A temperature sensing system and method

Abstract

This invention belongs to the field of solution temperature sensing and detection technology, and discloses a temperature sensing system and method, including an active-passive mode-locked fiber laser; the active-passive mode-locked fiber laser has a D-type multimode fiber, which is fully immersed in the solution to be tested; a spectrometer is used to receive the active-passive mode-locked laser output from the active-passive mode-locked fiber laser and obtain the spectral image of the active-passive mode-locked laser; a temperature characterization module is used to receive and analyze the spectral image of the active-passive mode-locked laser and obtain the current wavelength offset of the active-passive mode-locked laser; based on the pre-calibrated fitting calibration curve of the wavelength offset and the solution temperature change and the current wavelength offset of the active-passive mode-locked laser, the current temperature sensing result of the solution to be tested is obtained; this invention uses a D-type multimode fiber as a sensing probe for measuring solution temperature, which can meet the requirements of reliable measurement of the temperature of solutions with large surface areas and achieve high-precision and large-range temperature monitoring.

Description

Technical Field

[0001] This invention belongs to the field of solution temperature sensing and detection technology, and specifically relates to a temperature sensing system and method. Background Technology

[0002] With the development of optical detection technology, fiber optic sensing technology has developed rapidly. Among them, fiber optic sensors have been extensively and deeply studied due to their simple structure, high sensitivity, and resistance to electromagnetic interference. They are applicable to remote measurement and process control fields for sensing and detecting temperature, pressure, vibration, strain, magnetic field, refractive index, and species composition.

[0003] Currently, existing temperature measurement processes employ fiber Bragg gratings or cover the end of a single-mode fiber pigtail with a material whose refractive index changes with temperature. This alters the Fresnel reflectivity at the interface between the fiber pigtail and the covering material, and temperature is measured by measuring the shift in the Bragg wavelength or the change in Fresnel reflectivity. However, the drawback of these methods is their small sensing area, which is unsuitable for stable measurement of solutions with large surface areas and hinders large-scale applications. Summary of the Invention

[0004] In view of the technical problems existing in the prior art, the present invention provides a temperature sensing system and method to solve the technical problem that the existing temperature measurement methods have a small sensing area surface area, which is not conducive to the stable measurement of solutions with large surface areas.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] This invention provides a temperature sensing system, including an active and passive mode-locked fiber laser, a spectrometer, and a temperature characterization module; the output end of the active and passive mode-locked fiber laser is connected to the input end of the spectrometer, and the output end of the spectrometer is connected to the input end of the temperature characterization module.

[0007] The active-passive mode-locked fiber laser has a D-type multimode fiber, which is fully immersed in the solution to be tested; wherein the beam transmitted in the D-type multimode fiber can generate an evanescent field effect with the solution to be tested on the surface of the D-type multimode fiber.

[0008] The spectrometer is used to receive the active and passive mode-locked laser output from the active and passive mode-locked fiber laser and obtain the spectral image of the active and passive mode-locked laser.

[0009] The temperature characterization module is used to receive and analyze the spectral image of the active and passive mode-locked laser to obtain the current wavelength offset of the active and passive mode-locked laser; and to obtain the current temperature sensing result of the solution under test based on the pre-calibrated fitting calibration curve of the wavelength offset and the solution temperature change and the current wavelength offset of the active and passive mode-locked laser.

[0010] Furthermore, the active and passive mode-locked fiber laser includes a pump laser source, a NALM circuit, a central optical coupler, and an oscillating cavity circuit;

[0011] The output of the pump laser source is connected to the input of the NALM circuit; the NLAM circuit is coupled to the oscillating cavity circuit through the central optical coupler to form a figure-eight cavity laser; the output of the oscillating cavity circuit is connected to the input of the spectrometer; wherein, the D-type multimode fiber is located in the oscillating cavity circuit.

[0012] Furthermore, the NALM loop includes a first single-mode fiber, a first wavelength division multiplexer, an erbium-doped fiber, a second wavelength division multiplexer, a first polarization controller, and an active electro-optic modulator;

[0013] Both ends of the first single-mode fiber are connected to the central optical coupler to form a first cavity; the first wavelength division multiplexer, the erbium-doped fiber, the second wavelength division multiplexer, the first polarization controller, and the active electro-optic modulator are connected in series in the path of the first single-mode fiber; wherein, the coupling input ends of the first wavelength division multiplexer and the second wavelength division multiplexer are both connected to the output end of the pump laser source.

[0014] Furthermore, the splitting ratio of the central optical coupler is 50:50, and the splitting ratio of the oscillating cavity optical coupler is 90:10.

[0015] Furthermore, the active electro-optic modulator is a lithium niobate electro-optic modulator; wherein the operating bandwidth of the lithium niobate electro-optic modulator is 12.5 Gb / s.

[0016] Furthermore, the oscillating cavity circuit also includes a second single-mode fiber, an optical isolator, an oscillating cavity optical coupler, and a second polarization controller;

[0017] Both ends of the second single-mode fiber are connected to the central optical coupler to form a second cavity; the D-type multimode fiber, the optical isolator, the oscillating cavity optical coupler, and the second polarization controller are connected in series in the path of the second single-mode fiber; wherein, the output end of the oscillating cavity optical coupler is connected to the input end of the spectrometer.

[0018] Furthermore, the center wavelength of the pump laser source is 980nm, and the power of the pump laser source is adjustable in the range of 1-999mW.

[0019] Furthermore, the D-type multimode fiber adopts a GIMF-SIMF-GIMF structure; wherein, the GIMF-SIMF-GIMF structure is obtained by fusion splicing multimode step fiber, graded fiber and step fiber, and the graded fiber is a side-polished D-type structure.

[0020] The present invention also provides a temperature sensing method, utilizing the aforementioned temperature sensing system; wherein, the temperature sensing method includes the following steps:

[0021] The D-type multimode fiber in the active-passive mode-locked fiber laser is fully immersed in a standard solution, and the standard solution is kept at a constant temperature.

[0022] When the temperature of the standard solution changes, the active and passive mode-locked laser output from the active and passive mode-locked fiber laser is received by a spectrometer to obtain the spectral image of the active and passive mode-locked laser corresponding to the standard solution.

[0023] The spectral images of the active and passive mode-locked lasers corresponding to the standard solution are analyzed to obtain the wavelength shift of the active and passive mode-locked lasers at different temperatures of the standard solution.

[0024] Based on the wavelength shift of the active and passive mode-locked lasers at different temperatures of the standard solution, the fitting calibration of the solution temperature and the wavelength shift is performed to obtain the pre-calibrated fitting calibration curve of the wavelength shift and the solution temperature change.

[0025] Replace the standard solution with the test solution, and fully immerse the D-type multimode fiber in the active and passive mode-locked fiber laser into the test solution;

[0026] The active and passive mode-locked laser output from the active and passive mode-locked fiber laser is received again using a spectrometer, and the spectral image of the active and passive mode-locked laser corresponding to the solution to be tested is obtained.

[0027] The spectral image of the active and passive mode-locked laser corresponding to the standard solution is analyzed to obtain the current wavelength shift of the active and passive mode-locked laser in the test solution.

[0028] The current wavelength offset of the active and passive mode-locked laser in the test solution is substituted into the pre-calibrated fitting calibration curve of the wavelength offset and the solution temperature change to calculate the current temperature sensing result of the test solution.

[0029] Furthermore, the standard solution has the same composition and concentration as the test solution; wherein, the temperature change of the standard solution is a known quantity.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0031] This invention provides a temperature sensing system and method. It utilizes a D-type multimode fiber embedded in a passively mode-locked fiber laser. Based on the principles of multimode interference of the laser beam within the D-type multimode fiber and the evanescent field effect generated on the surface of the fiber by the beam and the solution being measured, the system detects the spectral image of the passively mode-locked laser after multimode interference and evanescent field effect using a spectrometer. By analyzing the spectral image, the current wavelength shift of the passively mode-locked laser is obtained. Then, based on the correlation between the wavelength shift and the solution temperature change, the solution temperature is measured. The use of a D-type multimode fiber as the sensing probe effectively increases the surface area of ​​the sensing region, enabling reliable temperature measurement of solutions with large surface areas and achieving high-precision, wide-range temperature monitoring, thus meeting the requirements for real-time stable monitoring in industrial production. This invention features a simple structure and principle, is easy to manufacture, has low cost, and is convenient to operate. Furthermore, it effectively avoids cross-sensitivity caused by measuring different parameters, improving measurement accuracy. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the active and passive mode-locked fiber laser in this invention;

[0033] Figure 2 This is the fitting calibration curve of wavelength shift versus solution temperature change in this invention;

[0034] Figure 3 These are the test spectral curves of NaCl solution at different temperatures in this invention.

[0035] Among them, 1 is the first pump laser source, 2 is the second pump laser source, 3 is the first single-mode fiber, 4 is the first wavelength division multiplexer, 5 is the erbium-doped fiber, 6 is the second wavelength division multiplexer, 7 is the first polarization controller, 8 is the active electro-optic modulator, 9 is the central optical coupler, 10 is the second single-mode fiber, 11 is the D-type multimode fiber, 12 is the optical isolator, 13 is the oscillating cavity optical coupler, and 14 is the second polarization controller. Detailed Implementation

[0036] To make the technical problems solved by the present invention, the technical solutions, and the beneficial effects clearer, the following specific embodiments provide a further detailed description of the present invention. It should be understood that the specific embodiments described herein are merely illustrative and are not intended to limit the scope of the invention.

[0037] This invention provides a temperature sensor system, including an active and passive mode-locked fiber laser, a spectrometer, and a temperature characterization module; the output end of the active and passive mode-locked fiber laser is connected to the input end of the spectrometer, and the output end of the spectrometer is connected to the input end of the temperature characterization module.

[0038] As attached Figure 1 As shown, the active-passive mode-locked fiber laser includes a pump laser source, a NALM circuit, a central optical coupler (OC) 9, and an oscillating cavity circuit. The output end of the pump laser source is connected to the input end of the NALM circuit, and the pump laser source is used to input pump light into the NALM circuit. The NALM circuit is coupled to the oscillating cavity circuit through the central optical coupler 9 to form a figure-eight cavity laser. The output end of the oscillating cavity circuit is connected to the input end of the spectrometer, and the oscillating cavity circuit is used to output active-passive mode-locked laser light to the spectrometer.

[0039] The center wavelength of the pump laser source is 980nm, and the power of the pump laser source is adjustable in the range of 1-999mW. Specifically, the pump laser source includes a first pump laser source 1 and a second pump laser source 2, and the output terminals of the first pump laser source 1 and the second pump laser source 2 are both connected to the input terminal of the NALM circuit. Preferably, both the first pump laser source 1 and the second pump laser source 2 are pump tabletop single-mode laser sources (Laser Pump Power, Pump) used to emit pump light with a wavelength of 980nm and a power adjustable in the range of 0-999mW.

[0040] The NALM loop includes a first single-mode fiber (SMF) 3, a first wavelength division multiplexer (WDM) 4, an erbium-doped fiber (EDF) 5, a second wavelength division multiplexer (WDM) 6, a first polarization controller (PC) 7, and an active electro-optic modulator (RF) 8.

[0041] Both ends of the first single-mode fiber 3 are connected to one end of the central optical coupler 9 to form a first cavity; the first wavelength division multiplexer 4, the erbium-doped fiber 5, the second wavelength division multiplexer 6, the first polarization controller 7, and the active electro-optic modulator 8 are connected in series in the path of the first single-mode fiber 3; wherein, the coupling input end of the first wavelength division multiplexer 4 is connected to the output end of the first pump laser source 1, and the coupling input end of the second wavelength division multiplexer 6 is connected to the output end of the second pump laser source 2.

[0042] The first single-mode fiber 3 serves as the transmission fiber in the NALM loop; preferably, the first single-mode fiber 3 is an SMF-28e single-mode fiber.

[0043] The first wavelength division multiplexer 4 and the second wavelength division multiplexer 6 are key components of the erbium-doped fiber laser, used to multiplex optical signals of different wavelengths in an optical fiber system. Preferably, both the first wavelength division multiplexer 4 and the second wavelength division multiplexer 6 are 980 / 1550nm wavelength division multiplexers, that is, both the first wavelength division multiplexer 4 and the second wavelength division multiplexer 6 can transmit optical signals of 980nm and 1550nm wavelengths simultaneously, and have the advantages of low insertion loss, high return loss and stable reliability.

[0044] The erbium-doped fiber 5 employs an erbium-doped gain medium. By doping the gain medium with the rare-earth element erbium, the passive transmission fiber can be transformed into an active fiber with amplification capabilities. The erbium-doped fiber 5 is positioned between the first wavelength division multiplexer 4 and the second wavelength division multiplexer 6. The first wavelength division multiplexer 4 and the second wavelength division multiplexer 6 are used to bidirectionally pump the erbium-doped fiber 5, thereby improving the absorption of pump light emitted by the pump laser source by the erbium-doped fiber 5. Preferably, the performance parameters of the erbium-doped fiber 5 are Er110-4 / 125.

[0045] The first polarization controller 7 is used to adjust the laser polarization of the NALM circuit; preferably, the first polarization controller 7 is a three-loop polarization controller.

[0046] The active electro-optic modulator 8 is used to add a stable electro-modulation signal to the NALM circuit to achieve stable generation of active mode-locked adjustable pulses in the NALM circuit; preferably, the active electro-optic modulator 8 is a lithium niobate electro-optic modulator with a working bandwidth of 12.5 Gb / s.

[0047] The splitting ratio of the central optical coupler 9 is 50:50; wherein, the central optical coupler 9 is used to couple 50% of the optical signal transmitted in the first single-mode fiber 3 to the oscillating cavity circuit, and retain the remaining 50% of the optical signal in the NALM circuit.

[0048] The resonant cavity circuit includes a second single-mode fiber (SMF) 10, a D-shaped multimode fiber (GSG-DSF) 11, an optical isolator (ISO) 12, an optical coupler (OC) 13, and a second polarization controller (PC) 14.

[0049] Both ends of the second single-mode fiber 10 are connected to the other end of the central optical coupler 9 to form a second cavity; the D-type multimode fiber 11, the optical isolator 12, the oscillating cavity optical coupler 13 and the second polarization controller 14 are connected in series in the path of the second single-mode fiber 10; wherein, the output end of the oscillating cavity optical coupler 13 is connected to the input end of the spectrometer.

[0050] The second single-mode fiber 10 serves as the transmission fiber in the oscillation cavity circuit; preferably, the second single-mode fiber 10 is an SMF-28e single-mode fiber.

[0051] The D-type multimode fiber 11 is fully immersed in the test solution. The light beam transmitted in the D-type multimode fiber 11 can generate an evanescent field effect with the test solution on the surface of the D-type multimode fiber 11. The D-type multimode fiber 11 adopts a GIMF-SIMF-GIMF structure. Specifically, the GIMF-SIMF-GIMF structure is obtained by fusion splicing multimode step fiber (GIMF), graded index multimode fiber (SIMF), and step fiber. The graded index fiber has a side-polished D-type structure, that is, the sides of the graded index fiber are polished. At both ends of the GIMF-SIMF-GIMF structure, the length of the multimode step fiber is 19.9-20.1 cm, and the length of the graded index fiber is in the centimeter range.

[0052] It should be noted that by performing side polishing on the graded fiber in the D-type multimode fiber 11 to expose the evanescent field of the light-guiding fiber in the D-type multimode fiber 11, the refractive index between the core and cladding is significantly reduced, thus preventing the light from being trapped and propagating in the fiber. Therefore, the evanescent field is exposed around the side polishing area so that the light beam transmitted in the D-type multimode fiber 11 can generate an evanescent field effect with the test solution on the surface of the D-type multimode fiber 11.

[0053] The optical isolator 12 is used to allow light to pass in one direction while blocking light in the opposite direction, ensuring that reflected or backscattered light will not re-enter the oscillation cavity circuit, thereby eliminating adverse effects and improving the stability of the oscillation cavity circuit.

[0054] The splitting ratio of the oscillating cavity optical coupler 13 is 90:10, and the oscillating cavity optical coupler 13 adopts a 10dB optical coupler; wherein, the central optical coupler 9 is used to couple 10% of the optical signal transmitted in the first single-mode fiber 3 to the spectrometer, and retain the remaining 90% of the optical signal in the oscillating cavity circuit.

[0055] The second polarization controller 14 is used to adjust the laser polarization of the oscillation cavity circuit; preferably, the second polarization controller 14 is a polarization three-loop controller.

[0056] It should be noted that when the central coupler 9 is connected to the first single-mode fiber 3 and the second single-mode fiber 10, a fusion splicer is used for splicing. In the NALM circuit, when the first wavelength division multiplexer 4, erbium-doped fiber 5, second wavelength division multiplexer 6, first polarization controller 7, and active electro-optic modulator 8 are connected to the first single-mode fiber 3, a fusion splicer is used for splicing. In the resonant cavity circuit, when the D-type multimode fiber 11, optical isolator 12, resonant cavity optical coupler 13, and second polarization controller 14 are connected to the second single-mode fiber 14, a fusion splicer is also used for splicing.

[0057] The following describes the temperature sensing method of the temperature sensing system using the temperature sensing and detection process of a 10% NaCl solution as an example. The specific steps include:

[0058] Step 1: Prepare a NaCl solution with the same composition and concentration as the solution to be tested to obtain a standard solution; place the standard solution on a heating platform; it should be noted that the standard solution has the same composition and concentration as the solution to be tested; the difference is that the temperature change of the standard solution is a known quantity, while the concentration of the solution to be tested is an unknown quantity.

[0059] Step 2: Immerse the D-type multimode optical fiber 11 fully into the standard solution, and use the heating platform to control the temperature of the standard solution at a constant temperature.

[0060] Step 3: When the temperature of the standard solution changes, the spectrometer is used to receive the active and passive mode-locked laser output from the oscillating cavity optical coupler 13 in the active and passive mode-locked fiber laser, and obtain the spectral image of the active and passive mode-locked laser corresponding to the standard solution.

[0061] Step 4: Analyze the spectral images of the active and passive mode-locked lasers corresponding to the standard solution, and determine the wavelength shift of the active and passive mode-locked lasers at different temperatures of the standard solution. Based on the wavelength shifts of the active and passive mode-locked lasers at different temperatures of the standard solution, perform a fitting calibration of the solution temperature and wavelength shifts to obtain a pre-calibrated fitting calibration curve of the wavelength shift versus solution temperature change, as shown in the attached figure. Figure 2 As shown; from the appendix Figure 2 As can be seen, the wavelength shift and the change in solution temperature exhibit a linear relationship.

[0062] Step 5: Replace the standard solution with the test solution and fully immerse the D-type multimode optical fiber 11 in the test solution.

[0063] Step 6: Use a spectrometer to receive and analyze the active and passive mode-locked laser output from the oscillating cavity optical coupler 13 in the active and passive mode-locked fiber laser again, and obtain the spectral image of the active and passive mode-locked laser corresponding to the solution to be tested, as shown in the attached figure. Figure 3 As shown.

[0064] Step 7: Analyze the spectral image of the active and passive mode-locked laser corresponding to the standard solution to obtain the current wavelength offset of the active and passive mode-locked laser in the test solution; substitute the current wavelength offset of the active and passive mode-locked laser in the test solution into the fitting calibration curve of the pre-calibrated wavelength offset and solution temperature change to calculate the current temperature sensing result of the test solution.

[0065] Working principle and sensing method:

[0066] The temperature sensing system described in this invention operates as follows:

[0067] Pump light with a wavelength of 980nm and a power adjustable in the range of 0-999mW is emitted by the first pump laser source 1 and the second pump laser source 2, respectively; wherein, the pump light emitted by the first pump laser source 1 enters the first wavelength division multiplexer 4, and the pump light emitted by the second pump laser source 2 enters the second wavelength division multiplexer 6.

[0068] When the pump light enters the erbium-doped fiber 5 through the first wavelength division multiplexer 4 or the second wavelength division multiplexer 6, the energy carried by the incident pump light will excite the electrons in the gain medium to a higher energy level due to the absorption effect of the gain medium in the erbium-doped fiber 5. Then, based on the relaxation phenomenon, the electrons at the higher energy level will release energy and emit photons when they transition to the ground state, thereby generating an optical signal with a wavelength of 1550 nm. The optical signal with a wavelength of 1550 nm will enter the NALM loop through the first wavelength division multiplexer 4 or the second wavelength division multiplexer 6. At this time, the optical signal with a wavelength of 1550 nm and the pump light with a wavelength of 980 nm will be transmitted simultaneously in the first single-mode fiber 3.

[0069] It should be noted that during the transmission of the optical signal with a wavelength of 1550nm and the pump light with a wavelength of 980nm in the first single-mode fiber 3, the polarization in the NALM circuit is adjusted by the first polarization controller 7; at the same time, a stable electro-optic modulator 8 is used to add a stable electro-modulation signal to the NALM circuit to achieve stable generation of active mode-locked adjustable pulses in the laser cavity.

[0070] Next, when the optical signal with a wavelength of 1550nm and the pump light with a wavelength of 980nm are transmitted simultaneously in the first single-mode fiber 3, 50% of the optical signal transmitted in the first single-mode fiber 3 is coupled to the oscillating cavity circuit using the central optical coupler 9, and the remaining 50% of the optical signal is retained in the NALM circuit.

[0071] After 50% of the optical signal in the NALM circuit is coupled to the oscillating cavity circuit through the central coupler 9, the 50% optical signal is transmitted in the second single-mode fiber 10. When the optical signal in the second single-mode fiber 10 is transmitted to the D-type multimode fiber 11, multimode interference can occur in the D-type multimode fiber 11. Since the graded fiber in the D-type multimode fiber 11 adopts a side-polished D-type structure, the optical signal can also generate an evanescent field effect at the interface between the fiber evanescent field of the D-type multimode fiber 11 and the test solution or standard solution. The optical signal after multimode interference and evanescent field effect will re-enter the D-type multimode fiber 11 to continue propagating and undergo multimode interference. Among them, the change in the fiber evanescent field causes a change in the refractive index.

[0072] After isolation by the optical isolator 12 and passing through the oscillating cavity coupler 13, a stable active and passive mode-locked laser output is formed. The oscillating cavity coupler 13 can output 10% of the active and passive mode-locked laser and send it to the spectrometer. The spectrometer can then observe the spectral signal of the active and passive mode-locked laser to obtain the spectral image of the active and passive mode-locked laser, while the remaining 90% of the active and passive mode-locked laser will continue to circulate in the oscillating cavity circuit. The polarization in the oscillating cavity circuit is adjusted by the second polarization controller 14.

[0073] It should be noted that after the optical signal enters the oscillating cavity circuit from the central optical coupler, it passes through the D-type multimode fiber optic cable 11, the optical isolator 12, the oscillating cavity optical coupler 13, and the second single-mode fiber optic cable 10 in sequence, and then re-enters the NALM circuit through the central optical coupler 9.

[0074] It should be noted that during the measurement, the entire D-type multimode fiber 11 is immersed in the NaCl solution; the working principle of the fiber evanescent field in the D-type multimode fiber 10 to sense the refractive index of the solution is as follows.

[0075] When the light beam propagates in the fiber core and passes through the D-type region of the D-type multimode fiber 11, total internal reflection occurs at the core-cladding interface; however, a portion of the energy still penetrates into the cladding at an exponentially decaying rate, and this portion of energy is the evanescent wave; where the fiber is considered as a special centrosymmetric waveguide, therefore:

[0076]

[0077] Where d is the diameter of the fiber core; λ is the wavelength of the incident light; n co n is the refractive index of the fiber core; cl denoted as cladding refractive index; N is the effective refractive index of the cladding mode; m is a positive integer representing the order of the cladding mode; p is 0 in TE mode and 1 in TM mode.

[0078] As light propagates in the D-type region, the binding force on the fundamental mode gradually weakens as the fiber core becomes thinner; in particular, when the side-projection region is sufficiently thin... When the value is less than the cutoff value of 0.84, the three-layer structure of core-cladding-external medium can be simplified to a two-layer structure of cladding-external medium. Because the V value increases sharply in this structure, the fundamental mode couples to multiple cladding modes; therefore, multiple modes can propagate simultaneously in the optical fiber, and the effective refractive index n... ex <N<n cl Furthermore, the finer the side-throw region, the more approximate this simplification becomes. In this case, the cladding and core refractive indices are the refractive indices of the external medium and the original cladding, respectively. Preferably, both the first single-mode fiber and the second single-mode fiber described in this invention are Corning SMF-28; wherein, the core refractive index n of SMF-28... co =1.4681, the cladding refractive index n of SMF-28 cl =1.4628, refractive index n of the surrounding medium ex =1.33.

[0079] Therefore, the energy fraction of the cladding mode penetrating into the surrounding medium is:

[0080]

[0081]

[0082] α c1 =1-(n ex / n c1 ) 2 X = 1 - (N / n) cl ) 2

[0083] From this, we can obtain the relationship between the effective refractive index of the fiber internal mode and the evanescent field. When the beam passes through the D-type fiber region, the evanescent wave is enhanced, while the effective refractive index of the fiber internal mode decreases. This is due to the inherent sensing mechanism of the intermodal interference refractive index sensor. The evanescent wave generated by the lower-order mode has a greater intensity. When the energy of the evanescent wave generated by the D-type region increases by the same amount, it causes a greater change in the effective refractive index of the higher-order mode, thus achieving higher refractive index sensitivity.

[0084] The relationship between the refractive index of a liquid and temperature can be summarized by the following empirical formula:

[0085]

[0086] Where α is the temperature coefficient, α = 4 × 10 -4 .

[0087] As can be seen from the above formula, the refractive index measured at a certain temperature can be converted to the refractive index at 20℃. Generally speaking, the refractive index of a liquid is inversely proportional to the temperature. The higher the temperature, the more active the molecules are, and therefore the lower the density between molecules, and the lower the refractive index.

[0088] In this invention, by controlling the standard solution at a constant temperature, the refractive index of the standard solution changes when the temperature increases, thereby causing the center wavelength of the active and passive mode-locked laser to drift. The spectrometer is used to detect the spectral image of the active and passive mode-locked laser with the center wavelength drift. By analyzing the spectral image of the active and passive mode-locked laser and performing fitting calibration of the solution temperature and wavelength shift, a pre-calibrated fitting calibration curve of the wavelength shift and solution temperature change can be obtained.

[0089] The temperature sensor system and method described in this invention can achieve high-precision and wide-range temperature monitoring, and has the potential to perform real-time and stable monitoring of industrial production. The structure and principle are simple, easy to manufacture, low in cost, and easy to operate. At the same time, it can effectively avoid cross-sensitivity caused by measuring different parameters and improve the accuracy of measurement.

[0090] The above embodiments are merely one of the implementation methods for achieving the technical solution of the present invention. The scope of protection claimed by the present invention is not limited to this embodiment, but also includes any variations, substitutions and other implementation methods that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention.

Claims

1. A temperature sensing system, characterized in that, It includes an active and passive mode-locked fiber laser, a spectrometer, and a temperature characterization module; the output of the active and passive mode-locked fiber laser is connected to the input of the spectrometer, and the output of the spectrometer is connected to the input of the temperature characterization module. The active and passive mode-locked fiber laser has a D-type multimode fiber (11), which is fully immersed in the solution to be tested; wherein the beam transmitted in the D-type multimode fiber (11) can generate an evanescent field effect with the solution to be tested on the surface of the D-type multimode fiber (11). The spectrometer is used to receive the active and passive mode-locked laser output from the active and passive mode-locked fiber laser and obtain the spectral image of the active and passive mode-locked laser. The temperature characterization module is used to receive and analyze the spectral image of the active and passive mode-locked laser to obtain the current wavelength offset of the active and passive mode-locked laser; and to obtain the current temperature sensing result of the solution under test based on the pre-calibrated fitting calibration curve of the wavelength offset and the solution temperature change and the current wavelength offset of the active and passive mode-locked laser. The active and passive mode-locked fiber laser includes a pump laser source, a NALM circuit, a central optical coupler (9), and an oscillating cavity circuit. The output end of the pump laser source is connected to the input end of the NALM circuit; the NALM circuit is coupled to the oscillating cavity circuit through the central optical coupler (9) to form a figure-eight cavity laser; the output end of the oscillating cavity circuit is connected to the input end of the spectrometer; wherein, the D-type multimode fiber (11) is located in the oscillating cavity circuit; The NALM loop includes a first single-mode fiber (3), a first wavelength division multiplexer (4), an erbium-doped fiber (5), a second wavelength division multiplexer (6), a first polarization controller (7), and an active electro-optic modulator (8). Both ends of the first single-mode fiber (3) are connected to the central optical coupler (9) to form a first cavity; the first wavelength division multiplexer (4), the erbium-doped fiber (5), the second wavelength division multiplexer (6), the first polarization controller (7) and the active electro-optic modulator (8) are connected in series in the path of the first single-mode fiber (3); wherein, the coupling input end of the first wavelength division multiplexer (4) and the coupling input end of the second wavelength division multiplexer (6) are both connected to the output end of the pump laser source; The oscillating cavity circuit also includes a second single-mode fiber (10), an optical isolator (12), an oscillating cavity optical coupler (13), and a second polarization controller (14). Both ends of the second single-mode fiber (10) are connected to the central optical coupler (9) to form a second cavity; the D-type multimode fiber (11), the optical isolator (12), the oscillating cavity optical coupler (13) and the second polarization controller (14) are connected in series in the path of the second single-mode fiber (10); wherein, the output end of the oscillating cavity optical coupler (13) is connected to the input end of the spectrometer.

2. The temperature sensing system according to claim 1, characterized in that, The active electro-optic modulator (8) is a lithium niobate electro-optic modulator; wherein the operating bandwidth of the lithium niobate electro-optic modulator is 12.5 Gb / s.

3. The temperature sensing system according to claim 1, characterized in that, The splitting ratio of the central optical coupler (9) is 50:50, and the splitting ratio of the oscillating cavity optical coupler (13) is 90:

10.

4. The temperature sensing system according to claim 1, characterized in that, The center wavelength of the pump laser source is 980nm, and the power of the pump laser source can be adjusted within the range of 1-999mW.

5. A temperature sensing system according to claim 1, characterized in that, The D-type multimode fiber (11) adopts a GIMF-SIMF-GIMF structure; wherein, the GIMF-SIMF-GIMF structure is obtained by fusion splicing multimode step fiber, graded fiber and step fiber, and the graded fiber is a side-polished D-type structure.

6. A temperature sensing method, characterized in that, The temperature sensing system according to any one of claims 1-5 is used; wherein the temperature sensing method includes the following steps: The D-type multimode fiber (11) in the active and passive mode-locked fiber laser is fully immersed in the standard solution, and the standard solution is subjected to constant temperature control. When the temperature of the standard solution changes, the active and passive mode-locked laser output from the active and passive mode-locked fiber laser is received by a spectrometer to obtain the spectral image of the active and passive mode-locked laser corresponding to the standard solution. The spectral images of the active and passive mode-locked lasers corresponding to the standard solution are analyzed to obtain the wavelength shift of the active and passive mode-locked lasers at different temperatures of the standard solution. Based on the wavelength shift of the active and passive mode-locked lasers at different temperatures of the standard solution, the fitting calibration of the solution temperature and the wavelength shift is performed to obtain the pre-calibrated fitting calibration curve of the wavelength shift and the solution temperature change. Replace the standard solution with the test solution, and fully immerse the D-type multimode fiber (11) in the active and passive mode-locked fiber laser into the test solution; The active and passive mode-locked laser output from the active and passive mode-locked fiber laser is received again using a spectrometer, and the spectral image of the active and passive mode-locked laser corresponding to the solution to be tested is obtained. The spectral image of the active and passive mode-locked laser corresponding to the standard solution is analyzed to obtain the current wavelength shift of the active and passive mode-locked laser in the test solution. The current wavelength offset of the active and passive mode-locked laser in the test solution is substituted into the pre-calibrated fitting calibration curve of the wavelength offset and the solution temperature change to calculate the current temperature sensing result of the test solution.

7. A temperature sensing method according to claim 6, characterized in that, The standard solution has the same composition and concentration as the test solution; wherein, the temperature change of the standard solution is a known quantity.

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