Widely tunable frequency-doubling optoelectronic oscillator using a polarization modulator and a phase-shifted fiber Bragg grating

A frequency-doubling optoelectronic oscillator with wideband frequency tunability implemented using a polarization modulator (PolM) and a phase-shifted fiber Bragg grating (PS-FBG) is proposed and experimentally demonstrated. In the proposed structure, the output from the PolM is divided into two branches by an optical coupler. In the lower branch, an optoelectronic feedback loop is formed in which a tunable microwave photonic filter (MPF) is constructed by the joint operation of the PolM, the PS-FBG, a tunable laser source (TLS), a polarization controller (PC), and a polarizer, the MPF serves as an oscillation mode selector that its central frequency is a function of the frequency difference between the TLS and the notch of the PS-FBG. In the upper branch, an equivalent Mach–Zehnder modulator (MZM) is performed by the joint use of the PolM, a second PC, and a second polarizer, the second PC is adjusted to let the bias point of the MZM located at the minimum transmission point (MITP), which generates an optical signal with two sidebands at the ±first orders. By beating the two sidebands at a second photodetector (PD), a frequency-doubled microwave signal is generated. By simply adjusting the wavelength of the TLS, the central frequency of the MPF is shifted, and the fundamental oscillation and its frequency-doubled signals can be tuned. The detailed theoretical study is achieved, and a verified experiment is established. A fundamental oscillation signal with a frequency tuning range from 9.39 to 15.4 GHz is generated in the OEO loop, which is multiplied to generate a tunable frequency-doubled signal. Their overall performances are also investigated.