Polymer-clad fibers

Single infrared fibers can be used as ATR probes and sensors in a wide variety of applications. Fibers are also used as waveguides to transfer radiation from the spectrometer to a remote location. Most often, chalcogenide fibers are used in bundles rather than single fibers. To reduce the attenuation found in polymer-clad fibers, the fibers are clad with another chalcogenide of lower refractive index than the core. As chalcogenide cladding has very low absorption, the attenuation can be reduced to as low as 0.1 dB... 

Yuan J, El-Sherif MA, MacDiarmid AG, Jones W (2001) Fiber optic chemical sensors using modified conducting polymer cladding. Proc SPIE 4205 170-179... 

This relationship ensures that the critical angle of incidence of the interface is not exceeded and that total internal reflection occurs (Levi, 1980). Typical values of NA for fused SiOj fibers with polymer cladding are in the range of 0.36-0.40. The typical values of = 14° used to insert the beam of the biomedical laser into the fiber is much smaller than those values ( 21-23°) corresponding to typical NA values. The maximum value of the propagation angle a typically used in biomedical laser systems is i 4.8°. 

The same restriction applies to all-polymer fibers. Modern, all-glass fibers may have losses of less than 0.5 dB km at a wavelength of 1.3 pm . Typical diameters of the core and the cladded fiber are 50 and 125 pm, respectively, but there is a trend to switch over to so-called monomode fibers which have cores with a diameter of 5-10 pm. 

Extrusion also allows the fabrication of core/clad preforms. Savage et al. successfully reported the coextrusion of a Ge-As-Se preform and its subsequent fiber drawing [152], The coextrusion technique produces preforms with optimum core/clad interface quality. Fibers with optical losses of 1.7 dB/m at 6.0 pm were fabricated. Recently Abouraddy et al. have extended the technology by coextruding composite macroscopic preforms made of two ChG materials and a thermoplastic polymer cladding [153], yielding mechanically robust fibers. 

A flexible optical fiber has a high-refractive-index, transparent core enclosed in a lower-refractive-index, transparent cladding (Figure 20-18a). The cladding is enclosed in a protective plastic jacket. The core and the coating can be made from glass or polymer. 

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