CN109347560B - Free tunable dual-passband microwave photonic filter device and implementation method - Google Patents
Free tunable dual-passband microwave photonic filter device and implementation method Download PDFInfo
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Abstract
The invention belongs to the technical field of photoelectricity, and particularly relates to a free tunable dual-passband microwave photonic filter device based on stimulated Brillouin scattering and an implementation method thereof. The invention adopts a laser, realizes the conversion from phase modulation to intensity modulation by virtue of a Brillouin absorption spectrum or a gain spectrum, realizes the free setting of the positions of two pass bands by setting the output frequencies of two microwave sources, and realizes the free tunable dual-pass band filtering with narrow bandwidth, high rejection ratio and high stability in the whole tunable range.
Description
Technical Field
The invention relates to the technical field of photoelectricity, in particular to a free tunable dual-passband microwave photonic filter device based on a stimulated Brillouin scattering effect and an implementation method thereof.
Background
The traditional microwave filter realizes filtering in an electrical domain through a single or a plurality of coupling resonant cavities, has poor tuning performance of a pass band, and cannot meet the application requirements of certain radio frequency systems for tuning a wide frequency band. The microwave photon filter modulates the electric signal to the light wave, processes the signal in the optical domain, and then converts the signal back to the electric domain to obtain the filtered electric signal, thereby effectively overcoming the problem of poor tuning performance of the electric domain microwave filter, having the advantages of wide frequency band work, reconstruction, electromagnetic interference resistance and the like, and being one of powerful alternative schemes for solving the technical bottleneck of the electric domain microwave filter. With the rapid development of wireless communication technology, the requirement for simultaneous operation of two frequency bands of a radio frequency system is more and more urgent, for example, an existing Wireless Local Area Network (WLAN) is a typical dual-band communication system, and in addition, part of the radio frequency system requires that two operating frequency bands are reconfigurable, so that a microwave filter, which is one of the key devices, is required to have two freely tunable pass bands. Microwave photonic filters also have significant advantages over electrical domain microwave filters in achieving tunable dual passband.
At present, some research results are obtained aiming at a dual-passband microwave photonic filter. In 2014, Liang Gao et al proposed a dual-passband microwave photonic filter scheme based on a phase modulator and an equivalent phase-shifted Bragg fiber grating (l.gao, et al. microwave photonic filter with two independent and tunable photonic waveguide using a phase modulator and an equivalent phase-shifted fiber grating, IEEE trans.micro.thermal tech.2014,62(2): 380-387). The scheme realizes the conversion from phase modulation to intensity modulation by using the equivalent phase shift Bragg fiber grating, and realizes the free tuning of two passbands by adjusting the working wavelengths of two lasers. In the structure, the difference between the working wavelength of the two lasers and the notch position of the grating reflection spectrum determines the position of the two passbands, so the working wavelength of the light source and the drift of the grating reflection spectrum cause the stability of the passbands to be poor. In 2015, Zuowei Xu et al proposed a tunable dual-passband microwave photonic filter scheme based on spectral splitting and dual-channel fiber delay lines (Z.W.Xu, et al. A tunable dual-pass band microwave filter based on optical slicing and dual-path fiber delay lines, Opt. Commun.2015,346: 10-15). According to the scheme, a wide-spectrum light source is subjected to spectrum segmentation by using a Mach-Zehnder optical fiber interferometer, dual-passband filtering is realized by using two optical fiber delay lines with different lengths, and the tuning of a passband is realized by adjusting the length difference of two arms of the Mach-Zehnder optical fiber interferometer. The main problem with this scheme is that the passband is wide in bandwidth (hundreds of MHz to around several GHz), and when the tuning frequency range is large, the passband shape and bandwidth can change significantly, and furthermore, the two passbands cannot be freely tuned. In 2017, Shuling Hu et al proposed a tunable dual-passband microwave photon filter scheme based on stimulated Brillouin scattering effect (s.l.hu, et al, tunable dual-bandpass microwave filter based on stimulated Brillouin scattering, IEEE photon. According to the scheme, two first-order sidebands generated by optical carrier under electro-optic modulation are used as double-pump light, amplitude balance of corresponding sidebands on two sides of a phase modulation light signal is broken through by a gain spectrum and an absorption spectrum of stimulated Brillouin scattering, conversion from phase modulation to intensity modulation is completed, double-passband filtering is achieved, and double-passband tuning can be achieved by adjusting the frequency of a microwave signal loaded on an electro-optic modulator. The scheme has the main problems that in the tuning process, two pass bands are always symmetrically distributed about the frequency corresponding to the Brillouin frequency shift amount, so that the tuning flexibility of the pass bands is poor, and free tuning of the two pass bands cannot be realized.
In summary, the tunable dual-passband microwave photonic filter schemes reported at present have more or less the following problems: firstly, the stability of the pass band is poor; second, the passband shape and bandwidth vary significantly during the tuning process; third, free tuning of both passbands cannot be achieved.
Disclosure of Invention
Aiming at the defects of the prior art scheme, the invention provides a free tunable dual-passband microwave photonic filter device based on the stimulated Brillouin scattering effect and an implementation method thereof.
The technical scheme of the invention is as follows: a free tunable dual-passband microwave photonic filter device based on a stimulated Brillouin scattering effect and an implementation method thereof are provided, wherein the device comprises: the laser device comprises a laser device (1), an optical coupler (2), an electro-optic phase modulator (3), a high-nonlinearity optical fiber (4), a double-parallel Mach-Zehnder electro-optic modulator (5), a direct-current power supply (6), a first microwave source (7), a 90-degree electric bridge (8), a first erbium-doped optical fiber amplifier (9), a double-parallel Mach-Zehnder electro-optic modulator (10), a direct-current power supply (11), a second microwave source (12), a second erbium-doped optical fiber amplifier (13), an optical circulator (14) and a photoelectric detector (15). The method is characterized in that all devices are connected according to the following sequence: the output of the laser (1) is connected with the input end of the optical coupler (2), one output end of the optical coupler (2) is connected with the optical input end of the electro-optical phase modulator (3) and serves as a signal optical branch, and the radio frequency input end of the electro-optical phase modulator (3) is the signal input end of the whole microwave photonic filter. The optical output end of the electro-optic phase modulator (3) is connected with the input end of the high nonlinear optical fiber (4), and the output end of the high nonlinear optical fiber (4) is connected with a port b of the optical circulator (9). The other output end of the optical coupler (2) is connected with the optical input end of the double-parallel Mach-Zehnder electro-optic modulator (5) and used as a pump light branch, the bias voltage input end and the radio frequency signal input end of the double-parallel Mach-Zehnder electro-optic modulator (5) are respectively connected with the output end of the direct current power supply (6) and the output end of the 90-degree electric bridge (8), the input end of the 90-degree electric bridge (8) is connected with the output end of the first microwave source (7), and the optical output end of the double-parallel Mach-Zehnder electro-optic modulator (5) is connected with the input end of the first erbium-doped optical fiber amplifier (9). The output end of the first erbium-doped fiber amplifier (9) is connected with the input end of a double-parallel Mach-Zehnder electro-optic modulator (10), the bias voltage input end and the radio frequency signal input end of the double-parallel Mach-Zehnder electro-optic modulator (10) are respectively connected with the output end of a direct current power supply (11) and the output end of a second microwave source (12), and the optical output end of the double-parallel Mach-Zehnder electro-optic modulator (10) is connected with the input end of a second erbium-doped fiber amplifier (13). The output end of the second erbium-doped fiber amplifier (13) is connected with the port a of the optical circulator (14). The port c of the optical circulator (14) is connected with the optical input end of the photoelectric detector (14), and the radio frequency output end of the photoelectric detector (15) is the signal output end of the whole microwave photon filter.
The implementation method of the free tunable dual-passband microwave photonic filter device based on the stimulated Brillouin scattering effect comprises the following steps:
step 2, a signal light branch is modulated by an input microwave signal through an electro-optic phase modulator (3), and the signal light after phase modulation enters a high-nonlinearity optical fiber (4) and is transmitted from left to right in a forward direction;
and 3, a pump light branch firstly passes through a first double-parallel Mach-Zehnder electro-optic modulator (5), the modulator is driven by a microwave signal with the frequency f' output by a first microwave source (7) through a 90-degree electric bridge (8), and the modulator works in a mode of inhibiting carrier waves by adjusting the output voltage of a direct current power supply (6)In single sideband modulation mode, thereby producing an optical carrier that is shifted up or down in frequency as shown in fig. 2. The optical carrier wave enters a second double parallel Mach-Zehnder electro-optic modulator (10) after passing through a first erbium-doped fiber amplifier (9), and the frequency f of the modulator output by a second microwave source (12)mBy adjusting the output voltage of the dc power supply (11), the microwave signal is driven to operate in a double-sideband modulation mode in which the carrier is suppressed, thereby generating the double pump light as shown in fig. 2. After passing through a second erbium-doped fiber amplifier (13), the double-pump light is input from an a port of an optical circulator (14) and output from a b port to enter a high nonlinear fiber (4) for reverse transmission from right to left;
and 4, when the pump light is transmitted in the high nonlinear optical fiber (4), due to the stimulated Brillouin scattering effect, generating a narrow-band gain peak at the Brillouin frequency shifted relative to the frequency of the pump light, and generating a narrow-band absorption peak at the Brillouin frequency shifted relative to the frequency of the pump light, so that amplitude balance of corresponding sidebands of the phase modulation signal light transmitted in opposite directions is broken, and conversion from phase modulation with frequency selectivity to intensity modulation is realized. When the pumping light is generated by reserving a low-frequency sideband as shown in fig. 2(a), two Brillouin narrow-band absorption peaks generated by the double pumping light are used for realizing the selective attenuation of the signal light sideband; when the pumping light is generated by reserving a high-frequency sideband as shown in fig. 2(b), two brillouin narrow-band gain peaks generated by the double pumping light are used for realizing the selective amplification of the signal light sideband;
and step 5, the signal light which is subjected to the stimulated Brillouin scattering effect enters a port b of the optical circulator (14), is output from a port c and enters a photoelectric detector (15) to complete photoelectric conversion. The microwave signals are recovered by the two groups of sideband frequencies after the conversion from phase modulation to intensity modulation is completed, and the microwave signals cannot be recovered by the sideband frequencies after the conversion from phase modulation to intensity modulation is not completed, so that dual-passband filtering is realized;
In summary, due to the adoption of the technical scheme, the invention has the beneficial effects that:
(1) the two cascaded double-parallel Mach-Zehnder electro-optic modulators are used for generating double pump light with freely tunable frequency, amplitude balance of a phase modulation light sideband is selectively broken through the stimulated Brillouin scattering effect, conversion from phase modulation to intensity modulation is completed, and a double-passband microwave photonic filter with freely tunable passband center is realized;
(2) the system only adopts one laser, and the tuning of the two pass bands is realized by changing the frequency of the microwave signal output by the microwave source.
Drawings
FIG. 1 is a schematic structural diagram of a free tunable dual-passband microwave photonic filter apparatus according to the present invention;
fig. 2 is a schematic diagram of the principle of a freely tunable dual-passband microwave photonic filter of the present invention, wherein fig. 2(a) is a schematic diagram of a microwave photonic filter based on brillouin loss spectrum; FIG. 2(b) is a schematic diagram of a microwave photonic filter based on Brillouin gain spectrum;
FIG. 3 shows the fixation fmAdjusting the frequency of the frequency response curve of the double-passband microwave photonic filter based on the Brillouin loss spectrum, which is obtained by measuring when f' is adjusted to 5 GHz;
fig. 4 shows that f' is fixed at 8GHz and is adjustedmMeasuring a frequency response curve of the obtained dual-passband microwave photonic filter based on the Brillouin loss spectrum;
FIG. 5 shows the fixation fmAdjusting the frequency response curve of the two-passband microwave photonic filter based on the Brillouin gain spectrum obtained by measuring when f' is adjusted to 5 GHz;
fig. 6 shows that f' is fixed at 8GHz and is adjustedmAnd measuring the frequency response curve of the obtained dual-passband microwave photonic filter based on the Brillouin gain spectrum.
The optical fiber laser device comprises a laser 1, an optical coupler 2, an electro-optic phase modulator 3, a high nonlinear optical fiber 4, a double-parallel Mach-Zehnder electro-optic modulator 5, a direct-current power supply 6, a microwave source 7, a 90-degree electrical bridge 8, an erbium-doped optical fiber amplifier 9, a double-parallel Mach-Zehnder electro-optic modulator 10, a direct-current power supply 11, a microwave source 12, an erbium-doped optical fiber amplifier 13, an optical circulator 14 and a photoelectric detector 15.
Detailed Description
The embodiments of the present invention are described below with reference to specific embodiments, and other advantages and effects of the present invention will be easily understood by those skilled in the art from the disclosure of the present specification. The invention is capable of other and different embodiments and of being practiced or of being carried out in various ways, and its several details are capable of modification in various respects, all without departing from the spirit and scope of the present invention.
The system principle of the present invention is further explained with reference to fig. 2 as follows:
as shown in FIG. 1, the light source output frequency is fcThe direct current optical signal is divided into two paths by the optical coupler, wherein one path is used as a signal optical branch and the other path is used as a pumping optical branch. The signal light enters the upper electro-optical phase modulator through the optical coupler and is modulated by the radio-frequency signal loaded on the electro-optical phase modulator. For small signal modulation, the output light field after signal light passes through the electro-optic phase modulator can be represented as
Wherein E iscAs amplitude of the light field, J0(m) and J1(m) Bessel functions of the first kind, zero order and first order respectively, and f is the frequency of the microwave signal input to the electro-optical phase modulator. For the phase modulation optical signal, if the amplitude balance of the sideband is broken by the stimulated Brillouin scattering effect, the radio frequency signal obtained after the positive and negative first-order sidebands and the carrier are beaten in the photoelectric detector has the characteristic of equal-magnitude reverse (phase difference pi),the mutual offset is realized, no radio frequency signal is output by the photoelectric detector, and the frequency response of the system has the full-impedance characteristic.
As shown in FIG. 2(a), the pump light enters the first double parallel Mach-Zehnder electro-optic modulator in the downstream through an optical coupler, the modulator is driven by a microwave signal with frequency f' output by a microwave source through a 90-degree bridge, the DC bias voltage is adjusted, the modulator works in a single-sideband modulation mode for suppressing the carrier, and the low-frequency lower sideband is reserved, and the frequency of output light of the modulator is fc-f'. After the power amplification is realized by the erbium-doped fiber amplifier, the amplified signal enters a second double-parallel Mach-Zehnder electro-optic modulator, and the frequency of the modulator output by the microwave source is fmThe microwave signal is driven, the DC bias voltage is adjusted, the modulator works in a double-sideband modulation mode of inhibiting carrier waves, and the frequencies of two generated sideband signals are respectively fc-f'+fmAnd fc-f'-fm. After power amplification of the two sideband signals is realized through the erbium-doped fiber amplifier, the two sideband signals are used as double-pump light, enter the high-nonlinearity fiber through the optical circulator and are transmitted in opposite directions with the phase modulation optical signals. Because the high nonlinear optical fiber has fixed Brillouin frequency shift fBFrequency f of double-pump light in phase modulated optical signalc-f'+fm+fBAnd fc-f'-fm+fBTwo narrow-band brillouin loss spectra are generated at the center. In the case of single wavelength pumping, the brillouin loss spectrum can be expressed as
Where v is the amount of frequency offset, g, relative to the amount of brillouin frequency shift0And ΓBRespectively brillouin peak gain factor and 3dB line width. When the sideband of the phase-modulated optical signal is at the center frequency fc-f'+fm+fBAnd fc-f'-fm+fBWhen in two Brillouin loss spectra, the output optical field can be expressed as
Wherein f is1＝fm-fB+f'-f，f2＝fm+fB-f' -f, L is the effective length of the highly nonlinear optical fiber, PPIs the pump light power. From the formula (3), the position f is determined by the Brillouin loss spectrumc-f'+fm+fBAnd fc-f'-fm+fBThe phase modulation optical sideband is attenuated, and the amplitude balance of the phase modulation optical signal sideband is broken, so that the conversion from phase modulation to intensity modulation is realized. After passing through the photoelectric detector, the obtained pass band is f ═ fm+fB-f' and f ═ fm-fBThe filter response at + f' is
|H(f)|2∝|A(f1)-A(f2)|2 (4)
Wherein
Because the high nonlinear optical fiber has fixed Brillouin frequency shift fBTherefore, the scheme only needs to reasonably set the modulation frequency f of the two double-parallel Mach-Zehnder electro-optic modulatorsmAnd f', arbitrary tuning of the location of the two filter passbands can be achieved.
Further, when the first double-parallel Mach-Zehnder electro-optic modulator is operated in a single-sideband modulation mode for suppressing the carrier wave and keeps the high-frequency upper sideband, the output optical signal frequency of the first double-parallel Mach-Zehnder electro-optic modulator is fc+ f', the system principle is shown in fig. 2 (b). In the case of single wavelength pumping, the brillouin gain spectrum can be expressed as:
when the sideband of the phase-modulated optical signal is at the center frequency fc+f'+fm+fBAnd fc+f'-fm+fBWhen the two Brillouin gain spectrums are within the two Brillouin gain spectrums, the two parts of sidebands can be selectively amplified, the amplitude balance of the sidebands of the phase modulation optical signal is broken, the conversion from the phase modulation to the intensity modulation is realized, and then the dual-passband filtering is realized. The same scheme as the filtering realized by using loss spectrum is adopted, and the modulation frequency f of two double-parallel Mach-Zehnder electro-optic modulators is reasonably setmAnd f', arbitrary tuning of the location of the two filter passbands can be achieved.
The feasibility of the invention will be explained below with reference to specific examples.
Example 1:
according to the structure shown in fig. 1, a free tunable dual-passband microwave photonic filter based on the brillouin absorption spectrum is realized by using the principle shown in fig. 2 (a). The light source was a DFB narrow linewidth laser centered at 1549.97 nm. The operating bandwidth of the electro-optic phase modulator is 10 GHz. The working bandwidths of the two double parallel Mach-Zehnder electro-optic modulators are both 40 GHz. The length of the high nonlinear optical fiber is 1km, and the Brillouin frequency shift amount is 9.644 GHz. The output frequency ranges of the two microwave sources are 100kHz-12.75GHz and 100kHz-40GHz respectively. The operating bandwidth of the photodetector is 20 GHz. In the experiment, a vector network analyzer is adopted to test the filtering performance of the microwave photonic filter, the sweep frequency range is 10MHz-50GHz, and the radio frequency output end and the radio frequency input end of the vector network analyzer are respectively connected with the radio frequency signal input end and the radio frequency signal output end of the microwave photonic filter.
In the experiment, the output frequency f of the second microwave source is fixedmThe tuning of the output passband spacing of the filter can be achieved by adjusting the first microwave source output frequency f'. FIG. 3 shows fmThe measured frequency responses of the microwave photonic filters were 6.5GHz, 7GHz, 7.5GHz, 8GHz, 8.5GHz, and 9GHz at 5GHz and f', respectively, the intervals of the two pass bands were 6.29GHz, 5.29GHz, 4.291GHz, 3.29GHz, 2.29GHz, and 1.28GHz, respectively, and the pass band bandwidths were 55.03 GHz6MHz, 55.887MHz, 56.936MHz, 54.334MHz, 57.752MHz and 54.387 MHz. From experimental results, it can be seen that by changing the modulation frequency of the first dual-parallel mach-zehnder electro-optic modulator, continuous tuning of the distance between the two pass bands can be realized.
Fixing the output frequency f' of the first microwave source by adjusting the output frequency f of the second microwave sourcemTuning of the center frequency between the two pass bands of the filter can be achieved. Fig. 3 shows f ═ 8GHz, fmWhen the frequency response of the microwave photonic filter is respectively 3GHz, 4GHz, 5GHz, 6GHz, 7GHz and 8GHz, the interval of the two pass bands is 3.2GHz, and the central frequencies between the two pass bands are respectively as follows: 2.9995GH, 3.994GHz, 5.0005GHz, 6.0005GHz, 7.0004GHz and 8 GHz. From the experimental results, it can be seen that by changing the modulation frequency of the second dual parallel mach-zehnder electro-optic modulator, continuous tuning of the center frequency position between the two pass bands can be achieved.
In summary, by setting fmAnd f', a freely tunable dual-passband microwave photonic filter can be realized based on the Brillouin loss spectrum.
Example 2:
according to the structure shown in fig. 1, a free tunable dual-passband microwave photonic filter based on the brillouin gain spectrum is realized by adopting the principle shown in fig. 2 (b). The device used in the experiment is the same as that in the embodiment 1, the vector network analyzer is adopted in the experiment to test the filtering performance of the microwave photonic filter, the sweep frequency range is 10MHz-50GHz, and the radio frequency output end and the radio frequency input end of the vector network analyzer are respectively connected with the radio frequency signal input end and the radio frequency signal output end of the microwave photonic filter.
In the experiment, the output frequency f of the second microwave source is fixedmThe tuning of the output passband spacing of the filter can be achieved by adjusting the first microwave source output frequency f'. FIG. 5 shows fmThe frequency response of the microwave photon filter is measured when the frequency response is respectively 6.5GHz, 7GHz, 7.5GHz, 8GHz, 8.5GHz and 9GHz at 5GHz and f', the intervals of the two pass bands are respectively 6.28GHz, 5.28GHz, 4.279GHz, 3.281GHz, 2.288GHz and 1.28GHz, and the pass bands are respectively 43.030MHz, 44.988MHz, 41.154MHz and 43.280MHz, 44.357MHz, and 49.045 MHz. From experimental results, it can be seen that by changing the modulation frequency of the first dual-parallel mach-zehnder electro-optic modulator, continuous tuning of the distance between the two pass bands can be realized.
Fixing the output frequency f' of the first microwave source by adjusting the output frequency f of the second microwave sourcemTuning of the center frequency between the two pass bands of the filter can be achieved. Fig. 3 shows f ═ 8GHz, fmWhen the frequency response of the microwave photonic filter is respectively 3GHz, 4GHz, 5GHz, 6GHz, 7GHz and 8GHz, the interval of the two pass bands is 3.2GHz, and the central frequencies between the two pass bands are respectively as follows: 3.000GH, 3.9975GHz, 4.9995GHz, 6.0005GHz, 6.9995GHz and 7.9995 GHz. From the experimental results, it can be seen that by changing the modulation frequency of the second dual parallel mach-zehnder electro-optic modulator, continuous tuning of the center frequency position between the two pass bands can be achieved.
In summary, by setting fmAnd f', a freely tunable dual-passband microwave photonic filter can be realized based on the Brillouin gain spectrum.
The results of the two embodiments verify the feasibility of the system structure and the method for realizing the free tunable dual-passband microwave photonic filter. No matter the loss spectrum or the gain spectrum generated by the stimulated Brillouin scattering effect is utilized, the free setting of the positions of two filter pass bands can be realized only by reasonably setting the output frequencies of two microwave sources, and the filter has the advantages of stability, narrow band, reconfigurability, flexible tuning and the like.
The foregoing embodiments are merely illustrative of the principles and utilities of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or change the above-mentioned embodiments without departing from the spirit and scope of the present invention. Accordingly, it is intended that all equivalent modifications or changes which can be made by those skilled in the art without departing from the spirit and technical spirit of the present invention be covered by the claims of the present invention.
Claims (3)
1. A free tunable dual-passband microwave photonic filter device based on stimulated Brillouin scattering is characterized by comprising a narrow linewidth laser (1), an optical coupler (2), an electro-optic phase modulator (3), a high nonlinear optical fiber (4), a double parallel Mach-Zehnder electro-optic modulator (5), a direct-current power supply (6), a first microwave source (7), a 90-degree electric bridge (8), a first erbium-doped optical fiber amplifier (9), a double parallel Mach-Zehnder electro-optic modulator (10), a direct-current power supply (11), a second microwave source (12), a second erbium-doped optical fiber amplifier (13), an optical circulator (14) and a photoelectric detector (15);
the devices are connected in the following order: the output end of the laser (1) is connected with the input end of the optical coupler (2), one output end of the optical coupler (2) is connected with the optical input end of the electro-optic phase modulator (3) and serves as a signal optical branch, and the radio frequency input end of the electro-optic phase modulator (3) is the signal input end of the whole microwave photonic filter; the optical output end of the electro-optic phase modulator (3) is connected with the input end of the high nonlinear optical fiber (4), and the output end of the high nonlinear optical fiber (4) is connected with a port b of the optical circulator (9); the other output end of the optical coupler (2) is connected with the optical input end of the double-parallel Mach-Zehnder electro-optic modulator (5) and used as a pump light branch circuit, the bias voltage input end and the radio frequency signal input end of the double-parallel Mach-Zehnder electro-optic modulator (5) are respectively connected with the output end of the direct current power supply (6) and the output end of the 90-degree electric bridge (8), the input end of the 90-degree electric bridge (8) is connected with the output end of the first microwave source (7), and the optical output end of the double-parallel Mach-Zehnder electro-optic modulator (5) is connected with the input end of the first erbium-doped optical fiber amplifier (9); the output end of the first erbium-doped fiber amplifier (9) is connected with the input end of a double-parallel Mach-Zehnder electro-optic modulator (10), the bias voltage input end and the radio frequency signal input end of the double-parallel Mach-Zehnder electro-optic modulator (10) are respectively connected with the output end of a direct current power supply (11) and the output end of a second microwave source (12), and the optical output end of the double-parallel Mach-Zehnder electro-optic modulator (10) is connected with the input end of a second erbium-doped fiber amplifier (13); the output end of the second erbium-doped fiber amplifier (13) is connected with the port a of the optical circulator (14); the port c of the optical circulator (14) is connected with the optical input end of the photoelectric detector (15), and the radio frequency output end of the photoelectric detector (15) is the signal output end of the whole microwave photon filter.
2. A method for realizing a free tunable dual-passband microwave photonic filter device based on stimulated Brillouin scattering is characterized by comprising the following steps:
step 1: the output frequency of the laser (1) is fcThe direct current light is divided into two paths by the optical coupler (2), one path is used as a signal light branch, and the other path is used as a pumping light branch;
step 2: the signal light branch is modulated by the input microwave signal through an electro-optic phase modulator (3), and the phase modulation light enters a high nonlinear optical fiber (4) and is transmitted forward from left to right;
and step 3: the pump light branch firstly passes through a first double-parallel Mach-Zehnder electro-optic modulator (5), the modulator is driven by a microwave signal with the frequency of f 'output by a first microwave source (7) through a 90-degree electric bridge (8), and the output voltage of a direct-current power supply (6) is adjusted to enable the modulator to work in a single-sideband modulation mode of inhibiting a carrier wave, so that an optical carrier wave with the frequency of moving up or down f' is generated; the optical carrier wave enters a second double-parallel Mach-Zehnder electro-optic modulator (10) after being subjected to power amplification through a first erbium-doped optical fiber amplifier (9), and the frequency f of the modulator output by a second microwave source (12)mThe microwave signal is driven, and the output voltage of the direct current power supply (11) is regulated to enable the microwave signal to work in a double-sideband modulation mode for inhibiting carrier waves, so that double pump light is generated; after power amplification of the double-pump light is realized through the second erbium-doped fiber amplifier (13), the double-pump light is input from an a port of the optical circulator (14) and is output from a b port to enter the high-nonlinearity fiber (4) for reverse transmission from right to left;
and 4, step 4: when the pump light is transmitted in the high nonlinear optical fiber (4), due to the stimulated Brillouin scattering effect, a narrow-band gain peak is generated at the Brillouin frequency shifted relative to the frequency of the pump light, and a narrow-band absorption peak is generated at the Brillouin frequency shifted relative to the frequency of the pump light, so that the amplitude balance of corresponding sidebands of the phase modulation signal light transmitted in opposite directions is broken, and the conversion from phase modulation with frequency selectivity to intensity modulation is realized; when the pumping light is generated in a mode of keeping a low-frequency sideband, two Brillouin narrow-band absorption peaks generated by the double pumping light are used for realizing the selective attenuation of the signal light sideband; when the pumping light is generated in a mode of reserving a high-frequency sideband, two Brillouin narrow-band gain peaks generated by the double pumping light are used for realizing the selective amplification of the signal light sideband;
and 5: the signal light after the action of the stimulated Brillouin scattering effect enters a port b of the optical circulator (14), and is output from a port c and enters a photoelectric detector (15) to complete photoelectric conversion; the microwave signals are recovered by the two groups of sideband frequencies after the conversion from phase modulation to intensity modulation is completed, and the microwave signals cannot be recovered by the sideband frequencies after the conversion from phase modulation to intensity modulation is not completed, so that dual-passband filtering is realized;
step 6: the position of the filter pass band is f', fmAnd fBThree quantities are determined together, wherein the Brillouin frequency shift quantity f of the high-nonlinearity optical fiberBFor fixed value, the modulation frequencies f' and f of two cascaded dual-parallel Mach-Zehnder electro-optic modulators are set reasonablymFree tuning of the two filter passbands can be achieved.
3. The method for implementing the free-tunable dual-passband microwave photonic filter device based on the stimulated brillouin scattering according to claim 2, wherein the free-tunable dual-passband microwave photonic filter device is the free-tunable dual-passband microwave photonic filter device according to claim 1.
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