Page 469 - High Power Laser Handbook

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

modes can be stripped off by the small coil diameters without sig-
nificant intermodal coupling.
Many lasers use linearly polarized beams. In an optical fiber with
perfect circular symmetry, the state of polarization can vary with propa-
gation due to the birefringence caused by random bends and stress and
phase-matched coupling between polarization modes. As a result,
uncertainties in the orientation of polarization, as well as in the state of
polarization, can arise. These uncertainties can be mitigated by PM fiber
designs. Polarization-maintaining large-core optical fibers are fabri-
cated by placing a pair of stress-inducing elements—typically, boron-
doped silica, which has a large thermal expansion coefficient mismatch
compared with that of silica—on either side of the core to create a polar-
ization-dependent refractive index (see Fig. 15.19b). Normally, the two
polarization modes of a circular fiber are degenerate; they become non-
degenerate with effective mode indices n and n , respectively, in a PM
y
x
fiber, providing birefringence ∆n =n – n . The beat length L = λ/(n – n )
B
y
x
x
y
is defined as the period of polarization mode beating. A cross section of
a commercial double-clad PM fiber is shown in Fig. 15.19b. The octago-
nal pump guide is designed to minimize skew rays, which do not pass
the doped core. The fibers are usually coated with a low-index polymer
to provide a pump NA as high as ~0.45.
Photonic Crystal Fibers
Photonic crystal fibers (PCFs) were first demonstrated in the late
1990s. 29,30 Its potential for achieving single-mode operation with large
31
cores was realized very early on. A PCF is drawn from a hexagonal
stack of capillaries, with typically one to seven capillaries replaced by
rods in the center. Pressurization of the airholes is typically used to
keep the holes from collapsing from surface tension during drawing.
The center rods form the core. The composite cladding material of
glass and air makes it easy to achieve a very low refractive index con-
trast between the core and the composite cladding, consequently pro-
viding much better control at achieving fibers with low NAs. Weak
32
guidance, however, makes PCFs very bend sensitive, especially at
large core diameters. In practice, they need to be kept straight beyond
core diameters of 40 µm. Fibers with these large core diameters are
made into rods with outer diameters anywhere from 1 to 2 mm and
are referred to as fiber rods. 33,34 Fiber rods with core diameters as large
as 100 µm and an effective mode area of ~4500 µm have been dem-
2
onstrated. The ytterbium-doped core glass for large core fibers
35
requires good uniformity and exceptional average refractive index
control. For PCFs, this control is achieved by stacking together ytter-
bium-doped glass, which has a high refractive index, and fluorine-
doped glass, which has a lower refractive index, in the correct area
ratio to achieve a high level of average refractive index control. The
process can be repeated many times in theory to achieve much
improved refractive index uniformity.