Wideband frequency-tunable optoelectronic oscillator with ultra-narrow linewidth based on stimulated Brillouin scattering

Abstract

We propose and theoretically analyze a wideband frequency-tunable optoelectronic oscillator based on stimulated Brillouin scattering (SBS) and gain–loss superposition with ultra-narrow linewidth. Typically, the linewidth of optoelectronic oscillator (OEO) based on SBS is difficult to narrow because of the relatively fixed 3-dB bandwidth of SBS. In the structure, three generated new pump lights with certain frequency intervals generated by optical modulation on the pump laser light are launched in a highly nonlinear fiber (HNLF) from the direction opposite to the phase modulated signals, where the SBS effect occurs. The gain spectrum of one pump light is superimposed with the loss spectra of the other two pump lights. As a result the linewidth of the output signal reduces highly compared with conventional OEOs based on SBS. Consequently, a microwave signal with wide tunable range of 1 GHz to 40 GHz and near 60% linewidth reduction at most is realized in theory. Particularly, the theoretical simulation of the phase noise is about −112.5 dBc/Hz at 10 kHz offset when the wavelength of the continuous wave laser is 1550 nm and the frequency of the output signal is 35 GHz. In addition, the proposed method can be generalized, theoretically, to any OEO based on SBS to effectively reduce its linewidth. Overall, the proposed structure improves the quality of generated signals prominently and has vast potential in microwave generation.