121 Material and Fabrication

The material of choice for low-loss optical fibers is pure silica glass synthesized by fusing SiO2 molecules. The refractive-index difference between the core and the cladding is realized by the selective use of dopants during the fabrication process. Dopants such as GeO2 and P2O5 increase the refractive index of pure silica and are suitable for the core, while materials such as boron and fluorine are used for the cladding because they decrease the refractive index of silica. Additional dopants can be used depending on specific applications. For example, to make fiber amplifiers and lasers, the core of silica fibers is codoped with rare-earth ions using dopants such as ErCl3 and Nd2O3. Similarly, Al2O3 is sometimes added to control the gain spectrum of fiber amplifiers.

The fabrication of optical fibers involves two stages [75]. In the first stage, a vapor-deposition method is used to make a cylindrical preform with the desired refractive-index profile and the relative core-cladding dimensions. A typical preform is 1-m long with 2-cm diameter. In the second stage, the preform

Figure 1.2 Schematic diagram of the MCVD process commonly used for fiber fabrication. (After Ref. [75].)

is drawn into a fiber using a precision-feed mechanism that feeds it into a furnace at a proper speed. During this process, the relative core-cladding dimensions are preserved. Both stages, preform fabrication and fiber drawing, involve sophisticated technology to ensure the uniformity of the core size and the index profile [75]—[77].

Several methods can be used for making a preform. The three commonly used methods are modified chemical vapor deposition (MCVD), outside vapor deposition (OVD), and vapor-phase axial deposition (VAD). Figure 1.2 shows a schematic diagram of the MCVD process. In this process, successive layers of SiO2 are deposited on the inside of a fused silica tube by mixing the vapors of SiCl4 and O2 at a temperature of ^ 1800° C. To ensure uniformity, the multiburner torch is moved back and forth across the tube length. The refractive index of the cladding layers is controlled by adding fluorine to the tube. When a sufficient cladding thickness has been deposited with multiple passes of the torch, the vapors of GeCl4 or POCl3 are added to the vapor mixture to form the core. When all layers have been deposited, the torch temperature is raised to collapse the tube into a solid rod known as the preform.

This description is extremely brief and is intended to provide a general idea. The fabrication of optical fibers requires attention to a large number of technological details. The interested reader is referred to the extensive literature on this subject [75]-[77].

1.2.2 Fiber Losses

An important fiber parameter is a measure of power loss during transmission of optical signals inside the fiber. If is the power launched at the input of a

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1 V-— LOSS

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\ PROFILE , \ \ /

INTRINSIC^ \ /

LOSS /

1

I 1 1 1 1 1

Figure 1.3 Measured loss spectrum of a single-mode silica fiber. Dashed curve shows the contribution resulting from Rayleigh scattering. (After Ref. [75].)

fiber of length L, the transmitted power PT is given by

where the attenuation constant a is a measure of total fiber losses from all sources. It is customary to express a in units of dB/km using the relation (see Appendix A for an explanation of decibel units)

10. fP2

As one may expect, fiber losses depend on the wavelength of light. Figure 1.3 shows the loss spectrum of a silica fiber made by the MCVD process [75]. This fiber exhibits a minimum loss of about 0.2 dB/km near 1.55 jm. Losses are considerably higher at shorter wavelengths, reaching a level of a few dB/km in the visible region. Note, however, that even a 10-dB/km loss corresponds to an attenuation constant of only a 2 10 5 cm 1, an incredibly low value compared to that of most other materials.

Several factors contribute to the loss spectrum of Fig. 1.3, with material absorption and Rayleigh scattering contributing dominantly. Silica glass has electronic resonances in the ultraviolet (UV) region and vibrational resonances in the far-infrared (FIR) region beyond 2 jm but absorbs little light in the wavelength region 0.5-2 jim. However, even a relatively small amount of impurities can lead to significant absorption in that wavelength window. From a practical point of view, the most important impurity affecting fiber loss is the

OH ion, which has a fundamental vibrational absorption peak at« 2.73 jm. The overtones of this OH-absorption peak are responsible for the dominant peak seen in Fig. 1.3 near 1.4 jm and a smaller peak near 1.23 jm. Special precautions are taken during the fiber-fabrication process to ensure an OH-ion level of less than one part in one hundred million [75]. In state-of-the-art fibers, the peak near 1.4 jm can be reduced to below the 0.5-dB level. It virtually disappears in especially prepared fibers [78]. Such fibers with low losses in the entire 1.3-1.6 jm spectral region are useful for fiber-optic communications and were available commercially by the year 2000 (e.g., all-wave fiber).

Rayleigh scattering is a fundamental loss mechanism arising from density fluctuations frozen into the fused silica during manufacture. Resulting local fluctuations in the refractive index scatter light in all directions. The Rayleigh-scattering loss varies as 4 and is dominant at short wavelengths. As this loss is intrinsic to the fiber, it sets the ultimate limit on fiber loss. The intrinsic loss level (shown by a dashed line in Fig. 1.3) is estimated to be (in dB/km)

where the constant CR is in the range 0.7-0.9 dB/(km-jm4) depending on the constituents of the fiber core. As 0Cr = 0.12-0.15 dB/km near X = 1.55 jm, losses in silica fibers are dominated by Rayleigh scattering. In some glasses, aR can be reduced to a level ~ 0.05 dB/km [79]. Such glasses may be useful for fabricating ultralow-loss fibers.

Among other factors that may contribute to losses are bending of fiber and scattering of light at the core-cladding interface [72]. Modern fibers exhibit a loss of« 0.2 dB/km near 1.55 jm. Total loss of fiber cables used in optical communication systems is slightly larger (by ~ 0.03 dB/km) because of splice and cabling losses.

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