Page 223 - High Power Laser Handbook

192 So l i d - S t at e La s e r s Zigzag Slab Lasers 193

Diffraction
In most applications, the slab represents a distributed aperture as the
propagating beam enters and exits. To minimize diffractive losses,
the slab’s Fresnel number must be on the order of 10 or larger. The
Fresnel number is given by

N = a / λL (8.6)
2
eff
where a is the half-thickness of the slab and L is the effective length
eff
of slab in zigzag propagation. Thus, for λ = 1 µm and a slab thickness
of ~2 mm, the slab length is limited to ~10 cm. To overcome this limi-
tation, slab architectures have been developed in which the beam
propagates across, instead of within, a thin gain medium. A recently
TM
developed architecture that uses this approach is the Thinzag archi-
tecture (Chap.9).
Slab Fabrication
Because the beam typically makes many bounces from the TIR faces
as it zigzags down the slab, the flatness (figure) and parallelism of
these surfaces is critical for the laser’s ultimate beam quality. Typical
polishing specification for these surfaces is λ/10 in zigzag transmis-
sion. Holding this type of specification for large aspect ratio/thin
slabs becomes very difficult. The polishing process stresses the slab,
and when released from the polishing fixture, YAG slabs can change
their shape in a phenomenon known as springing. A reasonable aspect
ratio of slab height to thickness that maintains the slab shape is on the
order of 20.

Slab Size
Before the development of ceramic laser host materials (c.f. Chap. 7),
crystalline host material sizes were limited by the crystal’s growth
process. For Nd:YAG, the largest commercially available boules yield
slabs that are ~3 cm tall. Ceramic Nd:YAG has increased this dimen-
sion by a factor of 5, and further increases will be possible in the near
future. Another method of overcoming the size limitation of the crys-
talline host material is diffusion bonding, which was used on the
Diode-Array Pumped Kilowatt Laser (DAPKL) laser in the mid-1990s
and is described later in this chapter.

8.3 Traditional Side-Pumped Slabs

8.3.1 Architecture and Technical Issues
Figure 8.1 showed a schematic diagram of a traditional side-pumped
slab. This type of slab is usually pumped by close-coupled diode
arrays through a coolant that flows over the slab’s TIR faces. The