Page 453 - High Power Laser Handbook

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

taken off a CVD system for solution doping, and an additional drying
process is required afterward.

Spectral Properties of Rare-Earth-Doped Fibers
Infrared transitions from trivalent rare earth ions originate from
f-shell electrons, which are relatively shielded and consequently do
not change significantly from one host to another. Upper-state life-
times, precise transition spectra, and nonradiative processes relying
on energy coupling to phonons of the surrounding lattice can, how-
3+
ever, be modified by the host glass. Lower-level transitions for Yb ,
3+
3+
Er , and Tm ions are illustrated in Fig. 15.5. Erbium-doped optical
fibers are the most studied due to their ability to amplify around the
telecommunication window of 1.55 µm. Initially, erbium-doped opti-
cal fibers were pumped in the visible by gas and dye lasers due to a
lack of suitable pumps at longer wavelength. The first diode-pumped
EDFAs were pumped around 1480 nm using diodes developed for
Raman amplifiers for telecommunications. 10,11 EDFAs spurred the
development of single-mode diode lasers around 980 nm, which
allows more efficient inversion and lower amplifier noise. Corre-
spondingly, high-power optical fiber lasers using these pump wave-
3+
lengths were also developed. Even though Nd -doped optical fiber
lasers were the first to be studied due to the four-level nature of Nd
3+
transitions, Yb ions have recently become the dopant of choice for
3+
3+
high-power fiber lasers. The first reason for this is the Yb ion’s sim-
ple system with low quantum defect. Low quantum defect equals
less heat generation, which is a huge plus for high-power lasers. The
simple system of ytterbium is also beneficial, because there is no need

3
H 4
4
3 I 11/2
F 5/2
3
H 5
4
I 13/2
3
F 4
3 4 3
F 7/2 I 15/2 H 6
Yb 3+ Er 3+ Tm 3+
Emission: 1010–1150 nm Emission: 1500–1620 nm Emission: 1700–2100 nm
Pump: 915 nm, 976 nm Pump: 976 nm, 1480 nm Pump: 790 nm, 1660 nm
3+
3+
3+
Figure 15.5 Low-level transitions of Yb , Er , and Tm in silica glass host.
Nonradiative processes are indicated by dotted lines. Pump transitions are
indicated by upward-pointing arrows, and emission is indicated by downward-
pointing arrows.