Ultra-low noise stable microwave generation system and method based on self-oscillation

Abstract

The invention relates to the technical field of microwave generation, in particular to an ultra-low noise stable microwave generation system and method based on self-oscillation, and the system comprises a pumping laser generation branch and an optical fiber ring cavity which is connected with a first optical fiber coupler and a second circulator. Comprising a third circulator connected with the first optical fiber coupler and the second circulator, a Brillouin gain optical fiber and an optical amplifier which are respectively connected with the third circulator, and an isolator, a second polarization controller, a second optical fiber coupler and a third optical fiber coupler which are arranged between the Brillouin gain optical fiber and the optical amplifier. The stable microwave generation system has the beneficial effects that the stable microwave generation system is novel in structure, ultra-low noise stable microwave signal output based on self-oscillation is realized, and the limitation that a traditional stable microwave signal generated based on optical heterodyne needs an external signal source and the noise of the microwave signal is relatively high is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of microwave generation, and in particular to an ultra-low noise stable microwave generation system and method based on self-oscillation. Background Art

[0002] The rapid development of microwave technology is profoundly promoting the progress of many research fields such as communications and high-resolution radar. Although electronic technology constitutes the technical basis of traditional microwave systems, its inherent shortcomings such as small transmission bandwidth, high phase noise and susceptibility to electromagnetic interference have made it difficult to adapt to the urgent needs of modern information society for real-time transmission and processing of massive amounts of information. In contrast, microwave photonic technology has effectively overcome the bottleneck of electronic technology with its significant advantages such as large bandwidth, low noise and resistance to electromagnetic interference, and has received widespread attention in various fields. It is well known that phase noise and frequency stability are two key parameters for measuring the performance of microwave signals. The former characterizes the short-term purity of the signal, while the latter determines the long-term consistency of the signal.

[0003] Normally, microwave signals can be generated in the optical domain according to the optical heterodyne method, that is, two light waves of different frequencies enter the photodetector at the same time to beat and generate microwave signals, and the frequency of the generated microwave signal depends on the frequency difference of the two lasers. The microwave signal generated by this method can achieve flexible frequency and power control, but the two independent lasers do not have coherence, which leads to the poor performance of the microwave signal generated by them in terms of phase noise performance. Stimulated Brillouin scattering can effectively suppress optical domain noise, thereby helping to reduce the electrical domain noise generated by photoelectric conversion. Coupled with the inherent narrow linewidth characteristics of Brillouin interaction, it has significant advantages in generating low phase noise microwave signals. In order to improve the coherence of the two optical signals, the researchers proposed a microwave signal generation method based on stimulated Brillouin scattering dual-wavelength Brillouin laser, that is, generating two Brillouin lasers in the same fiber ring cavity and generating microwave signals through beating. The two Brillouin lasers are generated in the same resonant cavity, which naturally have high coherence, can significantly reduce the phase noise of the microwave signal, and can achieve flexible microwave frequency tuning by adjusting the parameters in the cavity. However, if the microwave signal generated based on this scheme is to obtain higher frequency stability, it is usually necessary to use active feedback technology to lock the two Brillouin laser beams in the ring cavity to the longitudinal mode of the cavity, thereby effectively improving the stability of the system. However, this usually requires complex servo control circuits and precise calibration processes, which greatly increases the complexity and cost of the system.

[0004] Recently, researchers have proposed a new passive locking method based on frequency-shifted optical injection locking, which aims to combine stimulated Brillouin scattering, frequency shifting and optical injection locking. The core is to shift the generated Brillouin laser frequency to near the pump frequency, optimize the performance of the pump laser through optical injection locking technology, and simultaneously lock the Brillouin light to specific longitudinal modes, which can effectively suppress the mode hopping phenomenon and obtain ultra-low noise Brillouin laser. This scheme can also lock two laser beams to the same Brillouin ring cavity longitudinal mode at the same time, but the application of this technology to Brillouin cavity locking often requires the introduction of high-quality microwave signal sources. However, in order to obtain sufficient Brillouin gain, this type of Brillouin laser cavity usually uses a longer ring cavity. A longer ring cavity means a smaller longitudinal mode spacing, which makes the system susceptible to interference from changes in the external environment, making the changes in the cavity longitudinal mode and the Brillouin gain position unable to synchronize, resulting in mode hopping, affecting the system locking effect and long-term stability. It can be seen that although this scheme can achieve locking of pump light and generated Brillouin light to a certain extent, environmental changes cause the relative position of the Brillouin cavity longitudinal mode and the Brillouin gain spectrum to change, and the fixed microwave signal source cannot dynamically track its changes. In addition, the system requires an additional high-quality microwave signal source, which also increases the complexity and cost of the system.

[0005] In summary, it is still challenging to use a simple and reliable method to enable the system to generate ultra-low noise and high-stability microwave signals in a self-oscillating manner. Summary of the invention

[0006] Therefore, the technical problem to be solved by the present invention is: how to overcome the complexity and stability challenges faced by existing solutions while improving the phase noise performance of microwave signals, and to realize a microwave signal generation solution that does not require complex feedback control and can maintain high stability and low noise output in a long fiber ring cavity.

[0007] The above technical problems are solved by the following technical solutions: The present invention proposes an ultra-low noise stable microwave generation system based on self-oscillation, which includes a pump laser generation branch, which includes a first distributed feedback laser, a first circulator, a first fiber coupler, a first frequency shifter, a first polarization controller, a second circulator, and a second distributed feedback laser connected in sequence; a fiber ring cavity, which is respectively connected to the first fiber coupler and the second circulator, including a third circulator connected to the first fiber coupler and the second circulator, a Brillouin gain fiber and an optical amplifier respectively connected to the third circulator, and an isolator, a second polarization controller, a second fiber coupler, and a third fiber coupler arranged between the Brillouin gain fiber and the optical amplifier; and a frequency-shifted injection locking branch, which is connected between the second fiber coupler and the first circulator; the third fiber coupler is also connected to a photodetector.

[0008] In a preferred embodiment of the ultra-low noise stable microwave generation system based on self-oscillation described in the present invention: the frequency-shift injection locking branch includes a third polarization controller connected to the second optical fiber coupler, and a first filter, a second frequency shifter, and a second filter connected to the third polarization controller in sequence; wherein the second filter is also connected to the first circulator.

[0009] In a preferred embodiment of the ultra-low noise stable microwave generation system based on self-oscillation of the present invention: the photodetector can convert the received optical signal into a corresponding microwave signal and apply it to the first frequency shifter and the second frequency shifter respectively through the radio frequency beam splitter.

[0010] In a preferred embodiment of the ultra-low noise stable microwave generation system based on self-oscillation of the present invention: the first optical fiber coupler and the second circulator are respectively connected to the third circulator through a fourth optical fiber coupler.

[0011] In a preferred embodiment of the ultra-low noise stable microwave generation system based on self-oscillation of the present invention: the radio frequency of the first frequency shifter and the second frequency shifter is equal to the frequency interval between two Brillouin lights in the optical fiber ring cavity.

[0012] In a preferred embodiment of the ultra-low noise stable microwave generating system based on self-oscillation of the present invention: the first circulator, the second circulator, and the third circulator all include three ports, which are divided into port a, port b and port c, and port a to port b and port b to port c are unidirectionally conductive.

[0013] Another object of the present invention is to provide an ultra-low noise stable microwave generation method based on self-oscillation, and to achieve ultra-low noise stable microwave signal output based on self-oscillation through the above-mentioned ultra-low noise stable microwave generation system based on self-oscillation.

[0014] In order to solve the above technical problems, the present invention provides the following technical solutions: a method for generating ultra-low noise stable microwaves based on self-oscillation, characterized in that: the above ultra-low noise stable microwave generating system based on self-oscillation is adopted, comprising the following steps: generating two initial laser beams with a certain frequency interval through a pump laser generating branch, and the two initial laser beams enter the optical fiber ring cavity through a third circulator;

[0015] The laser entering the fiber ring cavity generates two first-order Brillouin lasers through the Brillouin gain fiber, which circulate in the fiber ring cavity.

[0016] Part of the two first-order Brillouin laser beams is then converted into photoelectric signals by a photodetector to generate high-quality microwave signals, and after being split by a radio frequency beam splitter, they are used as microwave signal sources for the first frequency shifter and the second frequency shifter in the system respectively;

[0017] Part of the remaining two first-order Brillouin laser beams enter the frequency-shift injection locking branch and enter the first distributed feedback laser through the first circulator to lock the wavelength of the first distributed feedback laser and transmit the narrow linewidth characteristic.

[0018] In a preferred embodiment of the method for generating ultra-low noise stable microwaves based on self-oscillation of the present invention: the initial laser is divided into two beams by a first circulator and a first optical fiber coupler;

[0019] A laser beam passes through a first frequency shifter, a first polarization controller, and a second circulator and enters a second distributed feedback laser, so as to lock the wavelength of the laser generated by the second distributed feedback laser and transmit the narrow line width characteristic;

[0020] Another initial laser beam and the laser beam generated by the distributed feedback laser enter the optical fiber ring cavity through the fourth optical fiber coupler.

[0021] In a preferred embodiment of the method for generating ultra-low noise stable microwaves based on self-oscillation of the present invention: the two first-order Brillouin laser beams are optimized in polarization state and stray light is filtered out by the third polarization controller and the first filter, and the first-order Brillouin laser generated in the optical fiber ring cavity with the initial laser is retained, and then injected into the first distributed feedback laser after passing through the second frequency shifter and the second filter in sequence, and the narrow linewidth characteristics of the Brillouin light are transmitted to the pump, so that the linewidth of the secondary laser output by the first distributed feedback laser becomes narrower; and the relative position between the initial pump light and the cavity longitudinal mode is maintained, and the wavelength of the output secondary laser is locked; at the same time, a part of the light output by the first distributed feedback laser passes through the first circulator , the first fiber coupler, the first frequency shifter, the first polarization controller, and the second circulator are then injected into the second distributed feedback laser to lock the frequency interval between the first distributed feedback laser and the second distributed feedback laser, and at the same time, the narrow linewidth characteristic of the first distributed feedback laser is transmitted to the second distributed feedback laser; after the secondary laser output, it enters the first distributed feedback laser and the second distributed feedback laser again after passing through the Brillouin gain fiber and the frequency shift injection locking branch, and the output laser linewidth is narrowed again; after several cycles, the first distributed feedback laser and the second distributed feedback laser output lasers with their wavelengths locked relative to the cavity longitudinal modes and their linewidths narrowed to the limit.

[0022] In a preferred embodiment of the ultra-low noise stable microwave generation method based on self-oscillation described in the present invention: the frequency of the high-quality microwave signal output by the photodetector depends on the frequency interval between the two initial laser beams output by the first distributed feedback laser and the second distributed feedback laser, and the frequency interval is equivalent to the Brillouin frequency shift of the initial laser on the Brillouin gain fiber in the fiber ring cavity.

[0023] The beneficial effects of the present invention are as follows: the ultra-low noise stable microwave generation system of the present invention has a novel structure, realizes ultra-low noise stable microwave signal output based on self-excited oscillation, solves the limitations of traditional stable microwave signals generated based on optical heterodyne, which require external signal sources and high microwave signal noise, and does not require complex active optoelectronic feedback to maintain stable operation of the system. At the same time, by accurately controlling the fiber cavity length or adjusting the wavelength of the pump laser, the frequency tuning of the ultra-low noise stable microwave signal can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings of the embodiments of the present invention are briefly introduced below. Obviously, the drawings described below only relate to some embodiments of the present invention, but are not intended to limit the present invention. Among them:

[0025] Figure 1 The overall structural diagram of the present invention is shown;

[0026] Figure 2 A schematic diagram showing the direction of the laser in the present invention is shown;

[0027] Figure 3 A schematic diagram of the structure when the input laser frequency interval is different multiples of the Brillouin frequency shift frequency interval in an embodiment of the present invention is shown;

[0028] Figure 4 The optical frequency comb spectrum diagram formed by the Brillouin laser cavity for high-quality microwave signal generation in the implementation example is shown;

[0029] Figure 5 The high-quality microwave signal spectrum generated by photoelectric conversion by a photoelectric detector in an implementation example is shown;

[0030] Figure 6 A frequency stability data diagram of a high-quality microwave signal generated in an implementation example within a certain period of time is shown;

[0031] Figure 7 shows a frequency noise diagram of the first beam of ultra-low noise Brillouin light generated in the implementation example under different conditions;

[0032] Figure 8 shows the frequency noise diagram of the second beam of ultra-low noise Brillouin light generated in the implementation example under different conditions;

[0033] Fig. 9 A data diagram comparing the phase noise of the ultra-low noise microwave signal generated in the implementation example and the external signal source is shown. DETAILED DESCRIPTION

[0034] In order to enable those skilled in the art to better understand the present invention, the present invention is further described in detail below in conjunction with specific implementation methods and drawings.

[0035] The terms used in the present invention are those general terms currently widely used in the art in consideration of the functions of the present invention, but these terms may vary according to the intention of a person of ordinary skill in the art, precedents, or new technologies in the art. In addition, specific terms may be selected by the applicant, and in this case, their detailed meanings will be described in the detailed description of the present invention. Therefore, the terms used in the specification should not be understood as simple names, but rather as a general description based on the meaning of the terms and the present invention.

[0036] Reference Figure 1 This embodiment provides an ultra-low noise stable microwave generation system based on self-oscillation, including a pump laser generation branch 1, which includes a first distributed feedback laser 11, a first circulator 12, a first fiber coupler 13, a first frequency shifter 14, a first polarization controller 15, a second circulator 16, and a second distributed feedback laser 17 connected in sequence; wherein the first distributed feedback laser 11 is a fixed wavelength or tunable distributed feedback laser; the first polarization controller 15 is used to control the polarization state of the frequency-shifted sideband of the first distributed feedback laser 11 to optimize the injection effect. The first frequency shifter 14 is used to frequency shift the laser output by the first distributed feedback laser 11. The first distributed feedback laser 11 generates a laser beam, which enters the first fiber coupler 13 through the first circulator 12 and is divided into two beams. One laser beam is frequency-shifted by the first frequency shifter 14, and then the polarization state of the post-frequency sideband is controlled by the first polarization controller 15 to optimize the laser state. The laser beam enters the second distributed feedback laser 17 through the second circulator 16 to lock the laser wavelength generated by the second distributed feedback laser 17 and transmit the narrow linewidth characteristics. The second distributed feedback laser 17 generates a laser beam, which enters the fiber ring cavity 2 together with the other laser beam produced by the first distributed feedback laser 11 through the second circulator 16.

[0037] The optical fiber ring cavity 2 is connected to the first optical fiber coupler 13 and the second circulator 16 respectively, and includes a third circulator 21 connected to the first optical fiber coupler 13 and the second circulator 16, a Brillouin gain optical fiber 22 and an optical amplifier 23 respectively connected to the third circulator 21, and an isolator 24, a second polarization controller 25, a second optical fiber coupler 26, and a third optical fiber coupler 27 arranged between the Brillouin gain optical fiber 22 and the optical amplifier 23; two laser beams generated by the pump laser generating branch 1 enter the Brillouin gain optical fiber 22 through the third circulator 21 to generate two first-order Brillouin laser beams propagating in opposite directions, and then enter the Brillouin gain optical fiber 22 again through the third circulator 21, the optical amplifier 23, the isolator 24, the second polarization controller 25, the second optical fiber coupler 26, and the third optical fiber coupler 27 in sequence.

[0038] Among them, the Brillouin gain fiber 22 is used to generate two counter-propagating first-order Brillouin lasers, and the optical amplifier 23 is used for the intracavity loss of the fiber ring cavity 2, thereby reducing the threshold of the stimulated Brillouin scattering effect. The isolator 24 is used to isolate the reverse high-order Brillouin laser generated after the first-order Brillouin laser passes through the Brillouin gain fiber again. The second polarization controller 25 is used to control the polarization in the Brillouin laser cavity, thereby controlling its output power. The second fiber coupler 26 divides the two passing first-order Brillouin lasers into two parts, one part enters the frequency-shifted injection locking branch 3, and the other part enters the third fiber coupler 27, and their coupling ratio is 10:90.

[0039] The laser light entering the third fiber coupler 27 is further divided into two parts, one part passes through the photodetector 4, and the other part continues to circulate in the fiber ring cavity 2, and their coupling ratio is also 10:90.

[0040] The frequency-shift injection locking branch 3 is connected between the second fiber coupler 26 and the first circulator 12. The frequency-shift injection locking branch 3 is used to lock the wavelength of the first distributed feedback laser 11 and transmit the narrow linewidth characteristic.

[0041] Among them, the pump laser generation branch 1 is used for the generation and output of lasers, and outputs two beams of lasers with a certain frequency interval. The fiber ring cavity 2 is an extended loop with a Brillouin cavity, which is used to generate two beams of high-quality Brillouin light; the frequency-shift injection locking branch 3 is used to lock the relative position of the laser output wavelength and the cavity longitudinal mode and to narrow the line width. A part of the two first-order Brillouin lasers generated in the fiber ring cavity 2 is converted into microwave signals by the photodetector 4, which serves as the microwave signal source for the two frequency shifters in the pump laser generation branch 1 and the frequency-shift injection locking branch 3.

[0042] As an optional embodiment: the frequency-shift injection locking branch 3 includes a third polarization controller 31 connected to the second optical fiber coupler 26, and a first filter 32, a second frequency shifter 33, and a second filter 34 connected to the third polarization controller 31 in sequence; wherein the second filter 34 is also connected to the first circulator 12.

[0043] The third polarization controller 31 is used to maintain the polarization state of the Brillouin light that is frequency-shifted and injection-locked; the second frequency shifter 33 is used to frequency-shift the Brillouin light so that it maintains the same frequency as the laser output by the original first distributed feedback laser 11. The first filter 32 is used to filter and retain the first-order Brillouin laser generated in the fiber ring cavity by the first distributed feedback laser 11, and the second filter 34 is used to filter and retain the laser with the same frequency as the laser generated by the first distributed feedback laser 11.

[0044] As an optional embodiment: the third optical fiber coupler 27 is also connected to a photodetector 4, which can receive optical signals and convert them into corresponding microwave signals, which are applied to the first frequency shifter 14 and the second frequency shifter 33 through a radio frequency beam splitter 41. The first optical fiber coupler 13 and the second circulator 16 are connected to the third circulator 21 through a fourth optical fiber coupler 5, respectively.

[0045] The photodetector 4 always maintains the optical signal input for photoelectric conversion, generates high-quality microwave signals, and after radio frequency splitting, serves as the microwave signal source of the first frequency shifter 14 and the second frequency shifter 33 in the system.

[0046] As an optional embodiment: the radio frequency of the first frequency shifter 14 and the second frequency shifter 33 is equal to the frequency interval of the two Brillouin light beams in the fiber ring cavity 2. Initially, the first frequency shifter 14 and the second frequency shifter 33 do not work. After the two initial pump laser beams generate two Brillouin laser beams in the fiber ring cavity 2, the microwave signal is synchronously output through the photodetector 4 to generate a microwave signal, and the generated microwave signal is used as the microwave signal source of the first frequency shifter 14 and the second frequency shifter 33. The first frequency shifter 14 and the second frequency shifter 33 are started to assist in completing the locking process. The radio frequency signal of the frequency shifter is a microwave signal generated after the two high-quality Brillouin light beams in the fiber ring cavity are photoelectrically converted by the photodetector 4. The frequency of the microwave signal depends on the frequency interval of the two Brillouin laser beams. Due to the injection locking effect, the signal light is locked to the vicinity of the maximum gain value of the Brillouin gain region. The locked distributed feedback laser, as a new light source, generates a Brillouin laser with a compressed line width again in the Brillouin laser cavity, and the cycle is continuously performed until the line width is compressed to the limit.

[0047] As an optional embodiment: the first circulator 12, the second circulator 16, and the third circulator 21 all include three ports, namely port a, port b, and port c, and the a-port to the b-port and the b-port to the c-port are unidirectional. When in use, the laser will only go from port a to port b or from port b to port c.

[0048] In the present invention, the b port of the first circulator 12 is connected to the first distributed feedback laser 11, the c port is connected to the first fiber coupler 13, and the a port is connected to the second filter 34. The a port of the second circulator 16 is connected to the first polarization controller 15, the b port is connected to the second distributed feedback laser 17, and the c port is connected to the fourth fiber coupler 5. The a port of the third circulator 21 is connected to the fourth fiber coupler 5, the b port is connected to the Brillouin gain fiber 22, and the c port is connected to the optical amplifier 23.

[0049] Reference Figure 1 , 2 As an optional embodiment: an ultra-low noise stable microwave generation system based on self-oscillation comprises the following steps:

[0050] S1: A beam of laser light is generated by the first distributed feedback laser 11. The laser light passes through the first circulator 12 and enters the first fiber coupler 13 to be divided into two beams. One beam of laser light passes through the first frequency shifter 14, the first polarization controller 15, and the second circulator 16 and enters the second distributed feedback laser 17. The sideband of the first distributed feedback laser 11 is injected into the second distributed feedback laser 17 to lock the laser wavelength generated by the second distributed feedback laser 17 and transmit the narrow linewidth characteristics. Another initial laser light is coupled with the laser light generated by the second distributed feedback laser 17 through the fourth coupler 5 and enters the fiber ring cavity 2. The two initial laser light beams generate two first-order Brillouin laser light beams that propagate in opposite directions through the Brillouin gain fiber 22;

[0051] S2: The two first-order Brillouin laser beams pass through the third circulator 21, the optical amplifier 23, the isolator 24, the second polarization controller 25, the second fiber coupler 26, and the third fiber coupler 27 and enter the Brillouin gain fiber 22 again, so that the two first-order Brillouin laser beams circulate in the fiber ring cavity 2. A fiber coupler is also provided in the fiber ring cavity 2 to output a part of the laser for performance characterization.

[0052] S3: The two first-order Brillouin laser beams are divided into one laser beam and two laser beams after passing through the second optical fiber coupler 26;

[0053] S4: One laser path enters the frequency-shift injection locking branch 3, and the other laser path is further divided into two parts by the third fiber coupler 27. One part circulates in the fiber ring cavity 2, and the other part enters the photodetector 4 for photoelectric conversion to generate high-quality microwave signals. After being split by the RF beam splitter 41, they are used as microwave signal sources for the first frequency shifter 14 and the second frequency shifter 33 in the system respectively.

[0054] S5: The laser light entering the frequency-shifted injection locking branch 3 is optimized by the third polarization controller 31, and then filtered by the first filter 32, retaining the first-order Brillouin laser generated by the first distributed feedback laser 11 in the fiber ring cavity, and then frequency-shifted and filtered by the second frequency shifter 33 and the second filter 34 to obtain a laser light having the same frequency as the initial laser light generated by the first distributed feedback laser 11, and entering the first distributed feedback laser 11 through the first circulator 12 to lock the wavelength of the first distributed feedback laser 11 and transmit the narrow linewidth characteristic; at the same time, a part of the light output by the first distributed feedback laser 11 is injected into the second distributed feedback laser 17 after passing through the first circulator 12, the first fiber coupler 13, the first frequency shifter 14, the first polarization controller 15, and the second circulator 16, so as to lock the frequency interval between the first distributed feedback laser 11 and the second distributed feedback laser 17, and at the same time transmit the narrow linewidth characteristic of the first distributed feedback laser 11 to the second distributed feedback laser 17, and lock the laser wavelength output by the distributed feedback laser 17;

[0055] S6: After several cycles, the first distributed feedback laser and the second distributed feedback laser can output lasers with wavelengths locked relative to the cavity longitudinal mode and line widths narrowed to a limit;

[0056] S7: After the microwave signal output by the photodetector acts on the locking loop, the system outputs a stable microwave signal with ultra-low noise characteristics.

[0057] After the secondary laser is output, it enters the distributed feedback laser again through the Brillouin gain fiber and the frequency shift injection locking branch, and the laser line width output by the distributed feedback laser is narrowed again;

[0058] After cycling through several groups, the distributed feedback laser can output a laser with its wavelength locked relative to the longitudinal mode of the cavity and its line width narrowed to the limit.

[0059] The frequency of the high-quality microwave signal output by the photodetector 4 depends on the frequency interval of the two initial laser beams output by the first distributed feedback laser 11 and the second distributed feedback laser 17, and the frequency interval is equivalent to the Brillouin frequency shift of the initial laser on the Brillouin gain fiber 22 in the fiber ring cavity 2.

[0060] In the present invention, initially the first frequency shifter 14 and the second frequency shifter 33 do not work, and the two initial laser beams with a certain frequency interval generated by the pump laser generating branch 1 are achieved by setting parameters such as the temperature of the first distributed feedback laser 11 and the second distributed feedback laser 17.

[0061] Reference Figures 4 to 9 As an optional embodiment: wherein the initial first distributed feedback laser 11 is set to about 1550.100nm, and the second distributed feedback laser 17 is set to about 1550.175nm.

[0062] After the system is built, the first distributed feedback laser 11 and the second distributed feedback laser 17 are turned on. The two initial laser beams are respectively transmitted through the first fiber coupler 13 and the second circulator 16 to enter the fourth fiber coupler 5, and then enter the fiber ring cavity 2 through the fourth fiber coupler 5. Two counter-propagating Brillouin laser beams are generated in the fiber ring cavity 2, and then output through the third fiber coupler 27. Part of the output light enters the photodetector 4 for photoelectric conversion to generate a high-quality microwave signal, and the other light passes through the third polarization controller 31 and the first filter 32 to enter the second frequency shifter 33 and the second filter 34. The RF signal is the microwave signal output by the photodetector. The frequency of the RF signal is the frequency interval of the two Brillouin laser beams in the fiber ring cavity 2, which is approximately around 9.383 GHz. The narrow bandwidth filter is adjusted, and the side mode suppression ratio reaches 20 dB. The frequency-shifted sideband returns to the first distributed feedback laser 11 through the first circulator 12 to complete the frequency-shifted light injection locking.

[0063] After the frequency-shifted light injection locking operation is completed through the frequency-shifted injection locking branch 3, the narrow linewidth characteristics of the Brillouin laser are transmitted to the first distributed feedback laser 11. The narrow linewidth pump light output by the first distributed feedback laser 11 enters the first frequency shifter 14 through the first fiber coupler 13 for frequency shifting operation. The radio frequency signal of the first frequency shifter 14 is the microwave signal output by the photodetector 4, and the frequency is equal to the frequency interval between the two Brillouin laser beams in the optical ring cavity. Then, the polarization state of the injected sideband is adjusted through the first polarization controller 15, and is injected into the second distributed feedback laser 17 through the second circulator 16 to lock the output wavelength of the second distributed feedback laser 17 and transmit the narrow linewidth characteristics.

[0064] Reference Figure 4, we can see the spectrum of the two Brillouin laser beams output from the fiber ring cavity 2. It is observed that the peak powers of the two Brillouin laser beams are almost equal, and small sidebands appear on both sides of the main peak. This is due to the four-wave mixing effect in the fiber ring cavity 2. Specifically, the wavelength interval of the two Brillouin light beams is about 0.07504nm. At this time, the frequency interval of the two Brillouin laser beams corresponds to 9.383GHz, which corresponds to the Brillouin frequency shift.

[0065] The optical signal output by the optical fiber ring cavity 2 is converted into photoelectricity by the photodetector 4. The spectrum of the generated microwave signal is as follows: Figure 5 As shown in the figure, it can be seen that at a resolution of 5MHz, the measured signal-to-noise ratio of the microwave signal is as high as 70dB, which proves the excellent quality of the output microwave signal. Such a high signal-to-noise ratio also shows that the generated signal has inherent low noise characteristics. At the same time, the signal frequency of the output microwave signal is strictly corresponding to the frequency interval of the two Brillouin laser beams in the fiber ring cavity, which is 9.383GHz.

[0066] Figure 6 The frequency stability data of the generated high-quality microwave signal over a certain period of time is shown in the figure. During the measurement time of up to 1500 seconds, the frequency fluctuation (jitter) of the microwave signal remained at the kilohertz level, highlighting the remarkable stability of the generated microwave signal. This excellent frequency stability makes the microwave signal very suitable for use as the RF signal source in the frequency shifting device described above. This stable and accurate RF signal plays a vital role in ensuring the precise and robust operation of the frequency shifting device, and helps to improve the overall stability and performance of the microwave signal generator.

[0067] Figures 7 to 9 The frequency noise and phase noise of the ultra-low noise microwave signal generated in the implementation example are shown in Figure 2. The frequency noise and microwave phase noise performance of the system are characterized using the delayed self-heterodyne method and the dual-wavelength delayed self-heterodyne method respectively. The measurement results show the difference between the system modulator being modulated by an external signal source and the system's self-generated microwave signal. Figure 7 and Figure 8 The frequency noise performance of the two Brillouin beams output by the fiber ring cavity is shown. The final output of the system has a stable Brillouin laser white noise background of about 0.015Hz 2 / Hz, which is equivalent to a fundamental frequency linewidth of about 0.048Hz, while without the frequency-shift injection locking process, the Brillouin optical noise would increase by about six orders of magnitude. It is worth noting that whether or not frequency-shift injection locking is performed will have a great impact on the noise characteristics of the output light, and the cascade narrowing mechanism achieved by combining stimulated Brillouin scattering and frequency-shift optical injection locking within the system is the reason for the significant compression of noise. More importantly, in both the external microwave source and the self-generated microwave source system, the frequency noise performance of the two Brillouin beams almost overlap. This shows that the system's self-generated microwave signal does not cause significant noise degradation when implementing the modulation of the modulator in the system. Fig. 9 The repetition rate phase noise of the generated microwave signal is compared with that of the driving RF signal. The two phase noise spectra are almost identical, indicating that the self-stabilized microwave signal generated in the resonant cavity does not bring about a significant phase noise boost. This similarity in the phase noise spectra indicates a high degree of phase coherence between the comb signals, highlighting the effectiveness of the system in maintaining signal phase integrity. In summary, due to the extremely low noise level, the self-generated microwave signal in this system can effectively replace the RF signal source and can be used as a high-quality microwave signal in fields such as communications and high-resolution radar.

[0068] Reference Figure 3 As an optional embodiment: when the frequency interval between the two initial laser beams is a multiple of 9.383 GHz, the frequency of the high-quality microwave signal finally output will also be a multiple of 9.383 GHz, thereby achieving frequency-adjustable ultra-low noise stable microwave signal output. At the same time, a radio frequency divider 42 is set between the photodetector 4 and the radio frequency beam splitter 41 to divide the microwave signal by multiples, and then serve as the microwave signal source for the first frequency shifter 14 and the second frequency shifter 33.

[0069] When in use, a beam of laser is generated by the first distributed feedback laser 11, and the laser passes through the first circulator 12 and enters the first fiber coupler 13 to be divided into two beams. One beam of laser passes through the first frequency shifter 14, the first polarization controller 15, and the second circulator 16 to enter the second distributed feedback laser 17, and the sideband of the first distributed feedback laser 11 is injected into the second distributed feedback laser 17 to lock the wavelength generated by the second distributed feedback laser 17 and transmit the narrow linewidth characteristics.

[0070] Another initial laser beam is coupled with the laser beam generated by the second distributed feedback laser 17 through the fifth fiber coupler 5 and enters the fiber ring cavity 2. The two initial laser beams generate two counter-propagating first-order Brillouin laser beams through the Brillouin gain fiber 22; at this time, assuming that the frequency interval between the initial laser beam and the initial laser beam generated by the second distributed feedback laser 17 is 3 times of 9.383 GHz, the frequency interval of the two generated first-order Brillouin laser beams is the same as that of the two initial laser beams, which is also 3 times of 9.383 GHz.

[0071] The two first-order Brillouin laser beams pass through the third circulator 21, the optical amplifier 23, the isolator 24, the second polarization controller 25, the second fiber coupler 26, and the third fiber coupler 27 and enter the Brillouin gain fiber 22 again, so that the two first-order Brillouin laser beams circulate in the fiber ring cavity 2.

[0072] The two first-order Brillouin laser beams are divided into one laser path and two laser paths after passing through the second optical fiber coupler 26;

[0073] The two laser beams are further divided into two parts by the third fiber coupler 27. One part circulates in the fiber ring cavity 2, and the other part enters the photoelectric detector 4 for photoelectric conversion to generate a high-quality microwave signal. At this time, the frequency of the microwave signal produced is also three times of 9.383 GHz. At this time, the microwave signal is processed by the radio frequency divider 42, and the microwave signal is divided by multiples to obtain a microwave signal with a frequency of 9.383 GHz.

[0074] After being split by the radio frequency beam splitter 41, they are used as microwave signal sources for the first frequency shifter 14 and the second frequency shifter 33 in the system respectively;

[0075] One laser enters the frequency-shifted injection locking branch 3, is optimized by the third polarization controller 31, and is filtered by the first filter 32 to retain the first-order Brillouin laser generated by the first distributed feedback laser 11 in the fiber ring cavity. Then, it is frequency-shifted and filtered by the second frequency shifter 33 and the second filter 34 to obtain a laser with the same frequency as the initial laser generated by the first distributed feedback laser 11, and enters the first distributed feedback laser 11 through the first circulator 12 to lock the wavelength of the first distributed feedback laser 11 and transmit the narrow linewidth characteristics. At the same time, a part of the light output by the first distributed feedback laser 11 is injected into the second distributed feedback laser 17 after passing through the first circulator 12, the first fiber coupler 13, the first frequency shifter 14, the first polarization controller 15, and the second circulator 16, so as to lock the frequency interval between the first distributed feedback laser 11 and the second distributed feedback laser 17, and at the same time, the narrow linewidth characteristics of the first distributed feedback laser 11 are transmitted to the second distributed feedback laser 17, and the laser wavelength output by the distributed feedback laser 17 is locked.

[0076] After several cycles, the first distributed feedback laser and the second distributed feedback laser can output lasers with wavelengths locked relative to the cavity longitudinal mode and line widths narrowed to a limit;

[0077] After the microwave signal output by the photodetector acts on the locking loop, the system outputs a stable microwave signal with ultra-low noise characteristics.

[0078] In summary, it can be seen from the above content that based on this system and method, the ultra-low noise microwave signal generated has a noise as low as -120dBc / Hz at a 10kHz offset frequency, and a frequency-stable microwave signal can be generated without the assistance of an external signal source. Frequency tuning can be achieved by precisely controlling the fiber cavity length or adjusting the pump laser wavelength. This technology is a promising solution for applications that require simple, high-quality and stable microwave signals, including advanced communication systems, high-resolution radars and other emerging technologies.

[0079] Finally, it should be pointed out that the methods and devices described in detail above are only embodiments, and those skilled in the art can modify these embodiments in different ways without departing from the scope of the present invention.

Claims

1. An ultra-low noise stable microwave generation system based on self-oscillation, characterized in that: include, A pump laser generating branch (1) comprises a first distributed feedback laser (11), a first circulator (12), a first optical fiber coupler (13), a first frequency shifter (14), a first polarization controller (15), a second circulator (16), and a second distributed feedback laser (17) connected in sequence; An optical fiber ring cavity (2), the optical fiber ring cavity (2) being connected to a first optical fiber coupler (13) and a second circulator (16) respectively, comprising a third circulator (21) connected to the first optical fiber coupler (13) and the second circulator (16), a Brillouin gain optical fiber (22) and an optical amplifier (23) respectively connected to two ends of the third circulator (21), and an isolator (24) arranged between the Brillouin gain optical fiber (22) and the optical amplifier (23), a second polarization controller (25), a second optical fiber coupler (26), and a third optical fiber coupler (27); A frequency-shift injection locking branch (3), wherein the frequency-shift injection locking branch (3) is connected between the second optical fiber coupler (26) and the first circulator (12); Wherein, the third optical fiber coupler (27) is also connected to a photoelectric detector (4).

2. The ultra-low noise stable microwave generation system based on self-oscillation according to claim 1, characterized in that: The frequency-shift injection locking branch (3) comprises a third polarization controller (31) connected to the second optical fiber coupler (26), and a first filter (32), a second frequency shifter (33), and a second filter (34) connected in sequence to the third polarization controller (31); The second filter (34) is also connected to the first circulator (12).

3. The ultra-low noise stable microwave generation system based on self-oscillation according to claim 2, characterized in that: The photoelectric detector (4) can convert the received optical signal into a corresponding microwave signal and apply it to the first frequency shifter (14) and the second frequency shifter (33) respectively through the radio frequency beam splitter (41).

4. The ultra-low noise stable microwave generation system based on self-oscillation according to claim 3, characterized in that: The first optical fiber coupler (13) and the second circulator (16) are respectively connected to the third circulator (21) via a fourth optical fiber coupler (5).

5. The ultra-low noise stable microwave generating system based on self-oscillation according to any one of claims 1 and 2, characterized in that: The radio frequency of the first frequency shifter (14) and the second frequency shifter (33) is equal to the frequency interval between two beams of Brillouin light in the optical fiber ring cavity (2).

6. The ultra-low noise stable microwave generation system based on self-oscillation according to claim 5, characterized in that: The first circulator (12), the second circulator (16) and the third circulator (21) all include three ports, namely port a, port b and port c, and port a to port b and port b to port c are unidirectionally conductive.

7. A method for generating ultra-low noise stable microwaves based on self-oscillation, characterized in that: The ultra-low noise stable microwave generation system based on self-oscillation according to any one of claims 1 to 6 comprises the following steps: Two initial laser beams with a certain frequency interval are generated through a pump laser generating branch (1), and the two initial laser beams enter the optical fiber ring cavity (2) through a third circulator (21); The laser light entering the optical fiber ring cavity (2) generates two beams of first-order Brillouin laser light through the Brillouin gain optical fiber, and circulates in the optical fiber ring cavity (2); A portion of the two first-order Brillouin laser beams is then subjected to photoelectric conversion by a photodetector (4) to generate high-quality microwave signals, and after being split by a radio frequency beam splitter (41), the signals are used as microwave signal sources for a first frequency shifter (14) and a second frequency shifter (33) in the system respectively. Part of the remaining two first-order Brillouin laser beams enter the frequency-shift injection locking branch (3) and enter the first distributed feedback laser (11) through the first circulator (12) to lock the wavelength of the first distributed feedback laser (11) and transmit the narrow linewidth characteristics.

8. The method for generating ultra-low noise stable microwaves based on self-oscillation according to claim 7, characterized in that: The laser light generated by the first distributed feedback laser (11) is divided into two beams through a first circulator (12) and a first optical fiber coupler (13); A laser beam passes through a first frequency shifter (14), a first polarization controller (15), and a second circulator (16) and enters a second distributed feedback laser (17) for locking the wavelength of the laser light generated by the second distributed feedback laser (17) and transmitting narrow line width characteristics; Another initial laser beam and the laser light generated by the distributed feedback laser (17) enter the optical fiber ring cavity (2) through the fourth optical fiber coupler (5).

9. The method for generating ultra-low noise stable microwaves based on self-oscillation according to claim 8, characterized in that: The two first-order Brillouin laser beams are optimized for polarization state and stray light is filtered out through a third polarization controller (31) and a first filter (32), and the first-order Brillouin laser beam generated in the optical fiber ring cavity with the initial laser beam is retained. The two first-order Brillouin laser beams are then injected into the first distributed feedback laser (11) after passing through a second frequency shifter (33) and a second filter (34) in sequence, and the narrow line width characteristic of the Brillouin light beam is transmitted to the pump, so that the line width of the secondary laser beam output by the first distributed feedback laser (11) becomes narrower; and the relative position between the initial pump light and the cavity longitudinal mode is maintained, and the output is locked. wavelength of the secondary laser; at the same time, a portion of the light output by the first distributed feedback laser (11) is injected into the second distributed feedback laser (17) after passing through the first circulator (12), the first fiber coupler (13), the first frequency shifter (14), the first polarization controller (15), and the second circulator (16), so as to lock the frequency interval between the first distributed feedback laser (11) and the second distributed feedback laser (17), and at the same time, transmit the narrow linewidth characteristic of the first distributed feedback laser (11) to the second distributed feedback laser (17); After the secondary laser is output, it passes through the Brillouin gain fiber (22) and the frequency shift injection locking branch (3) and enters the first distributed feedback laser (11) and the second distributed feedback laser (17), thereby narrowing the output laser line width again; After a number of cycles, the first distributed feedback laser (11) and the second distributed feedback laser (17) output lasers whose wavelengths are locked relative to the longitudinal mode of the cavity and whose line widths are narrowed to the limit.

10. The method for generating ultra-low noise stable microwaves based on self-oscillation according to claim 9, characterized in that: The frequency of the high-quality microwave signal output by the photodetector (4) depends on the frequency interval between two initial laser beams output by the first distributed feedback laser (11) and the second distributed feedback laser (17), and the frequency interval is equal to the Brillouin frequency shift of the initial laser beam on the Brillouin gain fiber (22) in the fiber ring cavity (2).

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