Switchable brillouin shift fiber laser high-frequency narrow-band microwave photonic filter

Abstract

The present application relates to narrow bandwidth filter, due to the limited gain bandwidth of erbium-doped fiber, the delay device such as optical fiber affects the period of transfer function in frequency, limited to the Brillouin frequency shift amount of material, the structure of traditional optical fiber ring resonator limits the narrow threshold of stimulated Brillouin scattering gain, the present application provides a kind of switchable Brillouin frequency shift fiber laser high frequency narrow band microwave photon filter, by using the laser generated by switchable Brillouin frequency shift fiber laser and the modulation light coupling, after beating, the narrow bandwidth microwave photon Brillouin filter of narrow bandwidth laser filtering can be realized;Brillouin frequency shift interval selection structure is accessed in switchable Brillouin frequency shift fiber laser, and then the purpose of Brillouin frequency interval selection is achieved.The present application utilizes switchable Brillouin frequency shift fiber laser, the working frequency of narrow linewidth laser output is selected by Brillouin frequency shift interval, and then single passband high frequency narrow bandwidth laser filtering is realized.

Description

Technical Field

[0001] This invention relates to narrow bandwidth filters, specifically a switchable Brillouin frequency shift fiber laser high-frequency narrowband microwave photonic filter. Background Technology

[0002] Microwave photonic filters pioneered direct signal processing in the optical domain, offering advantages such as high time-bandwidth product and high resolution, elimination of electronic bottlenecks, resistance to electromagnetic interference, large dynamic range, fast reconstruction speed, and good tunability. With the increasing demands for high performance in communication systems, such as capacity and data rate, microwave photonic filters are attracting growing attention from researchers. Microwave photonic filters with high frequency selectivity can better meet the data transmission rate demands driven by the surge in users and application scenarios in communication protocols, while those with narrow bandwidth can provide high frequency selectivity. Developing filters that combine high operating frequency and narrow bandwidth has become a significant challenge for the development of millimeter-wave communication technology. In 2017, Cao Hui et al. from Qingdao University designed a single-passband microwave photonic filter with a filtering range of 12.145 to 23.277 GHz when the temperature range varied from 20 to 80 degrees Celsius by cascading a finite impulse response filter and an infinite impulse response filter based on a high birefringence photonic crystal fiber. The filter's 3dB bandwidth was 350 MHz. Also in 2017, Han Chen et al. from the School of Instrument Science and Engineering at Southeast University reported a microwave photonic filter based on the birefringence effect of a semiconductor optical amplifier. The filter's filtering range was 15.44 to 19.44 GHz, and its 3dB bandwidth was 2.45 MHz. In 2019, Li Zhengkai et al. from the University of Electronic Science and Technology of China designed a tunable dual-passband microwave photonic filter that utilized stimulated Brillouin scattering phase intensity modulation conversion. The filter's filtering range was 0 to 9.644 GHz, and its 3dB bandwidth was 57 MHz.

[0003] Finite impulse response (FIR) filters based on high birefringence photonic crystal fibers adjust the wavelength spacing of the output laser by regulating the temperature and the effective length of the photonic crystal fiber, thereby adjusting the filter's filterable frequency. However, due to the limitation of the adjustable temperature range, the filter's filterable frequency threshold cannot be exceeded. Furthermore, since the gain bandwidth of erbium-doped fiber is limited, the number of laser output wavelengths is also limited, thus restricting the realization of infinite impulse response filters by increasing the number of laser wavelengths to narrow the bandwidth by 3dB. Microwave photonic filters based on the birefringence effect of semiconductor optical amplifiers suffer from a very limited range of usable filter response free spectrum due to the influence of delay devices such as optical fibers on the period of the transfer function within the frequency range. Consequently, the filter's filterable frequency range is limited. Moreover, the coupling loss between the semiconductor optical amplifier and the optical fiber is too high, resulting in significant noise and crosstalk, and the filter is easily affected by ambient temperature, leading to poor stability. Microwave photonic filters based on stimulated Brillouin scattering (SBS) are limited by the Brillouin frequency shift of the material, preventing the filter's filterable frequency range from exceeding the Brillouin frequency shift. Furthermore, the structure of the traditional fiber ring resonator limits the narrowing threshold of the stimulated Brillouin scattering gain, thus preventing the realization of high-frequency narrowband filtering in microwave photonic filters. Summary of the Invention

[0004] To address the problems existing in the prior art, the present invention aims to provide a switchable Brillouin frequency shift fiber laser high-frequency narrowband microwave photonic filter.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A switchable Brillouin frequency-shifting fiber laser high-frequency narrowband microwave photonic filter includes a narrow-linewidth continuous-wave fiber laser, a first beam splitter, a phase modulator, a spectrometer, a photodetector, and a vector network tester, as well as a switchable Brillouin frequency-shifting fiber laser for achieving frequency band switching.

[0007] The output of the narrow-linewidth continuous-wave fiber laser is connected to port a of the first optical splitter. Port b of the first optical splitter is connected to port a of the switchable Brillouin frequency-shifting fiber laser. Port c of the first optical splitter is connected to port a of the phase modulator. Port b of the switchable Brillouin frequency-shifting fiber laser is connected to port a of the second optical splitter. Port d of the switchable Brillouin frequency-shifting fiber laser is connected to port b of the phase modulator. Port c of the phase modulator is connected to the output of the vector network tester. Port b of the second optical splitter is connected to the input port of the spectrometer. Port c of the second optical splitter is connected to the input port of the photodetector. The output of the photodetector is connected to the input of the vector network tester.

[0008] Furthermore, the narrow-linewidth continuous-wave fiber laser is a continuous-running laser with a wavelength of 1550 nm, a linewidth of 0.1 Hz, and a maximum output power of 15 dBm; the first beam splitter has a splitting ratio of 90:10, with port a being the input port, port b being the 90% output port, and port c being the 10% output port; the second beam splitter has a splitting ratio of 50:50; the phase modulator has a modulation wavelength range of 1530 to 1625 nm, an electro-optical linewidth of 25 GHz, and an interpolation loss of 2.5 dB; the spectrometer has a wavelength range of 600 to 1700 nm, a wavelength resolution of 0.02 to 2, and a wavelength linearity of 0.0. The measurement power range is -90 to 20 dBm, with a power accuracy of ±0.4 and a maximum sampling point count of 50001. The photodetector has a linewidth of 50 GHz, a linear response of 10 dBm for optical input power, and a data rate of 40 Gbps. The vector network analyzer has a frequency range of 300 kHz to 20 GHz, a frequency resolution of 1 Hz, an intermediate frequency linewidth of 10 Hz to 1.5 MHz, a power range of -85 dBm to 10 dBm in the frequency range of 1 MHz to 6 GHz, a level resolution of 0.05 dB, a voltage range of 0 to ±35 V, and a maximum current of ±500 mA.

[0009] During operation, a narrow-linewidth continuous-wave fiber laser serves as the pump source. The pump light passes through a 90:10 splitter coupler and is input to the switchable Brillouin frequency-shifting fiber laser via port b. At port c, the pump light is input to a phase modulator. The phase modulator modulates the pump light with an RF signal from a vector network tester, generating modulated light. This modulated light passes through port d of the switchable Brillouin frequency-shifting fiber laser, where it is coupled with the narrow-linewidth laser. The resulting mixed light is output from port b and then output through a 50:50 splitter coupler. The output signal from port b is directly input to a spectrometer for observation, while the output signal from port c is transmitted to the vector network tester via a photodetector. By coupling the laser generated by the switchable Brillouin frequency-shifting fiber laser with the modulated light and then performing frequency matching, narrow-bandwidth laser filtering for a narrow-bandwidth microwave photonic Brillouin filter can be achieved.

[0010] A switchable Brillouin frequency-shifting fiber laser is disclosed for use in the aforementioned switchable Brillouin frequency-shifting fiber laser high-frequency narrowband microwave photonic filter. The switchable Brillouin frequency-shifting fiber laser includes a third optical splitter coupler, a polarization controller, a first erbium-doped fiber amplifier, a first optical circulator, and a Brillouin frequency shift spacing selection structure. The b-port of the first optical splitter coupler based on the switchable Brillouin frequency-shifting fiber laser is connected to the input of the polarization controller; the output of the polarization controller is connected to the input of the first erbium-doped fiber amplifier; the output of the first erbium-doped fiber amplifier is connected to the a-port of the third optical splitter coupler; the d-port of the third optical splitter coupler is connected to the a-port of the first optical circulator; and the c-port of the third optical splitter coupler... The c port of the switchable Brillouin frequency shift spacing selection structure is connected to the b port of the third optical splitter, which serves as the b output port of the switchable Brillouin frequency shift fiber laser and is connected to the a port of the second optical splitter of the switchable Brillouin frequency shift fiber laser high-frequency narrowband microwave photonic filter. The b port of the first optical circulator is connected to the b port of the Brillouin frequency shift spacing selection structure. The c port of the first optical circulator is connected to the input port of the second erbium-doped fiber amplifier. The output port of the second erbium-doped fiber amplifier is connected to the a port of the Brillouin frequency shift spacing selection structure. The d port of the Brillouin frequency shift spacing selection structure serves as the d port of the switchable Brillouin frequency shift fiber laser and is connected to the b port of the phase modulator of the switchable Brillouin frequency shift fiber laser high-frequency narrowband microwave photonic filter.

[0011] Furthermore, the third optical splitter has a splitting ratio of 99:1, with port b being the 1% output terminal and port d being the 99% output terminal. Ports a and c of the third optical splitter are the input terminals. The input power range of both the first and second erbium-doped fiber amplifiers is -20 to 15 dBm, and the maximum output power of both is 37 dBm.

[0012] During operation, the pump light, after being regulated by a polarization controller, is amplified by an erbium-doped fiber amplifier. It then passes through a beam splitter with a splitting ratio of 99:1, enters the resonant cavity via the d-port of the beam splitter, passes through the a-port of an optical circulator, and is output from the b-port of the circulator. This output is injected into a Brillouin frequency shift spacing selection structure, where it couples with the modulated light from the phase modulator of a switchable Brillouin frequency shift fiber laser high-frequency narrowband microwave photonic filter, which is input from the d-port of the Brillouin frequency shift spacing selection structure. The resulting mixed light, output from the c-port of the Brillouin frequency shift spacing selection structure, passes through the beam splitter and outputs a single-passband narrow-linewidth laser from its d-port. By selecting first-order, second-order, and third-order Brillouin structures to connect to the optical path, the high-frequency tunable function of the laser can be achieved. The selectable Brillouin frequency shifts are 10.75 GHz, 21.25 GHz, and 31.875 GHz, respectively.

[0013] A Brillouin frequency shift interval selection structure is provided for the aforementioned switchable Brillouin frequency shift fiber laser. The Brillouin frequency shift interval selection structure is connected to the optical path to achieve the purpose of Brillouin frequency interval selection. The Brillouin frequency shift interval selection structure can be selected from the following structures: first-order Brillouin structure, second-order Brillouin structure, and third-order Brillouin structure.

[0014] Furthermore, the first-order Brillouin structure includes a fourth optical splitter and a first single-mode fiber; the a-port of the fourth optical splitter, serving as the a-port of the Brillouin frequency shift spacing selection structure, is connected to the output port of the second erbium-doped fiber amplifier; the b-port of the fourth optical splitter is connected to the input end of the first single-mode fiber; the output end of the first single-mode fiber, serving as the b-port of the Brillouin frequency shift spacing selection structure, is connected to the b-port of the first optical circulator; the c-port of the fourth optical splitter, serving as the c-port of the Brillouin frequency shift spacing selection structure, is connected to the c-port of the third optical splitter; and the d-port of the fourth optical splitter, serving as the d-port of the Brillouin frequency shift spacing selection structure, is connected to the b-port of the phase modulator of the switchable Brillouin frequency shift fiber laser high-frequency narrowband microwave photonic filter; the splitting ratio of the fourth optical splitter is 50:50; and the first single-mode fiber is a 20m long single-mode fiber.

[0015] During operation, when a first-order Brillouin structure is connected to the optical path, the pump light is output from port b of the optical circulator and injected into port b of the first-order Brillouin structure. The pump light excites the stimulated Brillouin scattering effect in the 20-meter single-mode fiber, generating a reversed Stokes beam. After passing through port b of the optical circulator and exiting from port c of the optical circulator, it is coupled with the modulated light input from port d of the first-order Brillouin structure by the phase modulator of the switchable Brillouin frequency-shift fiber laser high-frequency narrowband microwave photonic filter. After coupling through the beam splitter, it is injected into port a of the first-order Brillouin structure to form a ring resonant cavity. When the power of the coupled light exceeds the threshold of the beam splitter, the mixed light is output from port c of the first-order Brillouin structure and outputs a single-passband narrow-linewidth laser with a frequency shift of 10.75 GHz through port b of the beam splitter.

[0016] Furthermore, the second-order Brillouin structure includes a fifth optical splitter coupler, a second optical circulator, and a second single-mode fiber. The a-port of the second optical circulator, serving as the a-port of the Brillouin frequency shift spacing selection structure, is connected to the output port of the second erbium-doped fiber amplifier. The b-port of the second optical circulator is connected to the input end of the second single-mode fiber. The c-port of the second optical circulator, serving as the c-port of the Brillouin frequency shift spacing selection structure, is connected to the c-port of the optical splitter coupler. The output end of the second single-mode fiber is connected to the a-port of the fifth optical splitter coupler. The b-port of the fifth optical splitter coupler, serving as the b-port of the Brillouin frequency shift spacing selection structure, is connected to the b-port of the first optical circulator coupler. The d-port of the optical splitter coupler, serving as the d-port of the Brillouin frequency shift spacing selection structure, is connected to the b-port of the phase modulator of the switchable Brillouin frequency shift fiber laser high-frequency narrowband microwave photonic filter. The splitting ratio of the fifth optical splitter coupler is 50:50. The second single-mode fiber is a 20m long single-mode fiber.

[0017] During operation, when the second-order Brillouin structure is connected to the optical path, the pump light is output from port b of the optical circulator and injected into port b of the second-order Brillouin structure. After passing through the beam splitter, the pump light excites the stimulated Brillouin scattering effect in the 20-meter single-mode fiber, generating a reversed Stokes beam. This beam passes through port b of the optical circulator and exits from port c. After being amplified by an erbium-doped fiber amplifier, the power is injected into port a of the second-order Brillouin structure. The Stokes beam then passes through port a of the optical circulator in the second-order Brillouin structure and exits from port b, further exciting the stimulated Brillouin scattering effect in the 20-meter single-mode fiber to generate a reversed second-order Stokes beam. After being output from port c of the optical circulator via port b, the light passes through a beam splitter and is injected into port a of the optical circulator, then outputs from port b. This output is coupled with the modulated light from the phase modulator of the switchable Brillouin frequency-shift fiber laser high-frequency narrowband microwave photonic filter, which is input from port d of the second-order Brillouin structure. After being coupled through the beam splitter, the light is output from port c of the second-order Brillouin structure to form a ring resonant cavity. When the power of the mixed light exceeds the threshold of the beam splitter, the mixed light is output from port c of the second-order Brillouin structure and outputs a single-passband narrow-linewidth laser with a frequency shift of 21.25 GHz through port b of the beam splitter.

[0018] Furthermore, the third-order Brillouin structure includes a sixth optical splitter, a third optical circulator, and two single-mode fibers. The a-port of the third optical circulator, serving as the a-port of the Brillouin frequency shift spacing selection structure, is connected to the output port of the second erbium-doped fiber amplifier. The d-port of the third optical circulator is connected to the input end of the third single-mode fiber, and the output end of the third single-mode fiber is disconnected. The b-port of the third optical circulator is connected to the input end of the fourth single-mode fiber. The c-port of the third optical circulator, serving as the c-port of the Brillouin frequency shift spacing selection structure, is connected to the c-port of the third optical splitter. The output end of the single-mode fiber is connected to port a of the sixth optical splitter. Port b of the sixth optical splitter, as the port b of the Brillouin frequency shift spacing selection structure, is connected to port b of the first optical circulator. Port d of the sixth optical splitter, as the port d of the Brillouin frequency shift spacing selection structure, is connected to port b of the phase modulator of the switchable Brillouin frequency shift fiber laser high-frequency narrowband microwave photonic filter. The splitting ratio of the sixth optical splitter is 50:50. The third single-mode fiber is a 5km long single-mode fiber. The fourth single-mode fiber is a 20m long single-mode fiber.

[0019] During operation, when the third-order Brillouin structure is connected to the optical path, the pump light is output from port b of the optical circulator and injected into port b of the third-order Brillouin structure. After passing through the beam splitter, the pump light excites the stimulated Brillouin scattering effect in the 20-meter single-mode fiber, generating a reversed Stokes beam. This beam passes through port b of the optical circulator and exits from port c of the optical circulator. After being amplified by an erbium-doped fiber amplifier, the power is injected into port a of the third-order Brillouin structure. The Stokes beam then passes through port a of the optical circulator in the third-order Brillouin structure and exits from port d of the optical circulator, exciting the stimulated Brillouin scattering effect in the 5-kilometer single-mode fiber to generate a reversed second-order Stokes beam. This second-order Stokes beam passes through port d of the optical circulator and exits from port b of the optical circulator, exciting the 20-meter single-mode fiber... Stimulated Brillouin scattering in a single-mode fiber generates a reversed third-order Stokes beam. This beam passes through port b of an optical circulator and exits from port c. After passing through a beam splitter, it is injected into port a of the optical circulator and exits from port b. This beam then couples with the modulated light input from port d of the third-order Brillouin structure via a phase modulator of a switchable Brillouin frequency-shift fiber laser high-frequency narrowband microwave photonic filter. After coupling through the beam splitter, the beam exits from port c of the third-order Brillouin structure to form a ring resonant cavity. When the power of the mixed light exceeds the threshold of the beam splitter, the mixed light exits from port c of the third-order Brillouin structure and outputs a single-passband narrow-linewidth laser with a frequency shift of 31.875 GHz through port b of the beam splitter.

[0020] In summary, the invention has the following beneficial effects:

[0021] Compared to existing microwave photonic filters, this invention utilizes a switchable Brillouin frequency-shifting fiber laser and achieves a narrow linewidth laser output operating frequency through a Brillouin frequency shift interval selection structure. The filtering frequency can be selected from 10.75, 21.25, and 31.875 GHz, thereby realizing single-passband high-frequency narrow-bandwidth laser filtering. Attached Figure Description

[0022] Figure 1 This diagram illustrates the structure of a switchable Brillouin frequency shift fiber laser high-frequency narrowband microwave photonic filter proposed in this invention.

[0023] Figure 2 This diagram illustrates the structure of a switchable Brillouin frequency-shifting fiber laser proposed in this invention.

[0024] Figure 3 This diagram illustrates a first-order Brillouin structure representing the Brillouin frequency shift interval selection structure proposed in this invention.

[0025] Figure 4 This is a schematic diagram of a second-order Brillouin structure representing the Brillouin frequency shift interval selection structure proposed in this invention.

[0026] Figure 5 This diagram illustrates a third-order Brillouin structure representing the Brillouin frequency shift interval selection structure proposed in this invention.

[0027] In the diagram: 1-Narrow linewidth continuous wave fiber laser, 2A-First optical splitter coupler, 2B-Second optical splitter coupler, 3-Switchable Brillouin frequency shift fiber laser, 4-Phase modulator, 5-Spectrometer, 6-Photodetector, 7-Vector network tester, 2C-Third optical splitter coupler, 8-Polarization controller, 9A-First erbium-doped fiber amplifier, 9B-Second erbium-doped fiber amplifier, 10A-First optical circulator, 11-Brillouin frequency shift spacing selection structure, 2D-Fourth optical splitter coupler, 2E-Fifth optical splitter coupler, 2F-Sixth optical splitter coupler, 10B-Second optical circulator, 10C-Third optical circulator, 12A-First single-mode fiber, 12B-Second single-mode fiber, 12C-Third single-mode fiber, 12D-Fourth single-mode fiber. Detailed Implementation

[0028] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0029] It should be noted that, for ease of description, the descriptions of direction in the following text are consistent with the directions in the accompanying drawings, but they do not limit the structure of the present invention.

[0030] like Figure 1As shown, the present invention discloses a switchable Brillouin frequency-shifting fiber laser high-frequency narrowband microwave photonic filter, comprising a narrow-linewidth continuous-wave fiber laser 1, a first beam splitter 2A, a phase modulator 4, a spectrometer 5, a photodetector 6, and a vector network tester 7, as well as a switchable Brillouin frequency-shifting fiber laser 3 for realizing frequency band switching.

[0031] The output of the narrow linewidth continuous wave fiber laser 1 is connected to port a of the first optical splitter 2A. Port b of the first optical splitter 2A is connected to port a of the switchable Brillouin frequency-shifting fiber laser 3. The output port c of the first optical splitter 2A is connected to port a of the phase modulator 4. Port b of the switchable Brillouin frequency-shifting fiber laser 3 is connected to port a of the second optical splitter 2B. Port d of the switchable Brillouin frequency-shifting fiber laser 3 is connected to port b of the phase modulator 4. Port c of the phase modulator 4 is connected to the output of the vector network tester 7. Port b of the second optical splitter 2B is connected to the input port of the spectrometer 5. The output port c of the second optical splitter 2B is connected to the input port of the photodetector 6. The output of the photodetector 6 is connected to the input of the vector network tester 7.

[0032] A narrow-linewidth continuous-wave fiber laser 1 serves as the pump source. The pump light passes through a first beam splitter 2A with a splitting ratio of 90:10. The pump light is input from port b of the first beam splitter 2A to a switchable Brillouin frequency-shifting fiber laser 3. The pump light then passes through port c of the first beam splitter 2A to port a of a phase modulator 4. The phase modulator 4 modulates the pump light with the radio frequency signal from the vector network tester 7 to generate modulated light. This modulated light is input from port b of the phase modulator 4 to port d of the switchable Brillouin frequency-shifting fiber laser 3. After coupling with the narrow-linewidth laser inside the switchable Brillouin frequency-shifting fiber laser 3, the mixed light output from port b passes through a second beam splitter 2B with a splitting ratio of 50:50. The output signal from port b of the second beam splitter 2B is directly input to the spectrometer 5 for observation. The output signal from port c of the second beam splitter 2B is output to the vector network tester 7 via a photodetector 6. By coupling the laser generated by the tunable Brillouin frequency-spaced Brillouin fiber laser 3 with the modulated light and then beating the light, narrow-bandwidth laser filtering of a narrow-bandwidth microwave photonic Brillouin filter can be achieved.

[0033] like Figure 2 As shown, the present invention discloses an adjustable Brillouin frequency-spaced Brillouin fiber laser, including a third optical splitter coupler 2C, a polarization controller 8, two erbium-doped fiber amplifiers 9, a first optical circulator 10, and a Brillouin frequency shift spacing selection structure 11.

[0034] The input terminal of the polarization controller 8 serves as port a of the adjustable Brillouin frequency-spaced Brillouin fiber laser 3. Port b of the first beam splitter 2A of the switchable Brillouin frequency-shifted fiber laser high-frequency narrowband microwave photonic filter is connected to the input terminal of the polarization controller 8. The output terminal of the polarization controller 8 is connected to the input terminal of the first erbium-doped fiber amplifier 9A. The output terminal of the first erbium-doped fiber amplifier 9A is connected to port a of the third beam splitter 2C. Port d of the third beam splitter 2C is connected to port a of the first optical circulator 10A. Port c of the third beam splitter 2C is connected to port c of the Brillouin frequency-shifted spacing selection structure 11. Port b of the third beam splitter 2C serves as the input terminal of the adjustable Brillouin frequency-spaced Brillouin fiber laser 3. The b-port of the fiber laser 3 is connected to the a-port of the second beam splitter 2B of the switchable Brillouin frequency-shift fiber laser high-frequency narrowband microwave photonic filter. The b-port of the first optical circulator 10A is connected to the b-port of the Brillouin frequency shift interval selection structure 11. The c-port of the first optical circulator 10A is connected to the input port of the second erbium-doped fiber amplifier 9B. The output port of the second erbium-doped fiber amplifier 9B is connected to the a-port of the Brillouin frequency shift interval selection structure 11. The d-port of the Brillouin frequency shift interval selection structure 11 serves as the d-port of the tunable Brillouin frequency-spaced Brillouin fiber laser 3 and is connected to the b-port of the phase modulator 4 of the switchable Brillouin frequency-shift fiber laser high-frequency narrowband microwave photonic filter.

[0035] After being adjusted by the polarization controller 8, the pump light is amplified by the first erbium-doped fiber amplifier 9A. After entering the resonant cavity through the d port of the third optical coupler 2C with a splitting ratio of 99:1, it passes through the a port of the first optical circulator 10A and is output from the b port of the first optical circulator 10A. It is then injected into the b port of the Brillouin frequency shift interval selection structure 11. The modulated light from the phase modulator 4 of the switchable Brillouin frequency shift fiber laser high-frequency narrowband microwave photonic filter is input from the d port of the Brillouin frequency shift interval selection structure 11. After the modulated light is coupled with the pump light, the mixed light output from the c port of the Brillouin frequency shift interval selection structure 11 is output as a single-passband narrow-linewidth laser through the b port of the third optical coupler 2C. In a tunable Brillouin frequency-spaced Brillouin fiber laser, the high-frequency tunable function of the laser can be achieved by selecting any one of the first-order, second-order, or third-order Brillouin structures to be connected to the optical path. The Brillouin frequency shifts of the three different Brillouin structures are 10.75 GHz, 21.25 GHz, and 31.875 GHz, respectively.

[0036] like Figure 3 As shown, the present invention also discloses a Brillouin frequency shift interval selection structure for accessing an optical path, thereby achieving the purpose of Brillouin frequency interval selection. The Brillouin frequency shift interval selection structure can be a first-order Brillouin structure, a second-order Brillouin structure, or a third-order Brillouin structure.

[0037] The first-order Brillouin structure is configured as follows: port a of the fourth optical splitter 2D serves as port a of the Brillouin frequency shift interval selection structure and is connected to the output port of the second erbium-doped fiber amplifier 9B; port b of the fourth optical splitter 2D is connected to the input end of the first single-mode fiber 12A; the output port of the first single-mode fiber 12A serves as port b of the Brillouin frequency shift interval selection structure and is connected to port b of the first optical circulator 10A; port c of the fourth optical splitter 2D serves as port c of the Brillouin frequency shift interval selection structure and is connected to port c of the third optical splitter 2C; and port d of the fourth optical splitter serves as port d of the Brillouin frequency shift interval selection structure and is connected to port b of the phase modulator 4 of the switchable Brillouin frequency shift fiber laser high-frequency narrowband microwave photonic filter.

[0038] When a first-order Brillouin structure is selected as the access optical path, the pump light is output from port b of the first optical circulator 10A and injected into port b of the first-order Brillouin structure. The pump light excites the stimulated Brillouin scattering effect of the 20-meter single-mode fiber, generating a reversed Stokes light. After passing through port b of the first optical circulator and being output from port c of the first optical circulator, it is coupled with the modulated light input from port d of the first-order Brillouin structure by the phase modulator 4 of the switchable Brillouin frequency-shift fiber laser high-frequency narrowband microwave photonic filter. After being coupled by the fourth optical splitter coupler 2D, it is injected into port a of the first-order Brillouin structure to form a ring resonant cavity. When the power of the coupled light exceeds the threshold of the fourth optical splitter coupler 2D, the mixed light is output from port c of the first-order Brillouin structure and outputs a single-passband narrow-linewidth laser with a frequency shift of 10.75 GHz through port b of the fourth optical splitter coupler 2D.

[0039] The second-order Brillouin structure is configured as follows: port a of the second optical circulator 10B serves as port a of the Brillouin frequency shift interval selection structure and is connected to the output port of the second erbium-doped fiber amplifier 9B; port b of the second optical circulator 10B is connected to the input end of the second single-mode fiber 12B; port c of the second optical circulator 10B serves as port c of the Brillouin frequency shift interval selection structure and is connected to port c of the third optical splitter coupler 2C; the output end of the second single-mode fiber 12B is connected to port a of the fifth optical splitter coupler 2E; port b of the fifth optical splitter coupler 2E serves as port b of the Brillouin frequency shift interval selection structure and is connected to port b of the first optical circulator 10A; and port d of the fifth optical splitter coupler 2E serves as port d of the Brillouin frequency shift interval selection structure and is connected to port b of the phase modulator 4 of the switchable Brillouin frequency shift fiber laser high-frequency narrowband microwave photonic filter.

[0040] When a second-order Brillouin structure is selected as the access optical path, the pump light is output from port b of the first optical circulator 10A and injected into port b of the second-order Brillouin structure. After passing through the beam splitter, the pump light excites the stimulated Brillouin scattering effect in the 20-meter single-mode fiber, generating a reversed Stokes beam. This beam passes through port b of the first optical circulator 10A and is output from port c. After being amplified by the second erbium-doped fiber amplifier 9B, the power is injected into port a of the second-order Brillouin structure. The Stokes beam passes through port a of the second optical circulator 10B in the second-order Brillouin structure and is output from port b. It also excites the stimulated Brillouin scattering effect in the 20-meter single-mode fiber, generating a reversed second-order Stokes beam. This second-order Stokes beam passes through the second optical circulator... After the output from port c of the second optical circulator 10B, the beam from port b of 10B passes through the third optical splitter 2C and is injected into port a of the first optical circulator 10A, and output from port b of the first optical circulator 10A. The modulated light input from port d of the phase modulator 4 of the switchable Brillouin frequency-shift fiber laser high-frequency narrowband microwave photonic filter is coupled through the fifth optical splitter 2E, and then output through port c of the second optical circulator 10B of the second Brillouin structure to form a ring resonant cavity. When the power of the mixed light exceeds the threshold of the fifth optical splitter 2E, the mixed light is output from port c of the second Brillouin structure and outputs a single-passband narrow-linewidth laser with a frequency shift of 21.25 GHz through port b of the fifth optical splitter 2E.

[0041] The configuration of the third-order Brillouin structure is as follows: the a port of the third optical circulator 10C serves as the a port of the Brillouin frequency shift interval selection structure and is connected to the output port of the second erbium-doped fiber amplifier 9B; the d port of the third optical circulator 10C is connected to the third single-mode fiber 12C; the b port of the third optical circulator 10C is connected to the input end of the fourth single-mode fiber 12D; the c port of the third optical circulator 10C serves as the c port of the Brillouin frequency shift interval selection structure and is connected to the c port of the third optical splitter coupler 2C; the output end of the fourth single-mode fiber 12D is connected to the a port of the sixth optical splitter coupler 2F; the b port of the sixth optical splitter coupler 2F serves as the b port of the Brillouin frequency shift interval selection structure and is connected to the b port of the first optical circulator 10A; and the d port of the sixth optical splitter coupler 2F serves as the d port of the Brillouin frequency shift interval selection structure and is connected to the b port of the phase modulator 4 of the switchable Brillouin frequency shift fiber laser high-frequency narrowband microwave photonic filter.

[0042] When a third-order Brillouin structure is selected for the optical path, the pump light is output from port b of the first optical circulator 10A and injected into port b of the third-order Brillouin structure. After passing through the beam splitter, the pump light excites the stimulated Brillouin scattering effect in the 20-meter single-mode fiber, generating a reversed Stokes beam. This beam passes through port b of the first optical circulator 10A and is output from port c. After being amplified by the second erbium-doped fiber amplifier 9B, the power is injected into port a of the third-order Brillouin structure. The Stokes beam passes through port a of the third optical circulator 10C in the third-order Brillouin structure and is output from port d. It also excites the stimulated Brillouin scattering effect in the 5-kilometer single-mode fiber, generating a reversed second-order Stokes beam. This second-order Stokes beam passes through port d of the third optical circulator 10C and is output from port b. This second-order Stokes beam then excites the 20-meter single-mode fiber. The stimulated Brillouin scattering effect of the fiber generates a reversed third-order Stokes beam. After passing through port b of the third optical circulator 10C and exiting from port c, the third optical splitter 2C is injected into port a of the first optical circulator 10A and exits from port b of the first optical circulator 10A. It is coupled with the modulated light from the phase modulator 4 of the high-frequency tunable narrowband microwave photonic filter based on the switchable Brillouin frequency shift fiber laser, which is input from port d of the third-order Brillouin structure. After being coupled through the sixth optical splitter 2F, it exits through port c of the third optical circulator 10C of the third-order Brillouin structure to form a ring resonant cavity. When the power of the mixed light exceeds the threshold of the sixth optical splitter 2F, the mixed light is output from port c of the third-order Brillouin structure and outputs a single-passband narrow-linewidth laser with a frequency shift of 31.875 GHz through port b of the sixth optical splitter 2F.

[0043] Based on the above specific embodiments, further specific implementations of the present invention are as follows:

[0044] The narrow linewidth continuous wave fiber laser 1 used is a continuous-running laser with a wavelength of 1550nm, a linewidth of 0.1Hz, and a maximum output power of 15dBm.

[0045] The first beam splitter 2A used is a beam splitter with a splitting ratio of 90:10, with port a being the input port, port b being the 90% output port, and port c being the 10% output port.

[0046] The second beam splitter 2B used has a beam splitting ratio of 50:50.

[0047] The third optical splitter 2C used has a splitting ratio of 99:1, with ports a and c being the input terminals, port b being the 1% output terminal, and port d being the 99% output terminal.

[0048] The fourth beam splitter used has a 2D splitting ratio of 50:50.

[0049] The fifth beam splitter 2E used has a beam splitting ratio of 50:50.

[0050] The sixth beam splitter 2F used has a beam splitting ratio of 50:50.

[0051] The working principle of the tunable Brillouin frequency-spaced Brillouin fiber laser is as follows:

[0052] In an optical fiber, the incident laser and the acoustic wave within the fiber undergo a nonlinear interaction, producing Stokes-Brillouin scattered light. This results in a Brillouin frequency shift v0 in a 100-meter optical fiber. B , represented as

[0053] ν B =(2V) A / c)ν P (1)

[0054] Where, ν P V is the pump light frequency. A where c is the speed of sound, c is the speed of light, and v is the Brillouin frequency shift. B The frequency is approximately 10 GHz around 1550 nm. As the pump light power increases, the pump light excites Brillouin scattering in the fiber, and its center frequency becomes...

[0055] f pass =v P -v B (2)

[0056] 3dB linewidth Δf of the output laser from an adjustable Brillouin frequency-spaced Brillouin fiber laser pass for

[0057] Δν=Δν P / (1+γ A / Γ C ) 2 (3)

[0058] Where Δν P The linewidth of the pump light, and

[0059] γ A =πΔν B , Γ C =-clnR / nL t (4)

[0060] Where Δν B Let be the linewidth of the Brillouin gain, c be the speed of light, R be the total loss of all resonant cavities, n be the refractive index of the optical fiber, and L be the linewidth of the Brillouin gain. t The sum of the fiber lengths in the dual-ring cavity. The filterable bandwidth is related to the laser linewidth and the linewidth of the Brillouin gain.

[0061] The phase modulator used has a modulation wavelength range of 1530 to 1625 nm, an electric beamwidth of 25 GHz, and an interpolation loss of 2.5 dB.

[0062] The spectrometer used has a wavelength range of 600 to 1700 nm, a wavelength resolution of 0.02 to 2, a wavelength linearity of 0.01 to 0.02, a measurement power range of -90 to 20 dBm, a power accuracy of ±0.4, and a maximum number of sampling points of 50001.

[0063] The photodetector used has a linewidth of 50 GHz, a linear response of 10 dBm for optical input power, and a data rate of 40 Gbps.

[0064] The vector network analyzer 7 used has a frequency range of 300kHz to 20GHz, a frequency resolution of 1Hz, an intermediate frequency linewidth of 10Hz to 1.5MHz, a power range of -85dBm to 10dBm when the frequency range is 1MHz to 6GHz, a level resolution of 0.05dB, a voltage range of 0 to ±35V, and a maximum current of ±500mA.

[0065] The two erbium-doped fiber amplifiers used have an input power range of -20 to 15 dBm and a maximum output power of 37 dBm.

[0066] The first single-mode fiber 12A used is a single-mode fiber with a length of 20m.

[0067] The second single-mode fiber 12B used is a single-mode fiber with a length of 20m.

[0068] The third single-mode fiber 12C used is a 5km long single-mode fiber.

[0069] The fourth single-mode fiber 12D used is a single-mode fiber with a length of 20m.

[0070] In practical operation, the narrow-linewidth continuous-wave fiber laser 1 serves as the pump source. The pump light passes through a first beam splitter coupler 2A with a splitting ratio of 90:10. The pump light is input to port a of the tunable Brillouin frequency-spaced Brillouin fiber laser 3 via port b. The pump light is also input to port a of the phase modulator 4 via port c. The phase modulator 4 modulates the pump light with the radio frequency signal from the vector network tester 7 to generate modulated light. This modulated light passes through port d of the tunable Brillouin frequency-spaced Brillouin fiber laser 3 and is coupled with the narrow-linewidth laser inside the tunable Brillouin frequency-spaced Brillouin fiber laser 3. The resulting mixed light is output from port b of the tunable Brillouin frequency-spaced Brillouin fiber laser 3 and output through a second beam splitter coupler 2B with a splitting ratio of 50:50. The output signal from port b is directly input to the spectrometer 5 for observation, while the output signal from port c is output to the vector network tester 7 via a photodetector 6. By coupling the laser generated by the tunable Brillouin frequency-spaced Brillouin fiber laser 3 with the modulated light and then beating the light, narrow-bandwidth laser filtering of a narrow-bandwidth microwave photonic Brillouin filter can be achieved.

[0071] The pump light output from port b of the first optical splitter 2A is adjusted by the polarization controller 8, amplified by the first erbium-doped fiber amplifier 9A, and then enters the resonant cavity through port d of the third optical splitter 2C with a splitting ratio of 99:1. After passing through port a of the first optical circulator 10A and outputting from port b of the first optical circulator 10A, it is injected into the Brillouin frequency shift spacing selection structure 11. After coupling with the modulated light input from port d of the phase modulator 4 of the high-frequency tunable narrowband microwave photonic filter based on the switchable Brillouin frequency shift fiber laser, the mixed light output from port c of the Brillouin frequency shift spacing selection structure 11 passes through the third optical splitter 2C and outputs a single-passband narrow-linewidth laser from port b. The Brillouin frequency shift spacing selection structure 11 achieves high-frequency tunable laser function by selecting any one of the first-order, second-order, and third-order Brillouin structures to be connected to the optical path. The Brillouin frequency shifts of the three Brillouin structures are 10.75 GHz, 21.25 GHz, and 31.875 GHz, respectively.

[0072] When a first-order Brillouin structure is connected to the optical path, the pump light is output from port b of the first optical circulator 10A and injected into port b of the first-order Brillouin structure. The pump light excites the stimulated Brillouin scattering effect of the 20-meter single-mode fiber, generating a reversed Stokes light. After passing through port b of the first optical circulator and being output from port c of the first optical circulator, it is coupled with the modulated light input from port d of the first-order Brillouin structure by the phase modulator 4 of the phase modulator 4 of the high-frequency tunable narrowband microwave photonic filter based on the switchable Brillouin frequency shift fiber laser. After coupling through the fourth optical splitter 2D, it is injected into port a of the first-order Brillouin structure to form a ring resonant cavity. When the power of the coupled light exceeds the threshold of the fourth optical splitter, the mixed light is output from port c of the first-order Brillouin structure and outputs a single-passband narrow-linewidth laser with a frequency shift of 10.75 GHz through port b of the fourth optical splitter.

[0073] When the second-order Brillouin structure is connected to the optical path, the pump light is output from port b of the first optical circulator 10A and injected into port b of the second-order Brillouin structure. After passing through the beam splitter, the pump light excites the stimulated Brillouin scattering effect in the 20-meter single-mode fiber, generating a reversed Stokes beam. This beam passes through port b of the first optical circulator and is output from port c. After being amplified by the second erbium-doped fiber amplifier, the power is injected into port a of the second-order Brillouin structure. The Stokes beam passes through port a of the second optical circulator in the second-order Brillouin structure and is output from port b of the second optical circulator, further exciting the stimulated Brillouin scattering effect in the 20-meter single-mode fiber to generate a reversed second-order Stokes beam. This second-order Stokes beam passes through the second optical circulator... After the output from port c of the second optical circulator, port b passes through the third optical splitter and is injected into port a of the first optical circulator, and then outputs from port b of the first optical circulator. It is coupled with the modulated light input from port d of the phase modulator 4 of the high-frequency tunable narrowband microwave photonic filter based on the switchable Brillouin frequency shift fiber laser through the fifth optical splitter. After being output from port c of the second optical circulator of the second Brillouin structure, it forms a ring resonant cavity. When the power of the mixed light exceeds the threshold of the fifth optical splitter, the mixed light is output from port c of the second Brillouin structure and outputs a single-passband narrow-linewidth laser with a frequency shift of 21.25 GHz through port b of the fifth optical splitter.

[0074] When a third-order Brillouin structure is connected to the optical path, the pump light is output from port b of the first optical circulator and injected into port b of the third-order Brillouin structure. After passing through a beam splitter, the pump light excites the stimulated Brillouin scattering effect in a 20-meter single-mode fiber, generating a reversed Stokes beam. This beam passes through port b of the first optical circulator and exits from port c. After being amplified by a second erbium-doped fiber amplifier, the power is injected into port a of the third-order Brillouin structure. The Stokes beam then passes through port a of the third optical circulator in the third-order Brillouin structure and exits from port d, exciting the stimulated Brillouin scattering effect in a 5-kilometer single-mode fiber to generate a reversed second-order Stokes beam. This second-order Stokes beam passes through port d of the third optical circulator and exits from port b. The second-order Stokes beam then excites the stimulated Brillouin scattering effect in the 20-meter single-mode fiber. The stimulated Brillouin scattering effect generates a reversed third-order Stokes beam. After passing through port b of the third optical circulator and exiting from port c, the third optical splitter is injected into port a of the first optical circulator and exits from port b. It is coupled with the modulated light from the phase modulator 4 of the high-frequency tunable narrowband microwave photonic filter based on the switchable Brillouin frequency shift fiber laser, which is input from port d of the third-order Brillouin structure. After being coupled through the sixth optical splitter, it exits through port c of the third optical circulator of the third-order Brillouin structure to form a ring resonant cavity. When the power of the mixed light exceeds the threshold of the sixth optical splitter, the mixed light is output from port c of the third-order Brillouin structure and outputs a single-passband narrow-linewidth laser with a frequency shift of 31.875 GHz through port b of the sixth optical splitter.

[0075] This invention utilizes first-order, second-order, and third-order Brillouin structures to achieve switchable single / double / third-octave frequency shift intervals. It employs a Brillouin frequency shift interval selection structure to switch the operating frequency of the narrow-linewidth laser output of the Brillouin laser, with selectable filtering frequencies of 10.75, 21.25, and 31.875 GHz. Compared to existing microwave photonic filters that cannot achieve switchable single / double / third-octave frequency shift intervals, this invention achieves single-passband high-frequency narrow-bandwidth laser filtering.

[0076] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although detailed descriptions have been made with reference to the examples of the present invention, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention and should be covered within the scope of protection of the claims of the present invention.

Claims

1. A switchable Brillouin frequency shift fiber laser high-frequency narrowband microwave photonic filter, characterized in that: It includes a narrow linewidth continuous wave fiber laser (1), a first beam splitter (2A), a phase modulator (4), a spectrometer (5), a photodetector (6) and a vector network tester (7), and a switchable Brillouin frequency-shifting fiber laser (3) for achieving frequency band switching. The output of the narrow linewidth continuous wave fiber laser (1) is connected to port a of the first optical splitter (2A), port b of the first optical splitter (2A) is connected to port a of the switchable Brillouin frequency shift fiber laser (3), port c of the first optical splitter (2A) is connected to port a of the phase modulator (4), port b of the switchable Brillouin frequency shift fiber laser (3) is connected to port a of the second optical splitter (2B), port d of the switchable Brillouin frequency shift fiber laser (3) is connected to port b of the phase modulator (4), port c of the phase modulator (4) is connected to the output of the vector network tester (7), port b of the second optical splitter (2B) is connected to the input port of the spectrometer (5), port c of the second optical splitter (2B) is connected to the input port of the photodetector (6), and the output of the photodetector (6) is connected to the input of the vector network tester (7).

2. The switchable Brillouin frequency shift fiber laser high-frequency narrowband microwave photonic filter according to claim 1, characterized in that: The narrow linewidth continuous wave fiber laser (1) is a continuous-running laser with a wavelength of 1550nm, a linewidth of 0.1Hz, and a maximum output power of 15dBm. The first optical splitter (2A) has a splitting ratio of 90:10, and port a is the input port, port b is the 90% output port, and port c is the 10% output port. The second beam splitter (2B) has a beam splitting ratio of 50:50; The phase modulator (4) has a modulation wavelength range of 1530 to 1625 nm, an electric beamwidth of 25 GHz, and an interpolation loss of 2.5 dB. The spectrometer (5) has a wavelength range of 600 to 1700 nm, a wavelength resolution of 0.02 to 2, a wavelength linearity of 0.01 to 0.02, a measurement power range of -90 to 20 dBm, a power accuracy of ±0.4, and a maximum number of sampling points of 50001. The photodetector (6) has a linewidth of 50 GHz, a linear response of 10 dBm for optical input power, and a data rate of 40 Gbps. The vector network tester (7) has a frequency range of 300kHz to 20GHz, a frequency resolution of 1Hz, an intermediate frequency linewidth of 10Hz to 1.5MHz, a power range of -85dBm to 10dBm when the frequency range is 1MHz to 6GHz, a level resolution of 0.05dB, a voltage range of 0 to ±35V, and a maximum current of ±500mA.

3. A switchable Brillouin frequency-shifting fiber laser, used in the switchable Brillouin frequency-shifting fiber laser high-frequency narrowband microwave photonic filter of claim 1, characterized in that: The switchable Brillouin frequency shift fiber laser (3) includes a third optical splitter (2C), a polarization controller (8), a first erbium-doped fiber amplifier (9A), a first optical circulator (10A), and a Brillouin frequency shift interval selection structure (11). The b port of the first beam splitter (2A) of the single-passband high-frequency narrow-linewidth microwave photonic filter of the switchable Brillouin frequency-shifting fiber laser is connected to the input of the polarization controller (8). The output of the polarization controller (8) is connected to the input of the first erbium-doped fiber amplifier (9A). The output of the first erbium-doped fiber amplifier (9A) is connected to the a port of the third beam splitter (2C). The d port of the third beam splitter (2C) is connected to the a port of the first optical circulator (10A). The c port of the third beam splitter (2C) is connected to the c port of the Brillouin frequency-shifting interval selection structure (11). The b port of the third beam splitter (2C) serves as the b port of the switchable Brillouin frequency-shifting fiber laser (3). The output port is connected to port a of the second optical splitter (2B) of the switchable Brillouin frequency-shifting fiber laser high-frequency narrowband microwave photonic filter. The port b of the first optical circulator (10A) is connected to port b of the Brillouin frequency shift interval selection structure (11). The port c of the first optical circulator (10A) is connected to the input port of the second erbium-doped fiber amplifier (9B). The output port of the second erbium-doped fiber amplifier (9B) is connected to port a of the Brillouin frequency shift interval selection structure (11). The port d of the Brillouin frequency shift interval selection structure (11) is connected to port b of the phase modulator (4) of the switchable Brillouin frequency-shifting fiber laser (3) as the port d of the switchable Brillouin frequency-shifting fiber laser high-frequency narrowband microwave photonic filter.

4. The switchable Brillouin frequency-shifting fiber laser according to claim 3, characterized in that: The third optical splitter (2C) has a splitting ratio of 99:1, with port b being the 1% output terminal and port d being the 99% output terminal. Ports a and c of the third optical splitter (2C) are the input terminals. The first erbium-doped fiber amplifier (9A) and the second erbium-doped fiber amplifier (9B) both have an input power range of -20 to 15 dBm and a maximum output power of 37 dBm.

5. A Brillouin frequency shift spacing selection structure for use in the switchable Brillouin frequency shift fiber laser of claim 3, characterized in that: The Brillouin frequency shift interval selection structure is connected to the optical path to achieve the purpose of Brillouin frequency interval selection. The Brillouin frequency shift interval selection structure can be selected from the following structures: first-order Brillouin structure, second-order Brillouin structure, and third-order Brillouin structure.

6. The Brillouin frequency shift interval selection structure according to claim 5, characterized in that: The first-order Brillouin structure includes a fourth optical splitter (2D) and a first single-mode fiber (12A). The fourth optical splitter (2D) has its a port connected to the output port of the second erbium-doped fiber amplifier (9B) as the a port of the Brillouin frequency shift interval selection structure (11). The fourth optical splitter (2D) has its b port connected to the input end of the first single-mode fiber (12A). The output end of the first single-mode fiber (12A) is connected to the b port of the first optical circulator (10A) as the b port of the Brillouin frequency shift interval selection structure (11). The fourth optical splitter (2D) has its c port connected to the c port of the third optical splitter (2C) as the c port of the Brillouin frequency shift interval selection structure (11). The fourth optical splitter (2D) has its d port connected to the b port of the phase modulator (4) of the switchable Brillouin frequency shift fiber laser high-frequency narrowband microwave photonic filter as the d port of the Brillouin frequency shift interval selection structure (11). The fourth beam splitter (2D) has a beam splitting ratio of 50:50; The first single-mode fiber (12A) is a single-mode fiber with a length of 20m.

7. The Brillouin frequency shift interval selection structure according to claim 5, characterized in that: The second-order Brillouin structure includes a fifth optical splitter (2E), a second optical circulator (10B), and a second single-mode fiber (12B). The a port of the second optical circulator (10B) is connected to the output port of the second erbium-doped fiber amplifier (9B) as the a port of the Brillouin frequency shift interval selection structure (11). The b port of the second optical circulator (10B) is connected to the input end of the second single-mode fiber (12B). The c port of the second optical circulator (10B) is connected to the c port of the third optical splitter (2C) as the c port of the Brillouin frequency shift interval selection structure (11). The output end of the second single-mode fiber (12B) is connected to the a port of the fifth optical splitter (2E). The b port of the fifth optical splitter (2E) is connected to the b port of the first optical circulator (10A) as the b port of the Brillouin frequency shift interval selection structure (11). The d port of the fifth optical splitter (2E) is connected to the b port of the phase modulator (4) of the switchable Brillouin frequency shift fiber laser high-frequency narrowband microwave photonic filter as the d port of the Brillouin frequency shift interval selection structure (11). The fifth beam splitter (2E) has a beam splitting ratio of 50:50; The second single-mode fiber (12B) is a single-mode fiber with a length of 20m.

8. The Brillouin frequency shift interval selection structure according to claim 5, characterized in that: The third-order Brillouin structure includes a sixth optical splitter (2F), a third optical circulator (10C), and two single-mode optical fibers; The a port of the third optical circulator (10C) is connected to the output port of the second erbium-doped fiber amplifier (9B) as the a port of the Brillouin frequency shift spacing selection structure (11). The d port of the third optical circulator (10C) is connected to the input end of the third single-mode fiber (12C). The output end of the third single-mode fiber (12C) is disconnected. The b port of the third optical circulator (10C) is connected to the input end of the fourth single-mode fiber (12D). The c port of the third optical circulator (10C) serves as the c port of the Brillouin frequency shift spacing selection structure (11). The output end of the fourth single-mode fiber (12D) is connected to the c port of the third optical splitter (2C), the a port of the sixth optical splitter (2F) is connected to the b port of the first optical circulator (10A) as the b port of the Brillouin frequency shift interval selection structure (11), and the d port of the sixth optical splitter (2F) is connected to the b port of the phase modulator (4) of the switchable Brillouin frequency shift fiber laser high-frequency narrowband microwave photonic filter as the d port of the Brillouin frequency shift interval selection structure (11). The sixth beam splitter (2F) has a beam splitting ratio of 50:50; The third single-mode fiber (12C) is a single-mode fiber with a length of 5km; The fourth single-mode fiber (12D) is a single-mode fiber with a length of 20m.

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