Page 476 - High Power Laser Handbook

444 Fi b er L a s er s Intr oduction to Optical Fiber Lasers 445

900 20
10
800
0
700
44-µm core diameter −10
A eff (µm 2 ) 600 −20 −20 0 20
500
400 20
27-µm core diameter
300 10
0
200
0 5 10 15 20 25 30 35 40 −10
Bend radius (cm)
−20
(a)
−20 0 20
(b)
Figure 15.25 (a) Effective mode area compression in bent fibers. (b) Top: Mode in
straight fiber with A = 862 µm ; Bottom: Mode in bent fiber with a bend radius of
2
eff
2
10 cm and A = 627 µm (core diameter is 44 µm). (Courtesy of John Fini)
eff
10000
Effective mode area (µm 2 ) 1000

100

1996 1998 2000 2002 2004 2006 2008
Year

Figure 15.26 Development of fiber mode areas of a variety of diffraction-
limited single-core optical fibers as a function of time. Circles: single-mode
step-index fibers; squares: multimode step-index fibers; triangles: photonic
crystal fibers and rods; diamonds: higher-order mode fiber; stars: leakage
channel fibers.

greater than 50 W, it is typically advisable to use silica optics to mini-
mize thermal beam distortions. In addition, water-cooled fiber hold-
ers and isolators may also be required at high powers.
Beam quality is usually measured by the M parameter, which
2
characterizes how much a beam’s diffraction angle is increased