Magnetic field sensing system based on polarization interference

Abstract

The invention provides a magnetic field sensing system based on polarization interference. A polarizer and an electro-optical modulator modulate an incoherent optical signal into two beams of linearly polarized light with different polarization states; the delay polarization adjustment module performs phase delay processing on the corresponding wavelength linear polarized light and performs polarization adjustment on another wavelength linear polarized light, and then the polarized light is combined by a polarization beam combiner and transmitted to the magneto-optical crystal; under the action of an external magnetic field, the polarization state of the beam-combined light changes when the beam-combined light passes through the magneto-optical crystal; the polarization analyzer guides the combined light with the changed polarization state to the same polarization direction, so that the intensity of the combined light is changed; the photoelectric detector carries out square law detection on the combined light with the changed intensity to obtain six paths of electric signals; the processor generates four polarization interferences based on the six paths of electric signals, two vernier envelopes are obtained, and the size of the external magnetic field is determined according to the frequency width of split valleys appearing in the lower envelope after the two vernier envelopes are superposed. The device is high in sensitivity and adjustable in measurement range and sensitivity.

Description

Technical Field

[0001] The present invention belongs to the field of magnetic field sensing, and particularly relates to a magnetic field sensing system based on polarization interference. Background Art

[0002] As a fundamental physical phenomenon in nature, the precise measurement and sensing technology of magnetic fields are key means to understand the essence of matter and drive the innovation of engineering technologies. From the protection of life activities by the Earth's magnetic field to the microscopic regulation of spin and magnetic moment at the quantum scale, magnetic field measurement has always run through humanity's exploration and utilization of natural laws. With the breakthroughs in materials science, microelectronics technology, and quantum physics, in recent years, magnetic sensors have received extensive attention due to their potential in various fields such as national defense, metal detection, intelligent electronics industry, and power system fault monitoring. Magnetic field measurement and magnetic sensors are the bridges connecting the physical world and the digital world, and their importance is not only reflected in basic scientific breakthroughs but also penetrates into the core links of industries such as industry, healthcare, environmental protection, and national defense. With the progress of materials science and quantum technology, magnetic sensors will further promote the development of frontier fields such as autonomous driving, precision medicine, and space exploration, becoming a key enabling technology for the future intelligent society.

[0003] Traditional magnetic sensors, such as Hall effect sensors, anisotropic magnetoresistive sensors, superconducting quantum interference device magnetometers, etc., are limited in practical applications due to disadvantages such as large volume, high cost, and complex operation. Fiber optic magnetic sensors have become the preferred alternative due to their compact size, low cost, high sensitivity, electromagnetic interference resistance, and the ability to operate in harsh environments.

[0004] However, fiber optic magnetic sensors have the following disadvantages:

[0005] 1. High system complexity and complex preparation

[0006] Traditional solutions need to use two separate interferometers to achieve the Vernier effect, and the free spectral range (FSR) needs to be precisely matched, resulting in complex optical path design and difficult debugging. Moreover, when preparing the interference structure with optical fibers, problems such as complex preparation processes are often faced.

[0007] 2. Insufficient sensitivity

[0008] The traditional Vernier effect needs to be achieved by cascading two interferometers, and the sensitivity amplification factor is determined by the ratio of the free spectral ranges (FSR) of the two interferometers. If the two FSRs are close but have slight differences, the sensitivity improvement is limited and cannot meet the high-precision requirements.

[0009] 3. Slow response speed due to wavelength demodulation dependence

[0010] Traditional sensors rely on an optical spectrum analyzer (OSA) for wavelength demodulation, with a slow response speed (millisecond level) and limited resolution (nanometer level).

[0011] 4. Structural and material limitations

[0012] They rely on special materials (such as magnetorheological fluid, magnetostrictive alloy) or complex preparation processes (such as micro-nano fiber structure), resulting in high costs and poor stability.

[0013] 5. Encapsulation and mechanical stability challenges

[0014] The fiber core diameter of the optical fiber is only at the micron level. The encapsulation process in the magnetic field sensitive area (such as coating magnetostrictive material or winding coils) is likely to cause micro-bending loss or fracture of the optical fiber, reducing reliability. Magnetostrictive fiber optic sensors are highly sensitive to mechanical stress. External pressure or deformation may be misjudged as a magnetic field change, requiring a high-precision stress isolation design, which increases the engineering difficulty.

[0015] It can be seen that the existing magnetic field sensing systems have problems such as low sensitivity, limited dynamic range and response bandwidth, complex system structure and poor stability, and the measurement results are easily affected by the external environment. Summary of the Invention

[0016] The present invention provides a magnetic field sensing system based on polarization interference to solve the problems of the current magnetic field sensing system, such as low sensitivity, limited dynamic range and response bandwidth, complex system structure and poor stability, and the measurement results are easily affected by the external environment.

[0017] According to the first aspect of the embodiments of the present invention, a magnetic field sensing system based on polarization interference is provided, which is characterized in that it includes a polarizer, an electro-optic modulator, a delay polarization adjustment module, a polarization beam combiner, a magneto-optic crystal, an analyzer, a photodetector and a processor connected in sequence;

[0018] The polarizer converts the incoherent optical signal into a linearly polarized light;

[0019] The electro-optic modulator modulates the linearly polarized light into two linearly polarized lights with different polarization states;

[0020] The delay polarization adjustment module performs phase delay processing on the linearly polarized light corresponding to the wavelength in the two linearly polarized lights and performs polarization adjustment on the linearly polarized light of the other wavelength;

[0021] The polarization beam combiner combines and transmits the linearly polarized light corresponding to the wavelength with two polarization states after the phase delay processing and the linearly polarized light of the other wavelength with two polarization states after the polarization adjustment to the magneto-optic crystal;

[0022] Under the action of an external magnetic field, the polarization state of the combined light changes when passing through the magneto-optic crystal;

[0023] The analyzer guides the combined light with a changed polarization state to the same polarization direction, causing the intensity of the combined light projected onto the transmission axis of the analyzer to change;

[0024] The photodetector performs square-law detection on the combined light with changed intensity to obtain six electrical signals;

[0025] Based on the six electrical signals, the processor generates four polarization interferences, obtains two cursor envelopes, superimposes the two cursor envelopes, and determines the magnitude of the external magnetic field according to the frequency width of the split valley that appears in the lower envelope after superposition.

[0026] Optionally, as the external magnetic field increases, the frequency width of the split valley becomes larger; the greater the phase delay, the wider the frequency width of the split valley that can appear, and the larger the measurement range of the system;

[0027] The intensity depth of the split valley, that is, the system sensitivity, is related to the intensity of the combined light projected onto the transmission axis of the analyzer; the greater the phase delay, the greater the degree of polarization adjustment, the greater the intensity of the corresponding beam in the combined light projected onto the transmission axis of the analyzer, the deeper the split valley, and the higher the system sensitivity within the measurement range of the system.

[0028] Optionally, when the magnitude of the phase delay is determined, that is, when the measurement range of the system is determined, the degree of polarization adjustment is controlled so that within the measurement range of the system, there is a constant phase difference between the corresponding beams after the combined light passes through the magneto-optic crystal, thereby ensuring the smooth progress of polarization interference.

[0029] Optionally, the delay polarization adjustment module includes a wavelength division multiplexer, a phase delay unit, and a first polarization controller. The input end of the wavelength division multiplexer is connected to the output end of the electro-optic modulator. The first output end is connected to the first input end of the polarization beam combiner through the phase delay unit, and the second output end is connected to the second input end of the polarization beam combiner through the first polarization controller;

[0030] The wavelength division multiplexer transmits the linearly polarized light corresponding to the wavelength in the two linearly polarized lights to the phase delay unit for phase delay processing, and transmits the linearly polarized light of the other wavelength to the first polarization controller for polarization adjustment.

[0031] Optionally, the phase delay unit includes a chirped fiber Bragg grating. The phase delay processing is performed by the chirped fiber Bragg grating. The refractive index of the chirped fiber Bragg grating changes in a gradient manner, and it performs different amounts of phase delay for linearly polarized lights of different wavelengths;

[0032] The greater the dispersion of the chirped fiber Bragg grating, the larger the free spectral range of the spectrum, the wider the frequency width of the split valley that can appear, and the larger the measurement range of the system.

[0033] Optionally, the phase delay unit further includes a coupler. The first output end of the wavelength division multiplexer is connected to the first input end of the coupler. The first output end of the coupler is connected to the second input end of the coupler through the chirped fiber Bragg grating. The second output end of the coupler is connected to the first input end of the polarization beam combiner.

[0034] Optionally, the magneto-optical crystal is arranged at an interval from the external magnetic field.

[0035] Optionally, it further includes a broadband light source. The broadband light source is sequentially connected to the delay polarization adjustment module through the polarizer and the electro-optic modulator;

[0036] The broadband light source provides an incoherent optical signal to the polarizer;

[0037] The electro-optic modulator performs electro-optical modulation on the linearly polarized light provided by the polarizer according to the microwave modulation signal. Since the electro-optic modulator is a polarization-dependent device and its modulation efficiency for linearly polarized light with different polarization states is different, two microwave-bearing optical beams with different polarization states are obtained. When the microwave signals in the microwave-bearing optical beams are transmitted along the delay polarization adjustment module, the polarization beam combiner, the magneto-optical crystal, and the analyzer, they undergo the same changes as the optical signals. The photodetector performs square-law detection on the microwave signals with changed intensities to obtain six electrical signals.

[0038] Optionally, the processor is a vector network analyzer. The vector network analyzer sends the microwave modulation signal to the electro-optic modulator and receives the six electrical signals provided by the photodetector. Based on the six electrical signals, four polarization interferences are generated to obtain two cursor envelopes. The two cursor envelopes are superimposed, and the magnitude of the external magnetic field is determined according to the frequency width of the split valley that appears in the lower envelope after the superposition.

[0039] Optionally, a first amplifier is provided between the electro-optic modulator and the delay polarization adjustment module, and a second amplifier is provided between the magneto-optical crystal and the analyzer.

[0040] The beneficial effects of the present invention are:

[0041] 1. The present invention obtains two linearly polarized light beams with different polarization states and incoherence based on incoherent optical signals, which is convenient for subsequent polarization interference. The delay polarization adjustment module performs phase delay processing on the linearly polarized light of the corresponding wavelength and adjusts the magnitude of the phase delay, which can facilitate subsequent polarization interference and can also achieve the adjustment of the system measurement range. Since the system measurement range is adjustable, the dynamic range and response bandwidth of the present invention are not limited. The delay polarization adjustment module also performs polarization adjustment on the linearly polarized light of another wavelength, controls the magnitude of the phase delay and the degree of polarization adjustment, and can achieve the sensitivity adjustment within the system measurement range. Controlling the degree of polarization adjustment can ensure a constant phase difference between the required signals for polarization interference, thereby ensuring the smooth progress of polarization interference. Since both the system measurement range and sensitivity are adjustable, the present invention has high reconfigurability and flexibility. The photodetector of the present invention performs square-law detection on the combined light after intensity change, and even a very small external magnetic field can be detected, thereby further improving the sensitivity. After the photodetector performs square-law detection on the combined light after intensity change, six electrical signals are obtained. Based on these six electrical signals, the processor can generate four polarization interferences. It can be seen that the present invention realizes multiple interferences using a single interference structure, greatly reducing the complexity of the system structure. The processor obtains two cursor envelopes according to the four polarization interferences, and superimposing the two cursor envelopes can significantly amplify the frequency shift caused by the magnetic field, thereby further improving the sensitivity. Under the action of an external magnetic field, the lower envelope formed after superimposing the two cursor envelopes will have a split valley. The present invention adopts a new demodulation method for demodulating the frequency width of the split valley, and determines the magnitude of the external magnetic field according to the frequency width of the split valley, realizing the demodulation conversion from the intensity domain to the frequency domain, thereby greatly improving the sensitivity.

[0042] 2. The present invention uses a chirped fiber Bragg grating for phase delay. Based on the characteristic that the chirped fiber Bragg grating has different magnitudes of phase delay for linearly polarized light of different wavelengths, when the wavelength of the linearly polarized light transmitted to the chirped fiber Bragg grating is fixed, the system measurement range and the phase of the linearly polarized light of the corresponding wavelength after passing through the chirped fiber Bragg grating can be accurately known. Thereafter, according to the phase of the linearly polarized light of the corresponding wavelength after passing through the chirped fiber Bragg grating, the degree of polarization adjustment can be accurately controlled so that within this system measurement range, there is a constant phase difference between the corresponding light beams after the combined light passes through the magneto-optical crystal.

[0043] 3. The present invention connects the chirped fiber Bragg grating and the coupler in series to form a ring-shaped chirped fiber Bragg grating, thereby further increasing the phase delay.

[0044] 4. The present invention arranges the magneto-optical crystal at an interval from the external magnetic field, so that the system measurement result is not easily affected by the external environment, thereby improving the measurement accuracy.

[0045] 5. The present invention utilizes an electro-optic modulator to convert an optical-domain signal into a microwave-domain signal. By taking advantage of the characteristics of high resolution, fast demodulation speed, and low demodulation cost of the microwave-domain signal, the system resolution and response speed can be significantly improved. At the same time, the electromagnetic interference resistance of the system is enhanced, making the system suitable for complex environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 is a schematic structural diagram of an embodiment of the magnetic field sensing system based on polarization interference of the present invention;

[0047] Figure 2 is a schematic diagram of the superposition of cursor envelopes and a fitting diagram of magnetic field sensitivity under different conditions;

[0048] Figure 3 is another schematic diagram of the superposition of cursor envelopes and a fitting diagram of magnetic field sensitivity under different conditions;

[0049] Figure 4 is a test diagram of system stability;

[0050] Figure 5 is a schematic structural diagram of another embodiment of the magnetic field sensing system based on polarization interference of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0051] In order to enable those skilled in the art to better understand the technical solutions in the embodiments of the present invention and make the above-mentioned objects, features, and advantages of the embodiments of the present invention more obvious and understandable, the technical solutions in the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0052] In the description of the present invention, unless otherwise specified and defined, it should be noted that the term "connection" should be understood in a broad sense. For example, it can be a mechanical connection or an electrical connection, or it can be the communication inside two components. It can be directly connected or indirectly connected through an intermediate medium. For those of ordinary skill in the art, the specific meanings of the above terms can be understood according to specific circumstances.

[0053] See Figure 1, which is a schematic structural diagram of an embodiment of the magnetic field sensing system based on polarization interference of the present invention. The magnetic field sensing system based on polarization interference may include a polarizer POL1, an electro-optic modulator EOM, a delay polarization adjustment module ODM, a polarization beam combiner PBC, a magneto-optic crystal MOC, an analyzer POL2, a photodetector PD, and a processor connected in sequence; the polarizer POL1 converts an incoherent optical signal into a linearly polarized light and transmits it to the electro-optic modulator EOM, and the electro-optic modulator EOM modulates the linearly polarized light into two linearly polarized lights with different polarization states and transmits the two linearly polarized lights to the delay polarization adjustment module ODM. Among them, the polarization states of the linearly polarized lights may be orthogonal, which are TE polarization state and TM polarization state respectively. Since the two linearly polarized lights are converted from an incoherent optical signal, the two linearly polarized lights have incoherence.

[0054] After receiving the linearly polarized light, the delay polarization adjustment module ODM first performs wavelength screening on the linearly polarized light, performs phase delay processing on the linearly polarized light corresponding to the wavelength in the two linearly polarized lights, and obtains the linearly polarized light corresponding to the wavelength with two polarization states after phase delay. After completing the phase delay, the phase of the linearly polarized light corresponding to the wavelength changes, and its two polarization states also change relative to the TE polarization state and the TM polarization state. The delay polarization adjustment module ODM also performs polarization adjustment on the linearly polarized light with the other wavelength in the two linearly polarized lights, and obtains the linearly polarized light with the other wavelength with two polarization states after polarization adjustment. After completing the polarization adjustment, the two polarization states of the linearly polarized light with the other wavelength change relative to the TE polarization state and the TM polarization state, and its phase also changes. The polarization beam combiner PBC combines and transmits the linearly polarized light corresponding to the wavelength with two polarization states after phase delay processing and the linearly polarized light with the other wavelength with two polarization states after polarization adjustment to the magneto-optic crystal MOC.

[0055] Under the action of an external magnetic field, the polarization state of the combined light changes when passing through the magneto-optic crystal MOC, and the magneto-optic crystal MOC transmits the combined light with the changed polarization state to the analyzer POL2. The magneto-optic crystal MOC converts the change in the magnetic field strength into a change in the light polarization state based on the Faraday effect. The analyzer POL2 guides the combined light with the changed polarization state to the same polarization direction, so that the intensity of the combined light projected onto the transmission axis of the analyzer changes; the photodetector PD performs square-law detection on the combined light with the changed intensity and obtains six electrical signals; the processor generates four polarization interferences based on the six electrical signals, obtains two vernier envelopes, superimposes the two vernier envelopes, and determines the magnitude of the external magnetic field according to the frequency width of the split valley appearing in the lower envelope after superposition. Among them, as the external magnetic field increases, the frequency width of the split valley becomes larger.

[0056] In this embodiment, the present invention obtains two linearly polarized light beams with different polarization states and incoherence based on incoherent optical signals, which facilitates subsequent polarization interference. After obtaining the two linearly polarized light beams, since polarization interference also requires the condition of a constant phase difference, the delay polarization adjustment module performs phase delay on the linearly polarized light of the selected corresponding wavelength. On the one hand, it can facilitate subsequent polarization interference. On the other hand, the magnitude of the phase delay determines the measurement range of the system. The larger the phase delay, the wider the frequency width of the split valley that can appear, and correspondingly, the larger the measurement range of the system. That is to say, by adjusting the magnitude of the phase delay, the adjustment of the system measurement range can be achieved. In addition, the intensity depth of the split valley (i.e., the obviousness of the split valley, the system sensitivity) is determined by the intensities of the two electrical signals in polarization interference, and the intensities of these two electrical signals are related to the intensity of the combined light projected onto the transmission axis of the analyzer. Therefore, both the magnitude of the phase delay and the degree of polarization adjustment in the delay polarization adjustment module are related to the system sensitivity. The larger the phase delay and the greater the degree of polarization adjustment, the greater the intensity of the corresponding light beam in the combined light projected onto the transmission axis of the analyzer, the deeper the split valley, and the higher the system sensitivity within the measurement range of the system. That is to say, by controlling the magnitude of the phase delay and the degree of polarization adjustment in the delay polarization adjustment module, the sensitivity adjustment within the system measurement range can be achieved.

[0057] To ensure the smooth progress of polarization interference, when the magnitude of the phase delay is determined, that is, when the measurement range of the system is determined, the degree of polarization adjustment is controlled so that there is a constant phase difference between the corresponding light beams after the combined light passes through the magneto-optical crystal within the measurement range of the system. It can be seen that when controlling the magnitude of the phase delay and the degree of polarization adjustment, it is necessary to comprehensively consider the required system measurement range, system sensitivity, and the constant phase difference of the signals required for polarization interference. The control of the degree of polarization adjustment can be used to change the extinction ratio to make the polarization interference pattern reach an ideal state.

[0058] In this embodiment, the delay polarization adjustment module may include a wavelength division multiplexer (WDM), a phase delay unit, and a first polarization controller (PC). The input end of the WDM is connected to the output end of the electro-optic modulator (EOM). The first output end is connected to the first input end of the polarization beam combiner (PBC) through the phase delay unit, and the second output end is connected to the second input end of the PBC through the first PC. The WDM transmits the linearly polarized light corresponding to the wavelength in the two linearly polarized light beams to the phase delay unit for phase delay processing, and transmits the linearly polarized light of the other wavelength to the first PC for polarization adjustment. Among them, the phase delay unit may include a chirped fiber Bragg grating (CFBG). The CFBG performs phase delay processing. The refractive index of the CFBG changes in a gradient manner, and it performs different amounts of phase delay for linearly polarized light of different wavelengths. The greater the dispersion of the CFBG, the larger the free spectral range of the spectrum, the wider the frequency width of the split valley that can appear, and the larger the measurement range of the system.

[0059] The present invention uses a chirped fiber Bragg grating for phase delay. Based on the characteristic that the chirped fiber Bragg grating has different amounts of phase delay for linearly polarized light of different wavelengths, when the wavelength of the linearly polarized light transmitted to the chirped fiber Bragg grating is fixed, the measurement range of the system and the phase of the linearly polarized light of the corresponding wavelength after passing through the chirped fiber Bragg grating can be accurately obtained. Thereafter, according to the phase of the linearly polarized light of the corresponding wavelength after passing through the chirped fiber Bragg grating, the degree of polarization adjustment can be accurately controlled so that within the measurement range of the system, there is a constant phase difference between the corresponding light beams after the combined light passes through the magneto-optical crystal.

[0060] In addition, the phase delay unit may further include an optical coupler (OC). The first output end of the WDM is connected to the first input end of the OC. The first output end of the OC is connected to the second input end of the OC through the CFBG, and the second output end of the OC is connected to the first input end of the PBC. The present invention connects the chirped fiber Bragg grating and the optical coupler in series to form a ring chirped fiber Bragg grating, thereby further increasing the phase delay.

[0061] As Figure 2 shown, Figure 2 (a) is a schematic diagram of the superposition of two cursor envelopes when there is no external magnetic field; Figure 2 (b) is a schematic diagram of the superposition of two cursor envelopes when the phase delay unit is a chirped fiber Bragg grating with a length of 1 km and the external magnetic field is 240 Oe; Figure 2(c) is a schematic diagram of the superposition of two cursor envelopes when the phase delay unit is a chirped fiber Bragg grating with a length of 1 km and the external magnetic field is 480 Oe; combined with Figure 2 It can be seen from (b) and (c) that as the external magnetic field increases, the frequency width of the split valley becomes larger. When the external magnetic field increases, correspondingly, the intensity of the combined beam projected onto the transmission axis of the analyzer increases, and the intensity depth of the split valley after polarization interference becomes deeper. In addition, Figure 2 (d) is a fitting diagram of the magnetic field sensitivity when the phase delay units are chirped fiber Bragg gratings with lengths of 1 km and 2 km respectively. Since the sensitivity where Δf is the changed frequency and ΔH is the change in the magnetic field. For the same magnetic field change, the smaller the frequency change, the smaller the sensitivity. It can be seen that the longer the length of the chirped fiber Bragg grating, the greater its dispersion, and correspondingly, the higher the sensitivity within the system measurement range.

[0062] As Figure 3 shown, Figure 3 (a) and (b) are the spectrum and its amplified spectrum of the superposition of two cursor envelopes when the external magnetic field is 0 Oe respectively, Figure 3 (c) is the spectrum of the superposition of two cursor envelopes when there is an external magnetic field, Figure 3 (d) is a fitting diagram of the magnetic field sensitivity. It can be seen from the figure that when there is an external magnetic field, there is a split valley phenomenon in the lower envelope of the superposition of two cursor envelopes. In addition, Figure 4 This is a system stability test diagram. It can be seen from the figure that the stability of the system during long-term operation (90 minutes) and repeated tests is such that the maximum frequency drift is only 0.3 MHz. The present invention has good stability, which provides a reliability guarantee for engineering applications. In addition, the magneto-optical crystal MOC can be arranged at intervals from the external magnetic field, so that the measurement result of the system is not easily affected by the external environment, thereby improving the measurement accuracy.

[0063] As can be seen from the above embodiments, the present invention obtains two linearly polarized light beams with different polarization states and incoherence based on incoherent optical signals, which facilitates subsequent polarization interference; the delay polarization adjustment module performs phase delay processing on the linearly polarized light of the corresponding wavelength and adjusts the magnitude of the phase delay, which can facilitate subsequent polarization interference and can also achieve adjustment of the system measurement range. Since the system measurement range is adjustable, the dynamic range and response bandwidth of the present invention are not limited; the delay polarization adjustment module also performs polarization adjustment on the linearly polarized light of another wavelength and controls the magnitude of the phase delay and the degree of polarization adjustment, which can achieve sensitivity adjustment within the system measurement range; controlling the degree of polarization adjustment can ensure a constant phase difference for the signals required for polarization interference, thereby ensuring the smooth progress of polarization interference; since both the system measurement range and sensitivity are adjustable, the present invention has high reconfigurability and flexibility; the photodetector of the present invention performs square-law detection on the combined light after intensity change, and even a very small external magnetic field can be detected, thereby further improving the sensitivity; after the photodetector performs square-law detection on the combined light after intensity change, six electrical signals are obtained. Based on these six electrical signals, the processor can generate four polarization interferences. It can be seen that the present invention realizes multiple interferences using a single interference structure, greatly reducing the complexity of the system structure; the processor obtains two cursor envelopes based on the four polarization interferences, and superimposing the two cursor envelopes can significantly amplify the frequency shift caused by the magnetic field, thereby further improving the sensitivity; under the action of an external magnetic field, the lower envelope formed after superimposing the two cursor envelopes will exhibit split valleys. The present invention adopts a new demodulation method for demodulating the frequency width of the split valleys, and determines the magnitude of the external magnetic field according to the frequency width of the split valleys, realizing the demodulation conversion from the intensity domain to the frequency domain, thereby greatly improving the sensitivity.

[0064] See Figure 5 , which is a schematic structural diagram of another embodiment of the magnetic field sensing system based on polarization interference of the present invention. Figure 5 And Figure 1The difference of the illustrated embodiment is that the magnetic field sensing system based on polarization interference may further include a broadband light source, and the broadband light source is sequentially connected to the delay polarization adjustment module through the polarizer and the electro-optic modulator; the broadband light source provides an incoherent optical signal to the polarizer; the polarizer converts the incoherent optical signal into a linearly polarized light; the electro-optic modulator performs electro-optical modulation on the linearly polarized light provided by the polarizer according to the microwave modulation signal. Since the electro-optic modulator is a polarization-related device and its modulation efficiency for linearly polarized lights with different polarization states is different, two microwave-carrying lights with different polarization states are obtained. The electro-optic modulator transmits the microwave-carrying light to the delay polarization adjustment module. When the microwave signal in the microwave-carrying light is transmitted along the delay polarization adjustment module, the polarization beam combiner, the magneto-optic crystal, and the analyzer, it undergoes the same changes as the optical signal; the photodetector performs square-law detection on the microwave signal with changed intensity to obtain six electrical signals. The present invention uses an electro-optic modulator to convert the optical domain signal into a microwave domain signal. By utilizing the characteristics of high resolution, fast demodulation speed, and low demodulation cost of the microwave domain signal, the system resolution and response speed can be significantly improved, and at the same time, the electromagnetic interference resistance of the system can be enhanced, making the system suitable for complex environments.

[0065] Among them, the processor may be a vector network analyzer VNA. The vector network analyzer VNA sends the microwave modulation signal to the electro-optic modulator and receives the six electrical signals provided by the photodetector. Based on the six electrical signals, four polarization interferences are generated to obtain two cursor envelopes. The two cursor envelopes are superimposed, and the magnitude of the external magnetic field is determined according to the frequency width of the split valley that appears in the lower envelope after the superposition. Among them, the vector network analyzer VNA can use the S21 parameter. The 2-port is responsible for collecting signals from the PD, and the 1-port outputs a swept signal of 0 - 3 GHz to the EOM. In addition, a first amplifier EDFA1 (such as an erbium-doped fiber amplifier) may be provided between the electro-optic modulator EOM and the delay polarization adjustment module ODM, and a second amplifier EDFA2 is provided between the magneto-optic crystal MOC and the analyzer POL2.

[0066] Based on the Faraday effect, the magneto-optic crystal will rotate the polarization state of the combined light, and the Faraday angle is: V is the Verdet constant related to the material, B is the magnetic flux density parallel to the light beam, and L is the optical path length passing through the Faraday material. The Jones vector of the magneto-optic crystal is:

[0067]

[0068] After the Faraday rotation, the combined light with the changed polarization state is:

[0069]

[0070] where the values of n are 1 and 2 respectively, W TE (t, λ n , τ n ) represents linearly polarized light of corresponding wavelengths after phase delay processing and linearly polarized light of another wavelength after polarization adjustment, both initially in the TE polarization state, W TM (t, λ n , τ n ) represents linearly polarized light of corresponding wavelengths after phase delay processing and linearly polarized light of another wavelength after polarization adjustment, both initially in the TM polarization state.

[0071] As can be seen from the above embodiments, the present invention obtains two linearly polarized lights with different polarization states and incoherence based on incoherent optical signals, which facilitates subsequent polarization interference; the delay polarization adjustment module performs phase delay processing on the linearly polarized light of the corresponding wavelength and adjusts the magnitude of the phase delay, which can facilitate subsequent polarization interference and can also realize the adjustment of the system measurement range. Since the system measurement range is adjustable, the dynamic range and response bandwidth of the present invention are not limited; the delay polarization adjustment module also performs polarization adjustment on the linearly polarized light of another wavelength and controls the magnitude of the phase delay and the degree of polarization adjustment, which can realize the sensitivity adjustment within the system measurement range; controlling the degree of polarization adjustment can ensure a constant phase difference between the signals required for polarization interference, thereby ensuring the smooth progress of polarization interference; since both the system measurement range and sensitivity are adjustable, the present invention has high reconfigurability and flexibility; the photodetector of the present invention performs square-law detection on the combined light after intensity change, and even a very small external magnetic field can be detected, thereby further improving the sensitivity; after the photodetector performs square-law detection on the combined light after intensity change, six electrical signals are obtained. Based on these six electrical signals, the processor can generate four polarization interferences. It can be seen that the present invention realizes multiple interferences using a single interference structure, greatly reducing the system structure complexity; the processor obtains two cursor envelopes based on the four polarization interferences, and superimposing the two cursor envelopes can significantly amplify the frequency shift caused by the magnetic field, thereby further improving the sensitivity; under the action of an external magnetic field, the lower envelope formed after superimposing the two cursor envelopes will exhibit split valleys. The present invention adopts a new demodulation method for demodulating the frequency width of the split valleys and determines the magnitude of the external magnetic field based on the frequency width of the split valleys, realizing the demodulation conversion from the intensity domain to the frequency domain, thereby greatly improving the sensitivity.

[0072] Other embodiments of the present invention will be readily apparent to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include known or customary techniques in the art not disclosed herein. The specification and examples are to be considered exemplary only, and the true scope and spirit of the invention are pointed out by the following claims.

[0073] It should be understood that the present invention is not limited to the exact structures described above and shown in the drawings, and various modifications and changes can be made without departing from its scope. The scope of the present invention is governed only by the appended claims.

Claims

1. A magnetic field sensing system based on polarization interferometry, characterized in that: It includes a polarizer, an electro-optic modulator, a time-delay polarization adjustment module, a polarization beam combiner, a magneto-optical crystal, an analyzer, a photodetector and a processor which are connected in sequence; The polarizer converts the incoherent optical signal into linearly polarized light; The electro-optic modulator modulates the linearly polarized light into two beams of linearly polarized light with different polarization states; The time-delay polarization adjustment module performs phase delay processing on the linear polarization light of the corresponding wavelength in the two linear polarization lights, and performs polarization adjustment on the linear polarization light of the other wavelength; The polarization beam combiner transmits the linear polarized light of corresponding wavelength with two polarization states after phase delay processing and the linear polarized light of another wavelength with two polarization states after polarization adjustment to the magneto-optical crystal; Under the action of external magnetic field, the polarization state of the combined light beam changes when passing through the magneto-optical crystal; The polarizer guides the combined light beams with changed polarization states to the same polarization direction, so that the intensity of the combined light beams projected onto the transmission axis of the polarizer changes; The photoelectric detector performs square law detection on the combined light beam after intensity change to obtain six electrical signals; The processor generates four polarization interferences based on the six electrical signals to obtain two vernier envelopes, superimposes the two vernier envelopes, and determines the size of the external magnetic field according to the frequency width of the splitting valley appearing in the lower envelope after superposition.

2. The magnetic field sensing system based on polarization interferometry according to claim 1, characterized in that: As the external magnetic field increases, the frequency width of the splitting valley increases; the greater the phase delay, the wider the frequency width of the splitting valley that can appear, and the larger the measurement range of the system; The intensity depth of the splitting valley, that is, the system sensitivity is related to the intensity of the combined light beam projected onto the transmission axis of the analyzer; the greater the phase delay, the greater the degree of polarization adjustment, the greater the intensity of the corresponding light beam in the combined light beam projected onto the transmission axis of the analyzer, the deeper the splitting valley, and the higher the system sensitivity within the measurement range of the system.

3. The magnetic field sensing system based on polarization interferometry according to claim 1, characterized in that: When the phase delay size is determined, that is, the system measurement range is determined, the polarization adjustment degree is controlled so that within the system measurement range, there is a constant phase difference between the corresponding light beams after the combined light passes through the magneto-optical crystal, thereby ensuring smooth polarization interference.

4. The magnetic field sensing system based on polarization interferometry according to any one of claims 1 to 3, characterized in that: The delay polarization adjustment module includes a wavelength division multiplexer, a phase delay unit and a first polarization controller, the input end of the wavelength division multiplexer is connected to the output end of the electro-optical modulator, the first output end is connected to the first input end of the polarization beam combiner through the phase delay unit, and the second output end is connected to the second input end of the polarization beam combiner through the first polarization controller; The wavelength division multiplexer transmits the linear polarized light of the corresponding wavelength in the two beams of linear polarized light to the phase delay unit for phase delay processing, and transmits the linear polarized light of the other wavelength to the first polarization controller for polarization adjustment.

5. The magnetic field sensing system based on polarization interferometry according to claim 4, characterized in that: The phase delay unit includes a chirped fiber Bragg grating, which performs phase delay processing. The refractive index of the chirped fiber Bragg grating changes in a gradient, and performs phase delays of different magnitudes for linearly polarized light of different wavelengths. The greater the dispersion of the chirped fiber Bragg grating, the larger the free spectral range of the spectrum, the wider the frequency width of the cleavage valley that can appear, and the larger the measurement range of the system.

6. The magnetic field sensing system based on polarization interferometry according to claim 5, characterized in that: The phase delay unit also includes a coupler, the first output end of the wavelength division multiplexer is connected to the first input end of the coupler, the first output end of the coupler is connected to the second input end of the coupler through the chirped fiber Bragg grating, and the second output end of the coupler is connected to the first input end of the polarization combiner.

7. The magnetic field sensing system based on polarization interferometry according to claim 1, characterized in that: The magneto-optical crystal is spaced apart from the external magnetic field.

8. The magnetic field sensing system based on polarization interferometry according to claim 1, characterized in that: It also includes a broadband light source, which is connected to the time-delay polarization adjustment module through the polarizer and the electro-optic modulator in sequence; The broadband light source provides an incoherent light signal to the polarizer; The electro-optic modulator performs photoelectric modulation on the linear polarized light provided by the polarizer according to the microwave modulation signal. Since the electro-optic modulator is a polarization-dependent device, its modulation efficiency for linear polarized light with different polarization states is different, so two light-borne microwaves with different polarization states are obtained; the microwave signal in the light-borne microwave undergoes the same changes as the optical signal when it is transmitted along the delay polarization adjustment module, the polarization beam combiner, the magneto-optical crystal, and the analyzer; the photoelectric detector performs square-law detection on the microwave signal after the intensity change to obtain six electrical signals.

9. The magnetic field sensing system based on polarization interferometry according to claim 8, characterized in that: The processor is a vector network analyzer, which sends the microwave modulated signal to the photoelectric modulator and receives six electrical signals provided by the photoelectric detector. Based on the six electrical signals, four polarization interferences are generated to obtain two vernier envelopes, which are superimposed. The size of the external magnetic field is determined according to the frequency width of the splitting valley that appears in the lower envelope after superposition.

10. The magnetic field sensing system based on polarization interferometry according to claim 9, characterized in that: A first amplifier is arranged between the photoelectric modulator and the time-delay polarization adjustment module, and a second amplifier is arranged between the magneto-optical crystal and the analyzer.

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