Photonic generation of programmable coherent linear frequency modulated signal and its application in X-band radar system

A photonic generation scheme of a programmable coherent linear frequency modulated (LFM) signal is proposed and experimentally demonstrated. By heterodyne beating an agile optical frequency comb (OFC) with a fixed OFC, LFM signals in different bands can be obtained. The two OFCs are generated independently from two arms of a single dual-drive Mach-Zehnder modulator (DDMZM) and are then propagated through the same optical path, thus naturally ensuring the coherence of the LFM pulses at the minimum cost. The generation of the agile OFC only requires a sweeping intermediate frequency (IF) signal with the far lower center frequency and smaller bandwidth than the generated LFM signal. And by precisely controlling this IF signal, parameters of the LFM signal can be flexibly adjusted such as the center frequency, bandwidth, time duty, and sweeping pattern. In addition, envelope-tailorable LFM pulsed signals are further realized by pre-distorting the IF signal, supporting future multifunctional radar applications. We then establish a complete X-band radar system. The pulse-to-pulse phase coherent integration of the generated LFM signal enhances the signal-to-noise ratio of echo signals by ∼24 dB. A 5.8 cm range resolution is finally achieved, which is comparable to more complex photonic radar systems. This compact DDMZM based scheme not only improves the radar performance by realizing coherence pulses but also provides full control of these pulses, evidently benefiting the system integration and multifunctionality in future advanced radar applications.