Tsunami Mode-Locked Ti:sapphire Laser
A-4
In passively mode-locked systems, the pulse itself generates the periodic
modulation. This can be accomplished with a saturable absorber dye that
responds to the instantaneous light intensity in a nonlinear manner. At low
light intensity the dye is opaque, but at higher intensities the dye is
bleached and becomes transparent. The bleaching time of the dye is the
effective time of the optical shutter. In the 1980's, a colliding pulse geome-
try was used with the passive modelocking technique to produce a colliding
pulse mode-locked (CPM) dye laser. When intracavity GVD compensation
(described laser in this chapter) was used with a CPM laser, sub-100 fs
pulses were generated for the first time.
Also, during the 1980's, several new developments in broad bandwidth,
solid-state laser materials occurred. The most notable of these was tita-
nium-doped sapphire which allowed lasers to be tuned over a continuous
range from < 700 to 1100 nm. In 1989, Spectra-Physics was the first com-
pany to offer a commercial CW Ti:sapphire laser.
The broad bandwidth and good thermal properties of this new material
motivated several new modelocking approaches. Additive pulse modelock-
ing (APM) used an interferometrically-coupled, external nonlinear fiber
cavity to induce modelocking. In 1991, self-modelocking in Ti:sapphire
was observed to be induced through the intensity-dependent, nonlinear
refractive index of the laser medium. At Spectra-Physics, the Tsunami laser
was developed, a commercial, mode-locked Ti:sapphire laser based upon a
regeneratively initiated technique.
Regenerative Modelocking in the Tsunami
Like active modelocking, regenerative modelocking in the Tsunami laser
employs an AOM within the cavity to generate a periodic loss. However,
unlike active modelocking, the RF drive signal used to drive the AOM is
derived directly from the laser cavity (Figure A-4). This removes one of the
greatest drawbacks of active modelocking, i.e., the requirement that the
cavity length match the external drive frequency. In the Tsunami, if the
laser cavity length L changes slightly, the drive signal to the modulator
changes accordingly.
When the Tsunami is initially aligned, the laser is operating CW with oscil-
lations from several longitudinal modes. These are partially phase-locked,
and mode beating generates a modulation of the laser output at a frequency
of
c
/
2L
. This mode beating is detected by a photodiode and then amplified.
Since this signal is twice the required
AOM modulation frequency (ω
ML
), it
is divided by two, then the phase is adjusted such that the modulator is
always at maximum transmission when the pulse is present. Finally, the
signal is reamplified and fed to the
AOM.
Most actively mode-locked systems run on resonance for maximum dif-
fraction efficiency. The
AOM in a Tsunami is operated off-resonance with a
diffraction efficiency of about 1%. The output pulse width is not controlled
by the
AOM diffraction efficiency. Rather, pulse shortening in the Tsunami
occurs through a combination of positive GVD and nonlinear effects (self
phase modulation) in the Ti:sapphire rod. Ultimately, the output pulse
width is controlled by adding net negative GVD to the cavity to balance
these effects. (Refer to the following section on GVD.)
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