Broadened waveguide design for separate confinement quantum well lasers

Waveguide thickness (W) in modern separate confinement quantum well (SCH QW) lasers is determined by the condition of maximum optical confinement factor for quantum wells and usually does not exceed 0.4 /spl mu/m. Studies of the dependence of parameters of SCH QW lasers on waveguide thickness have shown that laser response becomes slower with waveguide broadening. Recently studies have been carried out for GaAs-, InP-, and GaSb-based lasers. They have shown that the threshold current density (J/sub th/) increases very weakly (GaAs-based lasers) or decreases (InP- and GaSb-based lasers) with W increase up to 1 /spl mu/m, while the differential efficiency (/spl eta//sub d/) increases for all types of laser studied. The reason for the J/sub th/ decrease and increase in /spl eta//sub d/ is the decrease of the fraction of optical mode propagating in doped cladding layers and subsequent decrease of losses due to free carrier absorption. The positive effects are stronger in longer-wavelength lasers since cladding free carrier absorption at a given doping level increases with /spl lambda/. It is evident that the effect of J/sub th/ on output power becomes weaker with increase in operating current and positive contribution from /spl eta//sub d/ dominates at current densities well above J/sub th/ in all studied lasers.