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(Color online) Schematic diagram of the experimental setup to phase lock the beat signal of a two-mode terahertz QCL. Not shown is that the spectrum analyzer is also phase locked to the same microwave reference signal as for the source-locking microwave counter.
(Color online) The power spectra of the beat signal of two lateral-mode terahertz QCL that is phase locked to a microwave reference recorded by the spectrum analyzer with different resolution bandwidths (RBWs) and spans, but a fixed video bandwidth (VBW) of . Other lines appeared in (c) are due to the pickup of power-line signals. The linewidth of each spectrum is also indicated. The inset in (a) shows the beat frequency as a function of the bias current of the QCL at a heat-sink temperature of .
(Color online) Measured power spectrum of the beat signal under frequency stabilization (data points) (Ref. 12). A similar spectrum was obtained with a reduced RBW. The curve is a fit with a Lorentzian profile.
(Color online) Linewidth of the beat signal as a function of the heat-sink temperature of the QCL. The inset shows emission spectra of the two-mode QCL taken at several temperatures. For clarity an offset in the intensity for each spectrum is introduced.
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