Answer: When light is transmitted in the optical fiber, some distortion will also occur. The reasons for the distortion are as follows:

(1) In the optical fiber transmission system, due to the nonlinearity of the electrical/optical conversion characteristics of the semiconductor laser, the output optical signal is inconsistent with the change of the excitation current, resulting in distortion, which is called modulation distortion. The value of the modulation index M is not allowed to be too large. It is necessary to choose an optical transmitter with high performance and strong pre-distortion processing technology. The pre-distortion processing technology uses artificial design to generate pre-distortion to improve the modulation linearity, so as to eliminate and reduce the frequency of the optical fiber transmission system. Purpose of CSO vs CTB.

(2) In the optical transmission system, since the chance of distortion caused by the driving RF amplifier and the receiving RF amplifier is very small, the small distortion generated by the linear PIN photodiode can be ignored because the signal level is not too high, and the main reason is that Distortion of modulation characteristics of semiconductor lasers and dispersion of optical fibers.

(3) When the light intensity of the laser is modulated, the wavelength of the light will change, and additional frequency modulation will occur, which will broaden the signal frequency and cause a chirp effect, which is mainly manifested as CSO distortion.