Tuneable Photonic Microwave Waveforms Generation Based on a Dual-Loop Optoelectronic Oscillator

A novel scheme for generating full-duty-cycle triangular and square pulses based on an optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The scheme can generate a tuneable fundamental tone corresponding to the repetition rate of the pulses without any external radio frequency source. The "Vernier effect" is realised using two chirped fibre Bragg gratings, which can effectively improve the side-mode suppression ratio. The oscillation frequency is tuned from 2.5 to 6 GHz in an experiment to verify the theory, and the phase noise of the generated fundamental tone is measured to be -115 dBc/Hz at an offset frequency of 10 kHz. By appropriately setting the gain of the OEO loop, the power of the first- and third-order harmonics can be adjusted to meet the required power ratio. Periodical triangular pulses with repetition rates of 5 GHz, 6 GHz and square pulses with repetition rates of 2.5 GHz, 5 GHz were successfully generated. The root-mean-square errors are measured as 0.026 and 0.083 at a repetition rate of 5 GHz.