3.11. PHOTONIC CRYSTAL FIBERS

3.11. PHOTONIC CRYSTAL FIBERS

Traditional optical fibers, whose refractive index of core is higher than that of the cladding, confine light field by the total internal reflection. Yeh et al. proposed Bragg fiber (Fig. 3.51) in which rings of high- and low-refractive index are arranged around a central core [48]. In Bragg fibers, light cannot penetrate into cladding since light is reflected by the Bragg condition. Then, light beam, which is coupled into Bragg fiber at the input end, propagates along the fiber. Since light is confined by the Bragg condition, refractive index of the core could be lower than that of the cladding or core could be air [49, 50]. The forbidden frequency ranges in periodic dielectric structures of cladding are called photonic bandgaps. Bragg fibers use a one-dimensional transverse periodicity of concentric rings. There is another class of fibers that use a two-dimensional transverse periodicity [51, 52]. These fibers are called photonic crystal fibers (PCFs) or holey fibers (HFs). PCFs are classified into two categories: they are, solid-core PCF (Fig. 3.52) and hollow-core PCF (Fig. 3.53). A solid-core PCF refracts light at steep angles of incidence on the core cladding boundary. When the angle is shallow enough, light is trapped in the core and guided along the fiber. A hollow-core PCF with a proper cladding can guide light at angles of incidence where a photonic band gap operates. Therefore, hollow-core PCF requires strict control of the periodic cladding structures.

Figure 3.51: Bragg fiber.

Figure 3.52: Solid-core PCF.

Figure 3.53: