Sagnac fiber magneto-optic detector and system

Abstract

The application discloses a Sagnac fiber magneto-optic detector and system, comprising a light source module, an optical modulation module, a fiber transmission module and a photoelectric signal detection and data processing module, the light source module is used for generating and outputting incoherent light signals to the optical modulation module; the optical modulation module is used for splitting the incident incoherent light signals, and after the two-way signal light obtained by splitting is subjected to double-frequency phase modulation under the driving of two sawtooth signals with different slopes, the two-way signal light is combined to generate a beat carrier signal, the frequency of the beat carrier signal is related to the scanning rate of the sawtooth signal; the fiber transmission module is used for irradiating the incident beat carrier signal to a sample after magneto-optic modulation to generate sample reflection light; the photoelectric signal detection and data processing module is used for demodulating and processing the incident sample reflection light to output a magnetic field measurement result; the problem of excessively long fiber caused by the rigid constraint relationship between the modulation frequency and the fiber length in magneto-optic sensing is solved.

Description

Technical Field

[0001] This invention relates to the field of fiber optic magneto-optical sensing and precision measurement technology, specifically to a Sagnac fiber optic magneto-optical detector and system. Background Technology

[0002] Sagnac interferometric magneto-optical sensors utilize fiber optic interferometry and the Faraday magneto-optical effect to measure magnetic fields. Their core structure includes an optical fiber loop, an electro-optic modulator (EOM), and a photodetector. When linearly polarized light is split into two beams by the fiber coupler, phase modulation occurs under the drive of the EOM, propagating in clockwise (CW) and counterclockwise (CCW) directions. Under the influence of an external magnetic field, the Faraday effect of the magneto-optical material (such as terbium-doped fiber) disrupts the reciprocal phase of the bidirectional light, producing a non-reciprocal phase shift Δ. φm = 4VHL (where V is Wilder's constant and H is the magnetic field strength, l (This refers to the length / thickness of the magneto-optical material). The phase difference, after interference, outputs a change in light intensity, which can be quantized by demodulation to determine the magnetic field strength.

[0003] Compared with traditional Hall sensors, giant magnetoresistive (GMR) devices and other magnetoelectric sensors, Sagnac magneto-optical sensors have three significant advantages: (1) Complete resistance to electromagnetic interference: Quartz optical fiber has excellent insulation properties and no signal distortion in ultra-high voltage power transmission and transformation (>500 kV) or strong pulsed magnetic field (>10 T) environments; (2) Ultra-high sensitivity: The theoretical detection limit is 0.1 nT / √Hz, which can identify weak magnetic fields at the level of magnetoencephalography; (3) Distributed architecture potential: Multiple sensing units are multiplexed on a single optical fiber link to realize long-distance continuous monitoring of magnetic fields.

[0004] In the traditional Sagnac interferometer scheme, the fiber length (loop length) is inversely proportional to the optimal modulation frequency: (in The length of the optical fiber. At the speed of light, For refractive index, (For the modulation frequency), if the selected EOM operating frequency is 3.347 MHz, then the corresponding fiber length is approximately 30.5 m. However, excessively long optical fibers can cause a series of problems: (1) First, it significantly reduces the environmental stability of the system: long optical fibers accumulate more linear birefringence and are extremely sensitive to external temperature gradients and mechanical vibrations, leading to output signal drift and increased measurement errors. (2) Second, it restricts technical performance: traditional schemes require the modulation frequency to be inversely proportional to the fiber length. Long optical fibers force the system to operate in the low-frequency region, which not only limits the dynamic response bandwidth but also makes it more susceptible to low-frequency electronic noise interference. (3) Finally, the use of long optical fibers makes the sensor bulky, expensive to manufacture, and difficult to integrate into modern compact and highly integrated devices. Therefore, fiber length has become a key bottleneck restricting the accuracy, response speed, reliability, and miniaturization of Sagnac interferometric magneto-optical sensors.

[0005] Traditional Sagnac interferometric magneto-optical sensors require shortening the optical fiber, necessitating an increase in the modulation frequency f. For example, the Sagnac interferometric magneto-optical testing system proposed in patent application CN119757230A suffers from a transit time difference τ between the clockwise (CW) and counterclockwise (CCW) beams arriving at the modulator in the fiber loop. This necessitates strict control over the relationship between fiber length and modulation frequency to ensure the system operates at its highest sensitivity. While high-frequency operation of electro-optic modulators (EOMs) is technically feasible (e.g., waveguide-type lithium niobate modulators, reaching up to GHz), increasing the modulation frequency also increases the signal frequency, requiring wider bandwidth detectors and circuitry. This introduces more noise and reduces system sensitivity. Therefore, it is necessary to introduce other technologies to resolve the conflict between frequency and fiber length in Sagnac magneto-optical sensors.

[0006] In related technologies, the literature "Research on Dual-Loop Closed-Loop Detection Technology and Polarization Fluctuation Noise of Resonant Micro-Optical Gyroscopes" by Zhang Jianjie, PhD Electronic Journal, proposes a combination of sine wave and sawtooth wave modulation technology for real signal detection and frequency shifting on a single integrated optical phase modulator. The sine wave phase modulation demodulation is mainly used for gyroscope signal detection, while the digital sawtooth wave phase modulator acts as a frequency shifter to achieve the second closed loop. This literature focuses on dual-loop closed-loop signal detection technology for resonant gyroscopes, using sine waves for modulation and demodulation (for detecting resonance peaks) and digital sawtooth waves as feedback control signals (generating frequency shifts to offset Sagnac frequency differences, achieving closed loop). Its core lies in locking the resonant frequency. Sagnac interferometric magneto-optical detectors are typically open-loop or interferometric non-resonant cavity structures. Furthermore, in the traditional sine wave modulation + sawtooth wave feedback modulation scheme described in this literature, the sine wave is essential for traditional phase-locked demodulation, while the sawtooth wave is only used for closed-loop feedback. Traditional sine wave modulation is dependent on fiber length (intrinsic frequency). In addition, a photon-assisted measurement system based on sawtooth wave frequency shifting is proposed in patent application document CN120831627A. It is based on a Mach-Zehnder (MZM) link that is transmitted or dual-path parallel. It uses sawtooth waves to shift the frequency of the optical carrier to realize the down-conversion of the microwave signal for measuring Doppler frequency shift and angle of arrival. It is not for the magnetic field signal induced in the Sagnac closed loop (non-reciprocal phase shift caused by Faraday effect). Summary of the Invention

[0007] The technical problem to be solved by this invention is how to overcome the contradiction between the rigid constraints of fiber length and modulation frequency in Sagnac fiber magneto-optic detectors, and reduce the required physical fiber length while maintaining or improving detector performance.

[0008] The present invention solves the above-mentioned technical problems through the following technical means:

[0009] A Sagnac fiber magneto-optical detector is proposed, comprising a light source module, an optical modulation module, an optical fiber transmission module, and a photoelectric signal detection and data processing module, wherein: The light source module is used to generate and output incoherent optical signals to the optical modulation module; The optical modulation module is used to split the incident incoherent light signal into beams, and then perform dual-frequency phase modulation on the two signal lights obtained by splitting the beams under the drive of two sawtooth wave signals with different slopes, and then combine the beams to generate a beat carrier signal. The frequency of the beat carrier signal is related to the scanning rate of the sawtooth wave signal. The fiber optic transmission module is used to magneto-optically modulate the incident beat carrier signal and then irradiate it onto the sample to generate sample reflected light. The photoelectric signal detection and data processing module is used to demodulate and process the incident sample reflected light to output the magnetic field measurement results.

[0010] Furthermore, the light source module includes a broadband light source, a polarization controller, and an optical isolator. The polarization controller is arranged in the laser light path emitted by the broadband light source, and the optical isolator is arranged in the emitted light path of the polarization controller.

[0011] Furthermore, the optical modulation module includes a beam splitter, a first electro-optic modulator, a second electro-optic modulator, a signal generator, and a beam combiner; A beam splitter is used to split the incident incoherent optical signal into two signal beams. The first electro-optic modulator is used to generate, under the drive of the first sawtooth wave signal output by the signal generator, f A frequency shift of 1 is used to phase modulate one signal light to generate the first modulated coherent light; The second electro-optic modulator is used to generate, driven by the second sawtooth wave signal output from the signal generator. f The frequency shift of 2 is used to phase modulate another signal light, generating a second modulated coherent light; A signal generator is used to generate a first sawtooth wave signal and a second sawtooth wave signal and apply them to a first electro-optic modulator and a second electro-optic modulator, respectively. A beam combiner is used to combine the first modulated coherent light and the second modulated coherent light to generate a beat carrier signal; The amplitude of the first sawtooth wave signal is controlled by the 2π phase shift voltage of the first electro-optic modulator, and the amplitude of the second sawtooth wave signal is controlled by the 2π phase shift voltage of the second electro-optic modulator. The adjustment range of the slopes of the first and second sawtooth wave signals is set such that the difference frequency Δ... f Between 10MHz and 100MHz, Δ f = | f 1- f 2|, f 1. f 2 represents the frequency of the modulation signal.

[0012] Furthermore, an auxiliary driving circuit is provided between the signal generator and the corresponding electro-optic modulator, the auxiliary driving circuit including an automatic bias control unit and a broadband radio frequency drive amplifier; An automatic bias control unit is used to dynamically adjust the bias voltage applied to the DC port of the corresponding electro-optic modulator by monitoring the light component output by the corresponding electro-optic modulator and using a PID algorithm. A broadband radio frequency driver amplifier is used to amplify the amplitude of the corresponding sawtooth wave signal to the half-wave voltage of the corresponding electro-optic modulator.

[0013] Furthermore, both the first sawtooth wave signal and the second sawtooth wave signal are periodic linear ramp voltages. The slope of the ramp voltage Adjustable For time; And satisfy , is the modulation coefficient.

[0014] Furthermore, the formula for the beat carrier signal is expressed as follows: V ( t )= V DC + V AC ·cos(2 π ·Δ f · t + Φ nr ) In the formula, V ( t ) represents the beat carrier signal. V DC This is the DC component of the beat signal. V AC The modulation amplitude of the beat signal. Φ nr The phase difference Δ is caused by the magnetic field. f = Δω / 2 π Δω is the frequency difference, Δω = ω1 - ω2, where ω1 and ω2 are the angular frequencies of the two lights, respectively.

[0015] Furthermore, the optical fiber transmission module includes an optical fiber circulator, a polarization-maintaining fiber, a condenser lens, a quarter-wave plate, and a sample. The optical fiber circulator is located at the input end of the polarization-maintaining fiber, the condenser lens is located at the output end of the polarization-maintaining fiber, and the sample is placed at the focal plane. An optical fiber circulator is used to control the incident beat carrier signal to circulate unidirectionally in a fixed direction between multiple ports and to propagate to the condenser lens via polarization-maintaining fiber. A condenser lens is used to focus the output light from a polarization-maintaining fiber onto the sample surface via a quarter-wave plate to generate sample reflected light.

[0016] Furthermore, the photoelectric signal detection and data processing module includes a polarizer, a balanced photodetector, a lock-in amplifier, and a data processor; A polarizer is used to decompose the light reflected from a sample into two orthogonally polarized beams and generate interference output light. A balanced photodetector is used to differentially process interference light to convert it into a differential current signal; A lock-in amplifier is used to mix and filter a differential current signal based on an external reference frequency, and demodulate the phase difference caused by the magnetic field. The external reference frequency is the frequency of the beat carrier signal. The data processor is used to calculate and output the corresponding magnetic field strength and spin-related characteristic parameters of the magnetic material based on the demodulated phase difference.

[0017] Furthermore, the data processor integrates a dynamic feedback unit based on a PID algorithm, which is used to calculate the system phase drift based on the demodulated phase difference, and to generate a corresponding feedback control signal based on the system phase drift using the PID algorithm, which is then output to the optical modulation module to form a closed-loop control circuit.

[0018] In addition, the present invention also proposes a magnetic field monitoring system, which is equipped with a Sagnac fiber optic magneto-optical detector as described above.

[0019] The advantages of this invention are: (1) This invention uses two sawtooth wave signals with different slopes to form a dual-frequency modulation signal to perform phase modulation on the two signal lights obtained by splitting the incoherent optical signal, so as to generate a frequency shift in the signal light. This is to decouple the dependence of the modulation frequency on the transit time of the optical fiber loop through the dual-frequency modulation signal, and shift the signal spectrum to a high frequency through the sawtooth wave signal. Thus, after dual-frequency phase modulation, the signal is combined to construct a high-frequency beat carrier signal that is independent of the fiber length (transit time). The frequency of the beat carrier signal is related to the scanning rate of the sawtooth wave signal, and the scanning rate of the sawtooth wave signal determines the difference frequency of the phase modulation. Therefore, by locking on the frequency shift of the AC term, the phase difference caused by the magnetic field can be demodulated, so that the demodulation frequency of the system is no longer related to the transit time. This means that the operation of the entire system is not limited by the fiber length, thus completely eliminating the dependence of traditional sinusoidal modulation on fiber length (eigenfrequency). This allows the Sagnac fiber magneto-optic detector to work at the optimal sensitivity point without long fiber, thus achieving high sensitivity on short fiber and avoiding the extremely high equipment cost of ultra-high frequency demodulation.

[0020] (2) Since traditional magneto-optical detection signals are usually located in the low-frequency region (DC to several kHz), they are highly susceptible to laser intensity noise (RIN) and circuit 1 / f Noise coverage; this invention addresses noise by adjusting the slope. k Generating a high-frequency beat carrier greater than 10 MHz can shift the spectrum of the useful signal to a high-frequency quiet region with limited shot noise, thereby significantly improving the signal-to-noise ratio.

[0021] (3) The Sagnac interferometric fiber loop in the fiber optic transmission module adopts polarization-maintaining fiber, and its physical length can reach less than 10 meters. Through the synergistic effect of dual-frequency phase modulation and sawtooth wave frequency shifting technology, under the premise of ensuring (or even improving) the system sensitivity and dynamic range, the performance that requires hundreds of meters of fiber under the traditional single sinusoidal modulation scheme can be greatly reduced to the ten-meter level. This effectively solves the problems of high environmental sensitivity, large size, high cost and low-frequency noise sensitivity caused by long fiber, and realizes the miniaturization, high stability and strong environmental adaptability of the detector.

[0022] (4) The present invention integrates a dynamic feedback unit based on PID algorithm in the data processor to calculate the phase drift in real time and control the optical modulation module to apply the compensation phase shift, forming a closed-loop control loop. This dynamic feedback mechanism can effectively suppress the zero-point drift of the system, and its zero-drift suppression rate is greater than 98%, thereby ensuring that the detector has excellent long-term operational stability in complex environments.

[0023] (5) The Sagnac fiber magneto-optical detector designed in this invention is compatible with extremely low temperature and strong magnetic field environments, and its working temperature range covers 4K to 85℃. It can maintain phase stability under a strong magnetic field of not less than 45T.

[0024] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0025] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation thereof. Figure 1 This is a schematic diagram illustrating the working principle of a Sagnac fiber optic magneto-optical detector according to an embodiment of the present invention. Figure 2 This is a schematic diagram of the structure of a Sagnac fiber magneto-optical detector according to an embodiment of the present invention; Figure 3 This is a schematic diagram of a sawtooth wave frequency shift dual-frequency modulation process in one embodiment of the present invention; Figure 4 This is a schematic diagram of the signal demodulation and algorithm processing flow in one embodiment of the present invention. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] like Figure 1 As shown, the first embodiment of the present invention proposes a Sagnac fiber optic magneto-optical detector, which includes a light source module 1, an optical modulation module 2, an optical fiber transmission module 3, and a photoelectric signal detection and data processing module 4, wherein: Light source module 1 is used to generate and output incoherent optical signals to optical modulation module 2; Optical modulation module 2 is used to split the incident incoherent optical signal into beams, and then perform dual-frequency phase modulation on the two signal beams obtained by splitting the beams under the drive of two sawtooth wave signals with different slopes, and then combine the beams to generate a beat carrier signal. The frequency of the beat carrier signal is related to the scanning rate of the sawtooth wave signal. The fiber optic transmission module 3 is used to magneto-optically modulate the incident beat carrier signal and then irradiate it onto the sample to generate sample reflected light. The photoelectric signal detection and data processing module 4 is used to demodulate and process the incident sample reflected light to output the magnetic field measurement results.

[0028] It should be noted that in this embodiment, two sawtooth wave signals with a certain frequency difference are used to form a dual-frequency modulation signal to phase-modulate the two signal beams obtained by splitting the incoherent optical signal, so as to cause a frequency shift in the signal beam. This is to decouple the dependence of the modulation frequency on the fiber loop transit time through the dual-frequency modulation signal. After dual-frequency phase modulation, the beams are combined to construct a high-frequency beat carrier signal that is independent of the fiber length (transit time). The frequency of the beat carrier signal is related to the scanning rate of the sawtooth wave signal, and the scanning rate of the sawtooth wave signal determines the difference frequency of the phase modulation. Therefore, by locking on the difference frequency of the AC term, the phase difference caused by the magnetic field can be demodulated, so that the demodulation frequency of the system is no longer related to the transit time. This means that the operation of the entire system is not limited by the fiber length, thus completely eliminating the dependence of traditional sinusoidal modulation on fiber length (eigenfrequency).

[0029] Specifically, this embodiment achieves sawtooth wave frequency shift modulation technology by constructing a dual-frequency modulation signal from two sawtooth wave signals with a certain frequency difference. This technology utilizes the Serrodyne principle, applying a sawtooth wave driving voltage of a specific frequency to the EOM to achieve linear shift of the optical frequency. According to the formula Δ... = dV / dt τ Change the voltage slope dV / dt This will create an artificial non-reciprocal phase difference in the optical path. In the time domain, the ideal sawtooth wave phase modulation (slope is...) ω s The zero-return amplitude of 2π is equivalent to introducing a fixed frequency shift Δ on the light wave frequency. f Because the modulator produces different equivalent frequency shifts for the two beams, a beat signal with a frequency equal to the difference between the two modulation frequencies is generated at the detector. This causes the system's operating point to no longer be stationary, but rather to repeatedly sweep across the ±π / 2 highest sensitivity point at a constant speed. Therefore, the system's operating point no longer depends on the transit time τ of the light propagating in the fiber loop, thus achieving decoupling between the fiber length and the modulation frequency.

[0030] As a further preferred technical solution, such as Figure 2 As shown, the light source module 1 includes a broadband light source 11, a polarization controller 12, and an optical isolator 13. The polarization controller 12 is arranged in the laser light path emitted by the broadband light source 11, and the optical isolator 13 is arranged in the emitted light path of the polarization controller 12.

[0031] Among them, the broadband light source 11 adopts a superluminescent diode (SLED) with a wavelength range of approximately 1500±85 nm, a center wavelength of 1550 nm, and an output power of approximately 5 mW. Through its incoherent broadband characteristics, it effectively reduces optical path coherence noise and provides uniform spectral output, thereby ensuring the linearity of the interference response to the magnetic field. It can output approximately flat broadband light near the center wavelength and has a short coherence length, making it difficult for the reflected light from the end face to form stable interference, thereby reducing signal interference.

[0032] The polarization controller 12 is a three-ring polarization controller that supports manual or electronic adjustment of the polarization state with a minimum step angle of ≤0.1°. It is used to precisely control the polarization state of the optical path, ensure the linear polarization of the output light, and optimize the interference performance.

[0033] Optical isolator 13 is used to suppress backlighting and optical noise, with an isolation greater than 55 dB (bandwidth 1520~1570 nm), ensuring the stability of the forward signal throughput and preventing reflected light from damaging the light source. Specifically, optical isolator 13 can be a low-insertion-loss, high-isolation 1550 nm polarized fiber isolator with a bandwidth of 1500 ± 40 nm, achieving an isolation of 52 dB at 1550 nm. This effectively suppresses backlighting noise and prevents damage to the light source from causing a decrease in system stability.

[0034] It should be noted that the light source module in this embodiment is used to provide stable light input for detecting the weak magnetic field signal generated by Sagnac interference, and effectively suppress background noise, ensuring the reliability and accuracy of signal transmission, and adapting to the needs of complex testing environments.

[0035] As a further preferred technical solution, such as Figures 2 to 3 As shown, the optical modulation module 2 includes a beam splitter 21, a first electro-optic modulator 22, a second electro-optic modulator 23, a signal generator 24, and a beam combiner 25; Beam splitter 21 is used to split the incident incoherent optical signal into two signal beams; The first electro-optic modulator 22 is used to generate, under the drive of the first sawtooth wave signal output by the signal generator 24, f A frequency shift of 1 is used to phase modulate one signal light to generate the first modulated coherent light; The second electro-optic modulator 23 is used to generate, driven by the second sawtooth wave signal output from the signal generator 24. f The frequency shift of 2 is used to phase modulate another signal light, generating a second modulated coherent light; Signal generator 24 is used to generate a first sawtooth wave signal and a second sawtooth wave signal and apply them to the first electro-optic modulator 22 and the second electro-optic modulator 23, respectively. The beam combiner 25 is used to combine the first modulated coherent light and the second modulated coherent light to generate a beat carrier signal; The amplitude of the first sawtooth wave signal is controlled by the 2π phase shift voltage of the first electro-optic modulator 22, and the amplitude of the second sawtooth wave signal is controlled by the 2π phase shift voltage of the second electro-optic modulator 23. The adjustment range of the slope of the first sawtooth wave signal and the slope of the second sawtooth wave signal is set such that the difference frequency Δ f It is located between 10MHz and 100MHz, where the 2π phase shift voltage is twice the half-wave voltage, Δ f = | f 1- f 2|, f 1. f 2 represents the frequency of the modulation signal.

[0036] Therefore, it should be noted that this embodiment uses dual-frequency modulation signals, i.e., two different modulation frequencies, to drive two phase modulators respectively, synthesizing a time-varying phase difference during beam combining, thereby eliminating the influence of the loop transit time τ on the final synthesized signal. Thus, the loop transit time τ and the modulation frequency can be decoupled. fmThe strict binding relationship is 1 / (2τ). This allows for high-sensitivity measurements using a relatively low modulation frequency on a shorter fiber optic loop, effectively avoiding the need for ultra-high frequency EOM and ultra-wideband detection. Specifically, this embodiment uses dual-frequency modulation signals to enable two electro-optic modulators to generate... f 1 and f The frequency shift of 2, the system carrier frequency, i.e., the equivalent optical frequency shift, is Δ. f = | f 1- f 2|, which cancels out the oscillation factor in the interference output signal that is related to the loop transit time τ, thus allowing the use of a relatively low modulation frequency with a shorter fiber length.

[0037] Specifically, the first electro-optic modulator 22 and the second electro-optic modulator 23 can generally be fiber-integrated wide-range electro-optic modulators (EOMs) used to precisely phase modulate the two signal beams obtained from beam splitting. In terms of working principle, the EOM modulates the incident orthogonally polarized light (TM and TE components). An ideal EOM only modulates the TM component, and the resulting photoelectric field... The expression is:

[0038] In the formula, Indicating the modulation depth is crucial for magneto-optical measurements; , These represent the electric field components that propagate light along the TM and TE axes of the electro-optic modulator, respectively. , These represent the initial amplitude values ​​of the light wave on the fast and slow axes, respectively. It is a time-varying modulated signal loaded onto the electro-optic modulator. This indicates the initial angular frequency of the signal light before modulation. It is the symbol for a complex exponent.

[0039] As a further preferred technical solution, the first electro-optic modulator 22 and the second electro-optic modulator 23 can specifically be fiber-integrated lithium niobate (X-cut Y-transfer) titanium diffused dual polarization waveguide phase modulators, with a working wavelength covering 1550 nm ± 20 nm, insertion loss ≤ 4.0 dB, polarization correlation loss ≤ 1.0 dB, support for DC to 100 MHz phase modulation, half-wave voltage ≤ 4.0 V, RF impedance of 1 MΩ, and polarization-maintaining fiber and FC / APC connector at both ends.

[0040] As a further preferred technical solution, the signal generator 24 is a dual-channel 80 MHz function / arbitrary waveform generator with a sampling rate of 1.2GSa / S, used to generate a high linearity sawtooth wave signal. Its modulation frequency can usually be set in the tens of kHz to MHz range, which is much lower than the operating frequency of traditional sine modulation.

[0041] As a further preferred technical solution, the signal generator 24 precisely controls the modulation amplitude to a value (typically 2Vπ) corresponding to the 2π phase shift voltage of the first electro-optic modulator 22 and the second electro-optic modulator 23, to achieve a complete linear phase cycle from 0 to 2π, thereby realizing dynamic operating point scanning. The characteristic of this signal is that the interference signal output by the system no longer exhibits a DC or harmonic form, but is converted into a beat carrier signal whose frequency is related to the sawtooth wave scanning rate. At this point, the non-reciprocal phase difference caused by the magneto-optical effect... Φ nr It is directly encoded in the phase information of the beat carrier signal, rather than in the traditional amplitude.

[0042] Specifically, since traditional magneto-optical detection signals are typically located in the low-frequency region (DC to several kHz), they are easily obscured by laser intensity noise (RIN) and circuit 1 / f noise. This embodiment introduces a signal generator 24 as a sawtooth wave frequency shifting module, which can bring about noise spectrum shifting by adjusting the voltage slope. k The adjustment range is set such that the equivalent frequency shift Δ f Between 10 MHz and 100 MHz, by adjusting the slope k Generate a high-frequency carrier greater than 10 MHz, reducing the system noise floor from 1 / f By shifting the region (<100 Hz) to the white noise region, the useful signal spectrum can be moved to the high-frequency quiet region where shot noise is limited, thereby significantly improving the signal-to-noise ratio and significantly improving the noise suppression rate.

[0043] It should be noted that this embodiment controls the output frequency of the sawtooth wave generator. f s To change the voltage slope k The peak voltage of the sawtooth waveform must be kept strictly equal to 2V. π Under the premise of adjusting the output frequency of the sawtooth wave generator. f s A fixed frequency difference of more than 10 MHz can be achieved at >10 MHz, thereby completing the noise spectrum shift.

[0044] As a further preferred technical solution, both the first sawtooth wave signal and the second sawtooth wave signal are periodic linear ramp voltages. The slope of the ramp voltage Adjustable For time; And satisfy , The modulation coefficient (unit: V / Hz) is the amount of signal light that, under dual-frequency signal modulation, produces a fixed frequency difference, i.e., the equivalent frequency shift, after passing through two electro-optic modulators. .

[0045] As a further preferred technical solution, the beam splitter 21 and the beam combiner 25 are low insertion loss 1550 nm polarization beam splitters / combiners with a working wavelength covering 1550 nm ± 40 nm. They are used to couple a single output containing orthogonally linearly polarized light into two fiber outputs or to couple two orthogonally polarized beams into one output fiber.

[0046] It should be noted that this embodiment employs dual-frequency modulation decoupling technology in the optical modulation module 2, instead of aiming to offset the system operating point to the linear operating region through zero-difference detection at a single modulation frequency, it uses two frequencies. f 1 and f The combination of 2 is used to synthesize a time-varying phase difference, so that the operating point of the system is no longer stationary, but changes with Δ... f = | f 1- f By repeatedly sweeping the highest sensitivity point of ±π / 2 at a frequency of 2|, the system operating point no longer depends on the transit time τ of light propagating in the fiber optic loop. This technique uses a first electro-optic modulator 22 and a second electro-optic modulator 23 (such as EOM) to load... f 1 and f 2 frequency shift.

[0047] After the two optical fibers are combined, the signal light in the fiber contains two modulated orthogonal sinusoidal wave components, which propagate along the fast and slow axes of the polarization-maintaining fiber, respectively. Their photoelectric field... The expression is:

[0048] In the formula, , These represent the electric field components that propagate light along the fast and slow axes of the polarization-maintaining fiber, respectively. , These represent the initial amplitude values ​​of the light wave on the fast and slow axes, respectively. These are time-varying modulated signals of different frequencies loaded on two axes.

[0049] As a further preferred technical solution, the interference of two coherent light beams will form a beat carrier signal, expressed by the formula: V ( t )= V DC +V AC ·cos(2 π ·Δ f · t + Φ nr ) In the formula, V ( t ) represents the beat carrier signal. V DC This is the DC component of the beat signal. V AC The modulation amplitude of the beat signal. Φ nr The phase difference Δ is caused by the magnetic field. f = Δω / 2 π Δω is the frequency difference, Δω = ω1 - ω2, where ω1 and ω2 are the angular frequencies of the two lights, respectively. f = | f 1- f 2|=|ω1- ω2| / 2 π That is, Δω / 2 π .

[0050] Therefore, during demodulation, it is only necessary to lock the frequency Δ of the AC term using a lock-in amplifier. f The phase difference caused by the magnetic field can be demodulated by mixing and filtering with a reference signal of the same frequency. Φ nr At this point, the demodulation frequency of the system is no longer related to τ, meaning that the operation of the entire system is not limited by the length of the optical fiber.

[0051] As a further preferred technical solution, such as Figure 2 As shown, the optical fiber transmission module 3 includes an optical fiber circulator 31, a polarization-maintaining fiber 32, a condenser lens 33, a quarter-wave plate 34, and a sample 35. The optical fiber circulator 31 is located at the input end of the polarization-maintaining fiber 32, the condenser lens 33 is located at the output end of the polarization-maintaining fiber 32, and the sample 35 is placed at the focal plane. The fiber optic circulator 31 is used to control the incident beat carrier signal to be unidirectionally circulated between multiple ports in a fixed direction and propagated to the condenser lens 33 via the polarization-maintaining fiber 32. Condenser 33 is used to focus the output light of polarization-maintaining fiber 32 onto the surface of sample 35 via quarter-wave plate 34 to generate sample reflected light.

[0052] Specifically, the fiber optic circulator 31 is a 1550 nm three-port polarization-maintaining fiber circulator with low insertion loss and high isolation. The operating wavelength covers 1550 nm ± 40 nm, and the isolation at 1550 nm reaches 52 dB. It is used to enable light to circulate unidirectionally in a fixed direction between multiple ports (such as 1→2→3), thereby isolating the forward and reverse optical paths and realizing the separation of signal light transmission and reception.

[0053] Meter-level polarization-maintaining fiber 32 uses 1550 nm panda-type polarization-maintaining fiber, which features precise refractive index distribution, good cross-sectional geometric symmetry, good longitudinal uniformity, and excellent optical performance. Meter-level polarization-maintaining fiber 32 (length ≤10 m) typically uses polarization-maintaining fiber with low birefringence characteristics to suppress environmental stress interference, ensure polarization transmission stability, and achieve polarization drift of less than 0.05 pm in a strong magnetic field.

[0054] Condenser 33 is a finite conjugate uncoated molded glass aspherical lens with a designed wavelength of 1550 nm. The numerical aperture on the fiber side is 0.12, which is optimally matched with the polarization-maintaining fiber. The numerical aperture on the sample side is 0.5 to ensure high light collection efficiency. Condenser 33 is located at the fiber output end, and the NA value is precisely matched with the fiber output end. It is responsible for accurately focusing the fiber output light onto the sample surface (spot size ≤ 5 μm).

[0055] The quarter-wave plate 34 is an ultra-thin quartz crystal air gap zero-order quarter-wave plate (100 μm thick), with a fixed tilt angle of 45° and a designed wavelength of 1550 nm. It has high delay accuracy and damage threshold, and good parallelism. It can convert incident linearly polarized light into left / right circularly polarized light, and then achieve a 90° optical axis flip during reflection, so that the two orthogonal components of the reflected light exchange the propagation optical axis to form an optical fiber loop.

[0056] Sample 35 uses a surface-polished rare-earth iron garnet thin film with a strong magneto-optical effect. Placing it on the focal plane supports the detection of both strongly and weakly reflective materials.

[0057] As a further preferred technical solution, the following... Figure 2 As shown, the photoelectric signal detection and data processing module 4 includes a polarizer 41, a balanced photodetector 42, a lock-in amplifier 43, and a data processor 44. Polarizer 41 is used to decompose the sample reflected light into two orthogonally polarized beams and generate interference output interference light; A balanced photodetector 42 is used to perform differential processing on the interference light to convert the interference light into a differential current signal; The lock-in amplifier 43 is used to mix and filter the differential current signal according to an external reference frequency, and demodulate the phase difference caused by the magnetic field. The external reference frequency is the frequency of the beat carrier signal. The data processor 44 is used to calculate and output the corresponding magnetic field strength and spin-related characteristic parameters of the magnetic material based on the demodulated phase difference.

[0058] Specifically, the photoelectric signal detection and data processing module 4 is used to achieve high-precision conversion and demodulation from optical signals to electrical signals and then to digital signals. The polarizer 41 employs a low-insertion-loss 1550 nm in-line polarization-maintaining fiber 45° polarizer, with a working wavelength covering 1550 nm ± 40 nm and an extinction ratio of 40 dB, ensuring interference between two beams propagating orthogonally along the fast and slow axes of the fiber. The balanced photodetector 42 adopts a dual-channel differential structure, specifically a differential design using InGaAs dual photodiodes, with a bandwidth of no less than 10 MHz and a common-mode rejection ratio exceeding 60 dB, capable of rapidly converting interference light intensity into a differential current signal. The lock-in amplifier 43 performs mixing and filtering based on an external reference frequency, extracting the phase information of the wave at a specific modulation frequency. Φ nr The lock-in amplifier 43 is a digital lock-in amplifier with a frequency range of DC - 500 kHz. It employs a digital quadrature demodulation algorithm, using an FPGA or high-speed ADC to acquire and digitize the beat signal in real time, and then extracts the instantaneous phase value of the carrier signal in the digital domain, thereby achieving [the desired function]. Φ nr The high-precision, high-linearity solution is achieved. The data processor 44 uses a 24-bit ADC combined with an anti-aliasing filter, with a sampling rate of 2 MSa / s and a signal-to-noise ratio of 80 dB. With a sampling rate of no less than 1 MSa / s and a 16-bit resolution, combined with an anti-aliasing filter with a cutoff frequency of 100 kHz, microsecond-level signal acquisition is achieved. This allows the data processing computer to run a dedicated algorithm and calculate and output the corresponding magnetic field strength, spin-related characteristic parameters of the magnetic material, and other information in real time based on the demodulated phase information.

[0059] As a further preferred technical solution, the data processor 44 has undergone adaptive improvements for the composite modulation method: the photoelectric signal detection and data processing module adopts a two-stage demodulation architecture. First, a bandpass filter is used to extract the sawtooth wave frequency as the center frequency. f s First, a high-frequency carrier signal is used to filter out low-frequency background noise. Second, using digital down-conversion (DDC) technology or an analog mixer, the carrier signal is mixed with a reference signal of the same frequency and down-converted to intermediate frequency or baseband. Finally, the difference frequency of the dual-frequency modulation signal is used to... Δf =∣ f 1 f 2 | Using the reference frequency, the signal reflected from the sample end is synchronously demodulated using phase lock, thereby extracting the phase change proportional to the magnetic field. This processing method differs from the traditional method of locking only a single fundamental frequency.f The demodulation mode can effectively utilize the signal-to-noise ratio advantage brought about by frequency shifting.

[0060] As a further preferred technical solution, the data processor integrates a dynamic feedback unit based on a PID algorithm, which is used to calculate the system phase drift based on the demodulated phase difference, and uses the PID algorithm to generate a corresponding feedback control signal based on the system phase drift and output it to the optical modulation module to form a closed-loop control circuit.

[0061] It should be noted that, in order to solve the low-frequency phase noise problem introduced by ambient temperature drift and fiber stress changes during long-term measurements, the amplitude of the sawtooth wave frequency-shifting signal is precisely controlled to correspond to the 2π phase shift voltage of the electro-optic modulator, so as to achieve dynamic operating point scanning; its repetition frequency... f s Determines the equivalent optical frequency shift Δ f By adjusting f s or voltage slope k The system dynamically compensates for the real non-reciprocal phase shift caused by the magnetic field, forming a closed-loop detection. Therefore, this embodiment integrates a dynamic feedback control function based on the PID algorithm in the photoelectric signal detection and data processing module 4. This function is specifically implemented by the control program in the data processor 44 or the FPGA logic unit inside the lock-in amplifier 43. During operation, this dynamic feedback unit monitors the demodulated phase signal in real time. Φ nr The baseline change is analyzed, and the quasi-static drift component is extracted as an error signal. A PID (proportional-integral-derivative) algorithm is used to calculate the error in real time, generating a reverse compensation voltage signal. This compensation signal is fed back to the optical modulation module 2 and superimposed on the DC bias port of phase modulator 22 or 23. By dynamically fine-tuning the modulator's operating point, an artificial phase shift equal in magnitude but opposite in sign to the environmental drift is generated in the optical path, thus forming a closed-loop phase lock. This mechanism achieves a zero-point drift suppression rate of over 98%, effectively ensuring the long-term baseline stability of the detector in complex environments.

[0062] Specifically, such as Figure 4 As shown, the core workflow of the photoelectric signal detection and data processing module 4 is as follows: the sample reflected light is decomposed into two orthogonally polarized beams by the polarizer 41 and interference occurs, with the light intensity following... I = I 0[1 + cos ( + Φ nr The wave equation of ], where I 0 represents the average light intensity (or reference light intensity) of the interference field. This indicates the phase difference caused by the sample. Φ nr =2 θ K This is a non-reciprocal phase shift caused by a magnetic field. θ K The Kernel angle is used. The balanced photodetector 42 effectively eliminates common-mode noise through differential output. Subsequently, the lock-in amplifier 43 can significantly suppress 1 / f Low-frequency noise (e.g., 30 dB@1 Hz) enables phase demodulation accuracy to reach 10 nrad, sufficient to detect weak magnetic field changes of 0.01 rad / Gauss. Furthermore, to improve the baseline stability of the system during long-term measurements, the photoelectric signal detection and data processing module 4 integrates a dynamic feedback unit based on a PID algorithm. This unit analyzes the demodulated phase data in real time to calculate the system phase drift caused by factors such as ambient temperature changes and mechanical vibrations; then, it uses the PID algorithm to generate a corresponding feedback control signal and transmits this signal to the optical modulation module 2. By dynamically adjusting the DC bias voltage applied to the electro-optic modulator, a precise reverse compensation phase shift is applied to the optical path, forming a closed-loop control circuit. This dynamic feedback mechanism effectively suppresses zero-point drift of the system, with a zero-drift suppression rate greater than 98%, thereby ensuring excellent long-term operational stability of the detector in complex environments.

[0063] It should be noted that this embodiment is the first to synergistically introduce dual-frequency modulation and sawtooth wave frequency shifting technology into the Sagnac magneto-optical measurement system. Through the synergistic modulation effect of these two technologies, the rigid constraint contradiction between fiber length (L) and modulation frequency (f) in traditional Sagnac magneto-optical sensors (i.e., L∝1 / f) is resolved. This achieves a significant reduction in fiber length (lowering system cost and environmental sensitivity) while enhancing magnetic field response bandwidth and noise suppression capabilities, addressing the issues of low-frequency noise sensitivity, limited dynamic response, and low integration caused by long fibers. While maintaining or even improving system sensitivity and dynamic range, it effectively reduces the physical dependence on long fiber loops. Its significance lies in: firstly, significantly reducing system size, weight, and cost, improving engineering practicality; secondly, weakening environmental noise and birefringence drift introduced by long-distance fibers, enhancing system stability; and finally, opening a new path for deploying high-precision optical sensing in space-constrained extreme environments (such as deep space exploration and low-temperature strong magnetic field platforms), promoting the development of miniaturized, high-performance integrated magneto-optical sensors.

[0064] The Sagnac fiber optic magneto-optical detector in this embodiment can achieve high-precision measurement of magnetic field strength at the nanoradian level, with a response lower limit of approximately 0.01 rad / Gauss. It effectively solves the problems of low-frequency noise sensitivity, limited dynamic response, and low integration caused by long optical fibers due to rigid constraints on fiber length and modulation frequency in traditional Sagnac magneto-optical sensors. Furthermore, the detector is compatible with extremely low temperature and strong magnetic field environments, operating within a temperature range of 4K to 85℃, and can maintain phase stability under strong magnetic fields of no less than 45T.

[0065] Furthermore, the second embodiment of the present invention also proposes a magnetic field monitoring system, which is equipped with the Sagnac fiber magneto-optical detector as described in the first embodiment above.

[0066] It should be noted that the Sagnac fiber optic magneto-optical detector proposed in this invention has outstanding application value in aerospace (miniaturized gyroscopes and spaceborne magnetic field monitoring), quantum computing (compact flux sensing in extremely low temperature and strong magnetic field environments), and the power Internet of Things (embedded magnetic field monitoring and fault early warning for substation equipment). It can reduce the fiber length requirement of traditional Sagnac systems from hundreds of meters to tens of meters or even meters while maintaining nanoradian-level phase demodulation accuracy and high environmental stability. This significantly reduces sensor size, power consumption, and cost, and improves robustness under complex environments such as vibration and temperature changes, providing key technical support for the engineering and widespread application of high-performance optical sensing.

[0067] It should be understood that various parts of the present invention can be implemented in hardware, software, firmware, or a combination thereof. In the above embodiments, multiple steps or methods can be implemented in software or firmware stored in memory and executed by a suitable instruction execution system. For example, if implemented in hardware, as in another embodiment, it can be implemented using any one or a combination of the following techniques known in the art: discrete logic circuits having logic gates for implementing logical functions on data signals, application-specific integrated circuits (ASICs) having suitable combinational logic gates, programmable gate arrays (PGAs), field-programmable gate arrays (FPGAs), etc.

[0068] In the description of this specification, references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0069] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" or "several" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0070] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A Sagnac fiber optic magneto-optical detector, characterized in that, It includes a light source module, an optical modulation module, an optical fiber transmission module, and a photoelectric signal detection and data processing module, among which: The light source module is used to generate and output incoherent optical signals to the optical modulation module; The optical modulation module is used to split the incident incoherent light signal into beams, and then perform dual-frequency phase modulation on the two signal lights obtained by splitting the beams under the drive of two sawtooth wave signals with different slopes, and then combine the beams to generate a beat carrier signal. The frequency of the beat carrier signal is related to the scanning rate of the sawtooth wave signal. The fiber optic transmission module is used to magneto-optically modulate the incident beat carrier signal and then irradiate it onto the sample to generate sample reflected light. The photoelectric signal detection and data processing module is used to demodulate and process the incident sample reflected light to output the magnetic field measurement results.

2. The Sagnac fiber optic magneto-optical detector as described in claim 1, characterized in that, The light source module includes a broadband light source, a polarization controller, and an optical isolator. The polarization controller is arranged in the laser light path emitted by the broadband light source, and the optical isolator is arranged in the emitted light path of the polarization controller.

3. The Sagnac fiber optic magneto-optical detector as described in claim 1, characterized in that, The optical modulation module includes a beam splitter, a first electro-optic modulator, a second electro-optic modulator, a signal generator, and a beam combiner; A beam splitter is used to split the incident incoherent optical signal into two signal beams. The first electro-optic modulator is used to generate, under the drive of the first sawtooth wave signal output by the signal generator, f A frequency shift of 1 is used to phase modulate one signal light to generate the first modulated coherent light; The second electro-optic modulator is used to generate, driven by the second sawtooth wave signal output from the signal generator. f The frequency shift of 2 is used to phase modulate another signal light, generating a second modulated coherent light; A signal generator is used to generate a first sawtooth wave signal and a second sawtooth wave signal and apply them to a first electro-optic modulator and a second electro-optic modulator, respectively. A beam combiner is used to combine the first modulated coherent light and the second modulated coherent light to generate a beat carrier signal; The amplitude of the first sawtooth wave signal is controlled by the 2π phase shift voltage of the first electro-optic modulator, and the amplitude of the second sawtooth wave signal is controlled by the 2π phase shift voltage of the second electro-optic modulator. The adjustment range of the slopes of the first and second sawtooth wave signals is set such that the equivalent frequency shift Δ f Between 10MHz and 100MHz, Δ f = | f 1- f 2|, f 1. f 2 represents the frequency of the modulation signal.

4. The Sagnac fiber optic magneto-optical detector as described in claim 3, characterized in that, An auxiliary driving circuit is provided between the signal generator and the corresponding electro-optic modulator. The auxiliary driving circuit includes an automatic bias control unit and a broadband radio frequency drive amplifier. An automatic bias control unit is used to dynamically adjust the bias voltage applied to the DC port of the corresponding electro-optic modulator by monitoring the light component output by the corresponding electro-optic modulator and using a PID algorithm. A broadband radio frequency driver amplifier is used to amplify the amplitude of the corresponding sawtooth wave signal to the half-wave voltage of the corresponding electro-optic modulator.

5. The Sagnac fiber optic magneto-optical detector as described in claim 3, characterized in that, Both the first sawtooth wave signal and the second sawtooth wave signal are periodic linear ramp voltages. The slope of the ramp voltage Adjustable For time; And satisfy , is the modulation coefficient.

6. The Sagnac fiber optic magneto-optical detector as described in claim 3, characterized in that, The formula for the beat carrier signal is as follows: V ( t )= V DC + V AC ·cos(2 π ·D f · t + Φ nr ) In the formula, V ( t ) represents the beat carrier signal. V DC This is the DC component of the beat signal. V AC The modulation amplitude of the beat signal. Φ nr The phase difference Δ is caused by the magnetic field. f = Δω / 2 π Δω is the frequency difference, Δω = ω1 - ω2, where ω1 and ω2 are the angular frequencies of the two lights, respectively.

7. The Sagnac fiber optic magneto-optical detector as described in claim 1, characterized in that, The optical fiber transmission module includes an optical fiber circulator, a polarization-maintaining fiber, a condenser lens, a quarter-wave plate, and a sample. The optical fiber circulator is located at the input end of the polarization-maintaining fiber, the condenser lens is located at the output end of the polarization-maintaining fiber, and the sample is placed at the focal plane. An optical fiber circulator is used to control the incident beat carrier signal to circulate unidirectionally in a fixed direction between multiple ports and to propagate to the condenser lens via polarization-maintaining fiber. A condenser lens is used to focus the output light from a polarization-maintaining fiber onto the sample surface via a quarter-wave plate to generate sample reflected light.

8. The Sagnac fiber optic magneto-optical detector as described in claim 1, characterized in that, The photoelectric signal detection and data processing module includes a polarizer, a balanced photodetector, a lock-in amplifier, and a data processor. A polarizer is used to decompose the light reflected from a sample into two orthogonally polarized beams and generate interference output light. A balanced photodetector is used to differentially process interference light to convert it into a differential current signal; A lock-in amplifier is used to mix and filter a differential current signal based on an external reference frequency, and demodulate the phase difference caused by the magnetic field. The external reference frequency is the frequency of the beat carrier signal. The data processor is used to calculate and output the corresponding magnetic field strength and spin-related characteristic parameters of the magnetic material based on the demodulated phase difference.

9. The Sagnac fiber optic magneto-optical detector as described in claim 8, characterized in that, The data processor integrates a dynamic feedback unit based on a PID algorithm, which is used to calculate the system phase drift based on the demodulated phase difference, and uses the PID algorithm to generate a corresponding feedback control signal based on the system phase drift and output it to the optical modulation module to form a closed-loop control circuit.

10. A magnetic field monitoring system, characterized in that, It is equipped with a Sagnac fiber magneto-optical detector as described in any one of claims 1-9.

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