Page 454 - High Power Laser Handbook

422 Fi b er L a s er s Intr oduction to Optical Fiber Lasers 423

20

15
Attenuation (dB/km) 10 Ytterbium Erbium Thulium

5

0
0.75 1.00 1.25 1.50 1.75 2.00
Wavelength (µm)
Figure 15.6 Emission wavelengths of optical fiber lasers plotted with a
typical attenuation spectrum of a silica optical fiber.

to worry too much about excited state absorption and cooperative
upconversion, both of which are channels for power loss. (Note that
3+
there is still some level of upconversion in Yb system showing up as
blue fluorescence.) The upconversion process can also lead to photo-
darkening, which compromises the long-term reliability of an optical
fiber laser. Tm -doped optical fiber lasers have recently attracted
3+
much interest for applications around 2 µm. The most preferred
wavelength of pumping is 790 nm, which allows an efficient one-for-
two process and access to higher-power pump diodes. The one-for-
3
two process allows one ion excited to the H level to generate two
4
3
ions excited to the F level.
4
The emission wavelengths of Yb , Er , and Tm fiber lasers are
3+
3+
3+
illustrated in Fig. 15.6 and are compared with a typical silica fiber
attenuation spectrum. Most optical fiber lasers use a meter to a few
meters of rare-earth-doped optical fibers. Optical fiber attenuation is
3+
not a significant issue, even for Tm -doped optical fiber lasers around
the start of the silica phonon absorption band.
3+
The energy level diagram of Yb ions is shown in Fig. 15.7a. The
3
upper manifold F 5/2 and lower manifold F 7/2 have three and four
3
sublevels, respectively, due to Stark splitting. Unlike for Er ions,
3+
these manifolds give distinct transitions in absorption and emission
spectra. For a pump wavelength of 976 nm, a → e is used, especially
for cladding pumped optical fibers due to high pump absorption. For
pump wavelengths around 915 nm, a → f is sometimes used to pro-
vide more tolerance in pump wavelength control; this also allows
higher gain per unit length due to the much higher inversion possible
when pumped around 915 nm. Two transitions—one at 1025 nm, e → b,