Page 478 - High Power Laser Handbook

446 Fi b er L a s er s Intr oduction to Optical Fiber Lasers 447

lasers were pumped by more powerful gas and solid-state lasers,
with up to a few tens of watts of pump power, which is much more
powerful than the subwatt level available from a single-mode pump
diode. These optical fiber lasers are, however, impractical for com-
mercial use. Cladding pumping by multimode pump diodes, first
13
demonstrated by Po et al. in 1988, was a major milestone that even-
tually led to high-power single-mode lasers using the much higher
pump powers available from multimode pump diodes. We have
focused our discussions on high-power optical fiber lasers based on
cladding pumping configurations.
Typical cavity designs used commercially for higher-power appli-
cations are based on Fabry-Pérot cavity designs with fiber Bragg grat-
ings (FBG) as reflectors. Photosensitivity in germanium-doped silica
fibers was first discovered by Hill et al. in 1978. It was the work by
54
Meltz et al. in 1989 that led to intensive development of FBG. The
55
significance of the work by Meltz et al. is the first demonstration of a
Bragg grating in an optical fiber core for wavelengths besides the
wavelength of the writing laser by two interfering ultraviolet (UV)
beams (see Fig. 15.27). The pitch of an FBG can be varied by adjusting
the angle between the interfering beams. Even though the initial work
was motivated by fiber sensors for temperature and strain measure-
ments, FBGs received immense immediate interest mainly due to their
application as filters in wavelength-division-multiplexing (WDM)
systems for telecommunications. The application in optical fiber
lasers as reflectors was also quite straightforward. Chirped FBGs are
also used for intracavity dispersion management in mode-locked
lasers, as will be discussed later. FBGs were, in fact, a critical ingredi-
ent in enabling the development and eventual commercial success of
fiber lasers (see Ref. 56 for a review).

UV beam UV beam
Reflection

FBG Λ
Wavelength Transmission
Core
Input k b k f Wavelength

Wavelength k g
Figure 15.27 A fiber Bragg gratings (FBG) writing process.