Time and frequency transfer in optical fibers

The development towards more services in the digital domain, based on computers and server logs at different locations and in different networks, increases the need for high precision time indication. Even though GPS can support this with sufficient precision, many users do not have access to outdoor antennas. Furthermore, there is vulnerability in the weak radio-transmission from the satellites (NSTAC) as well as the dependence on the continuous replacement of old and outdated satellites (Chaplain). Therefore, alternative systems to support precise time are needed. Standardization of time transfer of a master clock is done for example in the IRIG system, but this one-way time transfer system do not take variations in transfer time into account, mainly because it is supposed to work on short distances (IRIG). In additional efforts to meet this request, several time and frequency transfer methods using optical fibers have been developed or are under development, using dedicated fibers (Kihara; Jefferts; Ebenhag2008; Kefelian), dedicated capacity in existing fiber networks (Calhoun) or already existing synchronization in active fiber networks (Emardson, Ebenhag2010a). A similarity of all these techniques is the need for two-way communication to compensate for the inevitable variations of propagation time, such as variation of temperature and mechanical stress along the transmission path. A two-way connection may however be undesirable when many users are connected in one network, or when user privacy is requested. As an alternative, a one-way transmission over fiber optic wavelength division multiplexing network with detection of variation in propagation time has been presented (Ebenhag2010b, Hanssen). The general conception of fiber optic communication is the transmission of digital data from one user to another, and through recovery of the phase variation of the bit-slots after reception, the exact time it has taken to transfer the data is of low importance. The individual packets of the data may even follow different paths with different propagation time, and still be interpreted correctly at the user end. Physical effects such as noise, dispersion and polarization dependence are important, but as long as each bit can be detected correctly, slow variations in propagation time do not affect the communication. When the fiber is used to transmit time or frequency however, the physical properties of the transmission link become very important. Even though time and frequency may appear as two faces of the same parameter, there are differences in the requirement of a transmission link. For time transfer, any variations in the delay through the link must be compensated for, either in a real time compensator or through post processing. For frequency transfer, the frequency shift caused by the rapidity of a change in the fiber delay must be handled.