Stable single mode operation of quantum cascade lasers by complex-coupled second-order distributed feedback grating

• Complex-coupled feedback is provided by the second-order grating.
• Both index- and loss-dominated coupling scheme are adopted.
• Stable single mode operation without mode hopping was observed.
• A side-mode suppression ratio ∼30 dB is reported.

Stable single mode operation of distributed feedback quantum cascade lasers operating in room-temperature continuous-wave mode is reported. Feedback is provided by the second-order grating in a both index- and loss-dominated coupling scheme. Reliable single mode emission at λ ∼ 4.7 μm with a side-mode suppression ratio ∼30 dB is observed. A linear tuning of wavelength with heat-sink temperature or injection current indicates the stable single mode operation without mode hopping.