  We demonstrate a laser-based underwater transfer of radio frequency clock
signal with a phase compensation technique. With this frequency transfer
scheme, a 100 MHz signal has been transferred over 5 m underwater link. Timing
jitter power spectral density, timing fluctuations and instabilities were all
measured to evaluate the quality of the transferred frequency signal. The
experimental results show the root-mean-square (RMS) timing fluctuations of the
transferred frequency signal over 5 m underwater link with and without phase
compensation are 2.1 ps and 9.6 ps within 5000 s, respectively. The relative
fractional frequency instabilities are on the order of 5E-13 at 1 s and 7E-16
at 1000 s for phase-compensated link, and 2E-12 at 1 s and 7E-15 at 1000 sfor
uncompensated link, respectively. The proposed frequency transfer scheme has a
potential application that disseminating Cs and H-master clocks signal over the
underwater transmission link since the stability of the transmission link is
superior to these atomic clocks.
