What is frequency chirp?
A chirp is a signal in which the frequency increases or decreases with time. This picture shows a linear chirp waveform; a sinusoidal wave that increases in frequency linearly over time.
Then what is frequency chirp in an optical pulse?
The chirp of an optical pulse is defined as the time dependence of its instantaneous frequency. In this picture, it shows the electric field of a strongly up-chirped pulse, where the instantaneous frequency grows with time.
An up-chirp means that the instantaneous frequency rises with time, and an down-chirp means that the instantaneous frequency decreases with time.
Frequency chirp can limit the performance of a fiber optic system even with DFB lasers with large values of Mode Suppression Ratio (MSR), for example, about 40 dB.
This is because when we directly modulate the semiconductor laser, it is invariably accompanied by phase modulation. This phase modulation is caused by the carrier-induced change in the refractive index of the waveguide. This effect is governed by the linewidth enhancement factor.
As a result of the frequency chirp imposed on an optical pulse, its spectrum is considerably broadened. However, the use of an external modulator essentially eliminates frequency chirp. This is because there is no turning on and off of the DFB laser, it is on all the time.
A pulse is said to be chirped if its carrier frequency changes with time. The frequency change is related to the phase derivative. There is a parameter called parameter C, which governs the linear frequency chirp imposed on the pulse.
Chirped pulses may broaden or compress. But pulse broadening is definitely undesirable. Broadening of a pulse spills pulse energy into the next bit position. And if the broadening is enough, it will cause at least 1 bit in error if the bit that is supposed to be in that position is a binary 0.
Frequency chirp affects the shape of an optical pulse. This picture shows a typical waveform from a directly modulated semiconductor laser.
The y-axis is the amplitude, and the x-axis is time. We need to pay close attention to the initial rise of the pulse and its overshoot. This is chirp-related.
The fact that the following pulse peak are different than the initial pulse is also caused by chirp. Each pulse top will be different from the next in a random fashion.
Pulse rise time is an extremely important parameter. It sets a limit maximum on system bit rate. Setting of threshold affects system performance. Threshold should be set as low as possible without losing the lasing condition. This will affect the extinction ratio, which we want as high as possible.