HARMONI is the first light visible and near-IR integral field spectrograph for the ELT. It covers a large spectral range from 450 nm to 2450 nm with resolving powers from 3500 to 18000 and spatial sampling from 60 mas to 4 mas. It can operate in two Adaptive Optics modes-SCAO (including a High Contrast capability) and LTAO-or with no AO mode. To prepare the final design reviews, we have built an optical bench to emulate and characterize the performance of the laser guide star (LGS) wavefront sensor (WFS) to be used in HARMONI. The WFS is a classic Shack-Hartmann, nonetheless pushed to the extreme due to the size of the primary mirror of the ELT (39 m). The WFS is composed of a 80 × 80 double side microlens array (MLA), an optical relay made of 6 lenses in order to re-image the light coming from the MLA on the detector, and a CMOS camera using a Sony detector with 1608 × 1104 pixels, RON< 3e, and a frame rate of 500Hz. The sensor has a large number of pixels to provide a field-of-view wider than 15 arcsec per subaperture over the full pupil, which is required to image the elongated LGS spots. An innovative feature of our bench is the use of a spatial light modulator (SLM) which allows us to emulate the M4 deformable mirror (DM) and the real position of its actuators, together with the projected spiders in the pupil plane. We report on the design and performance of our bench, including the first interaction matrices using the ELT-M4 influence functions and a non-elongated source. We expect to implement a system to emulate an elongated source in order to grasp a better understanding of its effects on wavefront sensing.