High-Power Quantum Cascade Lasers for 8 μm Spectral Range

The development of high-power mid-infrared laser sources is highly desired for a number of applications in free-space optical communication, laser imaging, detection, and ranging (LIDAR) and environmental monitoring. Quantum cascade lasers (QCLs) hold solid position among these technologies, however up to now their highest powers are demonstrated in 4.5 - 5
$\mu \mathrm{m}$
spectral range [1], while the results at other mid-IR wavelength may differ by an order of magnitude. In this work, we consider three different designs of high-power QCLs grown by a two-stage MBE and MOCVD epitaxy. The quality of all fabricated heterostructures is similar to that of structures produced solely by the MBE technique. The active region remains the same as in [2] in all three types of structures, while the main difference lies in the design of the upper cladding and contact layer. In particular, we discuss two structures with thick uniformly doped InP upper cladding together with InP or InGaAs contact layer (Types I and II correspondingly), and design with gradient doping of the InP upper cladding accompanied by InGaAs contact layer (Type III). All three structures were subjected to post-growth processing and fabrication of QCL chips with 40 and 60
$\mu \mathrm{m}$
stripes and 3–5 mm cavity lengths. All samples were tested under 150 ns pulsed pumping with a 12 kHz repetition rate. Our experiments show that QCLs based on both structures with InGaAs contact layer with uniform and gradient cladding doping (Types II and III) demonstrate better efficiency while the lasers based on Type I design with InP contact layer and uniformly doped upper cladding feature improved power characteristics resulting in the record-high power value
$> 16\ \mathrm{W}(> 8\mathrm{W}/\text{facet})$
. We claim the latter is directly related to the better thermal conductivity of InP contact layer comparing to InGaAs counterpart. This was confirmed by the chirp measurements demonstrating the lower heating rate of the active region in structure with InP contact layer (Type I), see Fig. 1a. At the same time, in our experiments InP contact layer had lower electrical conductivity, that finally affected the laser efficiency as shown in Fig 1.b.