Page 468 - High Power Laser Handbook

436 Fi b er L a s er s Intr oduction to Optical Fiber Lasers 437

general consequence of all large-core designs for fundamental
mode operation. A small NA requires a small refractive index con-
trast between core and cladding. The minimum NA is also limited
by how reliably the refractive index can be controlled in the CVD
processes. An NA of 0.06 is considered to be the minimum NA pos-
sible with standard fiber fabrication processes and reasonably good
controls.
Single-mode operation can be achieved in optical fibers that sup-
port a few modes, typically for V < 12. The fundamental mode can be
excited by underfilling the optical fiber’s acceptance cone, which is
determined by the fiber NA. In an optical fiber with a few modes, the
fundamental node’s acceptance cone is much smaller than the maxi-
mum fiber acceptance cone (see Fig. 15.19a). Even though a few high-
er-order modes can propagate free of loss in such optical fibers, it has
been shown that selective excitation of the fundamental mode can
15
allow single-mode operation. In addition, small coils can be used to
introduce higher losses to higher-order modes to suppress their prop-
16
agation. Scaling up the core diameter with this approach is mainly
limited by the increasing number of guided modes, which makes it
increasingly difficult to achieve fundamental mode launching and to
introduce differential modal losses by coiling. Commercial fibers
with 50-µm core diameters are available with conventional designs
for ytterbium-doped fiber amplifiers (see Fig. 15.19b), with an effec-
tive mode area of ~1000 µm . With core-pumped designs, mode areas
2
2
27
up to ~1800 µm are possible for erbium-doped fiber amplifiers.
Experimental results with 80-µm core diameters have been demon-
strated for ytterbium-doped fiber amplifiers in cladding-pumped
configurations. However, most of the fiber was coiled to a very
28
small diameter with a much smaller effective mode area than that in
a straight fiber (more discussion on mode area compression in bent
fibers later). Nevertheless, a straight section at the output end can be
used for its large effective mode area, assuming all higher-order

2θ fiber
2θ FM

(a) (b)
Figure 15.19 (a) Excitation of a fundamental mode in a fiber that supports
few modes. (b) Cross section of a commercial double-clad polarization-
maintaining (PM) optical fiber of conventional design made by Nufern.