  We present GaSb-based interband cascade lasers emitting at a center
wavelength of 6.12 $\mu$m at 20$^\circ$C in continuous-wave operation up to a
maximum operating temperature of 40$^\circ$C. Pulsed measurements based on
broad area devices show improved performance by applying the recently published
approach of adjusting the GaInSb layer thickness in the active region to reduce
the valence intersubband absorption. The W-quantum well design adjustment and
the optimization of the electron injector, to rebalance the electron and hole
concentrations in the active quantum wells, improved the device performance,
yielding room temperature current densities as low as 0.5 kA/cm$^2$ for broad
area devices under pulsed operation. As a direct result of this improvement the
long wavelength limit for GaSb-based ICLs in continuous wave operation could be
extended. For an epi-side down mounted 23 $\mu$m wide and 2 mm long device with
9 active stages and high-reflectivity back facet the threshold power is below 1
W and the optical output power is over 25 mW at 20$^\circ$C in continuous-wave
mode. Such low-threshold and low-power consumption interband cascade lasers are
especially attractive for mobile and compact sensing systems.
