Reduction of Spectral Linewidth of Resonant-Tunneling-Diode THz Oscillators Due to External Feedback

We theoretically show reduction of the spectral linewidth of resonant-tunneling-diode THz oscillators by the external feedback of output power. A simple relation between the frequency change due to the feedback and the spectral linewidth is found. The linewidth has minimum values at the feedback distance equal to integral multiple of half wavelength. The linewidth at the minimum points is narrower for larger amount of the feedback. Reduction of the linewidth becomes ~ 1/10 even with relatively small amount of the feedback. However, as the feedback amount increases, multimode oscillation due to the external cavity occurs in the middle between the minimum points of the linewidth and spread out.