Phase-coded coherent microwave pulse generation based on an actively mode-locked optoelectronic parametric oscillator.

An approach for generating phase-coded coherent microwave pulse trains at high frequencies is proposed and demonstrated based on an actively mode-locked optoelectronic parametric oscillator (AML-OEPO), where an electrical mixer is inserted into the cavity of an optoelectronic oscillator (OEO) to achieve both mode locking and parameter oscillation. The driving signal applied to the mixer is a low-frequency sinusoidal signal with voltage polarity coding, where the frequency is the same as the free spectral range (FSR) of the OEO cavity, and the duration of each voltage polarity coding bit is identical to the loop delay. As a result, phase-coded coherent microwave pulse trains can be generated, where the pulse interval is equal to the loop delay due to the active mode locking effect, and the phase coding period is equal to a multiple integer of the loop delay due to parameter oscillation. The enhancement of the signal period and the highly coherent characteristic are beneficial for breaking the contradiction between unambiguous detection range and ranging resolution in pulse radars. In the experiment, phase-coded microwave pulse trains with either 13-bit barker codes or 7-bit M codes are generated at 15.026 GHz. The autocorrelation calculation result of the phase-coded microwave pulse train with 13-bit barker codes shows a high peak-to-sidelobe ratio, verifying high coherence.