Reduction of Spurious Tones and Phase Noise in Dual-Loop OEO by Loop-Gain Control

Although a long loop delay in an optoelectronic oscillator (OEO) is essential for achieving a low-phase-noise signal, it generates narrowly spaced spurious modes that cannot be filtered out. Conventional dual-loop OEOs proposed to suppress the spurious modes exhibit degraded phase-noise performance compared with single-loop OEOs because of the short fiber loop in dual-loop OEOs. To overcome the decrease in phase-noise performance, we investigated both theoretically and experimentally the open-loop characteristics of the dual-loop OEO. We found that the spurious modes can be eliminated without degradation in the phase-noise performance by properly controlling the open-loop gain of the OEO. Through gain control, we successfully demonstrated the suppression of spurious modes by more than 21 dB.