Abstract
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 (half width at half maximum) 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. While increasing the injection amplitude, we observed a decrease in the noise component of the power spectrum, which manifests the free-running state, and an alternative increase in 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 × 10−4 of the oscillation amplitude. This threshold behavior can be qualitatively explained by Maffezzoni’s model of noise reduction in general limit-cycle oscillators.
Highlights
Compact and stable terahertz (THz) sources are highly desired for future THz imaging and THz wireless telecommunication systems
The noise component and the injectionlocked component had the same power at the threshold injection amplitude as small as 5 × 10−4 of the oscillation amplitude
Its frequency range was 10 Hz–26.5 GHz. It can operate in real-time spectrum analyzer (RTSA) mode, in which we can capture the signal without dead time and obtain a spectrogram, which is a series of spectra over time
Summary
Compact and stable terahertz (THz) sources are highly desired for future THz imaging and THz wireless telecommunication systems. One of the major concerns in regard to the RTD THz oscillator is its large linewidth in the free-running state It is typically 10 MHz for an oscillator operating around several hundred GHz,[8,9] where the statistical property and the origin of the noise have yet to be determined. This is because an inevitable small return light always exists and affects the RTD THz oscillator, resulting in a complex behavior This may have limited the previous studies on the injection locking in the middle- or large-signal injection regime to have a well-defined locking behavior. We investigated the injection-locking properties of the RTD THz oscillator in the small-signal injection regime without an optical feedback effect. The noise reduction and injection-locking behavior can be qualitatively explained by Maffezzoni’s model for general limit-cycle oscillators.[25]
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