A type of object in which this structure has a spiral shape efficiently couples the modes of a diode laser, allowing it to be used as a pump source. One system that uses multiple examples of this structure has very low microbending loss (-5[1])and can be tuned to compensate for dispersion. To prevent damage to it and increase its stability, this structure is coated by a buffer and a jacket. Two layers of this type of structure allow for propagation of multiple (*) wavelengths in the “double”-this structure geometry. This component is generated by drilling patterned holes in silica in a photonic crystal-based design for the larger device it is part of. This structure has a lower refractive index than the innermost layer, allowing for signals to be propagated via total internal reflection. For 10 points, name this layer of an optical fiber that surrounds the core. (10[1])■END■ (10[1]0[1])