  The microwave signal at the output of a photodiode that detects a modulated
optical beam contains the phase noise phi(t) and the amplitude noise alpha(t)
of the detector. Beside the white noise, which is well understood, the spectral
densities S_phi(f) and S_alpha(f) show flicker noise, proportional to 1/f.
  We report on the measurement of the phase and amplitude noise of high-speed
p-i-n photodiodes. The main result is that the flicker coefficient of the
samples is approximately 1E-12 rad^2/Hz (-120dB) for phase noise, and
approximately 1E-12 Hz^-1 (-120dB) for amplitude noise. These values could be
observed only after solving a number of experimental problems and in a
protected environment. By contrast, in ordinary conditions insufficient EMI
isolation, and also insufficient mechanical isolation, are responsible for
additional noise to be taken in. This suggests that if package and EMC are
revisited, applications can take the full benefit from the surprisingly low
noise of the p-i-n photodiodes.
