A microwave photonic frequency conversion method and system based on injection-locked OEO

Abstract

The present invention relates to an optical fiber communication system and method, and particularly to a microwave photon frequency conversion method and system based on injection-locked OEO, which solves the problems of poor tunability and low conversion efficiency existing in the existing all-optical mixing technology based on ultrafast optical nonlinear effects, and the technical problem that although the electro-optic modulation mixing technology based on the electro-optic effect has a simple structure, the frequency conversion effect is limited by the characteristics of the local oscillator signal and the modulator, making it difficult to flexibly frequency-convert broadband microwave signals. The microwave photon frequency conversion system based on injection-locked OEO includes a first laser, a first optical coupler, an upper branch module, a lower branch module, a band-stop filtering module, and a second photodetector; the upper branch module is used for optically suppressing the carrier double-sideband modulation of the signal to be frequency-converted; the lower branch module is used for realizing the optically suppressing the carrier single-sideband modulation of the tunable local oscillator signal and the oscillation signal; the present invention can achieve the flexible frequency conversion characteristics of broadband microwave signals.

Description

Technical Field

[0001] The present invention relates to an optical fiber communication system and method, and particularly to a microwave photon frequency conversion method and system based on injection-locked OEO. Background Art

[0002] To alleviate the increasingly scarce spectrum resources, commercial fifth-generation mobile communication will continue to develop millimeter waves (24.25 - 71 GHz) or even terahertz on the basis of the Sub-6G band. Therefore, how to efficiently generate and receive broadband microwave signals has become a research hotspot.

[0003] In traditional wireless microwave communication systems, a multi-stage electrical mixer is first used to convert the signal frequency to a higher radio frequency band or a lower intermediate frequency band, and then the signal is transmitted or received. Therefore, such frequency conversion schemes have problems such as narrow operating bandwidth, high phase noise, poor tunability, large loss, low isolation, and susceptibility to electromagnetic interference. The optical carrier microwave frequency conversion technology realizes the generation and processing of high-carrier-frequency microwave signals in the optical domain by electro-optically mixing the signal to be frequency-converted with a microwave local oscillator signal, and has advantages such as large bandwidth, tunability, low transmission loss, and anti-electromagnetic interference.

[0004] According to the electro-optic mixing physical effect, existing microwave photon mixing technologies are mainly divided into two categories: all-optical mixing technology based on ultrafast optical nonlinear effects and electro-optic modulation mixing technology based on the electro-optic effect. The all-optical mixing technology uses the nonlinear effects of media such as the four-wave mixing effect to perform frequency conversion processing on the optical carrier microwave signal. Its operating bandwidth can reach more than the order of hundreds of GHz, but there are problems such as poor tunability and low conversion efficiency. The electro-optic modulation mixing technology modulates the microwave local oscillator signal and the signal to be frequency-converted onto the optical carrier simultaneously or separately, and generates a mixing signal through the beat of a photodetector. This type of scheme has a simple structure, but the frequency conversion effect is limited by the characteristics of the local oscillator signal and the modulator.

[0005] In summary, the existing all-optical mixing technology based on ultrafast optical nonlinear effects has technical problems of poor tunability and low conversion efficiency; while the electro-optic modulation mixing technology based on the electro-optic effect, although having a simple structure, the frequency conversion effect is limited by the characteristics of the local oscillator signal and the modulator, making it difficult to flexibly frequency-convert broadband microwave signals. Summary of the Invention

[0006] The object of the present invention is to provide a microwave photon frequency conversion method and system based on injection-locked OEO for the problems that the existing all-optical mixing technology based on ultrafast optical nonlinear effects has poor tunability and low conversion efficiency, and the electro-optic modulation mixing technology based on the electro-optic effect, although having a simple structure, the frequency conversion effect is limited by the characteristics of the local oscillator signal and the modulator, making it difficult to flexibly frequency-convert broadband microwave signals, so as to achieve the flexible frequency conversion characteristic of broadband microwave signals.

[0007] To solve the above technical problems, the technical solution adopted by the present invention is as follows:

[0008] A microwave photonic frequency conversion method based on injection-locked OEO, which is characterized in that it includes the following steps:

[0009] Step S1: Divide the linearly polarized continuous light with a frequency of f0 and a polarization direction along the x-axis into a first optical carrier and a second optical carrier;

[0010] Step S2: Load a microwave signal, that is, the signal to be frequency-converted with a frequency of f1, on the first optical carrier, and perform suppressed-carrier double-sideband modulation to obtain a suppressed-carrier double-sideband modulation signal with a frequency of f0±f1 and a polarization direction along the y-axis; at the same time, perform suppressed-carrier single-sideband modulation on the second optical carrier to obtain a frequency-shifted signal with a frequency of f0+f2; where f2 is the frequency of the microwave signal;

[0011] Step S3: Divide the frequency-shifted signal into a first frequency-shifted optical signal and a second frequency-shifted optical signal;

[0012] Step S4: Load a local oscillator signal with a frequency of f3 and an oscillation signal with a frequency of f osc generated by the injection-locked OEO on the first frequency-shifted optical signal respectively, and perform suppressed-carrier single-sideband modulation to obtain suppressed-carrier single-sideband modulation signals with frequencies of f0+f2+f3 and f0+f2+f osc and a polarization direction along the x-axis, and then perform polarization beam combination with the suppressed-carrier double-sideband modulation signal with a frequency of f0±f1 and a polarization direction along the y-axis; the local oscillator signal with a frequency of f3 is a microwave signal;

[0013] The injection-locked OEO generates an oscillation signal with a frequency of f osc specifically as follows:

[0014] Mix and filter the local oscillator signal with a frequency of f3 and the oscillation signal with a frequency of f osc generated by the injection-locked OEO to generate a difference frequency signal with a frequency of |f osc -f3|; separate a path of the modulated signal after polarization beam combination in Step S4 for polarization analysis, and then combine it with the second frequency-shifted optical signal, and beat to recover the local oscillator signal with a frequency of f3 and the oscillation signal with a frequency of f osc ; mix the difference frequency signal with a frequency of |f osc -f3| and the local oscillator signal with a frequency of f3 to obtain a sum frequency signal with a frequency of f osc , and then perform amplification and filtering to achieve single-mode oscillation startup, and obtain an oscillation signal with a high frequency, a high side-mode suppression ratio, and a low phase noise and a frequency of f osc ;

[0015] Step S5: The modulated signal after polarization beam combination in step S4 is successively subjected to polarization, depolarization, and filtering, and then combined with the second frequency-shifted optical signal, and then beat to output a frequency-converted signal, completing microwave photonic frequency conversion.

[0016] Further, in step S2, an optical carrier suppression double-sideband modulated signal with a frequency of f0±f1 and a polarization direction along the y-axis direction is obtained, specifically:

[0017] The first optical carrier is intensity modulated and loaded with the signal to be frequency-converted with a frequency of f1 output by the first microwave source. By biasing the intensity modulation at the minimum transmission point, optical carrier suppression double-sideband modulation is performed to obtain an optical carrier suppression double-sideband modulated signal with a frequency of f0±f1.

[0018] The optical carrier suppression double-sideband modulated signal with a frequency of f0±f1 is power-amplified, then rotated by 90° in polarization, and then the polarization direction of the rotated optical carrier suppression double-sideband modulated signal is rotated from the x-axis direction to a direction perpendicular to the original direction, obtaining an optical carrier suppression double-sideband modulated signal with a frequency of f0±f1 and a polarization direction along the y-axis direction.

[0019] Further, step S5 is specifically:

[0020] Another modulated signal is separated from the modulated signal after polarization beam combination in step S4 and successively passes through polarization and depolarization to generate linearly polarized light with frequencies of f0 + f2 + f3, f0 + f2 + f osc and f0±f1, and a polarization direction of φ. Then, filtering is performed to generate linearly polarized light with frequencies of f0 + f2 + f osc and f0±f1, and through photoelectric conversion, a frequency-converted signal with a frequency of f2 + f osc ±f1 is output, completing microwave photonic frequency conversion.

[0021] Further, in step S5, filtering to generate linearly polarized light with frequencies of f0 + f2 + f osc and f0±f1 is specifically:

[0022] The linearly polarized light with a frequency of f0 + f2 + f3 - f B -f5 is loaded with a microwave signal with a frequency of f5, and modulation is performed to complete the optical carrier suppression single-sideband modulation of the microwave signal, generating a modulated optical signal with a frequency of f0 + f2 + f3 - f B . The modulated optical signal generates an attenuation spectrum at a frequency of f0 + f2 + f3 due to the stimulated Brillouin scattering effect, and the linearly polarized light with a frequency of f0 + f2 + f3 is absorbed to obtain linearly polarized light with frequencies of f0 + f2 + f osc and f0±f1.

[0023] In addition, the present invention also provides a microwave photonic frequency conversion system based on injection-locked OEO for implementing the above-mentioned microwave photonic frequency conversion method based on injection-locked OEO, which is characterized in that it includes a first laser, a first optical coupler, an upper branch module, a lower branch module, a band-stop filtering module, and a second photodetector;

[0024] The upper branch module is used to implement optical suppressed-carrier double-sideband modulation of the signal to be frequency-converted, and generate a carrier-suppressed double-sideband modulation signal;

[0025] The lower branch module is used to implement optical suppressed-carrier single-sideband modulation of the tunable local oscillator signal and the oscillation signal; the lower branch module includes a frequency shift unit and an injection-locked OEO unit;

[0026] The frequency shift unit is used to generate a frequency shift signal;

[0027] The injection-locked OEO unit is used to generate an oscillation signal with a frequency of f osc to realize optical suppressed-carrier single-sideband modulation of the local oscillator signal, and perform polarization beam combination with the carrier-suppressed double-sideband modulation signal;

[0028] The band-stop filtering module is used to filter out the carrier-suppressed single-sideband signal of the injection signal in the modulated signal after polarization beam combination;

[0029] The output end of the first laser is connected to the input end of the first optical coupler; the output end of the first optical coupler is respectively connected to the input end of the upper branch module and the input end of the frequency shift unit;

[0030] The output ends of the upper branch module and the frequency shift unit are respectively connected to the input end of the injection-locked OEO unit;

[0031] The output end of the injection-locked OEO unit is successively connected with a polarization controller and a second polarizer; the output end of the second polarizer is connected to the input end of the band-stop filtering module;

[0032] The output end of the band-stop filtering module is connected to the input end of the second photodetector, and the output end of the second photodetector is used to output the frequency-converted signal.

[0033] Further, the injection-locked OEO unit includes a third microwave source, a first electrical coupler, a second electrical coupler, a second 90° electrical bridge, a first electrical mixer, a first band-pass filter, and a second optical coupler, a second dual-parallel Mach-Zehnder modulator, a polarization beam combiner, a single-mode fiber, a third optical coupler, a first polarizer, and a fourth optical coupler, a first photodetector, a second electrical mixer, an electrical amplifier, a second band-pass filter, and a third electrical coupler connected in sequence;

[0034] The output end of the third microwave source is connected to the input end of the first electrical coupler; one output end of the first electrical coupler is connected to one input end of the second electrical coupler; the other output end of the first electrical coupler is connected to one input end of the first electrical mixer; the output end of the first electrical mixer is sequentially connected to the input end of the first band-pass filter and the input end of the second electrical mixer;

[0035] The input end of the second optical coupler is connected to the output end of the frequency shift unit; the other output end of the second optical coupler is connected to the other input end of the fourth optical coupler;

[0036] A path of modulation signal split by the third optical coupler passes through the first polarization analyzer to filter out the suppressed carrier double-sideband modulation signal with a frequency of f0±f1 and a polarization direction along the y-axis direction, and then enters the fourth optical coupler and is combined with the second path of frequency-shifted optical signal, and the local oscillator signal with a frequency of f3 and the oscillation signal with a frequency of f osc are recovered through beat frequency by the first photodetector;

[0037] The local oscillator signal with a frequency of f3 output by the third microwave source and the oscillation signal generated by the injection-locked OEO module with a frequency of f osc are mixed by the first electrical mixer, and then filtered by the first band-pass filter to obtain a difference frequency signal with a frequency of |f osc -f3|; the difference frequency signal and the local oscillator signal recovered by beat frequency of the first photodetector are mixed by the second electrical mixer, and then amplified by the electrical amplifier and filtered by the second band-pass filter to achieve single-frequency oscillation, and the oscillation signal with a frequency of f osc is input to the third electrical coupler;

[0038] One output end of the third electrical coupler is connected to the other input end of the first electrical mixer; the other output end of the third electrical coupler is connected to the other input end of the second electrical coupler; the output end of the second electrical coupler is connected to the radio frequency input end of the second dual-parallel Mach-Zehnder modulator through the second 90° electrical bridge;

[0039] The output end of the upper branch module is connected to the other input end of the polarization beam combiner;

[0040] The other output end of the third optical coupler is connected to the input end of the polarization controller.

[0041] Furthermore, the upper branch module includes a first microwave source, and an intensity modulator, an optical amplifier, and a 90° polarization rotator connected in sequence;

[0042] The output end of the first microwave source is connected to the radio frequency input end of the intensity modulator;

[0043] The input end of the intensity modulator is connected to one output end of the first optical coupler;

[0044] The output end of the 90° polarization rotator is connected to another input end of the polarization beam combiner.

[0045] The frequency shift unit includes a second microwave source, a first 90° hybrid coupler, and a first dual-parallel Mach-Zehnder modulator connected in sequence.

[0046] The input end of the first dual-parallel Mach-Zehnder modulator is connected to another output end of the first optical coupler; the output end of the first dual-parallel Mach-Zehnder modulator is connected to the input end of the second optical coupler.

[0047] Furthermore, the band-stop filtering module includes a highly nonlinear optical fiber, an optical circulator, a second laser, and a fourth microwave source, a third 90° hybrid coupler, and DPMZM3 connected in sequence.

[0048] The input end of DPMZM3 is connected to the output end of the second laser, and the output end of the third dual-parallel Mach-Zehnder modulator is connected to the first port of the optical circulator.

[0049] One end of the highly nonlinear optical fiber is connected to the output end of the second polarizer, and the other end of the highly nonlinear optical fiber is connected to the second port of the optical circulator.

[0050] The third port of the optical circulator is connected to the input end of the second photodetector.

[0051] Furthermore, the direction of the transmission axis of the first polarizer is the x-axis direction.

[0052] The center frequency of the first band-pass filter is f IF =|f osc -f3|, and the passband bandwidth is Δf IF ;

[0053] The polarization direction of the second polarizer forms an angle φ with the x-axis, where φ≠0° and φ≠90°.

[0054] Compared with the prior art, the beneficial effects of the technical solution of the present invention are as follows:

[0055] 1. By controlling the suppression-carrier single-sideband modulation signal of the local oscillator signal with a frequency of f0 + f2 + f3 and a polarization direction of the x-axis direction to be combined with the second path of frequency-shifted optical signal, the phase of the local oscillator signal with a frequency of f3 and the phase difference of the difference-frequency signal with a frequency of |f osc -f3| are recovered by beating, and the influence of the injection signal phase noise on the injection-locked OEO unit is eliminated, thereby obtaining an oscillation signal with high frequency, low phase noise, and high side-mode suppression ratio, providing a high-quality oscillation signal for the system of the present invention.

[0056] 2. In the method of the present invention, the modulated optical sidebands of the signal to be frequency-converted and the modulated optical sidebands of the oscillation signal of the injection-locked OEO unit are transmitted through a long single-mode optical fiber to a photodetector for beat frequency, completing frequency conversion. Among them, when the pump light passes through the highly nonlinear optical fiber, due to the attenuation spectrum generated by stimulated Brillouin scattering, the modulated optical sidebands of the local oscillator signal output by the injection-locked OEO unit are eliminated, improving the system dynamic range.

[0057] 3. The system of the present invention uses an injection-locked OEO unit based on optoelectronic delay compensation as a high-quality oscillation signal. By respectively loading the signal to be frequency-converted and the oscillation signal on two optical carriers for beat frequency, the up-conversion and / or down-conversion of the signal to be frequency-converted are realized, overcoming the limitations of the electrical bottleneck, and reducing the phase noise and side modes of the frequency-converted signal.

[0058] 4. In the upper branch module of the system of the present invention, through an intensity modulator and a 90° polarization rotator, optical modulation of the signal to be frequency-converted is realized and its polarization direction is rotated to a direction perpendicular to the original direction.

[0059] 5. In the frequency shift unit of the system of the present invention, by using an optical suppressed-carrier single-sideband modulator (the first dual-parallel Mach-Zehnder modulator) and supplemented with a second microwave source with adjustable frequency, an adjustable frequency conversion range is realized.

[0060] 6. In the band-stop filtering module of the system of the present invention, by using the narrowband loss spectrum of stimulated Brillouin scattering and supplemented with a fourth microwave source with adjustable frequency, the suppressed-carrier single-sideband modulation signal of the local oscillator signal is filtered out, improving the system dynamic range. Description of the Drawings

[0061] Figure 1 is a schematic diagram of an embodiment of the microwave photonic frequency conversion system based on injection-locked OEO of the present invention;

[0062] Figure 2 is Figure 1 the output schematic diagram at point a in

[0063] Figure 3 is Figure 1 the output schematic diagram at point b in

[0064] Figure 4 is Figure 1 the output schematic diagram at point c in

[0065] Figure 5 is Figure 1 the output schematic diagram at point d in

[0066] Figure 6 is Figure 1 the output schematic diagram at point e in

[0067] Figure 7 isFigure 1 Output schematic diagram of point f in

[0068] Figure 8 For Figure 1 Output schematic diagram of point g in

[0069] Figure 9 For Figure 1 Output schematic diagram of point h in

[0070] Figure 10 For Figure 1 Output schematic diagram of point i in

[0071] Figure 11 For Figure 1 Output schematic diagram of point j in

[0072] Figure 12 For Figure 1 Output schematic diagram of point k in

[0073] Figure 13 For Figure 1 Output schematic diagram of point l in

[0074] Figure 14 For Figure 1 Output schematic diagram of point m in

[0075] Figure 15 For Figure 1 Output schematic diagram of point o in

[0076] Figure 16 For Figure 1 Output schematic diagram of point n in

[0077] The reference signs in the figure are:

[0078] 1 - First laser, 2 - First optical coupler, 3 - Intensity modulator, 4 - Optical amplifier, 5 - 90° polarization rotator, 6 - First microwave source, 7 - First dual-parallel Mach-Zehnder modulator, 8 - Second optical coupler, 9 - Second dual-parallel Mach-Zehnder modulator, 10 - Second microwave source, 11 - First 90° hybrid, 12 - Polarization beam combiner, 13 - Single-mode fiber, 14 - Third optical coupler, 15 - First polarizer, 16 - Fourth optical coupler, 17 - First photodetector, 18 - Third microwave source, 19 - First electrical coupler, 20 - Second electrical coupler, 21 - Second 90° hybrid, 22 - First electrical mixer, 23 - First bandpass filter, 24 - Second electrical mixer, 25 - Electrical amplifier, 26 - Second bandpass filter, 27 - Third electrical coupler, 28 - Polarization controller, 29 - Second polarizer, 30 - Highly nonlinear fiber, 31 - Optical circulator, 32 - Third dual-parallel Mach-Zehnder modulator, 33 - Second laser, 34 - Fourth microwave source, 35 - Third 90° hybrid, 36 - Second photodetector. Detailed implementation manner

[0079] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the technical solutions in the present invention without creative efforts shall fall within the protection scope of the present invention.

[0080] As Figure 1 shown, a microwave photonic frequency conversion system based on injection-locked OEO includes a first laser 1, a first optical coupler 2, an intensity modulator 3, an optical amplifier 4, a 90° polarization rotator 5, a first microwave source 6, a first dual-parallel Mach-Zehnder modulator 7 (i.e., DPMZM1), a second optical coupler 8, a second dual-parallel Mach-Zehnder modulator 9 (i.e., DPMZM2), a second microwave source 10, a first 90° hybrid 11, a polarization beam combiner 12, a single-mode fiber 13, a third optical coupler 14, a first polarizer 15, a fourth optical coupler 16, a first photodetector 17, a third microwave source 18, a first electrical coupler 19, a second electrical coupler 20, a second 90° hybrid 21, a first electrical mixer 22, a first bandpass filter 23, a second electrical mixer 24, an electrical amplifier 25, a second bandpass filter 26, a third electrical coupler 27, a polarization controller 28, a second polarizer 29, a highly nonlinear fiber 30, an optical circulator 31, a third dual-parallel Mach-Zehnder modulator 32 (i.e., DPMZM3), a second laser 33, a fourth microwave source 34, a third 90° hybrid 35, and a second photodetector 36.

[0081] Among them, the first microwave source 6, intensity modulator 3, optical amplifier 4, and 90° polarization rotator 5 constitute the upper branch module, which is used for optical suppressed-carrier double-sideband modulation of the signal to be frequency-converted to generate a carrier-suppressed double-sideband modulation signal; the lower branch module is used to realize tunable local oscillator signal optical suppressed-carrier single-sideband modulation; the lower branch module includes a frequency shift unit and an injection-locked OEO unit (optoelectronic oscillator); the second microwave source 10, first 90° hybrid 11, and first dual-parallel Mach-Zehnder modulator 7 constitute the frequency shift unit, which is used to generate a frequency-shifted signal; the injection-locked OEO unit is used to generate an oscillation signal with a frequency of f osc to realize optical suppressed-carrier single-sideband modulation of the local oscillator signal and perform polarization beam combination with the carrier-suppressed double-sideband modulation signal; the highly nonlinear fiber 30, optical circulator 31, second laser 33, fourth microwave source 34, third 90° hybrid 35, and third dual-parallel Mach-Zehnder modulator 32 constitute a band-stop filtering module, which is used to filter out the carrier-suppressed single-sideband signal in the modulated signal after polarization beam combination; the band-stop filtering module uses the attenuation spectrum based on stimulated Brillouin scattering to filter out the modulation optical sidebands generated by the third microwave source 18, improving the system dynamic range. The frequency shift unit and the first laser 1, first optical coupler 2 together constitute a coherent light source with a tunable center frequency, and the corresponding center frequency is jointly determined by the center frequency of the first laser 1 and the output frequency of the second microwave source 10. The second optical coupler 8, second dual-parallel Mach-Zehnder modulator 9, polarization beam combiner 12, single-mode fiber 13, third optical coupler 14, first polarizer 15, fourth optical coupler 16, first photodetector 17, third microwave source 18, first electrical coupler 19, second electrical coupler 20, second 90° hybrid 21, first electrical mixer 22, first band-pass filter 23, second electrical mixer 24, electrical amplifier 25, second band-pass filter 26, and third electrical coupler 27 constitute an injection-locked OEO unit based on optoelectronic delay compensation, which uses optoelectronic delay compensation to eliminate the influence of the phase noise of the injection signal in the injection-locked OEO unit, thereby realizing the generation of a local oscillator signal with high frequency, low phase noise, and high side-mode suppression ratio. At the same time, the second dual-parallel Mach-Zehnder modulator 9 adopts the suppressed-carrier single-sideband modulation method to avoid the periodic attenuation of the power of the frequency-converted signal caused by the dispersion effect in the long optical fiber link.

[0082] The output end of the first laser 1 is connected to the input end of the first optical coupler 2. One output end of the first optical coupler 2 is sequentially connected to the intensity modulator 3, optical amplifier 4, and 90° polarization rotator 5; the output end of the 90° polarization rotator 5 is connected to one input end of the polarization beam combiner 12; the other output end of the first optical coupler 2 is sequentially connected to the first dual-parallel Mach-Zehnder modulator 7, second optical coupler 8, second dual-parallel Mach-Zehnder modulator 9, and the other input end of the polarization beam combiner 12.

[0083] The output end of the first microwave source 6 is connected to the radio frequency input end of the intensity modulator 3. The output end of the second microwave source 10 is successively connected to the radio frequency input end of the first 90° hybrid coupler 11 and the first dual parallel Mach-Zehnder modulator 7. The output end of the third microwave source 18 is connected to the input end of the first electrical coupler 19. One output end of the first electrical coupler 19 is successively connected to the input ends of the first electrical mixer 22, the first band-pass filter 23, and the second electrical mixer 24; the other output end of the first electrical coupler 19 is connected to one input end of the second electrical coupler 20; the output end of the second electrical mixer 24 is successively connected to the input end of the electrical amplifier 25, the input end of the second band-pass filter 26, and the input end of the third electrical coupler 27. One output end of the third electrical coupler 27 is connected to the other input end of the first electrical mixer 22, the other output end of the third electrical coupler 27 is connected to the other input end of the second electrical coupler 20, and the output end of the second electrical coupler 20 is successively connected to the radio frequency input ends of the second 90° hybrid coupler 21 and the second dual parallel Mach-Zehnder modulator 9. The output end of the fourth microwave source 34 is successively connected to the radio frequency input ends of the third 90° hybrid coupler 35 and the third dual parallel Mach-Zehnder modulator 32.

[0084] The other output end of the second optical coupler 8 is connected to one input end of the fourth optical coupler 16, and the output end of the fourth optical coupler 16 is successively connected to the input end of the first photodetector 17 and one input end of the second electrical mixer 24; the output end of the polarization beam combiner 12 is successively connected to the input ends of the single-mode fiber 13 and the third optical coupler 14, and one output end of the third optical coupler 14 is successively connected to the input ends of the first polarization analyzer 15 and the fourth optical coupler 16; the other output end of the third optical coupler 14 is successively connected to the polarization controller 28, the second polarization analyzer 29, the highly nonlinear fiber 30, and the second port of the optical circulator 31.

[0085] The second laser 33 is successively connected to the third dual parallel Mach-Zehnder modulator 32 and the first port of the optical circulator 31; the third port of the optical circulator 31 is connected to the input end of the second photodetector 36, and the output end of the second photodetector 36 is used to output the frequency-converted signal.

[0086] A microwave photon frequency conversion method based on injection-locked OEO according to the present invention uses the above-mentioned microwave photon frequency conversion system based on injection-locked OEO, and includes the following steps:

[0087] Step S1, as Figure 2 shown, use the first laser 1 to output linearly polarized continuous light with a frequency of f0 and a polarization direction along the x-axis direction, and use the first optical coupler 2 to divide the linearly polarized continuous light into a first optical carrier and a second optical carrier;

[0088] Step S2, as Figure 3As shown, a signal to be frequency-converted with a frequency of f1 is loaded onto the first optical carrier wave to complete suppressed-carrier double-sideband modulation, obtaining a suppressed-carrier double-sideband modulation signal with frequencies of f0±f1 and a polarization direction along the y-axis direction; meanwhile, suppressed-carrier single-sideband modulation is performed on the second optical carrier wave to obtain a frequency-shifted signal with a frequency of f0 + f2;

[0089] 2.1 Pass the first optical carrier wave through the intensity modulator 3 and load the signal to be frequency-converted with a frequency of f1 output from the first microwave source 6. By biasing the intensity modulator 3 at the minimum transmission point, complete suppressed-carrier double-sideband modulation to obtain a suppressed-carrier double-sideband modulation signal with frequencies of f0±f1;

[0090] 2.2 Pass the suppressed-carrier double-sideband modulation signal with frequencies of f0±f1 through the optical amplifier 4 for power amplification, and then through the 90° polarization rotator 5 to rotate the polarization direction of the amplified suppressed-carrier double-sideband modulation signal from the x-axis direction to a direction perpendicular to the original one, obtaining a suppressed-carrier double-sideband modulation signal with frequencies of f0±f1 and a polarization direction along the y-axis direction;

[0091] 2.3 As Figure 4 shown, pass the second optical carrier wave through the first dual-parallel Mach-Zehnder modulator 7, and bias the two sub-modulators of the first dual-parallel Mach-Zehnder modulator 7 at the minimum transmission point and the main modulator of the first dual-parallel Mach-Zehnder modulator 7 at the quadrature operating point. With the assistance of the first 90° electrical bridge 11, complete suppressed-carrier single-sideband modulation to obtain a frequency-shifted signal with a frequency of f0 + f2, that is, realize the flexible frequency shift of the linearly polarized continuous light f0 with a polarization direction along the x-axis direction, and the frequency shift amount is determined by the output frequency f2 of the second microwave source 10;

[0092] Step S3: Divide the frequency-shifted signal into the first frequency-shifted optical signal and the second frequency-shifted optical signal;

[0093] Step S4: As Figure 5 shown, load a local oscillator signal with a frequency of f3 and an oscillation signal with a frequency of f osc onto the first frequency-shifted optical signal respectively, perform suppressed-carrier single-sideband modulation on the local oscillator signal and the oscillation signal to obtain suppressed-carrier single-sideband modulation signals with frequencies of f0 + f2 + f3 and f0 + f2 + f osc respectively and a polarization direction along the x-axis direction. As Figure 6 shown, then perform polarization beam combination on the suppressed-carrier single-sideband modulation signals with frequencies of f0 + f2 + f3 and f0 + f2 + f osc respectively and a polarization direction along the x-axis direction and the suppressed-carrier double-sideband modulation signal with frequencies of f0±f1 and a polarization direction along the y-axis direction, and sequentially pass through the single-mode optical fiber 13 and the third optical coupler 14; the local oscillator signal with a frequency of f3 is a microwave signal;

[0094] 4.1. On the first frequency-shifted optical signal, the local oscillator signal with a frequency of f3 generated by the third microwave source 18 and the oscillation signal with a frequency of f osc generated by the injection-locked OEO unit are respectively loaded through the second dual-parallel Mach-Zehnder modulator 9;

[0095] The local oscillator signal with a frequency of f3 and the oscillation signal with a frequency of f osc generated by the injection-locked OEO are mixed and filtered to generate a difference-frequency signal with a frequency of |f osc - f3|; A path of the modulated signal after polarization beam combination in step S4 is polarization-analyzed, then combined with the second frequency-shifted optical signal, and the local oscillator signal with a frequency of f3 and the oscillation signal with a frequency of f osc are recovered by beat frequency. The difference-frequency signal with a frequency of |f osc - f3| and the local oscillator signal with a frequency of f3 are mixed to obtain a sum-frequency signal with a frequency of f osc , which is then amplified and filtered to achieve single-mode oscillation startup, and an oscillation signal with a high frequency, a high side-mode suppression ratio, and a low phase noise and with a frequency of f osc is obtained.

[0096] 4.2. Then, the two sub-modulators of the second dual-parallel Mach-Zehnder modulator 9 are biased at the minimum transmission point, and the main modulator of the second dual-parallel Mach-Zehnder modulator 9 is biased at the quadrature operating point to complete the suppressed-carrier single-sideband modulation of the local oscillator signal and the oscillation signal, and suppressed-carrier single-sideband modulation signals with frequencies of f0 + f2 + f3 and f0 + f2 + f osc and with a polarization direction along the x-axis are obtained;

[0097] 4.3. Then, the suppressed-carrier single-sideband modulation signals with frequencies of f0 + f2 + f3 and f0 + f2 + f osc and with a polarization direction along the x-axis and the suppressed-carrier double-sideband modulation signal with a frequency of f0 ± f1 and a polarization direction along the y-axis are combined by a polarization beam combiner 13, and then sequentially pass through a single-mode optical fiber 13 and a third optical coupler 14.

[0098] Step S5. As Figure 7 , Figure 8 shown, another path of the modulated signal split by the third optical coupler 14 passes through a first polarization analyzer 15 to filter out the suppressed-carrier double-sideband modulation signal with a frequency of f0 ± f1 and a polarization direction along the y-axis, and then enters a fourth optical coupler 16 to be combined with the second frequency-shifted optical signal, and beat frequency is performed through a first photodetector 17.

[0099] As Figure 11 , Figure 12 shown, the local oscillator signal with a frequency of f3 output by the third microwave source 18 and the oscillation signal with a frequency of f oscThe oscillating signal is mixed by the first electrical mixer 22 and then filtered by the first band-pass filter 23 to obtain a difference-frequency signal with a frequency of |f osc -f3|, as Figure 9 , Figure 10 shown. The difference-frequency signal with a frequency of |f osc -f3| is mixed with the local oscillator signal with a frequency of f3 recovered by beating with the first photodetector 17 through the second electrical mixer 24, and then amplified by the electrical amplifier 25 and filtered by the second band-pass filter 26 to achieve single-mode oscillation startup, obtaining an oscillating signal with a high frequency, a high side-mode suppression ratio, and a low phase noise with a frequency of f osc . The oscillating signal is input to the third electrical coupler 27.

[0100] As Figure 13 , Figure 14 shown, another path of light split by the third optical coupler 14 passes through the polarization controller 28 and the second analyzer 29 in sequence to generate linearly polarized light with frequencies of f0 + f2 + f3, f0 + f2 + f osc and f0 ± f1, and with a polarization direction of φ. Then, the linearly polarized light with frequencies of f0 + f2 + f osc and f0 ± f1 is filtered out through the highly nonlinear fiber 30, as Figure 16 shown, and the frequency-converted signal with a frequency of f2 + f osc ±f1 is output through the second photodetector 36 to complete microwave photon frequency conversion.

[0101] Preferably, as Figure 15 shown, the linearly polarized light with a frequency of f0 + f2 + f3 - f B -f5 output by the second laser 33 passes through the third dual-parallel Mach-Zehnder modulator 32, and a microwave signal with a frequency of f5 generated by the fourth microwave source 34 is loaded. By biasing the two sub-modulators of the third dual-parallel Mach-Zehnder modulator 32 at the minimum transmission point and the main modulator of the third dual-parallel Mach-Zehnder modulator 32 at the quadrature operating point, suppressed-carrier single-sideband modulation of the microwave signal is completed, generating modulated light with a frequency of f0 + f2 + f3 - f B . The modulated light is input from the first port of the optical circulator 31 and output from the second port, and enters the highly nonlinear fiber 30. An attenuation spectrum is generated at a frequency of f0 + f2 + f3 through the stimulated Brillouin scattering effect, and the linearly polarized light with a frequency of f0 + f2 + f3 is absorbed, thereby realizing band-stop filtering based on the stimulated Brillouin scattering loss spectrum and obtaining linearly polarized light with a frequency of f2 + f osc ±f1, that is, the frequency-converted signal. The transmission axis direction of the first analyzer 15 is the x-axis direction; the polarization direction of the second analyzer 29 forms an angle of φ with the x-axis, where φ ≠ 0° and φ ≠ 90°; the center frequency of the attenuation spectrum is f0 + f2 + f3, and the bandwidth is less than |f osc-f3|; The center frequency of the first band-pass filter 23 is f IF , and the passband bandwidth is Δf IF .

[0102] In this embodiment, for the injection-locked OEO unit based on optoelectronic delay compensation, assuming f osc > f3, as Figure 11 shown, when the local oscillator signal with a frequency of f3 output by the third microwave source 18 is mixed with the oscillation signal with a frequency of f osc through the first electrical mixer 22, a sum-frequency signal with a frequency of f3 + f osc and a difference-frequency signal with a frequency of f osc - f3 are generated. As osc shown, and then, through the first band-pass filter 23 with a center frequency of f Figure 12 and a passband bandwidth of Δf IF , the difference-frequency signal with a frequency of f IF - f3 is filtered out, and the electrical signal time domain of the difference-frequency signal can be obtained: osc In the formula, f

[0103]

[0104] ≈ f IF - f3, osc is the initial phase of the local oscillator signal, is the initial phase of the oscillation signal, τ1 represents the link delay from the first electrical mixer 22 to the second electrical mixer 24, which is mainly determined by the bandwidth Δf of the first band-pass filter 23 IF .

[0105] The local oscillator signal with a frequency of f3 and the oscillation signal with a frequency of f osc are used to perform small-signal modulation on the input frequency-shifted optical signal through the second dual-parallel Mach-Zehnder modulator 9, and the output optical field of the second dual-parallel Mach-Zehnder modulator 9 is obtained as

[0106]

[0107] In the formula, J0(·) represents the zero-order Bessel function of the first kind, J1(·) represents the first-order Bessel function of the first kind, j represents a complex number, m3 represents the modulation coefficient of the local oscillator signal, m osc represents the modulation coefficient of the oscillation signal, t represents time, and E in (t) represents the optical field input to the second dual-parallel Mach-Zehnder modulator 9

[0108] Then, the frequencies are f0 + f2 + f3 and f0 + f2 + f oscAfter the carrier-suppressed single-sideband modulation signal with the polarization direction along the x-axis and the carrier-suppressed double-sideband modulation signal with the frequency of f0±f1 and the polarization direction along the y-axis are combined by the polarization beam combiner 13, they sequentially pass through the third optical coupler 14, the first polarizer 15, the fourth optical coupler 16 and the second frequency-shifted light and enter the first photodetector 17 together. The local oscillator signal recovered by beat frequency is

[0109]

[0110] In the formula, τ2 represents the link delay from the output end of the second dual parallel Mach-Zehnder modulator 9 to the second electrical mixer 24, which is mainly determined by the length of the single-mode fiber 13 in the injection-locked OEO unit.

[0111] As Figure 9 shown, when the difference frequency signal with the frequency of |f osc -f3| output by the first band-pass filter 23 and the local oscillator signal generated by the beat frequency of the first photodetector 17 pass through the second electrical mixer 24, the mixer bandwidth is set, and the sum frequency signal is

[0112]

[0113] It can be seen from Equation (4) that when the condition f IF +f3≈f osc is satisfied, in the injection-locked OEO unit, the oscillation mode with the frequency of f osc is locked. As Figure 10 shown, through filtering by the electrical amplifier 25 and the second band-pass filter 26, single-mode oscillation is achieved; at this time, if the delay amount satisfies the condition τ1 = τ2, the phase noise of the injection signal f IF +f3 generated by mixing the local oscillator signal twice is only related to the phase noise of the injection-locked OEO unit itself, that is, the phase noise influence introduced by the local oscillator signal is eliminated, and an oscillation signal f osc with high frequency, high side-mode suppression ratio and low phase noise is generated, thereby improving the phase noise and side-mode suppression ratio of the frequency-converted signal.

Claims

1. A microwave photonic frequency conversion method based on injection-locked OEO, characterized in that, It includes the following steps: Step S1: Divide the linearly polarized continuous light with a frequency of f0 and a polarization direction along the x-axis into a first optical carrier wave and a second optical carrier wave; Step S2: Load a microwave signal, i.e., the signal to be frequency-converted with a frequency of f1, on the first optical carrier wave, perform suppressed-carrier double-sideband modulation, and obtain a suppressed-carrier double-sideband modulation signal with a frequency of f0±f1 and a polarization direction along the y-axis; meanwhile, perform suppressed-carrier single-sideband modulation on the second optical carrier wave to obtain a frequency-shifted signal with a frequency of f0 + f2; where f2 is the frequency of the microwave signal; Step S3: Divide the frequency-shifted signal into a first frequency-shifted optical wave and a second frequency-shifted optical wave; Step S4: Load the local oscillator signal with a frequency of f3 and the injection-locked OEO signal with a frequency of f3 on the first frequency-shifted light. osc The oscillation signal is modulated by suppressed carrier single sideband modulation to obtain frequencies of f0+f2+f3 and f0+f2+f osc , a suppressed carrier single-sideband modulation signal with a polarization direction in the x-axis direction, and then polarization combining it with a suppressed carrier double-sideband modulation signal with a frequency of f0±f1 and a polarization direction in the y-axis direction; the local oscillator signal with a frequency of f3 is a microwave signal; The frequency generated by the injection-locked OEO is f osc The specific oscillation signal is as follows: Mix the local oscillator signal with a frequency of f3 and the oscillation signal with a frequency of f osc generated by the injection-locked OEO, and after mixing and filtering, generate a difference-frequency signal with a frequency of |f osc - f3|; separate a path of the modulated signal after polarization beam combining in step S4 for polarization analysis, then combine it with the second path of frequency-shifted light, and beat-frequency to recover the local oscillator signal with a frequency of f3 and the oscillation signal with a frequency of f osc ; mix the difference-frequency signal with a frequency of |f osc - f3| and the local oscillator signal with a frequency of f3 to obtain a sum-frequency signal with a frequency of f osc , then amplify and filter it to achieve single-mode oscillation startup, and obtain an oscillation signal with a frequency of f osc with high frequency, high side-mode suppression ratio and low phase noise; Step S5: Sequentially perform polarization, polarization analysis, filtering, and beat frequency on the modulated signal after polarization beam combination in Step S4, and then output the frequency-converted signal to complete microwave photon frequency conversion.

2. The microwave photonic frequency conversion method based on injection-locked OEO according to claim 1, characterized in that In Step S2, to obtain a suppressed-carrier double-sideband modulation signal with a frequency of f0±f1 and a polarization direction along the y-axis, specifically: Pass the first optical carrier wave through intensity modulation and load the signal to be frequency-converted with a frequency of f1 output by the first microwave source. By biasing the intensity modulation at the minimum transmission point, perform suppressed-carrier double-sideband modulation to obtain a suppressed-carrier double-sideband modulation signal with a frequency of f0±f1; Amplify the power of the suppressed-carrier double-sideband modulation signal with a frequency of f0±f1, then perform a 90° polarization rotation, and rotate the polarization direction of the rotated suppressed-carrier double-sideband modulation signal from the x-axis direction to a direction perpendicular to the original direction, to obtain a suppressed-carrier double-sideband modulation signal with a frequency of f0±f1 and a polarization direction along the y-axis.

3. A microwave photonic frequency conversion method based on injection-locked OEO according to claim 2, characterized in that, Step S5 specifically is: Separate another modulated signal from the polarization-combined modulated signal in step S4 and sequentially pass it through polarization and depolarization to generate linearly polarized light with frequencies of f0 + f2 + f3, f0 + f2 + f osc and f0 ± f1, and linearly polarized light with a polarization direction of φ, and then filter to generate linearly polarized light with frequencies of f0 + f2 + f osc and f0 ± f1, and output a frequency-converted signal of f2 + f osc ±f1 through photoelectric conversion to complete microwave photon frequency conversion.

4. A microwave photonic frequency conversion method based on injection-locked OEO according to claim 3, characterized in that In step S5, the linearly polarized light with frequencies of f0 + f2 + f osc and f0 ± f1 is specifically as follows: The linearly polarized light with a frequency of f0 + f2 + f3 - f B - f5 is loaded with a microwave signal with a frequency of f5, and the suppressed-carrier single-sideband modulation of the microwave signal is completed, generating a modulated light with a frequency of f0 + f2 + f3 - f B The modulated light generates an attenuation spectrum at a frequency of f0 + f2 + f3 due to the stimulated Brillouin scattering effect, absorbs the linearly polarized light with a frequency of f0 + f2 + f3, and obtains linearly polarized lights with frequencies of f0 + f2 + f osc and f0 ± f1 respectively.

5. A microwave photonic frequency conversion system based on injection-locked OEO, which is used to implement a microwave photonic frequency conversion method based on injection-locked OEO according to any one of claims 1-4, and is characterized in that: It includes a first laser (1), a first optical coupler (2), an upper branch module, a lower branch module, a band-stop filtering module, and a second photodetector (36); The upper branch module is used to implement optical suppressed-carrier double-sideband modulation of the signal to be frequency-converted and generate a carrier-suppressed double-sideband modulation signal; The lower branch module is used to implement optical suppressed-carrier single-sideband modulation of the tunable local oscillator signal and the oscillating signal; the lower branch module includes a frequency-shifting unit and an injection-locked OEO unit; The frequency-shifting unit is used to generate a frequency-shifted signal; The injection-locked OEO unit is used to generate an oscillation signal with a frequency of f osc to implement optical single-sideband modulation of the local oscillator signal with suppressed carrier, and perform polarization beam combination with the carrier-suppressed double-sideband modulation signal; The band-stop filtering module is used to filter out the carrier-suppressed single sideband of the injection signal in the modulated signal after polarization beam combination; The output end of the first laser (1) is connected to the input end of the first optical coupler (2); the output end of the first optical coupler (2) is respectively connected to the input end of the upper branch module and the input end of the frequency-shifting unit; The output end of the upper branch module and the output end of the frequency-shifting unit are respectively connected to the input end of the injection-locked OEO unit; The output end of the injection-locked OEO unit is sequentially connected with a polarization controller (28) and a second polarization analyzer (29); the output end of the second polarization analyzer (29) is connected to the input end of the band-stop filtering module; The output end of the band-stop filtering module is connected to the input end of the second photodetector (36), and the output end of the second photodetector (36) is used to output the frequency-converted signal.

6. The microwave photon frequency conversion system based on injection-locked OEO according to claim 5, wherein: The injection-locked OEO unit includes a third microwave source (18), a first electrical coupler (19), a second electrical coupler (20), a second 90° electrical bridge (21), a first electrical mixer (22), a first band-pass filter (23), and a second optical coupler (8), a second dual-parallel Mach-Zehnder modulator (9), a polarization beam combiner (12), a single-mode optical fiber (13), a third optical coupler (14), a first polarizer (15), and a fourth optical coupler (16), a first photodetector (17), a second electrical mixer (24), an electrical amplifier (25), a second band-pass filter (26), and a third electrical coupler (27) connected in sequence; The output end of the third microwave source (18) is connected to the input end of the first electrical coupler (19); one output end of the first electrical coupler (19) is connected to one input end of the second electrical coupler (20); the other output end of the first electrical coupler (19) is connected to one input end of the first electrical mixer (22); the output end of the first electrical mixer (22) is sequentially connected to the first band-pass filter (23) and the other input end of the second electrical mixer (24); The input end of the second optical coupler (8) is connected to the output end of the frequency shift unit; the other output end of the second optical coupler (8) is connected to the other input end of the fourth optical coupler (16); One path of the modulation signal split by the third optical coupler (14) passes through the first polarization analyzer (15) to filter out the suppressed-carrier double-sideband modulation signal with a frequency of f0±f1 and a polarization direction along the y-axis direction, and then enters the fourth optical coupler (16) to be combined with the second path of frequency-shifted optical signal, and the local oscillator signal with a frequency of f3 and the oscillation signal with a frequency of f osc are recovered by beat frequency through the first photodetector (17); The local oscillator signal with a frequency of f3 output by the third microwave source (18) and the oscillation signal generated by the injection-locked OEO module with a frequency of f osc are mixed by the first electrical mixer (22), and then filtered by the first band-pass filter (23) to obtain a difference frequency signal with a frequency of |f osc - f3|; The difference frequency signal and the local oscillator signal recovered by beating with the first photodetector (17) are mixed by the second electrical mixer (24), and then amplified by the electrical amplifier (25) and filtered by the second band-pass filter (26) to achieve single-frequency oscillation, and an oscillation signal with a frequency of f osc is input into the third electrical coupler (27); One output end of the third electrical coupler (27) is connected to the other input end of the first electrical mixer (22); the other output end of the third electrical coupler (27) is connected to the other input end of the second electrical coupler (20); the output end of the second electrical coupler (20) is connected to the RF input end of the second dual-parallel Mach-Zehnder modulator (9) through the second 90° electrical bridge (21); The output end of the upper branch module is connected to the other input end of the polarization beam combiner (12); The other output end of the third optical coupler (14) is connected to the input end of the polarization controller (28).

7. A microwave photonic frequency conversion system based on injection-locked OEO according to claim 6, characterized in that: The upper branch module includes a first microwave source (6), and an intensity modulator (3), an optical amplifier (4), and a 90° polarization rotator (5) connected in sequence; The output end of the first microwave source (6) is connected to the RF input end of the intensity modulator (3); The input end of the intensity modulator (3) is connected to one output end of the first optical coupler (2); The output end of the 90° polarization rotator (5) is connected to the other input end of the polarization beam combiner (12); The frequency shift unit includes a second microwave source (10), a first 90° electrical bridge (11), and a first dual-parallel Mach-Zehnder modulator (7) connected in sequence; The input end of the first dual-parallel Mach-Zehnder modulator (7) is connected to the other output end of the first optical coupler (2); the output end of the first dual-parallel Mach-Zehnder modulator (7) is connected to the input end of the second optical coupler (8).

8. The microwave photonic frequency conversion system based on injection-locked OEO according to claim 7, wherein: The band-stop filtering module includes a highly nonlinear optical fiber (30), an optical circulator (31), a second laser (33), and a fourth microwave source (34), a third 90° hybrid (35), and a third dual-parallel Mach-Zehnder modulator (32) connected in sequence; The input end of the third dual-parallel Mach-Zehnder modulator (32) is connected to the output end of the second laser (33), and the output end of the third dual-parallel Mach-Zehnder modulator (32) is connected to the first port of the optical circulator (31); One end of the highly nonlinear optical fiber (30) is connected to the output end of the second polarization analyzer (29), and the other end of the highly nonlinear optical fiber (30) is connected to the second port of the optical circulator (31); The third port of the optical circulator (31) is connected to the input end of the second photodetector (36).

9. The microwave photonic frequency conversion system based on injection-locked OEO according to claim 8, wherein: The light transmission axis direction of the first polarization analyzer (15) is the x-axis direction; The center frequency of the first band-pass filter (23) is f IF = |f osc - f3|, and the passband bandwidth is Δf IF ; The polarization direction of the second polarization analyzer (29) forms an angle φ with the x-axis, where φ≠0° and φ≠90°.

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