Instabilities and nonlinear L-I characteristics in complex-coupled DFB lasers with antiphase gain and index gratings

Complex-coupled DFB lasers can be designed to provide large bandwidths and low chirp by adjusting the strength and phase of their index and gain gratings. In this paper we develop a self-consistent method of calculating the coupling coefficient of complex-coupled DFB lasers with a corrugated active region. Basic geometrical and structural parameters are used as inputs to the model. We show that antiphase coupling implies lasing on the short-wavelength side of the Bragg wavelength which in turn leads to instabilities and/or nonlinearities in the light-current characteristic. The critical output power for onset of instabilities is typically in the order of a few milliwatts. Time-domain simulations are used to assess the potential effects of these instabilities on optical communication systems. We find that transitions from a state of equal output powers from the facets to a state of dramatically different output powers occur within a few nanoseconds of turn-on. The origin of these instabilities is explained using a simple physical model and possible ways of increasing the critical power for instabilities are discussed. For example, we clearly show that the critical power for instability increases when the carrier lifetime is decreased.