Stabilized Fiber Links
The main challenge to synchronization of the elements in a large accelerator complex is the thermal variation of the lengths of the connections between various elements. A stabilized link actively compensates for this variation. Our design for a stabilized link achieves this by setting up a large interferometer along the length of the optical fiber used for transmitting the signals. A slow feedback system accounts for the change in the fiber length by measuring the relative optical phase of the transmitted and reflected optical reference. This approach has proven to be extremely effective and has allowed stabilization of the fiber down to the femtosecond level over several kilometers.
Fig. 1.: Laser light at near-infrared frequencies is reflected by a Faraday rotating mirror, which compensates for changes in polarization caused by the optical fiber. Before the light is reflected, its frequency is slightly shifted to distinguish true path length changes from other changes accruing from intermediate discontinuities. The original and reflected optical waves are combined and their phases compared, generating a signal that introduces compensation in the path length.