Research On Generation And Transmission Performance Of Chirp Signal Based On Microwave Photonics Technology

In modern radar systems,chirped microwave waveform is widely favored with powerful pulse compression capabilities.Conventionally,chirped microwave waveform generation schemes in the electrical domain have limitations of their low center frequency and small bandwidth due to the electrical bottleneck effect.Compared with the methods based on electrical domain,the microwave photonic technology shows great potential to generate the chirped signal with high center frequency,large bandwidth,wide tunability and low loss.Therefore,on the basis of microwave photonics technology,this paper proposes a dual-polarization Mach-Zehnder modulator(DPol-MZM),a cascaded polarization modulator(PolM),a parallel structure of a polarization modulator(PolM)and a phase modulator(PM)to generate dual-chirp signal.The specific schemes and innovations are as follows:1.A simple scheme to generate a frequency and bandwidth-doubling dual-chirp signal based on a DPol-MZM is presented,and the influence of fiber chromatic dispersion during transmission is analyzed in detail.In this scheme,the lightwave is split into two beams in the DPol-MZM,and one beam is modulated via sub-MZM1 by the RF signal to generate two second-order sidebands along with the optical carrier,while the other beam is modulated via sub-MZM2 by a single-chirp microwave with single sideband modulation.By adjusting the angle between the PBC and the principal axis of Pol,the output lightwave from the DPol-MZM is linearized and the optical carrier is eliminated.After optical to electrical conversion,a dual-chirp signal with doubled bandwidth and carrier frequency is generated.The generated waveform not only has a higher center frequency and good pulse compression capability,but also can effectively avoid the power fading caused by chromatic dispersion as it is transmitted through the fiber.According to the simulation show that after 30km fiber transmission,the dual-chirp microwave waveform with a center frequency of 20GHz,a bandwidth of 4GHz,and a pulse compression ratio(PCR)of 341 is generated and the spectrum power is only attenuated by 12dB compared to the back-to-back transmission.2.In this paper,we have presented a simple scheme to generate multi-frequency dual-chirp signals with improved time-bandwidth product(TBWP)based on two cascaded PolMs.In this scheme,the optical frequency comb(OFC)with four lines is generated by RF signal modulating the lightwave via one optical polarization modulator,and then is modulated by a parabolic signal via a second PolM.After heterodyne beating in balanced photodetector(BPD),the parabolic phase-shifted OFC is converted to three-band dual-chirped signals.In order to improve the TBWP,the parabolic electrical signal is divided into multi-segments and phase encoding encoded to increase multiply both the bandwidth and temporal duration of the generated dual-chirp signals,respectively.According to the simulation show that multi-frequency dual-chirp microwave waveforms with center frequency-bandwidth of 10GHz-3.2GHz,20GHz-3.2GHz,and 30GHz-3.2GHz,covering X,K and Ka bands,are simultaneously generated.The generated waveform has the TBWP of 5133 approximately.Their auto-correlation results manifest high peak-to-sidelobe ratio(PSR)and good pulse compression capabilities.These agree well with our theoretical prediction.The generated multi-frequency dual-chirp microwave waveforms with high-frequency,large-bandwidth,and large-TBWP is are expected to be used in multi-band radar system to improve its range resolution and detection range.3.A scheme based on the parallel structure of PolM and PM to generate multi-band dual-chirp signals is proposed.In this scheme,the upper branch lightwave is modulated by the RF signal via two PolMs,and the two orthogonal polarized lightwaves from PolM are projected onto the linear polarization state through pol.By adjusting the modulation indexes of polarization modulators,flat OFC with 12 lines are generated.The lower branch lightwave is modulated by a parabolic signal via a phase modulator(PM);and the lightwave of the upper branch is orthogonally combined through the PBC.Then the obtained polarization multiplexed lightwave is incident into the PBS,and the lightwave is reversed and recombined with equal amplitude by adjusting the PBS.After heterodyne beating in balanced photodetector(BPD),a multi-band dual-chirp signals can be obtained.The scheme has a novel structure,simple principle and tunable.In the simulation,the center frequency-bandwidth of the generated multi-band dual-chirp signals are 10GHz-4.8GHz,30GHz-4.8GHz,50GHz-4.8GHz,70GHz-4.8GHz,90GHz-4.8GHz and 100GHz-4.8GHz.It can cover the X-V band(6 bands)of the radar at the same time.