# Coupled Dual-Loop Optoelectronic Oscillator Based on Stimulated Brillouin Scattering
## 1 Introduction  
An optoelectronic oscillator (OEO) as a new type of microwave signal source, can generate high-quality microwave signals of tens or even hundreds of GHz [1]. It can be applied in many fields to improve the performance of electronic systems, such as radar [2], communication [3], remote sensing and telemetry [4]. The microwave signals generated by the OEO have low phase noise, high spectral purity and a quality factor. In the early research on OEO, a fixed center frequency bandpass filter was used as the frequency selective device to realize microwave signal generation, but this method would limit the tunability of the OEO and only generate a fixed frequency microwave signal. In order to generate frequency tunable microwave signals, many tunable optoelectronic oscillator (TOEO) schemes have been proposed [5; 6; 7; 8]. Common to these schemes is the introduction of a microwave photonic bandpass filter (MPBF) into the OEO, which utilizes the MPBF for frequency selection and frequency tunability. In [5], the MPBF is formed by an injection-locked Fabry-Perot laser diode (FP-LD) that realizes frequency tuning by adjusting the cavity length or the wavelength of the incident light. This solution enables microwave signal generation with a frequency tuning range of 6.41 to 10.85 GHz. However, the stability of the microwave signals generated by the FP-LD are poor. In [6], the MPBF is implemented by a polarization beam splitter (PBS), chirped fiber Bragg grating (CFBG) and a polarization modulator. Frequency tuning can be obtained by adjusting the polarization before the PBS using a polarization controller (PC), which can obtain microwave signals from 5.8 to 11.8 GHz. However, the PC is sensitive to both temperature and environmental vibrations, thus resulting in microwave signal instability. In [7], the MPBF is structured by cascading two phase modulators (PM) and a phase-shifted fiber Bragg grating (PS-FBG). The narrow notch of PS-FBG suppresses the sideband of phase modulation to realize conversion between phase modulation and intensity modulation. The tunability of the microwave signals can be achieved by changing the wavelength interval between the optical carrier and the notch of PS-FBG. The scheme achieves a wide tunability from 3 to 28 GHz, but the PS-FBG is also sensitive to environmental fluctuations, and it is difficult to generate stable microwave signals. In [8], the MPBF is realized by a very simple method that a phase modulator is directly connected with an optical bandpass filter (OBPF) to realize the phase modulation to the intensity modulation conversion. A large range of microwave signals is generated from 4.74 to 38.38 GHz, but this method requires an OBPF with a very narrow passband width. The OBPF is a complicated instrument and difficult to apply in practice. Stimulated Brillouin scattering (SBS) is also considered as an MPBF solution with great potential because it can be integrated into photonic chips [9; 10]. The link research of SBS-based TOEO is significant for SBS integrated on photonic chips, which can guide the link structure of the integrated SBS chips. The SBS-based TOEO needs many improvements in side mode and stability. A variety of methods have been proposed for improving performance of OEO [11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21]. In [11; 12; 13; 14], the researchers increase the side mode suppression ratio (SMSR) by multi-loop structure. In [15], a dual-loop SBS-based TOEO have been proposed and the SMSR is only 35 dB at 10 GHz. An improved SBS-based TOEO has been researched and the SMSR is increased to 45 dB at 10 GHz by employing a polarization multiplexed dual-loop in [16]. However, the scheme still has a high side mode for a high-quality microwave signal and needs to be improved. In fact, the SBS-based TOEOs also have a problem with the stability of frequency and power. The microwave signal generated in the scheme [15; 16] is easy to hop or multi-mode oscillate. In order to improve the stability of OEO, numerous schemes have been proposed [17; 18; 19; 20; 21].