Distribution feedback laser chirp depending on pulse pattern in high‐speed modulation and transmission performance

The wavelength generated by a single mode distribution feedback (DFB) laser varies over time inside pulses generated at high modulation speeds (chirp). This worsens the transmission characteristics of a long-distance communication link that uses the laser as a light source. Because the modulation of the DFB laser is used to produce a pulse train signal to be transmitted, it is important to determine the dependence of the wavelength fluctuation on the modulated pulse pattern. The dependence of the chirp on the pulse pattern and its degrading effect on the transmission characteristics will be demonstrated here. It will be shown that the pulse pattern dependent wavelength fluctuation is due both to temperature variations caused by the pulse density and carrier storage effects at gigabit per second order modulation speed. The carrier storage effect of the pattern on the wavelength dependence can be reduced by increasing the bias current above the threshold value. However, raising the bias current does not improve the thermal dependent wavelength fluctuation. For example, it will be shown that for a modulation current of 90 mA, the generated center wavelength varies on the order of 0.08 nm. A model for the degradation of the transmission characteristics due to the wavelength fluctuation of the light source through the interaction with chromatic dispersion in the fiber will be presented. Degradation will be quantified experimentally and analytically.