Modelocking
A-9
Figure A-7: Group delay time as a function of wavelength for a GTI
with a 40 µm spacing and a 4% reflector.
For a given reflectivity r, the magnitude of Tg is proportional to t
0
and,
thus, the spacing d (GVD is proportional to d
2
). By increasing the spacing
between the GTI surfaces, the amount of GVD can be increased signifi-
cantly. The bandwidth over which a GTI provides linear GVD is inversely
proportional to the spacing d. Thus, for large values of GVD, the bandwidth
over which the GVD is linear will become quite small. By using several
GTIs with different spacings, it is possible to generate transform-limited
pulses from the Tsunami laser with a duration as short as 1 ps to as broad as
80 ps. Pulse widths beyond 10 ps require a 3-plate birefringent filter to
eliminate the next order of the GTI. For a given GTI, the pulse width can be
varied by fine adjustments of the spacing d since different portions of the
dispersion curve generate varying values of
GVD. Typically, it is possible
to change the pulse width by a factor of about two for any particular GTI.
760 770 780 790
0
50
100
150
200
250
300
Wavelength (nm)
Tg Group Delay (fs)
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