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Chapter One: Introduction
to 50ºC. Once this temperature is identified and fixed, the temperature of the
crystal can be stabilized to ±0.1 ºC with the TEC controller.
The Nd:YAG laser amplifier is capable of storing a considerable amount of
energy due to the long upper state lifetime of the Nd ion. The Patara laser uses an
acousto-optic (AO) Q-switch to prevent the buildup of optical power in the
oscillator cavity by introducing a loss greater than the available gain allowing
energy to accumulate in the gain medium. The Q-switch then suddenly removes
the loss allowing the laser power to increase exponentially with each round trip of
the oscillator cavity until much of the stored energy is exhausted and the gain
once again falls below the loss (it takes light only a few nanoseconds to transit the
laser cavity and results in releasing stored energy in a short duration pulse.) The
high peak power achieved in this manner is also essential to achieving an
adequate fraction of conversion to 532 nm. The conversion process is dependent
on the intensity of the circulating 1064 nm power.
The laser beam that circulates inside the oscillator cavity is determined by a
process of self-consistency. The beam characteristics at any given location in the
cavity are repeated after the beam completes a round trip back to that location.
Oscillator beams are composed of transverse modes of simple structure related to
a fundamental Gaussian intensity profile mode, which is of the smallest lateral
extent and divergence in the set.
The aperture in the oscillator path limits the laser beam's transverse spatial extent.
To achieve maximum power and efficiency, this aperture should coincide with the
aperture of the amplifier medium so that the oscillating beam fills the gain
medium to the maximum extent. Laser beam quality will degrade if the diameter
of the fundamental mode is much smaller than the limiting aperture, causing
higher order modes to oscillate with a larger transverse extent and a higher
divergence than the fundamental mode.
Beam quality is dependent upon the product of the diameter of the beam and its
angular divergence as it propagates into the far distance. This product is
conserved with propagation and transformation by optical elements such as lenses
and mirrors. Beam quality can also be rated by the reduction of spot size achieved
by simple focusing with higher quality beams providing spots with smaller
diameters and higher intensities.
This factor also impacts the amount of achievable harmonic conversion and
favors restricting laser operation to its fundamental Gaussian mode, such as is
used in the Patara laser. The design of the laser cavity for fundamental mode
operation involves the selection of cavity length and the characteristics of the
pumped amplifier medium's diameter and focal power.
Once the design parameters are selected for optimum fundamental mode
operation, they cannot be altered without disrupting the fundamental mode size in
the amplifier leading to unacceptable changes in performance. This is particularly
important to the intensity of amplifier excitation, pump power, or drive current
© 2018 Cutting Edge Optronics, Inc.
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Patara I and II Laser User Manual
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