Tunable frequency-doubled optoelectronic oscillator based on a phase modulator in a Sagnac loop

A tunable frequency-doubled optoelectronic oscillator (FD-OEO) scheme based on a phase modulator (PM) located in a Sagnac loop is proposed and experimentally demonstrated. A light wave is halved and sent to the Sagnac loop along the clockwise (CK) and counter-clockwise (CCK) directions. The light wave along the CK direction is effectively modulated in the PM, while the one along the CCK direction is not modulated due to velocity mismatch. The optical output from the Sagnac loop is separated into two parts. One part is fed back to the radio-frequency (RF) port of the PM to form an OEO loop and obtain a fundamental signal. The other part is used to obtain a frequency-doubled signal by suppressing the optical carrier. The fundamental frequency of the OEO is continuously adjustable within the range from 3 to 20 GHz using a tunable electric filter, so a frequency-doubled signal with a frequency ranging from 6 to 40 GHz can be obtained. The phase noise performance of the generated fundamental and frequency-doubled signal is also studied.