Improved direct-modulation characteristics of a semiconductor laser by FM/IM conversion through an interferometer

The behavior of the light signal converted by means of a Mach-Zehnder interferometer is analyzed under the assumption of a three-tone modulation of the emitting laser source. Illustrations are given of the signal at the interferometer output, showing the frequency response, the second- and third-order intermodulation distortions (IMDs), with a direct comparison between intensity modulation (IM) and optical frequency modulation (FM). The FM characteristics are plotted for the different cases where the spurious IM component and the term of chirp formula involving the gain compression factor are taken into account or not. Successful results are obtained in agreement with previous studies. In addition, this work shows that the FM-to-IM signal would exhibit lower distortion levels than the conventional direct IM, placing less stringent requirements on the laser linearity.