  Gravitational lensing by massive objects along the line of sight to the
source causes distortions of gravitational wave-signals; such distortions may
reveal information about fundamental physics, cosmology and astrophysics. In
this work, we have extended the search for lensing signatures to all binary
black hole events from the third observing run of the LIGO--Virgo network. We
search for repeated signals from strong lensing by 1) performing targeted
searches for subthreshold signals, 2) calculating the degree of overlap amongst
the intrinsic parameters and sky location of pairs of signals, 3) comparing the
similarities of the spectrograms amongst pairs of signals, and 4) performing
dual-signal Bayesian analysis that takes into account selection effects and
astrophysical knowledge. We also search for distortions to the gravitational
waveform caused by 1) frequency-independent phase shifts in strongly lensed
images, and 2) frequency-dependent modulation of the amplitude and phase due to
point masses. None of these searches yields significant evidence for lensing.
Finally, we use the non-detection of gravitational-wave lensing to constrain
the lensing rate based on the latest merger-rate estimates and the fraction of
dark matter composed of compact objects.
