Different linear effects that occur in the physical medium are studied and analyzed in this chapter. Specifically, much attention is paid to fiber attenuation, Amplified Spontaneous Emission (ASE) noise due to optical amplifiers, and fiber dispersive effects that cause pulse broadening, which may represent a serious problem in high-speed optical transmission systems. In order to reduce fiber dispersive effects effectively, dispersion compensation fiber is employed. Other effects such as linear crosstalk, which causes distortion and interferes with the filtered channel, can be reduced by optimizing the optical filter bandwidth and shape to obtain a compromise between WDM linear crosstalk and filter induced intersymbol interference (ISI).
TopFiber Attenuations
Light traveling down an optical fiber experiences attenuation or loss as it propagates as shown in Figure 1. This is a fundamental limit on optical communications as it limits the possible distance of transmission. The attenuation or the fiber loss is defined as the ratio of the optical output power, Pout from a fiber of length L to the input power Pin and is expressed asα*L = 10 log10 [Pin / Pout] (1.1)
Figure 1. Signal progression through optical fiber with attenuation of 0.2 dB/km (Hiew, Abbou, & Chuah 2006)
in units of decibels (dB). The loss of fiber is typically expressed using the attenuation factor
α in units of dB/km
α = 10 log
10 [(
Pin /
Pout) /
L]
(1.2)Thus, an optical signal propagating through the optical fiber will suffer a finite loss that can be described fully using the attenuation factor.
There are a few factors that lead to attenuation of the signal in an optical fiber. These factors are coupling losses between the source–fiber, fiber-fiber losses, fiber-detector losses, fiber bending losses, and losses due to absorption, scattering and radiation losses (Keiser, 2000).
Coupling losses between the source–fiber, fiber-fiber and fiber-detector and fiber bending losses are extrinsic in nature and not inherent properties of the optical fiber. However, these losses can add up to significant amounts in many real-life situations. For example, the coupling between the laser and the fiber leads to high losses due to aperture differences between the two mediums. However, these losses can be reduced with additional precaution being taken during system installation and are normally considered apart from the design of optical networks.
On the other hand, the losses due to scattering, absorption and radiation depend on fundamental characteristics of the fiber. Absorption of light in an optical fiber may be intrinsic or extrinsic. Intrinsic absorption is due to material absorption and electron absorption that are fundamental properties of the fiber medium. The extrinsic absorption is due to the presence of transition metal impurities known as impurity absorption. These impurities nowadays may be reduced to amounts that are negligibly small in effect for expensive, top-quality optical fiber that employs the latest fabrication and purification technology.
However, intrinsic absorption cannot be reduced, as it is property of the fiber material. Material absorption is a loss mechanism related to the material composition and the fabrication process of the fiber that results in the dissipation of some of the transmitted optical power into heat in the optical fiber. An absolutely pure silicate glass has little intrinsic absorption due to its basic material structure in the near infrared region and this is one of the advantages of optical fiber communication. It has two intrinsic absorption mechanisms at optical wavelengths that leave a low intrinsic absorption window over the 0.8 μm to 1.7 μm wavelength range that lies across the optical and infrared bands. Though the intrinsic loss is small, it is significant nowadays as it represents the last attenuation mechanism in optical fibers after the other mechanisms are dealt with, and thus becomes a limiting factor.