  Gravitational waves from binary black holes that are gravitationally lensed
can be distorted by small microlenses along the line of sight. Microlenses with
masses of a few tens of solar masses, and that are close to a critical curve in
the lens plane, can introduce a time delay of a few millisecond. Such time
delay would result in distinctive interference patterns in the gravitational
wave that can be measured with current experiments such as LIGO/Virgo. We
consider the particular case of primordial black holes with masses between 5
and 50 solar masses acting as microlenses. We study the effect of a population
of primordial black holes constituting a fraction of the dark matter, and
contained in a macrolens (galaxy or cluster), over gravitational waves that are
being lensed by the combined effect of the macrolens plus microlenses. We find
that at the typical magnifications expected for observed GW events, the
fraction of dark matter in the form of compact microlenses, such as primordial
black holes, can be constrained to percent level. Similarly, if a small
percentage of the dark matter is in the form of microlenses with a few tens of
solar masses, at sufficiently large magnification factors, all gravitational
waves will show interference effects. These effects could have an impact on the
inferred parameters. The effect is more important for macroimages with negative
parity, which usually arrive after the macroimages with positive parity.
