Multi-longitudinal mode fiber laser weak current detection method based on magnetic rotation sensitive material

Abstract

The invention provides a multi-longitudinal-mode fiber laser weak current detection method based on a magnetic rotation sensitive material, and the method comprises the following steps: building a multi-longitudinal-mode fiber laser weak current detection system; a relationship between the magnetic field intensity and the current in the DC coil; determining the relationship between the intra-cavity loss and the Faraday rotation angle of the multi-longitudinal-mode fiber laser resonant cavity within the magnetic field range which does not reach the magnetic saturation intensity of the magneto-optical material; calculating a current value when the magnetic field reaches the magnetic saturation intensity value of the magneto-optical material; when the to-be-measured current value on the coil is between 0 and the critical current, calculating the to-be-measured current value; the magneto-optical sensor is arranged in the multi-longitudinal-mode fiber laser resonant cavity, and the corresponding relation between light intensity change and weak current is constructed through the matching relation between the magneto-optical sensor and the multi-longitudinal-mode fiber laser resonant cavity, so that small current change which is difficult to measure by a traditional sensor can be detected, and high-sensitivity measurement is realized.

Description

Technical Field

[0001] The present invention relates to the fields of fiber laser technology and sensing technology, and particularly to a method for detecting weak current of multi-longitudinal mode fiber laser based on magneto-optical rotation sensitive material. Background Art

[0002] In many application scenarios of industry, scientific research, and power systems, accurate measurement of small current is crucial. Traditional current sensors, typically represented by electromagnetic current transformers, have many drawbacks. Physically, they are large in size and heavy in weight, which limits their use in some occasions with strict requirements on equipment size and weight, such as aerospace and miniaturized electronic devices. In terms of performance, they are extremely vulnerable to electromagnetic interference, and it is difficult to guarantee the measurement accuracy in a complex electromagnetic environment, and large deviations may occur in the measurement results.

[0003] Optical fiber sensors have gradually become a research hotspot in the field of current sensing due to their advantages of anti-electromagnetic interference, high sensitivity, and long-distance transmission. However, existing optical fiber current sensors have prominent problems when measuring small current. On the one hand, the sensitivity is insufficient, and it is difficult to accurately detect the change of tiny current. On the other hand, the structure design is complex, which not only increases the manufacturing cost but also reduces the stability and reliability of the device. Summary of the Invention

[0004] Aiming at the problems in the prior art, the present invention provides a method for detecting weak current of multi-longitudinal mode fiber laser based on magneto-optical rotation sensitive material, aiming to achieve high-sensitivity measurement of small current.

[0005] The method for detecting weak current of multi-longitudinal mode fiber laser based on magneto-optical rotation sensitive material includes the following steps:

[0006] Step 1: Build a multi-longitudinal mode fiber laser weak current detection system, including a multi-longitudinal mode fiber laser resonator and a magneto-optical sensor. The magneto-optical sensor is arranged in the optical path at the end of the reflection end of the multi-longitudinal mode fiber laser resonator. The magneto-optical sensor includes a capillary glass tube and an optical fiber collimator. A magneto-optical material matching the inner hole of the capillary glass tube is placed in the capillary glass tube. One end of the optical fiber collimator is connected to the single-mode fiber at the end of the reflection end of the multi-longitudinal mode fiber laser resonator, and the other end is connected to the capillary glass tube. After aligning the optical fiber collimator, the capillary glass tube, and the mirror at the reflection end of the multi-longitudinal mode fiber laser resonator, glue and fix the optical fiber collimator, the capillary glass tube, and the multi-longitudinal mode fiber laser resonator with optical resin glue; wind a DC coil around the outer circumference of the capillary glass tube and make the DC coil correspond to the magneto-optical rotation sensitive material;

[0007] Step 2: The relationship between the magnetic field strength H and the current I in the DC coil is:

[0008]

[0009] where N is the number of turns of the DC coil winding, and l coil is the effective length of the DC coil;

[0010] Step 3: Determine the relationship between the intracavity loss α (θ) and the Faraday rotation angle θ within the magnetic field range where the magneto-optical material does not reach magnetic saturation intensity;

[0011] α (θ) = α0 - k1θ + k2θ 2

[0012] where α0 is the initial intracavity loss without magnetic field, and k1 and k2 are proportionality coefficients;

[0013] Step 4: Calculate the current value when the magnetic field reaches the magnetic saturation intensity value of the magneto-optical material, and use it as the critical current I cr , at this time, the intracavity loss drops to the lowest value α min ;

[0014]

[0015] where l is the path length of light propagating in the magneto-optical material (i.e., the length of the magneto-optical material); V is the Verdet constant;

[0016] Step 5: When the measured current value I on the coil is between 0 and the critical current I cr , the measured current value I is:

[0017]

[0018] where L is the cavity length of the multi-longitudinal-mode fiber laser resonator, g is the gain coefficient of the erbium-doped fiber, L EDF is the length of the erbium-doped fiber, I out and I in I in are respectively the output light intensity and the initial light intensity after light travels back and forth once in the multi-longitudinal-mode fiber laser resonator.

[0019] Furthermore: The multi-longitudinal-mode fiber laser resonator includes an erbium-doped fiber and a fiber Bragg grating. One end of the erbium-doped fiber is connected to a fiber collimator, the other end of the erbium-doped fiber is connected to one end of the fiber Bragg grating, and the other end of the fiber Bragg grating is respectively connected to a 976 nm pump light source and an optical power meter through a wavelength division multiplexer.

[0020] Furthermore: The erbium-doped fiber is a low-gain erbium-doped fiber with a length of 3 meters and a gain coefficient of 4 dB / m; the central wavelength of the fiber Bragg grating is 1550 nm.

[0021] Further, the length and diameter of the magneto-optical material are both less than 5 mm.

[0022] Further, the magneto-optical rotation ratio of the magneto-optical material is greater than 1000 ° / cm / T.

[0023] Further, the magneto-optical material is yttrium iron garnet.

[0024] Further, the magnetic saturation value of yttrium iron garnet is 45 mT.

[0025] The beneficial effects of the present invention: By arranging a magneto-optical inductor in a multi-longitudinal mode fiber laser resonator and through the cooperation relationship between the two, a corresponding relationship between the light intensity change and the weak current is constructed, so that the tiny current change that is difficult to measure by traditional sensors can be detected, and high-sensitivity measurement can be realized. Description of the Drawings

[0026] Figure 1 is a flowchart of the present invention;

[0027] Figure 2 is a diagram of a multi-longitudinal mode fiber laser weak current detection system;

[0028] Figure 3 is a relationship diagram of light intensity and current - simulation diagram;

[0029] Figure 4 is a non-linear regression analysis diagram of experimental results. Detailed Embodiments

[0030] The following will describe the present invention in detail with reference to the drawings. The embodiments of the present invention will be described in detail below. The examples of the embodiments are shown in the drawings, where the same or similar reference numerals represent the same or similar elements or elements with the same or similar functions throughout. The embodiments described below with reference to the drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention. The azimuth terms such as left, middle, right, up, and down in the embodiments of the present invention are only relative concepts to each other or are referenced based on the normal use state of the product, and should not be considered as restrictive.

[0031] The multi-longitudinal mode fiber laser weak current detection method based on magneto-optical rotation sensitive material, as shown in 1, includes the following steps:

[0032] Step 1: Build a multi-longitudinal mode fiber laser weak current detection system, including a multi-longitudinal mode fiber laser resonator and a magneto-optical inductor, as Figure 2As shown, the magneto-optical sensor is arranged in the optical path at the end of the reflection end of the multi-longitudinal-mode fiber laser resonator. The magneto-optical sensor includes a capillary glass tube 8 and an optical fiber collimator 6. A magneto-optical material 7 matching the inner hole of the capillary glass tube is placed in the capillary glass tube. The length and diameter of the magneto-optical material are both less than 5 mm. In this embodiment, the length is selected as 2 mm to reduce the loss when light passes through the magneto-optically sensitive material while ensuring the rotation angle of polarization. Its diameter is 2 mm, which matches the diameter of the optical fiber collimator. The magneto-optical rotation ratio of the magneto-optical material is greater than 1000° / cm / T. When the magneto-optical rotation ratio is relatively high, the magneto-optical material is more sensitive to the magnetic field, and thus the present invention can accurately measure weak current. Among them, the higher the order of magnitude of the magneto-optical rotation ratio, the smaller specifications of the magneto-optical material can be selected to meet the requirements. In this embodiment, the magneto-optical material is preferably yttrium iron garnet, and the magnetic saturation value of yttrium iron garnet is 45 mT. One end of the optical fiber collimator is connected to the single-mode optical fiber 4 at the end of the reflection end of the multi-longitudinal-mode fiber laser resonator, and the other end is connected to the capillary glass tube. After aligning the optical fiber collimator, the capillary glass tube and the mirror 9 at the reflection end of the multi-longitudinal-mode fiber laser resonator, they are glued and fixed through optical resin glue. A DC coil is wound around the outer circumference of the capillary glass tube and corresponds to the magneto-optically sensitive material. The multi-longitudinal-mode fiber laser resonator includes an erbium-doped fiber 5 and a fiber Bragg grating 3. One end of the erbium-doped fiber is connected to the optical fiber collimator, and the other end of the erbium-doped fiber is connected to one end of the fiber Bragg grating. The other end of the fiber Bragg grating is respectively connected to a 976 nm pump light source 1 and an optical power meter through a wavelength division multiplexer 2. The erbium-doped fiber is a low-gain erbium-doped fiber with a length of 3 meters and a gain coefficient of 4 dB / m. The center wavelength of the fiber Bragg grating is 1550 nm. The erbium-doped fiber and the fiber Bragg grating are fusion-spliced, and the output light intensity and the initial light intensity are measured by the optical power meter.

[0033] Step 2: The relationship between the magnetic field strength H and the current I in the DC coil is:

[0034]

[0035] Wherein, N is the number of turns of the DC coil winding, l coil is the effective length of the DC coil;

[0036] Step 3: In the magnetic field range of 0 - 45 mT, when linearly polarized light passes through a magneto - optical crystal in a magnetic field, its polarization plane will rotate. The rotation angle θ is related to the magnetic field strength and the magneto - optical properties of the crystal. The optical properties of the magneto - optical crystal, such as refractive index and absorption coefficient, will change under the action of the magnetic field, and these changes will affect the propagation of light in the crystal, including losses. In a multi - longitudinal - mode fiber laser resonator, the intracavity losses include absorption loss, scattering loss, coupling loss, etc., and these losses will affect the output power of the laser. Construct a function of the intracavity loss with respect to the Faraday rotation angle. To simplify the model, we use a quadratic polynomial form to determine the intracavity loss α (θ) of the multi - longitudinal - mode fiber laser resonator

[0037] α (θ) = α0 - k1θ + k2θ 2 (2)

[0038] where α0 is the initial intracavity loss without a magnetic field, representing the reference loss without the magneto - optical effect. k1 and k2 are proportionality coefficients; the linear term k1θ represents the linear influence of the Faraday rotation angle on the intracavity loss, resulting from the direct change of the refractive index or absorption coefficient of the magneto - optical crystal under the action of the magnetic field; the quadratic term k2θ 2 , represents the non - linear influence of the Faraday rotation angle on the intracavity loss, resulting from the complex interaction between the magneto - optical effect and other intracavity loss mechanisms (such as scattering, mode competition, etc.); According to the Faraday magneto - optical effect, when light passes through a magneto - optical material, the rotation angle of the light vector vibration plane can be expressed as: θ = VHl, and according to the relationship between the magnetic field strength H and the current I in the DC coil, we can get: Then, formula (2) can be transformed into:

[0039]

[0040] The output light intensity I out after light travels back and forth once in the resonator in and the initial light intensity I

[0041]

[0042] Step 4: Calculate the current value when the magnetic field reaches 45 mT and use it as the critical current I cr , at this time, the light intensity reaches saturation and no longer increases, and the intracavity loss drops to the minimum value α min ;

[0043]

[0044] where l is the path length of light propagation in the magneto - optical material, that is, the length of the magneto - optical material; V is the Verdet constant;

[0045] Step 5: When the measured current value I on the coil is between 0 and the critical current I cr , that is, when (0 < I < I cr ), the output light intensity I out increases exponentially with the current I; when I ≥ I cr , the output light intensity is constant, and the output light intensity at this time is:

[0046]

[0047] Therefore, when 0 < I < I cr , by measuring the output light intensity I out , the current value I flowing through the DC coil can be measured, and the measured current value I is: IμW out

[0048]

[0049] where L is the cavity length of the multi-longitudinal-mode fiber laser resonator, I out and I in I in are the output light intensity and the initial light intensity after light travels back and forth once in the multi-longitudinal-mode fiber laser resonator respectively; the total gain G of the erbium-doped fiber can be expressed as:

[0050] G = g × L EDF

[0051] where: g is the gain coefficient of the erbium-doped fiber, and L EDF is the length of the erbium-doped fiber; then, the total gain factor G lin of light traveling back and forth once in the resonator can be expressed as:

[0052] Experimental tests

[0053] 1. Build a test platform: Connect the multi-longitudinal-mode fiber laser weak current detection system to the optical power meter 10 to measure its output light intensity; at the same time, connect the DC coil to the adjustable DC current source to provide DC currents of different magnitudes.

[0054] 2. Weak current simulation test: According to the derivation process of the formula, the relationship between the output light intensity and the current flowing through the coil was simulated and calculated when the critical current was not reached. The results are as Figure 3 shown. The circles represent the initial light intensity; the squares represent the light intensity when the critical current is reached; when the critical current is not reached, the relationship between the light intensity and the current follows a quadratic exponential curve law.

[0055] 3. Weak current experimental test: Gradually increase the DC current from zero, and record the corresponding magnetic field strength and the output optical intensity of the laser at different current values. It is found in the experiment that when the current is small (0 A - 1.8 A), the corresponding magnetic field strength is in the range of 0 - 45 mT. The optical intensity of the laser continuously increases with the increase of the current, and a quadratic exponential curve model is used to perform nonlinear regression analysis on the experimental results. As Figure 4 shown, the dots in the figure are the data of the output optical intensity corresponding to different currents measured actually. Nonlinear fitting is performed on these data, and the curve relationship is consistent with the simulation results. The magneto-optical material used in the experiment is a ferromagnetic material. When the magnetic field strength exceeds 45 mT, it reaches magnetic saturation. At this time, the magneto-optical effect also tends to be stable and no longer increases significantly with the increase of the magnetic field strength. That is, the curve is the result of quadratic exponential regression analysis, and the coefficient of determination is 0.99872. When the current continues to increase and the magnetic field strength exceeds 45 mT, the optical intensity remains unchanged.

[0056] The present invention has the following performances compared with the prior art:

[0057] 1. High-sensitivity weak current measurement: Utilize the magneto-optical effect of the magneto-optical material with a high magnetic rotation ratio to convert the weak magnetic field change generated by a small current into the change of the optical intensity of the laser; then by detecting the change of the optical intensity, small currents can be accurately measured, and tiny current changes that are difficult to measure by traditional sensors can be detected, realizing high-sensitivity measurement;

[0058] 2. High-resolution measurement: In the range from 0 to the critical current, the optical intensity and the current present a complex curve relationship including a first-order exponential term and a second-order exponential term; this relationship enables the optical intensity to have obvious and distinguishable changes even when the current changes slightly, thus enabling high-precision measurement over a wide range; in the low-current section, the change of the optical intensity is relatively gentle but still measurable, and tiny currents can be accurately captured; when approaching the critical current, the change of the optical intensity speeds up, and it can also accurately reflect the change of a larger current, meeting the requirements of different application scenarios for the current measurement resolution; meanwhile, the critical current value can be freely determined by changing the number of turns of the coil;

[0059] 3. Saturation characteristic for current warning: When it is detected that the optical intensity no longer changes with the increase of the current and is in a saturated state, it can be judged that the current has reached or exceeded a certain threshold; in the power system, this can timely remind the staff that the equipment may be in an overloaded state, helping to take measures in advance to avoid equipment damage and accidents;

[0060] 4. Multi-parameter correlation provides more information: The relationship between optical intensity and current is not isolated. It is correlated with various physical parameters in the cavity (such as the erbium-doped fiber gain coefficient, intracavity loss, Faraday rotation angle, etc.). By analyzing the change in the relationship between optical intensity and current, the change in intracavity loss can be detected in a timely manner. This change can reflect the aging or contamination of the optical fiber, which is of great significance for evaluating the working state of the fiber laser, diagnosing potential faults, and optimizing system performance.

[0061] The above shows and describes the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. The above embodiments and the descriptions in the specification only illustrate the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed. The scope of the present invention claimed is defined by the appended claims and their equivalents.

Claims

1. A method for detecting weak current of multi-longitudinal-mode fiber laser based on magneto-optical sensitive material, characterized in that, Including the following steps: Step 1: Build a multi-longitudinal-mode fiber laser weak current detection system, including a multi-longitudinal-mode fiber laser resonator and a magneto-optical sensor. The magneto-optical sensor is arranged in the optical path at the end of the reflection end of the multi-longitudinal-mode fiber laser resonator. The magneto-optical sensor includes a capillary glass tube and an optical fiber collimator. A magneto-optical material matching the inner hole of the capillary glass tube is placed in the capillary glass tube. One end of the optical fiber collimator is connected to the single-mode fiber at the end of the reflection end of the multi-longitudinal-mode fiber laser resonator, and the other end is connected to the capillary glass tube. After aligning the optical fiber collimator, the capillary glass tube and the mirror at the reflection end of the multi-longitudinal-mode fiber laser resonator, the optical fiber collimator, the capillary glass tube and the multi-longitudinal-mode fiber laser resonator are glued and fixed with optical resin glue; a DC coil is wound around the outer circumference of the capillary glass tube and the DC coil corresponds to the magneto-rotation sensitive material; Step 2: The relationship between the magnetic field strength H and the current I in the DC coil is: where N is the number of turns of the DC coil winding, and l coil is the effective length of the DC coil; Step 3: Determine the relationship between the intracavity loss α of the multi-longitudinal-mode fiber laser resonator and the Faraday rotation angle θ within the magnetic field range where the magnetic saturation intensity of the magneto-optical material is not reached. (θ) and the Faraday rotation angle θ; α (θ) = α0 - k1θ + k2θ 2 where α0 is the initial intracavity loss without magnetic field, and k1 and k2 are proportionality coefficients; Step 4: Calculate the current value when the magnetic field reaches the magnetic saturation intensity value of the magneto-optical material, and use it as the critical current I cr , at this time, the intracavity loss drops to the minimum value α min ; where l is the path length of light propagating in the magneto-optical material (i.e., the length of the magneto-optical material); V is the Verdet constant; Step 5: When the measured current value I on the coil is between 0 and the critical current I cr , the measured current value I is: Among them, L is the cavity length of the multi-longitudinal-mode fiber laser resonator, g is the gain coefficient of the erbium-doped fiber, and L EDF is the length of the erbium-doped fiber, and I out and I in I in are the output optical intensity and the initial optical intensity after the light travels back and forth once in the multi-longitudinal-mode fiber laser resonator, respectively.

2. The multi-longitudinal-mode fiber laser weak current detection method based on a magneto-optical sensitive material according to claim 1, wherein: The multi-longitudinal-mode fiber laser resonator includes an erbium-doped fiber and a fiber Bragg grating. One end of the erbium-doped fiber is connected to the optical fiber collimator, the other end of the erbium-doped fiber is connected to one end of the fiber Bragg grating, and the other end of the fiber Bragg grating is connected to a 976nm pump light source and an optical power meter respectively through a wavelength division multiplexer.

3. The multi-longitudinal-mode fiber laser weak current detection method based on a magneto-optical sensitive material according to claim 2, characterized in that: The erbium-doped fiber is a low-gain erbium-doped fiber with a length of 3 meters and a gain coefficient of 4dB / m; the center wavelength of the fiber Bragg grating is 1550nm.

4. The multi-longitudinal-mode fiber laser weak current detection method based on magneto-optical sensitive material according to claim 1, characterized in that: The length and diameter of the magneto-optical material are both less than 5mm.

5. The multi-longitudinal-mode fiber laser weak current detection method based on a magneto-optical sensitive material according to claim 4, wherein: The magneto-rotation ratio of the magneto-optical material is greater than 1000° / cm / T.

6. The method for detecting weak current of multi-longitudinal mode fiber laser based on magneto-optical sensitive material according to claim 5, wherein: The magneto-optical material is yttrium iron garnet.

7. The method for detecting weak current of multi-longitudinal-mode fiber laser based on magneto-optical sensitive material according to claim 6, characterized in that: The magnetic saturation value of yttrium iron garnet is 45mT.

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