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

63
blue
red
prism 1
BPl
prism 2
BPl
Figure 3.8: Pulse propagation through Brewster prisms. The red and blue components
of the ultrafast pulse are not overlapped due to group velocity dispersion. This
is compensated by the prism pairs shown. The red frequency components traverse
more glass through the second Brewster prism than the blue frequency components.
This overlaps the red and blue frequency components by negative group velocity
dispersion. This figure is adapted from [51].
\Ve observed that the duration of the femtosecond pulse is strongly influenced by
the position of the Brewster prism pair. The Brewster prisms counteract the tendency
of the ultrafast pulse to spread out as it propagates through the cavity (see Fig. 3.8).
This phenomena, known as group velocity dispersion, occurs because the ultrafast
pulse is composed of a band of frequencies that have different group velocities. The
cw power of the laser is relatively insensitive to the positions of the prisms, but the
stability of the modelocking is highly sensitive to their positions. I found through trial
and error that optimal modelocking stability is achieved when the Ti:Ah03 laser pulse
is ,,-,2-3 mm from the front edge of BPI and ",,5-8 mm from the front edge of BP2.
BP2 is mounted on a micrometer stage to allow adjustment during spectroscopic
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.