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414 Fi b er L a s er s Intr oduction to Optical Fiber Lasers 415

of optical fiber lasers accelerated in the second half of the 1990s,
leading to the first commercialization effort by JDS Uniphase
Corporation (JDSU) to market watt-level fiber lasers for printing
applications.
The burst of the telecom bubble around 2001 was a blessing for
the development of high-power optical fiber lasers. Much of the
research and development demands in telecommunications suddenly
evaporated. The investments and people who had gathered to meet
those needs were looking for new business directions. The research
and development of multimode pump diodes led to significantly
more powerful and reliable multimode pump diodes at much lower
cost. Military-funded programs, in a drive for directed energy weapon
systems and countermeasures, also played a significant role in push-
ing for higher powers from fiber lasers and the development of
related technologies. It is worth noting that the U.S. directed energy
weapons program started by working on solid-state lasers in the
early 1960s. It went through gas lasers in the 1960s, chemical lasers in
the 1970s, x-ray lasers in the 1980s, and eventually back to solid-state
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lasers in recent years.
In the late 1990s, another technology development further advanced
peak-power scaling of fiber lasers. The small core of a single-mode
optical fiber leads to high optical intensities and, consequently, low
nonlinear thresholds. The solution to this issue comes in the form of
single-mode operation in multimode fibers with much larger cores,
significantly improving nonlinear thresholds. 15,16 With the convergence
of all these factors, a much accelerated rate of development took place,
culminating in the demonstration of single-mode diffraction-limited
multikilowatt-level fiber laser systems in recent years.

15.1.2 Advantages of Fiber Lasers
An optical fiber can be designed to support only the lowest-order
mode—that is, fundamental HE mode or LP mode, which elimi-
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01
nates the need for spatial mode control in an optical fiber laser con-
sisting of a rare-earth-doped single-mode core with two reflectors
placed at each end. Another significant benefit originates from better
heat dissipation, coming from the use of a long fiber with a large
heat-dissipating surface positioned just a few tens to a few hundreds
of micrometers (µm) away from the active region. In a rod laser, heat
is generated within a small volume in the rod center, with a limited
heat-dissipating surface some distance away. This limits power scal-
ing due to rod fracture by the large temperature gradient from a high
heat load. Disc lasers have been developed to combat this issue by
introducing a limited heat-dissipating surface on one side of a thin
disc, very close to the active region. The power scalability of disc
lasers is still an area of intense research. An additional benefit of fiber
lasers, especially from the perspective of practical high-power laser
systems, is high efficiency, mostly coming from better confinement