We demonstrate a broad gain,continuous-wave (CW) operation InP-based quantum cascade laser (QCL) emitting at 11.8 μm with a modified dual-upper-state (DAU) and diagonal transition active region design.A 3 mm cavity length,16.5 μm average ridge wide QCL with high-reflection (HR) coatings demonstrates a maximum peak power of 1.07 W at 283 K and CW output power of 60 mW at 293 K.The device also shows a broad and dual-frequency lasing spectrum in pulsed mode and a maximum average power of 258.6 mW at 283 K.Moreover,the full width at half maximum (FWHM)of the electroluminescent spectrum measured at subthreshold current is 2.37 μm,which indicates a broad gain spectrum of the materials.The tuning range of 1.38 μm is obtained by a grating-coupled external cavity (EC) Littrow configuration,which is beneficial for gas detection.