External Cavity Tuning of Coherent Quantum Cascade Laser Array Emitting at \sim7.6μm

An external cavity quantum cascade laser (QCL) array with a wide tuning range and high output power is presented. The coherent QCL array combined with a diffraction grating and gold mirror is tuned in the Littrow configuration. Taking advantage of the single-lobed fundamental supermode far-field pattern, the tuning capability of 30.6 cm^-1 is achieved with a fixed injected current of 3.5 A at room temperature. Single-mode emission can be observed in the entire process. The maximum single-mode output power of the external cavity setup is as high as 25 mW and is essential in real applications.