Comb-Locked Terahertz Spectroscopy of Critically Coupled Ultrahigh-Q Whispering-Gallery Mode Resonators

We present a novel frequency domain spectrometer, which is based on two external cavity diode lasers (ECDL) locked to a fibre-based erbium frequency comb, the repetition rate $\mathrm{f}_{rep}}$ of which is in turn referenced to a radio frequency (RF-) oscillator. By using an external endless frequency-shifter [1], one of the lasers is detuned by frequencies up to 10 THz, limited only by the spectral width of the comb. Within the locking bandwidth, the stability and phase noise of the RF reference is transferred to the difference frequency between both ECDLs [2].