The use of terahertz radiation bears great potential for various applications. We propose a method for obtaining terahertz radiation based on frequency difference generation from two-longitudinal-mode emission of monolithic distributed feedback lasers with modulated gratings. The frequency separation of the two dominating longitudinal modes of the proposed device structure can cover a wide range and several structural and operation point parameters can be used for coarse- and fine-tuning. Moreover, the nearly perfect overlapping of the dominant modes' optical field transverse distributions is enabling high-efficiency terahertz generation. The paper presents design formulas, simulation results and discusses the influences of the fabrication inaccuracies on the target frequency separation.© (2009) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.