Stability diagrams of two optically mutual-injected quantum cascade lasers

The dynamic properties and stability of optically mutual-injected arrays composed of terahertz quantum cascade lasers (THz QCLs) were investigated and compared with those of diode laser (DL) arrays. The influences of the coupling strength and frequency detuning on the working states of the arrays were analyzed using numerical simulations of the time evolutions of the electric fields and their corresponding Fourier-transform spectra. It was found that when the frequency detuning between individual lasers was zero, the QCL arrays could always maintain phase-locked operation. In contrast, the DLs were only able to function in a stable state with weak coupling strengths. With increasing coupling strength, periodic, quasi-periodic, multi-periodic, and aperiodic oscillations appeared. When the frequency detuning of the array lasers was nonzero, the QCL array could not be phase-locked at low coupling strengths, and it only entered the phase-locked region if the coupling strength was increased. However, the DL array could only work stably at low coupling strengths and quickly entered the aperiodic oscillation region as the coupling strength was increased. When we fixed the coupling strength and changed the frequency detuning, with large frequency detunings, both the QCL and DL arrays exhibited periodic oscillations. However, the QCLs were phase-locked at low frequency detunings, while the DLs exhibited periodic and multi-periodic oscillations across a broader frequency range. The results indicate that QCL arrays are more stable than DL arrays across a wide range of coupling strengths and frequency detuning parameters.