Fig. 1. Schematic diagram of the proposed photonics-based radar. LD, laser diode; ESG, electrical signal generator; OC, optical coupler; MZM, Mach–Zehnder modulator; OBPF, optical bandpass filter; PA, power amplifier; LNA, low-noise amplifier; PD/BPD, (balanced) photodetector; ADC, analog-to-digital conversion; DSP, digital signal processing. (Insets, optical spectra at several key points in the system.)

Fig. 2. Optical spectra at several key points in the proposed photonic radar. Dotted blue line, output of the MZM in the transmitter (point A); solid red line, output of the OBPF (point B); dashed green line, output of the MZM in the receiver (point C).

Fig. 3. Spectrograms of the signals (a) before and (b) after the photonic frequency doubling in the transmitter. The power profiles are projected to the time and frequency domains.

Fig. 4. Comparison between the de-chirped signals obtained by single-end PD (SPD, dotted blue line) and BPD (solid red line), including time-domain waveforms of (a) the I channel and (b) the Q channel, and the corresponding spectra of (c) the I channel and (d) the Q channel by FFT.

Fig. 5. Results of the photonic I/Q radar receiver. (a) Zoom-in view of the captured waveforms; (b) spectra of the real de-chirped signal from the I channel and the combined complex de-chirped signal; (c) and (d) zoom-in views of the spectra around the peaks for indicating the range resolution of the proposed photonic radar.

Fig. 6. (a) and (b) Experimental setup, and (c)–(e) results of the ISAR demonstration.

Fig. 7. ISAR imaging results when using a laser source with (a) <2-fm wavelength dither and (b) 800-fm wavelength dither.

Fig. 8. Simulated results on the FoM of the image with different laser wavelength dithers and different uncompensated differential delays between two branches. The hyperbolic fit of the contour line at 0.5, which is considered as the threshold of the acceptable imaging, is plotted as the dotted blue line. Images under four typical conditions at points A, B, C, and D are also depicted as the insets.