  During the last years the transfer of frequency signals through optical
fibers has shown ultra low instabilities in various configurations. The
outstanding experimental results of such point-to-point connections is
motivation to develop a means to extend the frequency transfer to accurate time
transfer. We aim at the synchronization of clocks located at different places
of the PTB campus with an over-all uncertainty of less than 100 ps. Such an
installation can be used as a part of the infrastructure connecting local time
scales with the ground station setup during forthcoming ACES experiments and
the local two-way satellite time and frequency transfer (TWSTFT) installations.
This paper reports on the progress on time transfer through optical fibers
(TTTOF) similar to the well known and long established TWSTFT scheme: A 10 MHz
signal is transferred through an optical fiber connection to a remote site for
realization of a time scale using a 10 MHz to 1PPS divider. For time transfer
the 1PPS output is initially synchronized and then monitored by TWSTFT
equipment connected by a second optical fiber to observe instabilities and
uncertainties. Depending on further needs in future the transferred signal can
be monitored only (for software correction) or controlled in real time by
adding adequate phase shifters. We discuss procedures for a proper calibration
of such TTTOF links and show results of experiments using fiber lengths up to
two kilometers which prove that the proposed method is well suitable for the
envisaged purpose.
