Tsunami - Beckman Institute Laser Resource Center

Tsunami
The modulation frequency 2wmL must be precisely matched to the repetition
rate of output pulses which is C/2L. The rf signal used to drive the - -
AOM is, thus, usually generated by a temperature-stabilized crystal oscillator,
and the cavity length of the laser is adjusted to obtain the appropriate
frequency.
The duration of the mode-locked pulses depends on several factors including
the gain bandwidth of the laser and the modulation depth of the AOM.
Laser media with greater gain bandwidth have the capability of generating
shorter pulses. Consequently, active mode locking a Nd:YAG laser produces
pulse widths of 30 to 150 ps, while for an ion laser, durations are
usually 120 to 200 ps.
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 geometry was used
with the passive mode locking technique to produce a colliding pulse modelocked
(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 198OYs, several new developments in broad bandwidth,
solid-state laser materials occurred. The most notable of these was
titanium-doped sapphire which allowed lasers to be tuned over a continu- -
ous range from < 700 to 1100 nm. In 1989, Spectra-Physics was the first
company to offer a commercial cw Ti:sapphire laser.
The broad bandwidth and good thermal properties of this new material
motivated several new mode locking approaches. Additive pulse mode
locking (APM) used an interferometrically-coupled, external nonlinear
fiber cavity to induce mode locking. In 1991, self-mode locking in
Tisapphire was observed to be induced through the intensity-dependent,
nonlinear refractive index of the laser medium. At Spectra-Physics Lasers
we developed the Tsunami laser, a commercial, mode-locked Ti:sapphire
laser based upon a regeneratively initiated technique.
Regenerative Mode Locking in Tsunami
Like active mode locking, regenerative mode locking in the Tsunami laser
employs an AOM within the cavity to generate a periodic loss. However,
unlike active mode locking, 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 mode locking, 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 - -
oscillations from several longitudinal modes. These are partially phase-