THEORETICAL INVESTIGATION OF THE 2ND-ORDER HARMONIC DISTORTION IN THE AM RESPONSE OF 1.55-MU-M F-P AND DFB LASERS

Numerical calculations of the second-order harmonic distortion in the amplitude modulation-response of Fabry-Perot and distributed feedback lasers are presented and the influence of several nonlinerities, such as longitudinal spatial hole burning, gain suppression, and relaxation oscillations is considered in detail. Our analysis is valid for modulation frequencies ranging from a few megahertz to well beyond the resonance frequency of the relaxation oscillation. The numerical calculation of the distortion is based on the laser model CLADISS [1] and consists of an extended small signal solution (up to second-order) of the set of coupled wave equations and the local carrier density rate equations. The distortion is investigated for Fabry-Perot lasers for which the effects of spontaneous emission and gain suppression can be clearly illustrated and for DFB lasers where the emphasis is on the influence of spatial hole burning and its combination with other nonlinearities. Various effects are discussed, e.g., the occurrence of a dip in the frequency dependence of the distortion resulting from the combination of spatial hole burning and relaxation oscillation contributions in some cases and the occurrence of a dip in the bias dependence when spatial hole burning and gain suppression contributions cancel each other.