Two-way time and frequency transfer using SONET/SDH

This article presents a novel design for an optical system for time-and-frequency transfer via optical fibres, developed at the Telecommunication Laboratory of Chunghwa Telecom Co. The system includes a transceiver, a controller, and a timing circuit. We determined the number of unused overhead bytes of the SONET/SDH frame, in transmitting a timing signal. The system completed the loop-back test in temperature-controlled conditions, at 23 ± 0.3°C. Experimental results show that short-term time stability can be achieved at 5 ps and long-term frequency stability is better than 1 × 10^-14, regardless of whether the loop-back is a 5-m or 35-km long optical fibre. Moreover, the delay variation in the 35-km loop-back test is less than 1.4 ns.