Attenuation in silica and plastic optical fibers

Silica (SiO2) has excellent optical properties including great long-term stability. A large variety of glasses and fibers are available. The attenuation of silica fibers is shown in Fig. 22.2. Inspection of Fig. 22.2 reveals that a minimum loss of 0.2 dB occurs at a wavelength of 1.55 |im.

There are several optical “windows” for communication over silica fibers. These communication windows are at 0.85, 1.3, and 1.55 |im. The 0.85 |im communication window is suitable for communication with GaAs-based LEDs and lasers. However, this window is limited to short distances due to the material dispersion and to the high attenuation of silica fibers at that wavelength. The 1.3 |im communication window is also suited for communication with LEDs and lasers. This window has relatively low loss and zero dispersion, allowing for high-bit-rate transmission, in particular in graded-index and single-mode fibers. The 1.55 |im communication window is characterized by the lowest loss of all three windows. Consequently, this window is used for long-distance high-bit-rate communication. To allow for high bit rates, single-mode fibers must be used. Since it is difficult to efficiently couple light emerging from an LED into a single-mode fiber, lasers are preferred over LEDs at 1.55 |im.

Plastic optical fibers are becoming increasingly popular for short-di stance communication (Neyer et al., 1999; Kibler et al., 2004). However, plastic fibers have losses that are about 1 000 times greater than the losses in silica fibers. Therefore, the transmission distances are limited to just a few meters to a few hundred meters, e. g. for communication within an automobile (Kibler et al., 2004) or airplane.

The attenuation in plastic fibers is shown in Fig. 22.3. The preferred communication window of plastic fibers is at 650 nm, where the loss is of the order of 0.1-0.2 dB per meter. At even shorter wavelengths, the attenuation in plastic fibers decreases. However, the material dispersion increases, thus making the 650 nm wavelength the preferred communication wavelength in plastic optical fibers.

Fig. 22.3. Attenuation of a PMMA step-index plastic op­tical fiber. At 650 nm, the pre­ferred communication wave­length, the attenuation is about 150 dB/km (after data sheet of Toray Industries Ltd., 2002).

Wavelength X (nm)





Fastest mode


Core /7|



Cladding n.

Fig. 22.4. Geometry used for calculation of the modal disper­sion in a multimode fiber waveguide.

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