Specialty Optical Fibers Handbook

are patented. A photonic crystal structure as an overlay on the side-polished

D-shape fiber is proposed [152]. The devices can be tunable if electro-optic active

materials are used. A method to fabricate electrodes on D-shape fibers has also

been proposed and patented [153].

16.8 RARE EARTH-DOPED ELLIPTICAL CORE FIBER

Among the several methods reported to introduce rare earth dopant into the

fiber, the solution doping technique, developed by Townsend et al. [154] in 1987,

is the simplest and, therefore, the most popular. The MCVD process for the

germania core deposition is carried out at a reduced temperature so that a

partially sintered germania layer inside the quartz tube is formed. A solution

of salt of the rare earth (usually aqueous or alcoholic ...