Tsunami - Beckman Institute Laser Resource Center

Tsunami
GVD Compensation
The materials in the Tsunami contribute positive GVD and, in combination
with SPM in the Ti:sapphire rod, induce a positive chup on the pulse.
Consequently, the circulating pulse continues to broaden as it propagates
through the cavity unless negative GVD is present to balance these effects.
As discussed earlier, a material exhibits GVD when the second derivative
of its refraction index, with respect to wavelength (d2n/d;12), is non-zero.
This is a special case that is only valid when all wavelengths follow the
same path through a material. This can be extended to any optical system
having a wavelength dependent path length (P). GVD is then governed by
the second derivative of the optical path with respect to wavelength
(d2P/d;l2).
For this reason, a prism pair can be used to produce negative GVD. This is
generally the preferred intracavity compensation technique for ultrashort
pulse lasers because (a) losses can be minimized by using the prisms at
Brewster's angle, and (b) the negative GVD is nearly linear over a large
bandwidth. Ideally, for stable short-pulse formation, the round trip time in
the cavity must be frequency independent, i.e., Tg(o) = d@/dw = constant,
where Tg(w) is the group delay time, $ is the phase change, and cc, is the
frequency. In reality, dispersion is not purely linear, and higher order
dispersion terms become significant for shorter output pulse widths (larger
bandwidths).
In the fs Tsunami laser, a four-prism sequence configuration is used to
provide negative GVD (Figure A-6).
-
Positive
Chirp
Chirp
Free
B A R
A
1
Tuning
Slit
Figure A-6: The four prism sequence used for dispersion compensation
in the Tsunami laser. An input pulse with a positive chirp (red
frequencies at the leading edge of the pulse) experiences negative GVD
(red frequencies have longer group delay time) in the prism sequence.
The net effect is that the prism sequence compensates for the positive --
GVD and produces a pulse which has no chirp.