CN109638621B - kHz-magnitude single-passband microwave photonic filter - Google Patents

Abstract

The present disclosure provides a kHz-level single-passband microwave photonic filter, comprising: a laser, a first optical coupler, a single-sideband suppression carrier modulation module, a single-sideband modulation module, a microwave signal source, a first optical amplifier, A second optical amplifier, a second optical coupler, a single-frequency Brillouin fiber laser, a photodetector and a vector network analyzer. The present disclosure utilizes the gain cavity of a single-frequency Brillouin fiber laser to realize ultra-narrow single-passband microwave photonic filtering, and solves the technical problem that the existing microwave photonic filter cannot realize single-passband filtering below the MHz order, and the filtering provided by the present disclosure The 3dB bandwidth of the device has achieved a significant breakthrough, which can reach the order of kHz. At the same time, its center frequency is stable, tunable, and has a high out-of-band rejection ratio.

Description

kHz order single passband microwave photonic filter

technical field

The present disclosure relates to the field of microwave photonic signal processing and electronic countermeasures, and in particular, to a kHz-level single-passband microwave photonic filter.

Background technique

Microwave photonics is the product of the fusion of microwave and photonic technologies, and has broad application prospects in the generation, transmission and processing of radio frequency signals. The microwave photonic filter uses photonic technology to process the microwave signal modulated on the optical carrier in the optical domain, and finally realizes the filtering function. It has the incomparable advantages of traditional electrical domain filters in terms of bandwidth, tuning and reconstruction. The core technology in the field of photonics has caused extensive research.

The single-bandpass filtering technology with one passband overcomes the shortcoming of periodic frequency response, and has been widely used in wireless communication, sensing, biology and military fields. The key indicator to measure the characteristics of single-bandpass filtering is single-passband 3dB bandwidth Δf _3dB , for a single passband filter, the smaller the 3dB bandwidth Δf _3dB , the better the frequency selection characteristics of the filter, and the desired frequency can be filtered out more accurately, so the ultra-narrow 3dB bandwidth Δf _3dB single passband is realized Microwave photonic filters have become a research hotspot.

At present, there are many solutions for realizing ultra-narrow single-passband microwave photonic filtering in the prior art. For example, a single-passband microwave photonic filter based on a broadband light source and a Mach-Zehnder interference structure is proposed. Signal sampling and Mach-Zehnder interferometer weights the sampled signal to obtain multiple tap coefficients, thereby realizing single-pass band filtering, and its 3dB bandwidth Δf _3dB is several hundred MHz; a single-pass microwave photon based on stimulated Brillouin scattering A band filter is proposed, which uses the narrow-band gain characteristic of stimulated Brillouin scattering to process the microwave modulated optical signal to achieve single-pass band filtering, and its 3dB bandwidth Δf _3d can reach 24.4MHz; Bulletin No. 103715480B The patent document discloses an invention called "a single-band-pass tunable microwave photonic filter with ultra-high quality factor", which effectively reduces the The gain spectrum width of stimulated Brillouin scattering realizes a single-passband microwave photon filtering technology with ultra-high quality factor, the 3dB bandwidth Δf _3dB is 4.14MHz, and the center frequency can be adjusted in the range of 0.3GHz-29.7GHz.

Although there are many single-passband microwave photonic filter solutions, its key index 3dB bandwidth Δf _3dB is limited, which is still in the order of MHz, and it cannot achieve narrower (kHz or even Hz order) single-passband microwave photonic filtering. , which cannot meet the application fields of high-purity spectrum microwave signal generation, high-resolution microwave photonic sensing, and high-performance microwave photonic radar.

SUMMARY OF THE INVENTION

(1) Technical problems to be solved

Existing microwave photonic filter technical solutions are difficult to achieve single-passband filtering with a bandwidth below 3dB MHz, which cannot meet the application fields of high-purity spectrum microwave signal generation, high-resolution microwave photonic sensing, and high-performance microwave photonic radar.

(2) Technical solutions

An embodiment of the present disclosure provides a kHz-level single-passband microwave photonic filter, including: a laser, a first optical coupler, a single-sideband suppressed carrier modulation module, a single-sideband modulation module, a microwave signal source, a first optical Amplifier, second optical amplifier, second optical coupler, single frequency Brillouin fiber laser, photodetector and vector network analyzer, wherein:

The laser with frequency _fc is generated by the laser, and after passing through the first optical coupler, it is divided into a first laser beam and a second laser beam. The first laser beam enters the single sideband suppression carrier modulation module, and the second laser beam Enter the SSB modulation module;

The microwave signal with the frequency f _p output by the microwave signal source is modulated onto the first laser beam by the single sideband suppression carrier modulation module to form a single sideband suppression carrier modulation signal, and the single sideband suppression carrier modulation signal includes the first upper side band f _c +f _p , the first upper sideband f _c +f _p enters the single-frequency Brillouin fiber laser after passing through the first optical amplifier and the second optical coupler in sequence, and the first upper side band f _c +f The power of _p is less than the excitation power threshold of the single-frequency Brillouin fiber laser, and a gain spectrum with a 3dB bandwidth of the order of kHz is formed in the single-frequency Brillouin fiber laser, and the center frequency of the gain spectrum is f _c +f _p -f _B , the 3dB bandwidth is Δf _SFBFL , where f _B is the Brillouin frequency shift.

The swept-frequency microwave signal with the frequency f _RF output by the vector network analyzer is modulated onto the second laser beam by the single sideband modulation module to form a single sideband modulation signal. The single sideband modulation signal includes the optical carrier f _c and the th The two upper sidebands f _c +f _RF , the optical carrier f _c and the second upper side band f _c +f _RF enter the single-frequency Brillouin fiber laser after passing through the second optical amplifier and the second optical coupler in sequence, and the optical The power of the carrier f _c is greater than the excitation power threshold of the single-frequency Brillouin fiber laser, and a laser with a frequency of f _c -f _B is excited; the optical power of the second upper sideband f _c +f _RF is located in the single-frequency distribution. Below the excitation power threshold of the Liouin fiber laser, a backscattered signal is generated. When the frequency of the backscattered signal is in the first upper sideband f _c +f _p , the gain spectrum (the center frequency is f _c + f _p -f _B. When the 3dB bandwidth is within the frequency range of Δf _SFBFL ), the gain obtained is amplified, the laser signal is excited, and beat frequency with the laser of frequency f _c -f _B in the photodetector to generate a microwave signal;

The microwave signal generated by the photodetector is input into a vector network analyzer to obtain the frequency response characteristic of the microwave photonic filter.

Optionally, the single passband center frequency f _pass of the filter is equal to f _p ; the 3dB bandwidth Δf _pass of the filter is equal to the 3dB bandwidth Δf _SFBFL of the gain cavity of the single frequency Brillouin fiber laser.

Optionally, the splitting ratio of the first optical coupler is 50%:50%, and the splitting ratio of the second optical coupler is 50%:50%.

Optionally, the single-frequency Brillouin fiber laser includes: an optical circulator, a third optical coupler, a fourth optical coupler, a fifth optical coupler, an optical isolator, and a high nonlinear fiber connected in sequence, And the high nonlinear fiber is connected with the optical circulator to form an optical circuit and form an optical resonant cavity.

Optionally, the splitting ratio of the third optical coupler is 50%:50%, the splitting ratio of the fourth optical coupler is x%:(100-x)%, and the fifth optical coupler has an optical splitting ratio of x%:(100-x)%. The splitting ratio is x%:(100-x)%, preferably, x≥90.

Optionally, the fourth optical coupler and the fifth optical coupler form a fiber ring cavity;

The fourth optical coupler includes port 42 and port 44; the fifth optical coupler includes port 52 and port 54;

Connecting port 42 and port 52 through optical fiber, and connecting port 44 and port 54 through optical fiber, forming the optical fiber annular cavity;

The light beams split by the fourth optical coupler with a ratio of (100-x)% and the beam split by the fifth optical coupler with a ratio of x% are transmitted in the fiber ring cavity.

Optionally, the free spectral range FSR1 of the optical fiber ring cavity and the 3dB bandwidth Δf _ring of the resonance peak satisfy the following conditions:

FSR1≥Δf _B

Δf _ring ≤FSR2

Wherein, Δf _B is the 3dB bandwidth of the Brillouin gain spectrum of the highly nonlinear fiber, FSR2 is the mode spacing of the single-frequency Brillouin fiber laser, and preferably, the cavity length of the fiber ring cavity is less than 8 meters .

(3) Beneficial effects

It can be seen from the above technical solutions that the kHz-level single-passband microwave photonic filter of the present disclosure has at least one or a part of the following beneficial effects:

1. The single-sideband modulated optical signal is injected into the single-frequency Brillouin fiber laser, and the ultra-narrow optical gain cavity of the single-frequency Brillouin fiber laser is used for optical amplification, which solves the problem that the existing microwave photonic filter cannot achieve MHz To solve the problem of single passband filtering below the level, the 3dB bandwidth Δf _3dB of the microwave photonic filter of the present disclosure has achieved a significant breakthrough, which can reach the order of kHz or even the order of Hz.

2. By adjusting the frequency of the pump light injected into the single-frequency Brillouin fiber laser, the center frequency of the filter can be adjusted, which has the advantage of finely tunable center frequency.

3. The output laser frequency of the single-frequency Brillouin fiber laser is stable, so that the center frequency of the microwave photonic filter is stable and is less affected by the environment.

4. The radio frequency signal within the passband of the microwave photonic filter is generated by two laser beat frequencies. Outside the passband, only one laser beam is generated, and there is no beat frequency radio frequency signal. Therefore, the microwave photonic filter has a band. The advantage of high external inhibition ratio.

Description of drawings

For a more complete understanding of the present disclosure and its advantages, reference will now be made to the following description taken in conjunction with the accompanying drawings, in which:

1 is a schematic structural diagram of a kHz-level single-passband microwave photonic filter provided by an embodiment of the present disclosure;

2 is a schematic structural diagram of a single-frequency Brillouin fiber laser in a kHz-level single-passband microwave photonic filter provided by an embodiment of the present disclosure;

3 is a schematic diagram of the principle of a single-frequency Brillouin fiber laser in a kHz-level single-passband microwave photonic filter provided by an embodiment of the present disclosure;

4 is a schematic diagram of the principle of a kHz-level single-passband microwave photonic filter provided by an embodiment of the present disclosure;

5 is a beat frequency spectrum obtained by entering a detector of laser light output by a single-frequency Brillouin fiber laser in a kHz-level single-passband microwave photonic filter provided by an embodiment of the present disclosure;

6 is a frequency response characteristic diagram of a kHz-level single-passband microwave photonic filter provided by an embodiment of the present disclosure;

7 is a tunable characteristic diagram of a center frequency of a kHz-level single-passband microwave photonic filter provided by an embodiment of the present disclosure;

FIG. 8 is another solution of a single-frequency Brillouin fiber laser in a kHz-level single-passband microwave photonic filter provided by an embodiment of the present disclosure.

Among them, the reference numerals are:

100, laser; 200, first optical coupler; 300, single sideband suppression carrier modulation module; 400, single sideband modulation module; 500, microwave signal source; 600, second optical coupler; 700, single frequency bridle Yuan fiber laser; 800, photodetector; 900, vector network analyzer;

210, port one one; 220, port one two; 230, port one three;

310, a first optical input port; 320, a first optical output port; 330, a first microwave input port;

410, a second optical input port; 420, a second optical output port; 430, a second microwave input port;

610, port two one; 620, port two two; 630, port two three;

710, optical circulator; 720, third optical coupler; 730, fourth optical coupler; 740, fifth optical coupler; 750, optical isolator; 760, high nonlinear optical fiber;

711, port ① (single frequency Brillouin fiber laser input port); 712, port ②; 713, port ③;

721, port three one; 722, port three two (single frequency Brillouin fiber laser output port); 723, port three three;

731, port four one; 732, port four two; 733, port four three; 734, port four four;

741, port five one; 742, port five two; 743, port five three; 744, port five four;

770, a first fiber grating; 780, a second fiber grating.

Detailed ways

The present disclosure provides a kHz-level single-passband microwave photonic filter, which utilizes an ultra-narrow gain cavity of a single-frequency Brillouin fiber laser (SFBFL, single-frequency Brillouin fiber laser) to realize ultra-narrow single-passband microwave photonic filtering, To solve the technical problem that the existing microwave photonic filter cannot realize single-pass band filtering below the MHz order, the 3dB bandwidth Δf _3dB of the microwave photonic filter of the present invention can reach the order of kHz or even Hz, and at the same time its center frequency is stable, tunable, and has a high bandwidth. External inhibition ratio is high.

In order to make the objectives, technical solutions and advantages of the present disclosure clearer, the present disclosure will be further described in detail below with reference to the specific embodiments and the accompanying drawings.

An embodiment of the present disclosure provides a kHz-scale single-passband microwave photonic filter. FIG. 1 is a schematic diagram of the system structure of the Hz-scale single-passband microwave photonic filter. Referring to FIG. 1 , the microwave photonic filter includes : laser 100, first optical coupler 200, SSB modulation module 300, SSB modulation module 400, microwave signal source 500, first optical amplifier, second optical amplifier, second optical coupler 600, single Frequency Brillouin fiber laser 700 , photodetector 800 and vector network analyzer 900 .

The laser 100 is connected to the first optical coupler 200, and the first optical coupler 200 is connected to the SSB modulation module 300; the first optical coupler 200 is simultaneously connected to the SSB modulation module 400 connection; the single sideband suppression carrier modulation module 300 is connected with the microwave signal source 500; the single sideband suppression carrier modulation module 300, the first optical amplifier, the second optical coupler 600 and the single frequency Brillouin fiber laser 700 connected in sequence; the single sideband modulation module 400 is connected to the vector network analyzer 900; the single sideband modulation module 400, the second optical amplifier, the second optical coupler 600 and the single frequency Brillouin fiber The lasers 700 are connected in sequence; the single-frequency Brillouin fiber laser 700, the photodetector 800 and the vector network analyzer 900 are connected in sequence.

In a feasible manner of the present disclosure, the light splitting ratio of the first optical coupler 200 is 50%:50%, which includes: a port 210 connected to the laser 100, modulated with a SSB carrier Ports one and two 220 are connected to the module 300 , and ports one and three 230 are connected to the single sideband modulation module 400 .

The working principle of the embodiment of the present disclosure is as follows: the laser 100 generates a laser with a frequency fc, which is input from the port _1-210 of the first optical coupler 200, and then splits into two laser beams with a coupling ratio of 50%:50% , one of the laser beams enters the SSB modulation module 300 from the port 12 220 output, and the other laser beam enters the SSB modulation module 400 from the port 13 230 output; the frequency output from the microwave signal source 500 is f _p The microwave signal is modulated onto the laser fc by the _SSB carrier modulation module 300, and the output signal of the SSB suppression carrier modulation module 300 is the upper sideband (ie the first upper sideband) with the frequency _fc + _fp . After being amplified by the first optical amplifier, it enters the single-frequency Brillouin fiber laser 700 through the second optical coupler 600 as the pump light of the microwave photonic filter; the output from the vector network analyzer 900 is used to measure the frequency response characteristics of the filter The swept-frequency microwave signal f _RF is modulated onto the laser f _c by the single sideband modulation module 400, and the output signal of the single sideband modulation module 400 is an optical carrier with a frequency of f _c and an upper sideband with a frequency of f _c +f _RF ( That is, the second upper sideband), after being amplified by the second optical amplifier, it enters the single-frequency Brillouin fiber laser 700 through the second optical coupler 600; the output signal of the single-frequency Brillouin fiber laser 700 enters the photodetector 800, and the photoelectric detection The microwave signal output by the device 800 is input into the vector network analyzer 900, so as to obtain the frequency response characteristic of the microwave photonic filter of the present disclosure.

In a feasible manner of the present disclosure, the laser 100 is a tunable narrow linewidth laser, which provides an optical carrier for the microwave signal. At the same time, after frequency shifting, as the pump light of the microwave photonic filter, the same laser is used to avoid The center frequency of the microwave photonic filter is unstable due to laser wavelength drift.

In a feasible manner of the present disclosure, the single sideband suppression carrier modulation module 300 includes: a first optical input port 310 connected to the first port two 220, a first optical input port 310 connected to the first optical amplifier The output port 320 and the first microwave input port 330 connected with the microwave signal source 500 .

In a feasible manner of the present disclosure, the single sideband modulation module 400 includes: a second optical input port 410 connected to the port one three 230, and a second optical output port 420 connected to the second optical amplifier , and the second microwave input port 430 connected to the vector network analyzer 900 .

In a feasible manner of the present disclosure, the splitting ratio of the second optical coupler 600 is 50%:50%, which includes: a port 2-610 connected to the single-frequency Brillouin fiber laser 700, Port two 2 620 connected to the first optical amplifier, and port two three 630 connected to the second optical amplifier.

Among them, the SSB modulation module 300 and the SSB modulation module 400 have many schemes that can be implemented, which are not specifically limited in this embodiment of the present disclosure. For example, schemes such as phase modulators and optical filters are adopted, which are not omitted here Repeat.

FIG. 2 is a schematic structural diagram of a single-frequency Brillouin fiber laser in a kHz-level single-passband microwave photonic filter provided by an embodiment of the present disclosure. Referring to FIG. 2 , the single-frequency Brillouin fiber laser 700 includes: sequentially connected optical circulator 710, third optical coupler 720, fourth optical coupler 730, fifth optical coupler 740, optical isolator 750, high nonlinear optical fiber 760; and the high nonlinear optical fiber 760 and the optical ring shaper 710 is connected. The highly nonlinear fiber 760 provides stimulated Brillouin gain.

The optical circulator 710 includes: a port ① 711 connected to the port 2-1 610, a port ① 711 as an input port of the single-frequency Brillouin fiber laser 700, a port ② 712 connected to the high nonlinear fiber 760, and a port ② 712 connected to the high nonlinear fiber 760. The third optical coupler 720 is connected to the port ③ 713 .

The optical splitting ratio of the third optical coupler 720 is 50%:50%, which includes: port three-one 721 connected to port ③ 713 of the optical circulator 710 , port three-two connected to the photodetector 800 722 , the port three-two 722 is used as the output port of the single-frequency Brillouin fiber laser 700 , and the port three-three 723 is connected to the fourth optical coupler 730 .

The light splitting ratio of the fourth optical coupler 730 is x%:(100-x)%, which includes: ports four one 731 connected to ports three three 723 of the third optical coupler 720, respectively The fifth optical coupler 740 is connected to the port four two 732 and the port four four 734, and the port four three 733, and the port four three 733 is a port discarded in the embodiment of the present disclosure.

The optical splitting ratio of the fifth optical coupler 740 is x%:(100-x)%, which includes: port five one 741 connected to the optical isolator 750, port five connected to the port four two 732 Two 742, the port five four 744 connected to the port four four 734, and the port five three 743, the port five three 743 is a port that is discarded in the embodiment of the disclosure.

In the embodiment of the present disclosure, the fourth optical coupler 730 and the fifth optical coupler 740 form an optical fiber ring cavity, and the light beam splitting by the fourth optical coupler 730 with a ratio of (100-x)%, The optical coupler 740 splits the beam with a ratio of x% and transmits in the fiber ring cavity.

In the embodiment of the present disclosure, the optical circulator 710 , the third optical coupler 720 , the fourth optical coupler 730 , the fifth optical coupler 740 , the optical isolator 750 and the high nonlinear optical fiber 760 constitute an optical circuit, forming a Optical resonator, high nonlinear fiber 760 provides stimulated Brillouin gain, and the Brillouin pump light input from port ①711 is used as the excitation source, so as to satisfy the three elements of laser; the fourth optical coupler 730 is coupled with the fifth optical The fiber ring cavity composed of the laser 740 plays the role of frequency selection and stable output laser frequency, thereby ensuring that the output light of the Brillouin fiber laser 700 is stable single-frequency light.

In the embodiment of the present disclosure, the principle of the single-frequency Brillouin fiber laser 700 is described by taking the optical carrier of frequency f _c as an example. The optical carrier f _c is input from port ① 711 , passes through port ② 712 of the optical circulator 710 , and enters the high nonlinearity The optical fiber 760 generates a frequency f _c -f _B in the highly nonlinear optical fiber 760 (f _B is the Brillouin frequency shift of the optical carrier f _c in the highly nonlinear optical fiber 760 ), and the self-published Brillouin scattering signal transmitted in the opposite direction , the spontaneous Brillouin scattering signal passes through the ports ② 712 and ③ 713 of the optical circulator 710, passes through the third optical coupler 720, passes through the fiber ring cavity composed of the fourth optical coupler 730 and the fifth optical coupler 740, and passes through the isolation 750 enters the highly nonlinear optical fiber 760, and the self-published Brillouin scattering signal fc _{- fB} is just within the Brillouin gain spectrum range of the optical carrier _fc , and the transmission direction is opposite to that of the optical carrier _fc ; if the optical carrier fc The power of _c meets the threshold of stimulated Brillouin scattering, the optical carrier f _c interacts with the spontaneous Brillouin scattering signal f _c -f _B in the highly nonlinear fiber 760, and stimulated Brillouin scattering occurs, and spontaneous Brillouin scattering occurs. The scattered signal f _c -f _B is amplified; the spontaneous Brillouin scattering signal f _c -f _B is transmitted multiple times in the optical resonator, if the gain provided by the stimulated Brillouin scattering is greater than the loss of the ring cavity, it will produce a frequency of f _c -f _B laser signal, output from port 32722.

In addition, FIG. 3 is a schematic diagram of the principle of a single-frequency Brillouin fiber laser in a kHz-level single-passband microwave photonic filter provided by an embodiment of the present disclosure. Referring to FIG. 3 , in order to realize a single-frequency Brillouin fiber laser 700 single-frequency laser Output, the fourth optical coupler 730 and the fifth optical coupler 740 form a fiber ring cavity, and the free spectral range FSR1 of the fiber ring cavity and the 3dB bandwidth Δf _ring of the resonance peak satisfy the following conditions:

FSR1≥Δf _B (1)

Δf _ring ≤FSR2 (2)

Among them, Δf _B is the 3dB bandwidth of the Brillouin gain spectrum of the highly nonlinear fiber 760 , FSR2 is the mode spacing of the single-frequency Brillouin fiber laser 700 , and is determined by the optical resonance of the single-frequency Brillouin fiber laser 700 The cavity length L _cav is determined; the 3dB bandwidth Δf _B of the Brillouin gain spectrum of the high nonlinear fiber 760 is about 25MHz. In order to satisfy the condition of formula (1), according to the calculation formula of the free spectral range FSR=c/nL, it can be known from The cavity length of the optical fiber ring cavity formed by the fourth optical coupler 730 and the fifth optical coupler 740 should be less than 8 meters. In addition, the 3dB bandwidth Δf _ring of the resonant peak of the optical fiber ring cavity is inversely proportional to the loss of light transmitting one circle in the ring cavity. )% of the fourth optical coupler 730 and the fifth optical coupler 740 where x≧90.

FIG. 4 is a schematic diagram of the principle of a kHz-level single-passband microwave photonic filter provided by an embodiment of the present disclosure. Referring to FIG. 4 , FIG. 4 (a) is a single sideband suppression carrier modulation module 300 and a single sideband A schematic diagram of the spectrum of the output signal after the output signal of the modulation module 400 is amplified by the first optical amplifier and the second optical amplifier respectively; the output signal of the single sideband modulation module 400 includes an optical carrier with a frequency of f _c and a frequency of f _c + The upper sideband of f _RF , where f _RF is the frequency of the swept microwave signal output by the vector network analyzer 900; after the optical carrier with frequency f _c is amplified by the second optical amplifier, its power value is located in the single-frequency Brillouin fiber Above the excitation power threshold of the laser 700, a laser with a frequency of f _c -f _B is excited, and f _B is the Brillouin frequency shift of the optical carrier f _c in the highly nonlinear fiber 760; the frequency is f _c +f _RF After the upper sideband is amplified by the second optical amplifier, its optical power is below the excitation power threshold of the single-frequency Brillouin fiber laser 700 to generate a backscattered signal; the single-sideband suppresses the output of the carrier modulation module 300 The signal includes an upper sideband with a frequency of f _c +f _p , where f _p is the frequency of the microwave signal output by the microwave signal source 500 . After being amplified by the first optical amplifier, the frequency of the single-frequency Brillouin fiber laser is A gain spectrum is formed in the resonant cavity of 700, the center frequency of the gain spectrum is f _c +f _p -f _B , and the 3dB bandwidth is Δf _SFBFL ;

Referring to Figure (b) in Figure 4, Figure ( _b ) in Figure 4 is a schematic diagram of the output signal spectrum of the single-frequency Brillouin fiber laser 700. The optical carrier power with frequency fc meets the requirements of the single-frequency Brillouin fiber laser. The excitation power threshold of 700 excites the laser with frequency f _c -f _B ; the power of the upper sideband with frequency f _c +f _p is close to the excitation power threshold of single-frequency Brillouin fiber laser 700, forming an ultra-narrow gain spectrum , the center frequency of the gain spectrum is f _c +f _p -f _B , the 3dB bandwidth is Δf _SFBFL ; the upper sideband of the frequency f _c +f _RF does not reach the excitation power threshold of the single-frequency Brillouin fiber laser 700, and only generates power very low backscattered signal; however, when the frequency of the backscattered signal is in the range of the gain spectrum produced by fc + fp (centered at _{fc + fp - fB} , with a 3dB bandwidth of _ΔfSFBFL ), The obtained gain is amplified to excite a laser signal, and this laser signal is coherent with the laser with frequency f _c -f _B , and the two laser signals beat frequency in the photodetector 800 to generate a microwave signal.

Referring to Fig. 4 (c), Fig. 4 (c) is that the microwave signal obtained after the beat frequency of the photodetector 800 is input into the vector network analyzer 900, thereby obtaining the frequency response characteristic of the microwave photonic filter of the present disclosure schematic diagram. The passband center frequency of the microwave photonic filter f _pass =(f _c +f _p -f _B )-(f _c -f _B )=f _p , so by adjusting the frequency f _p , the pass band of the microwave photonic filter can be adjusted The band center frequency f _pass has the advantage of being able to tune the center frequency of a single pass band; it is worth noting that the value of the Brillouin frequency shift f _B is related to the frequency of the pump light, and the Brillouin frequency corresponding to the pump light of different frequencies The brillouin frequency shift f _B is different. In the embodiment of the present disclosure, the difference between the Brillouin frequency shift f _B caused by the frequency difference between the upper sideband with frequency f _c +f _p and the optical carrier with frequency f _c No consideration; the 3dB bandwidth Δf _pass of the microwave photonic filter is equal to the 3dB bandwidth Δf _SFBFL of the gain cavity of the single-frequency Brillouin fiber laser 700 , ie Δf _pass =Δf _SFBFL .

In the embodiment of the present disclosure, the laser 100 is a narrow linewidth laser, and when its linewidth is in the order of Hz, the gain cavity 3dB bandwidth Δf _SFBFL of the single-frequency Brillouin fiber laser 700 can be expressed by the following formula:

Among them, c is the speed of light in vacuum, n _eff is the effective refractive index, L _cav and r are the cavity length of the single-frequency Brillouin fiber laser 700 and the optical amplitude feedback coefficient of the cavity, respectively.

It can be seen from equation (3) that the 3dB width of the gain cavity of the single-frequency Brillouin fiber laser 700, Δf _SFBFL, is determined by its cavity length L _cav and the optical amplitude feedback coefficient r of the cavity, and Δf _SFBFL is inversely proportional to L _cav and r, through Increasing L _cav and increasing r can achieve an ultra-narrow gain cavity Δf _SFBFL .

Ideally, the optical loss of the single-frequency Brillouin fiber laser 700 is equal to the gain when laser resonance occurs, that is, r=1, the gain spectrum Δf _SFBFL =0, and the gain cavity is infinitely narrow. However, due to the influence of spontaneous scattering noise, spontaneous radiation noise, etc., r cannot reach 1, but it can still be close to 1.

Assuming that r=0.99 and the cavity length L _cav is 5 km, then Δf _SFBFL =132.2 Hz. It can be seen from the above theoretical derivation that the single-frequency Brillouin fiber laser 700 can achieve an ultra-narrow 3dB bandwidth Δf _SFBFL . Since the 3dB bandwidth Δf _pass = Δf _SFBFL of the microwave photonic filter, the 3dB bandwidth of the microwave photonic filter of the present disclosure is obtained Significant breakthrough, can reach the order of kHz or even Hz.

As a specific embodiment of the present disclosure, the length of the highly nonlinear optical fiber 760 is selected to be 530 meters, the splitting ratio of the fourth optical coupler 730 and the fifth optical coupler 740 is x=90, and the fourth optical coupler 730 and the fifth optical coupler 740 have a splitting ratio x=90. The cavity length of the optical fiber ring cavity composed of the optical coupler 740 is 4 meters, which satisfies the conditions of formulas (1) and (2). Under the above conditions, the single-frequency characteristics of the single-frequency Brillouin fiber laser 700 are measured, and the microwave Frequency response and center frequency tuning characteristics of photonic filters.

In the above-mentioned specific embodiment, FIG. 5 is the beat frequency spectrogram of the output laser input of the single frequency Brillouin fiber laser 700 into the photodetector, the beat frequency noise is very small, and there is no RF frequency of the adjacent mode beat frequency. The laser 700 is a single frequency laser output.

In the above specific embodiment, FIG. 6 shows the frequency response characteristics of the single-pass band microwave photonic filter of the kHz level. As shown in FIG. 6 , the center frequency of the pass band is f _pass =10 GHz, and Δf _pass ≈ 10 kHz. Corresponding to the optical amplitude feedback coefficient r≈0.92, the 3dB bandwidth of 10kHz conforms to the theoretical calculation of equation (3). Therefore, a significant breakthrough has been made in the 3dB bandwidth Δf _3dB of the microwave photonic filter of the present disclosure, which can reach the order of kHz or even Hz.

In the above-mentioned specific embodiment, FIG. 7 shows the tunable characteristics of the single-passband center frequency of the single-passband microwave photonic filter of the kHz magnitude, and its center frequency _fpass is 5GHz, 10GHz, 15GHz, 20GHz, 25GHz, 30GHz respectively. , 35GHz, 40GHz, the present invention has the advantages of adjustable center frequency, out-of-band rejection ratio and so on.

In the embodiment of the present disclosure, the single-frequency Brillouin fiber laser 700 can also be implemented by other solutions, for example, as shown in FIG. (composed of the fourth optical coupler 730, the fifth optical coupler 740 and the fiber ring) can be composed of a fiber grating Fabry-Perot cavity (the first fiber grating 770 with high reflectivity, the second fiber connected by the fiber grating 780) instead. Furthermore, in order to lower the excitation power threshold of the single frequency Brillouin fiber laser 700, EDFA can be added in the resonator. The improvement scheme of the single-frequency Brillouin fiber laser 700 in the embodiment of the present disclosure will not be repeated one by one.

The specific embodiments described above further describe the purpose, technical solutions and beneficial effects of the present invention in further detail. It should be understood that the above descriptions are only specific embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention, for example: implementing the single-frequency Brillouin fiber laser in the present invention by other means; The highly nonlinear optical fibers of SBS are replaced by other types of optical fibers or by other types of integrated optical waveguides such as chalcogenide optical waveguides; etc., should be included in the protection scope of the present invention.

Claims (6)

1. A single-passband microwave photonic filter of kHz magnitude, comprising: a laser (100), a first optical coupler (200), a single-sideband suppression carrier modulation module (300), and a single-sideband modulation module (400) , a microwave signal source (500), a first optical amplifier, a second optical amplifier, a second optical coupler (600), a single-frequency Brillouin fiber laser (700), a photodetector (800) and a vector network analyzer ( 900), where: A laser with a frequency _fc is generated by a laser (100), and is divided into a first laser beam and a second laser beam after passing through a first optical coupler (200), and the first laser beam enters a single sideband suppression carrier modulation module ( 300), the second laser beam enters the single sideband modulation module (400); The microwave signal with the frequency _fp output by the microwave signal source (500) is modulated onto the first laser beam by the single sideband suppressed carrier modulation module (300) to form a single sideband suppressed carrier modulation signal, the single sideband suppressed carrier The modulated signal includes a first upper sideband f _c +f _p , and the first upper sideband f _c +f _p sequentially passes through the first optical amplifier and the second optical coupler (600) and then enters the single-frequency Brillouin fiber laser (700) ), the power of the first upper sideband f _c + f _p is less than the excitation power threshold of the single-frequency Brillouin fiber laser (700), forming a 3dB bandwidth in the single-frequency Brillouin fiber laser (700) of the order of kHz The gain spectrum of , the center frequency of the gain spectrum is f _c +f _p -f _B , and the 3dB bandwidth is Δf _SFBFL , where f _B is the Brillouin frequency shift; The swept-frequency microwave signal with the frequency f _RF output by the vector network analyzer (900) is modulated onto the second laser beam by the single sideband modulation module (400) to form a single sideband modulation signal, and the single sideband modulation signal includes The optical carrier f _c and the second upper sideband f _c +f _RF , the optical carrier f _c and the second upper sideband f _c +f _RF enter the single frequency after passing through the second optical amplifier and the second optical coupler (600) in sequence a Brillouin fiber laser (700), the power of the optical carrier _fc is greater than the excitation power threshold of the single-frequency Brillouin fiber laser (700), and a laser with a frequency of fc _{- fB} is excited; the second upper edge The optical power with f _c +f _RF is below the excitation power threshold of the single frequency Brillouin fiber laser (700), resulting in a backscattered signal, when the frequency of the backscattered signal is above the first When the gain spectrum is within the frequency range of the gain spectrum produced by f _c +f _p , the gain obtained is amplified, the laser signal is excited, and the laser signal with the frequency f _c -f _B beats in the photodetector (800) to generate microwaves Signal; Wherein, the single-frequency Brillouin fiber laser (700) includes: an optical circulator (710), a third optical coupler (720), a fourth optical coupler (730), and a fifth optical coupler ( 740), an optical isolator (750), a high nonlinear optical fiber (760), and the high nonlinear optical fiber (760) is connected with the optical circulator (710) to form an optical circuit and form an optical resonant cavity; The microwave signal generated by the photodetector (800) is input into a vector network analyzer (900) to obtain the frequency response characteristic of the microwave photonic filter. 2. The single-pass band microwave photonic filter of kHz order of magnitude according to claim 1, wherein the single-pass band center frequency f _pass of the filter is equal to f _p ; the 3dB bandwidth Δf _pass of the filter is equal to The 3dB bandwidth Δf _SFBFL of the gain cavity of the single frequency Brillouin fiber laser (700). 3. The kHz-level single-pass-band microwave photonic filter according to claim 1, wherein the splitting ratio of the first optical coupler (200) is 50%:50%, and the second optical coupler The splitting ratio of the device (600) is 50%:50%. 4. The kHz-level single-pass-band microwave photonic filter according to claim 1, characterized in that, the light splitting ratio of the third optical coupler (720) is 50%:50%, and the fourth optical coupling The light splitting ratio of the optical coupler (730) is x%:(100-x)%, and the light splitting ratio of the fifth optical coupler (740) is x%:(100-x)%, wherein x≥90. 5. The kHz-level single-pass-band microwave photonic filter according to claim 4, wherein the fourth optical coupler (730) and the fifth optical coupler (740) form a fiber ring cavity; The fourth optical coupler (730) includes port four two (732) and port four four (734); the fifth optical coupler (740) includes port five two (742) and port five four (744); Port four two (732) and port five two (742) are connected by optical fiber, and port four four (734) and port five four (744) are connected by optical fiber to form the optical fiber annular cavity; The light beam split by the fourth optical coupler 730 with a ratio of (100-x)% and the beam split by the fifth optical coupler (740) with a ratio of x% are transmitted in the fiber ring cavity. 6. The kHz-order single-passband microwave photonic filter according to claim 5, wherein the free spectral range FSR1 of the optical fiber ring cavity and the 3dB bandwidth Δf _ring of the resonance peak satisfy the following conditions: FSR1≥Δf _B Δf _ring ≤FSR2 Wherein, Δf _B is the 3dB bandwidth of the Brillouin gain spectrum of the highly nonlinear fiber (760), FSR2 is the mode spacing of the single-frequency Brillouin fiber laser (700), and the fiber ring cavity The cavity length is less than 8 meters.

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