  The threshold current density of narrow (1.5 {\mu}m) ridge-waveguide InGaN
multi-quantum-well laser diodes, as well as the shape of their lateral
far-field patterns, strongly depend on the etch depth of the ridge waveguide.
Both effects can be attributed to strong index-antiguiding. A value of the
antiguiding factor R = 10 is experimentally determined near threshold by
measurements of the current-dependent gain and refractive index spectra. The
device performances are simulated self-consistently solving the
Schr\"odinger-Poisson equations and the equations for charge transport and
waveguiding. Assuming a carrier-induced index change which matches the
experimentally determined antiguiding factor, both the measured high threshold
current and the shape of the far-field pattern of lasers with shallow ridges
can be reproduced theoretically.
