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

kHz-magnitude single-passband microwave photonic filter Download PDF

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CN109638621B
CN109638621B CN201910057155.1A CN201910057155A CN109638621B CN 109638621 B CN109638621 B CN 109638621B CN 201910057155 A CN201910057155 A CN 201910057155A CN 109638621 B CN109638621 B CN 109638621B
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frequency
optical
laser
optical coupler
brillouin
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CN109638621A (en
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文花顺
祝宁华
李明
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Institute of Semiconductors of CAS
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/06754—Fibre amplifiers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08—Construction or shape of optical resonators or components thereof
    • H01S3/081—Construction or shape of optical resonators or components thereof comprising three or more reflectors
    • H01S3/083—Ring lasers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/102—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the active medium, e.g. by controlling the processes or apparatus for excitation
    • H01S3/1022—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the active medium, e.g. by controlling the processes or apparatus for excitation by controlling the optical pumping

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  • Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)
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Abstract

The present disclosure provides a kHz-order single-passband microwave photonic filter, comprising: the device comprises 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, a second optical coupler, a single-frequency Brillouin optical fiber laser, a photoelectric detector and a vector network analyzer. The gain cavity of the single-frequency Brillouin fiber laser is utilized to realize ultra-narrow single-pass band microwave photon filtering, the technical problem that the existing microwave photon filter cannot realize single-pass band filtering below the MHz magnitude is solved, the 3dB bandwidth of the filter provided by the disclosure is remarkably broken through, the kHz magnitude can be reached, and meanwhile, the central frequency of the filter is stable and tunable, and the out-of-band rejection ratio is high.

Description

kHz-magnitude single-passband microwave photonic filter
Technical Field
The disclosure relates to the field of microwave photon signal processing and electronic countermeasure, in particular to a kHz-magnitude single-passband microwave photon filter.
Background
Microwave photonics is a product of microwave and photon technology fusion, and has wide application prospects in the aspects of radio-frequency signal generation, transmission, processing and the like. The microwave photonic filter processes a microwave signal modulated on an optical carrier in an optical domain by adopting a photonic technology to finally realize a filtering function, has the advantages that the traditional electric domain filter is difficult to compare with in the aspects of bandwidth, tuning, reconstruction and the like, is a core technology in the field of microwave photonics, and causes extensive research of people.
The single-band-pass filtering technology with one pass band overcomes the defect of frequency response periodicity, is widely applied to the fields of wireless communication, sensing, biology, military and the like, and the key index for measuring the filtering characteristic of the single-band-pass is the 3dB bandwidth delta f of the single pass band3dBFor a single pass band filter, the 3dB bandwidth Δ f3dBThe smaller the frequency selective characteristic of the filter, the better the desired frequency can be filtered out more accurately, thus achieving an ultra-narrow 3dB bandwidth Δ f3dBSingle-passband microwave photonic filters have become a research hotspot.
At present, there are various schemes for implementing ultra-narrow single-pass band microwave photon filtering in the prior art, such as: a single-passband microwave photon filter based on a broadband light source and a Mach-Zehnder interference structure is providedAnd obtaining a plurality of tap coefficients by sampling the broadband optical signal and weighting the sampled signal by the Mach-Zehnder interferometer, thereby realizing single-pass band filtering with a 3dB bandwidth delta f3dBSeveral hundred MHz; a microwave photon single-pass band filter based on stimulated Brillouin scattering is provided, which utilizes the narrow-band gain characteristic of stimulated Brillouin scattering to process a microwave modulated optical signal to realize single-pass band filtering, and the bandwidth delta f of the filter is 3dB3dCan reach 24.4 MHz; the patent document with publication number 103715480B discloses an invention named as 'a single-bandpass tunable microwave photonic filter with ultra-high quality factor', which effectively reduces the gain spectrum width of stimulated Brillouin scattering by a method of superposing the gain spectrum and the loss spectrum of the stimulated Brillouin scattering in an optical fiber, realizes the single-bandpass microwave photonic filtering technology with ultra-high quality factor, and has a 3dB bandwidth delta f3dB4.14MHz, and the adjustable range of the central frequency is 0.3GHz-29.7 GHz.
Although a plurality of single-passband microwave photonic filter schemes exist at present, the key index of the single-passband microwave photonic filter is 3dB bandwidth delta f3dBThe filter is still in MHz level due to limitation, narrower (kHz level or even Hz level) single-passband microwave photon filtering cannot be realized, and the application fields of high-purity spectrum microwave signal generation, high-resolution microwave photon sensing, high-performance microwave photon radar and the like cannot be met.
Disclosure of Invention
Technical problem to be solved
The technical scheme of the existing microwave photon filter is difficult to realize single-pass band filtering below 3dB bandwidth MHz magnitude and cannot meet the application fields of high-purity spectrum microwave signal generation, high-resolution microwave photon sensing, high-performance microwave photon radar and the like.
(II) technical scheme
The disclosed embodiment provides a kHz-magnitude single-passband microwave photonic filter, which comprises: laser instrument, first optical coupler, single sideband restrain carrier modulation module, single sideband modulation module, microwave signal source, first optical amplifier, second optical coupler, single-frequency Brillouin fiber laser ware, photoelectric detector and vector network analyzer, wherein:
generated by a laser at a frequency fcThe laser light is divided into a first laser beam and a second laser beam after passing through the first optical coupler, the first laser beam enters the single-sideband suppression carrier modulation module, and the second laser beam enters the single-sideband modulation module;
the frequency of the output of the microwave signal source is fpThe microwave signal is modulated on the first laser beam through the single-sideband suppressed carrier modulation module to form a single-sideband suppressed carrier modulation signal, and the single-sideband suppressed carrier modulation signal comprises a first upper sideband fc+fpSaid first upper sideband fc+fpThe single-frequency Brillouin fiber laser enters a single-frequency Brillouin fiber laser after sequentially passing through a first optical amplifier and a second optical coupler, and the first upper sideband fc+fpThe power of the single-frequency Brillouin optical fiber laser is smaller than the excitation power threshold value of the single-frequency Brillouin optical fiber laser, a gain spectrum with the 3dB bandwidth of the kHz magnitude is formed in the single-frequency Brillouin optical fiber laser, and the center frequency of the gain spectrum is fc+fp-fB3dB bandwidth of delta fSFBFLWherein f isBIs a brillouin frequency shift.
The output frequency of the vector network analyzer is fRFThe swept frequency microwave signal is modulated onto the second laser beam through a single-sideband modulation module to form a single-sideband modulation signal, and the single-sideband modulation signal comprises an optical carrier fcAnd a second upper sideband fc+fRFSaid optical carrier fcAnd a second upper sideband fc+fRFThe optical carrier f enters a single-frequency Brillouin optical fiber laser after passing through a second optical amplifier and a second optical coupler in sequence, andcthe power of the fiber is larger than the excitation power threshold of the single-frequency Brillouin fiber laser, and the excitation frequency is fc-fBThe laser of (1); the second upper sideband fc+fRFIs below the excitation power threshold of the single-frequency brillouin fiber laser, generates a backscatter signal, and when the frequency of the backscatter signal is in a first upper sideband fc+fpResulting gain spectrum (center frequency f)c+fp-fB3dB bandwidth of delta fSFBFL) In the frequency range of (1), obtainGain is amplified, laser signal is excited, and frequency is fc-fBThe laser of (2) beats in the photoelectric detector to generate a microwave signal;
and microwave signals generated by the photoelectric detector are input into a vector network analyzer to obtain the frequency response characteristic of the microwave photon filter.
Optionally, the filter has a single passband center frequency fpassIs equal to fp(ii) a 3dB bandwidth of the filter Δ fpassEqual to the 3dB bandwidth delta f of the gain cavity of the single-frequency Brillouin fiber laserSFBFL。
Optionally, the first optical coupler has a splitting ratio of 50% to 50% and the second optical coupler has a splitting ratio of 50% to 50%.
Optionally, the single-frequency brillouin fiber laser comprises: the optical fiber coupler comprises an optical circulator, a third optical coupler, a fourth optical coupler, a fifth optical coupler, an optical isolator and a high nonlinear optical fiber which are sequentially connected, wherein the high nonlinear optical fiber is connected with the optical circulator to form an optical loop and form an optical resonant cavity.
Optionally, the third optical coupler has a splitting ratio of 50% to 50%, the fourth optical coupler has a splitting ratio of x% to (100-x)%, and the fifth optical coupler has a splitting ratio of x% to (100-x)%, preferably, x is greater than or equal to 90.
Optionally, the fourth optical coupler and the fifth optical coupler constitute a fiber ring cavity;
the fourth optical coupler comprises port four and port four; the fifth optical coupler comprises a port five two and a port five four;
connecting the port four to the port five to the port two through optical fibers, and connecting the port four to the port five to the port four through the optical fibers to form an optical fiber ring cavity;
the light beam split by the fourth optical coupler in proportion of (100-x)% and the light beam split by the fifth optical coupler in proportion of x% are transmitted in the optical fiber looping cavity.
Optionally, the free spectral range FSR1 of the fiber ring cavity is shared with3dB bandwidth delta f of vibration peakringThe following conditions are satisfied:
FSR1≥ΔfB
Δfring≤FSR2
wherein, Δ fBThe FSR2 is the mode spacing of the single-frequency brillouin fiber laser for a 3dB bandwidth of the brillouin gain spectrum of the high nonlinearity fiber, preferably the cavity length of the fiber ring cavity is less than 8 meters.
(III) advantageous effects
According to the technical scheme, the kHz-magnitude single-passband microwave photonic filter disclosed by the invention has at least one or part of the following beneficial effects:
1. the single-sideband modulated optical signal is injected into the single-frequency Brillouin optical fiber laser, and the ultra-narrow optical gain cavity of the single-frequency Brillouin optical fiber laser is used for optical amplification, so that the problem that the existing microwave photonic filter cannot realize single-passband filtering below the MHz level is solved, and the 3dB bandwidth delta f of the microwave photonic filter is disclosed3dBThe method makes a remarkable breakthrough and can reach the kHz magnitude and even the Hz magnitude.
2. By adjusting the frequency of the pumping light injected into the single-frequency Brillouin fiber laser, the center frequency of the filter can be adjusted, and the center frequency of the filter has the advantage of fine and adjustable center frequency.
3. The single-frequency Brillouin fiber laser has stable output laser frequency, so that the microwave photon filter has stable center frequency and is slightly influenced by the environment.
4. The radio frequency signal in the pass band range of the microwave photon filter is generated by beating two beams of laser, only one beam of laser is generated outside the pass band range, and no beating radio frequency signal exists, so that the microwave photon filter has the advantage of high out-of-band rejection ratio.
Drawings
For a more complete understanding of the present disclosure and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a schematic structural diagram of a kHz-level single-passband microwave photonic filter provided by an embodiment of the present disclosure;
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;
FIG. 3 is a schematic diagram of a single-frequency Brillouin fiber laser in a kHz-scale single-passband microwave photonic filter provided by an embodiment of the present disclosure;
FIG. 4 is a schematic diagram of a kHz scale single passband microwave photonic filter provided by embodiments of the present disclosure;
FIG. 5 is a beat frequency spectrum diagram obtained by a detector entering laser output by a single-frequency Brillouin fiber laser in a kHz-magnitude single-passband microwave photonic filter provided by the embodiment of the present disclosure;
FIG. 6 is a graph of the frequency response characteristic of a kHz scale single passband microwave photonic filter provided by an embodiment of the present disclosure;
FIG. 7 is a center frequency tunable characteristic diagram of a kHz-scale single-passband microwave photonic filter provided by an embodiment of the disclosure;
fig. 8 is another scheme of a single-frequency brillouin fiber laser in a kHz-order single-passband microwave photonic filter provided by an embodiment of the present disclosure.
Wherein the reference numerals are:
100. a laser; 200. a first optical coupler; 300. a single sideband suppression carrier modulation module; 400. a single sideband modulation module; 500. a microwave signal source; 600. a second optical coupler; 700. a single-frequency Brillouin fiber laser; 800. a photodetector; 900. a vector network analyzer;
210. one port is provided; 220. a first port; 230. a first port and a third port;
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. a first port; 620. a second port; 630. a third port;
710. an optical circulator; 720. a third optical coupler; 730. a fourth optical coupler; 740. a fifth optical coupler; 750. an optical isolator; 760. a highly nonlinear optical fiber;
711. port ① (single frequency Brillouin fiber laser input port) 712, port ②; 713, port ③;
721. a port three is one; 722. port three and two (single-frequency brillouin fiber laser output port); 723. a third port;
731. a port four is one; 732. port four or two; 733. port four and port three; 734. port four;
741. a port five; 742. a fifth port; 743. port five and port three; 744. port five four;
770. a first fiber grating; 780. a second fiber grating.
Detailed Description
The invention provides a kHz-magnitude single-passband microwave photonic filter, which realizes ultra-narrow single-passband microwave photonic filtering by utilizing an ultra-narrow gain cavity of a single-frequency Brillouin fiber laser (SFBF L), solves the technical problem that the conventional microwave photonic filter cannot realize single-passband filtering below MHz magnitude, and has 3dB bandwidth delta f of the microwave photonic filter3dBThe frequency can reach kHz or even Hz magnitude, and meanwhile, the center frequency is stable and tunable, and the out-of-band rejection ratio is high.
For the purpose of promoting a better understanding of the objects, aspects and advantages of the present disclosure, reference is made to the following detailed description taken in conjunction with the accompanying drawings.
The embodiment of the present disclosure provides a kHz-level single-passband microwave photonic filter, fig. 1 is a schematic diagram of a system structure of the Hz-level single-passband microwave photonic filter, and referring to fig. 1, the microwave photonic filter includes: the single-sideband optical fiber broadband microwave signal processing device comprises a laser 100, a first optical coupler 200, a single-sideband suppressed carrier modulation module 300, a single-sideband modulation module 400, a microwave signal source 500, a first optical amplifier, a second optical coupler 600, a single-frequency Brillouin optical fiber laser 700, a photoelectric detector 800 and a vector network analyzer 900.
The laser 100 is connected with a first optical coupler 200, and the first optical coupler 200 is connected with a single sideband suppressed carrier modulation module 300; the first optical coupler 200 is simultaneously connected with the single sideband modulation module 400; the single sideband suppressed carrier modulation module 300 is connected with a microwave signal source 500; the single-sideband suppressed carrier modulation module 300, the first optical amplifier, the second optical coupler 600 and the single-frequency brillouin fiber laser 700 are connected in sequence; the single sideband modulation module 400 is connected with 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 laser 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 one possible aspect of the present disclosure, the first optical coupler 200 has a splitting ratio of 50% to 50%, and includes: ports 210 to which the laser 100 is connected, one port two 220 to which the single sideband suppressed carrier modulation module 300 is connected, and one port three 230 to which the single sideband modulation module 400 is connected.
The working principle of the embodiment of the disclosure is as follows: generation of a frequency f by laser 100cThe laser beams are input from the ports 210 of the first optical coupler 200 one by one, and then split into two laser beams according to the coupling ratio of 50% to 50%, wherein one laser beam is output from the port two and 220 and enters the single-sideband suppression carrier modulation module 300, and the other laser beam is output from the port one and three 230 and enters the single-sideband modulation module 400; the frequency f output from the microwave signal source 500pThe microwave signal is modulated to the laser f by the single sideband suppression carrier modulation module 300cThe output signal of the single sideband suppressed carrier modulation module 300 is at frequency fc+fpThe upper sideband (i.e. the first upper sideband) of the microwave photonic filter is amplified by the first optical amplifier and then enters the single-frequency brillouin fiber laser 700 through the second optical coupler 600 to be used as the pump light of the microwave photonic filter; swept-frequency microwave signal f for measuring filter frequency response characteristics output from vector network analyzer 900RFModulation to laser f by single sideband modulation module 400cThe output signal of the single-sideband modulation module 400 is at frequency fcOptical carrier wave of fc+fRFUpper sideband (i.e., secondary upper sideband) ofThe amplified second optical amplifier 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 microwave signal output by the photodetector 800 enters the vector network analyzer 900, so as to obtain the frequency response characteristic of the microwave photonic filter of the present disclosure.
In one possible embodiment of the present disclosure, the laser 100 is a tunable narrow linewidth laser, which provides a microwave carrier for the microwave signal, and the same laser is used as the pump light of the microwave photonic filter after frequency shift, so as to avoid unstable center frequency of the microwave photonic filter due to wavelength drift of the laser.
In one possible implementation manner of the present disclosure, the single sideband suppressed carrier modulation module 300 includes: a first optical input port 310 connected to the second port 220, a first optical output port 320 connected to the first optical amplifier, and a first microwave input port 330 connected to the microwave signal source 500.
In one possible manner of the present disclosure, the single sideband modulation module 400 includes: a second optical input port 410 connected to the first port three 230, a second optical output port 420 connected to the second optical amplifier, and a second microwave input port 430 connected to the vector network analyzer 900.
In one possible aspect of the present disclosure, the second optical coupler 600 has a splitting ratio of 50% to 50%, and includes: the first port 610 connected with the single-frequency brillouin fiber laser 700, the second port 620 connected with the first optical amplifier, and the second port 630 connected with the second optical amplifier.
The single-sideband suppressed carrier modulation module 300 and the single-sideband modulation module 400 have many schemes that can be implemented, which are not specifically limited in the embodiments of the present disclosure, for example, schemes such as a phase modulator and an optical filter are adopted, and are not described herein again.
Fig. 2 is a schematic structural diagram of a single-frequency brillouin fiber laser in a kHz-order single-passband microwave photonic filter provided in an embodiment of the present disclosure, and referring to fig. 2, the single-frequency brillouin fiber laser 700 includes: an optical circulator 710, a third optical coupler 720, a fourth optical coupler 730, a fifth optical coupler 740, an optical isolator 750 and a high nonlinear optical fiber 760 which are connected in sequence; and the high nonlinear fiber 760 is connected to the optical circulator 710. Where the high nonlinear fiber 760 provides stimulated brillouin gain.
The optical circulator 710 includes a port ① 711 connected to the first port 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 ③ 713 connected to the third optical coupler 720.
The third optical coupler 720 has a splitting ratio of 50% to 50%, and includes three ports, one 721, connected to the port ③ 713 of the optical circulator 710, three ports, two ports 722 connected to the photodetector 800, the three ports, two ports 722 serving as output ports of the single-frequency brillouin fiber laser 700, and three ports, three ports 723 connected to the fourth optical coupler 730.
The splitting ratio of the fourth optical coupler 730 is x%: (100-x)%, and comprises: a port four-one 731 connected to a port three-three 723 of the third optical coupler 720, a port four-two 732 and a port four-four 734 respectively connected to the fifth optical coupler 740, and a port four-three 733, the port four-three 733 being a port that is abandoned in the embodiments of the present disclosure.
The splitting ratio of the fifth optical coupler 740 is x%: (100-x)%, and includes: a port fife-one 741 connected to the optical isolator 750, a port fife-two 742 connected to the port forty-two 732, a port fife-four 744 connected to the port forty-four 734, and a port fife-three 743, the port fife-three 743 being a port that is not used in the disclosed embodiment.
In the embodiment of the present disclosure, the fourth optical coupler 730 and the fifth optical coupler 740 form a fiber ring cavity, and the light beam split by the fourth optical coupler 730 is transmitted in the fiber ring cavity in proportion to (100-x)% and the light beam split by the fifth optical coupler 740 is transmitted in proportion to x%.
In the embodiment of the 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 form an optical loop to form an optical resonant cavity, the high nonlinear optical fiber 760 provides a stimulated brillouin gain, the brillouin pump light input from the port ① 711 is used as an excitation source, so as to satisfy three elements of laser, and the optical fiber loop cavity formed by the fourth optical coupler 730 and the fifth optical coupler 740 plays roles in frequency selection and stable output of laser frequency, so as to ensure that the output light of the brillouin fiber laser 700 is stable single-frequency light.
In the disclosed embodiment, at frequency fcThe optical carrier f is an example illustrating the principle of the single-frequency brillouin fiber laser 700cInput from port ① 711, through port ② 712 of optical circulator 710, into high nonlinear fiber 760, where a frequency f is generated in high nonlinear fiber 760c-fB(fBAs optical carrier fcBrillouin frequency shift in high nonlinear optical fiber 760), a backward-transmitted spontaneous brillouin scattering signal that passes through port ② 712 and port ③ 713 of optical circulator 710, passes through third optical coupler 720, passes through the fiber ring cavity composed of fourth optical coupler 730 and fifth optical coupler 740, passes through isolator 750, enters high nonlinear optical fiber 760, and a spontaneous brillouin scattering signal fc-fBJust at the optical carrier fcAnd with the optical carrier fcThe transmission direction is opposite; if the optical carrier fcSatisfies the stimulated Brillouin scattering threshold value, the optical carrier fcWith spontaneous brillouin scattering signal fc-fBInteract in the highly nonlinear optical fiber 760 to generate stimulated Brillouin scattering and spontaneous Brillouin scattering signal fc-fBIs amplified; spontaneous brillouin scattering signal fc-fBMultiple transmission in the optical resonant cavity, if the gain provided by the stimulated Brillouin scattering is larger than the loss of the ring cavity, the frequency f is generatedc-fBThe laser signal of (2) is output from the port three or two 722.
Additionally, FIG. 3 illustrates kHz-scale single-passband microwave photons provided by embodiments of the present disclosureReferring to fig. 3, in order to realize single-frequency laser output of the single-frequency brillouin fiber laser 700, 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 Δ f of the formant are in the same sizeringThe following conditions are satisfied:
FSR1≥ΔfB(1)
Δfring≤FSR2 (2)
wherein, Δ fBThe FSR2 is the mode spacing of the single-frequency Brillouin fiber laser 700 at a 3dB bandwidth of the Brillouin gain spectrum of the highly nonlinear fiber 760, measured by the optical cavity length L of the single-frequency Brillouin fiber laser 700cavDetermining; the 3dB bandwidth Δ f of the Brillouin gain spectrum of the highly nonlinear optical fiber 760BAbout 25MHz, the cavity length of the fiber ring cavity formed by the fourth optical coupler 730 and the fifth optical coupler 740 should be less than 8 meters according to the free spectral range calculation formula FSR ═ c/n L to satisfy the condition of formula (1)ringIn inverse proportion to the loss of light transmitting one turn in the ring cavity, the fourth optical coupler 730 and the fifth optical coupler 740 having lower loss and splitting ratio x ≧ 90 in x%: 100-x%) are selected in order to satisfy the condition of expression (2).
Fig. 4 is a schematic diagram of a kHz-scale single-passband microwave photonic filter provided in an embodiment of the present disclosure, referring to fig. 4, where (a) in fig. 4 is a schematic diagram of a spectrum of an output signal of the single-sideband suppressed carrier modulation module 300 and the single-sideband modulation module 400 after the output signal is amplified by the first optical amplifier and the second optical amplifier, respectively; the output signal of the single sideband modulation module 400 comprises a frequency fcOptical carrier wave of fc+fRFOf (b) wherein fRFIs the frequency of the swept microwave signal output by the vector network analyzer 900; frequency fcAfter the optical carrier is amplified by the second optical amplifier, the power value of the optical carrier is above the threshold of the excitation power of the single-frequency brillouin fiber laser 700, and the excitation frequency is fc-fBLaser of (f)BAs optical carrier fcA brillouin frequency shift at the highly nonlinear optical fiber 760; frequency fc+fRFAfter the upper sideband of the single-frequency brillouin fiber laser 700 is amplified by the second optical amplifier, the optical power of the upper sideband is below the threshold of the excitation power of the single-frequency brillouin fiber laser 700, and a backscattering signal is generated; the output signal of the single sideband suppressed carrier modulation module 300 comprises a frequency fc+fpWherein, fpA gain spectrum is formed in the resonant cavity of the single-frequency brillouin fiber laser 700 after the frequency of the microwave signal output by the microwave signal source 500 is amplified by the first optical amplifier, and the center frequency of the gain spectrum is fc+fp-fB3dB bandwidth of Δ fSFBFL；
Referring to a graph (b) in fig. 4, the graph (b) in fig. 4 is a schematic diagram of an output signal spectrum of the single-frequency brillouin fiber laser 700 at a frequency fcThe optical carrier power of (a) satisfies the excitation power threshold of the single-frequency brillouin fiber laser 700, and the excitation frequency is fc-fBThe laser of (1); frequency fc+fpThe power of the upper sideband approaches the excitation power threshold of the single-frequency Brillouin fiber laser 700 to form an ultra-narrow gain spectrum, and the center frequency of the gain spectrum is fc+fp-fB3dB bandwidth of delta fSFBFL(ii) a Frequency fc+fRFThe upper sideband of (a) does not reach the excitation power threshold of the single-frequency brillouin fiber laser 700, and only generates a backscattering signal with very low power; however, when the frequency of the backscattered signal is at fc+fpResulting gain spectrum (center frequency f)c+fp-fB3dB bandwidth of delta fSFBFL) Within the range, gain is amplified to excite laser signal with frequency fc-fBThe two laser signals are beaten in the photodetector 800 to generate a microwave signal.
Referring to fig. 4 (c), fig. 4 (c) is a diagram of microwave signals obtained after the beat frequency of the photodetector 800 is input into the vector network analyzer 900, so as to obtain microwave photon filtering according to the present disclosureSchematic diagram of the frequency response characteristic of the device. The passband center frequency f of the microwave photonic filterpass＝(fc+fp-fB)-(fc-fB)＝fpThereby by adjusting the frequency fpThe center frequency f of the passband of the microwave photonic filter can be adjustedpassThe single-pass band has the advantage of tunable center frequency; notably, the brillouin frequency shift fBThe value of (A) is related to the frequency of the pump light, and the Brillouin frequency shift f corresponding to the pump light with different frequenciesBWith some difference, in the disclosed embodiment, the frequency is fc+fpHas an upper sideband with a frequency of fcIs generated due to the frequency differenceBThe difference of (a) is not considered; the 3dB bandwidth delta f of the microwave photon filterpassEqual to the 3dB bandwidth Δ f of the gain cavity of the single frequency Brillouin fiber laser 700SFBFLI.e. Δ fpass＝ΔfSFBFL。
In the embodiment of the present disclosure, the laser 100 is a narrow linewidth laser, and the gain cavity 3dB bandwidth Δ f of the single-frequency brillouin fiber laser 700 is in the case that the linewidth is in the Hz orderSFBFLCan be represented by the following formula:
Figure BDA0001952244070000111
wherein c is the vacuum light velocity, neffL as effective refractive indexcavAnd r are the cavity length of the single-frequency brillouin fiber laser 700 and the optical amplitude feedback coefficient of the cavity, respectively.
From the equation (3), it can be seen that the gain cavity of the single-frequency brillouin fiber laser 700 has a 3dB width Δ fSFBFLBy its lumen length LcavAnd the optical amplitude feedback coefficient r of the cavity, and Δ fSFBFLAnd LcavR is inversely proportional, by increasing LcavAnd r is increased, the ultra-narrow gain cavity delta f can be realizedSFBFL。
Ideally, when the single-frequency brillouin fiber laser 700 generates laser resonance, the optical loss is equal to the gain, i.e., r is 1, and the gain spectrum Δ f isSFBFL0, the gain cavity is infiniteAnd (3) narrow. But r may not reach 1 but may still approach 1 due to the effects of spontaneous scattering noise, spontaneous radiation noise, etc.
Suppose r is 0.99 and the cavity length is LcavAt 5km, Δ fSFBFL132.2 Hz. The theoretical derivation shows that the single-frequency Brillouin optical fiber laser 700 can realize the ultra-narrow 3dB bandwidth delta fSFBFLDue to the 3dB bandwidth Δ f of the microwave photonic filterpass＝ΔfSFBFLThus, the 3dB bandwidth of the microwave photonic filter of the present disclosure can reach kHz and even Hz levels with a significant breakthrough.
As a specific embodiment of the present disclosure, the length of the high nonlinear optical fiber 760 is 530 meters, the splitting ratio x of the fourth optical coupler 730 to the fifth optical coupler 740 is 90, the cavity length of the fiber ring cavity formed by the fourth optical coupler 730 and the fifth optical coupler 740 is 4 meters, and the conditions of the formulas (1) and (2) are satisfied, under which the single-frequency characteristic of the single-frequency brillouin fiber laser 700, the frequency response of the microwave photon filter, and the center frequency tuning characteristic are measured.
In the foregoing specific embodiment, fig. 5 is a beat frequency spectrum diagram of the single-frequency brillouin fiber laser 700 with output laser input into the photodetector, where beat frequency noise is very small and there is no radio frequency beat frequency from an adjacent mode, which indicates that the laser 700 is single-frequency laser output.
In the above embodiment, FIG. 6 shows the frequency response characteristic of the kHz-scale single-passband microwave photonic filter, as shown in FIG. 6, with the passband center frequency fpass＝10GHz，Δf pass10 kHz. The corresponding optical amplitude feedback coefficient r ≈ 0.92, and the 3dB bandwidth 10kHz conforms to the theoretical calculation of the formula (3). Thus, the 3dB bandwidth Δ f of the microwave photonic filter of the present disclosure3dBThe method makes a remarkable breakthrough and can reach the kHz or even Hz magnitude.
In the above embodiment, fig. 7 shows the tunable characteristic of the center frequency of the single passband of the kHz-order single-passband microwave photonic filter, where the center frequency fpass5GHz, 10GHz, 15GHz, 20GHz, 25GHz, 30GHz, 35GHz, 40GHz respectively, the invention has the advantages of adjustable center frequency, high out-of-band rejection ratio, etc.
In the embodiment of the present disclosure, the single-frequency brillouin fiber laser 700 may also be implemented by other schemes, for example, as shown in fig. 8, a fiber ring cavity (composed of the fourth optical coupler 730, the fifth optical coupler 740, and a fiber ring) in the single-frequency brillouin fiber laser 700, which plays a role in frequency selection and stabilization, may be replaced by a fiber grating fabry-perot cavity (composed of the first fiber grating 770 and the second fiber grating 780 which are connected by optical fibers and have high reflectivity). Furthermore, to lower the excitation power threshold of the single frequency brillouin fiber laser 700, an EDFA may be added to the cavity. The details of the improved scheme of the single-frequency brillouin fiber laser 700 in the embodiment of the present disclosure are not repeated.
The above-mentioned embodiments, objects, technical solutions and advantages of the present invention are further described in detail, it should be understood that the above-mentioned embodiments are only examples of the present invention, and are not intended to limit the present invention, and any modifications, equivalent substitutions, improvements and the like can be made within the spirit and principle of the present invention, for example: the single-frequency Brillouin fiber laser is realized by other modes; the highly nonlinear optical fiber for generating stimulated brillouin scattering in the present invention is replaced by other kinds of optical fibers or other types of integrated optical waveguides such as chalcogenide (chalcogenide) optical waveguides; and the like, are intended to be included within the scope of the present invention.

Claims (6)

1. A kHz-scale single-passband microwave photonic filter comprising: laser (100), first optical coupler (200), single sideband suppression carrier modulation module (300), single sideband modulation module (400), microwave signal source (500), first optical amplifier, second optical coupler (600), single-frequency brillouin fiber laser (700), photoelectric detector (800) and vector network analyzer (900), wherein:
generating a frequency f by a laser (100)cThe laser is divided into a first laser beam and a second laser beam after passing through a first optical coupler (200), the first laser beam enters a single sideband suppression carrier modulation module (300), and the second laser beam enters a single sideband suppression carrier modulation moduleA sideband modulation module (400);
the frequency of the output of the microwave signal source (500) is fpThe microwave signal is modulated on the first laser beam through a single sideband suppressed carrier modulation module (300) to form a single sideband suppressed carrier modulation signal, and the single sideband suppressed carrier modulation signal comprises a first upper sideband fc+fpSaid first upper sideband fc+fpThe single-frequency Brillouin optical fiber laser enters a single-frequency Brillouin optical fiber laser (700) after passing through a first optical amplifier and a second optical coupler (600) in sequence, and the first upper sideband fc+fpIs less than the excitation power threshold of the single-frequency Brillouin fiber laser (700), a gain spectrum with a 3dB bandwidth of kHz magnitude is formed in the single-frequency Brillouin fiber laser (700), and the center frequency of the gain spectrum is fc+fp-fB3dB bandwidth of Δ fSFBFLWherein f isBIs a Brillouin frequency shift;
the output frequency of the vector network analyzer (900) is fRFThe swept frequency microwave signal is modulated onto a second beam of laser light by a single sideband modulation module (400) to form a single sideband modulation signal, which comprises an optical carrier fcAnd a second upper sideband fc+fRFSaid optical carrier fcAnd a second upper sideband fc+fRFThe optical carrier wave F enters a single-frequency Brillouin optical fiber laser (700) after passing through a second optical amplifier and a second optical coupler (600) in sequence, and the optical carrier wave fcIs greater than the excitation power threshold of the single-frequency Brillouin fiber laser (700), and the excitation frequency is fc-fBThe laser of (1); the second upper sideband fc+fRFIs below an excitation power threshold of the single-frequency brillouin fiber laser (700), producing a backscatter signal, when the frequency of the backscatter signal is at the first upper sideband fc+fpIn the frequency range of the generated gain spectrum, the gain is amplified, a laser signal is excited, and the frequency is fc-fBThe laser of (2) beats in the photoelectric detector (800) to generate a microwave signal;
wherein the single-frequency Brillouin fiber laser (700) comprises: the optical fiber coupler comprises an optical circulator (710), a third optical coupler (720), a fourth optical coupler (730), a fifth optical coupler (740), an optical isolator (750) and a high nonlinear optical fiber (760) which are sequentially connected, wherein the high nonlinear optical fiber (760) is connected with the optical circulator (710) to form an optical loop and form an optical resonant cavity;
microwave signals generated by the photoelectric detector (800) are input into a vector network analyzer (900) to obtain the frequency response characteristic of the microwave photon filter.
2. The single passband microwave photonic filter on the order of kHz, according to claim 1, wherein the filter has a single passband center frequency fpassIs equal to fp(ii) a 3dB bandwidth of the filter Δ fpassA 3dB bandwidth Δ f equal to a gain cavity of the single frequency Brillouin fiber laser (700)SFBFL。
3. The kHz-scale single-passband microwave photonic filter of claim 1, wherein the first optical coupler (200) has a splitting ratio of 50% to 50% and the second optical coupler (600) has a splitting ratio of 50% to 50%.
4. The kHz scale single passband microwave photonic filter of claim 1, wherein the third optical coupler (720) has a split ratio of 50% to 50%, the fourth optical coupler (730) has a split ratio of x% to (100-x)%, and the fifth optical coupler (740) has a split ratio of x% to (100-x)%, wherein x is greater than or equal to 90.
5. The single-passband microwave photonic filter on the order of kHz, 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 (734); the fifth optical coupler (740) includes port five two (742) and port five four (744);
connecting a port four (732) with a port five (742) through an optical fiber, and connecting a port four (734) with a port five (744) through an optical fiber to form the optical fiber ring cavity;
a proportion of (100-x)% of the light beam split by the fourth optical coupler 730 and a proportion of x% of the light beam split by the fifth optical coupler (740) are transmitted in the fiber circulation cavity.
6. The single-passband microwave photonic filter on the order of kHz, according to claim 5, wherein the free spectral range FSR1 of the fiber ring cavity is associated with a 3dB bandwidth of the formants, Δ fringThe following conditions are satisfied:
FSR1≥ΔfB
Δfring≤FSR2
wherein, Δ fBFor a 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 annular cavity has a cavity length of less than 8 meters.
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