Page 394 - Fiber Bragg Gratings

8.3 The fiber Bragg grating rare-earth-doped fiber laser 371

ions of neodymium and dielectric end mirrors, which produced pulsed
radiation when pumped with a flash lamp. Although it may be argued
that the device was not a true fiber laser, the difference is merely in the
detail, since it was a waveguide. With the advent of low-loss single-mode
fibers [42] and the subsequent rare-earth-doped single-mode fibers [43],
the early lasers still depended on external mirrors to choose the appro-
priate emission line. Although fibers enabled efficient end-pumping of
these lasers, there were cases, as with Nd-doped fiber, in which the sup-
pression of lasing of an adjacent competing line became difficult owing
to the discrimination required of the resonator mirrors [44]. The problems
were further compounded by the use of bulk reflectors or by the deposition
of mirrors directly on the end faces of fibers [45]. In- and out-coupling
had to be achieved through some form of mechanical alignment of mirrors,
with high power densities at the mirror/fiber interface that usually needed
a layer of index-matching oil for good optical contact. The first demonstra-
tion of the use of a narrowband in-line fiber reflector for a fiber laser used
a 70% reflectivity grating etched into the core of a side-polished fiber [46].
While this scheme had the advantage of producing a narrow-bandwidth
reflector (—0.8 nm FWHM), it had the disadvantage of being a device
that needed considerable mechanical processing. The principle of tunabil-
ity had already been demonstrated using a bulk diffraction grating as a
frequency-selective reflector [47].
Thus, the photosensitive fiber Bragg grating, which was narrowband,
low loss, and of adjustable bandwidth, was wholly suited to application
in fiber lasers. The availability of compact high-power pump lasers at
1480 and 980 nm has made the fiber laser simple to fabricate. The high
gain available in rare-earth-doped fibers allowed a nominal 0.5% reflection
grating and a 100% bulk mirror with 30 m of erbium-doped fiber laser to
produce 300 mW of power at 1537.5 nm when pumped with 600 mW of
980-nm pump [48] in the first photoinduced grating-based fiber laser.
The ability to write Bragg gratings directly into rare-earth-doped fibers
compatible with photosensitive fibers has resulted in a variety of success-
ful demonstrations using either single or multiple gratings to form the
resonator [49]. In the following section, generic examples of some of these
lasers along with their properties will be examined.
Figure 8.15 shows a simple configuration of a single-cavity design of a
fiber laser. The rare-earth-doped fiber has Bragg gratings as wavelength-
selective mirrors. The laser is end-pumped via a WDM coupler normally
through an optical isolator.