The main objective of this proposal is to build novel, robust light sources for a transportable optical clock and to contribute, thereby, to the future redefinition of the second by providing a low-uncertainty link between institutes operating optical clocks. This grant will also enable the applicant to continue to collaborate with his former group and collaborators in the field of attosecond XUV physics. The applicant hopes to apply techniques from his former field to push frequency comb technology into the XUV regime and thus to contribute to the emergence of precision XUV spectroscopy/frequency metrology and nuclear clocks.Time and frequency metrology is going through the greatest changes since the first demonstration of the Caesium atomic clocks in the 1950’s. This revolution is caused by the arrival of optical atomic clocks, the development of which was sparked by the demonstration of self-referencing femtosecond frequency comb technology in 2000. In essence, frequency combs make it possible to count optical cycles.The potential for great performance improvements over Caesium atomic clocks have been known for a long time, but it was not until suitable clockwork was found that considerable progress was made. Today, several groups already report fractional uncertainties down to 10E-17, thus beating the best Caesium clocks by an order of magnitude. Time and frequency are the two quantities that can be measured the most precisely, and the unit of time is considered the most fundamental base unit. Thus, the development of optical clocks is expected to have widespread impact on science and our daily lives; however, before that happens, several technological and fundamental challenges must be overcome. A clear challenge is to develop more accurate methods to compare the frequencies of existing optical clocks, which are scattered around the world. Transportable clocks are thus needed, which in turn require robust light sources - hence the objectives of this work