A laser source assembly comprises a laser diode for generating a laser beam and a focusing lens for focusing the light beam. A spacer is provided for keeping the diode and the lens at a predetermined distance, and is made of a material of low expansion coefficient. Preferably, the subassembly constituted of the diode, the spacer and the lens is housed in a case made of another material that may have a higher expansion coefficient. A resilient element is provided to continuously keep the diode and the lens abutting on the opposite ends of the spacer.