Specialty Optical Fibers Handbook

while operating within the limiting ratio is more challenging. In such cases, a

higher coefficient of thermal expansion difference and, hence, higher birefrin-

gence can be achieved by adjusting the composition of stress members so they are

similar to those used for gyroscope fibers. Indeed a broad range of ytterbium-

doped LMA DCFs, whose characteristics are optimized for various output

powers, are now commercially available [33]. An optical image showing the

cross-section of such a fiber, with a 20-mm core and 400-mm inner-cladding

diameter and a birefringence exceeding 3:5  10

4

, is presented in Fig. 22.2.

22.5 FIBER LASERS: STATE OF THE ART

LMA fibers with core diameters of 20–30 mm and NAs of around 0.06 have

become the industry standard for