The noise power spectral density in interferometric systems was studied theoretically and experimentally. The influence of 1/f frequency modulation noise, white noise, and source dispersion are shown in the conversion of phase noise into intensity noise. The direct relative intensity noise (RIN) is compared with the RIN caused by phase-to-intensity noise conversion, and the root mean square of equivalent interferometer phase error was studied. The influence of the interferometer operating point is shown on the noise power spectrum. The results can be employed to estimate the influence of the these effects in phase-to-intensity noise conversion in radio-frequency-modulated optical links, which use either self-delayed interferences or interferometers. These results can be applied to optical signal processing, radar signal processing, etc., which often use interferometers.