ABSTRACT
The frequency chirping is caused by the refractive index change of an active layer due to carrier density modulation in the direct modulation of semiconductor lasers. This chapter describes the frequency chirping for different types of external modulators. Some transmission experiments employing an external modulation technique have been reported for the purpose of eliminating the frequency chirping. The variation of the lasing wavelength of directly modulated semiconductor lasers is caused by the temperature change and the carrier density modulation in the active layer. The chapter provides the measured spectral width of a directly modulated semiconductor laser as a function of modulation frequency. Some nonlinear effects such as spectral hole burning and spatial hole burning cause additional term in frequency chirping. These nonlinear effects give the nonlinear gain, which is proportional to the output intensity of a semiconductor laser. Thus, the carrier density of an active layer is changed to be proportional to the output.
