I. Introduction
Optoelectronic oscillator (OEO) is suitable for generating ultra-low phase noise microwave source [1], which can be used in communication and measurement instruments. Previously reported literature has reached the phase noise performance of −163 dBc/Hz at 6 kHz offset when oscillating at 10GHz [2]. To achieve such low phase noise, high cavity Q factor is needed. One way of improving cavity Q is to increase the length of fiber. However, mode spacing is inversely proportional to cavity length. The fiber longer than 1km corresponds to the mode spacing less than 200 kHz, thereby spurious modes are difficult to be filtered out both optically and electrically. To reduce the spurious level, inject-locking and dual-loop OEO have been widely studied [3]–[6]. The injection locking caused 55dB suppression of spur [4] while dual-loop structure achieves 21dB [7] typically.