Frequency Dependence of the Harmonic Distortion in InGaAsP Distributed Feedback Lasers

We report measurements of the second harmonic distortion in 1.3microns InGaAsP distributed feedback lasers. At low modulation frequencies (~50 MHz), the second harmonic distortion exhibits a minimum at a particular bias current. The minimum, in general, is not observed at high modulation frequencies (~1 GHz). A model of the harmonic distortion, based on the equivalent electrical circuit of the laser, is presented to explain the observed behavior.