A Wavelength Discriminator Method for Measuring Dynamic Chirp in DFB Lasers.

A new method of measuring chirp in a modulated DFB laser is described. The technique measures the mean wavelength of the laser as a function of time for an arbitrary modulating waveform. Chirp induced wavelength fluctuations (in a modulated DFB laser) are transformed into intensity fluctuations using a Fabry-Perot interferometer used as a wavelength discriminator. The intensity fluctuations contain the chirp information. The advantage of this method over the full time resolved spectrum technique is its ability to measure wavelength changes over many bits in a pseudorandom data stream, thus resolving pattern dependent chirp characteristics. The technique has been effective in explaining the dominant mode of degradation of regenerator sensitivity for transmission spans with a large amount of chromatic dispersion, and has contributed to an understanding of some of the basic laser parameters that influence device performance. Using this method we have measured the chirp in 20 DFB lasers. For the devices that did not exhibit an error rate floor there was a high correlation between system penalty and the chirp- chromatic dispersion product. A pattern dependent wavelength drift phenomenon, that has not been reported in the literature, has been observed. For a number of devices the lasing wavelength has been found to drift by a significant amount after the transient effects have died away. This drift causes contiguous transitions of a binary pseudo random bit stream to occur at different wavelengths.