  Passive mode-locking critically relies on a saturable loss mechanism to form
ultrashort pulses. However, in Kerr-lens mode-locking (KLM), no actual
absorption takes place, but rather losses appear due to diffraction, and actual
light must escape the cavity. The Kerr-lens effect works to generate through
diffraction an effective instantaneous saturable absorber that delicately
depends on the interplay between the spatial and temporal profiles of the
pulse. Despite the importance of KLM as a technique for generating ultrafast
pulses and the fundamental role of the diffraction losses in its operation,
these losses were never directly observed. Here, we measure the light that
leaks out due to diffraction losses in a hard-aperture Kerr-lens mode-locked
Ti:Sapphire laser, and compare the measured results with a numerical theory
that explicitly calculates the spatio-temporal behavior of the pulse.
