  We report on the characterization, including residual phase noise and
fractional frequency instability, of fiber-coupled PPLN non-linear crystals.
These components are devoted to frequency doubling 871 nm light from an
extended-cavity diode laser to produce a 435.5 nm beam, corresponding to the
ytterbium ion electric quadrupole clock transition. We measure doubling
efficiencies of up to 117.5 %/W. Using a Mach-Zehnder interferometer and an
original noise rejection technique, the residual phase noise of the doublers is
estimated to be lower than ${\rm -35\, dBrad^2/Hz}$ at 1 Hz, making these
modules compatible with up-to-date optical clocks and ultra-stable cavities.
The influence of external parameters such as pump laser frequency and intensity
is investigated, showing that they do not limit the stability of the
frequency-doubled signal. Our results demonstrate that such compact,
fiber-coupled modules are suitable for use in ultra-low phase noise
metrological experiments, including transportable optical atomic clocks.
