n Polarization maintaining Different material barrier Microstructured cladding Protective layer Photonic crystal fibers Dispersion compensation High nonlinear or doped FIGURE 4.11 Schematic cross section of different special fibers for use in telecommunication systems. Polarization maintain- ing and single-polarization fibers are sketched in the upper part of the figure, examples of PCFs are reported in the lower part. Technology for Telecommunications: Optical Fibers, Amplifiers, and Passive Devices Polarization maintaining fibers are generally characterized by a minimum loss greater than 1 dB/km (typical for a PANDA fiber is .5dB/km) that, besides the high cost, makes them unsuitable for transmission. However, there are several applications requiring small pieces of a PM fiber, such as optical components pigtailing, fiber optic sensing, interferom- etry, and quantum key distribution. They are also commonly used in telecommunications for the connection between a source laser and a modulator, since the modulator requires polarized light at the input. Single-polarization fibers [28,29] are special optical fibers that can transmit light with a certain linear polarization direction, whereas light with the other polarization direc- tions is either not guided or experiences strong optical losses. Such fibers should not be confused with PM fibers, which guide light with any polarization state, but can preserve a linear polarization state when the polarization direction is properly aligned with the birefringence axis. In many cases, single-polarization guidance occurs in only a limited wavelength range. Outside that range, both polarization directions or no light at all may be guided. Also, some fibers exhibit a limited extinction ratio. Different principles of operation can be utilized for single-polarization fibers. A common approach is the use an elliptical core, which introduces strong birefringence and also a polarization dependence of a cutoff wavelength, SO that only light with one polarization direction is guide