An optical transmitter is disclosed wherein a directly modulated laser outputs a frequency modulated signal through a semiconductor optical amplifier. Both the optical transmitter and semiconductor amplifier are modulated according to an output of a digital data source. An optical filter is positioned to receive an output of the semiconductor optical amplifier and has a frequency dependent transmission function effective to convert the amplified signal into a filtered signal having enhanced amplitude modulation. In some embodiments, the semiconductor optical amplifier is coupled to the digital data source by means of a compensation circuit configured to high-pass filter the output of the digital signal source. In other embodiments, the semiconductor optical amplifier is coupled to the digital data source by means of a compensation circuit configured to produce an output including a first-order time derivative of the output of the digital signal source.