Comparison of DFB laser linewidth measurement techniques results from COST 215 round robin

An interlaboratory comparison on four different linewidth measurement techniques is performed for twelve DFB lasers from various manufacturers. The measured linewidths agree very well for the lasers that have a flat frequency noise spectrum. Differences of more than a factor of 2 in measured linewidth for some of the other lasers can be explained by an increased level of the low frequency noise spectrum recognised by a Michelson interferometer. The conclusion is made that the application of the laser determines what is the best definition of linewidth and which is the best method for its measurement.