Page 484 - High Power Laser Handbook
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452 Fi b er L a s er s Intr oduction to Optical Fiber Lasers 453
Pump Pump
Pump Pump
Pump Pump
Pump Output
Pump Pump
Pump Pump
Pump Pump
Pump core Pump
Output
HR Doped core OC
Pump
cladding
Figure 15.32 Schematic illustration of a high-power continuous wave fiber laser.
the major limit. Instead, pump coupling and heat management are
the main design constraints. Bidirectional pumping can also be used
to provide maximum pump powers. A number of CW fiber lasers
with kilowatt power levels have been demonstrated in recent years
with both end-pumping schemes and pump combiners. 61,62
Single-Frequency Fiber Lasers
Single-frequency lasers offering laser spectral line widths of few tens
of kilohertz are desirable in applications requiring long coherence
lengths, such as optical sonar and high-power coherent combining.
FBGs combined with highly doped rare earth fibers provide a perfect
platform for single-frequency laser oscillators. Two designs are usu-
ally used—distributed Bragg reflector (DBR) and distributed feed-
back (DFB) configurations (Fig. 15.33). In the DBR configuration, a
cavity length of just a few centimeters is formed between two FBGs
with spectral line widths of a fraction of a nanometer. The short cav-
ity length increases the spectral spacing of the cavity modes and
allows single-frequency operation when the narrow spectral line
width of the FBGs selects a single cavity mode to go beyond thresh-
old. In a DFB configuration, a longer FBG with a built-in π/2 phase
jump constitutes the cavity. Operation of a DFB is not different from
a DBR design, except that precise phase control over the long FBG, in
combination with the π/2 phase jump, allows much narrower overall
spectral line width and, consequently, more robust single-frequency
operation. The FGB phase jump in a DFB laser can either be imple-
mented by moving the FBG’s line position or by introducing an addi-
tional average index change after the FBG is written. In the latter case,
the appropriate average index change can be achieved by exposure to
