Thin Disk Lasers
(source: Encyclopedia of Laser Physics and
Technology)
The thin disk laser (sometimes called thin disc
laser or active mirror laser) is a special kind of diode-pumped high power
laser. The difference to conventional rod lasers is the geometry of the gain
medium: it is a thin disk, where the thickness is considerably smaller than
the laser beam diameter, and the generated heat is extracted dominantly
through one end face, rather than in the transverse direction. The cooled end
face has a dielectric coating which is reflecting both the laser radiation and
the pump radiation. The thin disk is also often called active mirror, because
it acts as a mirror with laser gain.
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| Fig.: Schematic setup of a thin disk laser head.The pump optics (not shown) are arranged for multiple passes of the pump radiation. The heat is extracted in the longitudinal direction, which minimizes thermal lensing effects |
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Due to the small thickness of the disk (e.g.
100-200 ìm for Yb:YAG), the temperature rise due during laser action is small.
In addition, the temperature gradients are dominantly in a direction
perpendicular to the disk surface and thus cause only rather weak thermal
lensing. This allows for operation with very good beam quality due to the weak
thermal beam distortions. Well over 200 W of output power in a
diffraction-limited continuous-wave beam have been obtained. Thin disk lasers
with 4 kW output power in a multimode beam are commercially available. Thin
disk lasers have also lead to the highest average output power of 80 W from a
mode-locked laser.
The small thickness of the disk typically leads to
inefficient pump absorption in a single or double pass. This problem is
normally solved by using a multi-pass pump arrangement, which can be made
rather compact when using a well designed optical setup, typically containing
a parabolic mirrors and prism retroreflectors.
The thin disk laser concept allows for further
variations. For example, side pumping of the disk may allow for even higher
output powers while reducing the requirements on the pump beam quality. This
approach, developed at the Lawrence Livermore National Laboratory, is based on
a composite laser crystal. An undoped YAG disk, which is bonded to an Yb:YAG
disk, brings various benefits: It reduces the beam quality requirements for
the pump source, reduces the tendency for transverse amplified spontaneous
emission (ASE) and parasitic lasing in large disks, increases the mechanical
strength and may improve the cooling. The side-pumped concept may thus allow
the scaling to even higher powers, even though not necessarily with
diffraction-limited beam quality.