Heavy metal ion concentration sensing system based on parallel few-mode optical fiber vernier interferometer

Abstract

The invention provides a heavy metal ion concentration sensing system based on a parallel few-mode optical fiber vernier interferometer. A few-mode optical fiber MI is innovatively constructed as a vernier sensibilization unit and is connected in parallel with an MZI sensing unit to realize sensibilization, and the invention relates to the technical field of optical fiber sensing. The few-mode optical fiber can only transmit several optical fiber modes, the inter-mode interference types are few, the spectrum shape can be obviously simplified, and the contrast ratio is improved. Meanwhile, the reflective structure of the MI can reduce the cavity length, namely the size of the sensor. On the other hand, the cutting and welding deviation of a unit interferometer in a traditional series vernier can cause spectral distortion of the whole series structure, and the sensor manufacturing efficiency is low. In order to overcome the problem, the MI and the MZI are innovatively changed from series connection to parallel connection, so that the two unit interferometers can be manufactured separately, mutual influence is avoided, and the controllability of sensor manufacturing is greatly improved.

Description

Technical Field

[0001] The invention relates to the technical field of optical fiber sensing, and in particular to a heavy metal ion concentration sensing system based on a parallel few-mode optical fiber vernier interferometer. Background Art

[0002] Fiber optic sensing technology is an important part of modern information technology. It uses light waves as carriers and optical fibers as media. Based on the physical properties of optical fibers, it achieves high-precision measurement of various physical quantities and signal transmission by observing the changes in internal light waves. The basic principle of fiber optic sensing is that when the external environment (temperature, refractive index, displacement, etc.) changes, the amplitude, phase, polarization state and other parameters of the light transmitted in the optical fiber will change, thereby affecting the output spectrum of the sensing system. Tracking these changes can achieve the sensing of external environmental parameters.

[0003] The Chinese patent with the announcement number CN109959403B discloses a multi-parameter large-capacity sensing system, including a narrow linewidth pulse laser, an erbium-doped optical amplifier, a circulator, a matching interferometer, a reflected light detection unit and a computer connected in sequence through optical fibers; the circulator is connected to an eccentric optical fiber, and an ultra-low reflectivity grating array is engraved on the eccentric optical fiber. The sensing system provided by the above scheme will have a disordered interference spectrum, so it is difficult to apply the vernier effect to amplify its sensitivity, resulting in low concentration detection sensitivity. Therefore, it is very necessary to provide a heavy metal ion concentration sensing system based on a parallel few-mode optical fiber vernier interferometer to improve the detection sensitivity of the heavy metal ion concentration sensing system. Summary of the invention

[0004] In view of this, the present invention proposes a heavy metal ion concentration sensing system based on parallel few-mode fiber vernier interferometer. By connecting the few-mode MI fiber interferometer and the single-mode MZI fiber interferometer in parallel through an optical circulator, the spectra of the two are superimposed to form an envelope spectrum, so that the heavy metal ion concentration sensing system is more sensitive to tiny wavelength changes, thereby improving the detection sensitivity of the heavy metal ion concentration sensing system.

[0005] The present invention provides a heavy metal ion concentration sensing system based on parallel few-mode fiber vernier interferometer, comprising a narrow-band light source, a transmission fiber, an optical circulator, a few-mode MI fiber interferometer, a single-mode MZI fiber interferometer, a photoelectric detector and a data acquisition module, wherein:

[0006] The first optical signal emitted by the narrow-band light source is coupled to the first end of the optical circulator through the transmission optical fiber;

[0007] The second end of the optical circulator is connected to the few-mode MI fiber interferometer, the first optical signal is transmitted to the few-mode MI fiber interferometer through the second end of the optical circulator, and the few-mode MI fiber interferometer returns the second optical signal to the second end of the optical circulator;

[0008] The third end of the optical circulator is connected to the single-mode MZI fiber interferometer, and the single-mode MZI fiber interferometer is immersed in the solution to be tested, and the second optical signal is output to the single-mode MZI fiber interferometer through the third end of the optical circulator. When the ion concentration of the solution to be tested changes, the peak wavelengths of the few-mode MI fiber interferometer and the single-mode MZI fiber interferometer move in opposite directions and form an envelope spectrum, so that the light intensity of the third optical signal emitted by the single-mode MZI fiber interferometer to the photodetector changes;

[0009] The single-mode MZI fiber interferometer is connected to the data acquisition module through the photoelectric detector, so that the third optical signal is photoelectrically converted, and the converted electrical signal is stored in the data acquisition module.

[0010] On the basis of the above technical solution, preferably, the few-mode MI fiber interferometer includes a first single-mode fiber and a few-mode fiber, the first single-mode fiber and the few-mode fiber are fused directly against each other, the length of the few-mode fiber is 7.2 cm, and the few-mode fiber is a four-mode fiber.

[0011] On the basis of the above technical solution, preferably, the single-mode MZI fiber interferometer includes a second single-mode optical fiber, a third single-mode optical fiber and a fourth single-mode optical fiber arranged in sequence, and the second single-mode optical fiber, the third single-mode optical fiber and the fourth single-mode optical fiber are staggered and fused in pairs, and the length of the third single-mode optical fiber is 6 cm.

[0012] More preferably, the fusion bias between each of the second single-mode optical fiber, the third single-mode optical fiber and the fourth single-mode optical fiber is 10-12 μm.

[0013] Further preferably, the core diameter of the few-mode optical fiber is 14 μm, the cladding diameter is 125 μm, the core refractive index is 1.449, and the cladding refractive index is 1.444; the core diameters of the second single-mode optical fiber, the third single-mode optical fiber, and the fourth single-mode optical fiber are all 9 μm, the cladding diameters are all 125 μm, the core refractive indexes are all 1.468, and the cladding refractive indexes are all 1.463.

[0014] More preferably, the single-mode MZI fiber interferometer is covered with a glass capillary having a diameter of 3 mm and a length of 10 cm.

[0015] More preferably, it also includes a sensor device box, a slide and a glass rod, the end of one side of the slide is provided with an extension portion, the single-mode MZI fiber interferometer is installed on the slide, the sensor device box includes a first box body and a second box body, the second box body is provided on one side of the first box body, the second box body is used to place the optical circulator and the few-mode MI fiber interferometer, the first box body is provided with a plurality of pairs of grooves, the extension portion of the slide and the glass rod are both provided in the corresponding grooves, and the glass rod and the slide are arranged opposite to each other at a distance, the glass rod is used to support the slide, and the first box body is used to receive waste liquid.

[0016] More preferably, the multiple pairs of grooves include a first groove, a second groove and a third groove, the extension of the carrier is clamped in the first groove, the glass rod is arranged in the second groove or the third groove, and the depth of the first groove is the same as that of the second groove, and the depth of the first groove is less than the depth of the third groove.

[0017] More preferably, when the glass rod is disposed in the second groove, the carrier and the single-mode MZI fiber interferometer are in a horizontal state; when the glass rod is disposed in the third groove, the carrier and the single-mode MZI fiber interferometer are in an inclined state.

[0018] More preferably, the wavelength of the output light of the narrow-band light source is 1540 nm, and the operating wavelength of the optical circulator is 1500 nm to 1570 nm.

[0019] The heavy metal ion concentration sensing system based on parallel few-mode optical fiber vernier interferometer provided by the present invention has the following beneficial effects compared with the prior art:

[0020] (1) The spectrum of the few-mode MI fiber interferometer and the single-mode MZI fiber interferometer is superimposed in parallel through an optical circulator to form an envelope spectrum. The envelope peak in the envelope spectrum has a narrower bandwidth and a higher peak value, which makes the heavy metal ion concentration sensing system more sensitive to tiny wavelength changes, improves the detection sensitivity of the heavy metal ion concentration sensing system, and the free spectrum area of ​​the envelope spectrum is larger than that of a single interferometer. The heavy metal ion concentration sensing system has a wider dynamic measurement range. At the same time, the two interferometers are connected in a parallel structure, which optimizes the interferometer spectrum. The two interferometers do not affect each other, which is beneficial to the spectrum control and structural size optimization of the interferometer.

[0021] (2) By making the few-mode MI fiber interferometer and the single-mode MZI fiber interferometer separately and connecting them in parallel through an optical circulator, the problems of spectral superposition and mutual influence of the two interferometers caused by the series connection of traditional fiber interferometers, as well as the disadvantage of spectral control and structural size optimization of the unit interferometer, are avoided. At the same time, the two interferometers are connected in parallel to optimize the interferometer spectrum. The two interferometers do not affect each other, which is beneficial to the spectral control and structural size optimization of the unit interferometer. At the same time, the parallel structure reduces the volume of the sensor.

[0022] (3) By setting the core diameter and refractive index of few-mode fiber and single-mode fiber differently, it is helpful to optimize mode field matching and reduce the loss at the fiber connection. Compared with multi-mode fiber, few-mode fiber has fewer transmission modes and fewer types of inter-mode interference, which makes it easier to determine the interference components. This overcomes the defects of traditional interferometers made of multi-mode fiber and photonic crystal fiber, which have many modes and are not easy to regulate mode excitation and control spectral FSR. At the same time, a reflective few-mode MI fiber interferometer is constructed based on the inter-mode interference of few-mode fiber. Compared with the MZI fiber interferometer with the same interference effect, the optical transmission path required by the few-mode MI fiber interferometer can be reduced by half, thereby greatly reducing the size of the sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0024] Figure 1 A schematic diagram of the framework of the heavy metal ion concentration sensing system provided by the present invention;

[0025] Figure 2 A schematic diagram of the structure of an optical circulator, a few-mode MI fiber interferometer, and a single-mode MZI fiber interferometer provided by the present invention;

[0026] Figure 3 A spectrum diagram of the heavy metal ion concentration sensing system provided by the present invention;

[0027] Figure 4 A schematic diagram of the structure of the sensor device box provided by the present invention;

[0028] Figure 5 This is a concentration response straight line graph and a sensitivity test result graph of the heavy metal ion concentration sensing system provided by the present invention.

[0029] Explanation of the reference numerals in the accompanying drawings: 1. narrow-band light source; 2. transmission optical fiber; 3. optical circulator; 4. few-mode MI fiber interferometer; 5. single-mode MZI fiber interferometer; 6. photodetector; 7. data acquisition module; 8. sensor device box; 81. first box body; 811. first groove; 812. second groove; 813. third groove; 82. second box body; 9. carrier; 91. extension portion; 10. glass rod. DETAILED DESCRIPTION

[0030] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] In the description of the present invention, "multiple" means two or more, unless otherwise clearly and specifically limited. Reference to "embodiment" in this article means that the specific features, structures or characteristics described in conjunction with the embodiment may be included in at least one embodiment of the present invention. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0032] like Figure 1 As shown, the present invention provides a high-power nanosecond ultraviolet laser, including a narrowband light source 1, a transmission optical fiber 2, an optical circulator 3, a few-mode MI fiber interferometer 4, a single-mode MZI fiber interferometer 5, a photodetector 6 and a data acquisition module 7, wherein:

[0033] The first optical signal emitted by the narrowband light source 1 is coupled to the first end of the optical circulator 3 through the transmission optical fiber 2. The wavelength of the output light of the narrowband light source 1 is 1540 nm. The narrowband light source 1 can be an ultra-narrow linewidth light source.

[0034] In one example, since the wavelength of the light emitted by the narrowband light source 1 needs to be used as the signal demodulation working point of the sensor, the working point selection method adopts a three-wavelength demodulation method, including: first estimating the concentration of the solution to be tested to be c1, selecting 1540nm as the pending working point, and selecting two wavelengths with a phase difference of 10nm according to the size of the sensor's free spectral range, that is, selecting 1543.3nm and 1536.7nm as the other two pending working points, and using LabVIEW software to respectively collect the light intensity corresponding to the solution with a concentration of c1 measured at the three pending working points, and selecting the wavelength with a light intensity value in the middle as the demodulation working point to be used.

[0035] See also Figure 2 The second end of the optical circulator 3 is connected to the few-mode MI fiber interferometer 4, the first optical signal is transmitted to the few-mode MI fiber interferometer 4 through the second end of the optical circulator 3, and the few-mode MI fiber interferometer 4 returns the second optical signal to the second end of the optical circulator 3. The transmission optical fiber 2 is a single-mode optical fiber, the operating wavelength of the optical circulator 3 is 1500nm~1570nm, and the internal optical fiber of the optical circulator 3 is a single-mode optical fiber.

[0036] Further, the structure of the few-mode MI fiber interferometer 4 is a single-mode fiber-few-mode fiber that is directly fused, and the few-mode MI fiber interferometer 4 is a few-mode fiber Michelson interferometer (MI). It can be understood that the few-mode MI fiber interferometer 4 includes a first single-mode fiber and a few-mode fiber, the first single-mode fiber and the few-mode fiber are directly fused, the length of the few-mode fiber is 7.2 cm, and the few-mode fiber is a four-mode fiber. The core diameter of the few-mode fiber is 14μm, the cladding diameter is 125μm, the core refractive index is 1.449, and the cladding refractive index is 1.444.

[0037] The third end of the optical circulator 3 is connected to the single-mode MZI fiber interferometer 5, and the single-mode MZI fiber interferometer 5 is immersed in the solution to be tested. The second optical signal is output to the single-mode MZI fiber interferometer 5 through the third end of the optical circulator 3. When the ion concentration of the solution to be tested changes, the peak wavelengths of the few-mode MI fiber interferometer 4 and the single-mode MZI fiber interferometer 5 move in opposite directions and form an envelope spectrum, so that the light intensity of the third optical signal emitted by the single-mode MZI fiber interferometer 5 to the photodetector 6 changes.

[0038] When the target ion concentration changes, the peak wavelengths of the few-mode MI fiber interferometer 4 and the single-mode MZI fiber interferometer 5 move in opposite directions to form an envelope spectrum, and the peak wavelength of the envelope spectrum shifts. By chasing the drift of the wavelength, the sensitivity of the heavy metal ion concentration can be detected. It can be understood that the light intensity value corresponding to the heavy metal ion concentration near the concentration range is obtained, and a linear fitting line is obtained, and the light intensity value of the fitting line is tracked to achieve the sensitivity detection of the heavy metal ion concentration.

[0039] Further, the structure of the single-mode MZI fiber interferometer 5 is a single-mode fiber-single-mode fiber-single-mode fiber that is staggered and fused, and the single-mode MZI fiber interferometer 5 is a single-mode fiber Mach–Zehnder interferometer (MZI). It can be understood that the single-mode MZI fiber interferometer 5 includes a second single-mode fiber, a third single-mode fiber, and a fourth single-mode fiber that are arranged in sequence, and the second single-mode fiber, the third single-mode fiber, and the fourth single-mode fiber are staggered and fused in pairs, and the length of the third single-mode fiber is 6 cm. The core diameters of the second single-mode fiber, the third single-mode fiber, and the fourth single-mode fiber are all 9 μm, the cladding diameters are all 125 μm, the core refractive indexes are all 1.468, and the cladding refractive indexes are all 1.463. Among them, the third single-mode optical fiber in the single-mode MZI optical fiber interferometer 5 is stripped of the coating layer and coated with a chitosan film sensitive to the concentration of heavy metal ions, which is used as a sensing unit, and the fusion bias between the second single-mode optical fiber, the third single-mode optical fiber and the fourth single-mode optical fiber is 10-12μm.

[0040] See also Figure 3 , Figure 3 (a) is the spectrum diagram of the few-mode MI fiber interferometer 4. Figure 3 (b) is the spectrum diagram of the single-mode MZI fiber interferometer 5. Figure 3 (c) is a spectrum diagram after the few-mode MI fiber interferometer 4 and the single-mode MZI fiber interferometer 5 are connected in parallel through the optical circulator 3.

[0041] In this embodiment, a glass capillary with a diameter of 3 mm and a length of 10 cm is provided on the outer sleeve of the single-mode MZI fiber interferometer 5. The glass capillary can be used to inject the solution to be tested. Due to the capillary action, a liquid column will be formed in the tube. When the glass capillary is tilted, only a small amount of the liquid column will flow out, which will not affect the measurement of the liquid by the sensor in the glass capillary. And when the injection time exceeds 4 to 5 seconds, the influence of the original liquid in the glass capillary can be completely eliminated. Therefore, continuous injection of the glass capillary can eliminate the cleaning step when replacing the solution.

[0042] The single-mode MZI fiber interferometer 5 is connected to the data acquisition module 7 through the photoelectric detector 6, so that the third optical signal is photoelectrically converted, and the converted electrical signal is stored in the data acquisition module 7. The data acquisition module 7 includes a data acquisition card and a display screen, and the display screen is installed with LabVIEW software to obtain the light intensity value data in the data acquisition card, and the display screen is used to display the detection results of heavy metal ions in the solution to be tested.

[0043] By connecting the few-mode MI fiber interferometer 4 and the single-mode MZI fiber interferometer 5 in parallel through the optical circulator 3, the spectrum of the two is superimposed to form an envelope spectrum, and the envelope peak in the envelope spectrum has a narrower bandwidth and a higher peak value, so that the heavy metal ion concentration sensing system is more sensitive to small wavelength changes, and the detection sensitivity of the heavy metal ion concentration sensing system is improved. The free spectrum area of ​​the envelope spectrum is larger than the free spectrum area of ​​a single interferometer, and the heavy metal ion concentration sensing system has a wider dynamic measurement range. The few-mode MI fiber interferometer 4 and the single-mode MZI fiber interferometer 5 are connected in parallel, which avoids the traditional serial connection between fiber interferometers, reduces the problem of superposition and mutual influence of the spectrum of the two interferometers, and optimizes the interferometer spectrum. The few-mode MI fiber interferometer 4 and the single-mode MZI fiber interferometer 5 do not affect each other, which is beneficial to the spectrum control and structural size optimization of the unit interferometer. At the same time, the parallel structure reduces the volume of the sensor and has good practical value.

[0044] like Figure 4 As shown, it also includes a sensor device box 8, a carrier plate 9 and a glass rod 10. An extension portion 91 is provided at the end of one side of the carrier plate 9. The single-mode MZI fiber interferometer 5 is installed on the carrier plate 9. The sensor device box 8 includes a first box body 81 and a second box body 82. The second box body 82 is provided on one side of the first box body 81. The second box body 82 is used to place the optical circulator 3 and the few-mode MI fiber interferometer 4. A plurality of pairs of grooves are provided on the first box body 81. The extension portion 91 of the carrier plate 9 and the glass rod 10 are both provided in the corresponding grooves, and the glass rod 10 and the carrier plate 9 are arranged opposite to each other at a distance. The glass rod 10 is used to support the carrier plate 9, and the first box body 81 is used to receive waste liquid.

[0045] In this embodiment, the plurality of pairs of grooves include a first groove 811, a second groove 812, and a third groove 813. The extension 91 of the slide 9 is clamped in the first groove 811. The glass rod 10 is disposed in the second groove 812 or the third groove 813. The depths of the first groove 811 and the second groove 812 are the same, and the depth of the first groove 811 is less than the depth of the third groove 813. When the glass rod 10 is disposed in the second groove 812, the slide 9 and the single-mode MZI fiber interferometer 5 are in a horizontal state. The horizontal state is used to stabilize the liquid in the glass capillary and ensure the stability of the sensor and the solution to be measured during the measurement process. When the glass rod 10 is disposed in the third groove 813, the slide 9 and the single-mode MZI fiber interferometer 5 are in an inclined state. The inclined state is used to replace the solution to be measured. The tilt of the glass capillary facilitates the outflow of the liquid in the tube during injection. After the outflow, the liquid drips into the first box body 81 along the glass slide.

[0046] In one example, a slide 9 with a glass slide carrying a sensor, a first box body 81 containing measurement waste liquid, an optical circulator 3 and a second box body 82 of a few-mode MI fiber interferometer 4 are fixed, and the end extension of the slide 9 is placed in a first groove 811 opened in the first box body 81, one end of the slide 9 is fixed, and a glass rod 10 is placed in any other groove, and the slide 9 is placed on the glass rod 10, so that the slide 9 can be fixed. The edge of the slide 9 has a 1mm high baffle, which is flush with the height of the slide, and does not affect the extension of the optical fiber while fixing the slide.

[0047] It can be understood that when the glass rod 10 is placed in different grooves, the slide 9 will have two different states. When the glass rod 10 is placed in the second groove 812, the slide 9 is in a horizontal state, which is convenient for the heavy metal ion concentration measurement of the test liquid, and the liquid in the glass capillary is stable; when the glass rod 10 is placed in the third groove 813, the slide 9 is in an inclined state, which is convenient for the liquid in the glass capillary to flow out, and is often used to discharge the last liquid when replacing the liquid. At this time, the liquid flows into the first box body 81 along the inclination of the glass slide. The first box body 81 is waterproofed and used to receive waste liquid. After the slide 9 and the glass rod 10 are disassembled, the first box body 81 can be directly cleaned. The overall volume of the device is small and easy to wash.

[0048] The bottom of the second box body 82 is slightly wider than the glass slide, and can be used to place the glass slide fixed with the few-mode MI fiber interferometer 4. The optical circulator 3 needs to be connected to the single-mode MZI fiber interferometer 5, the few-mode MI fiber interferometer 4, and the narrow-band light source 1. The optical circulator 3 is placed in the second box body 82 so that the three are connected to the circulator at the shortest distance. The bottom of the second box body 82 is opened, so that the optical circulator 3 can be connected to the narrow-band light source 1 from the bottom, which is more conducive to experimental operation.

[0049] It can be understood that when measuring the heavy metal ion concentration of the solution, the narrow-band light source 1 is connected to the first end of the optical circulator 3 from the bottom of the second box body 82, the few-mode MI fiber interferometer 4 is connected to the second end of the optical circulator 3 from the bottom of the second box body 82, the end extension of the carrier 9 is placed in the first groove 811 of the first box body 81, one end of the carrier 9 is fixed, the glass rod 10 is placed in the second groove 812, the carrier 9 is placed above the glass rod 10, the glass slide carrying the single-mode MZI fiber interferometer 5 is fixed on the carrier 9, and the single-mode MZI fiber interferometer 5 is connected to the third end of the optical circulator 3.

[0050] In one example, the material of the sensor device box 8 is an acrylic plate, the first box body 81 is 12 cm long, 5 cm wide, and 10 cm high, the first box body 81 is waterproofed inside, the acrylic plate is 3 mm thick, a first groove 811 is set 2 cm away from the long edge of the first box body 81, the groove depth is 5 mm, the width is 5 mm, and a second groove 812 is set at an interval of 6 cm on the basis of the groove, the groove depth is 5 mm, and the width is 5 mm. In addition, a third groove 813 is set at an interval of 1 cm. The groove depth is 1cm and the width is 5mm; the length of the carrier 9 is selected to be 85mm and the width is 30mm. The two sides of the end of one side of the carrier 9 are respectively extended by 7.5cm, and the width of the extended part is 5mm. The thickness of the acrylic board used for the carrier 9 is 4mm, and a 1mm high baffle is provided on the edge of the board surface where it is not extended, and the width of the baffle is 2mm; the second box body 82 is fixed to one side of the first box body 81, and the height is set to 5cm, the length is 12cm, the width is 3cm, there is a 2cm high opening at the lower end, and the thickness of the acrylic board is 3mm.

[0051] The sensitivity detection method of the heavy metal ion concentration sensor system adopts the following steps:

[0052] 1) Fix the glass slide carrying the single-mode MZI fiber interferometer 5 on the slide 9, fix the glass slide carrying the few-mode MI fiber interferometer 4 and the optical circulator 3 in the second box body 82, place the end extension of the slide 9 in the first groove 811, place the glass rod 10 in the second groove 812, and place the slide 9 on the glass rod 10;

[0053] 2) connecting a first end of the optical circulator 3 to a narrowband light source 1 through a flange, connecting a second end of the optical circulator 3 to one end of a few-mode MI fiber interferometer 4, connecting a third end of the optical circulator 3 to one end of a single-mode MZI fiber interferometer 5, and connecting the other end of the single-mode MZI fiber interferometer 5 to a photodetector 6, connecting the photodetector 6 to a data acquisition card, and connecting the data acquisition card to a display;

[0054] 3) Use a syringe to inject the copper sulfate solution to be tested from one end of the glass capillary outside the single-mode MZI fiber interferometer 5. The liquid concentration increases from 2.75% to 3.75%, and the concentration gradient is 0.25%. At each concentration point, the experimental light intensity value is recorded by a data acquisition card;

[0055] 4) Performing linear fitting on the experimental data to obtain the concentration response line of the cascade interferometer to heavy metal ions;

[0056] 5) disconnecting the optical circulator 3 from the few-mode MI fiber interferometer 4 and the single-mode MZI fiber interferometer 5, and connecting the second end of the optical circulator 3 to the single-mode MZI fiber interferometer 5;

[0057] 6) The heavy metal ion concentration response characteristics of the single-mode MZI fiber interferometer 5 are tested separately. The incident end and the output end of the single-mode MZI fiber interferometer 5 are connected to the second end of the optical circulator 3 and the photodetector 6 respectively. Step (3) is repeated, and the light intensity value shown on the display screen connected to the data acquisition card is read to obtain the heavy metal ion concentration sensitivity of the single-mode MZI fiber interferometer 5.

[0058] See also Figure 5 , Figure 5 (a) is the heavy metal ion concentration response straight line graph. Figure 5 (b) is a heavy metal ion concentration sensitivity test result diagram. The heavy metal ion concentration sensitivity based on the parallel cascade of dual interferometers can reach -1.0064dB / %, which is 2 times larger than the sensitivity of a single single-mode-single-mode interferometer (0.50329dB / %), achieving a significant improvement in the heavy metal ion concentration sensitivity; the parallel cascade of the two interferometers improves the spectral quality, which is beneficial to the spectral regulation and structural size optimization of the unit interferometer. The sensor device box 8 can place and fix the sensor and adjust the sensor's tilt state to facilitate the measurement of heavy metal ion concentration. The sensor device box 8 is small in size and easy to carry, which is beneficial to the integration of the sensor system. The corresponding sensor system realizes a simple measurement of the heavy metal ion concentration.

[0059] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A heavy metal ion concentration sensing system based on parallel few-mode fiber vernier interferometer, characterized in that: It comprises a narrowband light source (1), a transmission optical fiber (2), an optical circulator (3), a few-mode MI optical fiber interferometer (4), a single-mode MZI optical fiber interferometer (5), a photodetector (6) and a data acquisition module (7), wherein: The first optical signal emitted by the narrow-band light source (1) is coupled to the first end of the optical circulator (3) through the transmission optical fiber (2); The optical circulator (3) connects the few-mode MI optical fiber interferometer (4) and the single-mode MZI optical fiber interferometer (5) in parallel; The second end of the optical circulator (3) is connected to the few-mode MI optical fiber interferometer (4), the first optical signal is transmitted to the few-mode MI optical fiber interferometer (4) through the second end of the optical circulator (3), and the few-mode MI optical fiber interferometer (4) returns the second optical signal to the second end of the optical circulator (3); The third end of the optical circulator (3) is connected to the single-mode MZI optical fiber interferometer (5), and the single-mode MZI optical fiber interferometer (5) is immersed in the solution to be tested, and the second optical signal is output to the single-mode MZI optical fiber interferometer (5) through the third end of the optical circulator (3); when the ion concentration of the solution to be tested changes, the peak wavelengths of the few-mode MI optical fiber interferometer (4) and the single-mode MZI optical fiber interferometer (5) move in opposite directions and form an envelope spectrum, so that the light intensity of the third optical signal emitted by the single-mode MZI optical fiber interferometer (5) to the photodetector (6) changes; The single-mode MZI optical fiber interferometer (5) is connected to a data acquisition module (7) via the photoelectric detector (6) so that the third optical signal is subjected to photoelectric conversion, and the converted electrical signal is stored in the data acquisition module (7).

2. The heavy metal ion concentration sensing system according to claim 1, characterized in that: The few-mode MI fiber interferometer (4) comprises a first single-mode fiber and a few-mode fiber, the first single-mode fiber and the few-mode fiber are directly fused, the length of the few-mode fiber is 7.2 cm, and the few-mode fiber is a four-mode fiber.

3. The heavy metal ion concentration sensing system according to claim 1, characterized in that: The single-mode MZI fiber interferometer (5) comprises a second single-mode optical fiber, a third single-mode optical fiber and a fourth single-mode optical fiber which are arranged in sequence, and the second single-mode optical fiber, the third single-mode optical fiber and the fourth single-mode optical fiber are staggered and fused in pairs, and the length of the third single-mode optical fiber is 6 cm.

4. The heavy metal ion concentration sensing system according to claim 3, characterized in that: The fusion bias between the second single-mode optical fiber, the third single-mode optical fiber and the fourth single-mode optical fiber is 10-12 μm.

5. The heavy metal ion concentration sensing system according to claim 4, characterized in that: The core diameter of the few-mode optical fiber is 14 μm, the cladding diameter is 125 μm, the core refractive index is 1.449, and the cladding refractive index is 1.444; the core diameters of the second single-mode optical fiber, the third single-mode optical fiber, and the fourth single-mode optical fiber are all 9 μm, the cladding diameters are all 125 μm, the core refractive indexes are all 1.468, and the cladding refractive indexes are all 1.

463.

6. The heavy metal ion concentration sensing system according to claim 1, characterized in that: The single-mode MZI optical fiber interferometer (5) is provided with a glass capillary with a diameter of 3 mm and a length of 10 cm.

7. The heavy metal ion concentration sensing system according to claim 6, characterized in that: The invention also comprises a sensor device box (8), a carrier (9) and a glass rod (10); an extension portion (91) is arranged at the end of one side of the carrier (9); the single-mode MZI optical fiber interferometer (5) is mounted on the carrier (9); the sensor device box (8) comprises a first box body (81) and a second box body (82); the second box body (82) is arranged on one side of the first box body (81); the second box body (82) is used to place the optical circulator (3) and the few-mode MI optical fiber interferometer (4); the first box body (81) is provided with a plurality of pairs of grooves; the extension portion (91) of the carrier (9) and the glass rod (10) are arranged in corresponding grooves; the glass rod (10) and the carrier (9) are arranged opposite to each other at a distance; the glass rod (10) is used to support the carrier (9); and the first box body (81) is used to receive waste liquid.

8. The heavy metal ion concentration sensing system according to claim 7, characterized in that: The multiple pairs of grooves include a first groove (811), a second groove (812) and a third groove (813); the extension portion (91) of the carrier (9) is clamped in the first groove (811); the glass rod (10) is arranged in the second groove (812) or the third groove (813); the depth of the first groove (811) is the same as that of the second groove (812); and the depth of the first groove (811) is less than the depth of the third groove (813).

9. The heavy metal ion concentration sensing system according to claim 8, characterized in that: When the glass rod (10) is arranged in the second groove (812), the carrier (9) and the single-mode MZI optical fiber interferometer (5) are in a horizontal state; when the glass rod (10) is arranged in the third groove (813), the carrier (9) and the single-mode MZI optical fiber interferometer (5) are in an inclined state.

10. The heavy metal ion concentration sensing system according to claim 1, characterized in that: The wavelength of the output light of the narrow-band light source (1) is 1540 nm, and the operating wavelength of the optical circulator (3) is 1500 nm to 1570 nm.

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