  Ultrahigh-resolution optical strain sensors provide powerful tools in various
scientific and engineering fields, ranging from long-baseline interferometers
to civil and aerospace industries. Here we demonstrate an ultrahigh-resolution
fibre strain sensing method by directly detecting the time-of-flight (TOF)
change of the optical pulse train generated from a free-running passively
mode-locked laser (MLL) frequency comb. We achieved a local strain resolution
of 18 p{\epsilon}/Hz1/2 and 1.9 p{\epsilon}/Hz1/2 at 1 Hz and 3 kHz,
respectively, with largedynamic range of >154 dB at 3 kHz. For remote-point
sensing at 1-km distance, 80 p{\epsilon}/Hz1/2 (at 1 Hz) and 2.2
p{\epsilon}/Hz1/2 (at 3 kHz) resolution is demonstrated. While attaining both
ultrahigh resolution and large dynamic range, the demonstrated method can be
readily extended for multiple-point sensing as well by taking advantage of the
broad optical comb spectra. These advantages may allow various applications of
this sensor in geophysical science, structural health monitoring, and
underwater science.
