a numerical aperture of 0.22; the diameter of the second silica core 21 is 60 μm, and the diameter of the second silica cladding 22 is 125 μm.

[0043] The seven-core optical fiber 3 has a central symmetry structure and comprises seven identical cores, specifically including a third silica core 31, a fourth silica core 32, a fifth silica core 33, a sixth silica core 34, a seventh silica core 35, an eighth silica core 36, and a ninth silica core 37, the diameters of the seven silica cores being all 8 μm; the fourth silica core 32, the fifth silica core 33, the sixth silica core 34, the seventh silica core 35, the eighth silica core 36, and the ninth silica core 37 are annularly arranged around the third silica core 31, wherein the third core is located at the axis of the seven-core optical fiber 3, and the other six cores are annularly arranged outside the central core; the distance between the central core and the outer core is 42 μm, and the distance between two adjacent outer cores is also 42 μm.

[0044] Each core of the seven-core optical fiber 3 is annularly covered by a stress layer with a lower refractive index than the cladding material, specifically including a first silica stress layer 38, a second silica stress layer 39, a third silica stress layer 310, a fourth silica stress layer 311, a fifth silica stress layer 312, a sixth silica stress layer 313, and a seventh silica stress layer 314, which respectively cover the third silica core 31, the fourth silica core 32, the fifth silica core 33, the sixth silica core 34, the seventh silica core 35, the eighth silica core 36, and the ninth silica core 37.

[0045] The seven-core fiber 3 further comprises a third quartz cladding 315 with a diameter of 150 μm; the third quartz cladding 315 is coated on the outer surface of the first quartz stress layer 38, the second quartz stress layer 39, the third quartz stress layer 310, the fourth quartz stress layer 311, the fifth quartz stress layer 312, the sixth quartz stress layer 313 and the seventh quartz stress layer 314, as shown in the attached Figure 3

[0046] The length of the seven-core fiber 3 is 5 mm, and the length of the graded-index multimode fiber 4 is 20 cm. The diameter of each stress layer of the seven-core fiber 3 is 27 μm. Because the radius of the stress layer is large and the refractive index is low, the light energy in the cladding is divided into six parts, which is transmitted in the partial cladding area surrounded by every three cores (the central core and any two adjacent outer cores); these different parts of cladding light energy will be regarded as point light sources for injecting light into the right graded-index multimode fiber 4.

[0047] The radius (30 μm) of the step multimode fiber 2 is smaller than the minimum distance (42 μm) from the axis of the seven-core fiber 3 to the edge of the outer core stress layer, and larger than the radius (13.5 μm) of the first quartz stress layer 38 of the central core, so that when light is injected into the seven-core fiber 3, it will not enter the six outer cores, but only enter the third quartz core 31 and the third quartz cladding 315, and when transmitted to the graded-index multimode fiber 4, different light modes are excited, which are received by the receiving single-mode fiber 5, and interference occurs. Because the length of the seven-core fiber 33 is 5 mm, a large optical path difference is generated after the light is transmitted in the cladding and core of the seven-core fiber 33, so that the width of the interference valley can be reduced.

[0048] The energy of each beam of light in the cladding and core of the seven-core fiber 3 is close, so that the energy of the two interference modes is evenly distributed; at the same time, due to the offset cascade structure, the two interference modes are close, and the energy received by the receiving single-mode fiber 5 is close, so that the extinction ratio can be increased.

[0049] The angular offset amount when the seven-core fiber 3 is connected with the graded-index multimode fiber 4 can be selected as 0° or 30°.

[0050] ​Due to the offset cascade structure, adjusting the angular offset of the seven-core fiber 3 will cause the initial radial offset of the light injected from the seven-core fiber 3 into the graded-index multimode fiber 4 to change, and the mode of the light transmitted in the graded-index multimode fiber 4 will change. Due to the different doping materials of the seven-core fiber 3 and the graded-index multimode fiber 4 used in the new sensor, the strain sensing characteristics are opposite. When the sensor is modulated by strain, if the change in the optical path difference of the two beams of light in the graded-index multimode fiber 4 is larger, the strain sensing characteristic is red shift of the interference valley; otherwise, the strain sensing characteristic is blue shift of the interference valley. Therefore, adjusting the angular offset of the seven-core fiber 3 can change the direction of the strain sensing interference valley displacement.

[0051] The graded-index multimode fiber 4 includes a tenth quartz core 41 and a fourth quartz cladding 42, the fourth quartz cladding 42 annularly covers the tenth quartz core 41; the graded-index multimode fiber 4 is a central symmetric structure, the numerical aperture is 0.30, and the radial refractive index function is square law distribution; the diameter of the tenth quartz core 41 is 105 μm, and the diameter of the fourth quartz cladding 42 is 125 μm.

[0052] The length of the seven-core fiber 3 and the length of the graded-index multimode fiber 4 are quite different. The group velocity of the two light modes can be obtained by weighted average of the group velocity in each fiber according to the length ratio of the fiber. Since the graded-index multimode fiber 4 is not processed and its refractive index distribution is not destroyed, the group velocity of each propagation mode in the graded-index multimode fiber 4 can still be close, and the short seven-core fiber 3 will not affect the overall group velocity, so that the sensor maintains high sensitivity characteristics; at the same time, the offset cascade structure formed by the seven-core fiber 3 and the graded-index multimode fiber 4 can excite close modes of the two beams of light in the graded-index multimode fiber 4, further reducing the group velocity difference, so the sensitivity is high.

[0053] The light receiving single-mode fiber 5 includes an eleventh quartz core 51, a fifth quartz cladding 52, and a second coating layer 53, the fifth quartz cladding 52 annularly covers the eleventh quartz core 51, and the second coating layer 53 annularly covers the fifth quartz cladding 52, and the second coating layer 53 is located outside the preset surface area of the right end of the fifth quartz cladding 52.

[0054] Specific implementation process:

[0055] Because the seven-core fiber 3 is a central symmetric structure, it contains seven identical cores, and the outside of the cores is coated with a stress layer with a lower refractive index than the cladding material, and the diameter of the stress layer is 27 μm, so that the optical energy in the cladding is divided into six parts, which is concentrated in the partial cladding area surrounded by every three cores (the central core and any two adjacent outer cores). Each part of the optical energy can be regarded as a point light source similar to the light beam in the middle third quartz core 31, which injects light into the right side graded-index multimode fiber 4.

[0056] The radius of the step multimode fiber 2 (30 μm) is smaller than the minimum distance from the axis of the seven-core fiber 3 to the edge of the outer core stress layer (42 μm), and larger than the radius of the first quartz stress layer 38 of the central core (13.5 μm), so that when the seven-core fiber 3 is injected with light, the light will not enter the outer six cores, but only enter the middle third quartz core 31 and the third quartz cladding 315, and when it is transmitted to the graded-index multimode fiber 4, it will excite different optical modes, which are received by the light receiving single-mode fiber 5 and interfere.

[0057] Because the length of the seven-core fiber 3 is 5 mm, the optical path difference is large after the light is transmitted in the cladding and core of the seven-core fiber 3, so that the width of the interference valley can be reduced.

[0058] Because the energy of each light beam in the cladding and core of the seven-core fiber 3 is close in size, the energy of the two interference modes is evenly distributed; at the same time, due to the offset cascade structure, the two interference modes are close, and the energy received by the light receiving single-mode fiber 5 is close, so that the extinction ratio can be increased.

[0059] Because the length of the seven-core fiber 3 is 5 mm, and the length of the graded-index multimode fiber 4 is 20 cm, the difference between the two is large. The group velocity of the two optical modes can be obtained by weighting the group velocity in each fiber according to the size of the fiber length. Because the graded-index multimode fiber 4 is not processed, the refractive index distribution is not destroyed, so that the group velocity of each propagation mode in the graded-index multimode fiber 4 can still be close in size, and the short distance of the seven-core fiber 3 will not affect the overall group velocity, so that the sensor maintains high sensitivity characteristics; at the same time, due to the offset structure, the two light beams can excite modes close in the graded-index multimode fiber 4, further reducing the group velocity difference, so that the sensitivity is high.

[0060] Because there is an offset of 12.5 μm between the axes of the seven-core fiber 3 and the graded-index multimode fiber 4, multiple injection light beams will excite multiple transmission modes in the graded-index multimode fiber 4. By changing the injection position of the point light source, the modes excited in the graded-index multimode fiber 4 can be adjusted.

[0061] Due to the offset cascade structure, adjusting the angular offset of the seven-core optical fiber 3 will cause the initial radial offset of the light injected from the seven-core optical fiber 3 into the graded-index multimode optical fiber 4 to change, and the mode of the light transmitted in the graded-index multimode optical fiber 4 to change. Due to the different doping materials of the seven-core optical fiber 3 and the graded-index multimode optical fiber 4 used in the new sensor due to the need of the processing technology, the strain sensing characteristics are opposite. When the sensor is modulated by strain, if the change amount of the optical path difference of the two beams of light in the graded-index multimode optical fiber 4 is larger, the strain sensing characteristic is red shift of the interference valley; otherwise, the interference valley is blue shift. Therefore, adjusting the angular offset of the seven-core optical fiber 3 can change the displacement direction of the interference valley of the strain sensing.

[0062] The utility model discloses the beneficial effect lies in:

[0063] 1. The utility model provides a kind of M-Z interference sensor of seven-core optical fiber 3 offset cascade graded-index multimode optical fiber 4 structure, and the characteristic that the group velocity difference of different modes is small is not influenced by short-distance seven-core optical fiber 3, so that sensor can be sensed strain and temperature with high sensitivity.

[0064] 2. There is larger refractive index difference between the cladding and core material of seven-core optical fiber 3, can increase optical path difference, reduce the width of interference valley, reduce the length of required sensing area, so that sensor has good stability.

[0065] 3. Offset cascade seven-core optical fiber 3 can make the energy size of two beams of light that interfere close, increase the depth of interference valley, increase interference extinction ratio.

[0066] 4. The angular offset of seven-core optical fiber 3 can be changed to adjust the sensitivity of sensor, even change the moving direction of interference valley, provide more selectivity for the design of sensor, add potential practical scene of sensor.

[0067] 5. Sensing optical fiber is not treated by microprocessing means, and the connection mode is simple, and the structural strength is high, and the sensor has practical value.

[0068] The present scheme also provides a manufacturing method of the above-mentioned M-Z interference sensor of seven-core optical fiber and graded-index multimode optical fiber cascade, which is specifically shown as follows:

[0069] A section of single-mode optical fiber (SMF-28e, Corning) and a step multimode optical fiber 2 (SI 60 / 125-22 / 250, Changfei) with a core diameter of 60 μm are cut flat at the end face after stripping the coating layer at both ends, and the single-mode optical fiber right end and the step multimode optical fiber 2 left end are fused together with the axis center opposite, and the step multimode optical fiber 2 is cut off at 1 mm right of the fusion point using a fiber fixed-length cutting device;

[0070] The right end of the step multimode fiber 2 is fused with a seven-core fiber 3 (MCF 7-42 / 150 / 250, Changfei), and the seven-core fiber 3 is cut to a length of 5 mm by using a fiber stress cutter (FL-500, Nanjing Guangbei);

[0071] The end face of the cut seven-core fiber 3 is cleaned and placed in a rotating clamp of a polarization maintaining fusion splicer (FL-4000, Nanjing Guangbei), and the end face of the seven-core fiber 3 is observed in the display screen of the polarization maintaining fusion splicer, and the rotating clamp is adjusted to a specified angular offset, as shown in Figure 3 The seven-core fiber 3 is rotated to an angular offset of 0°, fixed on the left motor of the polarization maintaining fusion splicer, and a segment of graded-index multimode fiber 4 (GI 105 / 125-30 / 250, Changfei) is fixed on the right motor, and the right motor is controlled to offset 12.5 μm in the y direction, so that the lower edges of the two fibers are just flat, and the two fibers are fused. The right end of the graded-index multimode fiber 4 is cut by using a fiber cutter to a length of 20 cm, and the right end of the graded-index multimode fiber 4 is fused with the left end of another single-mode fiber.

[0072] The sensor with an angular offset of 0° is completed, the angular offset is changed to 30°, and the other steps remain unchanged to manufacture a second sensor.

[0073] The scheme also provides a test system of the M-Z interference sensor of the seven-core fiber and the graded-index multimode fiber, which specifically comprises: a broadband light source 6 (HY-ASE-CL-17-N-B-FP, Haoyuan Optoelectronics), a strain measurement table 7, a temperature measurement table 8, and a spectrum analyzer 9 (AQ6370D, Yokogawa).

[0074] The left end port of the light injection single-mode fiber 1 of the sensor with an angular offset of 0° is placed in a bare fiber adapter and connected to the broadband light source 6, the sensing area is placed on the strain measurement table 7 and fixed, the right end port of the light receiving single-mode fiber 5 of the sensor is placed in a bare fiber adapter and connected to the spectrum analyzer 9, the broadband light source 6 is turned on to pass light to the sensor, and the output spectrum is collected. The parameters of the strain measurement table 7 are increased by 100 με, and the output spectrum is collected again. The above steps are repeated until six groups of strain sensing data are recorded. The parameters of the strain measurement table 7 are adjusted to the initial state, the parameters of the temperature measurement table 8 are adjusted to 30℃, and the output spectrum is collected. The parameters of the temperature measurement table 8 are increased by 5℃, and the output spectrum is collected again. The above steps are repeated until six groups of temperature sensing data are recorded, and the test results of the sensor with an angular offset of 0° are plotted as shown in Figure 5 The sensor with an angular offset of 0° is replaced with a sensor with an angular offset of 30°, and the above steps are repeated to obtain six groups of strain sensing data and six groups of temperature sensing data again, and the test results of the sensor with an angular offset of 30° are plotted as shown inFigure 6 As shown.

[0075] The experimental results show that the M-Z interference sensor with the seven-core fiber 3 cascaded gradient refractive index multimode fiber 4 structure can maintain the high sensitivity characteristics of the gradient refractive index multimode fiber interference sensor, and at the same time, the defects of the low interference valley width and the low extinction ratio are eliminated. The larger the optical path difference generated in the seven-core fiber 3 cladding and the core of the two interference modes, the lower the interference valley width, so the cascaded seven-core fiber 3 can effectively shorten the required gradient fiber length; the cascaded seven-core fiber 3 can effectively average the energy of the two modes and increase the extinction ratio; the length of the gradient sensing fiber has little effect on the interference valley width, so the length of the gradient fiber can be reduced to the decimeter level, and a compact sensor can be made; the length of the gradient sensing fiber is large enough, and the group velocity difference of the two modes is still small, so the high sensitivity characteristics of the sensor can be maintained. Adjusting the angular offset of the seven-core fiber 3 can fine-tune the injection position of the cladding light beam while keeping the center core light beam injection position unchanged, change the moving direction of the interference valley during strain sensing, and selectively adjust the sensitivity.

[0076] The above is only an embodiment of the present application, and the specific technical solutions and / or common knowledge of the scheme are not described in detail. It should be noted that for those skilled in the art, without departing from the technical scheme of the present application, a number of modifications and improvements can be made, and in the present application, unless otherwise specified and limited, the terms "installation", "connection", "connection", "fixing" and other terms should be understood in a broad sense, for example, it can be fixedly connected, or it can be detachably connected, or integrally connected; it can be directly connected, or indirectly connected through an intermediate medium, or the communication between two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances. The protection scope claimed in the present application should be subject to the content of its claims, and the specific implementation mode and the like recorded in the specification can be used to explain the content of the claims.

Claims

1. A M-Z interferometric sensor of a seven-core fiber concatenated graded-index multimode fiber, characterized in that, Comprise: sequentially connected light injection single-mode optical fiber, step multimode optical fiber, seven-core optical fiber, graded refractive index multimode optical fiber and light receiving single-mode optical fiber; The seven-core optical fiber is connected with the graded refractive index multimode optical fiber at the axial center offset by a preset offset amount; the seven-core optical fiber comprises seven identical cores, and one of the cores is located at the center of the seven-core optical fiber, and the remaining six cores are symmetrically arranged outside the central core; the outside of the core of the seven-core optical fiber is covered with a stress layer; The core radius of the step multimode optical fiber is greater than the stress layer radius of the central core of the seven-core optical fiber, and is less than the minimum distance from the central core to the stress layer of the outer core of the seven-core optical fiber; the length of the seven-core optical fiber is much smaller than the length of the graded multimode optical fiber; the refractive index of the stress layer material outside each core of the seven-core optical fiber is less than the refractive index of the cladding material of the seven-core optical fiber.

2. A M-Z interferometric sensor of a seven-core fiber cascaded graded-index multimode fiber according to claim 1, characterized in that: The light injection single-mode optical fiber comprises a first quartz core, a first quartz cladding and a first coating layer; the first quartz cladding annularly covers the first quartz core, and the first coating layer annularly covers the first quartz cladding; the first coating layer is located outside a preset surface area of the first quartz cladding away from the step multimode optical fiber.

3. The M-Z interferometric sensor of a seven-core fiber cascaded graded-index multimode fiber according to claim 1, characterized in that: The step multimode optical fiber comprises a second quartz core and a second quartz cladding; the second quartz cladding annularly covers the second quartz core; the step multimode optical fiber has a central symmetry structure.

4. A M-Z interferometric sensor of a seven-core fiber cascaded to a graded-index multimode fiber according to claim 3, characterized in that: The diameter of the second quartz cladding is greater than 2 times the diameter of the second quartz core.

5. The M-Z interferometric sensor of a seven-core fiber tapered graded-index multimode fiber according to claim 1, characterized in that: The seven-core optical fiber comprises a third quartz core, a fourth quartz core, a fifth quartz core, a sixth quartz core, a seventh quartz core, an eighth quartz core, a ninth quartz core, a first quartz stress layer, a second quartz stress layer, a third quartz stress layer, a fourth quartz stress layer, a fifth quartz stress layer, a sixth quartz stress layer, a seventh quartz stress layer and a third quartz cladding; the fourth quartz core, the fifth quartz core, the sixth quartz core, the seventh quartz core, the eighth quartz core and the ninth quartz core are arranged around the third quartz core, and the first quartz stress layer, the second quartz stress layer, the third quartz stress layer, the fourth quartz stress layer, the fifth quartz stress layer, the sixth quartz stress layer and the seventh quartz stress layer respectively cover the third quartz core, the fourth quartz core, the fifth quartz core, the sixth quartz core, the seventh quartz core, the eighth quartz core and the ninth quartz core; the third quartz cladding covers the outer surface of the first quartz stress layer, the second quartz stress layer, the third quartz stress layer, the fourth quartz stress layer, the fifth quartz stress layer, the sixth quartz stress layer and the seventh quartz stress layer.

6. The M-Z interferometric sensor of a seven-core fiber tapered graded-index multimode fiber according to claim 1, characterized in that: The angular offset amount of the seven-core optical fiber when connected with the graded refractive index multimode optical fiber comprises 0° or 30°.

7. The M-Z interferometric sensor of a seven-core fiber tapered graded-index multimode fiber according to claim 1, characterized in that: The graded refractive index multimode optical fiber comprises a tenth quartz core and a fourth quartz cladding; the fourth quartz cladding annularly covers the tenth quartz core; the graded refractive index multimode optical fiber has a central symmetry structure; the radial refractive index function of the graded refractive index multimode optical fiber is in a square law distribution.

8. The M-Z interferometric sensor of a seven-core fiber tapered graded-index multimode fiber according to claim 1, characterized in that: The light-receiving single-mode optical fiber comprises an eleventh quartz core, a fifth quartz cladding and a second coating layer; the fifth quartz cladding annularly covers the eleventh quartz core; and the second coating layer annularly covers the fifth quartz cladding, and the second coating layer is located outside a preset surface area of the fifth quartz cladding away from the end of the graded-index multimode optical fiber.

9. The M-Z interferometric sensor of a seven-core fiber tapered graded-index multimode fiber according to claim 1, characterized in that: The first quartz core of the light-injecting single-mode optical fiber is axially aligned with the second quartz core of the step multimode optical fiber when connected, the second quartz core of the step multimode optical fiber is axially aligned with the third quartz core of the seven-core optical fiber when connected; the axis of the third quartz core of the seven-core optical fiber is axially offset from the tenth quartz core of the graded-index multimode optical fiber by a preset offset amount when connected; and the tenth quartz core of the graded-index multimode optical fiber is axially aligned with the eleventh quartz core of the light-receiving single-mode optical fiber when connected.

Images