 can be obtained in different ways. For example, there can be a polarization-dependent fundamental mode cutoff, SO that the fiber guides only light with one polarization in a certain wavelength range. Similarly, it is possible to suppress Raman scattering [41] by strongly attenuating longer-wavelength light. Multicore designs are possible, for example, with a regular pattern of core struc- tures in a single fiber, where there may or may not be some coupling between the cores. In the field of telecommunications however, probably the most important applica- tion of the holey fibers is in optical amplifiers. Laser-active PCFs for fiber lasers and amplifiers can be fabricated by using, for example, a rare-earth-doped rod as the central element of the preform assembly. Rare earth dopants (e.g., ytterbium or erbium) tend to increase the refractive index, but this can be precisely compensated, for example, with additional fluorine doping, SO that the guiding properties Technology for Telecommunications: Optical Fibers, Amplifiers, and Passive Devices are determined by the photonic microstructure only and not by a conventional-type refrac- tive index difference. Generally, PCFs are constructed by the same methods as other optical fibers: first, one constructs a "preform" on the scale of centimeters in size, and then heats the preform and draws it down to the fiber diameter, shrinking the preform cross section but maintaining the same features. In this way, kilometers of fiber can be produced from a single preform. Most PCFs have been fabricated in silica glass, but other glasses have also been used to obtain particular optical properties (such has high optical nonlinearity). Using this prin- ciple, for example, fibers optimized to have a very high Raman efficiency have been fabri- cated, which are conceived for the design of high-gain low-noise Raman amplifiers. However, special care is required in various respects: Ends of PCFs may not be cleaned with liquid solvents, such as ethanol, as capillary forces may pull them i