Effective refractive index and first-order-mode cutoff conditions in InGaAsP/InP DH laser structures (λ = 1.2-1.6 µm)

The virtual wavelength-invariance of the ratio between bandgap wavelength and the square root of the dielectric-constant step in InGaAsP/InP DH structures [i.e., ( for 1.2 μm m] and an analytical approximation for the transverse propagation constant , allow the derivation of an accurate, closed-form expression for the effective refractive index Neffof InGaAsP/InP planar DH lasers emitting in the m range. Then, Neffis only a function of two readily measurable parameters: emission wavelength and active-layer thickness. Furthermore, the mode cutoff conditions for various lateral waveguides: buried-rectangular, buried-crescent, and ridge-guide, become wavelength-independent analytical expressions. First-order-mode cutoff conditions for these lateral waveguides are derived, plotted and compared to experimental data from mode-stabilized 1.3 and 1.55 μm DH lasers.