In this paper we show why a non-linear curvature wavefront sensor (nlCWFS) is superior to both Shack-Hartmann wavefront sensor (SHWFS) and conventional curvature wavefront sensor (cCWFS) for sensing mv < 15 natural guide stars. We have developed, an experimental setup aimed at comparing the the rms wavefront error obtained, with the nlCWFS and the SHWFS. We describe our experimental setup and present results from the laboratory demonstration of the nlCWFS. The non-linear approach, builds on the successful curvature wavefront sensing concept. The wavefront is reconstructed from the defocused pupil images using the Gerchberg-Saxton (GS) phase diversity algorithm. We compare results obtained from reconstructing the wavefront using a Shack-Hartmann wavefront sensor (SHWFS) and a nlCWFS for a monochromatic source. We discuss approaches to overcome non-linearity issues and discuss the challenge of using two WFSs in the same spatiotemporal control regime and the implementation of the nlCWFS on the 6.5 m MMT.