Fig. 2
From: Farey tree locking of terahertz quantum cascade laser frequency combs
Farey tree of the THz QCL frequency comb and typical intermode beatnote spectra under the Farey tree locking conditions. a Farey tree structure of the THz QCL comb under microwave injection. The Farey fractions outlined by solid rectangles, dashed rectangles, and dashed rectangles with crosses represent firmly locked, temporarily locked, and unlocked states, respectively. b–d are measured intermode beatnote spectra at Farey fractions of 2/3, 3/5, and 4/7, respectively. The red curves are recorded under Farey tree locking conditions, while the black ones are obtained in free-running mode. The RBW and VBW are set as 300 Hz and 30 Hz, respectively. The microwave injection power is 30 dBm
