Reduction of second- and third-order harmonic distortion by nonlinear absorption in gain-coupled DFB lasers

Abstract

The analog modulation distortion due to nonlinear gain compression has been found to be reduced significantly in the gain-coupled DFB laser of absorptive grating type. It is because the gain compression is compensated by the loss compression in the grating. I. IntroductionLaser diodes (LDs) with low modulation distortion are very important for subcarrier multiplexed (SCM) systems such as optical CATV and microcellular networks [ 11. Distributed feedback (DFB) laser diodes are used for those applications due to their single and stable longitudinal mode operation capability, and resulting low noise characteristics. However, some nonlinear effects specific to DFB lasers, such as the longitudinal spatial hole burning (LSHB), deteriorate their linearity and modulation distortion characteristics 123.We previously reported that the LSHB could be reduced in gain-coupled (GC) DFB lasers with absorptive gratings [3]. In addition, there is a possibility that the nonlinear absorption compression of the grating compensates the nonlinear gain compression of the active material, thus resulting in smaller nonlinearity. Therefore, modulation distortion in GC DFB LDs is expected to be lower than that of conventional index-coupled (IC) DFB LDs.In order to investigate whether this effect really exists or not, we here analyzed second and third harmonic distortions (2HD and 3HD) in GC DFB LDs of absorptive grating type numerically, taking the dynamic nature of the abosorptive grating into consideration.