Tsunami Mode-Locked Ti:sapphire Laser
3-10
Pulse Width Selection
The pulse width tuning characteristics of the Ti:sapphire laser are influ-
enced by two factors: those inherent in the Ti:sapphire material itself and
those from cavity parameters. While we cannot readily modify the Ti:sap-
phire material to change pulse width, we can modify the net group velocity
dispersion (GVD) of the cavity. The optical components in the laser cavity
introduce positive GVD and cause pulse spreading. Further pulse spreading
is caused by self-phase modulation (SPM) in the Ti:sapphire rod, which
results from the interaction of the short optical pulse with the nonlinear
refractive index. In order to obtain stable, short output pulses, these effects
must be compensated with negative GVD.
fs systems. Prism pairs are used to produce a net negative intracavity GVD
in the fs system. Varying the amount and type of prism glass through which
the intracavity beam travels changes the net intracavity GVD. Pulse widths
from <35 to 150 fs can be obtained with various cavity configurations.
Pulse width also depends on the operating wavelength.
ps systems. In the ps configuration, a GTI is used to provide negative GVD
in a patented mode locking design. This device is similar to a Fabry-Perot
interferometer, except the first mirror is a partial reflector instead of a high
reflector. It has the advantage of being highly reflective over a broad spec-
tral range, yet provides a frequency-dependent optical phase shift. It also
provides adjustable negative GVD when the distance between the mirrors is
varied. The GTI produces a fairly large negative GVD that is linear over a
narrow bandwidth. It must, therefore, be readjusted whenever a new wave-
length is selected. The GTI POSITION control adjusts this spacing and
allows optimization of the pulse width. A range of < 2 to > 80 ps can be
obtained with different GTI spacings and bi-fi types over most of the Tsu-
nami wavelength regime.
For a full review of GVD and compensation, please refer to Appendix A,
“Mode Locking: Group Velocity Dispersion.”
Purging the Tsunami
The Tsunami head is sealed so it can be purged. The Model 3910 regulator/
filter purge unit is provided for filtering and drying bottled nitrogen gas.
Purging the laser cavity with this gas not only eliminates the typical prob-
lems associated with dust and contamination, but also prevents tuning dis-
continuities caused by oxygen and water vapor. Reduction of the latter is
important for operation in the long wavelengths (see Figure 3-11).
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