A single-resonance ring brillouin microwave photonic filter based on time-reversal symmetry

Abstract

The present application relates to the field of microwave photon technology, specifically to a single-resonant ring Brillouin microwave photon filter based on time-reversal symmetry, in order to solve the technical problems of insufficient bandwidth compression capability, low side mode suppression ratio and complex system structure of the prior art, a single-resonant ring Brillouin microwave photon filter based on time-reversal symmetry (PT) is provided, the filter comprises a pump light path unit, a signal light path unit and a Brillouin laser resonant cavity. The present application breaks through the complex structure of traditional multi-ring coupling by fusing the narrow-band filtering characteristics of Brillouin laser gain spectrum and the single-longitudinal-mode selectivity of PT symmetric system, realizes the construction of PT symmetric structure in a single-resonant ring, and solves the technical pain points of insufficient bandwidth compression capability, low side mode suppression ratio and complex system structure of the existing scheme.

Description

Technical Field

[0001] This invention relates to the field of microwave photonics technology, specifically to a single-resonant ring Brillouin microwave photonic filter based on parity-time symmetry. Background Technology

[0002] With the rapid development of 6G communication, radar detection, electronic warfare, and other fields, radio frequency (RF) systems are placing higher demands on the performance of microwave filters. These filters require ultra-narrow bandwidth, high side-mode rejection ratio, low in-band loss, and electromagnetic interference (EMI) immunity. However, traditional electrical filters are limited by electronic constraints, have high bandwidth compression limits, and weak EMI immunity, making them unsuitable for the application requirements of next-generation RF systems.

[0003] Microwave photonic filtering technology, with its advantages in optical domain signal processing, has overcome the performance bottleneck of electrical domain filters and has become a current research hotspot. Among them, microwave photonic filters based on stimulated Brillouin scattering utilize the frequency selectivity of the Brillouin gain spectrum to achieve filtering. However, the Brillouin gain spectrum linewidth is usually tens of megahertz, making it difficult to achieve narrowband filtering, and it also suffers from problems such as insufficient side-mode suppression ratio and wide passband.

[0004] Parity-time (PT) symmetric optical systems can achieve mode degeneracy and selection at singular points by adjusting the balance between gain and loss, and have excellent side-mode suppression and single-mode screening capabilities. However, existing PT symmetric microwave photonic filtering technologies mostly adopt dual-cavity structures, which have drawbacks such as high system complexity, poor stability, and difficulty in integration. Summary of the Invention

[0005] In order to solve the technical problems of insufficient bandwidth compression capability, low side-mode suppression ratio and complex system structure in the prior art, this invention provides a single resonant ring Brillouin microwave photonic filter based on parity-time symmetry.

[0006] This invention is achieved using the following technical solution:

[0007] A parity-time symmetric single-resonant ring Brillouin microwave photonic filter includes:

[0008] Pump optical path unit, used for output frequency of The pump light is amplified to a power exceeding the stimulated Brillouin scattering threshold of the single-mode fiber and then injected into the single-mode fiber to excite Stokes light propagating in the reverse direction within the single-mode fiber. The frequency of the Stokes light is... ,in The backward stimulated Brillouin frequency shift of the single-mode fiber;

[0009] Signal optical path unit, used for output frequency of The optical carrier, and apply a frequency of to the optical carrier. The radio frequency sweep signal is subjected to double-sideband modulation to generate a signal containing an optical carrier. Top band With the bottom band Multi-frequency optical signals;

[0010] A Brillouin laser resonator is a closed-loop cavity composed of a single-mode fiber, an optical circulator, a lithium niobate waveguide, a polarization control unit, and an optical coupling unit. Multi-frequency optical signals and Stokes light interact within the cavity. When the radio frequency sweep signal frequency... satisfy At this time, the upper sideband of the multi-frequency optical signal matches the Stokes optical frequency, achieving selective resonant amplification and forming a loop oscillation, thus realizing microwave photonic filtering.

[0011] Principle Explanation: The core principle of the microwave photonic filter described in this invention lies in integrating the narrowband filtering characteristics of the Brillouin laser gain spectrum with the single-mode selectivity of a PT-symmetric system. This allows for the construction of a PT-symmetric structure within a single resonant ring, addressing the technical pain points of existing solutions, such as insufficient bandwidth compression, low side-mode suppression ratio, and complex system structure. Specifically, the analysis is as follows: First, leveraging the stimulated Brillouin scattering effect, the linewidth of the Brillouin gain spectrum (20 MHz) is compressed to the kilohertz range through stimulated emission amplification of the Brillouin laser resonator. Second, a phase modulator loads the radio frequency signal onto the optical carrier to form a double-sideband modulated signal. Selective amplification of the Brillouin gain is achieved only when the upper sideband frequency matches the Brillouin-Stokes light frequency; the carrier and lower sideband have no gain due to frequency mismatch, thus achieving bandpass filtering in the optical domain. The amplified upper sideband beats with the optical carrier through a photodetector, demodulating the ultra-narrowband radio frequency signal to complete the microwave photonic filtering function. Simultaneously, utilizing the birefringence characteristics of the lithium niobate waveguide, a single-mode filter is used to achieve the desired effect. Two orthogonal polarization modes are generated within a single resonant cavity, enabling the construction of a PT-symmetric non-Hermitian optical system without the need for a complex dual-cavity structure. By adjusting the polarization control unit, one polarization mode experiences gain while the other experiences loss. The two orthogonal polarization modes exchange energy through polarization coupling via a lithium niobate waveguide. The balance between gain and loss within the cavity is controlled by the polarization control unit. When the gain and loss coefficients are equal and greater than the inter-mode coupling coefficient, the system enters the PT-symmetric spontaneously broken region. The mode degeneracy at the singularity is broken, and only the target frequency obtains net gain and achieves single-longitudinal-mode oscillation. The remaining side modes are significantly suppressed, significantly improving the side-mode rejection ratio of the filter.

[0012] Furthermore, the microwave photonic filter also includes a detection unit for characterizing the spectral response and radio frequency performance.

[0013] Furthermore, the pump optical path unit includes:

[0014] The first laser, used to output a frequency of Pump light;

[0015] The first polarization controller is connected to the output of the first laser and is used to optimize the polarization state of the pump light;

[0016] An erbium-doped fiber amplifier is connected to the output of the first polarization controller to amplify the pump light power to the target power.

[0017] Furthermore, the signal optical path unit includes:

[0018] The second laser is used to output a frequency of Optical carrier wave;

[0019] The second polarization controller is connected to the output of the second laser;

[0020] A phase modulator, connected to the output of the second polarization controller, is used to receive frequencies of... The radio frequency sweep signal is used to modulate the optical carrier on both sides.

[0021] Furthermore, in the Brillouin resonator unit:

[0022] The optical circulator has its port a connected to the output of an erbium-doped fiber amplifier, and its port b connected to one end of a single-mode fiber.

[0023] A single-mode optical fiber with a physical length of 5km serves as the gain medium for stimulated Brillouin scattering, providing stable Brillouin gain for the Brillouin resonator.

[0024] The polarization control unit employs a third polarization controller to regulate the polarization state of the intracavity optical signal in order to adjust the balance between gain and loss.

[0025] A lithium niobate waveguide is connected between the C-port of the optical circulator and one end of the third polarization controller to generate two orthogonal polarization modes using its birefringence properties.

[0026] The optical coupling unit includes a first optical coupler and a second optical coupler. Port a of the first optical coupler is connected to the output end of the phase modulator, and the output port of the first optical coupler is connected to the other end of the single-mode optical fiber. Port a of the second optical coupler is connected to port b of the first optical coupler, the input port of the second optical coupler is connected to the other end of the third polarization controller, and the b port of the second optical coupler is connected to the input port of the photodetector.

[0027] Furthermore, the detection unit includes a third optical coupler, a spectrometer, a photodetector, and a vector network analyzer. A third optical coupler is also provided between the photodetector and the b port of the second optical coupler. The input port of the third optical coupler is connected to the b port of the second optical coupler, the a port of the third optical coupler is connected to the input port of the spectrometer, the b port of the third optical coupler is connected to the input port of the photodetector, the output port of the photodetector is connected to the receiving port of the vector network analyzer, and the transmitting port of the vector network analyzer is connected to the phase modulator.

[0028] Furthermore, the splitting ratios at ports a and b of the first and third optical couplers are both 50:50, and the splitting ratio at ports a and b of the second optical coupler is 90:10.

[0029] Furthermore, the physical length of the single-mode fiber is 5 km.

[0030] Furthermore, the output power range of the erbium-doped fiber amplifier is adjustable from 17 to 27 dBm.

[0031] The beneficial effects of this invention are as follows: This invention utilizes the stimulated Brillouin scattering effect to achieve optical domain radio frequency signal filtering, and uses the linewidth compression effect of the Brillouin laser resonator to compress the passband bandwidth to the kilohertz level; it utilizes the birefringence characteristics of lithium niobate waveguides to construct a parity-time (PT) symmetric system in a single resonator, and breaks the PT symmetry by adjusting the gain and loss balance to improve the side-mode suppression capability, and the structure is simple. Attached Figure Description

[0032] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0033] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1 This is a schematic diagram of the structure of the present invention;

[0035] Figure 2 The spectrum showing the mixture of Brillouin and double-sideband modulation signals of the present invention;

[0036] Figure 3 This represents the spectrum of the single longitudinal mode output achieved by the present invention.

[0037] In the diagram: 1-First laser, 2-Second laser, 3-First polarization controller, 4-Second polarization controller, 5-Third polarization controller, 6-Erbium-doped fiber amplifier, 7-Optical circulator, 8-Single-mode fiber, 9-Phase modulator, 10-First optical coupler, 11-Second optical coupler, 12-Third optical coupler, 13-Lithium niobate waveguide, 14-Spectrometer, 15-Photodetector, 16-Vector network analyzer. Detailed Implementation

[0038] To better understand the above-mentioned objectives, features, and advantages of the present invention, the solutions of the present invention will be further described below. It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other.

[0039] Many specific details are set forth in the following description in order to provide a full understanding of the invention, but the invention may also be practiced in other ways different from those described herein; obviously, the embodiments in the specification are only some embodiments of the invention, and not all embodiments.

[0040] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0041] like Figure 1 As shown, a single-resonant ring Brillouin microwave photonic filter based on parity-time symmetry includes:

[0042] Pump optical path unit, used for output frequency of The pump light is amplified to a power exceeding the stimulated Brillouin scattering threshold of the single-mode fiber 8 and then injected into the single-mode fiber 8 to excite reverse-propagating Stokes light within the single-mode fiber 8. The frequency of the Stokes light is... ,in The backward stimulated Brillouin frequency shift of the single-mode fiber 8;

[0043] Signal optical path unit, used for output frequency of The optical carrier, and apply a frequency of to the optical carrier. The radio frequency sweep signal is subjected to double-sideband modulation to generate a signal containing an optical carrier. Top band With the bottom band Multi-frequency optical signals;

[0044] The Brillouin laser resonator is a closed-loop cavity composed of a single-mode fiber 8, an optical circulator 7, a lithium niobate waveguide 13, a polarization control unit, and an optical coupling unit. Multi-frequency optical signals and Stokes light interact within the cavity. When the radio frequency sweep signal frequency... satisfy At this time, the upper sideband of the multi-frequency optical signal matches the Stokes optical frequency, achieving selective resonant amplification and forming a loop oscillation, thus realizing microwave photonic filtering.

[0045] Principle Explanation: The core principle of the microwave photonic filter described in this invention lies in integrating the narrowband filtering characteristics of the Brillouin laser gain spectrum with the single-mode selectivity of a PT-symmetric system. This allows for the construction of a PT-symmetric structure within a single resonant ring, addressing the technical pain points of existing solutions, such as insufficient bandwidth compression, low side-mode suppression ratio, and complex system structure. Specifically, the analysis is as follows: First, leveraging the stimulated Brillouin scattering effect, the linewidth of the Brillouin gain spectrum (20 MHz) is compressed to the kilohertz range through stimulated emission amplification of the Brillouin laser resonator. Second, the RF signal is loaded onto the optical carrier via phase modulator 9 to form a double-sideband modulated signal. Selective amplification of the Brillouin gain is achieved only when the upper sideband frequency matches the Brillouin-Stokes light frequency; the carrier and lower sideband have no gain due to frequency mismatch, thus achieving bandpass filtering in the optical domain. The amplified upper sideband and optical carrier beat through photodetector 15, demodulating the ultra-narrowband RF signal to complete the microwave photonic filtering function. Simultaneously, the birefringence characteristics of the lithium niobate waveguide 13 are utilized... By generating two orthogonal polarization modes within a single resonant cavity, a PT-symmetric non-Hermitian optical system can be constructed without a complex dual-cavity structure. By adjusting the polarization control unit, one polarization mode experiences gain while the other experiences loss. The two orthogonal polarization modes exchange energy through polarization coupling via the lithium niobate waveguide 13. The balance between gain and loss within the cavity is controlled by the polarization control unit. When the gain and loss coefficients are equal and greater than the inter-mode coupling coefficient, the system enters the PT-symmetric spontaneously broken region. The mode degeneracy at the singularity is broken, and only the target frequency obtains net gain and achieves single-longitudinal-mode oscillation. The remaining side modes are significantly suppressed, significantly improving the side-mode rejection ratio of the filter.

[0046] Furthermore, the microwave photonic filter also includes a detection unit for characterizing the spectral response and radio frequency performance.

[0047] In specific implementation, the pump optical path unit includes: a first laser 1, used to output a frequency of The pump light; a first polarization controller 3, connected to the output of the first laser 1, is used to optimize the polarization state of the pump light; an erbium-doped fiber amplifier 6, connected to the output of the first polarization controller 3, is used to amplify the power of the pump light to the target power.

[0048] In specific implementation, the signal optical path unit includes: a second laser 2, used to output a frequency of The optical carrier; the second polarization controller 4, connected to the output of the second laser 2; the phase modulator 9, connected to the output of the second polarization controller 4, for receiving optical carriers with a frequency of ; the second polarization controller 4, connected to the output of the second laser 2; and ... The radio frequency sweep signal is used to modulate the optical carrier on both sides.

[0049] In specific implementation, in the Brillouin resonator unit, port a of the optical circulator 7 is connected to the output of the erbium-doped fiber amplifier, and port b of the optical circulator 7 is connected to one end of the single-mode fiber 8. The single-mode fiber 8, with a physical length of 5 km, serves as the gain medium for stimulated Brillouin scattering, providing stable Brillouin gain for the Brillouin resonator. The polarization control unit uses a third polarization controller 5 to regulate the polarization state of the optical signal within the cavity to adjust the balance between gain and loss. The lithium niobate waveguide 13 is connected to port c of the optical circulator 7 and one end of the third polarization controller 5. Between the ends, it is used to excite two orthogonal polarization modes by utilizing its birefringence characteristics; the optical coupling unit includes a first optical coupler 10 and a second optical coupler 11. The a port of the first optical coupler 10 is connected to the output end of the phase modulator 9, the output port of the first optical coupler 10 is connected to the other end of the single-mode fiber 8, the a port of the second optical coupler 11 is connected to the b port of the first optical coupler 10, the input port of the second optical coupler 11 is connected to the other end of the third polarization controller 5, and the b port of the second optical coupler 11 is connected to the input port of the photodetector 15.

[0050] In specific implementation, the detection unit includes a third optical coupler 12, a spectrum analyzer 14, a photodetector 15, and a vector network analyzer 16. A third optical coupler 12 is also provided between the photodetector 15 and the b port of the second optical coupler 11. The input port of the third optical coupler 12 is connected to the b port of the second optical coupler 11, the a port of the third optical coupler 12 is connected to the input port of the spectrum analyzer 14, the b port of the third optical coupler 12 is connected to the input port of the photodetector 15, the output port of the photodetector 15 is connected to the receiving port of the vector network analyzer 16, and the transmitting port of the vector network analyzer 16 is connected to the phase modulator 9.

[0051] In this specific embodiment, the splitting ratio of ports a and b of the first optical coupler 10 and ports a and b of the third optical coupler 12 is 50:50, and the splitting ratio of ports a and b of the second optical coupler 11 is 90:10.

[0052] In this specific embodiment, the physical length of the single-mode fiber 8 is 5km.

[0053] In this specific embodiment, the output power range of the erbium-doped fiber amplifier is adjustable and ranges from 17 to 27 dBm.

[0054] The specific workflow of the microwave photonic filter described above is as follows:

[0055] The first laser 1 outputs a center wavelength =1550 nm, corresponding frequency is The pump light, after being optimized in polarization state by the first polarization controller 3, is sent to the erbium-doped fiber amplifier 6 and amplified to 23 dBm. Then, it is injected into the 5km single-mode fiber 8 through the b port of the optical circulator 7. When the pump light power exceeds the stimulated Brillouin scattering threshold of the single-mode fiber 8, Stokes light propagating in the opposite direction will be excited in the single-mode fiber 8, with a Brillouin frequency shift of approximately 10.743 GHz.

[0056] The second laser 2 outputs a center wavelength The optical carrier with a wavelength of 1550 nm is polarized and then sent to the phase modulator 9 after polarization matching is completed by the second polarization controller 4. The vector network analyzer 16 outputs a radio frequency sweep signal of 100 kHz-20 GHz, which is loaded onto the phase modulator 9 to perform double-sideband modulation on the optical carrier and generate a multi-frequency optical signal containing the optical carrier, upper sideband and lower sideband.

[0057] The modulated multi-frequency optical signal is injected into the Brillouin laser resonator through port a of the 50:50 first optical coupler 10. When the frequency of the RF sweep signal approaches 10.743 GHz, the upper sideband frequency of the optical signal matches the Stokes frequency, falling into the peak position of the Brillouin gain spectrum and thus being amplified. The amplified optical signal is output along port c of the optical circulator 7 and enters the lithium niobate waveguide 13, generating two orthogonal polarization modes. The polarization state is adjusted by the third polarization controller 5 to balance the gain and loss, so that the system enters the PT-symmetric spontaneously broken region, realizing the target single longitudinal mode oscillation and suppressing the side modes. Subsequently, the optical signal passes through the 90:10 second optical coupler 11, with 90% of the optical signal returning to port b of the first optical coupler to maintain loop oscillation, and 10% of the optical signal being output to the second optical coupler 11.

[0058] The 10% optical signal output from port b of the second optical coupler 11 is split by the third optical coupler 12 (50:50). One path is sent to the spectrometer 14 to monitor the spectral characteristics of the Brillouin laser in the cavity in real time; the other path is sent to the photodetector 15. The amplified upper sideband beats with the optical carrier in the photodetector 15, and the corresponding frequency radio frequency signal is demodulated. The signal is then sent back to the vector network analyzer 16 to test the core performance of the filter, such as amplitude-frequency response, passband bandwidth, and side-mode rejection ratio.

[0059] Figure 2The above describes the spectral output of the spectrometer 14 in the microwave photonic filter device, where: the pump light curve is measured when the pump light power does not reach the stimulated Brillouin scattering threshold, and its function is to excite Brillouin scattering to generate Brillouin laser; the pump light-excited Brillouin curve is the Brillouin laser generated after the pump light power reaches and exceeds the stimulated Brillouin scattering threshold, which is also the key to the filtering of the filter described in this invention; the optical carrier and Brillouin laser curves are measured when the optical carrier and modulation signal are injected synchronously after the system successfully excites Brillouin scattering.

[0060] Figure 3 This invention demonstrates the transmission scattering parameters in both the unbroken and broken PT-symmetric states. The spectrum curves and linewidth fitting curves vary with frequency. The main plot shows that when the system is in a PT-symmetry broken state, the spectrum exhibits a single and prominent main peak at the center frequency of 10.743 GHz. The 14 dB marked in the figure represents the relative power difference between the peak power of the main peak and the maximum side mode in this state, i.e., the side mode suppression ratio. This intuitively illustrates that the PT-symmetry broken mechanism effectively suppresses the two side modes, achieving stable single-longitudinal-mode output. The inset is a magnified scatter plot at the center frequency of this single-longitudinal-mode, with the horizontal axis representing frequency and the vertical axis representing... The curve in the figure represents the Lorentz line fitting result. As can be seen from the figure, the actual measured curve matches the theoretical Lorentz line shape very well. The -3 dB linewidth marked with a double-headed arrow in the inset is 4 kHz, which fully demonstrates that the single longitudinal mode linewidth is extremely narrow and the spectral purity is extremely high.

[0061] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the present invention. Although detailed descriptions have been provided with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments, and they should all be covered within the protection scope of the claims.

Claims

1. A single-resonant ring Brillouin microwave photonic filter based on parity-time symmetry, characterized in that, include: Pump optical path unit, used for output frequency of The pump light is amplified to a power exceeding the stimulated Brillouin scattering threshold of the single-mode fiber (8) and then injected into the single-mode fiber (8) to excite reverse-propagating Stokes light within the single-mode fiber (8). The frequency of the Stokes light is... ,in The backward stimulated Brillouin shift of the single-mode fiber (8); Signal optical path unit, used for output frequency of The optical carrier, and apply a frequency of to the optical carrier. The radio frequency sweep signal is subjected to double-sideband modulation to generate a signal containing an optical carrier. Top band With the bottom band Multi-frequency optical signals; The Brillouin laser resonator is a closed-loop cavity composed of a single-mode fiber (8), an optical circulator (7), a lithium niobate waveguide (13), a polarization control unit, and an optical coupling unit. Multi-frequency optical signals and Stokes light interact within the cavity. When the frequency of the radio frequency sweep signal... satisfy At this time, the upper sideband of the multi-frequency optical signal matches the Stokes optical frequency, achieving selective resonant amplification and forming a loop oscillation, thus realizing microwave photonic filtering.

2. The single-resonant ring Brillouin microwave photonic filter based on parity-time symmetry according to claim 1, characterized in that, The microwave photonic filter also includes a detection unit for characterizing the spectral response and radio frequency performance.

3. A single-resonant ring Brillouin microwave photonic filter based on parity-time symmetry according to claim 2, characterized in that, The pump optical path unit includes: a first laser (1), used to output a frequency of Pump light; a first polarization controller (3), connected to the output end of the first laser (1), for optimizing the polarization state of the pump light; an erbium-doped fiber amplifier (6), connected to the output end of the first polarization controller (3), for amplifying the power of the pump light to the target power.

4. A single-resonant ring Brillouin microwave photonic filter based on parity-time symmetry according to claim 3, characterized in that, The signal optical path unit includes: a second laser (2), used to output a frequency of The optical carrier; the second polarization controller (4), connected to the output of the second laser (2); the phase modulator (9), connected to the output of the second polarization controller (4), for receiving optical carriers with a frequency of 1000 Hz; the second polarization controller (4), connected to the output of the second laser (2); and the phase modulator (9), connected to the output of the second polarization controller (4), for receiving optical carriers with a frequency of 1000 Hz. The radio frequency sweep signal is used to modulate the optical carrier on both sides.

5. A single-resonant ring Brillouin microwave photonic filter based on parity-time symmetry according to claim 4, characterized in that, In the Brillouin resonator unit, port a of the optical circulator (7) is connected to the output of the erbium-doped fiber amplifier, and port b of the optical circulator (7) is connected to one end of the single-mode fiber (8). The single-mode fiber (8) serves as the gain medium for stimulated Brillouin scattering, providing a stable Brillouin gain for the Brillouin resonator. The polarization control unit uses a third polarization controller (5) to regulate the polarization state of the optical signal within the cavity to adjust the balance between gain and loss. The lithium niobate waveguide (13) is connected between port c of the optical circulator (7) and one end of the third polarization controller (5) to utilize its dual polarization... The refractive properties generate two orthogonal polarization modes; the optical coupling unit includes a first optical coupler (10) and a second optical coupler (11). The a port of the first optical coupler (10) is connected to the output end of the phase modulator (9), the output port of the first optical coupler (10) is connected to the other end of the single-mode fiber (8), the a port of the second optical coupler (11) is connected to the b port of the first optical coupler (10), the input port of the second optical coupler (11) is connected to the other end of the third polarization controller (5), and the b port of the second optical coupler (11) is connected to the input port of the photodetector (15).

6. A single-resonant ring Brillouin microwave photonic filter based on parity-time symmetry according to claim 5, characterized in that, The detection unit includes a third optical coupler (12), a spectrometer (14), a photodetector (15), and a vector network analyzer (16). A third optical coupler (12) is also provided between the photodetector (15) and the b port of the second optical coupler (11). The input port of the third optical coupler (12) is connected to the b port of the second optical coupler (11), the a port of the third optical coupler (12) is connected to the input port of the spectrometer (14), the b port of the third optical coupler (12) is connected to the input port of the photodetector (15), the output port of the photodetector (15) is connected to the receiving port of the vector network analyzer (16), and the transmitting port of the vector network analyzer (16) is connected to the phase modulator (9).

7. A single-resonant ring Brillouin microwave photonic filter based on parity-time symmetry according to claim 6, characterized in that, The splitting ratios of ports a and b of the first optical coupler (10) and ports a and b of the third optical coupler (12) are both 50:50, and the splitting ratio of ports a and b of the second optical coupler (11) is 90:

10.

8. A single-resonant ring Brillouin microwave photonic filter based on parity-time symmetry according to claim 7, characterized in that, The physical length of the single-mode fiber (8) is 5 km.

9. A single-resonant ring Brillouin microwave photonic filter based on parity-time symmetry according to claim 8, characterized in that, The output power range of the erbium-doped fiber amplifier is adjustable from 17 to 27 dBm.

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