Low-noise radio-frequency (RF) signal sources play an essential role in many fields and applications, such as radar, wireless communication, aerospace, measurement instruments, and scientific research. The optoelectronic oscillator uses a long fiber link to replace the traditional RF resonator as the high-Q-energy storage element, but this leads to issues related to the large volume, control difficulty, and poor stability. The coupled optoelectronic oscillator (COEO) uses active regeneration mode-locking technology to significantly shorten the length of the fiber ring cavity, which effectively improves its integration and stability. It is necessary to solve the issues of polarization sensitivity and supermode spurious for turnkey-coupled optoelectronic oscillators to realize self-start-up and stable single-mode oscillation.

The proposed turnkey-coupled optoelectronic oscillator mainly comprises an active non-polarization-maintaining fiber mode-locked ring cavity and an RF feedback amplification loop. In the 180 m non-polarization-maintaining fiber ring cavity, an electric absorption modulator (EAM) and a semiconductor optical amplifier (SOA) are used as the regenerative mode-locked element and amplification gain medium, respectively, to achieve self-starting capability and stable oscillation. In addition, the polarization insensitivity of the electric absorption modulation and the rapid gain saturation effect of the semiconductor optical amplification can help realize anti-polarization disturbance and supermode spurious suppression for the turnkey-coupled optoelectronic oscillator.

A turnkey-coupled photoelectric oscillator is demonstrated. The modulation gain fluctuation for an active fiber ring cavity is lower than 2.07 dB in 60 min, which is within the tolerance range for the mode-locked gain of the RF feedback amplification loop. In addition, the issues of the mode spurious and polarization disturbance are resolved, and the proposed 10 GHz oscillator can maintain stable single-mode oscillation after startup. Finally, the phase noise and spurious rejection ratio of the generated signal can reach -125 dBc/Hz at 10 kHz and 76.3 dB, respectively, which would significantly facilitate the development of coupled optoelectric oscillators.

In this study, a turnkey-coupled optoelectronic oscillator based on a 180 m non-polarization-maintaining active fiber ring cavity has been constructed to solve the issues of self-starting and stable oscillation owing to anti-polarization fluctuation and supermode spurious suppression. Finally, a 10 GHz RF signal is successfully generated with phase noise at a 10 kHz frequency offset lower than -125 dBc/Hz and a signal spurious suppression ratio of approximately 76.3 dB.