Abstract
Frequency combs with equally spaced frequency lines show great potentials for applications in spectroscopy, imaging, communications, and so on. In the terahertz frequency region, the quantum cascade laser (QCL) is an ideal radiation source for frequency comb and dual-comb operation. The systematic evaluation of phase noise characteristics of terahertz QCL frequency comb and dual-comb sources is of great importance for high precision measurements. In this work, we present detailed measurements and analysis of the phase noise characteristics of terahertz QCL frequency comb and dual-comb sources emitting around 4.2 THz with repetition frequencies of ~6.2 GHz. The measurement results for the current noise of the direct current (DC) sources (that are used to electrically pump the terahertz QCLs) indicate that at 100 Hz, the current noise for DC-1 and DC-2 is 0.3895 and 0.0982 nA/Hz1/2, respectively. Such levels of current noise can be safely disregarded. The phase noise of radio frequency (RF) generators (that are employed for injection locking and phase locking), intermode beatnotes, and dual-comb signals with and without phase-locked loop (PLL) are all measured and compared. The experimental results show that in the free-running mode, the phase noise of the intermode beatnote signals is always lower than that of the dual-comb signals across all frequencies. Additionally, the phase noise induced by the RF generators is negligible. By employing the phase locking technique, the phase noise of the intermode beatnote and dual-comb signals in the low offset frequency band can be significantly suppressed. At an offset frequency of 100 Hz, the measured phase noise values of the dual-comb line without and with phase locking are 15.026 and −64.801 dBc/Hz, respectively.
Published Version
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call