An alternative to Quasi Phase Matching (QPM), called Semi Quasi Phase Matching (SQPM) is proposed, wherein the
segments of crystals with inverted axis are substituted by segments of small polycrystal grains of the same material.
These segments produce negligible nonlinear interaction, while still providing the π phase change necessary. A SQPM
device of length L should produce the same nonlinear output as a QPM device of length L/2. Crystals with higher
nonlinear coefficients and larger transparency ranges like III-V and II-VI compounds, not normally amenable to
inversion of axes, can be used. This is especially important for doubling of CO2 laser lines.
We report the Ga0.86In0.14As0.13Sb0.87 room temperature refractive index value obtained from direct reflectivity measurements and also estimated from laser transverse far field pattern measurements. The value n equals 3.78 obtained is higher than previous theoretical calculations and is high enough to support a good optical confinement in DH lasers with 27% Al in the confining layers. We also show that the active layer low resistivity gives the main contribution to the high threshold current (Ith) for narrow stripe lasers. This is partly solved by making the active region n-type. The minimum Ith obtained for n-type active layer lasers was 290 mA compared to 800 mA for p-type active layer lasers, the broad area threshold current being the same (3 kA/cm2) in both cases.
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