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The dielectric profile of stripe geometry injection lasers is modeled with an objective of structure design requirements for fundamental lateral mode operation. Heterostructure lasers are modeled with a dielectric step profile using an effective dielectric discontinuity based on the gain/loss profile of the active layer as well as the overall geometrical structure. The analysis provides a quantitative comparison of the performance of two important double-heterostructure lasers: 1) the oxide-stripe geometry laser and 2) the channeled-substrate planar (CSP) laser. Modes of oxide-stripe lasers have lateral gain confinement, whereas, modes of CSP devices have strong lateral index confinement. To isolate the influence of geometry on the effective dielectric profile we assume that the real refractive index of the active layer is position independent. Resulting calculations show that a stripe geometry laser inherently has a depressed effective index in the active region below the metallic contact. This phenomenon alone produces index anti-guiding. In actual devices, both geometry and free carrier injection into the active region produce lateral index antiguiding. Lateral mode cut-off conditions are calculated as functions of the effective complex dielectric step and the stripe width. The results show that cutoff is related in a unique fashion to the ratio of the real and imaginary parts of the complex dielectric step; the ratio is positive for index guided modes and negative for gain guided ones.