Fig. 1.

Largely-detuned DFB grating design for THz comb nonlinear generation from a mid-IR QCL. a Schematic of largely detuned DFB QCL design for THz frequency comb operation. b Calculated GDD spectra at different coupling coefficient κ and the DFB induced relative phase spectrum for κ = 15 cm⁻¹. c measured high-resolution GDD spectra of a DFB with κ = 35 cm⁻¹ and L = 1.5 mm and its comparison with the calculated results. d Measured low-resolution GDD spectra of the DFB and FP devices. The gray and pink bars indicate grating and comb spectral positions