Abstract: An optical transmission system has a directly modulated laser for modulating data directly on an optical signal, and a narrow band optical filter having a band center frequency offset from a central optical frequency of the optical signal, to reduce the phase difference between FM and AM of the modulated optical signal, the filter having a bandwidth sufficiently narrow to substantially remove frequencies outside a spectrum of adiabatic frequency chirp resulting from the modulation, combined with Fourier broadening caused by the data modulation. This is a cost effective way of improving the dispersion tolerance to give greatly improved system reach and to make it practical to use directly modulated lasers with existing NDSF. The narrow band filter can be located at the transmitter or the receiver, and can have a center frequency locked to a feature in the frequency spectrum of the laser.