All solid state laser types exhibit, more or less,
the effects of thermal gradients in the laser medium. As a
laser rod is being pumped, whether by flash lamp or by diodes,
not all pump energy is converted to useful laser light but a significant
portion is instead converted to heat. Since the surface can transfer
heat energy more readily than the center, it will be cooler than the center.
The thermal gradient, radial in the case of a rod, creates a stress gradient,
since a volume element in the center will expand more than a volume
element further out. This thermally induced stress creates
birefringence in Nd:YAG and also creates a thermal lens, since
the density now is different in the center of the rod compared
with further out along the radius. When designing Nd:YAG lasers
these effects need to be fully understood, characterized and
taken into account
Since
the very act of pumping the Nd:YAG medium has, as described
above, a negative effect on the optical properties
of the laser medium it is, obviously desirable to pump with
as little energy as possible and also to make sure that
most of the energy pumped with becomes useful laser energy
instead of heat.
Laser diodes are uniquely suited to accomplish
exactly this. The output from the pump diodes used is around
808nm, which is where the Nd:YAG material has its absorption
max. Pumping with diodes thus allows the same output energy
to be achieved with input of energy, and in particular
with less heat generated and thus less stress induced.
Furthermore, since usually many sets of diodes
are used, and can be arranged in a large number of geometries
around the rod, the gain uniformity can be made very high.
The figures to the right shows good gain uniformity (top),
as displayed by the BriteLight, and poor gain uniformity (bottom)
as seen when using diodes of poor quality, insufficient
number of diodes or a poorly designed pump geometry. Diode
pumped systems can be designed to display close to diffraction
limited beam performance, even when operated at high power
outputs and at high pulse energies. This is a direct consequence
of the fact that diode pumping generates much less heat
in the gain medium than does flash lamp pumping. Reduced
heating means less thermal stress which means reduced risk
of destroying the rod, less thermal lensing and less thermally
induced birefringence. The BriteLight uses end-pumping
in the Master Oscillator and radial pumping in the amplifiers
for best mode control.
Diode
pumped systems are vastly more efficient than flash lamp
pumped systems for two reasons. First, diodes consume far
less energy than flash lamps since their conversation efficiency
from electrical to optical (photons) energy is vastly
superior. Secondly, the output spectra from a diode (typically
808nm +/- 5nm) is very well matched to the absorption
spectra of Nd:YAG, which has an absorption max around 808nm,
resulting in a large fraction of the pump energy being
absorped as useful excitation energy rather than just converted
to heat. Flash lamps, on the other hand, have an output
spectra covering the entire visible and part of the UV
region and only a small portion is absorbed as useful excitation
energy while as much as 95% turns into waste heat, which
needs to be removed through active cooling. Besides being more
efficient diode pumped systems thus also require less cooling.
Besides
reaching end of life much sooner than diodes, which can run
for years even at 7x24x365, flash lamps often fail unpredictable
and without warning. Diodes, on the other hand, degrade slowly
at the end of their life giving time to plan for their
replacement. The benefit of diodes is thus not only a longer
life but also greater reliability and predictability. These
benefits can be especially important in applications requiring
continuous operation or in commercial applications.
Even though flash lamp technology is cheaper
it has a number of significant disadvantages. It is less
reliable, has shorter lifetimes and most importantly a spectral
output which is a poor match with the absorption spectra
of the Nd:YAG gain material. The poor spectral match results
in most of the light emitted from the flash lamp being wasted
and only heating up the gain medium. The heat generated
in the gain medium creates thermally induced stress which in
turns causes both thermal lensing and stress induced birefringence.
The magnitude of these effects preclude most solid state
lasers from operating at higher repetition rates while, at
the same time, producing any significant energy per pulse.
BriteLight: JMAR's Laser Technology 
