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 ca lculating the coupling coefficient of complex-coupled DFB lasers with a corrugated active region. Basic geometrical and structural parameter s are used as inputs to the model, We show that antiphase coupling imp lies lasing on the short-wavelength side of the Bragg wavelength which in turn leads to instabilities and/or nonlinearities in the light-cur rent characteristic, The critical output power for onset of instabilit ies is typically in the order of a few milliwatts, Time-domain simulat ions are used to assess the potential effects of these instabilities o n optical communication systems. We find that transitions from a state of equal output powers from the facets to a state of dramatically dif ferent output powers occur within a few nanoseconds of turn-on, The or igin 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 fo r instability increases when the carrier lifetime is decreased.