Fig. 3: Source noise characterisation in the RF domain.
From: Frequency stable and low phase noise THz synthesis for precision spectroscopy
a Power Spectral Density for a laser difference frequency of 1.965 GHz (4 cavity FSR) recorded by the acquisition card after down-frequency mixing to 100 kHz. The central frequency is numerically shifted to 0 for readability and the PSD is normalized to peak maximum. On the top right, a zoom on the central region, the dotted line indicating the width at −3 dB/Hz, i.e., the FWHM. b Phase noise spectrum for a 1.965 GHz difference frequency (purple line) compared to the phase noise observed for a similar frequency generated using an RF synthesizer (green line) together with the spectrum analyzer phase noise specification at 3 GHz (red dots).
