William Angerer - Department of Physics and Astronomy - University ...

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The production of ultrafast, high peak power pulses in the Ti:Ah03 laser occurs
through a passive modelocking process called Kerr-lens modelocking. The cw laser
light in the laser cavity is composed of a spectrum of frequencies. The width of
this spectrum is determined by the active medium and the optics in the laser cavity.
These frequencies, or longitudinal modes, are discrete as a result of the boundary
conditions at the high reflector and output coupler mirrors. The modes are separated
by a frequency '!rei L, where L is the length of the cavity. vVhen the laser is operating
in cw mode, the phases between the modes are random. vVe can add a mechanism
to the laser cavity that favors the propagation of a pulsed laser source as opposed to
a cw source. The cw laser source will evolve to a pulsed laser source due to the gain
differential between the two modes. This transformation from cw to pulsed mode
occurs by fixing or locking the phase relationships between the modes to non-random
values. In other words, by defining phase relationships, the longitudinal modes can
add together to form a high intensity laser pulse.
For the Ti:AI 2 0 3 laser, the laser crystal not only functions as the acti\·e medium.
but also as a Kerr lens, i.e. the active medium has an intensity dependent index of
refraction that is exploited to produce ultrafast pulses. By placing a partially closed
shutter in front of the output coupler, a loss is added to the cavity. If a noise spike. i.e.
an intense pulse of light, is introduced into the cavity, the higher intensity of the spike
will activate the Kerr lens modelocking and the light associated with the noise spike
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