Page 506 - High Power Laser Handbook

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

values when reaching the vulnerable air-end cap interface. No distur-
bance to the beam quality is incurred, as long as the freely expanding
beam does not overfill the refractive-index-homogeneous portion of
the end cap’s cross-sectional area. Fiber end capping is now ubiquitous
and represents a key enabler for both pulsed and CW high-power lasers
and amplifiers.

16.3 Fiber Laser Trades for High-Pulse-Power Operation

This section reviews design solutions that must be considered in
the development of high pulse energy and peak power fiber-based
laser sources. Such solutions are conceived to maximize perfor-
mance while addressing and overcoming the challenges described
in Sec. 16.2. Many of the traditional criteria for well-architected
pulse fiber amplifiers originated in the early application of those
amplifiers to optical telecommunications; thus, they emphasize
gain maximization, noise management, low dispersion, and operation
at wavelengths of ultra-low loss for fused silica (such as ~1.5 μm).
Within this framework, fibers represented a very different medium
as compared with bulk DPSS lasers. In the cases of interest in this
chapter, however, such differences become less pronounced; there-
fore, a primary goal is to correctly negotiate the trades that enable
power scaling without giving up key advantages, including effi-
ciency and beam quality, of fibers over other solid-state sources.
The following discussion spans three main areas: fiber designs,
ASE management, and source architecture.

16.3.1 Type of Fiber
The selection criteria for the best fiber candidates to enable high peak
power operation are, in principle, quite simple: Using a large core
(and correspondingly a large mode field area A) and relatively short
fiber is the most straightforward approach to minimizing the NLE
strength given by Eq. (16.1). Enlarging the doped core region is also a
valid strategy for pulse energy scaling, because it leads to higher sat-
uration energy [see Eq. (16.7)]. Unfortunately, it also amounts to
increasing the number of guided modes and, therefore, degrading
beam quality, unless the core NA is reduced or additional special
solutions are implemented. A classic approach involves LMA fibers,
or multimode fibers forced into fundamental mode (LP ) operation
01
by matching the launched optical field to the fiber’s LP mode and
01
using selective bend loss to suppress higher-order modes. Unfortu-
nately, the scalability is practically limited to mode field areas (MFAs)
2
of ~700 μm , due to vanishing intermodal bend-loss discrimination.
Moreover, tight bending distorts the guided mode field, thus greatly
20
reducing its effective area, which ultimately offsets the benefit of a
large core. Finally, even in low-M fibers, the spatial interference of
2