# Opto-Electronic Oscillators for Micro- and Millimeter Wave Signal Generation
## 3 Recent Development of the OEO's Application
### Application for SMSR, FSR and Phase Noise Measurements  
As mentioned several times in the text, one of the main challenges associated with the OEO is multimode operation. There are some analytical approaches to calculating the SMSR [98; 99]. These analytical approaches are challenging and require some system values. We have developed an application of the OEO based on the single-loop OEO with an optical fiber path selector published in 2020 [100]. The application allows rapid measurements of the SMSR, FSR and phase noise of the OEO for different optical lengths. In addition, various electrical bandpass filters can be tested to find the optimum one for the OEO. The application is shown in Figure 13.  
In the configuration shown in Figure 13, a spectrum analyzer (SA) is used to verify that the Barkhausan conditions are satisfied, and a signal source analyzer (SSA) is operated to measure the phase noise, FSR and SMSR. The proposed application has the advantage of verifying, measuring and testing the various electrical bandpass filters in different optical delay lines to find the optimum one. In this application, the optical lengths are changed without the requirement to exchange the optical connectors due to the optical path selector's capability to switch between different fiber speeds. Figure 14 is a photograph of the proposed application setup, while Table 3 shows the experimental results of the OEO around 9.63 GHz, where the different optical lengths are formed.  
Figure 13: OEO application to measure the phase noise, SMSR and free spectral range (FSR) of the electrical bandpass filters at different optical fiber lengths. Reprinted from Reference [100].  
According to the results in Table 3, the tradeoff between the phase noise and the SMSR is clearly seen when the length of the optical delay line is alternated. This application can be used to test various electrical bandpass filters in the OEO loop to find the optimum filter in terms of phase noise and SMSR. Also, the optimum optical delay line can be determined with the use of this application. In addition, with the proposed application, the phase noise result of the OEO can be obtained with the SSA. For instance, in Figure 15, the phase noise performance of an OEO with a length of 7.5 km is presented.  
Figure 14: Photograph of the proposed application of the OEO combined with the optical fiber path selector.  
Figure 15: Phase-noise performance of an OEO with a 7.5 km optical delay line length.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Fiber Length**} & \multirow{2}{*}{**FSR**} & \multirow{2}{*}{**SMSR**} & \multicolumn{2}{c}{**Phase Noise @1 kHz**} & \multirow{2}{*}{**Phase Noise @10**} \\  & & **(kHz)** & & **Offset** & **kHz Offset** \\  & & & **(dBc/Hz)** & & **(dBc/Hz)** \\ \hline
1.25 & 158.5 & 78.1 & \(-\)73.8 & \(-\)120.7 \\
2.50 & 79.3 & 71.1 & \(-\)80.6 & \(-\)123.3 \\
5.00 & 40.2 & 62.5 & \(-\)100.9 & \(-\)123.3 \\
7.50 & 27.2 & 39.9 & \(-\)104.7 & \(-\)125.0 \\ \hline \hline \end{tabular}
\end{table}
Table 3: FSR, SMSR and phase noise measurements of the single-loop OEO for different optical lengths [100].  
As can be clearly seen in Figure 15, \(-\)104.7 dBc/Hz and \(-\)125.0 dBc/Hz were at 1 kHz and 10 kHz offset from the 10.45 GHz carrier. In this application, the phase noise of the OEO for different optical fiber lengths could be easily measured by adjusting the optical fiber path selector.