# Opto-Electronic Oscillators for Micro- and Millimeter Wave Signal Generation
## 2 Current Progress of the Common Topologies of the OEO
### Progress of the OEO toward Lower SMSR and Phase Noise  
As mentioned earlier, one of the general challenges associated with OEO design is multimode operation. Since an electrical bandpass filter has bandwidth limitations, some optical solutions have been proposed. One of the most popular solutions is to form more than one optical delay line. These dual or multi-loop OEOs have seen widespread use for more than 20 years to eliminate multimode operation [63]. The typical configuration of a dual-loop OEO is shown in Figure 3. In this configuration, one loop is used as a long cavity, while the other behaves as a short cavity. This is achieved by using short and long fibers in different loops. One of the recent advances in the dual-loop OEO is the use of multicore fibers and the self-polarization-stabilization technique [64]. Conventionally, two or more single-mode fibers (SMFs) are used to form a dual- or multi-loop configuration. In this novel approach, the combination of cores in a multicore fiber is used to form a short and a long cavity [64]. With this novel approach, an SMSR of 61 dB was achieved for a microwave OEO oscillating at 7.8 GHz. Another recent approach is parity symmetry of the OEO with a dual-loop configuration [65]. The parity symmetry is achieved by using two optical carriers with different optical powers. Parity symmetry provides additional gain for the main mode. With the combination of parity symmetry and a dual loop, a 60.71 dB SMSR was achieved for a 10 GHz central frequency. Figure 2: Comparison of phase noise and side-mode suppression ratio (SMSR) performance of OEO with 1 km and 15 km delay line length [1]. Reprinted with permission from ref. [1]. Copyright 2015 IEEE. An injection-locked OEO is another configuration to improve the SMSR performance of the OEO. It was first proposed in 2005 [63; 66]. The typical configuration of an injection-locked OEO is shown in Figure 4. In the typical configuration of the OEO, it consists of two oscillator blocks. One of them is classified as the master OEO and other as the slave OEO [63]. This approach is used to suppress the spurious side modes and at the same time maintain the quality factor (Q-factor) of the OEO [63]. The slave OEO is used to suppress the spurious side modes as it employs a short fiber, while the master OEO employs a long fiber to keep the high Q-factor. In reference [67], a tunable, dual-loop, injection-locked OEO was presented. The OEO was tunable from \(11.1\) GHz to \(12.1\) GHz, and the spurious side modes were suppressed below \(-115\) dBC/Hz [67]. Another approach was to form a microwave frequency divider based on the injection-locked OEO [68]. In this implementation, the \(10\) GHz free-running OEO was injected with a \(20\) GHz microwave signal. A single-sideband (SSB) phase noise of \(-130\) dBC/Hz at a \(10\) kHz offset was achieved. In Reference [69], an injection-locked OEO based on stimulated Brillouin scattering (SBS) is described. This approach provided a frequency tunability up to \(40\) GHz with an SMSR of \(60\) dB and an SSB noise of \(-116\) dBc/Hz at a \(10\) Hz offset. The coupled OEO, introduced in 1997 [63; 70], is another type of commonly used OEO configuration to improve SMSR. In this configuration, the OEO consists of an optical loop and an opto-electronic loop coupled via an electro-optic modulator [63]. The coupled OEO is used to improve the phase noise characteristics and the SMSR. The typical configuration of a coupled OEO is shown in Figure 5. An example of a recent development in the coupled OEO was the synthesis of a \(90\) GHz signal using a \(30\) GHz coupled OEO [71]. The \(90\) GHz signal was obtained by biasing the MZM to the third harmonic. An SSB phase noise of \(-104\) dBc/Hz at a \(1\) kHz offset from the \(90\) GHz carrier was achieved in this configuration [71]. On the other hand, a novel, coupled OEO with an erbium-doped fiber (EDF) has been proposed [72]. The novel configuration allows a large spatial hole burning (SHB) using the unpumped EDF to improve the SMSR and the phase noise.