Time and frequency transfer in an optical fiber network: First results

A technique for time and frequency transfer over an asynchronous fiber-optical TCP/IP network is being developed in Sweden by SP Measurement Technology together with STUPI. The technique is based on passive listening to existing data traffic at 10 Gbit/s in the network. Since the network is asynchronous, intermediate supporting clocks will be located and compared at each router. We detect with a specially-designed Header Recogniser high-speed optoelectronic device the frame alignment bytes of the SONET/SDH protocol as references for the supporting clock comparison. The goal of the project is to establish a time transfer system with a precision on the ns level. We have performed an assessment of the technique in a lab environment. The root mean square (rms) difference between the values measured with the fiber network technique and using GPS phase observations is approximately 40 ps. Hence, we conclude that the noise contribution from the hardware will not be a limiting factor in the time transfer system.