Page 474 - High Power Laser Handbook

442 Fi b er L a s er s Intr oduction to Optical Fiber Lasers 443

(a) (b) (c)
Figure 15.24 Cross section of double-clad LCFs with (a) low-index polymer
pump guide, (b) fluorine-doped silica pump guide, and (c) fluorine-doped silica
pump guide and a PM core. All cores are ytterbium doped.

drawing process. As a result, the second core follows a helical pattern
around the primary core, which is sometimes referred to as chirally
coupled core fibers. 45,46 One major issue with such designs is that all
mode coupling requires modal spatial overlap as well as phase match-
ing. As the primary core gets larger and becomes more multimode,
all modes are increasingly confined to the primary core, which makes
it difficult for coupling to take place over practical length scales due
to diminished spatial modal overlap. A second issue is that the
mode density in the modal index space gets increasingly larger as
the core gets more multimode (see Fig. 15.2), which makes it hard to
phase match only the unwanted modes without affecting the nearby
desired mode. Optical fibers with overall performance that is signifi-
cantly better than conventional fibers have not yet been demonstrated
with these approaches.
A second approach is based on the propagation of a higher-
order mode in a multimode fiber. It has been shown that a higher-
order mode can be excited using a mode converter based on a
long-period grating that is fabricated in a fiber designed to have a
central single-mode core surrounded by a much larger multimode
core. The long-period grating couples the fundamental mode input
guided in the small single-mode core to a co-propagating higher-
order mode guided mostly in the larger multimode core. It has been
shown that higher-order modes can propagate with low sensitivity
to external perturbations. Effective mode areas as high as 3200 to
2100 µm have been demonstrated with LP to LP modes, respec-
2
04
07
tively. It was found that higher-order modes propagate more
47
robustly than lower ones in the same fiber. (Note that a higher-order
mode also has a smaller effective mode area than lower-order modes
in the same fiber.) A second mode converter is required at the out-
put end to convert the higher-order mode back to the fundamental
mode. Recently, active fibers have been demonstrated with this
approach. ASE limits these fibers to a low gain of just more than
48