Page 485 - High Power Laser Handbook

452 Fi b er L a s er s Intr oduction to Optical Fiber Lasers 453

Doped core
Pump Output
HR OC
(a)

Doped core
Pump Output
Phase jump
(b)
Figure 15.33 (a) Distributed Bragg reflector (DBR) design and (b) distributed
feedback (DFB) design.

a UV beam, while monitoring the FBG’s transmission. Single-polar-
ization operation can also be achieved by introducing birefringence
in the phase jump by having the polarization of the UV writing beam
perpendicular to the fiber axis. DBR fiber lasers can be easily imple-
mented by writing FBGs in photosensitive fibers and then splicing
them to a section of highly rare-earth-doped fiber. A DFB fiber laser,
however, requires the writing of FBGs directly in a highly rare-earth-
doped fiber, which requires photosensitivity as well as high doping
levels, which are conflicting requirements. This conflict can however
be resolved by engineering photosensitive claddings with a highly
rare-earth-doped core. 63
A single-frequency fiber laser is usually pumped by a single-mode
pump diode to provide an output power in the range of a few tens to a
64
few hundred milliwatts. The laser can be amplified in a double-clad
fiber to a few hundred watts. Currently, SBS is the key limiting factor in
amplification of a single-frequency seed to high-output powers. The
reason for this is the low SBS threshold with narrow spectral line width
operation. The use of large effective mode areas increases the SBS
threshold. Recently, an amplified single-frequency output power as
65
high as 500 W has been demonstrated. It was also found that core
temperature variation along the amplifier arising from pump decay
raises the SBS threshold due to the temperature dependence of the SBS
process, which leads to an effective SBS spectral broadening. Very
recently, a 1.7-KW single-frequency fiber was demonstrated; further-
18
more, it was found that ~10 dB SBS suppression can be obtained through
SBS spectral broadening by dopant variations across the core. 18,19
15.3.2 Q-Switched Lasers
In a pulsed rare-earth-doped optical fiber, the extractable energy is
ultimately limited by ASE, which leads to a reduction of population
inversion due to interpulse ASE (see Sec. 15.2.3). The extractable
energy scales linearly with the effective mode area, as do the nonlinear