PCF is categorized into two types by waveguiding principles. One is a photonics band gap fiber (PBF), a photonics band-gap PCF. PBF has two-dimensional Bragg reflection structure, and allows light to propagate in the air core. Light with a specific wavelength can not penetrate in the medium, in which air holes are aligned with a period close to light wavelength, with large change in the refraction index, while light with different wavelength can penetrate the medium. The restrict structural periodicity and hole size homogeneity are required for manufacturing PBF. The other type of PCF is index guiding PCF, and is also called as micro-structured fiber. The both types control brand-new propagation properties by forming periodic microstructure in a wavelength order in the optical fiber.

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Figure 3.16: Cross-section diagram of sorts of photonics crystal fibers. (a) Index guiding PCF, (b) air-clad fiber, (c) hole-assisted fiber, (d) photonics bandgap fiber

Fig.3.16 schematically represents the section of various types of PCF. FIg.3.16(a-c) shows the index guiding PCFs; (a) is a typical one, (b) simplifies (a) and is used in the case that some limited properties are required, and (c) dopes impurity such as Ge as well as a normal fiber and is used especially for suppressing the bending loss. Fig.3.16(d) is PBF and is used for the communication with short wavelength, UV range, which does not propagate in the previously used quartz fiber, or high power transmission, since the core, the part that light propagates in, is the air.

The commercially available PCFs are briefly described below.

(1) Hollow core PCF, PBF

In a general optical fiber, the core part where light propagates is composed of quartz glass. In the hollow core (HC) PCF, since the core is filled with air, the fiber is hardly damaged, and nonlinear properties are avoided. HC-PCF will be applicable to long-distance transmission since it has little nonlinearity.

(2) LMA PCF

In a general fiber, light exhibits multimode propagation when MFD is large. On the other hand, an LMA fiber allows singlemode propagation in spite of the large MDF. Since the MFD is large, LMA PCF is not affected by the nonlinear property and the material damage. Therefore, the high-power light transmission is possible with LMA PCF.

(3) Highly nonlinear PCF

In a general optical fiber, the nonlinear coefficient is small, and the wideband light is not generated. In a highly non-linear (HNL) PCF, since the nonlinear coefficient is large, supercontinuum (SC) light extending in the visible and near-infrared range is generated.

(4) Endless single mode PCF

In a general SMF, light with shorter wavelength than the cut-off wavelength exhibits multimode propagation. On the other hand, in the endless single mode (ESM) PCF, light can exhibit singlemode propagation regardless of the wavelength and the core diameter.

(5) PM PCF

In a general PM fiber, the stress yields birefringence, and the birefringence maintains the polarization. In a PM PCF, a large difference in refractive indices between air and glass, and the elliptical shape yields a large birefringence. Compared with a general PM fiber, since the beat length is shorter, the polarization is less disturbed by the bending and the temperature change.