Tsunami - Beckman Institute Laser Resource Center

Mode Locking
The net intracavity GVD can be changed by translating prisms Pr2 and Pr3
perpendicular to their bases. This is achieved using a single micrometer
adjustment. By translating Pr2 and Pr3 further into the intracavity beam,
more optical material is inserted into the cavity and the net intracavity
GVD becomes less negative.
The different spectral components of the pulse are spatially spread between
prisms Pr2 and Pr3. This allows wavelength selection to be conveniently
accomplished by moving a slit between these two prisms in the direction
of the spectral spread. Varying the slit separation selects the output bandwidth
(and hence pulse width). As the output wavelength is tuned, it is
necessary to optimize intracavity dispersion by adjusting Pr2 and Pr3, in
order to maintain the shortest output pulse width. To a first approximation,
the prisms need to be translated at the same rate as the slit to maintain the
minimum pulse duration.
In the picosecond configuration of the Tsunami laser, wavelength selection
and tuning are achieved by a birefringent filter, and dispersion compensation
is accomplished with a Gires-Tournois interferometer (GTI). The GTI
introduces a larger net negative GVD than a prism pair, but it is linear over
a smaller wavelength region.
A GTI is a pair of parallel surfaces where the first surface is a partial
reflector and the second has 100% reflectivity. The two qualities which
characterize these interferometers are the spacing between the surfaces
(4, and the reflectivity (r) of the first surface. The round-trip time through
the GTI may be expressed as:
For a short pulse, the group delay time is given by:
Figure A-7 shows Tg for a typical GTI as a function of wavelength. The
GVD is proportional to the slope of this curve (actually, - dTg/dil), and is
periodic with regions of both positive and negative GVD.
When a GTI is used in a tunable laser, the spacing between the surfaces of
the GTI must be adjusted to obtain the appropriate dispersion at the lasing
wavelength. In the Tsunami laser, this is achieved by using a piezo-electric
transducer (PZT) between the GTI plates. By changing the applied voltage
to the PZT, the distance between the plates is varied (which varies the
dispersion) and, for any given wavelength, the pulse width can be optimized.