A closed loop control direct current fiber optic current sensor

Abstract

The application discloses a closed-loop control direct-current fiber current sensor, which comprises a grating light valve type fiber direct-current current sensor and a polarization interference type fiber temperature sensor; the polarization interference type fiber temperature sensor and the sensing head of the grating light valve type fiber direct-current current sensor are integrated in the same shell; the temperature at the primary sensing head of the grating light valve type fiber direct-current current sensor is measured through the polarization interference type fiber temperature sensor; error correction is carried out according to the measured temperature information; and the influence of the environmental temperature change on the measurement accuracy of the grating light valve type fiber direct-current current sensor is corrected. Meanwhile, the micro-power light control signal source technology is adopted to realize the closed-loop light feedback control of the grating light valve type fiber direct-current current sensor, the measurement linearity of the grating light valve type fiber direct-current current sensor is improved in the closed-loop demodulation algorithm, and the dynamic measurement range is increased.

Description

Technical Field

[0001] This invention relates to an optical fiber current sensor. Background Technology

[0002] In power generation, transmission, and the operation of power equipment, various physical quantities need to be monitored, and current is one of the most important parameters. Currently, the traditional electromagnetic current transformers widely used in power systems suffer from a series of problems, including magnetic saturation, high electromagnetic interference, small dynamic range, narrow bandwidth, and flammability / explosiveness. Furthermore, as the operating voltage level of the power grid increases, the design of the insulation structure of traditional transformers becomes extremely complex, leading to a sharp increase in cost and size.

[0003] With the in-depth development of substation automation technology, a new type of intelligent primary equipment has emerged, which integrates and merges primary and secondary equipment. Replacing traditional electromagnetic instrument transformers with electronic instrument transformers, transmitting digital signals via optical fibers, and integrating relay protection and measurement and control functions locally, combined with circuit breakers or fully enclosed combined electrical appliances, forms a new generation of intelligent primary equipment. This enhances the reliability and flexibility of real-time monitoring and control of substation automation systems, reduces construction and operation investment, and facilitates maintenance.

[0004] Optical current transformers (OCTs) use optical media as the medium for current sensing and / or signal transmission, overcoming the shortcomings of traditional current transformers. They offer advantages such as good insulation performance, small size, no magnetic saturation or ferroresonance, wide frequency response range, and good electromagnetic compatibility. Furthermore, they can seamlessly integrate with existing communication networks, aligning with the development trend of networked and intelligent power systems.

[0005] Traditional optical current transformers (OCTs) consist of Faraday magnetic field sensing elements, which can be categorized into all-fiber current transformers and glass-block fiber current transformers based on the type of sensing element. All-fiber current transformers are costly, while glass-block fiber current transformers have lower reliability in engineering applications. Furthermore, because the polarization state of light waves is highly susceptible to environmental interference, Faraday-effect-based OCTs face significant technical challenges and complexities in resisting environmental interference. Therefore, researching novel, highly reliable, and low-cost OCTs has become a key issue that needs to be addressed. Summary of the Invention

[0006] Purpose of the invention: In view of the above-mentioned prior art, a closed-loop control DC fiber optic current sensor is proposed, which can eliminate the influence of ambient temperature changes on the measurement accuracy of the fiber optic DC current sensor, and make the measurement have high linearity and a large dynamic measurement range.

[0007] Technical Solution: A closed-loop controlled DC fiber optic current sensor includes a grating optical valve type fiber optic DC current sensor and a polarization interferometric fiber optic temperature sensor. The sensing heads of the polarization interferometric fiber optic temperature sensor and the grating optical valve type fiber optic DC current sensor are integrated in the same housing. The temperature at the sensing head of the grating optical valve type fiber optic DC current sensor is measured by the polarization interferometric fiber optic temperature sensor. The grating optical valve type fiber optic DC current sensor employs a closed-loop demodulation algorithm, and the detected current is obtained by compensating for the phase shift of the demodulated output. In the closed-loop demodulation algorithm, error correction is performed based on the measured temperature information to correct the influence of ambient temperature changes on the measurement accuracy of the grating optical valve type fiber optic DC current sensor.

[0008] Furthermore, in the closed-loop demodulation algorithm, the micro-power laser outputs a laser signal composed of a square wave signal and a stepped wave signal under the control of the signal processor. The laser signal is transmitted through a single-mode fiber to a photodiode and converted into an electrical signal. The electrical signal is amplified by a transformer and then applied to a grating-type optical valve fiber DC current sensor. The square wave signal corresponds to a non-reciprocal π / 2 modulation phase shift, and the stepped wave signal corresponds to the compensation phase shift.

[0009] Furthermore, the current sensing head of the grating-type optical valve fiber DC current sensor includes a first polarization-maintaining fiber collimator, a second polarization-maintaining fiber collimator, a sensing optical valve, a control optical valve, and a substrate; the first polarization-maintaining fiber collimator is disposed opposite to the sensing optical valve, and the second polarization-maintaining fiber collimator is disposed opposite to the control optical valve. The surfaces of the sensing optical valve and the control optical valve relative to the polarization-maintaining fiber collimator are coated with reflective films, which serve as electrodes. An air gap is provided between the sensing optical valve and the control optical valve and the substrate; the sensing optical valve is used to load the bus current signal to be measured, and the control optical valve is used to load the modulation phase shift and the compensation phase shift.

[0010] Furthermore, the sensing light valve and the control light valve are made of silicon nitride material, with a width of 1-10 μm, a length of 100-1000 μm, and a thickness of 200-300 nm.

[0011] Furthermore, the reflective film is made of aluminum or a dielectric material with a reflectivity greater than 92%.

[0012] Furthermore, the grating-type optical valve fiber DC current sensor includes a broadband light source, a depolarizer, a 1×3 beam splitter, a signal processor, and also includes a first circulator, a first polarizer, a Faraday rotator, a polarization beam splitter, and a current sensing head.

[0013] The output of the broadband light source is connected to the input of the depolarizer. The output of the depolarizer is connected to the input of the 1×3 beam splitter. The first output of the 1×3 beam splitter is connected to the first end of the first circulator. The second end of the first circulator is connected to the input of the first polarizer. The output of the first polarizer is connected to the input of the Faraday rotator. The output of the Faraday rotator is connected to the input of the polarization beam splitter. The two outputs of the polarization beam splitter are respectively connected to the first polarization-maintaining fiber collimator and the second polarization-maintaining fiber collimator of the current sensing head.

[0014] The third end of the first circulator is connected to the first input end of the signal processor, the first output end of the signal processor is connected to the input end of the low-power laser, the output end of the low-power laser is connected to the input end of the photodiode, the output end of the photodiode is connected to the input end of the transformer, and the output end of the transformer is connected to the control light valve of the current sensor head.

[0015] Furthermore, the polarization interference fiber optic temperature sensor shares the broadband light source, depolarizer, 1×3 beam splitter, and signal processor of the grating optical valve type fiber optic DC current sensor, and also includes a second circulator and a fiber optic temperature sensing probe; the third output end of the 1×3 beam splitter is connected to the first end of the second circulator, the second end of the second circulator is connected to the fiber optic temperature sensing probe, and the third end of the second circulator is connected to the third input end of the signal processor.

[0016] Furthermore, the grating-type optical valve fiber DC current sensor also includes a wavelength demodulation module. The input end of the wavelength demodulation module is connected to the second output end of the 1×3 beam splitter, and the output end of the wavelength demodulation module is connected to the second input end of the signal processor. The second output end of the signal processor is connected to the feedback control end of the broadband light source. The signal processor outputs a wavelength feedback control signal to the broadband light source to stabilize the output wavelength of the broadband light source.

[0017] Beneficial Effects: This invention provides a closed-loop controlled DC fiber optic current sensor. A polarization interferometric fiber optic temperature sensor and a grating optical valve type fiber optic DC current sensor are integrated into the same housing. The polarization interferometric fiber optic temperature sensor measures the temperature at the primary sensing head of the grating optical valve type fiber optic DC current sensor. Error correction is performed based on the measured temperature information to mitigate the impact of ambient temperature changes on the measurement accuracy of the grating optical valve type fiber optic DC current sensor. Simultaneously, low-power optical control signal source technology is used to achieve closed-loop optical feedback control of the grating optical valve type fiber optic DC current sensor. This improves the measurement linearity of the grating optical valve type fiber optic DC current sensor and increases the dynamic measurement range in the closed-loop demodulation algorithm.

[0018] A wavelength demodulation module is used to monitor the light source wavelength of the grating optical valve type fiber optic DC current sensor in real time, and the driving current of the light source is controlled in real time according to the wavelength fluctuation to maintain the stability of the light source output wavelength, thereby further improving the measurement accuracy of the grating optical valve type fiber optic DC current sensor.

[0019] The polarization interferometric fiber optic temperature sensor and the grating optical valve type fiber optic DC current sensor share a broadband light source, depolarizer, 1×3 beam splitter and signal processor, which can reduce system cost and improve system integration capability, thereby reducing cost. Attached Figure Description

[0020] Figure 1 This is a structural diagram of the closed-loop control DC fiber optic current sensor of the present invention;

[0021] Figure 2 This is a structural diagram of the sensing head of the grating-type optical valve fiber optic DC current sensor in this invention;

[0022] Figure 3 This is a structural diagram of the sensing head of the polarization interference fiber optic temperature sensor of the present invention;

[0023] Figure 4 This is a structural diagram of the wavelength demodulation module of the present invention;

[0024] Figure 5 This is a schematic diagram of the closed-loop optical feedback control principle based on a low-power optical control signal source in this invention. Detailed Implementation

[0025] The invention will now be further explained with reference to the accompanying drawings.

[0026] A closed-loop controlled DC fiber optic current sensor includes a grating-type optical valve fiber optic DC current sensor and a polarization interferometric fiber optic temperature sensor.

[0027] like Figure 2 As shown, the current sensing head 10 of the grating optical valve type fiber optic DC current sensor includes a first polarization-maintaining fiber collimator 101, a second polarization-maintaining fiber collimator 102, a sensing optical valve 103, a control optical valve 104, and a substrate 105. The first polarization-maintaining fiber collimator 101 is positioned opposite to the sensing optical valve 103, and the second polarization-maintaining fiber collimator 102 is positioned opposite to the control optical valve 104. Reflective films 106 are deposited on the surfaces of the sensing optical valve 103 and the control optical valve 104 relative to the polarization-maintaining fiber collimators. These reflective films 106 serve as electrodes for the two optical valves. A 1μm thick air gap is provided between the sensing optical valve 103 and the control optical valve 104 and the substrate 105. The sensing optical valve 103 is used to load the bus current signal to be measured, and the control optical valve 104 is used to load the modulation phase shift and compensate for the phase shift.

[0028] The substrate 105 is made of semiconductor Si and electromagnetic materials, while the sensing light valve 103 and the control light valve 104 are made of silicon nitride material, with a width of 1-10 μm, a length of 100-1000 μm, and a thickness of 200-300 nm. The reflective film 106 is made of aluminum or a dielectric material with a reflectivity greater than 92%.

[0029] like Figure 1 As shown, the grating-type optical valve fiber DC current sensor includes a broadband light source 1, a depolarizer 2, a 1×3 beam splitter 3, and a signal processor 8. It also includes a first circulator 4, a first polarizer 5, a Faraday rotator 6, a polarization beam splitter 7, a current sensing head 10, a low-power laser 15, a photodiode 16, and a transformer 17. The output terminal 11 of the broadband light source 1 is connected to the input terminal of the depolarizer 2. The output terminal of the depolarizer 2 is connected to the input terminal of the 1×3 beam splitter 3. The first output terminal 31 of the 1×3 beam splitter 3 is connected to the first terminal 41 of the first circulator 4. The second terminal 42 of the first circulator 4 is connected to the input terminal of the first polarizer 5. The output terminal of the first polarizer 5 is connected to the input terminal of the Faraday rotator 6. The output terminal of the Faraday rotator 6 is connected to the input terminal of the polarization beam splitter 7. The two output terminals of the polarization beam splitter 7 are respectively connected to the first polarization-maintaining fiber collimator 101 and the second polarization-maintaining fiber collimator 102 of the current sensing head 10. The third end 43 of the first circulator 4 is connected to the first input end 81 of the signal processor 8. The first output end of the signal processor 8 is connected to the input end of the low-power laser 15. The output end of the low-power laser 15 is connected to the input end of the photodiode 16. The output end of the photodiode 16 is connected to the input end of the transformer 17. The output end of the transformer 17 is connected to the control light valve 104 of the current sensing head 10.

[0030] The polarization interferometric fiber optic temperature sensor shares the broadband light source 1, depolarizer 2, 1×3 beam splitter 3, and signal processor 8 of the grating-type fiber optic DC current sensor. The polarization interferometric fiber optic temperature sensor also includes a second circulator 9 and a fiber optic temperature sensing probe 14. The third output terminal 33 of the 1×3 beam splitter 3 is connected to the first terminal of the second circulator 9, the second terminal 92 of the second circulator 9 is connected to the fiber optic temperature sensing probe 14, and the third terminal 93 of the second circulator 9 is connected to the third input terminal 83 of the signal processor 8.

[0031] In the grating-type fiber optic DC current sensor of the present invention, the light emitted by the broadband light source 1 is linearly polarized after passing through the first polarizer 5. The main axis of the first polarizer 5 and the polarization-maintaining fiber connected to its output end are fused at 45°. Therefore, the output linearly polarized light simultaneously excites two orthogonal linearly polarized beams with equal amplitude in the polarization-maintaining fiber connected to the output end of the first polarizer 5, and they propagate along the fast and slow axes of the polarization-maintaining fiber. The two orthogonal linearly polarized beams are rotated 45° by the Faraday rotator 6 and then enter the polarization beam splitter 7. The polarization beam splitter 7 separates the two orthogonal linearly polarized beams and they respectively enter the first current sensing head 10. Two orthogonal linearly polarized beams are reflected at the reflective films 106 of the sensing optical valve 103 and the control optical valve 104, respectively, by the polarization-maintaining fiber collimator 101 and the second polarization-maintaining fiber collimator 102. The reflected light then passes through the first and second polarization-maintaining fiber collimators 101 and 102, the polarization beam splitter 7, and the Faraday rotator 6. The orthogonal linearly polarized light is rotated by 90°, meaning that the incident light signal along the fast / slow axis of the polarization-maintaining fiber is reflected along the slow / fast axis of the polarization-maintaining fiber. Therefore, the two reflected interference beams exhibit equal-path interference.

[0032] The DC current to be measured is shunted by the shunt 12 connected in series with the DC current bus 11, and then converted into a voltage signal by the sampling resistor 13. This voltage signal is applied to the sensing light valve 103. Under the action of the electrostatic force generated by the voltage, the sensing light valve 103 deforms. At this time, the distance between the sensing light valve 103 and the substrate 105 and the first polarization-maintaining fiber collimator 101 changes, and a relative displacement occurs between it and the control light valve 104. Therefore, when the light signal incident on the sensing light valve 103 is reflected from the reflective film 106, its optical path changes with the voltage, that is, with the magnitude of the DC current to be measured.

[0033] Specifically, after the two beams of polarized light reflected from the sensing light valve 103 and the control light valve 104 interfere, they are sent to the optical signal processor 8 after passing through the first circulator 4. The received optical wave interference signal is:

[0034]

[0035] Where A is the attenuation coefficient, P0 is the input light intensity, λ is the incident light wavelength, and Δy = y1 - y2 is the displacement difference between the sensing light valve 103 and the control light valve 104. Here, y1 is the displacement of the sensing light valve 103 under the action of the voltage to be measured, which is linearly related to the voltage to be measured within the elastic deformation range; y2 is the displacement of the control light valve 104, the magnitude of which is related to the feedback control voltage applied to the control light valve 104. Therefore, by detecting the magnitude of the interference signal intensity, the position of the sensing light valve 103 can be deduced, and thus the magnitude of the DC current to be measured can be inferred.

[0036] For the grating optical valve, the difference in the thermal expansion coefficients of the aluminum reflective film and the silicon substrate on the surfaces of the sensing optical valve 103 and the control optical valve 104 will cause the optical loss coefficient and optical path difference of the grating optical valve to change with temperature, thereby introducing errors into the optical interference signal received by the photoelectric receiver. Using δ to represent the equivalent error coefficient, the optical interference signal received by the photoelectric receiver is:

[0037]

[0038] As can be seen from equation (3), the interference output is a cosine function, which has defects such as low response sensitivity near zero phase difference, limited measurement range, and the interference result not reflecting the directionality of the input current. In order to solve the problems of cosine sensitivity and directionality, improve the linearity of the detection system, and increase the dynamic range, this invention adds a closed-loop optical feedback control structure based on a micro-power optical control signal source to the closed-loop demodulation algorithm.

[0039] Specifically, under the control of the signal processor 8, the low-power laser 15 outputs a laser signal composed of a square wave modulation signal and a stepped-wave feedback signal. This output laser signal is transmitted to the photodiode 16 via a single-mode optical fiber. The photodiode 16 converts the light energy into a proportional electrical signal, which is also composed of the square wave modulation signal and the stepped-wave signal. This electrical signal is amplified by the transformer 17 and then acts on the control light valve 104, causing it to generate a displacement y2. y2 is generated by the combined action of the square wave modulation signal and the stepped-wave signal, corresponding to the modulation phase shift φ of the system. s and compensation phase shift φ l Therefore, equation (3) can be transformed into:

[0040]

[0041] Control the amplitude of the square wave modulation signal so that φ s =±π / 2, then:

[0042]

[0043] After digital demodulation, the outputs of the positive and negative half-cycle photodetectors of the square wave are subtracted, then:

[0044]

[0045] According to the closed-loop demodulation algorithm, the stepped wave generates a compensating phase shift φ. l , making P out =0, then:

[0046]

[0047] In the closed-loop case, φ l and Since the value is small, it satisfies the small angle approximation condition, and we can obtain:

[0048]

[0049] so,

[0050]

[0051] like Figure 5 As shown, at this time, the compensated phase shift φ l This represents both the step height of the stepped wave signal and the demodulated output. It can be seen that the demodulated output is related to the error coefficient δ, necessitating error correction measures.

[0052] In this invention, the sensing heads of a polarization interferometric fiber optic temperature sensor and a grating optical valve type fiber optic DC current sensor are integrated into the same housing. The temperature at the sensing head of the grating optical valve type fiber optic DC current sensor is measured by the polarization interferometric fiber optic temperature sensor. In the closed-loop demodulation algorithm, error correction is performed based on the measured temperature information to correct the influence of ambient temperature changes on the measurement accuracy of the grating optical valve type fiber optic DC current sensor, i.e., to eliminate the error coefficient δ.

[0053] First, polarization interferometric fiber optic temperature sensors are based on the temperature birefringence effect of polarization-maintaining fibers, such as... Figure 3 As shown, the light emitted from the broadband light source 1 becomes linearly polarized after passing through the single-mode fiber collimator 141 and the polarizing prism 142, and then enters the polarization-maintaining fiber 143. The reflective film 145 on the polarization-maintaining fiber collimator 144 reflects the incident light. When the ambient temperature changes, the temperature birefringence effect alters the propagation constant difference between the two eigenmodes in the polarization-maintaining fiber 143, causing the phase difference between the eigenmodes to change with temperature. The signal processor 8 detects the energy change of the interference field caused by the phase difference, thereby obtaining the temperature change information.

[0054] The output light intensity P of the interference light incident on signal processor 8 T for:

[0055]

[0056] Where, δ x and δ y To reduce the phase delay of the fast and slow axes of polarization-maintaining fiber 143; P o This is the input light intensity.

[0057] The phase difference between linearly polarized light propagating along the fast and slow axes of polarization-maintaining fiber 143 is:

[0058] δ x -δ y =L(β) x -β y )=LΔβ (11)

[0059] Where L is the length of polarization-maintaining fiber 133; Δβ is the difference in propagation constants between the fast and slow axes of polarization-maintaining fiber 143, and Δβ is linearly related to temperature within the range of -200℃ to 400℃. Due to temperature changes, the difference in propagation constants between the fast and slow axes changes. This difference is then reflected by the output light intensity P. T The detection can obtain information on temperature changes.

[0060] The sensing heads of the polarization interferometric fiber optic temperature sensor and the grating optical valve type fiber optic DC current sensor are integrated in the same housing. The temperature sensor obtains the temperature information of the current sensing head in the same temperature field. Based on the temperature information and the variation law of the system output with temperature, the signal processor 8 generates an error correction coefficient (1-δ), and multiplies it by equation (9) to obtain:

[0061]

[0062] In this invention, the sensing head of the grating-type optical valve fiber optic DC current sensor is placed in an environmental test chamber beforehand to obtain the system output variation with temperature. The error correction coefficient (1-δ) can be determined based on the pre-obtained variation pattern. It can be seen that the corrected demodulated output is independent of temperature, thus achieving temperature compensation for the current sensor.

[0063] Furthermore, when a continuous driving current is applied to the broadband light source 1, the temperature of the active region of the broadband light source 1 will rise due to internal thermal effects, causing the output wavelength of the broadband light source 1 to drift. Therefore, in this invention, a wavelength feedback control signal is output to the broadband light source 1 through the signal processor 8 to stabilize the output wavelength of the broadband light source 1. Specifically, the grating-type optical valve fiber DC current sensor also includes a wavelength demodulation module 18. The input terminal of the wavelength demodulation module 18 is connected to the second output terminal 32 of the 1×3 beam splitter 3, the output terminal of the wavelength demodulation module 18 is connected to the second input terminal 82 of the signal processor 8, and the second output terminal of the signal processor 8 is connected to the feedback control terminal 12 of the broadband light source 1.

[0064] like Figure 4 As shown, the wavelength demodulation module 18 consists of an optical splitter 181, an edge filter 182, a photodetector 183, and a photodetector 184. After the output light from one source enters the wavelength demodulation module 18, it is split into two paths by the optical splitter 181. One path passes through the edge filter 182 and then enters the photodetector 183, where the optical power signal received by the photodetector 183 is I(λ). The other path directly enters the photodetector 184, where the optical power signal received by the photodetector 184 is I0. During demodulation by the edge filter 182, in the linear variation region between wavelength and output light intensity, different wavelengths correspond to different output light intensities, i.e.:

[0065] I(λ)=I0H(λ) (13)

[0066] Where H(λ) is the wavelength-dependent spectral transmittance of the edge filter, which varies with different wavelengths. To reduce the impact of changes in the optical power of the input edge filter 182 on the wavelength detection accuracy, the signals received by photodetectors 183 and 184 are divided, yielding:

[0067] V=I0H(λ) / I0=H(λ) (14)

[0068] Obviously, V is only related to the wavelength that changes with temperature, and is not related to the fluctuation of the output power of the light source. The wavelength change information can be calculated based on the V value, and a feedback control signal can be generated. The feedback control signal is sent to the broadband light source 1 through port 85 to adjust the driving current of the broadband light source 1 in real time, so as to ensure the stability of the output wavelength of the broadband light source 1.

[0069] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A closed loop controlled DC fiber optic current sensor characterized by, The application relates to a grating light valve type optical fiber direct current sensor and a polarization interference type optical fiber temperature sensor; the polarization interference type optical fiber temperature sensor and the sensing head of the grating light valve type optical fiber direct current sensor are integrated in a same shell, the temperature of the sensing head of the grating light valve type optical fiber direct current sensor is measured through the polarization interference type optical fiber temperature sensor; the grating light valve type optical fiber direct current sensor adopts a closed loop demodulation algorithm, and a compensation phase shift of a demodulation output is used to obtain a detected current; in the closed loop demodulation algorithm, error correction is carried out according to the measured temperature information, so as to correct the influence of environmental temperature change on the measurement accuracy of the grating light valve type optical fiber direct current sensor. The current sensing head (10) of the grating light valve type optical fiber direct current sensor comprises a first polarization maintaining optical fiber collimator (101), a second polarization maintaining optical fiber collimator (102), a sensing light valve (103), a control light valve (104) and a substrate (105); the first polarization maintaining optical fiber collimator (101) is arranged opposite to the sensing light valve (103), the second polarization maintaining optical fiber collimator (102) is arranged opposite to the control light valve (104), the surfaces of the sensing light valve (103) and the control light valve (104) are plated with a reflecting film (106) opposite to the polarization maintaining optical fiber collimator, the reflecting film (106) serves as an electrode, and an air gap is arranged between the sensing light valve (103) and the control light valve (104) and the substrate (105); the sensing light valve (103) is used for loading a bus current signal to be measured, and the control light valve (104) is used for loading a modulation phase shift and a compensation phase shift. In the closed loop demodulation algorithm, the micro-power laser (15) outputs a laser signal superimposed by a square wave signal and a step wave signal under the control of the signal processor (8), the laser signal is transmitted to the photodiode (16) to be converted into an electric signal, the electric signal is amplified by the transformer (17) and then loaded to the control light valve (104) in the grating light valve type optical fiber direct current sensor; wherein the square wave signal corresponds to produce non-reciprocal π / 2 modulation phase shift, and the step wave signal corresponds to produce the compensation phase shift.

2. The closed loop controlled DC fiber-optic current sensor of claim 1, wherein, The sensing light valve (103) and the control light valve (104) are made of silicon nitride material, have a width of 1-10 mu m, a length of 100-1000 mu m and a thickness of 200-300 nm.

3. The closed loop controlled DC fiber optic current sensor of claim 1, wherein, The reflecting film (106) is made of aluminum or dielectric material with a reflectivity greater than 92%.

4. The closed loop controlled DC fiber optic current sensor of claim 1, wherein, The grating light valve type optical fiber direct current sensor comprises a wide spectrum light source (1), a depolarizer (2), a 1*3 beam splitter (3), a signal processor (8), a first circulator (4), a first polarizer (5), a Faraday rotator (6), a polarization beam splitter (7) and a current sensing head (10). The output end (11) of the wide spectrum light source (1) is connected with the input end of the depolarizer (2), the output end of the depolarizer (2) is connected with the input end of the 1*3 beam splitter (3), the first output end (31) of the 1*3 beam splitter (3) is connected with the first end (41) of the first circulator (4), the second end (42) of the first circulator (4) is connected with the input end of the first polarizer (5), the output end of the first polarizer (5) is connected with the input end of the Faraday rotator (6), the output end of the Faraday rotator (6) is connected with the input end of the polarization beam splitter (7), and the two output ends of the polarization beam splitter (7) are connected with the first polarization maintaining optical fiber collimator (101) and the second polarization maintaining optical fiber collimator (102) of the current sensing head (10) respectively. The third end (43) of the first circulator (4) is connected to a first input end (81) of a signal processor (8), a first output end of the signal processor (8) is connected to an input end of the micro-power laser (15), an output end of the micro-power laser (15) is connected to an input end of the photodiode (16), an output end of the photodiode (16) is connected to an input end of the transformer (17), and an output end of the transformer (17) is connected to a control light valve (104) of the current sensing head (10).

5. The closed loop controlled DC fiber optic current sensor of claim 4, wherein, The polarization interference type fiber temperature sensor shares the wide spectrum light source (1), the depolarizer (2), the 1×3 beam splitter (3), and the signal processor (8) of the grating light valve type fiber DC current sensor, and further comprises a second circulator (9) and a fiber temperature sensing probe (14); a third output end (33) of the 1×3 beam splitter (3) is connected to a first end of the second circulator (9), a second end (92) of the second circulator (9) is connected to the fiber temperature sensing probe (14), and a third end (93) of the second circulator (9) is connected to a third input end (83) of the signal processor (8).

6. The closed-loop controlled DC fiber-optic current sensor according to claim 4 or 5, characterized in that The grating light valve type fiber DC current sensor further comprises a wavelength demodulation module (18), an input end of the wavelength demodulation module (18) is connected to a second output end (32) of the 1×3 beam splitter (3), an output end of the wavelength demodulation module (18) is connected to a second input end (82) of the signal processor (8), and a second output end of the signal processor (8) is connected to a feedback control end (12) of the wide spectrum light source (1); the signal processor (8) outputs a wavelength feedback control signal to the wide spectrum light source (1) for stabilizing the output wavelength of the wide spectrum light source (1).

Images