The emerging new concepts and technologies based on microwave photonics have led to an ever-increasing interest in developing innovative radar systems with a net gain in functionality,bandwidth /resolution,size,mass,complexity and cost when compared with the traditional implementations. This paper describes the techniques developed in the last few years in microwave photonics that might revolutionize the way to design multifunction radar systems,with an emphasis on the recent advances in optoelectronic oscillators( OEOs),arbitrary waveform generation,photonic mixing,phase coding,filtering,beamforming,analog-to-digital conversion,and stable radio-frequency signal transfer. Challenges in implementation of these components and subsystems for meeting the technique requirements of the multifunction radar applications are discussed.
An approach to implementing optical single sideband(OSSB)polarization modulation,which is a combination of two orthogonally polarized OSSB modulations with complementary phase differences between the optical carrier and the sideband,is demonstrated based on two cascaded polarization modulators(PolMs).The two PolMs are driven by two RF signals that are 90°out of phase.By properly adjusting the polarization state between the two PolMs,OSSB polarization modulation with large operation bandwidth can be realized.An experiment is performed.OSSB polarization modulation with an operation bandwidth from 2 to 35 GHz is successfully demonstrated.The spectral profile of the OSSB polarization-modulated signal is observed through an optical spectrum analyzer,and its complementary phase properties are analyzed by sending the signal to a photodetector(PD)for square-law detection.Due to the complementary phase differences between the optical carrier and the sideband along the two polarization directions,no microwave frequency component is generated after the PD.The generated OSSB polarization-modulated signal is transmitted through 25 and 50 km single-mode fiber with 50 Mbaud 16 quadrature amplitude modulation baseband data to investigate the transmission performance of the proposed system in radio-over-fiber applications,and very small error vector magnitude degradation is observed.OSSB polarization modulation is also employed to realize a microwave photonic phase shifter.A full-range tunable phase shift is obtained for 2 and 35 GHz microwave signals.
A tunable and dispersion-insensitive two-tap microwave photonic filter(MPF) with a complex coefficient is proposed and demonstrated.By simply adjusting a polarization controller(PC) in one tap,the filter can be tuned over the entire free spectral range(FSR) while keeping the shape of the filter unchanged.Because the two taps are both single-sideband(SSB) modulated,the filter has high tolerance to fiber dispersion.The transmission response of a system with 25-or 40-km single-mode fiber(SMF) was measured and compared with an MPF with one double sideband(DSB) modulation tap.The proposed MPF shows much better performance for long-distance fiber transmission.
