  An opto-electronic radio-frequency oscillator that is based on forward
scattering by the guided acoustic modes of a standard single-mode optical fiber
is proposed and demonstrated. An optical pump wave is used to stimulate
narrowband, resonant guided acoustic modes, which introduce phase modulation to
a co-propagating optical probe wave. The phase modulation is converted to an
intensity signal at the output of a Sagnac interferometer loop. The intensity
waveform is detected, amplified and driven back to modulate the optical pump.
Oscillations are achieved at a frequency of 319 MHz, which matches the
resonance of the acoustic mode that provides the largest phase modulation of
the probe wave. Oscillations at the frequencies of competing acoustic modes are
suppressed by at least 40 dB. The linewidth of the acoustic resonance is
sufficiently narrow to provide oscillations at a single longitudinal mode of
the hybrid cavity. Competing longitudinal modes are suppressed by at least 38
dB as well. Unlike other opto-electronic oscillators, no radio-frequency
filtering is required within the hybrid cavity. The frequency of oscillations
is entirely determined by the fiber opto-mechanics.
