A brillouin fiber laser narrow-band tunable double-pass microwave photonic filter

Abstract

This invention relates to the fields of optical fiber communication and microwave photonics. Existing tunable dual-passband microwave photonic filters suffer from poor passband stability and cannot meet practical application requirements. This invention provides a narrow-band tunable dual-passband microwave photonic filter using a Brillouin fiber laser. It generates dual-tone pump light to excite the Brillouin using a tunable laser source and an intensity modulator. Combined with a Brillouin fiber oscillator and a cascaded Fabry-Perot cavity, the Brillouin gain spectrum is narrowed to the kHz level. Simultaneously, the different periodic resonant frequencies of the two ring cavities effectively suppress side modes, filtering out the desired frequency band signal. Furthermore, by synchronously tuning the center frequencies of the two passbands of the filter by changing the pump light wavelength, and simultaneously controlling the output frequency of the signal generator, the frequency interval between the two passbands is changed, ultimately realizing a narrow-linewidth tunable dual-passband microwave photonic filter.

Description

Technical Field

[0001] This invention relates to the fields of optical fiber communication and microwave photonics technology, specifically to a Brillouin fiber laser narrowband tunable dual-passband microwave photonic filter. Background Technology

[0002] The limited bandwidth of the microwave frequency band restricts the communication capacity and transmission rate of communication systems. Microwave photonic filters, however, combine the advantages of both microwave and optoelectronic technologies, offering numerous benefits such as high bandwidth, low loss, light weight, reconfigurable spectrum, wide tuning range, and strong resistance to electromagnetic interference. Furthermore, microwave photonic filters process microwave signals in the optical domain, thus overcoming the various limitations of traditional electrical filters and, to a certain extent, replacing them. Therefore, they have found widespread application in modern radar, electronic warfare, and wireless communication, gradually becoming a key technology for high-frequency broadband signal control and processing.

[0003] Due to the increasing communication transmission rates and the high precision requirements of information processing, high frequency and high Q value are important trends in the development of current microwave photonic filters. The quality factor (Q value) is an important indicator of a filter's ability to select the target frequency band; the higher the Q value, the better its frequency selectivity. The key indicator determining the Q value is the 3dB bandwidth of the filter's passband. ,because ,therefore The smaller the Q factor, the larger the Q value, which allows for the filtering out of fine frequency components of the signal under test. Simultaneously, to achieve rational allocation and full utilization of resources, microwave photonic filters have developed the need for continuously tunable center frequencies and dual-frequency operation. Therefore, the development of microwave filters with high Q factors, narrow linewidths, and tunable multi-passbands has become a significant challenge for the development of microwave communication technology.

[0004] Extensive research has been conducted by experts and scholars both domestically and internationally on achieving wide-range tuning of the center frequency and high Q-value in narrow bandwidth bandpass microwave photonic filters. To date, experimental schemes for achieving bandwidth narrowing or center frequency tuning in microwave photonic filters include a tunable microwave photonic filter based on dual light sources and dual phase-shift fiber gratings, whose 3dB passband bandwidth can be adjusted from 180MHz to 319MHz, and whose center frequency is adjustable from 1-7GHz; and a method employing an all-pass micro-ring resonator combined with the principle of phase modulation converted to intensity modulation, utilizing optical single-sideband modulation and optical carrier separation to achieve microwave photonic filtering, with measured filter bandwidth and out-of-band rejection ratio of 726MHz and 37dB, respectively, and a filter frequency tuning range of 1.64- 23.41GHz; Patent document CN109347560A discloses an invention entitled "Freely Tunable Dual-Passband Microwave Photonic Filter Device and Implementation Method". This filter generates frequency-free tunable dual pump light by using two cascaded dual parallel Mach-Zehnder electro-optic modulators. Through stimulated Brillouin scattering effect, it selectively breaks the amplitude balance of the phase modulation light sideband, and completes the conversion from phase modulation to intensity modulation. This realizes a dual-passband microwave photonic filter with freely tunable passband center. Its passband bandwidth is as narrow as 43MHz and the passband tuning range is 3-8GHz.

[0005] In summary, most of the tunable dual-passband microwave photonic filter schemes reported so far are concentrated in the MHz range, with poor passband stability, which cannot meet the practical application requirements. With the development of optical radio technology, the research on dual-passband microwave photonic filters with wide tuning range, narrow bandwidth, and high Q value has a wider significance. Summary of the Invention

[0006] To address the problems existing in the prior art, the present invention aims to provide a narrowband tunable dual-passband microwave photonic filter for Brillouin fiber lasers. This filter generates dual-tone pump light to excite Brillouin using a tunable laser source and an intensity modulator. Combined with a Brillouin fiber oscillator and a cascaded Fabry-Perot cavity, the Brillouin gain spectrum is narrowed to the kHz level.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] A narrow-linewidth tunable dual-passband microwave photonic filter based on a Brillouin fiber laser includes two ring cavities. The second ring cavity R2 is cascaded with the first ring cavity R1 to form a Fabry-Perot cavity. The polarization direction of the pump light and Stokes light in the ring cavity is adjusted by adjusting the polarization controller in the first ring cavity R1, thereby improving the SBS coupling efficiency. At the same time, the different periodic resonant frequencies of the two ring cavities are used to effectively suppress side modes and filter out the desired frequency band signal.

[0009] Furthermore, it includes two tunable lasers, three polarization controllers, a phase modulator, an optical isolator, three fiber couplers, two single-mode fibers, an intensity modulator, a signal generator, a voltage source, an erbium-doped fiber amplifier, an optical circulator, a photodetector, and an electric vector network analyzer; wherein: the first tunable laser emits a center frequency of ƒ c1 The light, acting as a carrier wave, passes through the first polarization controller and enters the first input port a of the phase modulator. Then, the electric vector network analyzer generates a frequency of ƒ. RF The radio frequency signal is modulated by double-sideband sweep frequency modulation of the carrier through the second input port b of the phase modulator. The modulated signal enters the first input port a of the first fiber coupler through the first output port c of the phase modulator and the optical isolator, and then enters the first single-mode fiber through the second polarization controller through the first output port b of the fiber coupler.

[0010] The second tunable laser emits a center frequency of ƒ. c2 The light is used as the pump light to excite stimulated Brillouin scattering. The pump light passes through the third polarization controller and enters the first input port a of the intensity modulator. The signal generator receives the input frequency through the second input port b of the intensity modulator. f m A single-tone radio frequency signal is generated by carrier suppression via a voltage source through the third input port (c) of an intensity modulator, followed by double-sideband modulation supported by pump light, producing a center frequency of [value missing]. f c2 - f m and f c2 + f m The dual-tone pump light is amplified by an erbium-doped fiber amplifier, then enters the first input port a of an optical circulator, and then enters the first single-mode fiber through the first output port b of the optical circulator to excite stimulated Brillouin scattering at a frequency of f c2 Two Brillouin gain spectra are generated on the left and right sides, and the upper sideband of the modulated signal is... f c1 + f RF After Brillouin gain spectrum amplification, the center frequencies of the two Brillouin gain spectra are: f c2 - f m - f B and f c2 + f m - f B ,in,f B For Brillouin frequency shift.

[0011] Furthermore, the second ring cavity R2 includes a modulation signal amplified by Brillouin gain, which is input from the second output port b of the optical circulator to the first input port a of the second fiber coupler. The first output port b of the second fiber coupler, the second single-mode fiber, and the third fiber coupler form the ring cavity R2. After resonance in the cascaded Fabry-Perot cavity, the modulation signal is split into a first laser beam and a second laser beam by the fiber coupler. The first laser beam is injected counterclockwise into the cascaded Fabry-Perot cavity through the first output port b of the second fiber coupler for multiple resonances. The second laser beam is input to the photodetector through the second output port c of the second fiber coupler. The signal after photoelectric conversion by the photodetector is input to an electric vector network analyzer to measure the amplitude-frequency response, which is used to characterize the filtering characteristics of the proposed narrowband tunable microwave photonic filter.

[0012] Furthermore, the first ring cavity R1 is formed by sequentially connecting an optical circulator, a first single-mode fiber, a second polarization controller, a first fiber coupler, a third fiber coupler, and a second fiber coupler to form a Brillouin laser resonator. The modulation signal output from the first output port c of the phase modulator is sequentially input into the first ring cavity R1 loop through the optical isolator and the first output port b of the first fiber coupler. After interacting with the stimulated Brillouin scattering signal, it is output through the second output port c of the second fiber coupler.

[0013] Furthermore, the splitting ratio of the first and third fiber couplers is 50%:50%, and the splitting ratio of the second fiber coupler is 90%:10%, wherein the second output port c of the second fiber coupler is the 10% port.

[0014] Furthermore, the modulation signal is emitted from the first tunable laser, passes sequentially through the third polarization controller, intensity modulator and erbium-doped fiber amplifier, and is then connected to the loop of the first ring cavity R1 by an optical circulator to form a narrow linewidth Brillouin fiber laser. The narrow linewidth Brillouin laser is output from the second output port c of the second fiber coupler.

[0015] Furthermore, the free spectral ranges of the two ring cavities are FSR1 and FSR2, respectively. The effective FSR of the double-ring cavity structure formed by cascading the two ring cavities satisfies the following condition:

[0016]

[0017] Where FSR1 corresponds to annular cavity R1, and FSR2 corresponds to annular cavity R2. n m (m=1, 2) are integers, and the free spectral range of the two ring cavities is expressed by the following formula:

[0018]

[0019] in L m The ring length is indicated by m = 1, 2, which represent the ring numbers; n = 1.468 is the effective refractive index of the optical fiber; the first single-mode fiber in ring R1 is 100 meters long, and the second single-mode fiber in ring R2 is 10 meters long.

[0020] Furthermore, by adjusting the wavelength of the pump light from the second tunable laser, the center frequencies of the two passbands of the narrow-linewidth tunable dual-passband microwave photonic filter can be synchronously tuned; and by changing the output RF signal of the signal generator... f m The frequency of the narrow-linewidth tunable dual-passband microwave photonic filter can be changed to alter the frequency spacing between the two passbands.

[0021] In summary, the invention has the following beneficial effects:

[0022] This invention discloses a narrow-linewidth tunable dual-passband microwave photonic filter based on a Brillouin fiber laser. A narrow-linewidth Brillouin fiber laser is formed from a second tunable laser to a first single-mode fiber, achieving a 3dB bandwidth on the order of kHz, thereby greatly narrowing the passband bandwidth of the filter.

[0023] The pump light frequency ƒ can be changed by altering the laser wavelength of the second tunable laser. c2 This achieves synchronous tuning of the center frequencies of the two stimulated Brillouin scattering gain spectra; by changing the output RF signal of the signal generator. f m The frequency of the filter is adjusted to change the frequency interval between the two passbands, while also having the advantage of finely tuned passband center frequencies, ultimately achieving stable tuning of the dual passbands of the microwave photonic filter.

[0024] By cascading ring cavities R2 and R1 to form a ring Fabry-Perot resonator, a vernier effect is achieved. Compared to the linewidth compression characteristic of a single ring resonator in stimulated Brillouin scattering, the dual-ring cavity structure can provide a higher linewidth compression ratio under the action of the vernier effect, and greatly suppresses side modes, which helps to realize microwave photonic filters with higher Q values.

[0025] Ring cavities based on optical circulators have the advantage that the Brillouin pump frequency does not need to be matched with the cavity mode. Compared to self-inductive fiber Bragg gratings, cascaded ring Fabry-Perot cavities have no additional cavity attenuation besides the inherent losses of the devices, and due to the low nonlinear coefficient of single-mode fiber, it is difficult to generate high-order Stokes gratings. Attached Figure Description

[0026] Figure 1This diagram illustrates the structure of a narrow-linewidth tunable dual-passband microwave photonic filter based on a Brillouin fiber laser, as provided in an embodiment of the present invention.

[0027] Figure 2 This diagram illustrates the principle of a narrow-linewidth tunable dual-passband microwave photonic filter based on a Brillouin fiber laser, as provided in an embodiment of the present invention.

[0028] Figure 3 This is a schematic diagram of the vernier effect principle used in a narrow-linewidth tunable dual-passband microwave photonic filter based on a Brillouin fiber laser, provided by an embodiment of the present invention.

[0029] Figure 4 This is a diagram showing the tunable center frequency characteristics of a narrow-linewidth tunable dual-passband microwave photonic filter based on a Brillouin fiber laser, provided by an embodiment of the present invention.

[0030] Figure 5 This is a frequency response diagram of a narrow-linewidth tunable dual-passband microwave photonic filter based on a Brillouin fiber laser, provided by an embodiment of the present invention, with passband center frequencies of 5.74 GHz and 15.74 GHz, respectively.

[0031] In the diagram: 1A - First tunable laser, 1B - Second tunable laser, 2A - First polarization controller, 2B - Second polarization controller, 2C - Third polarization controller, 3 - Phase modulator, 4 - Optical isolator, 5A - First fiber coupler, 5B - Second fiber coupler, 5C - Third fiber coupler, 6A - First single-mode fiber, 6B - Second single-mode fiber, 7 - Intensity modulator, 8 - Signal generator, 9 - Voltage source, 10 - Erbium-doped fiber amplifier, 11 - Optical circulator, 12 - Photodetector, 13 - Electric vector network analyzer;

[0032] Figure 2 (a) The output frequency of the first tunable laser 1A is f c1 The optical carrier wave is generated by radio frequency signals from the electric vector network analyzer 13. f RF The spectrum of the double-sideband modulated signal is obtained by phase modulator 3, and then the modulated signal is transmitted to the narrow linewidth Brillouin laser through the first fiber coupler 5A.

[0033] Figure 2 (b) is the output center frequency of the tunable laser 1B. f c2 The pump light is subjected to a half-wave voltage applied by voltage source 9, which suppresses the carrier wave of the pump light through intensity modulator 7. Simultaneously, a signal generator 8 outputs a frequency of... f m Single-tone radio frequency signal, thus inf c2 - f m and f c2 + f m Two-tone pump light is generated at the point, and then enters the first single-mode fiber 6A through the optical circulator 11 to excite Brillouin;

[0034] Figure 2 (c) The stimulated Brillouin process occurs in the first single-mode fiber 6A, and the center frequency of the Brillouin gain spectrum is... f c2 - f m - f B and f c2 + f m - f B , f B This is the Brillouin frequency shift;

[0035] Figure 2 (d) represents the upper sideband of the modulated signal. f c1 + f RF Amplified by the Brillouin gain spectrum, the bandwidth of the Brillouin gain spectrum is Δ. f B ;

[0036] Figure 2 (e) represents the periodic resonance of the second ring cavity R2, where FSR is the free spectral range of the second ring cavity R2, and Δ f BFL The linewidth of the Brillouin laser;

[0037] Figure 2 (f) By adjusting the second polarization controller 2B, the polarization direction of the pump light and Stokes light in the ring cavity is adjusted, thereby improving the SBS coupling efficiency; at the same time, by utilizing the different periodic resonance frequencies of the two ring cavities, the free spectral ranges of the two ring cavities are matched to form a vernier effect, thereby suppressing the side modes, and the Brillouin gain spectrum is narrowed by the dual ring cavities R1 and R2. Detailed Implementation

[0038] The present invention will now be described in further detail with reference to the accompanying drawings.

[0039] 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.

[0040] This invention discloses a narrow-linewidth tunable dual-passband microwave photonic filter based on a Brillouin fiber laser. It utilizes a dual-ring resonator to achieve the vernier effect while simultaneously achieving an ultra-narrow linewidth through the Brillouin fiber laser within the structure, solving the technical problem that existing microwave photonic filters cannot achieve both a wide tuning range and an ultra-narrow bandwidth. By simply changing the pump light wavelength and adjusting the signal generator output frequency, the proposed filter's dual-passband can be stably tuned. This microwave photonic filter is stably tuned in the 0-20 GHz frequency range, with an out-of-band rejection ratio exceeding 20 dB, a 3-dB bandwidth of 3.1 kHz, and a maximum Q value of 6.445 × 10⁻⁶. 6 .

[0041] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0042] like Figure 1 The present invention describes a narrow-linewidth tunable dual-passband microwave photonic filter based on a Brillouin fiber laser, comprising two tunable lasers, three polarization controllers, a phase modulator 3, an optical isolator 4, three fiber couplers, two single-mode fibers, an intensity modulator 7, a signal generator 8, a voltage source 9, an erbium-doped fiber amplifier 10, an optical circulator 11, a photodetector 12, and an electric vector network analyzer 13.

[0043] Based on the above-mentioned constituent elements, the structural relationship of the present invention is as follows:

[0044] The system consists of a first tunable laser 1A connected to a first polarization controller 2A; the first polarization controller 2A connected to the first input port a of a phase modulator 3; the second input port b of the phase modulator 3 connected to an electric vector network analyzer 11; the first output port c of the phase modulator 3 connected to an optical isolator 4; the optical isolator 4 connected to the first input port a of a first fiber coupler 5A; the first output port b of the first fiber coupler 5A connected to a second polarization controller 2B; the second polarization controller 2B connected to a first single-mode fiber 6A; the first tunable laser 1B connected to a third polarization controller 2C; the third polarization controller 2C connected to the first input port a of an intensity modulator 7; and a signal generator 8 connected to an intensity modulator 7. The second input port b of the modulator 7 is connected; the voltage source 9 is connected to the third input port c of the intensity modulator 7; the first output port d of the intensity modulator 7 is connected to the erbium-doped fiber amplifier 10; the erbium-doped fiber amplifier 10 is connected to the first input port a of the optical circulator 11; the first output port b of the optical circulator 11 is connected to the first single-mode fiber 6A; the second output port c of the optical circulator 11 is connected to the first input port a of the second fiber coupler 5B; the first output port b of the second fiber coupler 5B, the third fiber coupler 5C, and the second single-mode fiber 6B are connected in sequence; the second output port c of the second fiber coupler 5B is connected to the photodetector 12; and the photodetector 12 is connected to the electric vector network analyzer 13.

[0045] In one feasible embodiment of the present invention, the first annular cavity R1 is formed by sequentially connecting an optical circulator 11, a first single-mode fiber 6A, a second polarization controller 2B, a first fiber coupler 5A, a third fiber coupler 5C, and a second fiber coupler 5B to form an optical circuit and an optical resonant cavity; wherein the splitting ratio of the first fiber coupler 5A and the third fiber coupler 5C is 50%:50%, and the splitting ratio of the second fiber coupler 5B is 90%:10%, wherein the second output port c of the second fiber coupler 5B is the 10% port.

[0046] Based on the above solution, further specific implementation schemes of the present invention are as follows:

[0047] Combination Figure 1 As shown, the first tunable laser 1A emits a center frequency of ƒ c1 The light, acting as a carrier wave, passes through the first polarization controller 2A and enters the first input port a of the phase modulator 3. Then, the electric vector network analyzer 11 generates a frequency of ƒ. RFThe radio frequency signal is modulated by double-sideband sweep frequency modulation of the carrier through the second input port b of the phase modulator 3. The modulated signal enters the first input port a of the first fiber coupler 5A through the optical isolator 4 from the first output port c of the phase modulator 3, and then enters the first single-mode fiber 6A through the second polarization controller 2B from the first output port b of the first fiber coupler 5A. The second tunable laser 1B emits a center frequency of ƒ. c2 The light is used as the pump light to excite stimulated Brillouin scattering. The pump light passes through the third polarization controller 2C and enters the first input port a of the intensity modulator 7. The signal generator 8 receives the frequency through the second input port b of the intensity modulator 7. f m The single-tone radio frequency signal, through the adjustment of voltage source 9, undergoes carrier suppression at the third input port c of intensity modulator 7, ultimately achieving significant double-sideband modulation in a pump-light-supported double-sideband modulation mode, thereby... f c2 - f m and f c2 + f m Two-tone pump light is generated at the point of origin, and after being amplified by the erbium-doped fiber amplifier 10, it is input into the optical circulator 11. Then, it is input from the first output port b of the optical circulator 11 into the first single-mode fiber 6A to excite stimulated Brillouin scattering. f c2 Two Brillouin gain spectra are generated nearby, with the upper sideband of the modulated signal... f c1 + f RF The modulated signal is amplified by the Brillouin gain spectrum. The amplified Brillouin gain signal is input from the second output port b of the optical circulator 11 to the first input port a of the second fiber coupler 5B. The first output port b of the second fiber coupler 5B, the second single-mode fiber 6B, and the third fiber coupler 5C form a ring cavity R2. Ring cavity R2 and ring cavity R1 form a cascaded Fabry-Perot cavity. After resonance in the cascaded Fabry-Perot cavity, the modulated signal is split into two laser beams by the second fiber coupler 5B at a ratio of 90%:10%. The first 90% laser beam is injected counterclockwise into the Fabry-Perot cavity through the first output port b of the second fiber coupler 5B for multiple resonances. The second 10% laser beam is input to the photodetector 12 through the second output port c of the second fiber coupler 5B. The signal after photoelectric conversion by the photodetector 12 is used to measure the amplitude-frequency response using an electric vector network analyzer 13, thereby obtaining the frequency response characteristics of the narrow-linewidth tunable dual-passband microwave photonic filter proposed in this invention.

[0048] In one feasible embodiment of the present invention, the first tunable laser 1A is a tunable narrow linewidth laser that provides an optical carrier for a microwave signal; the second tunable laser 1B is a laser of the same type as the first tunable laser 1A and is used to emit pump light from the microwave photonic filter.

[0049] The ring cavity R1 is formed by sequentially connecting an optical circulator 11, a first single-mode fiber 6A, a second polarization controller 2B, a first fiber coupler 5A, a third fiber coupler 5C, and a second fiber coupler 5B to form an optical resonant cavity.

[0050] Ring cavity R2 is formed by sequentially connecting the first output port b of the third fiber coupler 5C, the second single-mode fiber 6B, and the second input port c of the third fiber coupler 5C. Ring cavity R2 and ring cavity R1 constitute a cascaded Fabry-Perot cavity.

[0051] The splitting ratios of the first fiber coupler 5A and the third fiber coupler 5C are both 50%:50%, and the splitting ratio of the second fiber coupler 5B is 90%:10%, wherein the second output port c of the second fiber coupler 5B is the 10% port.

[0052] In one feasible embodiment of the present invention, a narrow-linewidth Brillouin fiber laser is formed by passing a first tunable laser 1A sequentially through a first polarization controller 2A, a phase modulator 3, an optical isolator 4, a first fiber coupler 5A, a second polarization controller 2B, a first single-mode fiber 6A, an optical circulator 11, and a first ring cavity R1. The second tunable laser 1B emits pump light to excite stimulated Brillouin scattering (SBS). The RF signal output from signal generator 8 and the half-wave voltage output from voltage source 9 are used to suppress the carrier and modulate the pump light through intensity modulator 7, generating a two-tone pump light. After being amplified by erbium-doped fiber amplifier 10, the power is input into the first single-mode fiber 6A via optical circulator 11 to excite stimulated Brillouin scattering. The modulated light signal, amplified by the stimulated Brillouin scattering effect, then enters the ring cavity R2 through optical circulator 11. The polarization direction of the pump light and Stokes light within the ring cavity is adjusted by adjusting the polarization controller within the first ring cavity R1, thereby improving the SBS coupling efficiency. Simultaneously, the different periodic resonant frequencies of the two ring cavities effectively suppress side modes, narrowing the Brillouin gain spectrum from the dual-ring cavity to the kHz level. Finally, the second fiber coupler 5B splits the laser beam into two beams. One beam is output to photodetector 8, converted by photodetector 9, and input to electric vector network analyzer 13 for measurement. The other beam is injected counterclockwise into the Fabry-Perot cavity for multiple resonances.

[0053] refer to Figure 2 This is a schematic diagram illustrating the principle of the narrow-linewidth tunable dual-passband microwave photonic filter based on a Brillouin fiber laser provided in an embodiment of the present invention.

[0054] Figure 2 (a) The output frequency of the first tunable laser 1A is f c1 The optical carrier is subjected to double-band modulation of the spectrum of the signal by the phase modulator 3 through the electric vector network analyzer 13; Figure 2 (b) In the Brillouin pump light input intensity modulator 7 output from the second tunable laser 1B, the radio frequency signal output by the signal generator 8 is adjusted. f m The pump light is double-sideband modulated, and then a half-wave voltage is applied to the intensity modulator 7 through voltage source 9, thereby suppressing the carrier signal, and finally... f c2 - f m and f c2 + f m Dual-tone pump light is generated at the location; Figure 2 (c) The pump light is amplified by erbium-doped fiber amplifier 10 and stimulated Brillouin scattering in the first single-mode fiber 6A. The center frequency of the Brillouin gain spectrum is... f c2 - f m - f B and f c2 + f m - f B , f B This is the Brillouin frequency shift; Figure 2 (d) represents the upper sideband of the Brillouin gain spectrum amplified modulation signal. f c1 + f RF . Figure 2 (e) represents the periodic resonance of the first ring cavity R1, and each resonance has an extremely narrow bandwidth. Due to its narrow resonant linewidth, it can greatly compress the bandwidth of the stimulated Brillouin scattering gain spectrum and increase the out-of-band suppression of the microwave photonic filter. Figure 2 (f) To utilize the different periodic resonant frequencies of the two ring cavities to form a vernier effect, effectively suppressing side modes and filtering out the desired frequency band signal, the Brillouin gain spectrum is narrowed by the dual ring cavities. To obtain the maximum Brillouin gain, two PCs are used to maintain the parallel polarization between the pump wave and the Stokes wave emitted by the laser, thereby improving the SBS coupling efficiency.

[0055] Figure 3The diagram illustrates the vernier effect principle used in the narrow-linewidth tunable dual-passband microwave photonic filter based on a Brillouin fiber laser, as provided in this embodiment of the invention.

[0056] Δ in the figure ƒ B The stimulated Brillouin gain spectral bandwidth is the frequency shift associated with the stimulated Brillouin scattering pump light. ƒ B Defined as ƒ B =(2ν A / c) ν P , where ν A ν is the speed of sound in the medium, c is the speed of light in a vacuum, and ν is the speed of sound in the medium. P It is the optical frequency of the pump light. ƒ B The effective free spectral range (FSR) of the dual-ring cavity structure is approximately 10.737 GHz in the 1550 nm wavelength range. According to the vernier effect, the FSR of the dual-ring cavity structure is the least common multiple of the ring cavity R1 and ring cavity R2.

[0057] (2)

[0058] Where FSR1 corresponds to 100 meters of SMF in the first annular cavity R1, and FSR2 corresponds to 10 meters of SMF in the second annular cavity R2. n m (m=1, 2) are integers. The free spectral range (FSR) of the two ring cavities is expressed as:

[0059] (3)

[0060] In the formula L m (m=1, 2) are the ring lengths of the two ring cavities, where m represents the ring cavity number, and n=1.468 is the effective refractive index of the fiber. Therefore, the free spectral ranges of the two ring cavities are 1.86 MHz and 18.6 MHz, respectively. According to formula (2), the effective free spectral range of the microwave photonic filter is 18.6 MHz. When the effective free spectral range exceeds the Brillouin gain bandwidth and the gain is greater than the loss, the laser mode oscillates only at the frequency at which the resonance conditions of both ring cavities are simultaneously satisfied.

[0061] In the above specific embodiments, Figure 4 The center frequency tunable characteristic diagram of the narrow-linewidth tunable dual-passband microwave photonic filter based on a Brillouin fiber laser provided in this embodiment of the invention is shown. The figure also shows the output RF signal of the modulation signal generator 8. f mWhen the frequency varies in the range of 1-9 GHz, the dual passband frequency spacing of the microwave photonic filter is stably tuned to 2-18 GHz, the left passband is stably tuned to 10-2 GHz, and the right passband is stably tuned to 12-20 GHz, while the out-of-band suppression exceeds 20 dB.

[0062] In the above specific embodiments, the dual-passband microwave photonic filter provided by the present invention can also synchronously tune the center frequencies of the two passbands of the filter by changing the pump light wavelength. Based on the relationship between wavelength and frequency... f =c / λ( f Let λ be the frequency of the stimulated Brillouin scattering pump light, λ be the wavelength of the stimulated Brillouin scattering pump light, and c = 3 × 10⁻⁶. 8 When the pump light wavelength output by the second tunable laser 1B is changed by 0.1 pm, the minimum tuning accuracy achieved in this embodiment of the invention is a frequency shift of 12.5 MHz.

[0063] In the above specific embodiments, Figure 5 Frequency response diagrams of a narrow-linewidth tunable dual-passband microwave photonic filter based on a Brillouin fiber laser, provided for embodiments of the present invention, at center frequencies of 5.74 GHz and 15.74 GHz. The microwave photonic filter has a minimum 3dB bandwidth of only 3.1 kHz and a maximum Q factor ( The value is calculated to be 6.445 × 10. 6 The images clearly show that the passband sidemodes of the microwave photonic filter are significantly suppressed, reaching the kHz level, with a sidemode suppression ratio exceeding 20 dB. Therefore, the microwave photonic filter of this invention achieves a significant breakthrough in 3 dB bandwidth narrowing, while also possessing extremely high selectivity and a large tunable range, demonstrating great potential in realizing kHz-level bandwidth, high suppression ratio, and tunable dual-passband filtering.

[0064] Finally, it should be noted that the above embodiments 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 embodiments 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 narrow-linewidth tunable dual-passband microwave photonic filter based on a Brillouin fiber laser, characterized in that, It includes two ring cavities, wherein the second ring cavity R2 and the first ring cavity R1 are cascaded to form a Fabry-Perot cavity. The polarization direction of the pump light and Stokes light in the ring cavity is adjusted by adjusting the polarization controller in the first ring cavity R1, thereby improving the SBS coupling efficiency. At the same time, the different periodic resonant frequencies of the two ring cavities are used to effectively suppress side modes and filter out the required frequency band signal. It includes two tunable lasers, three polarization controllers, a phase modulator (3), an optical isolator (4), three fiber couplers, two single-mode fibers, an intensity modulator (7), a signal generator (8), a voltage source (9), an erbium-doped fiber amplifier (10), an optical circulator (11), a photodetector (12), and an electric vector network analyzer (13). The pump light frequency ƒ can be changed by changing the laser wavelength of the second tunable laser. c2 This achieves synchronous tuning of the center frequencies of the two stimulated Brillouin scattering gain spectra; by changing the output RF signal of the signal generator. f m The frequency of the filter is adjusted to change the frequency interval between the two passbands, while the center frequency of the passband is finely resonant, ultimately achieving stable tuning of the dual passbands of the microwave photonic filter. in: The first tunable laser (1A) emits a center frequency of ƒ c1 The light, acting as a carrier wave, passes through the first polarization controller (2A) and enters the first input port a of the phase modulator (3). Then, the electric vector network analyzer (13) generates a frequency of ƒ. RF The radio frequency signal is modulated by double-sideband sweep frequency through the second input port b of the phase modulator (3). The modulated signal enters the first input port a of the first fiber coupler (5A) through the optical isolator (4) from the first output port c of the phase modulator (3), and then enters the first single-mode fiber (6A) through the second polarization controller (2B) through the first output port b of the first fiber coupler (5A). The second tunable laser (1B) emits a center frequency of ƒ c2 The light is used as the pump light to excite stimulated Brillouin scattering. The pump light enters the first input port a of the intensity modulator (7) through the third polarization controller (2C). The signal generator (8) inputs a frequency of through the second input port b of the intensity modulator (7). f m A single-tone radio frequency signal is generated by a voltage source (9) through the third input port c of an intensity modulator (7) for carrier suppression. Double-sideband modulation is achieved using a pump-light-supported double-sideband modulation mode, resulting in a center frequency of [value missing]. f c2 - f m and f c2 + f m The dual-tone pump light is amplified by an erbium-doped fiber amplifier (10), then enters the first input port a of an optical circulator (11), and then enters the first single-mode fiber (6A) through the first output port b of the optical circulator (11) to excite stimulated Brillouin scattering. f c2 Two Brillouin gain spectra are generated on the left and right sides, and the upper sideband of the modulated signal is... f c1 + f RF After Brillouin gain spectrum amplification, the center frequencies of the two Brillouin gain spectra are: f c2 - f m - f B and f c2 + f m - f B ,in, f B For Brillouin frequency shift.

2. The narrow-linewidth tunable dual-passband microwave photonic filter based on a Brillouin fiber laser according to claim 1, characterized in that, The second ring cavity R2 includes a modulated signal amplified by Brillouin gain, which is input from the second output port b of the optical circulator (11) to the first input port a of the second fiber coupler (5B). The first output port b of the second fiber coupler (5B), the second single-mode fiber (6B), and the third fiber coupler (5C) form the second ring cavity R2. After resonance of the cascaded Fabry-Perot cavity, the modulated signal is divided into a first laser beam and a second laser beam by the second fiber coupler (5B). The first laser beam is injected counterclockwise into the cascaded Fabry-Perot cavity through the first output port b of the second fiber coupler (5B) for multiple resonances. The second laser beam is input to the photodetector (12) through the second output port c of the second fiber coupler (5B). The signal after photoelectric conversion by the photodetector (12) is input to the electric vector network analyzer (13) to measure the amplitude-frequency response to characterize the filtering characteristics of the proposed narrowband tunable microwave photonic filter.

3. The narrow-linewidth tunable dual-passband microwave photonic filter based on a Brillouin fiber laser according to claim 1, characterized in that, The first annular cavity R1 is formed by sequentially connecting an optical circulator (11), a first single-mode fiber (6A), a second polarization controller (2B), a first fiber coupler (5A), a third fiber coupler (5C), and a second fiber coupler (5B) to form a Brillouin laser resonator. The modulated signal output from the first output port c of the phase modulator (3) passes sequentially through the optical isolator (4) and the first output port b of the first fiber coupler (5A) into the first ring cavity R1 loop. After interacting with the stimulated Brillouin scattering signal, it is output through the second output port c of the second fiber coupler (5B).

4. The narrow-linewidth tunable dual-passband microwave photonic filter based on a Brillouin fiber laser according to claim 3, characterized in that, The splitting ratio of the first fiber coupler (5A) and the third fiber coupler (5C) is 50%:50%, and the splitting ratio of the second fiber coupler (5B) is 90%:10%, wherein the second output port c of the second fiber coupler (5B) is the 10% port.

5. The narrow-linewidth tunable dual-passband microwave photonic filter based on a Brillouin fiber laser as described in claim 3, characterized in that, The modulation signal is emitted from the first tunable laser (1A), passes through the third polarization controller (2C), intensity modulator (7) and erbium-doped fiber amplifier (10) in sequence, and is then connected to the loop of the first ring cavity R1 by the optical circulator (11) to form a narrow linewidth Brillouin fiber laser. The narrow linewidth Brillouin laser is output from the second output port c of the second fiber coupler (5B).

6. The narrow-linewidth tunable dual-passband microwave photonic filter based on a Brillouin fiber laser as described in claim 1, characterized in that, The free spectral ranges of the two annular cavities are FSR1 and FSR2, respectively. The effective FSR of the double-annular cavity structure formed by cascading the two annular cavities satisfies the following conditions: Where FSR1 corresponds to the first annular cavity R1, and FSR2 corresponds to the second annular cavity R2. n m Here, m is an integer, where m = 1 or 2, representing the ordinal number of the ring cavity. The free spectral range of the two ring cavities is expressed by the following formula: in L m The ring length is indicated by m = 1, 2, which represent the ring number; n = 1.468 is the effective refractive index of the optical fiber; the first single-mode fiber (6A) in the first ring cavity R1 is 100 meters long, and the second single-mode fiber (6B) in the second ring cavity R2 is 10 meters long.

7. The narrow-linewidth tunable dual-passband microwave photonic filter based on a Brillouin fiber laser as described in any one of claims 1 to 6, characterized in that, By adjusting the wavelength of the pump light from the second tunable laser (1B), the center frequencies of the two passbands of the narrow-linewidth tunable dual-passband microwave photonic filter are synchronously tuned; by changing the output RF signal of the signal generator (8)... f m The frequency of the narrow-linewidth tunable dual-passband microwave photonic filter can be changed to alter the frequency spacing between the two passbands.

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