An Active Time-Domain Mode-Locked Optoelectronic Oscillator Based on Intracavity Polarization Control

An active time-domain mode-locked (TDML) optoelectronic oscillator (OEO) is proposed and experimentally demonstrated based on intracavity polarization control. In the TDML OEO, the optoelectronic feedback loop gain is periodically controlled to above or below the OEO oscillation threshold using an electrical-controlled optical attenuator, which consists of two polarizers and one electronically controlled optical polarization controller (OPC). The loss of the optical attenuator is electrically controlled by adjusting the OPC to tune the polarization angle difference between the polarizer after the OPC and the linearly polarized light from the OPC. The active mode-locking is implemented when the period of loop loss matches the loop delay. The harmonic mode-locking is performed by increasing the loop-loss control frequency to ensure the time loop delay is an integer multiple of the loop loss period. Then, a microwave pulse is generated from the TDML OEO, where the pulse duty factor is tuned by tuning the ratio of the loop gains duration above the oscillation threshold and the loop gains duration below the OEO oscillation threshold. Simulation analysis and experimental demonstration are performed. In the experiment, microwave pulses with different duty factors ranging from 20% to 80% are generated at the modulation shape of a square. Microwave pulse generation with sinusoidal and triangular modulation shapes are also studied.