Core and cladding

Vertica.1 Axia.1 Deposition. The vertical axial deposition (VAD) process (18) was developed by a consortium of Japanese cable manufacturers and Nippon Telephone and Telegraph (NTT). This process also forms a cylindrical soot form. However, deposition is achieved end-on without use of a mandrel and subsequent formation of a central hole. Both the core and cladding are deposited simultaneously using more than one torch (Fig. 12). 

Blanks can also be made using Vapor Axial Deposition (VAD) (Fig. 21). The process involves simultaneous flame deposition of both core- and cladding-glass soots onto the end (ie, axially) of a rotating fused-siUca target rod. The finished perform is then consoHdated in a process similar to the OVD process. 

The simplest case of fibre structure is the step-index one, characterized by a constant circular refractive index profile in the core and polymer cladding of lower refractive index (Figure 3). The refractive indexes of the core and cladding depend on the materials used. The cores of these structures can be prepared from melts as well as from preforms without radial and azimutal variations of the refractive index. To obtain suitable mechanical... 

Figure 1. Schematic of the optical fiber system. Excitation light is launched into the fiber. Due to the refractive index differences between the fiber core and cladding materials, the light is internally reflected and travels through the fiber with minimal loss (see inset). The emitted light is carried back from the fluorescent sensor located on the tip of the fiber to a CCD camera detector. Reprinted with permission from Science, 2000, 287, 451-452. Copyright 2000 AAAS.

In another approach, two cascaded microstructure-collapses on a photonic crystal fiber are intentionally introduced by C02 laser or electric arc heating31. In this way, the mismatch of core size along the fiber causes light coupling between the core and cladding modes. 

The dependence of PS porosity on doping density has been exploited to fabricate optical waveguides. Such structures require only a single variation of refractive index. If a p-type substrate with implanted p+ lines is anodized and partially oxidized, mesoporous lines of high refractive index are embedded in a low refractive index material. An obtained refractive index contrast between core and clad of 30% is promising for future applications [Tal2]. 

Core and cladding materials are selected not only on the basis of their refractive index, but also for their processability, attenuation loss, mechanical properties, and dispersion properties. However, density, p, and refractive index, n, are critical and have been correlated for over 200 optical glasses, for which the following formula... 

Figure 3.3-6 Law of refraction a at the interface between two media with n = 1 and 2 = 1.5 all rays with Q2 > 41.8° are totally reflected b if ag is the angle of total reflection at the interface between core and clad of an optical fiber, then is the largest angle of rays relatively to the optical axis inside the fiber and Qa the largest angle outside.

---