Incoherent beam combination of mid-wave infrared quantum cascade lasers

Quantum cascade laser (QCL) emitting in the mid-wave infrared atmospheric windows (3 to 5 mu m) will be of immediate use to several civilian applications, including airborne self-defense protection system and trace gas sensing and free space optical communications. When the output power of a single QCL is too low, the beams of different lasers can be combined by incoherent beam combining. For incoherent beam combining the laser beams are arranged side by side on the aperture of the laser system and combine in the farfield. Incoherent beam combining has been applied very successfully to diode lasers with high robustness and reliability due to it is neither limited to any number of lasers nor to any laser characteristics. This technique is demonstrated to be compatible with QCLs in this work. In this paper, the method of incoherent beam combination of 4 independent QCL emitters with a 0.1 W continuous wave power at room temperature each is studied. Results show that the incoherent power superposition of mid-infrared QCLs can be achieved by beam combining with an efficiency of not less than 90%. The output farfield divergence angle is about 5 mrad, which is consistent with the farfield divergence angle of the four sub-beams.