At present, with the rapid development of optoelectronic chip technology, high-frequency millimeter wave communication has become an important development technology in broadband access networks. Due to the high efficiency and stability of transmitting and receiving millimeter wave signals, the optical millimeter system using an external modulation frequency-doubling scheme can make full use of optical fiber resources in the existing telecommunication network, so it has potential application value in practice. The basic principle of frequency-doubling, typical frequency-doubling simulation or experimental schemes, and related test results are reviewed. Considering that the Mach-Zehnder Modulator (MZM) is a key electro-optical device for frequency-doubling optical generation of millimeter wave, therefore, the key operating parameters are analyzed. The application prospect of optical millimeter waves technology in Radio over Fiber (RoF) systems is also discussed.