Two-Way Time Transfer via Common-path Optical Link

Wen-Hung Tseng, Mingfu Li, Chia-Shu Liao Chunghwa Telecommunication Laboratories 12, Lane 551, Min-Tsu Road Sec.5 Yang-Mei, Taoyuan, Taiwan 326, R.O.C Tel: +886 3 4244185, Fax: +886 3 4245474, e-mail: whtseng@cht.com.tw Abstract We present the experiments of two-way time transfer through a fiber optical link. The fiber link, which is constructed to a common-path configuration, is used to replace the RF cable on the earth station of the two-way satellite time and frequency transfer (TWSTFT). The resulting data exhibits the time deviation of less than 20 ps at one-day averaging time. Introduction There exists a strict challenge to non-reciprocity in delay on the two-way time transfer. The Sagnac effect can be accurately corrected, and the propagation path delay in the satellite communication system can be cancelled out almost entirely. However, the delays on the earth station cannot be cancelled and needs to be carefully calibrated. And, the instabilities in cable delays will reduce the accuracy of the time and frequency transfer. In this paper, we propose a new method to improve the earth-station stability by propagating the RF signals through a common-path optical link. We employ two optical circulators and two pair of optical transceivers to construct the optical link. The RF signals are modulated to optical signals on the same light wavelength, and are transmitted bi-directionally along a single fiber. Thus, the common-path link can avoid the non-reciprocity of path and improve the delay stability of the transmit and receive path on earth stations. Common-Path Optical Link Fig. 1 shows the block diagram of a common-path optical link. It is composed of 1.31μ m directly modulated lasers, PIN photodiodes, optical circulators, and single mode fibers. The key component is optical circulators, which are passive, multi-port, and non-reciprocal optical devices. The circulators utilize the Faraday effect to route light form port 1 to port 2, port 2 to port 3 and are used here for bi-directional transmission. They also have the characteristics of low insertion loss and high isolation. So the signal to noise ratio will keep high and the reflected signal will be isolated to a large extent.