Fundamental Research On Technologies Of Photonic Microwave Signal Frequency Conversion Based On Electro-optic Mixing

Wideband and high-efficiency microwave frequency conversion with flexible operation is important and widely used in both civil and military applications such as wireless communication,radar communication,electronic reconnaissance and countermeasure.Photonic-assisted method for microwave frequency conversion based on electro-optic mixing in optical domain is recognized as the preferred scheme for ultra-wideband microwave signal frequency conversion,due to the advantages of large processing bandwidth,low insertion loss,high isolation and immunity to electromagnetic interference.However,the opeariton frequency range of the conventional method based on electro-optic mixing is limited by the frequency range of electrical local oscillator?LO?and the bandwidth of the corresponding optoelectronic devices.Therefore,methods that enable microwave frequency conversion with ultra-wide frequency range by using low-frequency electrical LO and narrow-band optoelectronic devices,and at the same time allow reconfigurable frequency conversion have great academic significance and application value.In this thesis,special focus are put on the theoretical and experimental research of microwave frequency conversion methods based on electro-optic modulation and four-wave mixing?FWM?,respectively.Moreover,the frequency conversion techniques based on electro-optic mixing are applied to characterization measurement of microwave photonic devices and microwave photonic frequency measurement.The main contents of the thesis are summarized as follows:?1?Research on microwave frequency conversion techniques based on electro-optic modulation has the following innovations:A photonic-assisted method for microwave frequency down-conversion is proposed based on parallel electro-optic modulation.It enables configurable and ultra-wide frequency operation with a low-frequency electrical LO by choosing two different comb lines of MLLD,whose operation frequency range theoretically can be about 193.95 GHz.In the demonstration,6.25⁴0 GHz RF signals are experimentally down-converted to IF signals below 1 GHz with a DC⁶.25 GHz LO.The SFDR₃ of the system is as high as 106.6 dBc?Hz^2/3.Another microwave frequency down-conversion method is proposed based on re-circulating electro-optic modulation?EOM?.Through re-circulating EOM frequency shifting in opposite directions with a low-frequency electrical LO,it allows flexible,tunable and ultra-wide microwave frequency down-conversion.In the demonstration,5⁴0 GHz RF signals are experimentally down-converted to IF signals at 0.5 GHz with a 3⁶ GHz LO.Therefore,the proposed two microwave frequency conversion methods enable wide frequency range with low-frequency electrical LO,which eliminates the bandwidth limitation of devices in the conventional EOM-based methods.?2?Research on microwave frequency conversion techniques based on FWM has the following innovations:A harmonic frequency down-conversion method is proposed based on cascaded four-wave mixing?CFWM?,in which the optical LO is harmonic intensified by using a harmonic intensification unit consisting of a Mach-Zehnder modulator?MZM?and an optical seminconductor amplifier?SOA?.In the experiment,the 3⁴0 GHz RF signals are down-converted to IF signals below 1 GHz with a low-frequency electrical LO?<5 GHz?.It enables a configurable and ultra-wide frequency down-conversion by tuning the frequency of the electrical LO.On this basis,another microwave frequency down-conversion method is proposed based on re-circulating FWM.With a re-circulating FWM feedback structure,a better harmonic intensification is achieved and the optical spectrum of the intensified harmonics is extended as wide as 1.5 nm in 25-dB reduction bandwidth.In the demonstration,5⁴0GHz RF signals are experimentally down-converted to IF signals below 2.44 GHz with a 4.88-GHz LO.Therefore,the proposed two harmonic down-conversion methods enable flexible and wideband microwave frequency conversion with low-frequency electrical LO and narrow-band optoelectronic devices.?3?Research on microwave frequency conversion measurement techniques based on electro-optic mixing has the following innovations:A frequency conversion measurement for individually measuring frequency response of directly modulated laser,MZM and photodetector?PD?is proposed based on cascaded EOM,which enables self-calibrated measurement by setting the frequency relationship between the cascaded EOM signals.As the experimental results indicated,the proposed method can reduce half bandwidth requirement of assisted devices or extend double measuring frequency range.Moreover,a three-in-one frequency response measurement is realized with the same measurement scheme.A self-calibrated frequency conversion measurement for measuring the frequency response of optical filters is proposed based on dual frequency-shifting,where a frequency range of 40 GHz and a frequency resolution as high as 50 kHz are achieved.A wide-band and high-resolution frequency conversion measurement for measuring the frequency response of optical filters is proposed based on low-frequency detection,where a frequency range of 140 GHz and a frequency resolution as high as 50 kHz are achieved through analyzing two mixing signals fixed at70±0.5 MHz.?4?Research on instantaneous microwave frequency measurement techniques based on electro-optic mixing has the following innovations:A cross-referenced method for microwave frequency measurement is proposed based on electro-optic mixing.Two pairs of down-converted signals are generated by electro-optic mixing between the microwave signal under test and two optical frequency combs?OFCs?with different repetition frequencies,respectively.It enables accurate and multi-tone instantaneous microwave frequency measurement by analyzing the intrinsic cross-referencing relationships among the down-converted components.Importantly,the measurement deadband of the conventional dual-comb-based method is eliminated by the proposed method.As the simulation and experimental results indicated,0-20 GHz multi-tone instantaneous microwave measurement with±0.25-MHz measurement error is achieved by using two OFCs with the repetition frequencies of 2.01 GHz and 2.047 GHz,respectively.Moreover,it enables expandability for the measurement range.