  Type-II W-lasers have made an important contribution to the development of
mid-infrared laser diodes. In this paper, we show that a similar approach can
yield high performance lasers in the optical communications wavelength range.
(GaIn)As/Ga(AsSb) type-II W structures emitting at 1255 nm have been realised
on a GaAs substrate and exhibit low room temperature threshold current
densities of 200-300 Acm$^{-2}$, pulsed output powers exceeding 1 W for 100
${\mu}$m wide stripes, and a characteristic temperature T${_0}$${\approx}$90 K
around room temperature. Optical gain studies indicate a high modal gain around
15-23 cm$^{-1}$ at 200-300 Acm$^{-2}$ and low optical losses of 8 ${\pm}$ 3
cm$^{-1}$. Analysis of the spontaneous emission indicates that at room
temperature, up to 24% of the threshold current is due to radiative
recombination, with the remaining current due to other thermally activated
non-radiative processes. The observed decrease in differential quantum
efficiency with increasing temperature suggests that this is primarily due to a
carrier leakage process. The impact of these processes is discussed in terms of
the potential for further device optimisation. Our results present strong
figures of merit for near-infrared type-II laser diodes and indicate
significant potential for their applications in optical communications.
