CN115267974B - Narrow-band tunable microwave photon filter based on Brillouin fiber laser - Google Patents
Narrow-band tunable microwave photon filter based on Brillouin fiber laser Download PDFInfo
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- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
- G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
- G02B6/29379—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device
- G02B6/29389—Bandpass filtering, e.g. 1x1 device rejecting or passing certain wavelengths
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- G02F1/35—Non-linear optics
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- 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
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- 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
- H01S3/30—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range using scattering effects, e.g. stimulated Brillouin or Raman effects
- H01S3/302—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range using scattering effects, e.g. stimulated Brillouin or Raman effects in an optical fibre
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Abstract
The invention relates to the field of microwave photon signal processing, and discloses a narrow-band tunable microwave photon filter based on a Brillouin optical fiber laser, wherein a narrow-linewidth continuous wave optical fiber laser I is sequentially connected with a polarization controller I, a phase modulator and an annular cavity I; the narrow linewidth continuous wave fiber laser II is sequentially connected with the polarization controller II, the erbium-doped fiber amplifier and the annular cavity I, and the annular cavity I is connected with the photoelectric detector, the vector network analyzer and the phase modulator. The invention provides a narrow linewidth double-loop Brillouin laser and a microwave photon filtering structure design, which are combined, the characteristic that a Brillouin optical fiber laser effectively presses a narrow stimulated Brillouin scattering gain spectrum is utilized to realize narrow-band filtering, the technical problem that the existing microwave photon filter realizes single-pass band filtering of a sub-kHz magnitude is solved, in addition, a double-wavelength structure is adopted to respectively provide modulated light and Brillouin pumping light, and the narrow-band tunable microwave photon filter is finally realized by changing the center frequency of a passband of a modulated light wavelength tuning filter.
Description
Technical Field
The invention relates to the field of microwave photon signal processing, in particular to a narrow-band tunable microwave photon filter based on a Brillouin fiber laser.
Background
The traditional electronic filter device is difficult to control and process the high-frequency broadband signal because of the limitation of the electronic transmission rate; the microwave photon filtering technology for processing the light-carried microwave signals based on various optical effects has been attracting attention for many years due to the excellent characteristics of large bandwidth, low loss, electromagnetic interference resistance and the like and the potential of overcoming the limitation of electronic transmission, and becomes a key technology in the next-generation wireless radio frequency system.
Due to the increasing complexity of radio frequency systems, microwave photon filters are evolving towards high frequencies, high Q values and tunable directions. The Q value of the quality factor is an important index for measuring the selection ability of the filter to the target frequency band, and the higher the Q value is, the stronger the frequency selection ability of the filter is. The key indicator for determining the Q value is the 3dB bandwidth Δf of the filter passband 3dB By the following constitutionObtainable, Δf 3dB The smaller the Q value is, the larger the Q value is, and the fine frequency component of the signal to be detected can be filtered out. Therefore, the realization of ultra-narrow bandwidth microwave photon filters with large tuning ranges has become the leading edge and hot spot of research in recent years.
Various tunable, narrow bandwidth microwave photon filters have been proposed. For example, a narrow-band and tunable microwave photon filter realized by a phase modulator and a super-structure fiber Bragg grating, wherein the 3dB bandwidth is 143MHz, and the tunable range is 0.4-6.4 GHz; a tunable microwave photon filter with a 3dB bandwidth and a side mode rejection ratio of 61.2MHz and 25dB respectively and a frequency tuning range of 27.72GHz is realized by using four-wave mixing based on a distributed feedback semiconductor laser under optical injection. The patent document with publication number of CN105589221B discloses a tunable double-passband microwave photon filter based on stimulated Brillouin scattering, which performs closed-loop control on an electro-optical modulator by using a bias feedback control module to output a stable narrow-wave-suppressed double-sideband modulation signal, effectively reduces the gain spectrum width of the stimulated Brillouin scattering by combining stimulated Brillouin scattering with Stokes gain and anti-Stokes loss in an optical fiber, realizes the amplitude response of the double-passband filter, can realize the narrow bandwidth of 20.1MHz, can realize tuning within the range of 1.8 GHz-20 GHz, and can obtain the stop-band suppression ratio exceeding 35 dB.
The bandwidth of the existing microwave photon filter is mostly concentrated on the MHz level, the structure for realizing the narrow-band and tuning functions is mostly complex, the existing microwave photon filter is mostly difficult to have the wide-frequency tuning range and the kHz and even sub-kHz narrow-band filtering functions, and the urgent requirements of the ultra-narrow bandwidth tunable microwave photon filter in the front technical fields of high-resolution microwave photon sensing, high-purity frequency spectrum microwave photon signal generation, high-precision microwave photon radar and the like can not be met. In order to solve the problems, the invention provides a narrow-band tunable microwave photon filter based on a Brillouin fiber laser.
Disclosure of Invention
The invention aims to solve the problem that the traditional electronic filter device is difficult to control and process high-frequency broadband signals at present; the narrow bandwidth tunable microwave photon filter based on the Brillouin fiber laser is provided by the embodiment of the invention; the narrow-band tunable microwave photon filter based on the Brillouin fiber laser has the advantages of ultra-narrow band, tunable structure, high out-of-band rejection ratio and the like by combining the narrow-line Brillouin laser with the microwave photon filter and utilizing the obvious gain linewidth compression characteristic of the Brillouin laser to narrow the passband of the filter to the sub-kHz.
The invention is realized by adopting the following technical scheme:
the narrow-band tunable microwave photon filter based on the Brillouin optical fiber laser comprises an output end of a narrow-linewidth continuous wave optical fiber laser I, an input end of a polarization controller I, an output end of the polarization controller I, an a port of a phase modulator, a c port of the phase modulator, an a port of an optical fiber coupler I and a b port of the optical fiber coupler I, wherein the narrow-band tunable microwave photon filter is connected with an input end of the polarization controller I; the output end of the narrow linewidth continuous wave optical fiber laser II is connected with the input end of the polarization controller II, the output end of the polarization controller II is connected with the input end of the erbium-doped optical fiber amplifier, the output end of the erbium-doped optical fiber amplifier is connected with the a port of the optical circulator, the b port of the optical circulator is connected with the b port of the single-mode fiber I, the c port of the optical circulator is connected with the a port of the optical fiber coupler II, the c port of the optical fiber coupler II is connected with the input end of the single-mode fiber II, the output end of the single-mode fiber II is connected with the d port of the optical fiber coupler II, the b port of the optical fiber coupler II is connected with the a port of the optical fiber coupler III, the b port of the optical fiber coupler III is connected with the input end of the polarization controller III, and the c port of the optical fiber coupler I of the output end of the polarization controller III; the c port of the optical fiber coupler III is connected with the input end of the photoelectric detector, the output end of the photoelectric detector is connected with the input end of the vector network analyzer, and the output end of the vector network analyzer is connected with the b port of the phase modulator.
In implementation, the narrow-band tunable microwave photon filter based on the Brillouin optical fiber laser is designed, the narrow-line-width continuous wave optical fiber laser I is a tunable narrow-line-width laser emitting an optical carrier, the output end of the narrow-line-width continuous wave optical fiber laser I is connected with the input end of the polarization controller I, the output end of the polarization controller I is connected with the a port of the phase modulator, the c port of the phase modulator is connected with the a port of the optical fiber coupler I, the optical fiber coupler I has a spectral ratio of 50:50, the b port of the optical fiber coupler I is connected with the a port of the single-mode optical fiber I, and the length of the single-mode optical fiber I is 100m; the narrow linewidth continuous wave optical fiber laser II is a tunable narrow linewidth laser for emitting pump light, the output end of the narrow linewidth continuous wave optical fiber laser II is connected with the input end of the polarization controller II, the output end of the polarization controller II is connected with the input end of the erbium-doped optical fiber amplifier, the output end of the erbium-doped optical fiber amplifier is connected with the a port of the optical circulator, the b port of the optical circulator is connected with the b port of the single-mode fiber I to form a narrow linewidth Brillouin optical fiber laser, 3dB bandwidth of kHz magnitude is obtained, the c port of the optical circulator is connected with the a port of the optical fiber coupler II, the optical fiber coupler II has a spectral ratio of 50:50, the c port of the optical fiber coupler II is connected with the input end of the single-mode fiber coupler II, the length of the single-mode fiber II is 10m, and the output end of the single-mode fiber II is connected with the d port of the optical fiber coupler II, and the c port of the single-mode fiber coupler II, the single-mode fiber II and the d port of the optical fiber coupler II are connected to form an annular cavity II; the b port of the optical fiber coupler II is connected with the a port of the optical fiber coupler III, and the optical fiber couplerIII is a split ratio of 90:10, an a port is an input port, a port is a 90% output port, a c port is a 10% output port, a b port of an optical fiber coupler III is connected with an input end of a polarization controller III, a c port of an output end of the polarization controller III is connected with an optical fiber coupler I, an optical circulator, an optical fiber coupler II, an optical fiber coupler III and the polarization controller III are connected to form a Brillouin laser resonant cavity to form an annular cavity I, the annular cavity I and the annular cavity II are cascaded to form an annular Fabry-Perot resonant cavity so as to form a vernier effect, and different periodic resonances of the two annular cavities are utilized to effectively inhibit a side mode and filter out a required frequency band signal; the c port of the optical fiber coupler III is connected with the input end of the photoelectric detector, the output end of the photoelectric detector is connected with the input end of the vector network analyzer, and the output end of the vector network analyzer is connected with the b port of the phase modulator. When in use, the narrow linewidth continuous wave fiber laser I emits a laser with the center frequency f c1 From the a port of the phase modulator to the phase modulator via the polarization controller I, the frequency generated from the vector network analyzer is f RF The radio frequency signal of the optical fiber coupler I enters the optical fiber coupler I through the port b of the phase modulator, the modulated signal is output through the port c of the phase modulator, enters the optical fiber coupler I through the port a of the optical fiber coupler I and is output through the port b, and enters the single-mode optical fiber I through the port a of the single-mode optical fiber I; the center frequency of the laser emitted by the narrow linewidth continuous wave fiber laser II is f c2 As the pumping light for exciting stimulated Brillouin scattering, the pumping light frequency f can be changed by changing the laser wavelength of the narrow linewidth continuous wave optical fiber laser II c2 Thereby achieving the purpose of changing the center frequency of the stimulated Brillouin scattering gain spectrum, realizing the stable and tunable function of the microwave photon filter, enabling pumping light to enter an erbium-doped optical fiber amplifier through a polarization controller II to amplify power to exceed the stimulated Brillouin scattering threshold, inputting the pumping light through an a port of an optical circulator and outputting the pumping light through a b port of the optical circulator, entering the pumping light through a b port of a single-mode fiber I, interacting in the single-mode fiber I to excite the stimulated Brillouin scattering, and modulating the upper sideband f of the signal c1 +f RF Amplified by a Brillouin gain spectrum, the center frequency of the Brillouin gain spectrum is f c2 -f B Wherein f B Is Brillouin frequency shift; the modulation signal amplified by the Brillouin gain is output from a port b of the optical circulator and a port c of the optical circulator, is injected into a port a of the optical fiber coupler II, enters an annular cavity II formed by connecting a port c of the optical fiber coupler II, a single-mode fiber II and a port d of the optical fiber coupler II through the port c, is output from a port b of the optical fiber coupler after the annular cavity II resonates, and enters a port a of the optical fiber coupler III to reach an optical fiber coupler III with a spectral ratio of 90:10; the modulating signal is divided into a first beam of laser and a second beam of laser after passing through an optical fiber coupler III, 90% of the first beam of laser is output through a b port of the optical fiber coupler III, then the first beam of laser is injected into the optical fiber coupler I anticlockwise through a c port of the optical fiber coupler I through a polarization controller III to complete primary resonance, then the first beam of laser is subjected to multiple resonance in an annular cavity I formed by connecting the optical fiber coupler I, a single-mode optical fiber I, an optical circulator, an optical fiber coupler II, the optical fiber coupler III and the polarization controller III, and the modulating signal output by the c port of the phase modulator is input into the annular cavity I through the b port of the optical fiber coupler I to interact with stimulated Brillouin scattering signals; 10% of the second laser is input into a photoelectric detector through a c port of the optical fiber coupler III, and signals after photoelectric conversion of the photoelectric detector are input into a vector network analyzer to measure amplitude frequency response, so that the filtering characteristic of the narrow-band tunable microwave photon filter is obtained.
Compared with the prior art, the invention has the following beneficial effects: the narrow linewidth Brillouin laser is combined with the microwave photon filter, the image is observed by utilizing the obvious gain linewidth compression characteristic of the Brillouin laser, the side mode of the microwave photon filter with a single-ring cavity structure can be obviously restrained, the side mode restraining ratio is 25dB, the remarkable breakthrough is achieved in the 3dB bandwidth narrowing, the filter passband is narrowed to the kHz magnitude, the 3dB bandwidth of the sub-kHz magnitude is obtained, and therefore the passband bandwidth of the filter is greatly narrowed. By simply changing the wavelength of the pumping light, the passband of the filter can be tuned stably, and the design difficulty of the microwave photon filter with a large tuning range and ultra-narrow filtering bandwidth is solved.The microwave photon filter can be tuned stably in the frequency range of 0-20 GHz, the out-of-band rejection ratio is about 20dB, and the 3dB bandwidth and the maximum Q value are 114Hz and 1.753 multiplied by 10 respectively 8 The filter has extremely high frequency selectivity and large tunable range, and has great potential in realizing filter bandwidth of the order of kHz and below, high rejection ratio and tunable single-passband filter.
Compared with the characteristic of single annular resonant cavity to the linewidth narrowing of stimulated Brillouin scattering gain, the vernier effect formed by the annular cavity R2 and the annular cavity R1 which are cascaded to form the annular Fabry-Perot resonant cavity can provide higher linewidth compression ratio, greatly inhibit side modes and be beneficial to realizing a microwave photon filter with higher Q value. Compared with self-induced fiber bragg gratings, circulator-based ring cavities have the advantage that the brillouin pump frequency does not need to be matched with the cavity modes, there is no extra cavity attenuation except for the inherent loss of the device, and it is difficult to generate high-order stokes due to the low nonlinear coefficient of single-mode fiber. A narrowband tunable microwave photon filter based on a brillouin fiber laser is an effective solution for obtaining a narrowband tunable microwave filter.
Drawings
Fig. 1 shows a schematic structure of the present invention.
Fig. 2 shows a schematic diagram of a narrowband tunable microwave photon filter based on a brillouin fiber laser according to an embodiment of the present invention;
wherein a is the output frequency f of the narrow linewidth continuous wave fiber laser I c1 Is an optical carrier of a signal from a vector network analyzer RF Performing a spectrogram of the double-sideband modulated signal by a phase modulator;
b is the frequency f of Brillouin pumping light output by the narrow linewidth continuous wave optical fiber laser II c2 The spectrum generated in the stimulated Brillouin process is emitted into a single-mode fiber I;
c is the upper sideband f of the modulated signal c1 +f RF A spectral diagram amplified by the brillouin gain spectrum; the center frequency and bandwidth of the Brillouin gain spectrum are f respectively c2 -f B And Δf B ；
d is a spectrogram of periodic resonance of the annular cavity I;
and e is the free spectral range matching of the annular cavity I and the annular cavity II, so that a vernier effect suppression side mode is formed, and a narrow Brillouin gain spectrogram is formed.
Fig. 3 shows a vernier effect schematic diagram adopted by the narrow-band tunable microwave photon filter based on the brillouin optical fiber laser in the embodiment of the present invention.
Fig. 4 shows a frequency response diagram of a narrow-band tunable microwave photon filter based on a brillouin fiber laser according to an embodiment of the present invention in a microwave photon filter corresponding to different structures.
Fig. 5 shows a tunable characteristic diagram of a center frequency of a narrow-band tunable microwave photon filter based on a brillouin fiber laser, specifically a diagram of a change in a wavelength range of 1550.2320 to 1550.3920nm of a pump light outputted from a narrow-linewidth continuous wave fiber laser ii.
In the figure: 1A-narrow linewidth continuous wave fiber laser I, 1B-narrow linewidth continuous wave fiber laser II, 2A-polarization controller I, 2B-polarization controller II, 2C-polarization controller III, 3-phase modulator, 4A-fiber coupler I, 4B-fiber coupler II, 4C-fiber coupler III, 5A-single mode fiber I, 5B-single mode fiber II, 6-erbium-doped fiber amplifier, 7-optical circulator, 8-photodetector, 9-vector network analyzer, R1-annular cavity I, R2-annular cavity II.
Detailed Description
Specific embodiments of the present invention will be described below with reference to the accompanying drawings.
As shown in FIG. 1, a narrow-band tunable microwave photon filter based on a Brillouin fiber laser, wherein a narrow-line-width continuous wave fiber laser I1A is a tunable narrow-line-width laser emitting an optical carrier, an output end of the narrow-line-width continuous wave fiber laser I1A is connected with an input end of a polarization controller I2A, an output end of the polarization controller I2A is connected with an a port of a phase modulator 3, a c port of the phase modulator 3 is connected with an a port of an optical fiber coupler I4A, the optical fiber coupler I4A has a splitting ratio of 50:50, a b port of the optical fiber coupler I4A is connected with an a port of a single-mode optical fiber I5A, and the length of the single-mode optical fiber I5A is 100m; the narrow linewidth continuous wave optical fiber laser II 1B is a tunable narrow linewidth laser for emitting pumping light, a single-mode optical fiber I5A and a single-mode optical fiber II 5B which are used for emitting a Brillouin scattering effect are arranged in the same constant temperature box, errors caused by temperature drift can be avoided, the output end of the narrow linewidth continuous wave optical fiber laser II 1B is connected with the input end of the polarization controller II 2B, the output end of the polarization controller II 2B is connected with the input end of the erbium-doped optical fiber amplifier 6, the output end of the erbium-doped optical fiber amplifier 6 is connected with an a port of the optical circulator 7, a B port of the optical circulator 7 is connected with a B port of the single-mode optical fiber I5A to form a narrow linewidth Brillouin optical fiber laser, a 3dB bandwidth of kHz is obtained, a c port of the optical circulator 7 is connected with an a port of the optical fiber coupler II 4B, a splitting ratio of the optical fiber II 4B is 50:50, a c port of the optical fiber coupler II 4B is connected with the input end of the single-mode optical fiber II 5B, the length of the single-mode optical fiber II 5B is 10m, and the output end of the optical fiber II 5B is connected with a port of the optical fiber coupler II 4B, and a port of the optical fiber II is connected with a port of the optical fiber II 2B; the B port of the optical fiber coupler II 4B is connected with the a port of the optical fiber coupler III 4C, the optical fiber coupler III 4C is in a split ratio of 90:10, the a port is an input port, the B port is a 90% output port, the C port is a 10% output port, the B port of the optical fiber coupler III 4C is connected with the input end of the polarization controller III 2C, the C port of the output end of the polarization controller III 2C is connected with the C port of the optical fiber coupler I4A, the single-mode optical fiber I5A, the optical circulator 7, the optical fiber coupler II 4B, the optical fiber coupler III 4C and the polarization controller III 2C are connected to form a Brillouin laser cavity, an annular cavity I R1 is formed, the annular cavity I R1 and the annular cavity II R2 are cascaded to form an annular Fabry-Perot resonant cavity, a vernier effect is formed, different periodic resonances are utilized by utilizing the two annular cavities, a side mode is effectively suppressed, a required frequency band signal is filtered, the C port of the optical fiber coupler III 4C is connected with the input end of the optical fiber coupler I4A, the optical fiber coupler I4B is connected with the input end of the optical fiber detector 8, the output end of the optical fiber coupler I4C is connected with the input end 9 of the optical fiber detector 8, the maximum bandwidth analyzer is connected with the input end 9 of the network analyzer 9, and the maximum bandwidth of the analyzer is connected with the input end 9 of the network analyzer by the analyzer.
When in use, the narrow linewidth continuous wave fiber laser I1A emits a laser with the center frequency f c1 As carrier wave, enters the phase modulator 3 from the port a of the phase modulator 3 through the polarization controller i 2A, and is generated from the vector network analyzer 9 with the frequency f RF The radio frequency signal of the (2) enters the carrier wave through the b port of the phase modulator 3 to carry out double-sideband sweep frequency modulation, the modulated signal is output by the c port of the phase modulator 3, enters the optical fiber coupler I4A through the a port of the optical fiber coupler I4A and is output by the b port, and enters the single-mode optical fiber I5A through the single-mode optical fiber I5 Aa port; the center frequency f is emitted by a narrow linewidth continuous wave fiber laser II 1B c2 As the pumping light for exciting stimulated Brillouin scattering, the pumping light frequency f can be changed by changing the laser wavelength of the narrow linewidth continuous wave optical fiber laser II 1B c2 Thereby achieving the purpose of changing the center frequency of the stimulated Brillouin scattering gain spectrum, realizing the stable and tunable function of the microwave photon filter, enabling pump light to be tuned to the optimal polarization state through a polarization controller II 2B, enabling the pump light to enter an erbium-doped optical fiber amplifier 6 to amplify power, amplifying the power to exceed the stimulated Brillouin scattering threshold value, enabling the pump light to be input through an a port of an optical circulator 7 and output through a B port, enabling the pump light to be emitted and enter through a B port of a single-mode optical fiber I5A along the opposite direction of a modulated light signal, enabling the pump light to interact in the single-mode optical fiber I5A to excite the stimulated Brillouin scattering, and enabling the upper sideband f of the modulated signal to be modulated c1 +f RF Amplified by a Brillouin gain spectrum, the center frequency of the Brillouin gain spectrum is f c2 -f B Wherein f B Is Brillouin frequency shift; the modulated signal amplified by the Brillouin gain is output from the B port of the optical circulator 7 and the C port of the optical circulator, is injected into the a port of the optical fiber coupler II 4B, enters an annular cavity II R2 formed by connecting the C port of the optical fiber coupler II 4B, the single-mode fiber II 5B and the d port of the optical fiber coupler II 4B from the C port, is output from the B port of the optical fiber coupler 4B after the annular cavity II R2 resonates, and enters the a port of the optical fiber coupler III 4C to reach the optical fiber coupler III 4C with the spectral ratio of 90:10; the modulated signal is divided into a first beam of laser and a second beam after passing through an optical fiber coupler III 4CThe laser, 90% of the first beam of laser is output through the B port of the optical fiber coupler III 4C, then is injected into the optical fiber coupler I4A anticlockwise through the C port of the optical fiber coupler I4A by the polarization controller III 2C to complete resonance once, and then resonates for many times in an annular cavity IR 1 formed by connecting the optical fiber coupler I4A, the single-mode optical fiber I5A, the optical circulator 7, the optical fiber coupler II 4B, the optical fiber coupler III 4C and the polarization controller III 2C, and a modulation signal output by the C port of the phase modulator 3 is input into the annular cavity IR 1 through the B port of the optical fiber coupler I4A to interact with stimulated Brillouin scattering signals; 10% of the second laser is input into the photoelectric detector 8 through the C port of the optical fiber coupler III 4C, and the signal after photoelectric conversion of the photoelectric detector 8 is input into the vector network analyzer 9 to measure amplitude frequency response, so that the filter characteristic of the narrow-band tunable microwave photon filter is obtained.
The working principle adopted is as follows: after the interaction of the Brillouin gain spectrum and the optical modulation signal, the narrow spectral linewidth of the Brillouin fiber laser is formed through the main resonant cavity, and the vernier effect formed by different periodic resonances of the double annular cavities is utilized to effectively inhibit the side modes, so that ultra-narrow band microwave photon filtering based on the Brillouin laser is realized.
As shown in fig. 2: the center frequency of the microwave photon filter can be expressed as:
f pass ＝f c2 -f B
from the above, by tuning the frequency f c2 The center frequency f of the microwave photon filter can be changed pass And tuning not to the modulated signal frequency f c1 Is limited by the number of (a).
As shown in fig. 3: Δf B A frequency shift f related to the stimulated Brillouin scattering pumping light for the stimulated Brillouin gain spectrum bandwidth B The definition is as follows:
f B ＝(2ν A /c)ν P
wherein v A Represents the sound velocity in the medium, c represents the vacuum light velocity, v P Representing the optical frequency of the pump light; f (f) B At 1550The wavelength range in nm is about 10.737GHz. Free spectral range FSR of two annular cavities according to vernier effect 1 ，FSR 2 The effective FSR with the double ring cavity structure satisfies the following conditions:
FSR＝n 1 FSR 1 ＝n 2 FSR 2
in the formula, FSR 1 Corresponding annular cavity IR 1 ；FSR 2 Corresponding annular cavity IIR 2 ；n m (m=1, 2) is an integer. That is, the effective free spectral range FSR of the dual ring cavity structure is FSR 1 And FSR (FSR) 2 Least common multiple of (2);
the free spectral range of annular cavities IR 1 and IIR 2 is expressed as:
wherein L is m (m=1, 2) is the ring length of the annular chambers ir 1 and ii R2; the effective refractive index of the fiber n=1.468, in this embodiment, L 1 ＝100m，L 2 =10m; the free spectral ranges of the annular cavity IR 1 and the annular cavity IIR 2 are 1.86MHz and 18.6MHz respectively; the effective free spectral range of the microwave photon filter is 18.6MHz. When the effective free spectral range exceeds the brillouin gain bandwidth and the gain is greater than the loss, the laser mode oscillates only at frequencies that satisfy both the ring cavity ir 1 and ring cavity ii R2 resonance conditions.
As shown in fig. 4: the 3dB bandwidth of the Brillouin gain spectrum is approximately 9.926MHz, and in this embodiment, only the annular cavity IR is added 1 When the 3dB bandwidth of the microwave photon filter is narrowed to the order of subKHz. But in the embodiment, the annular chambers IIR are connected in series 2 Then, a double-ring cavity structure is formed, and at the moment, the 3dB bandwidth of the microwave photon filter is further reduced to the hundred hertz level; by comparing the single-ring cavity structure with the double-ring cavity structure image, the side mode of the microwave photon filter with the single-ring cavity structure can be obviously inhibited, and the side mode inhibition ratio is 19dB, so that the microwave photon filter disclosed by the embodiment has obvious breakthrough in the 3dB bandwidth narrowing and can reach the subkHz magnitude. At the same time at 19.98GHzBy the formula q=f/Δf 3dB Calculated q=1.753×10 8 The microwave photon filter disclosed by the invention has extremely high selectivity and a larger tunable range, and has huge potential in realizing tunable single-pass band filtering with high suppression ratio of bandwidth in the sub-kHz magnitude.
As shown in fig. 5: when the wavelength is changed in the range of 1550.2320 to 1550.3920nm, the passband of the microwave photon filter is stably tuned to 2-20GHz, and the out-of-band rejection exceeds 25dB, therebyThe relation between the available wavelength lambda and the frequency f is inversely proportional, wherein f represents the frequency of the stimulated Brillouin scattering pumping light, lambda represents the wavelength of the stimulated Brillouin scattering pumping light, and the light velocity c=3×10 8 m/s. The minimum tuning accuracy in this embodiment depends on the minimum tuning accuracy of the narrow linewidth continuous wave optical fiber laser ii 1B, and the tuning range and the tuning accuracy can be further improved by improving the operating bandwidth and the tuning accuracy of the device.
The microwave photon filter in this embodiment has a 3dB bandwidth Δf pass The method is basically stable, and the side mode suppression ratio fluctuates less in a tuning range; the main reason for this variation is the temperature variation and the annular cavity IR in the examples 1 Annular cavity IIR 2 Matching problems between free spectral ranges. The matching problem between the two annular cavities and the maintenance of parallel polarization between the pump wave and stokes wave in the optical path can be solved by adjusting the polarization controller iii 2C.
The scope of the present invention is not limited to the above embodiments, and various modifications and alterations of the present invention will become apparent to those skilled in the art, and any modifications, improvements and equivalents within the spirit and principle of the present invention are intended to be included in the scope of the present invention.
Claims (5)
1. A narrow-band tunable microwave photon filter based on a Brillouin fiber laser is characterized in that: the optical fiber laser comprises a narrow linewidth continuous wave optical fiber laser I (1A) and a narrow linewidth continuous wave optical fiber laser II (1B), wherein the output end of the narrow linewidth continuous wave optical fiber laser I (1A) is connected with the input end of a polarization controller I (2A), the output end of the polarization controller I (2A) is connected with an a port of a phase modulator (3), a c port of the phase modulator (3) is connected with an a port of an optical fiber coupler I (4A), and a B port of the optical fiber coupler I (4A) is connected with an a port of a single-mode optical fiber I (5A); the output end of the narrow linewidth continuous wave optical fiber laser II (1B) is connected with the input end of the polarization controller II (2B), the output end of the polarization controller II (2B) is connected with the input end of the erbium-doped optical fiber amplifier (6), the output end of the erbium-doped optical fiber amplifier (6) is connected with the a port of the optical circulator (7), the B port of the optical circulator (7) is connected with the B port of the single-mode optical fiber I (5A), the C port of the optical circulator (7) is connected with the a port of the optical fiber coupler II (4B), the C port of the optical fiber coupler II (4B) is connected with the input end of the single-mode optical fiber II (5B), the output end of the single-mode optical fiber II (5B) is connected with the d port of the optical fiber coupler II (4B), the B port of the optical fiber coupler II (4B) is connected with the a port of the optical fiber coupler III (4C), the B port of the optical fiber coupler III (4C) is connected with the a port of the controller III (2C), and the output port of the optical fiber I (2C) is connected with the output port of the optical fiber I; the C port of the optical fiber coupler III (4C) is connected with the input end of the photoelectric detector (8), the output end of the photoelectric detector (8) is connected with the input end of the vector network analyzer (9), and the output end of the vector network analyzer (9) is connected with the b port of the phase modulator (3).
2. A brillouin fiber laser based narrowband tunable microwave photonic filter according to claim 1, wherein: the optical fiber coupler I (4A), the single-mode optical fiber I (5A), the optical circulator (7), the optical fiber coupler II (4B), the optical fiber coupler III (4C) and the polarization controller III (2C) are connected to form an optical resonant cavity to form an annular cavity I (R1); the c port of the optical fiber coupler II (4B), the single-mode optical fiber II (5B) and the d port of the optical fiber coupler II (4B) are connected to form an annular cavity II (R2).
3. A brillouin fiber laser based narrowband tunable microwave photonic filter according to claim 1, wherein: the optical fiber coupler III (4C) is characterized in that the split ratio is 90:10, the port a is an input port, the port b is a 90% output port, and the port C is a 10% output port; the split ratio of the optical fiber coupler I (4A) to the optical fiber coupler II (4B) is 50:50.
4. A brillouin fiber laser based narrowband tunable microwave photonic filter according to claim 1, wherein: the narrow linewidth continuous wave optical fiber laser I (1A) is a tunable narrow linewidth laser for emitting an optical carrier, and the narrow linewidth continuous wave optical fiber laser II (1B) is a tunable narrow linewidth laser for emitting pump light.
5. A brillouin fiber laser based narrowband tunable microwave photonic filter according to claim 1, wherein: the length of the single-mode optical fiber I (5A) is 100m, and the length of the single-mode optical fiber II (5B) is 10m.
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| US7272160B1 (en) * | 2005-01-24 | 2007-09-18 | Np Photonics, Inc | Single-frequency Brillouin fiber ring laser with extremely narrow linewidth |
| CN109638621A (en) * | 2019-01-21 | 2019-04-16 | 中国科学院半导体研究所 | KHz magnitude single-pass band microwave photon filter |
| CN114336236A (en) * | 2021-11-15 | 2022-04-12 | 太原理工大学 | Ultra-narrow microwave photon generator based on self-excitation forward Brillouin fiber laser |
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