Studies On Microwave Signal Generator Based On Recirculating Frequency-Shifting Loop

In recent years,microwave signals with high and tunable frequency have attracted lots of interests in the communication technology.A microwave signal generator based on recirculating frequency-shifting loop(RFSL)is proposed in this paper.The RFSL is constructed by a dual-parallel Mach-Zehnde modulator(DPMZM)based on the single sideband modulation effect.The optical carrier shifted by the RFSL in frequency domain beats with the initial optical carrier to generate a microwave signal.The frequency tunable range of the microwave signal is controlled by the frequency multiplication factor(FMF)generated in the loop.The frequency tunable accuracy is decided by the frequency of the radio-frequency(RF)signal applied on the DPMZM.In the prposed system,continuous or pulsed microwave signals with high frequency can be generated experimentally.Microwave signals are generated with a tunable range from 5 GHz to 470 GHz and a tunable accuracy on the order of 100 Hz.The main works of this paper are listed as follows:1.Studies on RFSL based on DPMZMThe working principle of DPMZM is studied in theory.The frequency of the optical carrier is shifted by the DPMZM experimentally.The frequency and power effects of RF signals applied on DPMZM are discussed.The principle of frequency-shifting in the RFSL is theoretical studied and the effects of the amplified spontaneous emission in the loop are analyzed.The output optical signals with 5 GHz,7.5 GHz and 10 GHz frequency-shifting are implemented by adjusting the frequency of the RF signal.2.Continuous microwave signal generation based on RFSLThe principle of continuous microwave signal generation is studied theoretically and the phase noise of the microwave signal is analyzed.The tunability of the FMF is achieved by an optical bandpass filter.A frequency doubling microwave signal with 10 GHz and a frequency quadrupling microwave signal with 20 GHz are generated.The FMF is successfully tuned from 1 to 30.The potential of generating continuous microwave signal frequency up to 300 GHz is proved experimentally.The stability and phase noise of the microwave signal are measured and analyzed.3.Pulsed microwave signal generation based on RFSLThe principle of pulsed microwave signal generation is studied and the time-domain-controlled method of the pulsed optical signal is analyzed.The feasibility of using DPMZM as an optical switch is studied theoretically and experimentally.Experimentally,a frequency doubling microwave signal with 10 GHz,a frequency tripling microwave signal with 15 GHz and a frequency quadrupling microwave signal with 20 GHz are generated by controlling the pulse width of the RF signal.The FMF is successfully tuned from 1 to 47.The potential of generating pulsed microwave signal frequency up to 470 GHz is proved experimentally.The effects of photodetector responsivity,power difference of optical signals and duty cycle of pulsed optical carrier on the microwave signal are experimentally studied.