  Femtosecond frequency combs have boosted progress in various fields of
precision metrology. Nevertheless, demanding applications such as front-end
frequency and time standards, ultrastable microwave generation or
high-resolution spectroscopy still necessitate improved stability. The spectral
bandwidth and absolute position of individual comb lines are crucial in this
context. Typically, both parameters are controlled on short and long time
scales by tight locking to external optical and microwave references which
represent costly and cumbersome additions to the entire setup. Here, we
demonstrate fully self-controlled stabilization of a fibre-based femtosecond
frequency comb requiring neither optical nor radio frequency external
references. In the first step, this technology allows us to optically eliminate
the carrier-envelope phase slip via ultrabroadband difference frequency
generation. The resulting amplification of intrinsically quantum-limited phase
noise from the mode-locked oscillator is elegantly addressed in the second
step. We efficiently suppress these excess fluctuations by a direct transfer of
the superior short-time noise properties of the fundamental oscillator to the
offset-free comb. Our combined scheme provides a high-precision frequency
reference operating completely autonomously, thus marking a new era for
fibre-based sources in advanced applications ranging from space exploration to
tests of the invariability of fundamental constants.
