High power thermoelectrically cooled and uncooled quantum cascade lasers with optimized reflectivity facet coatings

We present a method of preserving the device wall-plug efficiency by adjusting mirror losses with facet coatings for longer cavity quantum cascade lasers. An experimental study of output power and wall-plug efficiency as functions of mirror losses was performed by varying the front facet coating reflectivity with a high-reflectivity-coated rear facet. The use of optimized reflectivity coatings on 7-mm-long chips resulted in continuous-wave output power of 2.9 W at 293 K for thermoelectrically cooled devices mounted on AlN submounts and average and continuous-wave output power in excess of 1 W for uncooled devices emitting at 4.6 m. © 2009 American Institute of Physics. doi:10.1063/1.3246799 Midwave infrared MWIR quantum cascade lasers QCLs with watt-level continuous-wave cw output power at room temperature have recently been demonstrated. 1‐4 These lasers are useful for applications such as protecting aircraft from shoulder-fired missiles, free-space optical communications, infrared illuminators for night-vision applications, and chemical sensing. All these applications would benefit from higher power, improved reliability, and lower power consumption. The large design flexibility of QCLs allows one to scale up their output power in several ways: increase the number of active region stages, increase the active region sheet carrier density per stage, and increase the chip dimensions, i.e., width and/or length. All these approaches have been demonstrated to result in higher peak power in pulsed mode, but most of them are of limited usefulness for cw devices because of active region self-heating. An increase in the number of stages or the sheet carrier density result in a higher generated heat per unit area in the active region and, thus, in a higher temperature rise. An increase in the active region width results in an increase in thermal resistance and thus leads to a temperature rise. Moreover, unless special measures are taken, wider waveguides result in lower beam quality.