Tsunami - Beckman Institute Laser Resource Center

Tsunami
CW Breakthrough
Under certain circumstances, it is possible for a mode-locked Tsunami
laser to exhibit a small continuous wave (cw) component along with the
pulsed output. This phenomenon is generally restricted to systems operating
in the fs regime where intracavity prisms are used for dispersion
compensation. It usually occurs when either the input pump power is too
high or the intracavity dispersion compensation is incorrect. It can easily
be removed by reducing the ion laser pump power or by adjusting the
prism dispersion compensation control on the Tsunami laser. There
appears to be no analogous dual mode operation in the ps regime (where a
GTI is used for dispersion compensation). Too high an input power in a ps
system usually results in satellite temporal pulses.
Because of the dynamic range limitation of photodetectors, a direct
measurement of the cw background for a fs mode-locked Tsunami is
impossible. For a mode-locked repetition rate of 100 MHz (105 duty
cycle), with 1% of the energy in a cw component, this amounts to a base
line signal 10-7 times smaller than the peak pulse height. In order to detect
this cw signal level directly, a detector with a resolution of 100 fs and
dynamic range of > lo7 is required. A successful measurement can be
made if we take into account the fact that the cw component has a very
different bandwidth than that of a sub 100 fs pulse. It is possible to determine
a cw component of less than 1% with a conventional detector in
conjunction with a spectrally selective device. We recommend a monochromator
with a CCD detector. This also provides an accurate measurement
of the output wavelength (usually required for an experiment) and
the bandwidth of the pulse (required for determining the pulse time-bandwidth
product and, thus, its relationship to a transform-limited pulse).
Figure B-6 shows a standard setup. For a typical c 80 fs output pulse, the
ratio of bandwidth to the cw linewidth is about 100 (10 nm vs. 0.1 nm).
Consequently, a 1 % cw component has the same peak height as the
spectral peak of the pulse. The signal detection limit of this technique is
governed by the ultimate resolution of the monochromator but, with a
conservative signal detection limit of about 10% of the peak height, it is
possible to detect a cw component of about 0.1% (Figure B-7). During
final test procedures at Spectra-Physics Lasers, all Tsunami systems are
tested using an external monochromator and CCD diagnostic to ensure any
cw component is less than 0.1%.
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