The phase noise characteristics of single mode semiconductor lasers with optical feedback have been studied. A correlation is found between the suppression of the low‐frequency noise and the enhancement of the high‐frequency noise near the relaxation oscillation frequency. Contrary to previous analyses, the low‐frequency noise is minimized when the reflected light is 70°–90° out of phase with respect to that inside the diode laser cavity. A calculation of the laser frequency fluctuation spectrum, taking into account the amplitude‐phase coupling of the laser field, agrees with the observation. A new explanation is proposed for the feedback‐induced phase noise reduction effect.