Page 507 - High Power Laser Handbook

474 Fi b er L a s er s Pulsed Fiber Lasers 475

residual higher-order modes causes beam-centroid wander and far-
field beam-pointing instability, which especially hampers their use in
remote sensors and materials processing. 21
Another approach is to resort to waveguides designed to be effec-
tively single-mode despite very large cores. An important example is
photonic crystal fibers (PCFs). In these fibers, an air-clad multimode
waveguide (designed for pump guiding) embeds a hexagonal array
of axial holes in which a number of central holes are replaced with a
solid hexagonal array of rare-earth-doped elements to form the core.
The refractive indices of the pump waveguide and core are controlled
by the axial-hole diameter-to-spacing ratio and the amount of index-
lowering fluorine doping, respectively. They are also fine-tuned to
provide a core-to-pump waveguide index contrast that supports sin-
gle transverse mode operation. This microstructure is realized
through a stack-and-draw fabrication process that offers significantly
higher design precision compared with the chemical deposition and
solution doping that are normally used for standard fibers. In partic-
ular, very-low-NA cores can be reproducibly obtained. This enables
single-mode core size scaling (up to 100 μm diameter, to date), even
though bend loss ultimately becomes significant, and thus the PCF
must be enclosed in a rigidly straight overcladding that forms the
main feature of the so-called rod-type PCF. Performance examples of
this are reviewed in Sec. 16.4.
Similar to standard polarization-maintaining (PM) fibers, bire-
fringence along a preferential PCF core axis can be elasto-optically
imparted by the frozen-in stress resulting from incorporation of two
sets of borosilicate axial rods thermally mismatched to the PCF silica-
based cladding. Unlike in standard PM fibers, the stress rods con-
tribute to light confinement and are located in close proximity of the
fiber core, which affords high birefringence encompassing the large
core area without the need for excessive (and hence structurally
unsound) boron doping. Because of the exceedingly low core-to-
inner cladding index contrast, the induced birefringence can be suf-
ficient for light polarized along the PCF fast axis to experience an
effective index lower than the inner cladding and, as a consequence,
leak out of the core, which is known as polarizing behavior.
22
Alternative fiber concepts, such as leakage channel and chi-
rally coupled core fibers (which are discussed in Chap. 15) have
23
been proposed to foster MFA scaling with good beam quality. In the
limit of very large cores, however, such diverse single-mode solu-
tions seem inevitably to converge toward waveguides that must be
kept nearly straight to escape bend loss or bend-induced mode field
compression. Another adopted approach relies on fibers that selec-
tively operate in a single, cylindrically symmetric large-area high-
order mode (HOM), such as LP or LP . This mode is selected at the
07
04
24
fiber input by means of a judiciously designed long-period grating.
The rationale is that HOMs are subject to lower bend-induced area