Fig. 1. (Color online) (a) Experimental setup for phase noise measurements. Comb1 and Comb2 are two terahertz QCLs with a ridge width of 150 μm and a cavity length of 6 mm. The intermode beatnote and dual-comb signals are recorded using a spectrum analyzer. The insert is a photograph of two terahertz QCLs mounted on a Y-shaped sample holder, with a distance of 34 mm between the two laser facets. RF-1 is used to down-convert the dual-comb signal to 95 MHz. RF-2 acts as a local oscillator (LO) signal of the phase-locked loop (PLL) to beat with the down-converted dual-comb line to generate the error signal to control the drive current of the terahertz QCL for phase locking. AC: alternating current. DC: direct current. RF: radio frequency. IF: intermediate frequency. (b) and (c) Intermode beatnote maps of Comb1 and Comb2, respectively, measured with a resolution bandwidth (RBW) of 10 kHz and a video bandwidth (VBW) of 1 kHz. (d) Intermode beatnotes (or repetition frequencies) of Comb1 and Comb2 measured with a RBW of 100 kHz and a VBW of 10 kHz. f_rep1 and f_rep2 are the repetition frequencies of Comb1 and Comb2, respectively. The difference between f_rep1 and f_rep2 is 16.74 MHz. (e) Dual-comb spectrum measured with a RBW of 10 kHz and a VBW of 1 kHz. The mode spacing is 16.42 MHz. For the dual-comb spectral measurements, Comb1 and Comb2 are operated at 1000 and 920 mA, respectively. The heat sink temperature is stabilized at 18 K.

Fig. 2. (Color online) Current noise power spectral density of current sources DC-1 (Keysight E3644A) and DC-2 (QubeCL) measured at different currents. DC-1 and DC-2 are two DC sources employed for driving Comb1 and Comb2, respectively.

Fig. 3. (Color online) Phase noise plots measured for different RF sources used in the system. (a), (b), and (c) show the measured phase noise spectra of RF-1 (R&S SMA 100B), RF-2 (R&S SMCV 100B) and RF-3 (Anritsu MG3693C), respectively, at different microwave powers. The output frequency is set to 4 GHz.

Fig. 4. (Color online) (a) Dual-comb spectrum with one of the lines (indicated by the red arrow) phase locked to a stable RF source. (b) Spectrum of the phase locked line at 93.62 MHz measured with a RBW of 10 kHz and a VBW of 1 kHz. The inset is the high-resolution spectrum of the phase locked line measured with a RBW of 1 Hz and a VBW of 1 Hz.

Fig. 5. (Color online) Phase noise spectra of the intermode beatnote and dual-comb signals measured in free-running and/or phase locked conditions. For a clear comparison, the phase noise spectra of RF sources, i.e., RF-1 and RF-2, recorded at 0 dBm power are also plotted. The drive currents for Comb1 and Comb2 are set to 1000 and 920 mA, respectively.