Generation system and generation method of three-band coherent microwave signal

Abstract

The invention relates to a generation system and a generation method of a three-band coherent microwave signal. The system comprises a laser, an electro-optical modulator, an optical fiber ring, a photoelectric detector, a first bridge, a second bridge, a first broadband filter, a second broadband filter, a third broadband filter, a tunable filter, a first amplifier, a second amplifier, a third amplifier, a fourth amplifier, a coupler, an external signal source, a signal generator and the like. The generating system disclosed by the invention is a Fourier domain mode-locked photoelectric oscillator system for generating coherent microwave signals of three frequency bands based on difference frequency injection locking, and a Fourier domain mode-locked photoelectric oscillator autonomously-generated signal is combined with an external injection signal; therefore, multiple frequency band sweep frequency signals with good coherence and strong tunability can be output at the same time. The generated microwave signals of three frequency bands have good tunability and coherence, and the formed three-loop Fourier domain mode-locked optoelectronic oscillator system can be applied to multiple fields.

Description

Technical Field

[0001] The present invention relates to a microwave photon signal generating system, in particular to a generating system and method for three-band coherent microwave signals. Background Art

[0002] Microwave signals in a single frequency band are affected by range-Doppler coupling and signal congestion within a single band, making them inadequate for applications in fields such as radar communications. Generating multi-band microwave signals with good coherence has become a hot topic. The traditional method of generating multi-band microwave signals using multi-stage electronic oscillators is not only complex and costly, but also suffers from high phase noise. Consequently, this method has been eliminated.

[0003] Compared with the method of generating multi-band microwave signals by electronic oscillators, the multi-band microwave signals generated by microwave photonic technology have the advantages of diverse waveforms, high spectral purity, low phase noise, good anti-interference and stability. Several typical methods in microwave photonic technology are as follows:

[0004] 1. Generate multi-band microwave signals based on the optical heterodyne method. This method is to couple two optical signals of different frequencies (usually from two independent lasers) and enter a photodetector for beat frequency. If one of the optical signals contains several different frequency bands, then the final beat frequency signal of the two beams will also contain several different frequency bands. The system structure of the optical heterodyne method is relatively simple and easy to build. It is only necessary to quantitatively tune the output signals of the two output lasers to easily obtain signals of multiple frequency bands. However, since the two optical signals come from two completely independent lasers, the microwave signals generated by this method are not ideal in terms of stability and coherence.

[0005] 2. Use electro-optical modulation to generate multi-band microwave signals. This method uses the electro-optical effect to achieve mutual conversion of photoelectric signals. Its core is to modulate the intensity, phase, or polarization of the optical signal through an external signal, thereby adjusting the final output signal of the system. This method has the advantages of fast modulation speed and good output signal linearity in generating multi-band microwave signals, and is suitable for fields such as radar and high-precision measurement. However, this method is significantly affected by external modulation signals and usually requires a high-frequency swept source signal. It has high requirements in terms of experimental conditions and costs, making it less practical.

[0006] 3. Generate multi-band microwave signals using an optoelectronic oscillator. This method typically involves designing filters with different frequency passbands within the electrical loop of the optoelectronic oscillator system, thereby achieving multi-band microwave signal output. The advantage of this method is that the phase noise of the generated microwave signal does not deteriorate with increasing frequency, and it offers excellent interference immunity and stability. However, its disadvantages are the inability to generate broadband swept-frequency signals and limited tunability.

[0007] 4. Generate multi-band microwave signals based on a Fourier domain mode-locked optoelectronic oscillator. The Fourier domain mode-locked optoelectronic oscillator replaces the fixed-bandpass filter in the optoelectronic oscillator with a tunable filter. When the period of the tuned tunable filter is a certain multiple of the entire loop system, Fourier domain mode locking can be achieved, thereby achieving simultaneous oscillation of all longitudinal modes in the loop system. Compared to optoelectronic oscillator systems, the Fourier domain mode-locked optoelectronic oscillator has advantages in generating continuous broadband signals and tunable signals. However, existing Fourier domain mode-locked optoelectronic oscillators cannot simultaneously output microwave signals in multiple frequency bands with good coherence. Summary of the Invention

[0008] One of the objectives of the present invention is to provide a system for generating three-band coherent microwave signals to solve the problem of low signal quality of multi-band microwave signals generated by existing Fourier domain mode-locked optoelectronic oscillators.

[0009] A second object of the present invention is to provide a method for generating a three-band coherent microwave signal to improve the signal quality of a multi-band microwave signal.

[0010] One of the purposes of the present invention is achieved in that:

[0011] A three-band coherent microwave signal generation system, comprising:

[0012] The laser, whose output end is connected to the input end of the electro-optical modulator through an optical fiber, is used to provide an optical carrier signal for the system;

[0013] The electro-optic modulator, whose output is connected to the input of the photodetector through an optical fiber loop, is used to modulate the intensity and phase of the optical carrier signal input to the electro-optic modulator and provide delay compensation for the system loop through the optical fiber loop;

[0014] a photodetector, the output end of which is connected to the input end of the fourth amplifier, for converting the modulated and delayed optical signal into an electrical signal and outputting the electrical signal to the three branches;

[0015] a fourth amplifier, whose output terminal is connected to the input terminal of the first bridge, and is used to provide amplification gain for the electrical signals output to the three branches;

[0016] The first bridge has an output divided into three paths, the first output being connected to the input of the first broadband filter, the second output being connected to the input of the second broadband filter, and the third output being connected to the input of the third broadband filter; the first bridge cooperates with the second bridge to form three branches, and the first output of the first bridge is connected to the first branch, the second output is connected to the second branch, and the third output is connected to the third branch;

[0017] a first broadband filter, whose output terminal is connected to the input terminal of the tunable filter, for filtering out the band signal passing through the first branch and tuning the signal outside the bandwidth to ensure the instantaneous single-mode oscillation of the first branch;

[0018] a tunable filter, the output of which is connected to the input of the first amplifier, for adjusting the period of the driving signal to match the oscillation period of the system to achieve Fourier domain mode locking;

[0019] a signal generator, whose output terminal is connected to the control terminal of the tunable filter and is used to apply a voltage control signal to the tunable filter to control the microwave signal output by the system;

[0020] a first amplifier, whose output terminal is connected to the first input terminal of the second bridge, for providing gain compensation for the first branch so that the microwave signal of the closed loop where the first branch is located can oscillate normally;

[0021] a second broadband filter, whose output terminal is connected to the input terminal of the second amplifier, for filtering out the band signal passing through the second branch and tuning the signal outside the bandwidth to ensure instantaneous single-frequency oscillation of the second branch;

[0022] A second amplifier, whose output terminal is connected to the second input terminal of the second bridge, is used to provide gain compensation for the second branch so that the injection-locked up-converted microwave signal generated by the system can oscillate normally;

[0023] a third broadband filter, whose output terminal is connected to the input terminal of the third amplifier, for filtering out the band signal passing through the third branch and tuning the signal outside the bandwidth to ensure instantaneous single-frequency oscillation of the third branch;

[0024] a third amplifier, whose output terminal is connected to the third input terminal of the second bridge, for providing gain compensation for the third branch so that the injection-locked down-converted microwave signal generated by the system can oscillate normally;

[0025] The second bridge has an output divided into two paths, the first output path being connected to the input of the coupler, and the second output path being the output of the system; the second bridge is used to complete the setting of the three branches and transform the three branches into corresponding components of the three closed loops of the system;

[0026] A coupler, whose output is connected to the RF terminal of the electro-optical modulator, is used to complete the three closed loop settings of the system; and

[0027] The external signal source has an output end connected to the coupling end of the coupler and is used to input a single-frequency signal into the system to lock the microwave signal generated by the closed loop where the tunable filter is located.

[0028] The generation system of the present invention is a Fourier domain mode-locked optoelectronic oscillator system that generates coherent microwave signals in three frequency bands based on difference frequency injection locking. It combines the self-generated signal of the Fourier domain mode-locked optoelectronic oscillator with the external injection signal, thereby achieving the simultaneous output of multiple frequency band swept signals with good coherence and strong tunability.

[0029] In the generation system of the present invention, a Fourier domain mode-locked optoelectronic oscillator autonomously generates a microwave signal of one frequency band, and microwave signals of the other two frequency bands are generated by difference frequency injection up-conversion and down-conversion.

[0030] The present invention replaces the fixed-bandpass filter in the optoelectronic oscillator with a tunable filter. By tuning the period of the tunable filter to a certain multiple of the entire loop system, Fourier domain mode locking is achieved, enabling simultaneous oscillation of all longitudinal modes in the loop system. Compared to conventional optoelectronic oscillator systems, the present invention offers significant advantages in generating continuous broadband and tunable signals. Furthermore, based on the Fourier domain mode-locked optoelectronic oscillator, the present invention can generate multi-band microwave signals with good coherence, strong tunability, and across frequency bands.

[0031] The characteristics of the present invention are: 1. The bandwidth and center frequency changes of the microwave signals in the three frequency bands always remain the same, that is, they are output synchronously; 2. The microwave signals in the three frequency bands have good tunability and coherence; 3. The resulting three-loop Fourier domain mode-locked optoelectronic oscillator system has good structural symmetry and economy, and is suitable for applications in multiple fields.

[0032] The working principle of the generation system of the present invention is analyzed as follows:

[0033] The core component of the generation system of the present invention is a tunable filter, whose center frequency varies periodically. When the time required for the system output signal to circulate through the entire loop matches the modulation period of the tunable filter, or there is an integer multiple of this, the relationship is satisfied:

[0034] T 系统 =n×T 可调谐滤波器 (1)

[0035] Wherein, n is a positive integer.

[0036] The center frequency of the system's output signal is identical to that of the tunable filter, and changes synchronously with the center frequency of the tunable filter. Signal energy loss during this entire process is negligible. Within the tunable filter's swept frequency range, Fourier mode locking causes all longitudinal modes to oscillate sequentially. However, only signals aligned with the tunable filter's center frequency are allowed to pass through, while signals at other frequencies are suppressed or filtered out. Therefore, at any given moment, the system outputs only a single signal aligned with the tunable filter's center frequency.

[0037] When the Fourier domain mode-locked optoelectronic oscillator system stably outputs a microwave signal of a certain frequency band (assuming its frequency is ω osc ), a single-frequency signal is input from an external signal source (assuming its frequency is ω inj ). The microwave signal ω output by the system osc With external injection signal ω inj are input into the electro-optic modulator together, and the signal entering the electro-optic modulator can be expressed as:

[0038] V in (t) = V inj cos(ω inj t+β inj )+V osc cos(ω osc t+β osc ) (2)

[0039] Among them, V osc is the amplitude of the system output signal before the external signal is injected, ω osc is the angular frequency of the system output signal before the external signal is injected, β osc is the initial phase of the system output signal before the external signal is injected, V inj is the amplitude of the external injection signal, β inj is the initial phase of the external injection signal.

[0040] After the signal is modulated by the electro-optical modulator and subjected to gain compensation by the amplifier and filtering by the filter, the output signal is Bessel expanded and high-order harmonics are filtered out. The expression is:

[0041]

[0042] Among them, J n (*) is the nth order Bessel polynomial, is the modulation index of the injected signal, is the modulation index of the output signal of the system itself, η is the extinction coefficient, V ph is the photovoltage on the photodetector, V DC is the DC bias voltage of the electro-optic modulator, Vπ is the half-wave voltage of the electro-optic modulator.

[0043] According to formula (3), the output signal ω of the system itself osc (t) and external injection signal ω inj (t) is generated by up-conversion and down-conversion with a frequency of ω osc +ω inj and frequency ω osc -ω inj signal.

[0044] Furthermore, when the signal generator driving signal for the tunable filter is changed to a sawtooth wave and the period of the driving signal is adjusted to match the period of the entire system, the system can simultaneously output linear chirped swept-frequency microwave signals in three frequency bands.

[0045] Assume that the frequency of the microwave signal output by the system itself is:

[0046] ω osc (t)=2πkt

[0047] Where k is the derivative of frequency with respect to time, and t is time.

[0048] The frequency of the signal formed by up-conversion of the microwave signal and the external injection signal is:

[0049] ω 上变频 (t)=2πf inj t+2πkt 2

[0050] The frequency of the signal formed by down-conversion is:

[0051] ω 下变频 (t)=2πkt 2 -2πf inj t

[0052] Substituting the above results into formula (3), we can get the expression of the microwave signal output by the system after up-conversion and down-conversion:

[0053]

[0054] It can be seen from equations (4) and (5) that the three-band linear chirped swept-frequency microwave signals ultimately output by the generation system of the present invention are closely related and have good coherence.

[0055] The second purpose of the present invention is achieved in this way:

[0056] A method for generating a three-band coherent microwave signal comprises the following steps:

[0057] S1. Setting up a generation system of a three-band coherent microwave signal according to the present invention.

[0058] S2. The laser sends a continuous optical carrier signal to the electro-optical modulator. The electro-optical modulator modulates the intensity and phase of the optical carrier signal. The modulated optical signal is sent to the photodetector after delay compensation in the optical fiber ring. The photodetector converts the input optical signal into an electrical signal, which is then amplified by the fourth amplifier and output to the first bridge.

[0059] S3. The three branches arranged between the first bridge and the second bridge cooperate with other parts of the system to form three closed loops, thereby forming a three-loop Fourier domain mode-locked optoelectronic oscillator system.

[0060] S4. Before the external signal source injects a signal, the open-loop gain of each closed loop of the system is adjusted so that the closed loop where the tunable filter is located oscillates normally, while the other two closed loops are in a non-oscillating state.

[0061] S5. When the system forms a stable oscillation, that is, it outputs a microwave signal of only one frequency band, a single-frequency signal is input from an external signal source. The single-frequency signal and the microwave signal generated by the free oscillation of the system are simultaneously fed back to the electro-optical modulator and mutually locked.

[0062] S6. After the system forms a stable oscillation, the system outputs the microwave signal generated by the up-conversion and the microwave signal generated by the down-conversion. This allows the three-loop Fourier domain mode-locked optoelectronic oscillator system to simultaneously output three-band microwave signals with good coherence.

[0063] Furthermore, in step S5 , the frequency of the single-frequency signal input by the external signal source is equal to the frequency difference between the remaining two closed loops and the closed loop where the tunable filter is located.

[0064] The present invention's method for generating a three-band coherent microwave signal offers numerous technical advantages, primarily: 1. The three frequency bands of microwave signals output exhibit good coherence and synchronization; 2. No external high-frequency sweep signal source is required to output a cross-frequency sweep signal; and 3. The system structure is simple and economical. Consequently, the present invention's method for generating a three-band coherent microwave signal has promising application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 It is a system block diagram of the generation system of the present invention.

[0066] Figure 2 It is the spectrum diagram of the three-band linear chirp swept frequency signal output by the system.

[0067] Figure 3It is the spectrum of the three-band linear chirp swept signal output by the system.

[0068] In the figure: 1. Laser, 2. Electro-optic modulator, 3. Fiber ring, 4. Photodetector, 5. Fourth amplifier, 6. First bridge, 7. First broadband filter, 8. Tunable filter, 9. First amplifier, 10. Second bridge, 11. Second broadband filter, 12. Second amplifier, 13. Third broadband filter, 14. Third amplifier, 15. Coupler, 16. External signal source, 17. Signal generator. DETAILED DESCRIPTION

[0069] The present invention will be further described below in conjunction with the accompanying drawings.

[0070] like Figure 1 As shown, the multi-band microwave signal generation system of the present invention includes a laser 1, an electro-optical modulator 2, an optical fiber ring 3, a photodetector 4, a first bridge 6, a second bridge 10, a first broadband filter 7, a second broadband filter 11, a third broadband filter 13, a tunable filter 8, a first amplifier 9, a second amplifier 12, a third amplifier 14, a fourth amplifier 5, a coupler 15, an external signal source 16 and a signal generator 17.

[0071] The output of laser 1 is connected to the input of electro-optical modulator 2 via optical fiber, providing an optical carrier signal for the system. The output of electro-optical modulator 2 is connected to the input of photodetector 4 via optical fiber ring 3, modulating the intensity and phase of the optical carrier signal input to the electro-optical modulator and providing delay compensation for the system loop. The output of photodetector 4 is connected to the input of fourth amplifier 5, converting the modulated and delayed optical signal into an electrical signal and outputting it to three subsequent branches. The output of fourth amplifier 5 is connected to the input of first bridge 6, providing amplification gain for the electrical signals output to the three branches. The output of first bridge 6 is divided into three paths: the first path is connected to the input of first broadband filter 7, the second path is connected to the input of second broadband filter 11, and the third path is connected to the input of third broadband filter 13. The first bridge 6 and second bridge 10 cooperate to form three branches.

[0072] The first branch comprises a first broadband filter 7, a tunable filter 8, and a first amplifier 9 connected in series. One end of this branch is connected to the first output of the first bridge 6, and the other end is connected to the first input of the second bridge 10. In this branch, the output of the first broadband filter 7 is connected to the input of the tunable filter 8, which filters out the band signals passing through the first branch and tunes out-of-band signals to ensure instantaneous single-mode oscillation in the first branch. The output of the signal generator 17 is connected to the control terminal of the tunable filter 8, which applies a voltage control signal to the tunable filter 8 to control the microwave signal output by the system. The output of the tunable filter 8 is connected to the input of the first amplifier 9, which adjusts the period of the drive signal to match the period of the entire oscillator system to achieve Fourier domain mode locking. The output of the first amplifier 9 is connected to the first input of the second bridge 10, which provides gain compensation for the first branch to ensure proper initiation of the microwave signal in the closed loop of the first branch.

[0073] The second branch includes a second broadband filter 11 and a second amplifier 12 connected in series. One end of this branch is connected to the second output of the first bridge 6, and the other end is connected to the second input of the second bridge 10. In this branch, the output of the second broadband filter 11 is connected to the input of the second amplifier 12, which filters out the band signals passing through the second branch and tunes out-of-band signals to ensure instantaneous single-frequency oscillation in the second branch. The output of the second amplifier 12 is connected to the second input of the second bridge 10, providing gain compensation for the second branch to ensure normal oscillation of the injection-locked up-converted microwave signal generated by the system.

[0074] The third branch includes a third broadband filter 13 and a third amplifier 14 connected in series. One end of this branch is connected to the third output of the first bridge 6, and the other end is connected to the third input of the second bridge 10. Within this branch, the output of the third broadband filter 13 is connected to the input of the third amplifier 14, filtering out signals in the band passing through the third branch and tuning signals outside the bandwidth to ensure instantaneous single-frequency oscillation in the third branch. The output of the third amplifier 14 is connected to the third input of the second bridge 10, providing gain compensation for the third branch to ensure proper oscillation of the injection-locked down-converted microwave signal generated by the system.

[0075] The output of the second bridge 10 is divided into two paths. The first path is connected to the input of the coupler 15, and the second path serves as the system output. The second bridge 10 is used to complete the configuration of the three branches described above and transform the three branches into corresponding components of the system's three closed loops. The output of the coupler 15 is connected to the RF terminal of the electro-optical modulator 2 to complete the configuration of the three closed loops of the system. Of course, a duplexer can also be used here instead of the coupler. The output of the external signal source 16 is connected to the coupling terminal of the coupler 15 to input a single-frequency signal into the system to lock the microwave signal generated by the closed loop where the tunable filter is located.

[0076] When the optical carrier signal emitted by laser 1 enters electro-optical modulator 2, after intensity and phase adjustment, the optical signal, after delay compensation by optical fiber ring 3, is converted into an electrical signal for output upon reaching photodetector 4. The output electrical signal is first amplified by fourth electrical amplifier 5 before passing through first electrical bridge 6 and entering the system's three closed loops (1), (2), and (3). Closed loop (1) changes the type of drive signal sent by signal generator 17 to the tunable filter to a sawtooth wave, while simultaneously adjusting the drive signal's period to match the system's oscillation period. Once the system achieves stable oscillation, closed loop (1) will stably output a linearly chirped swept-frequency microwave signal across a frequency band. By adjusting the gains of closed loops (2) and (3), when closed loop (1) stably outputs a microwave signal, both closed loops remain in a non-oscillating state. Under the above conditions, a single-frequency signal is injected into the loop system via an external signal source 16, using coupler 15 as a carrier. The frequency of this single-frequency signal is determined by the difference between the first broadband filter 7, the second broadband filter 11, and the third broadband filter 13. According to the aforementioned injection locking theory, this injected signal and the microwave signal generated by closed loop (1) undergo up-conversion and down-conversion, respectively, to form a combined frequency signal and a difference frequency signal. The combined frequency signal is filtered to remove clutter in closed loop (2) and forms a stable oscillation, while the difference frequency signal is filtered to remove clutter in closed loop (3) and forms a stable oscillation. This allows the simultaneous output of three-band linear chirped swept-frequency microwave signals with good coherence in a three-loop Fourier domain mode-locked optoelectronic oscillator system.

[0077] like Figure 2 As shown in the spectrum diagram of the three-band linear chirp swept-frequency signal generated by the generation system of the present invention, the system simultaneously outputs microwave signals in three frequency bands, each with a sweep bandwidth of 400 MHz. The frequency ranges are 8.8 GHz-9.2 GHz, 13.8 GHz-14.2 GHz, and 3.8 GHz-4.2 GHz, respectively. In other words, the system simultaneously outputs linear chirp swept-frequency microwave signals in three different frequency bands: X, Ku, and C. These three microwave signals also maintain synchronization in output power and flatness.

[0078] like Figure 3 As shown in Figure 2, the spectrum measurement results of the microwave signal output by the system of the present invention are consistent with the theoretical derivation results. That is, the system simultaneously outputs linear chirped swept-frequency microwave signals in the X, Ku, and C frequency bands, and they maintain synchronization in bandwidth.

[0079] The method for generating a multi-band microwave signal of the present invention comprises the following steps:

[0080] S1. Settings Figure 1 The three-band coherent microwave signal generation system shown.

[0081] S2, laser 1 sends a continuous optical carrier signal to electro-optical modulator 2, electro-optical modulator 2 modulates the intensity and phase of the optical carrier signal, and the modulated optical signal is sent to photodetector 4 after delay compensation in optical fiber ring 3. Photodetector 4 converts the input optical signal into an electrical signal, which is then amplified by fourth amplifier 5 and output to first bridge 6.

[0082] S3. The three branches arranged between the first bridge 6 and the second bridge 10 cooperate with other parts of the system to form three closed loops (1), (2) and (3), thereby forming a three-loop Fourier domain mode-locked optoelectronic oscillator system.

[0083] S4. Before the external signal source 16 injects a signal, the open-loop gain of each closed loop of the system is adjusted so that the closed loop (1) where the tunable filter 8 is located oscillates normally, while the closed loop (2) and the closed loop (3) are in a non-oscillating state.

[0084] S5. When the system achieves stable oscillation, i.e., outputs a microwave signal in only one frequency band, a single-frequency signal is input from external signal source 16. This single-frequency signal and the microwave signal generated by the system's free oscillation are simultaneously fed back to electro-optical modulator 2, achieving mutual locking. The frequency of the single-frequency signal input by external signal source 16 is equal to the frequency difference between closed loops (2) and (3), and closed loop (1) where the tunable filter resides.

[0085] S6. After the system forms a stable oscillation, the system outputs the microwave signal of the sum frequency generated by up-conversion and the microwave signal of the difference frequency generated by down-conversion. This enables the three-loop Fourier domain mode-locked optoelectronic oscillator system to simultaneously output microwave signals in three bands with good coherence.

Claims

1. A system for generating three-band coherent microwave signals, characterized in that: include: The laser, whose output end is connected to the input end of the electro-optical modulator through an optical fiber, is used to provide an optical carrier signal for the system; The electro-optic modulator, whose output is connected to the input of the photodetector through an optical fiber loop, is used to modulate the intensity and phase of the optical carrier signal input to the electro-optic modulator and provide delay compensation for the system loop through the optical fiber loop; a photodetector, the output end of which is connected to the input end of the fourth amplifier, for converting the modulated and delayed optical signal into an electrical signal and outputting the electrical signal to the three branches; a fourth amplifier, whose output terminal is connected to the input terminal of the first bridge, and is used to provide amplification gain for the electrical signals output to the three branches; The first bridge has an output divided into three paths, the first output being connected to the input of the first broadband filter, the second output being connected to the input of the second broadband filter, and the third output being connected to the input of the third broadband filter; the first bridge cooperates with the second bridge to form three branches, and the first output of the first bridge is connected to the first branch, the second output is connected to the second branch, and the third output is connected to the third branch; a first broadband filter, whose output terminal is connected to the input terminal of the tunable filter, for filtering out the band signal passing through the first branch and tuning the signal outside the bandwidth to ensure the instantaneous single-mode oscillation of the first branch; a tunable filter, the output of which is connected to the input of the first amplifier, for adjusting the period of the driving signal to match the oscillation period of the system to achieve Fourier domain mode locking; a signal generator, whose output terminal is connected to the control terminal of the tunable filter and is used to apply a voltage control signal to the tunable filter to control the microwave signal output by the system; a first amplifier, whose output terminal is connected to the first input terminal of the second bridge, for providing gain compensation for the first branch so that the microwave signal of the closed loop where the first branch is located can oscillate normally; a second broadband filter, whose output terminal is connected to the input terminal of the second amplifier, for filtering out the band signal passing through the second branch and tuning the signal outside the bandwidth to ensure instantaneous single-frequency oscillation of the second branch; A second amplifier, whose output terminal is connected to the second input terminal of the second bridge, is used to provide gain compensation for the second branch so that the injection-locked up-converted microwave signal generated by the system can oscillate normally; a third broadband filter, whose output terminal is connected to the input terminal of the third amplifier, for filtering out the band signal passing through the third branch and tuning the signal outside the bandwidth to ensure instantaneous single-frequency oscillation of the third branch; a third amplifier, whose output terminal is connected to the third input terminal of the second bridge, for providing gain compensation for the third branch so that the injection-locked down-converted microwave signal generated by the system can oscillate normally; The second bridge has an output divided into two paths, the first output path being connected to the input of the coupler, and the second output path being the output of the system; the second bridge is used to complete the setting of the three branches and transform the three branches into corresponding components of the three closed loops of the system; The coupler, whose output is connected to the RF terminal of the electro-optical modulator, is used to complete the setting of the three closed loops of the system; as well as The external signal source has an output end connected to the coupling end of the coupler and is used to input a single-frequency signal into the system to lock the microwave signal generated by the closed loop where the tunable filter is located.

2. A method for generating a three-band coherent microwave signal, characterized in that: The following steps are involved: S1. Setting up the three-band coherent microwave signal generation system according to claim 1; S2. The laser sends a continuous optical carrier signal to the electro-optical modulator. The electro-optical modulator modulates the intensity and phase of the optical carrier signal. The modulated optical signal is delayed and compensated in the optical fiber ring before being sent to the photodetector. The photodetector converts the input optical signal into an electrical signal, which is then amplified by the fourth amplifier and output to the first bridge. S3. The three branches arranged between the first bridge and the second bridge cooperate with other parts of the system to form three closed loops of the system, thereby forming a three-loop Fourier domain mode-locked optoelectronic oscillator system; S4. Before the external signal source injects a signal, adjust the open-loop gain of each closed loop of the system so that the closed loop where the tunable filter is located starts oscillating normally, while the other two closed loops are in a non-oscillating state; S5. When the system forms a stable oscillation, that is, outputs a microwave signal of only one frequency band, a single-frequency signal is input from an external signal source. This single-frequency signal and the microwave signal generated by the system's free oscillation are simultaneously fed back to the electro-optical modulator and mutually locked. S6. After the system forms a stable oscillation, the system output end outputs the microwave signal generated by the up-conversion and the microwave signal generated by the down-conversion.

3. The method for generating a three-band coherent microwave signal according to claim 2, wherein: In step S5 , the frequency of the single-frequency signal input by the external signal source is equal to the frequency difference between the remaining two closed loops and the closed loop where the tunable filter is located.

Images