<p>Unwanted mode conversion caused by inhomogeneity in the refractive index is an issue in an asymmetric slab waveguide with a longitudinal varying index profile. In this paper, we discuss the effect of longitudinal and transverse inhomogeneity on the coupling between modes and, consequently, on the quantum crosstalk length. The simulation is discussed in detail, and random errors in RI are numerically studied. We have shown that the random longitudinal inhomogeneity in the waveguide’s refractive index can be modeled using the phase-match condition with an individual sinusoidal grating. Our investigations show that a bimodal quantum system based on waveguide TE₀ and TE₁ modes has a reliable quantum cross-talk length. Still, if the transverse refractive index distribution is imperfect, it is reduced to 250 [um]. We also compare the effect of core-clad wall imperfection with that of inhomogeneity in the waveguide’s refractive index and show that the wall imperfection has a severe destructive impact and can reduce the quantum cross-talk length from &gt; 30[mm] to 11.964[mm].</p>