Photonics-based radar with balanced I/Q de-chirping for interference-suppressed high-resolution detection and imaging

Photonics-based radar with a photonic de-chirp receiver has the advantages of broadband operation and real-time signal processing, but it suffers from interference from image frequencies and other undesired frequency-mixing components, due to single-channel real-valued photonic frequency mixing. In this paper, we propose a photonicsbased radar with a photonic frequency-doubling transmitter and a balanced in-phase and quadrature(I/Q)de-chirp receiver. This radar transmits broadband linearly frequency-modulated signals generated by photonic frequency doubling and performs I/Q de-chirping of the radar echoes based on a balanced photonic I/Q frequency mixer, which is realized by applying a 90° optical hybrid followed by balanced photodetectors. The proposed radar has a high range resolution because of the large operation bandwidth and achieves interference-free detection by suppressing the image frequencies and other undesired frequency-mixing components. In the experiment, a photonics-based K-band radar with a bandwidth of 8 GHz is demonstrated. The balanced I/Q de-chirping receiver achieves an image-rejection ratio of over 30 dB and successfully eliminates the interference due to the baseband envelope and the frequency mixing between radar echoes of different targets. In addition, the desired dechirped signal power is also enhanced with balanced detection. Based on the established photonics-based radar,inverse synthetic aperture radar imaging is also implemented, through which the advantages of the proposed radar are verified.

Ultra-high resolution microwave photonic radar with post-bandwidth synthesis

We demonstrate microwave photonic radar with post-bandwidth synthesis, which can realize target detection with ultra-high range resolution using relatively small-bandwidth radio frequency(RF) frontends. In the proposed radar, two temporal-overlapped linear frequency-modulated(LFM) signals with the same chirp rate and different center frequencies are transmitted. By post-processing the de-chirped echoes in the receiver, a signal equivalent to that de-chirped from an LFM signal with the combined bandwidth is achieved. In a proof-ofconcept experiment, two LFM signals with bandwidths of 8.4 GHz are exploited to achieve radar detection with an equivalent bandwidth of 16 GHz, and a range resolution of 1 cm is obtained.

In-band intermodulation induced by transient response of erbium-doped fiber amplifier

Transient response of an erbium-doped fiber amplifier(EDFA) is studied in an externally-modulated analog link.Double tones represented as transmitted radio frequency and dither signals are introduced.Extra modulation is generated owing to the EDFA’s transient response caused by a low-frequency dither signal.Therefore,the parasitic modulation is superposed to the output signals and may significantly affect in-band electrical spectra.Analytical and numerical solutions are both given,which agree well with experimental results.This work indicates that a suitable dither signal should be selected to maximize the carrier to intermodulation ratio.In-band spurious free dynamic range is optimized in the meantime.