The electrical
signal at the output of the
modulator is amplified using an RF
amplifier and filtered before it is fed
back to the modulator, thereby completing
a feedback loop with gain,
which generates sustained oscillation at
a frequency determined by the filter.
The OEO enjoys extremely high Qfactors
of its optic resonator system
suitable for the low phase noise RF
signal generation. A 10 GHz oscillator
with the phase noise of –160 dBc/Hz at
10 kHz offset has been reported.3 The
main advantage of the OEO is its
phase noise frequency independence
that enables the generation of a low
phase noise signal from microwaves to
upper millimeter-wave frequencies. A
disadvantage in its simple form is its
natural multimode operation resulting
in gaps in the frequency coverage as
well as relatively high spurs. A YIG filter
has been used to extend the tunability
of the OEO, while the spurious
problem has been addressed in a multiloop
design capable of undesired
mode suppression