Sentence:  Microstructural optical fibers have been developed that have utilized phosphate-based glasses due to their ability to solubilize rare earth components such as <WSD>ytterbium</WSD>, which give the fibers their high light absorption and amplification per unit length. 
Meaning of the word marked with <WSD> tags:  A metallic chemical element ("symbol" Yb) with an atomic number of 70.