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This article reports on extensive investigations of the influence of external optical feedback (OFB) on the spectral line shape and the associated spectra of relative intensity noise (RIN) and frequency noise (FN) in semiconductor lasers. The study is based on a generalized simulation model that treats the OFB as a multiple-roundtrip time delay of the lasing field in the external cavity. We present criteria to apply the famous Lang–Kobayashi model to predict the operation characteristics of lasers under OFB. The accuracy of approximated models of calculating the spectral linewidth from the low-frequency components of the frequency noise is examined. We characterize the spectral line shape and the spectra of the RIN and FN in five distinct operating regions, namely, continuous-wave (cw), weak OFB-induced pulsation, period-doubling route-to-chaos, chaos or coherence collapse, and strong OFB-induced pulsation. InGaAsP lasers emitting in a wavelength of 1.5 μm are included in the calculations. The results show that the line shape is most enhanced with sharp central and satellite peaks when strong OFB induces either high-frequency pulsation or cw operation. The moderate range of OFB induces coherence collapse operation, which is characterized by most suppressed and most broadened central and satellite peaks. The cw operation induced under very strong OFB is characterized by RIN and FN levels lower than those of the case of cw operation induced under low OFB. © 2004 American Institute of Physics.