A comparison of the single-mode condition for semiconductor rib waveguides with large cross section obtained by the effective index method and the corrected formula proposed by Soref et al.  with experimental data is carried out. It is found that the effective index method is in a better agreement with Rickman's  experimental data than the modified version. Moreover a stronger condition is proposed for single-mode design purposes.
Souren P. Pogossian, Lili Vescan, and Adrian Vonsovici, "The Single-Mode Condition for Semiconductor Rib Waveguides with Large Cross Section," J. Lightwave Technol. 16, 1851- (1998)