Ultrastable Frequency Transfer over Optical Fibers

The development of optical frequency standards has a strong impact on metrology, astronomy and on fundamental physics. Today’s optical frequency standards reach a relative uncertainty below 1 part in a quadrillion. For the dissemination of such stable frequencies, their optical form must be preserved during transfer. Therefore, optical fibers are used to disseminate ultra-stable frequencies to different places around the globe by exploiting the current optical fibers infrastructure that is used for optical communication. In this book, a detailed description for the optical frequency transfer over optical fibers will be introduced. The book will start by giving an introduction about the need for frequency transfer and the available optical frequency standards. Afterwards, an introduction about femtosecond frequency comb will be given and how it can be used for frequency transfer. Thereafter, transmission of light in optical fibers will be explained. The challenges that are faced while transmitting optical frequency through optical fibers such as sensitivity to environmental perturbations, attenuation, Brillouin scattering, chromatic dispersion and polarization mode dispersion will be discussed. The concept of phase noise in frequency domain and Allan Deviation in the time domain will be introduced. An explanation will be given for the main sources of phase noises and how the optical frequencies are changed during transfer by these sources. A compensation technique for these variations will be implemented that is based on Michelson interferometer. An example for optical frequency transfer over 73 km of optical fibers between two cities in Germany will be demonstrated. Thereafter, an efficient technique for optical amplification will be introduced which exploits the Brillouin scattering to amplify very low signals. This technique is implemented for the optical frequency transfer over 480 km fiber. Finally, an overview of the development in this field for longer distance dissemination of optical frequencies is introduced.