Page 452 - High Power Laser Handbook

420 Fi b er L a s er s Intr oduction to Optical Fiber Lasers 421

the fiber would only support a single mode for V < 2.405. The total
number of guided modes in a fiber can be estimated from

V 2
N = (15.8)
2

15.2.2 Properties of Rare-Earth-Doped Optical Fibers

Basics of Rare-Earth-Doped Glass
3+
3+
Most rare earth ions of interest, such as Yb , Er , Tm , and Nd ,
3+
3+
are trivalent and much larger than typical glass formers. This is par-
ticularly true in silica glass, which comprises the tetrahedral struc-
tures of SiO . Incorporation of rare earth ions requires a disruption of
2
the regular glass network, which ultimately limits the level of pos-
sible rare earth doping before the onset of clustering due to phase
3+
separation. A high level of Al ions can be added to a silica glass
host to form a homogeneously modified glass network, which allows
a much improved incorporation of rare earth ions. Incorporation of
5+
P ions in silica glass can have a similar effect. In fact, phosphate
glass, a glass made mostly of P O , is known to allow very high lev-
2
5
els of rare earth doping to few tens of weight percent (wt%) levels
before significant phase separation. In any case, much lower rare-
earth-doping levels and gain per unit length in a silica-based glass
host are expected in comparison with that in a crystal host. In an
EDFA, typical erbium-doping levels are in the few tens of mole parts
per million (mol ppm) to a few hundreds of mol ppm. Absorption
and emission spectra of rare earth ions in a glass host are usually
much broader in comparison with those in crystal hosts due to sig-
nificant inhomogeneous broadening in an amorphous host. This is
beneficial, as it relaxes the required wavelength control for diode-
pumped lasers and allows for broadband amplification, such as in
wavelength-division-multiplexing systems, ultrashort pulse genera-
tion, and widely tunable lasers.
Vapor-phase delivery of rare earth compounds has been used in
early demonstrations of rare-earth-doped single-mode optical fibers.
The key difficulty is a lack of compounds with high enough vapor
pressure near room temperature. Elaborate heated delivery systems
must be developed to incorporate both rare earth and aluminum ions.
The key benefit of a vapor-phase delivery system is its compatibility
with chemical vapor deposition (CVD) processes, which are widely
used for optical fiber fabrication. The solution doping method, based
on aqueous impregnation of soot formed by a CVD process at a low
deposition temperature well below sintering temperature, is much
easier to implement and consequently widely used. For most dopants
of interest, soluble compounds are easily available for common labo-
ratory solvents. The drawbacks are that a preform usually has to be