  We calculate the amplitude-to-phase (AM-to-PM) noise conversion in a modified
unitraveling carrier (MUTC) photodetector. We obtained two nulls as measured in
the experiments, and we explain their origin. The nulls appear due to the
transit time variation when the average photocurrent varies, and the transit
time variation is due to the change of electron velocity when the average
photocurrent varies. We also show that the AM-to-PM conversion coefficient
depends only on the pulse energy and is independent of the pulse duration when
the duration is less than 500 fs. When the pulse duration is larger than 500
fs, the nulls of the AM-to-PM conversion coefficient shift to larger average
photocurrents. This shift occurs because the increase in that pulse duration
leads to a decrease in the peak photocurrent. The AM-to-PM noise conversion
coefficient changes as the repetition rate varies. However, the repetition rate
does not change the AM-to-PM conversion coefficient as a function of input
optical pulse energy. The repetition rate changes the average photocurrent. We
propose a design that would in theory improve the performance of the device.
