Nonlinear Fiber Optics - 4 ed. Agrawal

440 Chapter 11. Highly Nonlinear Fibers
Figure 11.11: Scanning electron micrographs of four types of microstructured fibers. (After
Ref. [69]; c○2001 OSA.)
11.4 Microstructured Fibers
Narrow-core tapered fibers with an air cladding suffer from a practical problem. They
are fragile, are hard to handle, and are rarely much longer than 30 cm. This problem has
been solved to a large extent with the development of microstructured fibers in which
the narrow silica core is surrounded by a silica cladding with embedded air holes. For
this reason, such fibers are also known as “holey” fibers. For historical reasons, they
are also referred to as the photonic crystal fibers (PCFs). In fact, such a fiber was first
developed in 1996 in the form of a photonic-crystal cladding with a periodic array of
air holes [66]. It was realized later that the periodic nature of air holes is not critical for
silica-core fibers as long as the cladding has multiple air holes that effectively reduce
its refractive index below that of the silica core [67]–[71]. In this case, light is guided
by the total internal reflection, and the air holes are used to reduce the index of the
cladding region.
The periodic nature of the air holes become important in the so-called photonic
bandgap fibers in which the optical mode is confined to the core by periodic variations
of the refractive index within the cladding. The core of such fibers often contains air
to which light is confined by the photonic bandgap [72]. Such true PCFs can act as a
highly nonlinear medium if air is replaced with a suitable gas or liquid [73]. For example,
when air was replaced with hydrogen in such a fiber, stimulated Raman scattering
was observed at pulse energies that indicated that Raman threshold was more than 100
times lower than that of silica fibers [74].
Figure 11.11 shows four examples of microstructured fibers [69]. In design (a),
the narrow silica core is surrounded by a single ring of air holes, resulting in a high
refractive-index step. The rest of the cladding is made of silica. In design (b), the
narrow silica core is surrounded by multiple rings of periodic air holes, resulting in a
PCF structure. In design (c), air holes surrounding the narrow silica core are so large
in size that the core is mostly surrounded by air. This design is sometimes referred
to as the “grapefruit” structure because of its appearance. In design (d), the core is
surrounded by a ring of mostly air. The integrity of the structure is maintained by
narrow silica bridges that connect the core with the rest of the cladding. In all designs,
the size of air holes varies from structure to structure and can change from