Tunable step frequency microwave carrier system based on double-pumping Brillouin cavity

Abstract

The invention discloses a tunable step frequency microwave carrier system based on a double-pump Brillouin cavity. The system comprises a Brillouin laser cavity, a frequency shift injection locking loop and an optical circulation frequency shift ring. According to the system, two laser beams with tunable wavelengths serve as pump light, a Brillouin laser cavity is formed to generate first-order Brillouin laser, the frequency of the pump laser is locked and the line width of the pump laser is compressed by using a frequency shift injection locking technology, and dual-wavelength Brillouin laser with the compressed line width is generated; the dual-wavelength Brillouin laser interval is changed by tuning the laser frequency through a driver or replacing the DFB wavelength; in the optical circulation frequency shift ring, programming control is carried out on the acoustic optical modulator through a signal generator, and finally a microwave source with programmable sweep frequency bandwidth is generated through a photoelectric detector. The device can generate high-quality microwave signals with adjustable frequency and controllable stepping, and is suitable for the fields of radar, communication, measurement and the like.

Description

Technical Field

[0001] This invention relates to the field of microwave carrier systems, and more specifically to a tunable step-frequency microwave carrier system based on a dual-pump Brillouin cavity. Background Technology

[0002] In modern radar systems, to achieve versatility and adaptability to complex electromagnetic environments, it is necessary to generate multi-band swept microwave signals with tunable microwave center frequencies and wide sweep bandwidths. Traditional electronic methods face significant challenges in generating such signals due to limitations in frequency agility and narrow bandwidth. Stepped frequency signal generation methods relying on photonics have emerged as an alternative, capable of extending the broadband tunable microwave center frequency and wide sweep bandwidth that are unattainable by conventional electronic devices. In addition to extending the signal center frequency range, this method also provides arbitrary programmability in sweep bandwidth and frequency hopping rate. Furthermore, photonic methods are unaffected by electromagnetic interference, thus offering advantages in radar vital sign detection and high-precision radar ranging.

[0003] In recent years, although the center frequency agility and tuning range of stepped frequency signals have been expanded to some extent—for example, by using a single-frequency laser to adjust the step frequency carrier frequency through filtering in an optical loop—this method still has limitations. The method of tuning the center frequency by changing the center wavelength of the optical loop filter can only limit the center frequency tuning range of the stepped frequency signal to below the sweep bandwidth, resulting in a working capacity limited to 100 GHz. This limitation is particularly problematic for terahertz carrier generation, a key requirement for next-generation 6G communication systems. To address this issue, a dual-laser structure using two incoherent light sources has been proposed. However, the inherent incoherence between the two laser beams in this structure affects the phase stability and spectral purity of the signal. Furthermore, recent research has utilized soliton microcombs to extend the capabilities of stepped frequency signals, achieving frequency sweeping in the 4 GHz terahertz band through selective comb line extraction. Despite its advantages, this approach faces two key challenges: first, the microwave center frequency resolution is limited by the free spectrum range of the microcavity, typically in the hundreds of GHz range; second, operation requires precise optical filters and high-gain amplifiers, increasing system complexity, and generating soliton microcombs involves complex laser frequency sweeping, stabilization, and precise red detuning control processes, further complicating practical deployment.

[0004] Brillouin photonic platforms have opened new possibilities for high-performance carrier sources. Researchers have proposed a novel passive locking method based on frequency-shifted optical injection locking, aiming to combine stimulated Brillouin scattering, frequency shifting, and optical injection locking techniques. In this method, the generated Brillouin laser frequency is shifted to near the pump frequency, and the pump laser performance is optimized through injection locking technology. Furthermore, the Brillouin light is locked to specific longitudinal modes, effectively suppressing mode hopping and generating a signal center frequency with fine frequency tuning at the megahertz level and scalability in the terahertz range. While this method can tune frequencies, it still has limitations in terms of sweep speed and bandwidth. Summary of the Invention

[0005] Purpose of the invention: The purpose of this invention is to provide a tunable step-frequency microwave carrier system based on a dual-pump Brillouin cavity to solve the problem that the frequency sweep speed and bandwidth of microwave carrier cannot be flexibly adjusted, thereby generating a step-frequency microwave signal with programmable center frequency, sweep bandwidth, and frequency hopping speed.

[0006] Technical solution: According to one aspect of the present invention, a tunable stepped frequency microwave carrier system based on a dual-pump Brillouin cavity is provided, comprising a first optical circulator, a Brillouin gain fiber, a first fiber coupler, and a first polarization controller connected in a ring in sequence to form a Brillouin laser cavity. The Brillouin laser cavity receives two wavelength-tunable laser beams as pump light, one beam being emitted by a first DFB laser and passing through a second optical circulator and a second fiber coupler, and the other beam being emitted by a second DFB laser and passing through a third optical circulator and the second fiber coupler. The two laser beams jointly enter a first fiber amplifier and the first optical circulator sequentially from the second fiber coupler. The Brillouin laser cavity emits a first-order Brillouin laser to the first fiber coupler, and the first fiber coupler is connected to the third fiber coupler; wherein the third fiber coupler is connected to the fourth fiber coupler, the first frequency shifter, and the second optical circulator to form a first frequency shift injection lock loop, and the third fiber coupler, the second frequency shifter, and the third optical circulator are connected to form a second frequency shift injection lock loop. The fourth fiber coupler is connected in sequence to the fifth fiber coupler, the first acousto-optic modulator, the sixth fiber coupler, the seventh fiber coupler, and the photodetector; wherein the sixth fiber coupler, the second fiber amplifier, the optical filter, the second acousto-optic modulator, and the second polarization controller are connected in a ring to form an optical cyclic frequency shifting loop; the second acousto-optic modulator is connected to the signal generator, and the signal generator is connected to the first acousto-optic modulator.

[0007] Furthermore, an eighth fiber coupler can be added between the second optical circulator and the second fiber coupler. The eighth fiber coupler is sequentially connected to the second frequency shifter, the third optical circulator, and the second fiber coupler. The third optical circulator is connected to the second DFB laser.

[0008] Furthermore, the interval of the dual-wavelength Brillouin laser can be changed by tuning the frequency of the first wavelength tunable laser emitted by the first DFB laser through a driver connected to the first DFB laser or by changing the DFB wavelength.

[0009] Furthermore, after tuning the frequency of the first wavelength tunable laser emitted by the first DFB laser, the repetition frequency and pulse duration of the second acousto-optic modulator and the first acousto-optic modulator inside and outside the optical cyclic frequency shift loop are programmed by a signal generator, and finally the microwave source with a programmable sweep bandwidth is generated by the photodetector.

[0010] Furthermore, the first-order Brillouin laser propagates in the opposite direction to the received laser. The first polarization controller in the Brillouin laser cavity is used to adjust the polarization of the laser to maximize the Brillouin gain. The first fiber coupler splits the first-order Brillouin laser into a first laser path and a second laser path. The first laser path circulates within the Brillouin laser cavity. The second laser path enters the third fiber coupler and splits into a third laser path and a fourth laser path. The third laser path is modulated by the second frequency shifter and then enters the second DFB laser through the third optical circulator to lock the frequency of the second wavelength tunable laser and compress its linewidth. The fourth laser path enters the fourth fiber coupler and splits into a fifth laser path and a sixth laser path.

[0011] Furthermore, after being modulated by the first frequency shifter, the fifth laser enters the first DFB laser through the second optical circulator to lock the frequency of the first wavelength tunable laser and compress its linewidth. The locked first wavelength tunable laser and the second wavelength tunable laser serve as new light sources to generate a dual-wavelength Brillouin laser with further compressed linewidth in the Brillouin laser cavity. After multiple cycles, the linewidth of the dual-wavelength Brillouin laser reaches its limit, and the dual-wavelength Brillouin laser is output as the sixth laser, entering the fifth fiber coupler to split into the seventh and eighth lasers.

[0012] Furthermore, the seventh laser input is fed into the first acousto-optic modulator to generate an optical pulse, which enters the sixth fiber coupler to split into the ninth and tenth lasers. The ninth laser circulates within the optical frequency shift loop. The eighth and tenth lasers enter the photodetector together from the seventh fiber coupler to output a stepped-frequency optical signal that, together with another wavelength Brillouin laser, generates a stepped-frequency microwave signal.

[0013] Furthermore, the second, first, third, fourth, fifth, seventh, and eighth optical fiber couplers each have one input terminal and two output terminals, while the sixth optical fiber coupler has two input terminals and two output terminals.

[0014] Furthermore, the second optical circulator includes three ports, which are respectively coupled to the first frequency shifter, the first DFB laser, and the second fiber coupler; the third optical circulator includes three ports, which are respectively coupled to the second frequency shifter, the second DFB laser, and the second fiber coupler; the first optical circulator includes three ports, which are respectively coupled to the first fiber amplifier, the Brillouin gain fiber, and the first polarization controller.

[0015] Furthermore, the first frequency shifter and the second frequency shifter are electro-optic modulators.

[0016] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: 1. By programming the repetition frequency and pulse duration of the acousto-optic modulators inside and outside the optical cyclic frequency shift loop through an arbitrary wave signal generator, the programmable control of the sweep bandwidth is realized, thereby improving the programmability of the sweep bandwidth; 2. By changing the frequency interval of the dual-wavelength tunable laser, the center frequency of the step frequency signal can be tunable, thereby improving the tunability of the center frequency of the step frequency signal.

[0017] Compared with existing technologies, this invention overcomes the major challenges faced by traditional electronic methods in generating swept frequency microwave signals, solves the limitations of the center frequency tuning range in existing photon-assisted stepped frequency signal generation methods, avoids the influence of the inherent incoherence between sources in dual-laser structures on signal phase stability and spectral purity, breaks through the resolution limitations of microcavity-based methods, and overcomes the inflexibility of microwave carrier sources based on Brillouin platforms in terms of sweep speed and bandwidth. It has higher engineering value, can adapt to various environments, and thus generates stepped frequency signals with flexible programmable sweep bandwidth, tunable center frequency, and tunable frequency hopping rate. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the tunable step-frequency microwave carrier system structure provided in Embodiment 1 of the present invention;

[0019] Figure 2 This is a schematic diagram of another tunable step-frequency microwave carrier system structure provided in Embodiment 2 of the present invention;

[0020] Figure 3The image shows the spectrum and phase spectrum of the dual-wavelength Brillouin laser with a spacing of 260-300 GHz in Embodiment 3 of the present invention, as well as a schematic diagram of the phase noise of the microwave signal.

[0021] Figure 4 This is a time-frequency diagram of the 6.4GHz swept microwave signal in Embodiment 3 of the present invention;

[0022] Figure 5 This is a schematic diagram illustrating the time-frequency effect of increasing the frequency modulation speed of a step-frequency microwave signal by 10-20 times in Embodiment 3 of the present invention.

[0023] In the diagram: 1-First DFB laser, 2-DFB laser, 3-Second optical circulator, 4-Third optical circulator, 5-First optical circulator, 6-Second fiber coupler, 7-First fiber coupler, 8-Third fiber coupler, 9-Fourth fiber coupler, 10-Fiber coupler, 11-Sixth fiber coupler, 12-Seventh fiber coupler, 13-First polarization controller, 14-Second polarization controller, 15-First fiber amplifier, 16-Second fiber amplifier, 17-First frequency shifter, 18-Second frequency shifter, 19-First acousto-optic modulator, 20-Second acousto-optic modulator, 21-Driver, 22-Brillouin gain fiber, 23-Signal generator, 24-Optical filter, 25-Photodetector, 26-Eighth fiber coupler. Detailed Implementation

[0024] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings.

[0025] Example 1 To achieve tunability of the center frequency of the stepped frequency signal and programmability of the sweep bandwidth, such as Figure 1 As shown, this invention creatively proposes a tunable stepped-frequency microwave carrier system based on a dual-pump Brillouin cavity, comprising a first optical circulator 5, a Brillouin gain fiber 22, a first fiber coupler 7, and a first polarization controller 13 connected in a ring to form a Brillouin laser cavity. The Brillouin laser cavity receives two wavelength-tunable laser beams as pump light. One beam is emitted by a first DFB laser 1, passes through a second optical circulator 3 and a second fiber coupler 6, and the other beam is emitted by a second DFB laser 2, passes through a third optical circulator 4 and a second fiber coupler 6. The two laser beams are combined at the second fiber coupler 6 and then pass through a first fiber amplifier 15 and the first optical circulator 5 before entering the Brillouin laser cavity.

[0026] Optionally, in this embodiment, the first fiber amplifier 15 is selected as a C-band fiber amplifier to amplify the pump light signal.

[0027] In the Brillouin laser cavity, the first polarization controller 13 is used to adjust the polarization of the pump light to maximize the Brillouin gain of the Brillouin gain fiber 22, which is the main medium for Brillouin scattering. When two wavelength-tunable laser beams enter the Brillouin laser cavity, stimulated Brillouin effect occurs, generating a first-order Brillouin laser beam opposite to the laser propagation direction. The first fiber coupler 7 splits the generated first-order Brillouin laser beam into two paths. The first laser beam circulates within the Brillouin laser cavity, while the second laser beam is output to the third fiber coupler 8. The third fiber coupler 8 has one input and two outputs, splitting the input laser beam into a third and a fourth laser beam.

[0028] The first fiber coupler 7 is connected to the third fiber coupler 8. The third fiber coupler 8 is connected to the fourth fiber coupler 9, the first frequency shifter 17, and the second optical circulator 3 to form a first frequency shift injection lock loop. The third fiber coupler 8, the second frequency shifter 18, and the third optical circulator 4 are connected to form a second frequency shift injection lock loop. The first frequency shift injection lock loop and the second frequency shift injection lock loop can be understood as a two-way frequency shift injection lock system.

[0029] The generated intracavity Brillouin laser frequency is naturally located at the cavity resonance. The frequency of the first wavelength tunable laser source emitted by the first DFB laser 1 is tuned by the driver 21, the interval of the dual-wavelength Brillouin laser is changed, and the repetition frequency and pulse duration of the second acousto-optic modulator 20 and the first acousto-optic modulator 19 inside and outside the cyclic frequency shifting loop are programmed by the arbitrary wave signal generator 23. Finally, the microwave source with a programmable sweep bandwidth is generated by the photodetector 25.

[0030] Optionally, the interval between the two-wavelength Brillouin lasers can be changed by altering the frequency of the first wavelength tunable laser source emitted by the first DFB laser 1.

[0031] The optical cyclic frequency shifting loop consists of a sixth fiber coupler 11, a second fiber amplifier 16, an optical filter 24, a second acousto-optic modulator 20, and a second polarization controller 14 connected in a ring in sequence. The second acousto-optic modulator 20 is connected in sequence to a signal generator 23, a first acousto-optic modulator 19, a fifth fiber coupler 10, a fourth fiber coupler 9, and a third fiber coupler 8. The sixth fiber coupler 11 is connected in sequence to the first acousto-optic modulator 19, the fifth fiber coupler 10, the seventh fiber coupler 12, and a photodetector 25.

[0032] The third laser beam, split from the third fiber coupler 8, is modulated by the second frequency shifter 18 and then enters the second DFB laser 2 via the third optical circulator 4 to lock the wavelength and linewidth of the second tunable laser source emitted by the second DFB laser 2. The fourth laser beam is split into the fifth and sixth laser beams by the fourth fiber coupler 9. The fifth laser beam is modulated by the first frequency shifter 17 and then enters the first DFB laser 1 via the second optical circulator 3 to lock the frequency of the first tunable laser source. The locked first tunable laser source and the second tunable laser source, as a new source, generate a linewidth-compressed dual-wavelength Brillouin laser in the Brillouin laser cavity. After multiple cycles, the linewidth of the dual-wavelength laser reaches its limit and is output as the sixth laser beam. The sixth laser beam is split into the seventh and eighth laser beams via the fifth fiber coupler 10. The seventh laser beam is input to the first acousto-optic modulator 19 to generate light pulses of a certain duration and repetition frequency. It then enters the sixth fiber coupler 11 to split into the ninth and tenth laser beams. The ninth laser beam circulates within the optical cyclic frequency shift loop. It passes through the second fiber amplifier 16 for power amplification to compensate for loop loss, and then enters the optical filter 24 to suppress spontaneous emission noise generated by the optical cycle. It then passes through the second acousto-optic modulator 20 and forms a loop with the second polarization controller 14. The second polarization controller 14 is used to adjust the polarization state during optical cycle to suppress power fluctuations. The arbitrary wave signal generator 23 can arbitrarily program pulse signals of different durations and repetition frequencies. The input to the first acousto-optic modulator 19 and the second acousto-optic modulator 20 can arbitrarily program the number of times the light pulses circulate within the cyclic frequency shift loop. The eighth and tenth laser beams are input together to the photodetector 25 via the seventh fiber coupler 12, and the output stepped-frequency optical signal generates a stepped-frequency microwave signal with another Brillouin laser. Specifically, the first acousto-optic modulator 19 acts as a high-speed optical switch to control the light pulses entering the loop, and the second acousto-optic modulator 20 controls the number of times the light pulses circulate in the loop.

[0033] Optionally, both the first frequency shifter 17 and the second frequency shifter 18 are electro-optic modulators. An radio frequency (RF) signal is applied to the first frequency shifter 17 and the second frequency shifter 18, modulating the corresponding optical sideband of each Brillouin laser. The RF frequency of this sideband corresponds to the Brillouin frequency shift value, thus performing a blue shift modulation on the Brillouin laser wavelength, which is then injected into the DFB laser for cyclic locking. Specifically, the RF frequencies of the first frequency shifter 17 and the second frequency shifter 18 correspond to the Brillouin frequency shift values ​​of the first DFB laser 1 and the second DFB laser 2, respectively.

[0034] In some embodiments described above in this application, an optical circulator is proposed to realize directional transmission and signal circulation of the optical path. This application further proposes a second optical circulator 3 including three ports, which are respectively coupled to the first frequency shifter 17, the first DFB laser 1, and the second fiber coupler 6; a third optical circulator 4 including three ports, which are respectively coupled to the second frequency shifter 18, the second DFB laser 2, and the second fiber coupler 6; and a first optical circulator 5 including three ports, which are respectively coupled to the first fiber amplifier 15, the Brillouin gain fiber 22, and the first polarization controller 13.

[0035] In some of the embodiments described above in this application, this application further proposes that the second fiber optic coupler 6, the third fiber optic coupler 8, the fourth fiber optic coupler 9, the fifth fiber optic coupler 10, the seventh fiber optic coupler 12, and the eighth fiber optic coupler 26 have one input end and two output ends; the first fiber optic coupler 7 has two input ends and one output end; and the sixth fiber optic coupler 11 has two input ends and two output ends.

[0036] In some embodiments described above in this application, a Brillouin laser cavity is proposed for generating Brillouin lasers. During this process, maintaining the same polarization state within the Brillouin laser cavity aims to improve the pump utilization efficiency of generating Brillouin lasers with the same polarization and to ensure that the two Brillouin laser beams within the cavity are output with the same polarization. Further, this application proposes that after receiving two wavelength-tunable laser beams, the Brillouin laser cavity generates a first-order Brillouin laser with the opposite direction of laser propagation. A first polarization controller 13 is used to adjust the polarization of the laser to maximize the Brillouin gain. A first fiber coupler 7 splits the generated first-order Brillouin laser into a first laser path and a second laser path, with the first laser path circulating within the Brillouin laser cavity.

[0037] Specifically, after receiving two wavelength-tunable laser beams, the Brillouin laser cavity generates a first-order Brillouin laser beam with the opposite propagation direction to the pump light based on the stimulated Brillouin scattering mechanism. The first polarization controller 13 is used to adjust the polarization of the pump light to maximize the Brillouin gain of the Brillouin gain fiber 22. The first fiber coupler 7 splits the generated first-order Brillouin laser beam into two paths. The first laser beam forms a closed loop path in the Brillouin laser cavity, and the cavity loss is compensated by continuous feedback to maintain stable oscillation. The second laser beam is split into two laser beams by the third fiber coupler 8.

[0038] By combining frequency-shift injection locking, optical injection locking, and stimulated Brillouin effect, the mode hopping of the optical frequency comb teeth is suppressed, effectively reducing the phase noise of the optically generated microwave source. Furthermore, by changing the first wavelength tunable laser source through the driver 21, the frequency of the corresponding Brillouin laser can be changed, thereby arbitrarily altering the interval between the two wavelength Brillouin lasers. Another Brillouin laser beam is then fed into the optical cyclic frequency shift loop. The arbitrary wave signal generator 23 performs pulse modulation on the first acousto-optic modulator 19 and the second acousto-optic modulator 20, adjusting the repetition frequency and duration, thereby generating a microwave signal with a programmable step frequency, sweep bandwidth, and frequency hopping speed.

[0039] Example 2 To generate a microwave signal with ultra-low phase noise and precisely tunable frequency, such as Figure 2 As shown, this application further proposes another tunable step-frequency microwave carrier system based on a dual-pump Brillouin cavity, which includes the addition of an eighth fiber coupler 26 between the second optical circulator 3 and the second fiber coupler 6. The eighth fiber coupler 26 is sequentially connected to the second frequency shifter 18, the third optical circulator 4, and the second fiber coupler 6. The third optical circulator 4 is connected to the second DFB laser 2. Specifically, the technical solution provided in this embodiment reconstructs the connection path of the second frequency shifter 18 by adding an eighth fiber coupler 26 between the second optical circulator 3 and the second fiber coupler 6.

[0040] The working principle is as follows: The first wavelength tunable laser source emitted by the first DFB laser 1 enters the second optical circulator 3, the eighth fiber coupler 26, and the second fiber coupler 6 in sequence. The second wavelength tunable laser source generated by the second DFB laser 2 enters the third optical circulator 4 and the second fiber coupler 6. After the two laser beams merge in the second fiber coupler 6, they pass together through the first fiber amplifier 15 and the first optical circulator 5 into the Brillouin laser cavity.

[0041] Optionally, in this embodiment, the first fiber amplifier 15 is selected as a C-band fiber amplifier to amplify the pump light signal.

[0042] After two wavelength-tunable laser beams enter the Brillouin laser cavity, the stimulated Brillouin effect occurs, generating a first-order Brillouin laser beam opposite to the laser propagation direction. The first fiber coupler 7 splits the generated first-order Brillouin laser beam into two paths. The first laser beam circulates within the Brillouin laser cavity, while the second laser beam is output to the third fiber coupler 8. The second laser beam then enters the fourth fiber coupler 9 along the third fiber coupler 8. The fourth fiber coupler 9 splits the third laser beam into the fourth laser beam. The third laser beam is modulated by the first frequency shifter 17 and then enters the first DFB laser 1 through the second optical circulator 3, forming a frequency shift injection-locked loop that locks the first DFB laser 1 and its output first wavelength-tunable laser source. That is, the first DFB laser 1 remains synchronized with the frequency drift within the Brillouin laser cavity.

[0043] This embodiment changes the connection path of the second frequency shifter 18 by adding an eighth fiber optic coupler 26. The purpose is to achieve minute, continuous, and precise control over the final output microwave center frequency. The principle is as follows: A locked-in tunable laser source generates a series of precisely controlled modulated sideband lasers via a first frequency shifter 17. These modulated sideband lasers are then injected into a second DFB laser 2 to achieve synchronization. The frequency interval between the sideband lasers and the first DFB laser 1 is an integer multiple of the radio frequency applied to the first frequency shifter 17, thus synchronizing the second DFB laser 2 with the first DFB laser 1. The oscillation frequency of the second DFB laser 2 strictly follows this sideband frequency. By fine-tuning the radio frequency source signal frequency driving the second frequency shifter 18, precise and continuous tuning of the center frequency of the output microwave signal can be achieved. The radio frequency shift frequency can be understood as the tuning precision being an integer multiple of the cavity longitudinal mode.

[0044] Through the above technical solutions, the technical solutions provided in the embodiments of this application achieve fine and continuous tuning of the center frequency of the output microwave.

[0045] Example 3 In the technical solution provided in this embodiment, the working principle of the entire system is as follows: Two pump beams from different DFB lasers enter the Brillouin laser cavity, generating first-order Brillouin laser light in the Brillouin gain fiber. These Brillouin lasers are fed back to the DFB lasers through a frequency-shifting injection-locked loop to lock the frequency of the pump beams and compress their linewidth. The locked dual-wavelength pump beams generate dual-wavelength Brillouin laser light with compressed linewidth in the Brillouin laser cavity. After multiple cycles, the output linewidth reaches its limit. After being processed by an optical cyclic frequency-shifting loop, the laser light is converted into a microwave signal with tunable step frequency characteristics by a photodetector. By adjusting the frequency of the first DFB laser and controlling the operating parameters of the acousto-optic modulator using a programmable signal generator, the center frequency characteristics of the generated signal can be flexibly adjusted, thereby realizing a microwave source with programmable sweep bandwidth.

[0046] The first wavelength tunable laser source 1 and the second wavelength tunable laser source 2 are both narrow-linewidth single-frequency continuous-wave laser outputs, with center wavelengths ranging from 1549 nm to 1553 nm, spectral linewidths less than 400 kHz, maximum operating power greater than 10 mW, and side-mode suppression ratios greater than 30 dB. The dual-wavelength lasers are amplified by fiber amplifier 15, with a maximum amplification of 26 dB, and set to continuous power output mode. They are then injected into the Brillouin laser cavity through optical circulator 5. Polarization controller 13 adjusts the polarization of the pump light to maximize the Brillouin gain of the 20 m long Brillouin gain fiber 22.

[0047] like Figure 3 As shown, the spectrum and phase noise diagram of the dual-wavelength Brillouin laser with an interval of 260-300 GHz output by the technical solution of this invention are shown. The spectral signal-to-noise ratio is as high as 68 dB, and the phase noise of the microwave signal is as low as -126 dBc / Hz in the range of 10 kHz to 100 kHz. Figure 4 As shown, this is a time-frequency effect diagram of the programmable bandwidth of the stepped-frequency microwave sweep signal output by the technical solution of this invention. By adjusting the repetition frequency and duration of the pulse signal applied to the acousto-optic modulator 19 and acousto-optic modulator 20 through an arbitrary wave generator, the number of times the optical pulse circulates in the cyclic frequency shift loop is changed, thereby achieving a sweep bandwidth of 6.4 GHz for the stepped-frequency microwave signal within 14.875 μs.

[0048] like Figure 5 The diagram shows the programmable time-frequency effect of the step-frequency microwave modulation speed output by the technical solution of this invention. By adding 1km of optical fiber within the cyclic frequency shift loop and adjusting the repetition frequency and duration of the pulse signal applied to the acousto-optic modulator 19 and acousto-optic modulator 20 by changing the arbitrary wave generator, the number of times the optical pulse circulates within the cyclic frequency shift loop is changed, thereby increasing the modulation speed of the step-frequency microwave signal by 10-20 times.

Claims

1. A tunable stepped-frequency microwave carrier system based on a dual-pumped Brillouin cavity, comprising a first optical circulator (5), a Brillouin gain fiber (22), a first fiber coupler (7), and a first polarization controller (13) connected in a ring to form a Brillouin laser cavity, characterized in that, The Brillouin laser cavity receives two wavelength-tunable laser beams as pump light. One beam is emitted by the first DFB laser (1), passes through the second optical circulator (3) and the second fiber coupler (6), and the other beam is emitted by the second DFB laser (2), passes through the third optical circulator (4) and the second fiber coupler (6). The two laser beams enter the first fiber amplifier (15) and the first optical circulator (5) sequentially from the second fiber coupler (6). The Brillouin laser cavity emits a first-order Brillouin laser to the first fiber coupler (7), and the first fiber coupler (7) is connected to the third fiber coupler (8); wherein the third fiber coupler (8) is connected to the fourth fiber coupler (9), the first frequency shifter (17), and the second optical circulator (3) to form a first frequency shift injection lock loop, and the third fiber coupler (8), the second frequency shifter (18), and the third optical circulator (4) are connected to form a second frequency shift injection lock loop; The fourth fiber coupler (9) is connected in sequence to the fifth fiber coupler (10), the first acousto-optic modulator (19), the sixth fiber coupler (11), the seventh fiber coupler (12), and the photodetector (25); wherein the sixth fiber coupler (11), the second fiber amplifier (16), the optical filter (24), the second acousto-optic modulator (20), and the second polarization controller (14) are connected in sequence in a ring to form an optical cyclic frequency shifting ring; the second acousto-optic modulator (20) is connected to the signal generator (23), and the signal generator (23) is connected to the first acousto-optic modulator (19).

2. The tunable step-frequency microwave carrier system according to claim 1, characterized in that, An eighth fiber coupler (26) may be added between the second optical circulator (3) and the second fiber coupler (6). The eighth fiber coupler (26) is connected in sequence to the second frequency shifter (18), the third optical circulator (4), and the second fiber coupler (6). The third optical circulator (4) is connected to the second DFB laser (2).

3. The tunable step-frequency microwave carrier system according to claim 1, characterized in that, The spacing of the dual-wavelength Brillouin lasers can be changed by using a driver (21) connected to the first DFB laser (1) or by changing the DFB wavelength to tune the frequency of the first wavelength tunable laser emitted by the first DFB laser (1).

4. The tunable step-frequency microwave carrier system according to claim 3, characterized in that, After tuning the frequency of the first wavelength tunable laser emitted by the first DFB laser (1), the second acousto-optic modulator (20) and the first acousto-optic modulator (19) inside and outside the optical cyclic frequency shifting loop are programmed with repetition frequency and pulse duration by the signal generator (23), and finally the microwave source with programmable sweep bandwidth is generated by the photodetector (25).

5. The tunable step-frequency microwave carrier system according to claim 1, characterized in that, The first-order Brillouin laser propagates in the opposite direction to the received laser. The first polarization controller (13) in the Brillouin laser cavity is used to adjust the polarization of the laser to maximize the Brillouin gain. The first fiber coupler (7) splits the first-order Brillouin laser into a first laser and a second laser. The first laser circulates in the Brillouin laser cavity. The second laser enters the third fiber coupler (8) and splits into a third laser and a fourth laser. The third laser is modulated by the second frequency shifter (18) and then enters the second DFB laser (2) through the third optical circulator (4) to lock the frequency of the second wavelength tunable laser and compress its linewidth. The fourth laser enters the fourth fiber coupler (9) and splits into a fifth laser and a sixth laser.

6. The tunable step-frequency microwave carrier system according to claim 5, characterized in that, The fifth laser is modulated by the first frequency shifter (17) and then enters the first DFB laser (1) through the second optical circulator (3) to lock the frequency of the first wavelength tunable laser and compress its linewidth. The locked first wavelength tunable laser and the second wavelength tunable laser are used as new light sources to generate a dual-wavelength Brillouin laser with a further compressed linewidth in the Brillouin laser cavity. After multiple cycles, the linewidth of the dual-wavelength Brillouin laser reaches its limit. The dual-wavelength Brillouin laser is output as the sixth laser and enters the fifth fiber coupler (10) to split into the seventh and eighth lasers.

7. The tunable step-frequency microwave carrier system according to claim 6, characterized in that, The seventh laser input is fed into the first acousto-optic modulator (19) to generate an optical pulse, which enters the sixth fiber coupler (11) to split into the ninth and tenth lasers. The ninth laser circulates in the optical frequency shift loop. The eighth and tenth lasers enter the photodetector (25) together from the seventh fiber coupler (12) to output a stepped-frequency optical signal and generate a stepped-frequency microwave signal with another wavelength Brillouin laser.

8. The tunable step-frequency microwave carrier system according to claims 1-2, characterized in that, The second fiber coupler (6), the first fiber coupler (7), the third fiber coupler (8), the fourth fiber coupler (9), the fifth fiber coupler (10), the seventh fiber coupler (12), and the eighth fiber coupler (26) each have one input and two outputs, while the sixth fiber coupler (11) has two inputs and two outputs.

9. The tunable step-frequency microwave carrier system according to claim 1, characterized in that, The second optical circulator (3) includes three ports, which are coupled to the first frequency shifter (17), the first DFB laser (1) and the second fiber coupler (6) respectively; the third optical circulator (4) includes three ports, which are coupled to the second frequency shifter (18), the second DFB laser (2) and the second fiber coupler (6) respectively; the first optical circulator (5) includes three ports, which are coupled to the first fiber amplifier (15), the Brillouin gain fiber (22) and the first polarization controller (13) respectively.

10. The tunable step-frequency microwave carrier system according to claim 1, characterized in that, The first frequency shifter (17) and the second frequency shifter (18) are electro-optic modulators.

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