The theoretical construction of the fundamental ￡ NOR gate using two injection semiconductor laser diodes due to mutual coupling is presented. Two laser diodes that are commonly driven by a monochromatic light beam result in chaos; however, chaotic synchronization between the two lasers may be achieved by coupling them. The all-optical logic gate is finally implemented by synchronizing or un-synchronizing appropriately the two chaotic states under a phase modulator (PM) control. Numerical results validate the feasibility of the method.