Ultra-stable frequency transfer using optical fibers

Sammanfattning: A new era of precise time measurement came with the atomic clock. The technology is vital to navigations systems like GPS and to accurate physical measurements. Improvements in laser cooling and the development of frequency combs have combined to enable the construction of clocks using optical reference transitions, which are more stable than the Cesium standard with references in the microwave regime. Optical fiber networks have been used to transmit stable frequencies and compare clocks but setups so far use specialized equipment. It would be advantageous to be able to use the existing optical fiber infrastructure. In this thesis a frequency is stabilized over 80km of fiber in the lab, reaching an instability of 10^(-15) in 7000 seconds. With a 1542.14 nm laser, frequency shifted with an acoustic optical modulator, the signal is transmitted along the fiber. A heterodyne detection scheme allows phase information to be received. A phase-locked loop regenerates the signal after the fiber and a PI controller locks the phase to a stable reference. The maximum noise frequency that can be regulated is affected by the latency in long fibers so the delays in two pairs of fibers are measured. The 80km fiber displays a delay of 0.4ms, close to the theoretical calculated value. The 2x60km fiber that goes from SP Technical Research Institute of Sweden in Borås to Chalmers in Gothenburg display a delay of about 1ms, when the calculated value for such a distance of fiber is 0.6ms. Further on polarization changes in the 80km fiber and the 2x60km fiber between Boråas and Gothemburg is examined. The 2x60km fiber show regular variations in time at 200Hz.