Broadband microwave frequency comb based on optoelectronic oscillator assisted by acousto-optic frequency shift

Abstract

An acousto-optic frequency shift (AOFS)-assisted optoelectronic oscillator (OEO) for generating broadband microwave frequency combs (MFCs) has been proposed and experimentally demonstrated in this paper. Periodic gain modulation and main frequency oscillation are achieved using AOFS dual-loop and phase-shifted fiber Bragg gratings (PS-FBGs), respectively. The AOFS dual-loop is employed to assist in generating the optical carrier-based microwave interferometry (OCMI), thereby expanding the frequency-domain bandwidth of the MFC. Compared to traditional actively mode-locked optoelectronic oscillators (AML-OEOs) with bandwidth of only tens of MHz, the AOFS-assisted OEO can generate MFCs with bandwidth of the multiple GHz range. Moreover, the interference frequency produced by the OCMI is related to the optical path difference (OPD) of the OCMI. Thus, switchable modulation frequency can be achieved by adjusting the length of the OCMI arms. In the experiments, 5 GHz bandwidth MFCs with free spectral ranges (FSRs) of 4.92754 MHz, 3.62319 MHz, 2.75362 MHz, and 1.3913 MHz have been successfully generated.