Results of spectral measurements of λ = 1.5 μm quaternary laser diodes are presented. With regards to frequency chirp, frequency drift and emission bandwidth quite often considerable discrepancies have been observed between measured values and theoretical predictions of numerical laser modelling. Besides the well known tendency of DFB lasers to exhibit mode jumps or poor side mode suppression ratio depending on the fact reflection phase and magnitude sometimes a non-monotonous dependance of the emission frequency on the laser current is observed even for perfectly single mode emitting laser diodes. This behavior may partly be caused by stochastically distributed disturbances of the light guide and the grating structure within the laser due to unsufficient control of crystal growth during laser fabrication particularly when using the LPE method. Measurement results are given and compared to numerically simulated laser behavior. An outline of the laser simulation model is also presented.