The optical carrier suppression in optical quadruple frequency modulation by cascaded external modulators is investigated theoretically and experimentally. Theoretical analysis demonstrates that the optical carrier suppression ratio is related with not only the initial phase difference of electrical signals applied on the two modulators, but also the optical phase shift between the two modulators. The maximum suppression ratio can be achieved when the total phase difference is equal to n\pi+\pi/2 (n=1,2,...), which is verified by experiments. By properly controlling the total phase shift, 40-GHz millimeter-wave is generated by using a 10-GHz radio frequency (RF) source and the modulators.This work was supported by the National “973" Program of China under Grant No.2006CB302806. E-mail: zw-03@mails.tsinghua.edu.cn1. R. P. Braun, G. Grosskopf, D. Rohde, and F. Schmidt, Electron. Lett. 32, 626 (1996).2. Y. Li, A. J. C. Vieira, S. M. Goldwasser, and P. R. Herczfeld, IEEE Trans. Microwave Theory Techniq. 49, 2048 (2001).3. J. Marti, J. L. Corral, F. Ramos, V. Polo, and J. M. Fuster, Electron. Lett. 35, 517 (1999).4. H. Wen, L. Chen, Y. Pi, J. Yu, and S. Wen, Chinese J. Lasers (in Chinese) 34, 935 (2007).5. M. Bhattacharya, B. Sarkar, and T. Chattopadhyay, IEEE Photon. Technol. Lett. 14, 1611 (2002).6. L. A. Johansson and A. J. Seeds, IEEE Photon. Technol. Lett. 12, 690 (2000).7. D. Wake, C. R. Lima, and P. A. Davies, IEEE Trans. Microwave Theory Techniq. 43, 2270 (1995).8. G. Qi, J. Yao, J. Seregelyi, S. Paquet, and J. C. Bélisle, Proc. SPIE 5579, 598 (2004).9. Q. Ye, R. Qu, and Z. Fang, Chin. Opt. Lett. 5, 8 (2007).10. J. Zhang, H. Chen, M. Chen, T. Wang, and S. Xie, IEEE Photon. Technol. Lett. 19, 1057 (2007).