The function of digital optical mitter is to convert the digital base charged signal output by the electric terminal into optical signal, and effectively inject it into the optical fiber line with coupling technology. The electric / optical conversion is realized by modulating the light source with the digital electrical signal carrying information. Modulation is divided into direct modulation and external modulation. The characteristic parameters of the modulated light source include power, amplitude, frequency and phase. Direct light intensity (power) modulation is widely used in practical optical fiber communication systems. The function of the optical mitter is to convert the AM RF TV signal sent by the front end into an optical signal, and mit the optical signal to the distance through the optical fiber. According to different operating frequencies, optical mitters are divided into two categories: A and B: Class a optical mitter refers to the optical mitter that mits 59 PAL-D analog TV signals in the frequency range of 47 - 550 MHz and digital modulation signals in the frequency range of 550 - 750 (862) MHz; Class B optical mitter refers to an optical mitter that mits 84 PAL-D analog TV signals in the frequency range of 47 - 750 MHz. According to different modulation modes, optical mitters are divided into frequency modulation, amplitude modulation and digital modulation. At present, the most widely used is multi-channel AM optical mitter, which is divided into direct modulation and external modulation. Application of 1550nm external modulation optical mitter CATV optical fiber mission system, whether digital or analog, its important development goal is how to extend the mission distance of the trunk line, or let the optical fiber go deep into the user area as far as possible, so as to realize optical fiber to the roadside or optical fiber to the