The main reason of chirp is that the refractive index of the medium changes dynamically due to the influence of dynamic electric signal modulation, which leads to the change of the phase of the optical signal propagating in the medium, and this change is directly reflected in the dynamic change of the frequency of the output optical signal.
Chirp is a phenomenon that signal frequency changes with time, and it is a representation of pulse characteristics in time domain. In a pulse period, all frequency components do not arrive at the same time, that is, different frequency components of laser pulses are asynchronous in time domain, which are divided into positive chirp and negative chirp. Positive chirp means that the frequency increases with time, that is, the laser output is low frequency first and then high frequency, for example, the rising edge of the pulse is low frequency, the falling edge is high frequency, and the negative chirp is the opposite, such as normal dispersion and high frequency.