## 5 Common Systems
### Timing and Synchronization
#### 5.1.6 Continuous Wave Fiber-Optic Links for RF Reference Distribution  
The RF transmission to the LLRF stations is done with so-called \"CW optical links\" [198]. These are radio-over-fiber type links based on standard telecom single mode fibers as the transmission medium, which have been developed in collaboration between PSI and Instrumentation Technologies Ltd. (Solkan, Slovenia) The operating principle of the \"Libera Sync 3\" fiber-optic links is based on intensity modulation of CW laser diode light with additional group delay stabilization between the transmitter input signal and the photo-detected light reflected from the link end receiver. The performance goal of these links was set to less than 40 fs drift peak-peak over 24 h (the typically measured drift is approximately 20 fs peak-peak for a 500 m link) and less than 10 fs rms integrated \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Client (0)** & **Reference Signal at Client** & **Distribution (Link Type)** & \begin{tabular}{l} **Stability Goal** \\ **jitter 1/drift 2 ()** 3 \\ \end{tabular} \\ \hline Gun and Experiment Lasers (4) & 142.8 MHz optical fs pulses & stabilized pulsed optical & \begin{tabular}{l} few \(f_{5 ms/}\)/\(<\)10 \(f_{5p-p}\) \\ (\(<\)1 fs\({}_{rms/}\)/few fs\({}_{p-p}\)) \\ \end{tabular} \\ BAM (4, later 6) & 142.8 MHz optical fs pulses & stabilized pulsed optical & \begin{tabular}{l} few \(f_{5 ms/}\)/\(<\)10 fs\({}_{p-p}\) \\ (\(<\)1 fs\({}_{rms/}\)/few fs\({}_{p-p}\)) \\ \end{tabular} \\ LAM (2) & 142.8 MHz optical fs pulses & stabilized pulsed optical & \begin{tabular}{l} few \(f_{5 ms/}\)/\(<\)10 fs\({}_{p-p}\) \\ (\(<\)1 fs\({}_{rms/}\)/few fs\({}_{p-p}\) 5) \\ \end{tabular} \\ S-band RF (6) & 2998.8 MHz RF (21 \(\times\)\(t_{rep}\)) & stabilized CW optical & \begin{tabular}{l} \(<\)10 fs\({}_{rms/}\)/\(\sim\)30 fs\({}_{p-p}\) \\ (\(\sim\)3 fs\({}_{rms/}\)/\(<\)20 fs\({}_{p-p}\)) \\ \end{tabular} \\ C-band RF (27) & 5712 MHz RF (40 \(\times\)\(t_{rep}\)) & stabilized CW optical & \begin{tabular}{l} \(<\)10 fs\({}_{rms/}\)/\(\sim\)40 fs\({}_{p-p}\) 4 \\ \end{tabular} \\ X-band RF (S-band front end) (1) & 11,995.2 MHz RF (84 \(\times\)\(t_{rep}\)) & \
\begin{tabular}{l} stabilized CW optical \\ + quadrupuler \\ \end{tabular} &
\begin{tabular}{l} \(<\)10 fs\({}_{rms/}\)/\(\sim\)30 fs\({}_{p-p}\) \\ (\(<\)3 fs\({}_{rms/}\)/\(<\)30 fs\({}_{p-p}\)) \\ \end{tabular} \\ BPM (46) & 142.8 MHz RF & VHF CW optical, coaxial & not critical \\ \hline Event System (1) & 142.8 MHz RF 6 & coaxial & not critical \\ \hline \hline \end{tabular}  
* 10 Hz-10 MHz offset frequency range; 2 per 1/2 day – day; 3 potential of the technology; 4 up to 500 fs\({}_{p-p}\) (depends on station); 5 with polarization maintaining fibre as transmission medium; 6 eventually 1428 MHz.  
\end{table} 
Table 5: The clients of the Timing Reference Distribution, the types of links and their stability goals. jitter in the frequency range from 10 Hz to 10 MHz (typically measured jitter is around 2.4 fs rms for a 500 m link and is limited by the measurement setup). Nearly 90% of the links which are needed for SwissFEL operation are currently at PSI (no spares in case of failure), all of which have been tested for drift and jitter performance and have gone through a burn-in phase of about 1000 h. To date, about 70% of the links are installed in SwissFEL and are in operation. For the operation of the links, communication between transmitter and receiver over ethernet is necessary. Because of initial problems with high network traffic and ethernet module failures of the link devices, the CW links have their own virtual LAN along the entire machine. Furthermore, for a full maneuverability during setup and operation, an EPICS based remote control interface was built, which can be used in parallel for all links. For the injector, 6 slightly de-tuned European standard S-band frequencies and one European X-band frequency link are needed. The three linac sections need 9, 4 and 14 American standard C-band frequency links, correspondingly.