Embodiments described herein relate to the rapid thermal processing of substrates. A fiber coupled laser diode array is provided in an optical system configured to generate a uniform irradiance pattern on the surface of a substrate. A plurality of individually controllable laser diodes are optically coupled via a plurality of fibers to one or more lenses. The fiber coupled laser diode array generates a Gaussian radiation profile which is defocused by the lenses to generate a uniform intensity image. In one embodiment, a field stop is disposed within the optical system.