Specialty Optical Fibers Handbook

P

Th

¼ 21 

A

eff

L

eff

 g

B

, (22:2)

where A

eff

is the effective area of the fiber, L

eff

is the effective length of the fiber

given by L

eff

¼ [1- exp (-aL)]=a, and g

B

is the Brillouin gain coefficient. The

larger core sizes (i.e., effective areas) of LMA fiber raises the SBS threshold,

compared to single-mode fibers (by a few orders of magnitude) and enhances the

power handling capability of the laser. For example, when pumped at 915 nm a

single-mode ytterbium-doped fiber with a 0.15 NA, a core diameter of 5 mm,

and an ytterbium ion concentration of around 1 wt% has an SBS threshold of

around 40 W at 3-kHz line width, while a 20-mm core 0.06-NA fiber has a

threshold ...