System and method for real-time measurement of microwave polarization direction based on dual-local microwave electric field

Abstract

The present application relates to a system and method for real-time measurement of microwave polarization direction based on two local microwave electric fields, comprising two lasers, a microwave source and a microwave horn antenna, a Helmholtz coil, a rubidium atomic cell, a motorized rotating stage, a photodetector and a spectrum analyzer, the two lasers generate probe light and coupling light respectively, and the microwave horn antenna radiates microwave to the rubidium atomic cell. The probe light and the coupling light excite the atoms in the rubidium atomic cell to the Rydberg state under the influence of the microwave electric field. By measuring the transmittance change of the probe light with a frequency near the electromagnetically induced transparency resonance frequency, two beat frequency signal amplitudes of different frequencies generated by the two local microwave electric fields and the signal microwave electric field are obtained, and the real-time measurement of the polarization direction of the signal microwave electric field is realized according to the ratio of the two beat frequency signal amplitudes. By measuring the change of the beat frequency signal amplitude before and after applying a magnetic field, the angular resolution of the microwave electric field polarization direction in the range of 0 to 180 degrees is realized.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of weak signal detection, and particularly relates to a system and method for real-time measurement of microwave polarization direction based on double local microwave electric field. BACKGROUND

[0002] Compared with traditional sensors, quantum sensors have higher sensitivity and accuracy [Quantum sensing, C. L. Degen, F. Reinhard, and P. Cappellaro, Rev. Mod. Phys. 89, 035002 (2017)]. The application of quantum sensors in electromagnetic field measurement is particularly prominent, especially the measurement of microwave electric field, which has important significance for the fields of wireless communication, radar system, astronomy, etc. [Nature Physics, 8, 819-824 (2012)]. Accurate measurement of microwave polarization direction has important significance for understanding and optimizing microwave transmission characteristics. Precise measurement of microwave polarization direction can provide information about the propagation environment of electromagnetic waves, such as reflection, scattering and absorption, etc., and thus has important applications in radar, communication, remote sensing, non-destructive detection, etc.

[0003] In 2013, J. A. Sedlacek et al. first measured the polarization of the microwave electric field by using the Rydberg EIT in the Rb atomic vapor cell, and achieved an angular resolution of 0.5 degrees by the polarization of the microwave electric field relative to the polarization of the probe light and the coupling light [Phys. Rev. Lett. 111(6), 063001 (2013)]. This scheme is very dependent on the polarization of the light field, and complex modeling analysis is needed to get the results. In 2022, Zhang Linjie et al. based on the Rydberg atomic EIT-AT effect, realized the calibration of the polarization direction of the microwave electric field by analyzing the EIT-AT spectral characteristics of the room temperature cesium Rydberg atom, and the angular resolution reached 1.64 degrees [Nat. Phys. 16(9), 911-915 (2020)]. In 2023, Liu Xiubin et al. used the EIT-AT of Rydberg atoms to measure the microwave polarization in cold atoms [Journal of Spacecraft TT&C Technology, 2023, 43(3): 5-10]. In 2019, Simons et al. first demonstrated a measurement device for microwave polarization based on Rydberg atoms [IEEE Access 7, 164975-164985 (2019)], and in 2023, Wang Yuhand et al. used a mixer to accurately measure the microwave polarization [Opt. Express 31, 10449-10457 (2023)], and the results showed that the amplitude of the beat signal changed with the polarization of the microwave electric field within a period of 180 degrees. The current scheme for measuring the polarization direction of the microwave electric field using a Rydberg atomic mixer often needs to change the angle of the local microwave electric field or the signal microwave electric field horn antenna, and needs to rotate the polarization of the local microwave electric field by 180 degrees to get the polarization direction of the microwave field to be measured, which is difficult to operate. At the same time, due to the symmetry of the positive and negative polarization directions, a single measurement cannot distinguish whether the polarization direction of the microwave electric field is X or 180-X within the range of 0-180 degrees, and generally the change trend is used to distinguish by continuous measurement, which makes it difficult to realize real-time high-resolution measurement of the polarization direction of the microwave electric field. SUMMARY

[0004] In view of the defects in the prior art, the purpose of the present application is to provide a system and method for real-time measurement of microwave polarization direction based on two local microwave electric fields, two polarized orthogonal local microwave electric fields are introduced, the frequencies of the two local microwave electric fields are set respectively to be different from the frequency of the signal microwave electric field, so that the two local microwave electric fields will generate a beat frequency signal with the signal microwave electric field to be measured respectively. In this way, the amplitudes of the two beat frequency signals can be obtained in a single measurement, and the polarization angle of the signal microwave electric field is directly obtained through the ratio of the amplitudes of the two beat frequency signals. Further, in order to distinguish the polarization direction of the microwave electric field in the range of 0-180 degrees, the influence of the external magnetic field on the polarization measurement is studied to realize the polarization direction distinction of X degrees or 180-X degrees. This scheme greatly simplifies the theoretical analysis and experimental system of measuring microwave polarization using Rydberg atoms, realizes real-time high-resolution measurement of microwave polarization, and has important significance and value in microwave sensing and practical application.

[0005] To achieve the above purpose, the technical scheme adopted by the present application is:

[0006] In a first aspect, a system for real-time measurement of microwave polarization direction based on two local microwave electric fields, the system comprises a probe light laser and its frequency locking system, a coupling light laser and its frequency locking system, three microwave sources, three microwave horn antennas, two dichroic mirrors, a pair of Helmholtz coils, a rubidium atomic cell, a motorized rotation table, a photodetector, and a frequency spectrometer, the probe light laser and its frequency locking system are used to generate probe light, the coupling light laser and its frequency locking system are used to generate coupling light, the three microwave sources are used as inputs of two local microwave electric fields and a signal microwave electric field respectively, the three microwave horn antennas are connected to the three microwave sources respectively to radiate microwaves to the rubidium atomic cell, the microwave horn antennas on both sides output first and second local microwave electric fields, and the microwave horn antenna in the middle outputs a signal microwave electric field, the two dichroic mirrors transmit the probe light and the coupling light respectively, a pair of Helmholtz coils are arranged on both sides of the rubidium atomic cell, the pair of Helmholtz coils are used to provide a uniform magnetic field along the direction of laser propagation, the rubidium atomic cell is used to provide rubidium atoms to interact with laser and microwave, the motorized rotation table is used to change the polarization direction of the microwave horn antenna, the photodetector receives the probe light after passing through the rubidium atomic cell, and the frequency spectrometer receives the signal of the photodetector to perform frequency spectrum analysis on the probe light after passing through the rubidium atomic cell, the system measures the signal microwave electric field strength by detecting the change of the light intensity of the probe light near the electromagnetically induced transparency resonance of the Rydberg atom, and measures the polarization direction of the signal microwave electric field in real time according to the ratio of the amplitude of the beat frequency signal of the first frequency generated by the first local microwave electric field to the amplitude of the beat frequency signal of the second frequency generated by the second local microwave electric field.

[0007] Further, the polarization states of the probe light and the coupling light are the same direction of circular polarization.

[0008] Further, the three microwave horn antennas are arranged at a position far from the rubidium atom cell, the three microwave horn antennas are arranged adjacent to each other, the signal microwave electric field and the local microwave electric field have close frequencies, and the polarizations of the two local microwave electric fields are orthogonal and perpendicular.

[0009] Further, the polarizations of the two local microwave electric fields are linear polarizations, the polarization direction of the first local microwave electric field is perpendicular to the propagation direction of the laser, and the polarization direction of the second local microwave electric field is parallel to the propagation direction of the laser.

[0010] Further, the mutual orthogonality of the polarizations of the two local microwave electric fields is realized by adjusting the angles of the rectangular horn antennas of the two local microwave electric fields.

[0011] Further, the system continuously changes the linear polarization direction of the signal microwave electric field in a range of 360 degrees by fixing the rectangular horn antenna of the signal microwave electric field on the motorized rotating table and continuously rotating.

[0012] Further, the rubidium atom cell comprises a vacuum glass cell filled with rubidium atom vapor.

[0013] In a second aspect, a method for measuring a microwave polarization direction in real time based on double local microwave electric fields, the method measures the microwave polarization direction in real time by using the system for measuring a microwave polarization direction in real time based on double local microwave electric fields according to the first aspect and any one of the optional embodiments thereof, and the method comprises the following steps:

[0014] S1, the probe light and the coupling light of different wavelengths are propagated in opposite directions in the rubidium atom cell to form electromagnetically induced transparency of the Rydberg atom, after the application of a microwave electric field, the electromagnetically induced transparency of the Rydberg atom is split by Autler-Townes, and the change of the signal microwave electric field intensity is measured by the change of the transmittance of the probe light at the resonance position of the electromagnetically induced transparency of the Rydberg atom;

[0015] S2, the signal microwave electric field and the two orthogonal polarization local microwave electric fields of different frequencies form beat signals, respectively;

[0016] S3, the polarization direction of the signal microwave electric field is changed, and the polarization directions of the two local microwave electric fields remain unchanged;

[0017] S4, the polarization direction of the measured signal microwave electric field is converted into the ratio of the two beat signals;

[0018] S5, the ratio of the two beat signals obtained by the switching magnetic field is compared with a preset value to determine the polarization direction of the microwave electric field in the range of 0-180 degrees or 180-X.

[0019] Further, the relationship between the transmission rate of the probe light and the amplitude E LO of the local microwave electric field in step S1 is as follows: SIG

[0020]

[0021] T probe represents the transmission rate of the probe light, E mod represents the total electric field experienced by the atoms, including the local microwave electric field E LO and the signal microwave electric field E SIG , Δω represents the frequency difference between the signal and the local microwave, defined as Δω = ω LO - ω SIG , where ω SIG and ω LO are their angular frequencies, respectively, and Δφ represents the phase difference between the two microwaves, defined as Δφ = φ LO - φ SIG , where φ SIG and φ LO are their phases, respectively.

[0022] Further, the relationship between the amplitudes of the two beat signals and the amplitudes E LO1 , E LO2 of the two local microwave electric fields and the amplitude E SIG of the signal microwave electric field in step S2 is as follows:

[0023] A beat1 ∝ |E LO1 + E SIG cosθ|

[0024] A beat2 ∝ |E LO2 + E SIG sinθ|

[0025] where A beat represents the amplitude of the beat signal, and θ is defined as the angle between the polarization directions of the signal microwave and the local microwave electric fields.

[0026] ​The beneficial technical effect of the present application is that: based on the Rydberg atom mixer, two orthogonal polarization local microwave electric fields are used to measure the microwave polarization direction in real time. In the Rydberg atom mixer, the signal microwave electric field and the local microwave electric field with a frequency close to it form a beat signal, and people use the beat signal to show the accurate measurement of the microwave phase, intensity, frequency and polarization. On the basis of the previous, two polarization orthogonal vertical local microwave electric fields are introduced, and the frequency difference between the two local microwave electric fields and the signal microwave electric field is set respectively, and a beat signal is generated with the signal microwave electric field to be measured. In this way, the amplitudes of the two beat signals can be obtained in a single measurement, and the amplitudes of the two beat signals reflect the projection of the intensity of the signal microwave electric field on the two orthogonal polarization local microwave electric fields. Through the ratio of the amplitudes of the two beat signals, the polarization angle of the signal microwave electric field is directly obtained. Further, in order to distinguish the polarization direction of the microwave electric field in the range of 0-180 degrees, the influence of the external magnetic field on the polarization measurement is studied to realize the polarization direction distinction of X degrees or 180-X degrees. The scheme greatly simplifies the theoretical analysis and experimental system of measuring the microwave polarization by using the Rydberg atom, realizes the real-time high-resolution measurement of the microwave polarization, and has important significance and value in the microwave sensing and practical application. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 The energy level diagram of the system for measuring the microwave polarization direction in real time based on two local microwave electric fields according to the embodiment one of the present application is shown, and note that 47P 3 / 2 The energy level actually in 46D 5 / 2 In the following, for the convenience of display, it is drawn above;

[0028] Figure 2 The structure diagram of the system for measuring the microwave polarization direction in real time based on two local microwave electric fields according to the embodiment one of the present application is shown;

[0029] Figure 3 The theoretical diagram of the change of the amplitudes of the two beat signals with the polarization direction of the signal microwave electric field is shown, and the curve shows the change curve of the beat amplitude with the polarization angle in formula (2), and the upper right corner of the drawing is the relationship between the ratio of the amplitudes of the two beat signals and the polarization direction of the signal microwave electric field;

[0030] Figure 4 The typical beat signals generated by the signal microwave electric field and the two local microwave electric fields 3 when the signal microwave electric field has different polarization directions are shown, (upper) the polarization direction of the signal microwave electric field is the same as that of the first local microwave electric field, (middle) the polarization direction of the signal microwave electric field is the same as that of the second local microwave electric field, and (lower) the polarization direction of the signal microwave electric field is 45 degrees with the polarization directions of the first local microwave electric field and the second local microwave electric field;

[0031] Figure 5The graph shows the relationship between the beat frequency amplitude of 12kHz and 10kHz and the polarization angle of the signal microwave electric field when there is no magnetic field. The green and blue curves in the graph are the results of fitting according to formula (2).

[0032] Figure 6 The graph shows the relationship between the beat frequency amplitude at 12kHz and 10kHz and the polarization angle of the microwave electric field under a Gaussian static magnetic field.

[0033] Figure 7 The figure shows the fitting relationship between the ratio of the two beat frequency signals and the polarization direction of the microwave electric field under the two conditions of B=4G and no magnetic field.

[0034] Among them: 1-Detector laser and its frequency locking system, 2-Coupled laser and its frequency locking system, 3-Local microwave electric field, 4-Signal microwave electric field, 5-Microwave horn antenna, 6-Dichroic mirror, 7-Helmholtz coil, 8-Russian atomic vapor cell, 9-Photodetector, 10-Spectrum analyzer, 11-Electric rotary table. Detailed Implementation

[0035] The present invention will now be further described with reference to the accompanying drawings and specific embodiments.

[0036] Example 1

[0037] like Figure 1 and Figure 2 As shown, this embodiment of the invention provides a system for real-time measurement of microwave polarization direction based on dual local microwave electric fields, including a probe laser and its frequency locking system 1, a coupling laser and its frequency locking system 2, three microwave sources, three microwave horn antennas 5, two dichroic mirrors 6, a pair of Helmholtz coils 7, a rubidium atomic vapor cell 8, a photodetector 9, a spectrum analyzer 10, and an electric rotary table 11.

[0038] A probe laser and its frequency-locking system 1 are used to generate probe light, a coupling laser and its frequency-locking system 2 are used to generate coupling light, three microwave sources are used as inputs to two local microwave electric fields 3 and one signal microwave electric field 4, respectively, three microwave horn antennas 5 are connected to the three microwave sources respectively, radiating microwaves to a rubidium atom vapor cell 8, two dichroic mirrors 6 respectively direct the probe light and coupling light laser beams through the rubidium atom vapor cell 8, a pair of Helmholtz coils 7 are used to provide a uniform magnetic field along the laser propagation direction, the rubidium atom vapor cell 8 includes a vacuum glass cell filled with rubidium atom vapor, the rubidium atom vapor cell 8 is used to provide rubidium atom vapor to interact with the laser and microwaves, a photodetector 9 receives the probe light, a spectrum analyzer 10 is used to receive the signal from the photodetector and perform spectrum analysis, and an electric rotary table 11 is used to change the polarization direction of the microwave horn antenna.

[0039] The probe light and the coupling light are both split by the dichroic mirror, and the probe light and the coupling light split by the dichroic mirror are propagated in the rubidium atom cell in opposite directions, and the Helmholtz coil is arranged on the two sides of the rubidium atom cell, and the Helmholtz coil applies a magnetic field with a size of 4G along the positive direction of the z axis at the rubidium atom cell.

[0040] The three microwave horn antennas 5 are arranged at positions far from the rubidium atom cell, and the three microwave horn antennas 5 are arranged in parallel, the microwave horn antennas 5 on the two sides output first and second local microwave electric fields, and the microwave horn antenna 5 in the middle is fixed on the electric rotating table 11 to output a signal microwave electric field, the signal microwave electric field has a frequency close to that of the local microwave electric field, and the two local microwave electric fields are polarized orthogonally and vertically.

[0041] The photodetector 9 is used to measure the intensity information of the probe light after passing through the rubidium atom cell, the output of the photodetector 9 is connected to an oscilloscope to obtain spectral information, and the output of the photodetector 9 is connected to a frequency spectrometer 10 to obtain frequency spectrum information.

[0042] In the embodiment of the application, the probe light is a 780nm laser, and the coupling light is a 480nm laser, the 780nm probe light and the 480nm coupling light are propagated in opposite directions in the rubidium atom cell to form electromagnetically induced transparency (EIT) of Rydberg atoms, at this time, the EIT will be split by Autler-Townes after a microwave electric field is applied, and the intensity of the microwave electric field can be measured by detecting the change in the intensity of the probe light near the EIT resonance.

[0043] In the embodiment of the application, the power of the probe light is 60 microwatts, the diameter of the probe light in the rubidium atom cell is about 800 micrometers, the power of the coupling light is 80 milliwatts, and the diameter of the coupling light in the rubidium atom cell is about 900 micrometers, and in fact, the power and the diameter are not limited.

[0044] In order to avoid the influence of the polarization directions of the probe light and the coupling light on the beat signal after a magnetic field is introduced, the polarization states of the probe light and the coupling light are set as the same direction of circular polarization. Due to the transition selection rule, the probe light and the coupling light in the circular polarization state will not only change the population of the energy level, but also cause the atoms in some states to not interact with the microwave, at this time, the Rydberg EIT-AT spectrum is no longer a double-peak structure, but a three-peak structure of the mixed three-level EIT spectrum and four-level EIT-AT split spectrum. In order to avoid the influence of the atoms only participating in the three-level EIT process, that is, the atoms not interacting with the microwave, the frequency of the probe light is locked outside the EIT-AT split peak, and there is a shift of about 5MHz relative to the resonance position.

[0045] The signal microwave and the local microwave with a similar frequency are radiated to the rubidium atom cell 8 through two microwave horn antennas 5 respectively, and interference is realized in the rubidium atom cell 8. In the embodiment of the present application, the rectangular horn antenna 5 is used to realize the transmission of the microwave electric field to the rubidium atom cell 8, and the rectangular horn antenna 5 can provide a very good linearly polarized microwave signal under the far field condition. The beat frequency signal of the interference is measured by the Rydberg atom microwave electric field sensor, that is, the periodic sinusoidal change of the probe light intensity with time is obtained, and the frequency of the beat frequency is equal to the frequency difference of the two microwave electric fields. The relationship between the amplitude of the beat frequency signal and the amplitude E LO of the signal microwave electric field and the amplitude E SIG of the local microwave electric field is as follows:

[0046]

[0047] wherein A beat represents the amplitude of the beat frequency signal, and θ is defined as the included angle between the polarization directions of the signal microwave and the local microwave electric field.

[0048] In order to meet the far field condition, the three microwave horn antennas 5 are placed far away from the rubidium atom cell 8. The three microwave horn antennas 5 are placed in parallel and close to each other, the microwave horn antennas 5 on the two sides output the first local microwave electric field 3 and the second local microwave electric field 3, and the microwave horn antenna 5 in the middle outputs the signal microwave electric field 4.

[0049] The frequency settings of the three microwave electric fields are as follows: the frequency of the signal microwave electric field 4 is set to be resonant with the 46D-47P energy level, and the frequency is 22.067 GHz. The frequency of the first local microwave electric field 3 is set to be 22.067 GHz+12 kHz, and the frequency of the second local microwave electric field 3 is set to be 22.067 GHz-10 kHz, so that the first local microwave electric field 3 and the second local microwave electric field 3 form beat frequency signals with the signal microwave electric field 4 with frequencies of 10 kHz and 12 kHz respectively.

[0050] The polarizations of the two local microwave electric fields 3 are linear polarization, and the intensities and polarization directions of the two local microwave electric fields are fixed, wherein the polarization direction of the first local microwave electric field 3 is perpendicular to the propagation direction of the laser, and the polarization direction of the second local microwave electric field 3 is parallel to the propagation direction of the laser. And the polarization directions of the two local microwave electric fields 3 are realized to be orthogonal to each other by carefully adjusting the angles of the rectangular horn antennas of the two local microwave electric fields 3. The specific implementation method is to strictly realize by observing the 22 kHz beat frequency signal generated by the two local microwave electric fields 3, that is, when the polarization directions of the two local microwave electric fields 3 are perpendicular to each other, the 22 kHz beat frequency signal is the smallest.

[0051] The method for changing the polarization direction of the signal microwave electric field is as follows: the rectangular horn antenna of the signal microwave electric field is fixed on an electric rotary table, which can continuously rotate in a range of 360 degrees with a step precision less than 0.05 degrees, so that the polarization direction of the signal microwave electric field can be continuously changed in a range of 360 degrees. Meanwhile, the rectangular horn antenna of the local microwave electric field is fixed, that is, the polarization direction of the local microwave electric field is ensured to be unchanged. The change of the polarization direction of the signal microwave electric field is introduced for simplification, which is defined as the projection of the polarization of the signal microwave electric field on the polarization directions of the two local microwave electric fields. At this time, the relationship between the amplitude of the probe light intensity and the amplitude of the signal microwave electric field and the included angle of the polarization direction is as follows:

[0052]

[0053] wherein A beat represents the amplitude of the beat frequency signal, θ is defined as the included angle of the polarization directions of the signal microwave and the local microwave electric fields, E LO1 and E LO2 are the amplitudes of the two local microwave electric fields, and E SIG is the amplitude of the signal microwave electric field.

[0054] As can be seen from formula (2), when the polarization direction of the signal microwave electric field changes, the amplitude of the probe light intensity will change. Figure 3 is a curve of the amplitudes of the two beat frequency signals formed by the signal microwave electric field 4 and the two local microwave electric fields 3 with orthogonal polarization directions changing with the polarization direction of the signal microwave electric field. The upper right corner of the drawing is the ratio of the two beat frequency signals, and the polarization direction of the signal microwave electric field 4 can be directly obtained according to the measured ratio of the two beat frequencies. In the specific embodiment of the present application, the output power of the microwave source of the first local microwave electric field 3 is 7 dBm, the output power of the microwave source of the second local microwave electric field 3 is 9 dBm, and the output power of the microwave source of the signal microwave electric field 4 is 1 dBm.

[0055] The polarization direction of the signal microwave electric field 4 is changed every 5 degrees by using the electric rotary table, and then the amplitudes of the two beat frequency signals of 12 kHz and 10 kHz are read on the frequency spectrometer, so as to study the relationship between the amplitudes of the two beat frequencies and the polarization direction of the signal microwave electric field 4. The results are shown in Figure 4 and Figure 5 .

[0056] Figure 4 The changes of the amplitudes of the beat frequency signals formed by the signal microwave electric field 4 and the two local microwave electric fields 3 with different polarization directions of the signal microwave electric field 4 are shown. Specifically, as Figure 4As shown by the black curve, when the polarization direction of the signal microwave electric field 4 is the same as that of the first local microwave electric field 3, the beat frequency signal (12kHz) of the two is at its maximum. At this time, the polarization direction of the signal microwave electric field 4 is perpendicular to the polarization direction of the second local microwave electric field 3, that is, the beat frequency signal (10kHz) of the two is at its minimum. Note that the reason why the beat frequency signal with the local microwave electric field 3 is not zero at this time is that the polarization of the probe light and the dipole light is circularly polarized. Similarly, as Figure 4 As shown by the red curve, when the polarization direction of the signal microwave electric field 4 is the same as that of the second local microwave electric field 3, the 10kHz beat frequency signal is at its maximum value and the 12kHz beat frequency signal is at its minimum value. Figure 4 The blue curve shows that when the polarization directions of signal microwave electric field 4 and the first local microwave electric field 3 and the second local microwave electric field 3 are at a 45-degree angle, the intensity of the two beat frequency signals is almost the same.

[0057] Figure 5 The black curve represents the 12kHz beat frequency signal formed by the signal microwave electric field 4 and the first local microwave electric field, while the red curve represents the 10kHz beat frequency signal formed by the signal microwave electric field 4 and the second local microwave electric field. It can be clearly seen that within a 180-degree period, the amplitudes of the two beat frequency signals change with the polarization angle of the signal microwave electric field. When the angle of the signal microwave electric field 4 is 90 degrees, its polarization direction is parallel to the polarization direction of the second local microwave electric field 3 and perpendicular to the polarization direction of the first local microwave electric field 3. At this time, the amplitude of the 10kHz beat frequency signal is at its maximum, while the amplitude of the 12kHz beat frequency signal is at its minimum. When the angle of the signal microwave electric field 4 is 0 degrees or 180 degrees, its polarization direction is parallel to the polarization direction of the first local microwave electric field 3 and perpendicular to the polarization direction of the second local microwave electric field 3. At this time, the amplitude of the 12kHz beat frequency signal is at its maximum, while the amplitude of the 10kHz beat frequency signal is at its minimum. Figure 5 The blue and green curves are the results of fitting the experimental data according to formula (2). From the fitting results, the experimental results match the expected results of the theoretical equation well.

[0058] Next, the relationship between the ratio of the amplitude of the 12kHz beat frequency signal generated by the first local microwave electric field 3 to the amplitude of the 10kHz beat frequency signal generated by the second local microwave electric field 3 and the polarization direction of the signal microwave electric field was studied. The experimental and theoretical results are as follows: Figure 5 As shown by the black curve. Figure 7The ratio of the two beat signals and the polarization direction of the signal microwave electric field 4 are given. Using this measurement scheme, it is no longer necessary to change the angle of the signal microwave electric field 4 (or the local microwave electric field 3) horn antenna for multiple measurements, but only through a single measurement of the ratio of the amplitudes of the two beat signals of the two local microwave electric fields 3 with fixed angle, the polarization direction of the signal microwave electric field 4 to be measured can be obtained, so that high-resolution real-time polarization angle measurement is realized. Another advantage of this scheme is that the projections of the signal microwave electric field 4 on the two orthogonal components are measured at the same time, avoiding the influence of the amplitude disturbance of the signal microwave electric field 4 on the change of the strength of a single beat signal.

[0059] Due to the symmetry of the projections of the two polarization directions X degrees and 180-X degrees of the signal microwave electric field 4 on the local microwave electric field 3, the value of the beat frequency amplitude can be located to two possible angle values within a period of 180 degrees, that is, the actual measurement can only distinguish within 90 degrees, but cannot distinguish whether it is X degrees or 180-X degrees. In order to break this symmetry, a background magnetic field of 4G is introduced. A pair of Helmholtz coils is used to apply a magnetic field of 4G along the positive direction of the z axis at the rubidium atomic vapor cell. That is, a uniform magnetic field along the direction of laser propagation is generated by the Helmholtz coil to cause the Zeeman effect and polarization asymmetry of the atom, and the polarization direction of the signal microwave electric field is changed every 5 degrees by using the motorized turntable. Then the amplitudes of the two beat signals of 12 kHz and 10 kHz are read on the spectrum analyzer, and the results are shown in Figure 6 The EIT spectrum change caused by the magnetic field leads to the change of the transmission of the probe light, thereby causing the amplitude of the beat signal to decrease. Figure 6 It can be seen that compared with the case without magnetic field, the minimum point of the 12 kHz beat signal generated by the signal microwave electric field 4 and the first local microwave electric field has moved by 10 degrees after applying a magnetic field of 4G.

[0060] Further, the ratio of the amplitudes of the 12 kHz beat signals with B=4G and B=0 is studied with the polarization angle of the signal microwave electric field 4, and the results are shown in Figure 7 The ratio of the two beat signals measured by switching the magnetic field is that when the ratio is less than 0.22, the signal microwave electric field 4 is in the range of 0 to 90 degrees, and when the ratio is greater than 0.22, the signal microwave electric field 4 is in the range of 90 to 180 degrees. This ratio depends on the size of the magnetic field and the frequency shift of the probe light. Thus, through two measurements by switching the magnetic field, high-resolution real-time polarization angle measurement of the signal microwave electric field 4 to be measured can be realized within the range of 0-180 degrees.

[0061] Example Two

[0062] This invention provides a method for real-time measurement of microwave polarization direction based on a dual-local microwave electric field 3. The method uses the system provided in Embodiment 1 of this invention to measure microwave polarization direction in real time. The method includes the following steps:

[0063] S1. Constructing a Rydberg atomic microwave electric field sensor

[0064] use 87 The Rb atom, which involves a Rydberg atom, consists of four energy levels: 5s... 1 / 2 (F=2), 5P 3 / 2 (F=3), 46D 5 / 2 (F=4), 47P 3 / 2 (F=3), but not limited to specific atomic energy levels. The 780nm probe light acts on the 5S... 1 / 2 (F=2)5P 3 / 2 The transition at (F=3) is coupled with 480nm of light to the 5P phase. 3 / 2 (F=3)61D 5 / 2 The transition (F=4) and the 9.2 GHz microwave acting on 61D 5 / 2 (F=4)62P 3 / 2 The transition (F=3) occurs. The 780nm probe light and the 480nm coupling light propagate in opposite directions in the rubidium atom vapor cell, forming the electromagnetically induced transparency (EIT) of the Rydberg atom. When a microwave electric field is applied, the EIT will undergo Autler-Townes splitting. The change in microwave electric field intensity can be measured by the change in the transmittance of the probe light at the EIT resonance position.

[0065] In this embodiment of the invention, the probe light power is 60 microwatts, and the diameter in rubidium atomic vapor is about 800 micrometers. The coupled light power is 80 milliwatts, and the diameter in rubidium atomic vapor is about 900 micrometers. In fact, there is no limitation on these values.

[0066] To avoid interference from the polarization directions of the probe and coupling beams on the beat frequency signal after the introduction of a static magnetic field, the polarization states of the probe and coupling beams are set to be circularly polarized with the same direction. Due to the transition selection rule, the circularly polarized probe and coupling beams not only change the energy level population but also cause atoms in certain states to not interact with the microwaves. In this case, the Rydberg EIT-AT spectrum is no longer a bimodal structure but a trimodal structure that is a mixture of the three-level EIT spectrum and the four-level EIT-AT split spectrum. To avoid the influence of atomic energy levels that only participate in the three-level EIT process, i.e., atoms that do not interact with the microwaves, on the measurement, the probe beam frequency is locked outside the EIT-AT split peak, with an offset of about 5 MHz relative to the resonance position.

[0067] The intensity information of the probe light after passing through the atomic cell is measured by using a photoelectric detector, the output of the photoelectric detector is connected to an oscilloscope to obtain spectral information, and the output of the photoelectric detector is connected to a frequency spectrometer to obtain frequency spectrum information.

[0068] S2, the signal microwave electric field 4 and the two orthogonal polarization local microwave electric fields 3 form beat signals respectively

[0069] In order to meet the far field condition, the three microwave horn antennas are placed far away from the atomic vapor cell Rb. The three microwave horn antennas are placed in parallel, the microwave horn antennas on both sides output the first local microwave electric field and the second local microwave electric field, and the microwave horn antenna in the middle outputs the signal microwave electric field 4.

[0070] The frequencies of the three microwave fields are set as follows: the frequency of the signal microwave electric field 4 is set to be resonant with the 46D-47P energy level, and the frequency is 22.067 GHz. The frequency of the first local microwave electric field 3 is set to be 22.067 GHz+12 kHz, and the frequency of the second local microwave electric field 3 is set to be 22.067 GHz-10 kHz, so that the first local microwave electric field 3 and the second local microwave electric field 3 form beat signals with frequencies of 10 kHz and 12 kHz respectively with the signal microwave electric field 4.

[0071] The polarizations of the two local microwave electric fields 3 are linear polarization, and the intensities and polarization directions of the two local microwave electric fields are fixed, wherein the polarization direction of the first local microwave electric field 3 is perpendicular to the propagation direction of the laser, and the polarization direction of the second local microwave electric field 3 is parallel to the propagation direction of the laser. And by carefully adjusting the direction of the rectangular horn antenna of the two local microwave electric fields 3, the polarization directions of the two local microwave electric fields 3 are orthogonal to each other. Specifically, strict implementation is realized by observing the 22 kHz beat signal generated by the two local microwave electric fields 3, that is, when the polarization directions of the two local microwave electric fields 3 are perpendicular to each other, the 22 kHz beat signal is the smallest.

[0072] S3, change the polarization direction of the signal microwave electric field, and ensure that the polarization directions of the two local microwave electric fields remain unchanged.

[0073] The rectangular horn antenna of the signal microwave electric field is fixed on a motorized rotary table, which can rotate continuously within 360 degrees with a step accuracy of less than 0.05 degrees, so that the linear polarization direction of the signal microwave electric field can be changed continuously within 360 degrees. At the same time, the rectangular horn antennas of the two local microwave electric fields are fixed, that is, the polarization directions of the two local microwave electric fields remain unchanged.

[0074] S4, convert the measurement of the polarization direction of the signal microwave electric field 4 into the measurement of beat frequency

[0075] The polarization direction of the signal microwave electric field is changed every 5 degrees by using an electric turntable. Then, the amplitudes of the two beat frequency signals at 12kHz and 10kHz are read on the spectrum analyzer to study the relationship between the two beat frequency amplitudes and the polarization direction of the signal microwave electric field.

[0076] like Figure 5 As shown, the black curve represents the 12kHz beat frequency signal formed by the signal microwave electric field 4 and the first local microwave electric field, while the red curve represents the 10kHz beat frequency signal formed by the signal microwave electric field 4 and the second local microwave electric field. It can be clearly seen that within a 180-degree period, the amplitudes of the two beat frequency signals change with the polarization angle of the signal microwave electric field. When the angle of the signal microwave electric field 4 is 90 degrees, its polarization direction is parallel to the polarization direction of the second local microwave electric field 3 and perpendicular to the polarization direction of the first local microwave electric field 3. At this time, the amplitude of the 10kHz beat frequency signal is at its maximum, while the amplitude of the 12kHz beat frequency signal is at its minimum. When the angle of the signal microwave electric field 4 is 0 degrees or 180 degrees, its polarization direction is parallel to the polarization direction of the first local microwave electric field 3 and perpendicular to the polarization direction of the second local microwave electric field 3. At this time, the amplitude of the 12kHz beat frequency signal is at its maximum, while the amplitude of the 10kHz beat frequency signal is at its minimum. The blue and green curves represent the results of fitting the experimental data according to formula (2). The fitting results show that the experimental data matches the expected results of the theoretical equation quite well.

[0077] like Figure 5 As shown, the relationship between the ratio of the amplitude of the 12kHz beat frequency signal generated by the first local microwave electric field 3 to the amplitude of the 10kHz beat frequency signal generated by the second local microwave electric field 3 and the polarization direction of the signal microwave electric field was then investigated. Experimental and theoretical results are as follows: Figure 5 As shown by the black curve. Figure 7 The relationship between the ratio of two beat frequency signals and the polarization direction of the signal microwave electric field 4 is presented. Based on the fitted relationship between the ratio of the amplitudes of the two beat frequency signals and the polarization direction of the signal microwave electric field, it is no longer necessary to change the angle of the horn antenna of the signal microwave electric field 4 (or local microwave electric field 3) for multiple measurements. Only by performing a single measurement using two local microwave electric fields 3 at a fixed angle to obtain the ratio of the amplitudes of the two beat frequency signals, the polarization direction of the signal microwave electric field 4 under test can be obtained, thus achieving high-resolution real-time polarization angle measurement.

[0078] S5, resolution of microwave electric field polarization X or 180-X within the range of 0-180 degrees.

[0079] Because of the symmetry of the two polarization directions X and 180-X of the signal microwave electric field 4 in the local microwave electric field 3 projection, the value of the beat frequency amplitude can be located to two possible angle values within a period of 180 degrees, that is, the actual measurement can only distinguish within 90 degrees, and cannot distinguish X or 180-X. In order to break this symmetry, a background magnetic field of 4G is introduced. A magnetic field of 4G along the positive direction of the z axis is applied at the rubidium atomic cell through a Helmholtz coil. The polarization direction of the signal microwave electric field is changed every 5 degrees by using the motorized turntable. Then the corresponding 12 kHz and 10 kHz beat frequency signal amplitudes are read on the spectrum analyzer, and the results are shown in Figure 6 .

[0080] The EIT spectrum change caused by the magnetic field causes the change of the probe light transmittance, thereby causing the amplitude of the beat frequency signal to be smaller. As shown in Figure 6 , compared with the case without a magnetic field, the minimum point of the 12 kHz beat frequency signal generated by the first local microwave electric field of the signal microwave electric field 4 moves by 10 degrees after a magnetic field of 4G is applied. Further, the ratio of the 12 kHz beat frequency signal amplitudes when B=4G and B=0 changes with the polarization angle of the signal microwave electric field 4, and the results are shown in Figure 5 . The ratio of the two beat frequency signals measured by switching the magnetic field is that when the ratio is less than 0.22, the signal microwave electric field 4 is in the range of 0 to 90 degrees, and when the ratio is greater than 0.22, the signal microwave electric field 4 is in the range of 90 to 180 degrees. This ratio depends on the size of the magnetic field and the probe light frequency shift. Thus, the ratio of the two beat frequency signals measured by switching the magnetic field can realize high-resolution real-time polarization angle measurement of the signal microwave electric field 4 in the range of 0-180 degrees.

[0081] As can be seen from the above embodiments, the system and method for real-time measurement of microwave polarization direction based on double local microwave electric field disclosed in the present application introduces two orthogonal polarization local microwave electric fields 3 to realize real-time measurement of the microwave electric field polarization direction on the basis of the measurement of the microwave polarization of the Rydberg atom frequency mixer. Compared with the conventional method which needs to rotate the polarization of the local microwave electric field 3 by 180 degrees to obtain the polarization direction of the measured microwave field, this method obtains the projection of the intensity of the signal microwave electric field 4 on the two orthogonal polarization local microwave electric fields 3 through a single measurement, thereby obtaining the polarization direction of the signal microwave electric field 4 once, and realizing high-resolution real-time polarization angle measurement of the microwave polarization. Further, a weak static magnetic field is introduced to break the symmetry of the atomic energy level and the beat frequency signal, and the polarization X degrees or 180-X degrees of the microwave electric field in the range of 0-180 degrees is distinguished. This work first demonstrates the direct real-time measurement of the microwave polarization in the range of 0-180 degrees, which has important significance and value in practical applications such as microwave sensing and information transmission.

[0082] The system and method described in the present application are not limited to the embodiments described in the specific embodiments, and other embodiments can be derived by those skilled in the art according to the technical solutions of the present application, which also belong to the technical innovation range of the present application.

Claims

1. A system for real-time measurement of microwave polarization direction based on dual-local microwave electric field, characterized in that: The system includes a probe laser and its frequency-locking system, a coupling laser and its frequency-locking system, three microwave sources, three microwave horn antennas, two dichroic mirrors, a pair of Helmholtz coils, a rubidium atom vapor cell, an electric rotary stage, a photodetector, and a spectrum analyzer. The probe laser and its frequency-locking system generate probe light, and the coupling laser and its frequency-locking system generate coupling light. The three microwave sources serve as inputs for two local microwave electric fields and one signal microwave electric field, respectively. The three microwave horn antennas are connected to the three microwave sources, radiating microwaves into the rubidium atom vapor cell. The microwave horn antennas on both sides output a first local microwave electric field and a second local microwave electric field, while the middle microwave horn antenna outputs a signal microwave electric field. The two dichroic mirrors direct the probe light and coupling light laser beams through the rubidium atom vapor cell. A pair of Helmholtz coils are respectively disposed on both sides of the rubidium atom vapor cell. The pair of Helmholtz coils are used to provide a uniform magnetic field along the laser propagation direction. The rubidium atom vapor cell is used to provide an environment for the interaction between rubidium atom vapor and the laser and microwaves. The electric rotary stage is used to change the polarization direction of the microwave horn antenna. The photodetector receives the probe light after passing through the rubidium atom vapor cell. The spectrum analyzer is used to receive the signal from the photodetector to perform spectral analysis on the probe light after passing through the rubidium atom vapor cell. The system measures the intensity of the signal microwave electric field by detecting the change in light intensity near the electromagnetic induction transparent resonance of the Rydberg atom. The polarization direction of the signal microwave electric field is measured in real time based on the ratio of the amplitude of the beat frequency signal of the first frequency generated by the first local microwave electric field to the amplitude of the beat frequency signal of the second frequency generated by the second local microwave electric field.

2. The system for real-time measurement of microwave polarization direction based on dual local microwave electric field as described in claim 1, characterized in that: The polarization states of the probe light and the coupling light are circularly polarized with the same direction.

3. The system for real-time measurement of microwave polarization direction based on dual-local microwave electric field as described in claim 1, characterized in that: The three microwave horn antennas are positioned at a distance from the rubidium atomic vapor pool. The three microwave horn antennas are placed adjacent to each other and parallel to each other. The frequency of the signal microwave electric field is close to that of the local microwave electric field, and the polarizations of the two local microwave electric fields are orthogonal and perpendicular.

4. The system for real-time measurement of microwave polarization direction based on dual-local microwave electric field as described in claim 3, characterized in that: Both local microwave electric fields are linearly polarized. The polarization direction of the first local microwave electric field is perpendicular to the laser propagation direction, and the polarization direction of the second local microwave electric field is parallel to the laser propagation direction.

5. The system for real-time measurement of microwave polarization direction based on dual local microwave electric field as described in claim 3, characterized in that: By adjusting the angles of the rectangular horn antennas of the two local microwave electric fields, the mutual orthogonality of their polarizations can be achieved.

6. The system for real-time measurement of microwave polarization direction based on dual local microwave electric field as described in claim 5, characterized in that: The system continuously rotates a rectangular horn antenna of the signal microwave electric field on an electric rotary table to continuously change the polarization direction of the signal microwave electric field within a 360-degree range.

7. The system for real-time measurement of microwave polarization direction based on dual local microwave electric field as described in claim 6, characterized in that: The rubidium atom vapor cell includes a vacuum glass cell filled with rubidium atom vapor.

8. A method for real-time measurement of microwave polarization direction based on dual-local microwave electric field, characterized in that: The method measures the microwave polarization direction in real time using the system for real-time measurement of microwave polarization direction based on dual local microwave electric field as described in any one of claims 1-7. The method includes the following steps: S1. By using probe light and coupling light of different wavelengths to propagate in opposite directions in a rubidium atom vapor cell, electromagnetic induction transparency of Rydberg atoms is formed. After applying a microwave electric field, the electromagnetic induction transparency of Rydberg atoms undergoes Autler-Townes splitting. The change in signal microwave electric field intensity is measured by the change in transmittance of probe light at the electromagnetic induction transparency resonance position of Rydberg atoms. S2, the signal microwave electric field and two orthogonally polarized local microwave electric fields of different frequencies respectively form beat frequency signals; S3. Change the polarization direction of the signal microwave electric field to ensure that the polarization directions of the two local microwave electric fields remain unchanged; S4. Convert the polarization direction of the microwave electric field of the measurement signal into the ratio of the two beat frequency signals. S5. The ratio of the two beat frequency signals obtained by switching the magnetic field is compared with the preset value to distinguish the microwave electric field polarization X or 180-X in the range of 0-180 degrees.

9. The method for real-time measurement of microwave polarization direction based on dual local microwave electric field as described in claim 8, characterized in that: In step S1, the light transmittance and the amplitude E of the local microwave electric field are detected. LO And the amplitude of the microwave electric field E SIG The relationship is as follows: T probe E represents the transmittance of the probe light. mod This represents the total electric field experienced by the atom, including the local microwave electric field E. LO and signal microwave electric field E SIG Δω represents the frequency difference between the signal and the local microwave, defined as Δω = ω LO -ω SIG , where ω SIG and ω LO These are their angular frequencies, and Δφ represents the phase difference between the two microwaves, defined as Δφ = φ LO -φ SIG , where φ SIG and φ LO These are their phases.

10. The method for real-time measurement of microwave polarization direction based on dual local microwave electric field as described in claim 8, characterized in that: The amplitude of the beat frequency signal in S2 is related to the amplitude E of the two local microwave electric fields. LO1 E LO2 And the amplitude of the microwave electric field E SIG The relationship is as follows: TO beat1 ∝|E LO1 +E SIG cosθ| TO beat2 ∝|E LO2 +E SIG sinθ| Among them, A beat The amplitude of the beat frequency signal is represented by θ, which is defined as the angle between the polarization directions of the signal microwave and the local microwave electric field.

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