okay i got it.
Did you try externally modulated laser technique? It may prove to be less chirp introducing technique. In addition to that, the overall performance will be very good in externally modulated laser technique. I would suggest you to go for that technique and compare the results of the two techniques. Refer to the link provided below.
http://www.cpeu.dk/data/uploads/teaching/34129_e2009_cpeu_dml_eml.pdf
regards!