Research On Photonic-Assisted Microwave Mixing Technology Based On Electro-optic External Modulation

Microwave photonic technology is a critical part of signal processing at the transmission or receiving end for RF signals.With the continuous development of 5G communication technology,the millimeter-wave band is widely used,and requirements for high transmission rates,low latency,high network capacity density,etc.,are constantly being raised.Traditional microwave frequency mixing processing methods encounter electronic bottlenecks in handling high-frequency signals,resulting in problems such as low conversion efficiency and multiple output signal interference.Microwave photonics is an emerging discipline that combines microwave and photonics technologies,which can overcome the serious electronic bottlenecks in traditional microwave systems in terms of processing speed and transmission bandwidth and has advantages such as wide bandwidth,low loss and anti-electromagnetic interference,thus optimizing the performance of the mixing system.This article mainly uses the principles of microwave photonics to conduct research on photonic-assisted microwave frequency mixing technology based on electro-optic external modulation,and proposes three schemes to improve the performance of the mixing system and enhance the system’s functionality.The specific research content is as follows:To solve the complex issue of switching between up-conversion and down-conversion signals,a dual-output photonic-assisted microwave frequency mixing scheme is proposed.The lightwave emitted by the laser is divided into two paths by a power splitter,and is modulated by the RF signal and the local oscillator signal respectively through the MachZehnder modulator and the dual parallel Mach-Zehnder modulator.The two modulated signals are input to the 2×4 90° optical hybrid.The four optical signals output from the mixer are converted into electrical signals through two balanced detectors,and the generated photocurrent simultaneously generates up-conversion and down-conversion RF electrical signals by the 2×2 90° hybrid coupler,which can effectively suppress the mirrored down-conversion signal.Simulation results show that up-conversion and down-conversion signals are simultaneously output from the two output ports of the 2×2 90° hybrid coupler,and the spurious suppression ratio is 27.2 dB and 30.0 dB,respectively.Through tunable testing,when the local oscillator signal is fixed at 8 GHz,the up-conversion signal can be flexibly tuned between 17-23 GHz,and the down-conversion signal can be flexibly tuned between 1-7 GHz.For system linearity,the measured spurious-free dynamic range of this scheme is 92.8 dB·Hz2/3.For image rejection function,the maximum image rejection ratio can reach 65.2 dB.In order to address the limitation of single-frequency output in the aforementioned scheme,a dual-band dual-output photonic-assisted upconversion harmonic mixing scheme is proposed.The lightwave emitted by the laser is injected into a polarization-multiplexing dual-parallel MachZehnder modulator(PDM-DPMZM).The X branch of the PDM-DPMZM is modulated by the intermediate frequency(IF)signal,operating in carriersuppressed single-sideband mode,and only the+1st order sideband is retained;the Y branch is modulated by the local oscillator signal,operating in carrier-suppressed double-sideband mode,and only the ±2nd order sidebands are retained.The optical signal after polarization combiner is divided into two paths by a power splitter,and each path consists of a polarization controller(PC)and a polarization beam splitter(PBS).In order to build an I/Q orthogonal structure to achieve useless signal phase cancellation,the PC in the lower path is adjusted to introduce a 900 phase shift,and after balanced photodetection by BPD,I/Q mixing can be achieved.The resulting photocurrent is then output from the two channels of the 2×2 90° hybrid coupler as two different frequency electrical signals,thus achieving up-conversion harmonic mixing.Simulation results show that two different frequency electrical signals are simultaneously output from the two output ports of the 2×2 90° hybrid coupler,with spurious suppression ratios of 30.6 dB and 28.1 dB,respectively.Through tunable testing,this scheme can achieve frequency tuning of up-conversion signals in the frequency ranges of 8-11 GHz(X band)and 13-16 GHz(Ku band).The finally measured spurious-free dynamic range of this scheme is 96.8 dB·Hz2/3.Experimental validation was conducted based on the simulation,which further demonstrated the feasibility of the design scheme.Experimental results demonstrate that the spurious suppression ratio of the up-converted output signal can reach 19 dB.To address the issue of limited signal frequency caused by image signal suppression in the first approach,a dual-channel frequency conversion scheme with photonic-assisted microwave mixing is proposed.The lightwave emitted by the laser is injected into the PDM-DPMZM.The X arm of the PDM-DPMZM is modulated by the RF signal and operates in the carrier suppression single-sideband mode,retaining only the+1 st-order sideband;the Y arm is modulated by the local oscillator signal and operates in the carrier suppression single-sideband mode,retaining only the+1 storder sideband.The optical signal after polarization beam combining(PBC)is evenly divided into two paths by the power splitter and sent to two channels,each consisting of a polarization controller(PC)and a polarization splitter.In order to construct an I/Q orthogonal structure to achieve image signal phase cancellation,the PC in the lower path is adjusted to introduce a 90-degree phase shift,and after the BPD converts the optical signal into an electrical signal,I/Q mixing can be achieved.The resulting photocurrent is fed into a 2×2 90° hybrid coupler,which outputs two different frequency electrical signals from the two channels of the coupler,thus realizing dual-channel image rejection in down-conversion.According to simulation results,two electrical signals from two output ports of the 2×2 90° hybrid coupler are outputted to two different channels,where one channel outputs the down-converted signal with power enhancement,while the down-converted signal of the other channel is effectively suppressed.For the image rejection function,the measured image rejection ratio can reach up to 54 dB.