  We have developed a means for accurate time transfer using optical fibers and
aim at the synchronization of clocks located at different places on an
institute campus with an overall uncertainty of 100 ps or better. Such an
installation shall be used as a part of the infrastructure connecting the
ground station setups during forthcoming T2L2 and ACES experiments and the
local installations at the PTB time laboratory. Our target transmission length
is less than 1 km. To transfer time, a code-domain-multiple-access (CDMA)
signal is used for modulation of a laser. Optical signals are exchanged in the
two-way mode to cancel long-term fiber length variation. This is similar to the
well known two-way satellite time and frequency transfer (TWSTFT) scheme. We
discuss procedures for a proper calibration of such time transfer through
optical fibers links and show first promising results of an experiment using a
test loop on the PTB campus with a length of 2 km.
