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 spee ds (chirp). This worsens the transmission characteristics of a long-di stance communication link that uses the laser as a light source. Becau se the modulation of the DFB laser is used to produce a pulse train si gnal to be transmitted, it is important to determine the dependence of the wavelength fluctuation on the modulated pulse pattern. The depend ence of the chirp on the pulse pattern and its degrading effect on the transmission characteristics will be demonstrated here. It will be sh own that the pulse pattern dependent wavelength fluctuation is due bot h to temperature variations caused by the pulse density and carrier st orage effects' at gigabit per second order modulation speed. The carri er storage effect of the pattern on the wavelength dependence can be r educed by increasing the bias current above the threshold value. Howev er, raising the bias current does not improve the thermal dependent wa velength fluctuation. For example, it will be shown that for a modulat ion current of 90 mA, the generated center wavelength varies on the or der of 0.08 nm. A model for the degradation of the transmission charac teristics due to the wavelength fluctuation of the light source throug h the interaction with chromatic dispersion in the fiber will be prese nted. Degradation will be quantified experimentally and analytically.