Timing and Synchronization Systems
To produce an ultra-stable timing and synchronization system with jitter reduced to the few femtosecond level, we have developed a laser-based scheme with optical signals distributed over a stabilized optical fiber . Transmitting precise frequency and timing signals over distances of hundreds of meters, stabilized to a few femtoseconds (a few parts in 108), is accomplished by measuring the phase delay in an optical fiber and actively compensating for differences with a piezoelectric modulator.
In our scheme, illustrated in Figure 1, phase differences at optical frequency are down-converted to 110 MHz. Because phase information is preserved during the heterodyning process, phase differences at optical frequency can be detected at radio frequencies, using conventional RF electronics. The radiofrequency reference signal need not be provided with femtosecond accuracy at the far end of the fiber, because one degree of error at 110 MHz is equivalent to only one degree at the optical frequency, or 0.014 fs. The system is linear, and signals modulated onto the CW laser carrier at the fiber entrance do not intermodulate with each other. Moreover, the optical power level is significantly below any nonlinear threshold in the fiber. The laser frequency itself must be stabilized, so the laser is locked to an absorption line in an acetylene cell.
At present, a 3 km fiber link has been stabilized to the sub-femtosecond jitterlevel. 2 km of fiber in this link passes under several roads and through several buildings at LBNL, demonstrating that the fiber stabilization system is robust under real-world conditions. This technique will soon be used as a backbone to demonstrate synchronization of mode-locked lasers. Further developments will include integration with controls and low-level RF systems, and high-resolution diagnostics of photon and electron beams, to provide enhanced feedback control of the integrated laser/accelerator systems. We are developing such systems for implementation at the LCLS.
Figure
1. Schematic of the frequency-offset method for distribution of timing and
synchronization signals with femtosecond-scale timing stability over long
distances (100 m fiber in this example, extendable to km scale).
