Page 482 - High Power Laser Handbook

450 Fi b er L a s er s Intr oduction to Optical Fiber Lasers 451

Doped SM core
MM pump core
Pump core cladding
Skew rays

(a)

(b) (c) (d) (e)
Figure 15.29 A double-clad fiber with (a) a circular pump guide, (b) an
offset core, (c) a D-shaped pump guide, (d ) a square pump guide and (e) an
octagon pump guide. SM: single mode; MM: multimode.

Earlier double-clad fibers for cladding pumping used circular
pump guides. The existence of skew rays in circular multimode pump
guides—that is, rays propagating in helical patterns around the doped
core—can substantially affect efficient pump absorption. Coiling, espe-
cially coiling with varying curvatures along the fiber, was found to be
very useful for improving pump absorption. Eventually, it was found
that pump absorption could be substantially improved by designs of
offset signal core and, even more efficiently, by noncircular pump
guides. Some examples of these are illustrated in Fig. 15.29. It was later
found that the presence of low-index inclusions in the pump guides,
such as stress elements, could also improve pump absorption.

Pump Coupling Schemes
The easiest way to inject multimode pump light into the pump core
of a double-clad fiber is through the fiber end of the end-pumping
scheme (see Fig. 15.30a). A dichroic mirror can be used between the
two lenses to separate pump from signal light. Although an end-
pumping scheme is easy to implement, the maximum pump injection
points are limited to two. The thermal load from uncoupled pump
light must be removed from the fiber ends, which can be a significant
issue in very high power lasers. An alternative path is through the
60
side-pumping scheme, illustrated in Fig. 15.30b, in which pump
light is reflected off a coated V groove machined into the side of a
double-clad fiber. This scheme can be implemented multiple times
along a fiber and, in theory, allows multiple injection points. Because
the V groove also disrupts the propagating pump light, a laser must
be carefully designed for multiple pump injections. No high-power