High resolution printing systems that utilize high power laser diode bars and digital mirror devices (DMD) require side-by-side stacking of illumination modules to stitching of the image from each module to form a longer total image width. An inline illumination optical system having a refractive prism and Total Internal Reflection (TIR) prism pair with an air gap along with a light guide transporting light beams at a compound angle to the prism pair eliminates the need for any axial rotation of the laser and light guide, and enables side-by-side module stacking. The illumination optical system includes an illumination module having a light source, the light guide, a DMD array and a Refractive TIR (RTIR) prism. The system also includes a DMD housing containing the DMD array and having a width within which the illumination module is confined to allow side-by-side stacking.