This feature is a consequence of the fact that machining
with ultrafast laser pulses is a multiphoton process, as compared
with the thermal process that is characteristic of most ‘long
pulse’ micromachining tools. Long pulse machining is
a process wherein the electric field drives free electrons
trapped in defects within the material, causing energy to
be deposited in the form of heat through repeated collisions
with the surrounding atoms. Typically, however, the number
of free electrons will vary from spot-to-spot because the
number of defects also varies from spot-to-spot. This statistical
variability leads to large variations in the threshold at
different locations on the material.
In contrast, ultrafast laser pulse micromachining is a multiphoton
process which only depends on the fact that atoms are present
in the beam and the beam has enough fluence to drive the process
above threshold. The number of atoms in the size of the focal
spot on the material does not vary significantly from spot-to-spot.
Consequently, neither does the threshold.
When the threshold doesn’t vary and the beam fluence
doesn’t vary, the amount of material removed is precisely
the same shot-after-shot.
