Linden Photonics offers a standard single mode fiber with a special liquid crystal polymer (LCP) coating. This material has a negative coefficient of thermal expansion, which significantly reduces the change of signal delay due to temperature change. Other than with normal fiber cables, the thermal coefficient of delay (TCD) is not in the range from 40 to 130 ps/km/K, but below 7 ps/km/K. Thus it is nearly as good as the phase stabilized optical fiber (PSOF) by Furukawa (specification < 5 ps/km/K). The TCD of Linden Photonic cables have been measured at DESY. In order to make this comparable, the PSOF by Furukawa and furthermore three standard optical fiber cables were measured, too. As a result, a complete picture of the TCDs of different fibers will be presented. This information is very useful in order to design optical synchronization systems in general. It is planned to use Linden Photonics cables for the long links (fiber lengths in a range from 0.4 to 3.5 km) of the laser based synchronization system at XFEL. INTRODUCTION In the laser based synchronization system at FLASH [F], delay changes of 300 ps have been observed on fiber links with a length of up to 400 m. The latest link stabilizations units are able to compensate delay changes of about 666 ps [B]. Assuming the same delay changes at XFEL and scaling them to a length of 3.4 km, it is desirable to reduce the delay change by at least a factor of 5 to below 666 ps. Thus, the same link stabilization used at FLASH can be used at XFEL. The reduction of the delay change can be achieved by using a phase stable optical fiber instead of a conventional one. Furthermore, such cables are always useful on fiber links were an active stabilization is not possible. THEORY The TCD of a fiber that is mechanically coupled with the jacket can be calculated [P]