A Continuously-Tunable Optoelectronic Oscillator With Full Locking Range Utilizing Three Frequency Tuning Mechanisms

A continuously tunable optoelectronic oscillator (OEO) with three frequency tuning mechanisms is proposed to enable frequency locking covering the whole working band. Three frequency-tuning mechanisms are employed simultaneously in the OEO, including a course tuning of microwave photonic filter, a fine tuning of phase shifter and a frequency calibration of direct digital synthesizer (DDS). The introduction of frequency compensation signal achieved by DDS increases the locking range of OEO so that the lock-out problem caused by mode hopping can be effectively solved. In the proof-of-concept experiment, a 3-6 GHz tunable RF oscillating signal has been successfully verified. The locking range of the proposed OEO is extended over 11 times compared to conventional OEO. With a 500 m optical fiber inserted in the loop, the phase noise is measured to be -110 dBc/Hz at 10 kHz offset. The Allan deviation is improved from 1.7 x 10(-8) to 1.6 x 10(-11) at 1000 s averaging time. To the best of the authors' knowledge, this is the first time that a frequency tunable OEO with full locking range has been implemented.