  We studied the injection-locking properties of a resonant-tunneling-diode
terahertz oscillator in the small-signal injection regime with a
frequency-stabilized continuous THz wave. The linewidth of the emission
spectrum dramatically decreased to less than 120 mHz (HWHM) from 4.4 MHz in the
free running state as a result of the injection locking. We experimentally
determined the amplitude of injection voltage at the antenna caused by the
injected THz wave. The locking range was proportional to the injection
amplitude and consistent with Adler's model. As increasing the injection
amplitude, we observed decrease of the noise component in the power spectrum,
which manifests the free-running state, and alternative increase of the
injection-locked component. The noise component and the injection-locked
component had the same power at the threshold injection amplitude as small as
$5\times10^{-4}$ of the oscillation amplitude. This threshold behavior can be
qualitatively explained by Maffezzoni's model of noise reduction in general
limit-cycle oscillators.
