Photonic Stretch Receiver Using an Optical Phase Correlated Reference Signal in a Coherent-detection Link

A novel photonic technique to perform stretch processing for a linearly-frequency-modulated (LFM) radar based on a coherent-detection link is proposed and experimentally demonstrated. In the signal generation unit, a dual-polarization quadrature phase shift keying (DP-QPSK) modulator is used to generate two orthogonally polarized optical signals with one been intensity-modulated by echoes and the other intensity-modulated by a RF reference signal which acts as an optical reference signal. The orthogonally polarized optical signals are transmitted over a polarization-maintaining fiber (PMF) to a polarization and phase diversity coherent receiver (PPDCR). In the receiver, the orthogonally polarized signals are coherently detected with an optical oscillator (OL) which is generated by modulating the light wave with the RF reference signal on a null-biased Mach-Zehnder modulator (MZM). Since the orthogonally polarized optical echoes and the reference signal are transmitted over the same PMF, the optical phases are correlated, and the stretched echoes can be recovered based on a digital signal processing algorithm. The proposed technique is experimentally evaluated through a series of inverse synthetic aperture radar (ISAR) imaging tests.