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A pulse radar based on photonics-assisted signal generation and stretch processing is proposed and experimentally demonstrated. A radio frequency (RF) linear frequency modulated (LFM) pulse signal is generated by photonic frequency doubling from an intermediate frequency (IF) LFM pulse signal and fed into an antenna for transmitting. An optical reference signal is generated form another coherent IF-LFM pulse signal with an equaled bandwidth and a doubled pulse duration to IF-LFM used for generating the transmitted pulse, and the generated optical reference signal is split. The split optical reference signals are separately fed into a polarization and phase diversity coherent receiver (PPDCR) as an optical local oscillation signal (OLO) and a dual-polarization quadrature phase shift keying (DP-QPSK) modulator as an optical carrier. In the DP-QPSK modulator, the input light wave is split into two paths with light wave on one path being intensitymodulated by echoes and light wave on the other path not being modulated. The light waves are polarization orthogonally multiplexed at the output of the DP-QPSK modulator and coherently detected with the OLO in the PPDCR. The information of the echoes can be recovered by using a digital signal processing algorithm from the outputs of the PPDCR. Experiments verify the idea of a photonic dechirp pulse synthetic aperture radar (SAR), which can obtain microwave image with high resolution in a long-range.INDEX TERMS Photonic signal generation, photonic dechirp-on-receive, pulse synthetic aperture radar, long range, high resolution, microwave imaging.

A signal-to-noise ratio (SNR) improvement method for microwave photonic (MWP) links enhanced by optical injection locking (OIL) and channelized spectrum stitching (CSS) is investigated and experimentally demonstrated. By exploiting the resonant amplification characteristics of OIL, both optical gain and in-band noise suppression of the input radio frequency signal can be achieved. The injection bandwidth is channelized to further suppress noise during OIL, and the input signal can be well reconstructed by spectrum stitching in the digital domain. Experimental results show that the optimal improvement in SNR of 3.6 dB is achieved for linear frequency modulated signals and at least an additional improvement of 7.2 dB can be obtained by adopting CSS. Other broadband signals for radar and communication are used to further verify the ability to improve SNR. The potential for application scenarios with large operating bandwidth and high optical gain is also demonstrated.