We demonstrate InGaN violet light-emitting superluminescent diodes with large spectral width suitable for applications in optical coherence spectroscopy. This was achieved using the concept of nonlinear indium content profile along the superluminescent diode waveguide. A specially designed 3D substrate surface shape leads to a step-like indium content profile, with the indium concentration in the InGaN/GaN quantum wells ranging approximately between 6% and 10%. Thanks to this approach, we were able to increase the width of the spectrum in processed devices from 2.6 nm (reference diode) to 15.5 nm.