The effects of 1/f frequency noise on self-heterodyne detection are described and the results are applied to the problem of laser-diode-linewidth measurement. Laser diode linewidths determined by self-heterodyne methods are not adequate predictors of coherent communications system performance because the measurements often include significant broadening due to 1/f frequency noise. In this report, the autocorrelation function and power spectrum of thedetected self-heterodyne photocurrent are developed in terms of an arbitrary frequency noise. From numerical analysis, the power spectrum resulting from the 1/f frequency noise is shown to be approximately Gaussian and an empirical expression is given for its linewidth. These results are applied to the problem of self-heterodyne linewidth measurements for coherent optical communications, and the amount of broadening due to 1/f frequency noise is predicted. Two methods are then provided for estimating the portion of the measured self-heterodyne linewidth due to the white component of the frequency noise and the portion due to 1/f frequency noise.