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A method to experimentally verify the importance of non-symmetrical fluctuations of the longitudinal intensity distribution as a cause to linewidth rebroadening is proposed and tested on several DFB lasers. This is based on the measurement of cross correlation between the intensity noise from the two laser facets. Our results indicate that such fluctua tions, probably due to weak side mode suppression, can play an important roll in linewidth rebroadening. Linewidth floors were observed at high output powers without any noticeable cross-correlation.