Analytical formulas for intermodulation- and harmonic-distortion products of a directly intensity-modulated laser diode are presented. The Volterra theory of nonlinear systems is applied to the single-mode rate equations of the modulated carrier and photon densities. This analysis takes gain saturation and noncoherent external optical feedback into account and provides expressions for the frequency-dependent intermodulation- and harmonic-distortion products. The single-mode analysis has been applied to a InGaAsP/InP Fabry-Perot laser diode, and the theoretical values of the second- and third-order intermodulation products at high modulation frequencies show good agreement with measurements. Thus, the formulas seem to provide a convenient and accurate means of predicting the laser-induced distortions in an analog optical transmission link over a wide frequency range.