Cable TV transmitter according to the way of working can be divided into direct modulation light transmitter and outer modulation optical transmitter.
Directly modulated optical transmitter for more DFB laser, good linear, DFB laser can achieve better without the predistortion circuit compensation value of CTB and CSO value, but as a result of the direct modulation are additional frequency modulation, nonlinear distortion index (especially the CSO) it's hard to do is very high.
Directly modulated optical transmitter power generally not too big, within the 24 mw, therefore, limits the transmission distance, commonly used in the local distribution network and township-level optical cable transmission network.The type optical transmitter used at 1310 nm fiber net, 1310 nm fiber optic attenuation is 0.35 db/km, so the maximum transmission distance is less than 35 km.This kind of optical transmitter performance is stable, structure of the general rules, the price is low, has been widely used.
Outside modulation type generally with YAG laser, YAG laser modulation, outside using linear is very poor, must use the predistortion circuit to compensate.Due to its dispersion is less, so the YAG optical transmitter is very suitable for 1550 - nm wavelength optical fiber, multi-purpose at 1550 nm fiber net.YAG optical transmitter in 1550 nm fiber net, available with amplification relay, 1550 nm fiber attenuation and small (0.25 db/km), so the YAG optical transmitter used extreme distance transmission.
Modulation optical transmitter: the output power is big, can be more than 2 x 20 mw (two way output), low noise, there is no additional frequency modulation of similar LD and optical fiber dispersion characteristics of composite cso distortion.Therefore, often used for long distance transmission or large cable transmission system.Outside the modulated light hair price is high, close transmission optical fiber network rarely choose other production for light emission.