Current sensor based on U-shaped anti-resonance optical fiber and detection method

Abstract

The invention relates to the technical field of optical fiber sensing, and particularly discloses a current sensor based on a U-shaped anti-resonance optical fiber and a detection method, and the current sensor comprises a signal generator which is connected with a generation modulation unit; the optical fiber ring cavity with a U-shaped anti-resonance optical fiber is connected with the generation modulation unit and the photoelectric detector, and is used for receiving modulated signal light, circulating a first part of signal light in the received signal light in the optical fiber ring cavity, and enabling a second part of signal light to enter the photoelectric detector; the demodulation unit is connected with the photoelectric detector and the signal generator, and is used for receiving the signal light from the photoelectric detector and the reference light from the signal generator, calculating the phase difference between the signal light and the reference light, and deducing to-be-measured according to the relation between the phase difference and the loss. According to the current sensor based on the U-shaped anti-resonance optical fiber and the detection method provided by the invention, the detection sensitivity is higher, and the current sensor can work in a wider wavelength range.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing technology, and more specifically, to a current sensor based on a U-shaped anti-resonance optical fiber and a detection method. Background Art

[0002] Traditional current sensors mainly include electromagnetic current sensors based on Rogowski coils and resistor series sensors. Electromagnetic current sensors are used to monitor weak leakage currents of insulators, which have the disadvantages of low output voltage, high noise, poor anti-interference, and low precision. At the same time, their iron core structure also limits the high-frequency response of the overall device. Resistor series sensors must be installed in the ground wire of the equipment, which not only increases the insulation requirements of the shunt, but also causes the ground wire to be open if the resistor is damaged, threatening the safety of the equipment and staff. Due to the many drawbacks of the above-mentioned traditional current sensors, it is urgent to develop new current sensors, and at this time, the advantages of optical fiber current sensors are increasingly reflected.

[0003] The sensitivity and accuracy of fiber optic current sensors depend greatly on the manufacturing of their sensing structures and the selection of demodulation methods. In order to solve the intrinsic defects of quartz optical fibers, hollow-core optical fibers with air as the medium have been proposed and widely studied. Among them, hollow-core antiresonant optical fibers have the advantages of low delay, low dispersion, low nonlinearity, and high power damage threshold, but their loss is relatively high.

[0004] Cavity Ring Down (CRD) technology is a spectral detection technology. Compared with traditional spectral detection technology, signal light can be circulated in the cavity, which greatly enhances the absorption of light by the material and greatly improves the sensitivity of detection. However, the cavity built by high-reflection mirrors needs to be strictly aligned, and there are also problems such as mode matching. Optical fiber has low loss and is easy to couple with optical fiber. Therefore, optical fiber is introduced into the CRD system, and the fiber ring cavity ring-down technology is proposed. The phase shift fiber loop attenuation (PS-FLRD) method is that two couplers with high coupling ratios are connected to each other to form a loop. The optical signal can repeatedly pass through the detection area in the optical fiber loop. The intensity and phase changes of the signal caused by attenuation will accumulate in the loop. The loss in the ring cavity can be demodulated by detecting the phase change between the input light and the output light.

[0005] The current sensor in the prior art has large optical path loss, which leads to reduced detection accuracy and poor sensitivity. In addition, the original PS-FLRD equation uses the continuous cavity length condition in its derivation process, which leads to inherent errors in the equation and affects the loss detection accuracy. Summary of the invention

[0006] In order to solve the deficiencies in the prior art, the present invention provides a current sensor and a detection method based on a U-shaped anti-resonant optical fiber, which have higher detection sensitivity and can operate in a wider wavelength range.

[0007] The present invention adopts the following technical solution.

[0008] In a first aspect, the present invention provides a current sensor based on a U-shaped antiresonant optical fiber, comprising: a modulation generation unit for generating and modulating signal light and reference light, the current sensor also comprising: a signal generator, an optical fiber ring cavity with a U-shaped antiresonant optical fiber, a photodetector and a demodulation unit; the signal generator is connected to the modulation generation unit for receiving the modulated reference light; the optical fiber ring cavity with the U-shaped antiresonant optical fiber is connected to the modulation generation unit and the photodetector for receiving the modulated signal light, and circulating a first part of the received signal light in the optical fiber ring cavity, and a second part of the received signal light enters the photodetector, wherein the first part is larger than the second part; the demodulation unit is connected to the photodetector and the signal generator, and is used to receive the signal light from the photodetector and the reference light from the signal generator, and calculate the phase difference between the signal light and the reference light, and derive the quantity to be measured according to the relationship between the phase difference and the loss.

[0009] Preferably, the fiber ring cavity comprises: a U-shaped antiresonant fiber, a first coupler and a second coupler; the output ends of the first coupler and the second coupler are connected; the high coupling ratio end of the first coupler and the high coupling ratio end of the second coupler are respectively connected to the two ends of the U-shaped antiresonant fiber; the low coupling ratio end of the first coupler is connected to the modulation generating unit, and the low coupling ratio end of the second coupler is connected to the photodetector.

[0010] Preferably, both the first coupler and the second coupler are 1X2 couplers, and the coupling ratio of the second coupler is greater than that of the first coupler.

[0011] Preferably, the U-shaped antiresonant optical fiber comprises: an outer sleeve, a fiber core, a capillary and a U-shaped nested structure; a plurality of capillaries are evenly arranged around the fiber core at equal intervals, and each capillary is externally cut with the fiber core; the outer sleeve is coated on the outside of the plurality of capillaries, and each capillary is internally cut with the outer sleeve; the two ends of the outer sleeve are respectively connected to the high coupling ratio end of the first coupler and the high coupling ratio end of the second coupler; the U-shaped nested structure is arranged inside the capillary.

[0012] Preferably, the U-shaped nested structure includes a curved portion and two straight portions, the curved portion is semicircular, one end of each straight portion is connected to the capillary and the other end is connected to one end of the curved portion, and the two straight portions are parallel to each other.

[0013] Preferably, the number of the capillaries is greater than five.

[0014] Preferably, the modulation generating unit comprises: a light source and a modulator; the light source is used to generate original light, and the original light is divided into signal light and reference light; the modulator is connected to the light source, the signal generator and a U-shaped anti-resonant optical fiber, the modulator receives a radio frequency signal generated by the signal generator, and uses the radio frequency signal to modulate both the signal light and the reference light into sinusoidal light.

[0015] Preferably, the demodulation unit includes a phase-locked amplifier and a demodulation module; the phase-locked amplifier is connected to the signal generator and the photodetector respectively, and sends the signal light and the reference light to the demodulation module; the demodulation module calculates the phase difference between the signal light and the reference light, and derives the value to be measured based on the relationship between the phase difference and the loss.

[0016] In a second aspect, the present invention provides a current detection method based on a U-shaped anti-resonant optical fiber, using the aforementioned current sensor based on a U-shaped anti-resonant optical fiber, comprising the following steps:

[0017] Inputting the modulated signal light into the optical fiber loop having the U-shaped antiresonant optical fiber, so that the first part of the signal light received by the optical fiber ring cavity circulates in the optical fiber loop, and the second part of the light received by the optical fiber ring cavity enters the photodetector;

[0018] inputting the modulated reference light into the signal generator;

[0019] The signal light of the photodetector and the reference light of the signal generator are input to the demodulation unit;

[0020] The demodulation unit is used to calculate the phase difference between the signal light and the reference light, and the optical fiber loss is derived according to the relationship between the phase difference and the loss, thereby obtaining the measured value.

[0021] Preferably, the relationship between the phase difference and the loss is as follows:

[0022]

[0023] Among them, α is the loss, β = wl / c is the eigenfrequency of the system, which is a constant value when the modulation frequency is determined; l is the length of the optical fiber loop, ω is the angular modulation frequency, and c is the speed of light; is the phase difference detected by the demodulation unit, is the initial phase of the optical signal after being modulated by the modulator.

[0024] The beneficial effect of the present invention is that, compared with the prior art, the present invention sets a U-shaped anti-resonant optical fiber in the optical fiber loop, so that the light field can have a stronger interaction with the external medium, thereby improving the light field intensity in the optical fiber loop, so that high-sensitivity detection of external parameters to be measured (such as temperature, pressure, refractive index, etc.) can be achieved, and the anti-resonant structure has a wider working band, so that the sensor can work in a wider wavelength range and is suitable for a variety of application scenarios. At the same time, the present invention inputs the modulated signal light into the optical fiber optical path, so that the light can repeatedly traverse the sensing optical fiber in the loop, thereby improving the detection sensitivity of the sensor.

[0025] In order to further reduce the optical signal leakage of the hollow-core anti-resonant optical fiber, the U-shaped anti-resonant optical fiber provided in the present invention is a nested optical fiber. By changing the optical fiber structure and the additional anti-resonant layer, the limiting loss can be reduced while avoiding the loss fluctuation caused by Fano resonance, thereby expanding the low-loss working band, allowing the light to circulate multiple times in the loop, thereby improving the sensitivity of the system.

[0026] Furthermore, a U-shaped nested structure is nested inside the capillary, which can add additional anti-resonance layers to reduce light leakage; when the anti-resonance layers are close to each other, even if their surfaces are along the radial direction of the optical fiber, the surrounding energy leakage can be effectively suppressed.

[0027] In addition, the current detection method based on U-shaped antiresonant optical fiber provided by the present invention uses an improved loss and phase difference derivation formula in its demodulation unit, taking into account the fact that the output light in the PS-FLRD technology is discrete rather than continuous, and can calculate the phase shift under discontinuous conditions. Based on the relationship between the phase difference and the loss under the discontinuous condition, the optical fiber loss is demodulated, thereby eliminating the inherent error in the traditional demodulation equation, improving the accuracy and precision of the measurement, and thus achieving high-precision sensing measurement capabilities. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic structural diagram of a current sensor based on a U-shaped anti-resonant optical fiber in the present invention.

[0029] Figure 2 It is a schematic structural diagram of the cross section of the U-shaped antiresonant optical fiber in the present invention.

[0030] Figure 3 It is a schematic diagram of the structure of the U-shaped antiresonant optical fiber in the present invention.

[0031] Figure 4 The graphs are a relationship curve of current versus time, a relationship curve of phase shift versus time, and a relationship curve of phase shift versus modulation frequency obtained by using the current sensor of the present invention.

[0032] The reference numerals in the figure are: 1, light source; 2, modulator; 3, signal generator; 4, first coupler; 5, second coupler; 6, U-shaped antiresonant optical fiber; 601, outer sleeve; 602, fiber core; 603, capillary; 604, U-shaped nested structure; 7, photodetector; 8, phase-locked amplifier. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. The embodiments described in this application are only part of the embodiments of the present invention, not all of them. Based on the spirit of the present invention, other embodiments obtained by ordinary technicians in this field without creative work are all within the scope of protection of the present invention.

[0034] like Figure 1 As shown, embodiment 1 of the present invention provides a current sensor based on a U-shaped antiresonant optical fiber, comprising: a generation and modulation unit for generating and modulating reference light and signal light, a signal generator 3, a first coupler 4, a second coupler 5, a U-shaped antiresonant optical fiber 6, a photodetector 7 and a phase-locked amplifier 8.

[0035] The modulation generation unit includes a light source 1 and a modulator 2. The light source 1 is an amplified spontaneous radiation light source, which is used to generate original light, and the original light is divided into signal light and reference light through a third coupler; the signal generator 3 is used to generate a radio frequency signal, and the radio frequency signal includes a DC bias that enables the modulator 2 to work in a linear region; the modulator 2 is connected to the light source 1 and the signal generator 3, and is used to modulate both the signal light and the reference light into sinusoidal light using the radio frequency signal; the signal generator 3 receives the modulated reference light.

[0036] The coupling ratio of the second coupler 5 is greater than that of the first coupler 4. The output ends of the first coupler 4 and the second coupler 5 are connected, and the high coupling ratio end of the second coupler 4 and the high coupling ratio end of the second coupler 5 are respectively connected to the two ends of the U-shaped anti-resonance fiber 6. The first coupler 4, the U-shaped anti-resonance fiber 6 and the second coupler 5 are connected to form a fiber ring cavity. By connecting the high coupling ratio ends of the first coupler 4 and the second coupler 5 to the U-shaped anti-resonance fiber 6, most of the signal light received by the fiber ring cavity can circulate in the fiber ring cavity.

[0037] like Figure 2As shown, the U-shaped anti-resonant optical fiber 6 is the sensing unit of the present invention. The use of the U-shaped anti-resonant optical fiber allows the light field to have a stronger interaction with the external medium, thereby increasing the light field intensity in the optical fiber loop. Therefore, high-sensitivity detection of the measurement to be measured (such as temperature, pressure, refractive index, etc.) can be achieved, and the anti-resonant structure has a wider operating band, so that the sensor can operate in a wider wavelength range and is suitable for a variety of application scenarios.

[0038] In addition, by connecting the high coupling ratio end of the first coupler 4 and the high coupling ratio end of the second coupler 5 to the sensing unit, it is convenient to disconnect the sensing unit to measure the initial phase of the optical signal modulated by the modulator. If the sensing unit is connected to the low coupling ratio ends of the first coupler and the second coupler, the initial phase cannot be measured.

[0039] Furthermore, if Figure 3 As shown, in the hollow core anti-resonant fiber, the air region inside the hollow capillary of the cladding and between the capillaries also has a mode. If the mode of the air region of the cladding matches the effective refractive index of the core mode, the energy of the core will leak into the cladding. Therefore, in order to reduce the optical signal leakage of the hollow core anti-resonant fiber, in a further preferred but non-restrictive embodiment of the present invention, the U-shaped anti-resonant fiber 6 is a U-shaped nested fiber. By changing the fiber structure and the additional anti-resonant layer, the limiting loss can be reduced while avoiding the loss fluctuation caused by Fano resonance, expanding the low-loss working band, and allowing the light to circulate multiple times in the loop, thereby improving the sensitivity of the system.

[0040] Furthermore, the U-shaped nested optical fiber includes: an outer sleeve 601 made of quartz material, a core 602, a capillary 603 and a U-shaped nested structure 604. The core 602 is at the center of the U-shaped nested optical fiber, and its radius is R1. A plurality of capillaries 603 are evenly arranged around the core 602 at equal intervals, each capillary 603 is circumscribed with the core 602, and the radius of each capillary 603 is R3. The distance between two adjacent capillaries 603 is L1. The outer sleeve 601 is coated on the outside of the plurality of capillaries 603, each capillary 603 is circumscribed with the outer sleeve 601, and the capillaries 603 and the outer sleeve 601 together constitute the cladding of the core 602. The inside of the core 602, the inside of the capillary 603, the inside of the U-shaped nested structure 604, and the part between the core 602 and the outer sleeve 601 are all filled with air.

[0041] A plurality of U-shaped nested structures 604 are evenly spaced inside each capillary 603 , and both ends of each U-shaped nested structure 604 are connected to the inner surface of the capillary 603 .

[0042] Furthermore, the U-shaped nested structure 604 includes two curved portions and a straight portion, the curved portion is semicircular with a radius of R2, each straight portion is connected to the inner wall of the capillary 603 at one end and connected to one end of the curved portion at the other end, the two straight portions are parallel to each other and have a spacing of L2, satisfying L2=R2.

[0043] In a further preferred but non-limiting embodiment of the present invention, the number of the capillaries 603 is 6, and the number of the U-shaped nested structures 604 is 3.

[0044] Furthermore, R1 is preferably about 25 microns, R3 is preferably about 12 microns, R2 is preferably 8 microns, and the wall thickness of the U-shaped nested structure is 0.5 microns. The gap L1 between adjacent capillaries 603 is 0.4 mm, and the wall thickness of the capillary 603 is 0.8 microns.

[0045] The curvature of the boundary of the core 602 in the U-shaped nested optical fiber designed in the embodiment of the present invention is a negative value, which can reduce the light leakage of the hollow core anti-resonance optical fiber, reduce the overlap between the mode field of the mode and the glass structure, and suppress the coupling between the core mode and the cladding mode. In addition, the outer layer of the core 602 is provided with a cladding structure composed of a capillary 602 and an outer sleeve 601. Due to the increase in the boundary curvature of the cladding structure, the limiting loss of the optical fiber is greatly reduced. In addition, since the capillary of the hollow core anti-resonance optical fiber not only affects the light leakage, but also directly affects the coupling between the cladding mode and the core mode, the capillary in the embodiment of the present invention is arranged periodically around the core. Further, since there is a geometric relationship between the various structural parameters (core radius, glass thickness, capillary radius, number and gap) of the U-shaped anti-resonance optical fiber 6, the present invention sets the radius of the core, the number of capillaries, the capillary radius, the capillary spacing, and the capillary wall thickness. In addition, in order to improve the light leakage of the single-ring structure hollow-core anti-resonant optical fiber and further reduce the limiting loss, the embodiment of the present invention adds a U-shaped nested structure in the capillary. By adding the U-shaped nested structure in the capillary, an additional anti-resonant layer can be added to reduce light leakage; when the anti-resonant layers are close to each other, even if their surfaces are along the radial direction of the optical fiber, the surrounding energy leakage can be effectively suppressed. Figure 3 As shown in the figure, a gap is formed between the two straight parts of the U-shaped nested structure, and the connection node between the U-shaped nested structure and the capillary is far away from the fiber core, which can reduce the loss and avoid the loss fluctuation caused by Fano resonance. In addition, the nested structure will also change the distribution of the cladding mode to achieve the purpose of suppressing mode coupling.

[0046] Furthermore, if the adjacent capillaries are in contact, the nodes generated by the contact will introduce glass modes, increasing the loss of the optical fiber by several orders of magnitude. Therefore, the capillaries 603 in the U-shaped antiresonant optical fiber 6 in the embodiment of the present invention need to be kept separated from each other. As the capillary gap increases, the low-loss bandwidth of the optical fiber continues to increase, while the minimum loss first decreases and then increases. This is mainly because the increase in the capillary gap will increase the light leaking through the gap, and the capillary size is reduced and the protective layer is close to the core, thereby increasing the spatial overlap between the core mode and the cladding. Studies have shown that when the core radius, glass thickness, and capillary gap are all fixed, a lower loss can be obtained in a single-ring hollow-core antiresonant optical fiber when the number of capillaries N>5. This is due to the fact that the small-sized capillaries will reduce the mode coupling between the cladding mode and the fundamental mode. If a nested structure is added to the hollow-core antiresonant optical fiber, the air in the capillary is divided into two parts by the nested structure, resulting in a decrease in the effective refractive index of the cladding mode and suppression of the coupling with the fundamental mode. At this time, fewer capillaries and larger capillary sizes can be selected.

[0047] Due to the influence of the glass node, the operating bandwidth of the double negative curvature antiresonant fiber is narrower than that of the single ring hollow core antiresonant fiber or the circular nested hollow core antiresonant fiber. If the core radius, glass thickness and capillary gap of the U-shaped nested fiber and the circular nested fiber are the same, and the number of antiresonant layers is the same, then the boundary power density of the U-shaped nested hollow core antiresonant fiber proposed in the embodiment of the present invention is generally lower than that of the circular nested hollow core antiresonant fiber. Therefore, it can be considered that compared with the circular nested structure, the U-shaped nested structure does effectively reduce the limiting loss.

[0048] In a preferred but non-restrictive embodiment of the present invention, the U-shaped antiresonant optical fiber 6 includes a series of optical fibers based on the antiresonance mechanism, such as Bragg fiber, photonic crystal fiber, Kagomé fiber, negative curvature hollow core fiber, etc., and its light guiding mechanism can be explained as treating the high refractive index layer in the optical fiber as a FP resonant cavity. When the wavelength of light meets the resonance condition of this FP cavity, the high refractive index layer will resonate; when the wavelength of light is far away from the resonant wavelength, the light will be reflected back by the FP cavity, thereby being confined in the low refractive index layer and propagating forward along its axial direction.

[0049] In recent years, reports on hollow-core optical fibers have appeared frequently, including those for terahertz wave transmission and infrared waveguide transmission, but there are few reports on the production of sensors based on the anti-resonance mechanism of hollow-core optical fibers. The present invention coats a layer of magnetic fluid on the anti-resonance optical fiber for electric field sensing, and the anti-resonance optical fiber of the present invention is a U-shaped anti-resonance optical fiber, which enhances the interaction between light and the substance to be measured by changing the anti-resonance optical fiber structure in the optical fiber sensor and destroying the total reflection propagation condition of light. When light propagates from the core of the optical fiber to the curved part of the optical fiber, due to the bending of the optical fiber, the critical condition of total reflection of light is not met, and part of the light will directly leak into the surrounding environment to generate loss. And the smaller the bending diameter, the greater the loss. Part of the light will leak from the core into the cladding, and when the refractive index of the external medium is less than the refractive index of the cladding, a high-order cladding mode will be excited. When the light propagates to the end of the bending area, the light in the cladding will be partially coupled back to the core. Since the light propagating in the core and the light propagating in the cladding travel different optical paths, a phase difference will be generated between them, which will cause interference between the core mode and the cladding mode. Therefore, we combine the antiresonant fiber with the U-shaped fiber to improve the sensitivity of the system.

[0050] The U-shaped antiresonant fiber 6 proposed above is placed in the optical fiber loop, the modulator 2 is connected to the low coupling ratio end of the first coupler 4 in the optical fiber ring cavity, and most of the signal light modulated by the modulator 2 enters the ring cavity for circulation; in addition, the low coupling ratio end of the second coupler 5 in the optical fiber ring cavity is also connected to the photodetector 7, so that the first part of the signal light received by the optical fiber ring cavity can circulate in the optical fiber ring cavity and pass through the U-shaped antiresonant fiber 6, while the second part of the signal light received by the optical fiber ring cavity is led out of the loop through the second coupler 5 and reaches the photodetector 7, wherein the first part is larger than the second part.

[0051] The photodetector 7 is connected to the phase-locked amplifier 8, which is also connected to the signal generator 3. The photodetector 7 absorbs the optical signal output by the second coupler 5 and transmits it to the phase-locked amplifier 8. The radio frequency signal generated by the signal generator 3 is used as the reference signal of the phase-locked amplifier 8, and most of the other light enters the fiber ring cavity; the phase-locked amplifier 8 receives the reference light from the signal generator 3 and the signal light emitted by the photodetector 7, and operates on the two optical signals. Finally, the phase difference between the two light paths is obtained, and the relationship between the phase difference and the loss is deduced, thereby obtaining the loss information measured by the system.

[0052] In a preferred but non-limiting embodiment of the present invention, the light source 1 is a SLD light source in the 1310 nm band, because the Verdet constant in the sensing optical fiber is inversely proportional to the working wavelength, and the use of a 1310 nm light source can improve the Verdet constant. In addition, the SLD light source has a large output power, which can ensure that the all-optical current transformer can operate normally and stably after removing the optical loss, and has a wide spectrum width. The light source has a wide spectrum width, which effectively suppresses back reflection and back scattering.

[0053] Furthermore, the coherence length of the light source 1 cannot be longer than the length of the optical fiber ring cavity, otherwise the ring cavity will become an interference structure, and its coherence length cannot be too long, otherwise no obvious phase change can be seen when measuring current.

[0054] In a preferred but non-limiting embodiment of the present invention, the first coupler 4 and the second coupler 5 are both 1×2 couplers, and are high coupling ratio couplers (high coupling ratio means a coupling ratio of 90:10 and above).

[0055] The U-shaped sensing unit must be set at the two arm ends of the 1×2 coupler. The sensing unit needs to be disconnected when obtaining the initial phase. If the sensing unit is placed on the other side of the ring cavity, measurement cannot be performed.

[0056] like Figure 4 As shown in the figure, the relationship between the output phase and current under different modulation frequencies is tested. The details are as follows:

[0057] like Figure 4 As shown in (a), the current increases from 0A to 5A in steps of 500mA, and the modulation frequency is set to 100kHz, 167kHz, 300kHz, and 500kHz. Figure 4 As shown in (b), the phase shift angle measured at 167 kHz varies linearly and periodically with the current. Figure 4 As shown in (c), the current and output phase at different frequencies are linearly fitted, and the linear relationship is good, all greater than 0.9979. Figure 4 (c) It can be clearly seen from the current sensitivity of the curve that the sensitivity at 300kHz is relatively low because it is closer to the no-response point (334kHz). For 167kHz and 500kHz, the current sensitivities are 0.8518deg / A and 0.8131deg / A, respectively, which are roughly the same. Therefore, by selecting the modulation frequency, the system can operate at different current sensitivities. In addition, a significant increase in the phase shift at a current of 100mA can be observed in the experiment, so the minimum detectable current is at least 100mA. Embodiment 2 of the present invention provides a current detection method based on a U-type antiresonant optical fiber, using the current sensor based on the U-type antiresonant optical fiber in Embodiment 1, comprising the following steps:

[0058] The modulated signal light is input into the optical fiber loop having the U-shaped anti-resonant optical fiber 6, so that most of the signal light circulates in the optical fiber loop, and a small part of the light enters the photodetector 7;

[0059] Inputting the modulated reference light into the signal generator 3;

[0060] The signal light of the photodetector 7 and the reference light of the signal generator 3 are input to the demodulation unit;

[0061] The demodulation unit is used to calculate the phase difference between the signal light and the reference light, and the optical fiber loss is derived according to the relationship between the phase difference and the loss, thereby obtaining the measured value.

[0062] In a preferred but non-limiting embodiment of the present invention, the relationship between loss and phase difference is as follows:

[0063]

[0064] Where α is the loss, β = wl / c is the eigenfrequency of the system, which is determined by the modulation frequency of the system, the loop length and the speed of light. When the modulation frequency is determined, the eigenfrequency is customized. l is the length of the optical fiber loop, ω is the angular modulation frequency, and c is the speed of light; is the phase difference detected by the lock-in amplifier, is the initial phase of the optical signal after being modulated by the modulator. To determine the change in loss and further obtain the current size.

[0065] In a preferred but non-limiting embodiment of the present invention, the present invention can also measure other quantities to be measured, such as placing an air chamber at the sensing unit, passing different types of gases or gases of different concentrations into the air chamber for gas sensing; placing ordinary optical fibers at the sensing unit, and then increasing pressure on them for stress sensing; placing liquid in the sensing unit part for refractive index measurement; and realizing magnetic field measurement and temperature measurement, etc.

[0066] The improved loss and phase difference derivation formula proposed in the present invention takes into account the fact that the output light in the PS-FLRD technology is discrete rather than continuous, and can calculate the phase shift under discontinuous conditions. Based on the relationship between the phase difference and the loss under the discontinuous condition, the optical fiber loss is demodulated, thereby eliminating the inherent error in the traditional demodulation equation, improving the accuracy and precision of the measurement, and thus achieving high-precision sensing measurement capabilities.

[0067] The present invention uses the phase-locked amplifier 8 to compare the phases of the reference light and the signal light, which can suppress the influence of noise on the signal and reduce the interference of the external environment during the measurement process.

[0068] The beneficial effect of the present invention is that, compared with the prior art, the present invention sets a U-shaped anti-resonant optical fiber in the optical fiber loop, so that the light field can have a stronger interaction with the external medium, thereby improving the light field intensity in the optical fiber loop, so that high-sensitivity detection of external parameters to be measured (such as temperature, pressure, refractive index, etc.) can be achieved, and the anti-resonant structure has a wider working band, so that the sensor can work in a wider wavelength range and is suitable for a variety of application scenarios. At the same time, the present invention inputs the modulated signal light into the optical fiber optical path, so that the light can repeatedly traverse the sensing optical fiber in the loop, thereby improving the detection sensitivity of the sensor.

[0069] In order to further reduce the optical signal leakage of the hollow-core anti-resonant optical fiber, the U-shaped anti-resonant optical fiber provided in the present invention is a nested optical fiber. By changing the optical fiber structure and the additional anti-resonant layer, the limiting loss can be reduced while avoiding the loss fluctuation caused by Fano resonance, thereby expanding the low-loss working band, allowing the light to circulate multiple times in the loop, thereby improving the sensitivity of the system.

[0070] Furthermore, a U-shaped nested structure is nested inside the capillary, which can add additional anti-resonance layers to reduce light leakage; when the anti-resonance layers are close to each other, even if their surfaces are along the radial direction of the optical fiber, the surrounding energy leakage can be effectively suppressed.

[0071] In addition, the current detection method based on U-shaped antiresonant optical fiber provided by the present invention uses an improved loss and phase derivation formula in its demodulation unit, taking into account the fact that the output light in the PS-FLRD technology is discrete rather than continuous, and can calculate the phase shift under discontinuous conditions. Based on the relationship between the phase difference and the loss under the discontinuous condition, the optical fiber loss is demodulated, thereby eliminating the inherent error in the traditional demodulation equation, improving the accuracy and precision of the measurement, and thus achieving high-precision sensing measurement capabilities.

[0072] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents, and any modifications or equivalent replacements that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A current sensor based on a U-shaped anti-resonant optical fiber, comprising: A generating and modulating unit for generating and modulating signal light and reference light, characterized in that: The current sensor further comprises: a signal generator (3), an optical fiber ring cavity having a U-shaped anti-resonant optical fiber (6), a photodetector (7) and a demodulation unit; The signal generator (3) is connected to the modulation generating unit and is used to receive the modulated reference light; The optical fiber ring cavity with the U-shaped antiresonant optical fiber (6) is connected to a modulation generating unit and a photodetector (7), and is used to receive modulated signal light, circulate a first portion of the received signal light in the optical fiber ring cavity, and allow a second portion of the received signal light to enter the photodetector (7), wherein the first portion is larger than the second portion; The demodulation unit is connected to the photodetector (7) and the signal generator (3), and is used to receive signal light from the photodetector (7) and reference light from the signal generator (3), calculate the phase difference between the signal light and the reference light, and derive the value to be measured based on the relationship between the phase difference and the loss.

2. The current sensor based on U-shaped anti-resonant optical fiber according to claim 1, characterized in that: The optical fiber ring cavity comprises: a U-shaped anti-resonant optical fiber (6), a first coupler (4) and a second coupler (5); The output ends of the first coupler (4) and the second coupler (5) are connected; The high coupling ratio end of the first coupler (4) and the high coupling ratio end of the second coupler (5) are respectively connected to two ends of a U-shaped anti-resonance optical fiber (6); The low coupling ratio end of the first coupler (4) is connected to the modulation generating unit, and the low coupling ratio end of the second coupler is connected to the photoelectric detector (7).

3. The current sensor based on U-shaped anti-resonant optical fiber according to claim 2, characterized in that: The first coupler (4) and the second coupler (5) are both 1X2 couplers, and the coupling ratio of the second coupler (5) is greater than that of the first coupler (4).

4. The current sensor based on U-shaped anti-resonant optical fiber according to claim 2, characterized in that: The U-shaped antiresonant optical fiber (6) comprises: an outer sleeve (601), a fiber core (602), a capillary (603) and a U-shaped nested structure (604); A plurality of capillaries (603) are evenly arranged around the fiber core (602) at equal intervals, and each capillary (603) is externally tangent to the fiber core (602); an outer sleeve (601) is coated on the outside of the plurality of capillaries (603), and each capillary (603) is internally tangent to the outer sleeve (601); two ends of the outer sleeve (601) are respectively connected to a high coupling ratio end of a first coupler (4) and a high coupling ratio end of a second coupler (5); and a U-shaped nested structure (604) is arranged inside the capillary (603).

5. The current sensor based on U-shaped anti-resonant optical fiber according to claim 4, characterized in that: The U-shaped nested structure (604) includes a curved portion and two straight portions, the curved portion is semicircular; each straight portion has one end connected to the capillary (603) and the other end connected to one end of the curved portion, and the two straight portions are parallel to each other.

6. The current sensor based on U-shaped anti-resonant optical fiber according to claim 5, characterized in that: The number of the capillaries (603) is greater than five.

7. The current sensor based on U-shaped anti-resonant optical fiber according to any one of claims 1 to 6, characterized in that: The modulation generating unit comprises: a light source (1) and a modulator (2); The light source (1) is used to generate original light, and the original light is divided into signal light and reference light; The modulator (2) is connected to the light source (1), the signal generator (3) and the U-shaped anti-resonant optical fiber (6); the modulator (2) receives the radio frequency signal generated by the signal generator (3) and uses the radio frequency signal to modulate both the signal light and the reference light into sinusoidal light.

8. The current sensor based on U-shaped anti-resonant optical fiber according to any one of claims 1 to 6, characterized in that: The demodulation unit comprises a phase-locked amplifier (8) and a demodulation module; The lock-in amplifier (8) is connected to the signal generator (3) and the photodetector (7) respectively, and sends the signal light and the reference light to the demodulation module; The demodulation module calculates the phase difference between the signal light and the reference light, and derives the quantity to be measured according to the relationship between the phase difference and the loss.

9. A current detection method based on a U-shaped anti-resonant optical fiber, using the current sensor based on a U-shaped anti-resonant optical fiber according to any one of claims 1 to 8, characterized in that: The following steps are involved: Inputting the modulated signal light into an optical fiber loop having a U-shaped anti-resonant optical fiber (6), so that a first portion of the signal light received by the optical fiber ring cavity circulates in the optical fiber loop, and a second portion of the signal light received by the optical fiber ring cavity enters a photodetector (7); Inputting the modulated reference light into a signal generator (3); Inputting the signal light of the photodetector (7) and the reference light of the signal generator into a demodulation unit; The demodulation unit is used to calculate the phase difference between the signal light and the reference light, and the optical fiber loss is derived according to the relationship between the phase difference and the loss, thereby obtaining the measured value.

10. The current detection method based on U-shaped anti-resonant optical fiber according to claim 9, characterized in that: The relationship between the phase difference and the loss is as follows: Where α is the loss, β = wl / c is the eigenfrequency of the system, which is a constant when the modulation frequency is determined; l is the length of the fiber loop, ω is the angular modulation frequency, and c is the speed of light; is the phase difference detected by the demodulation unit, is the initial phase of the optical signal after being modulated by the modulator.

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