The idea of this chapter is to give an overview on optical communication systems. The most important devices for fiber-optic transmission systems are presented, and their properties discussed. In particular, we consider such systems working with those basic components which are necessary to explain the principle of operation. Among them is the optical transmitter, consisting of a light source, typically a low speed LED or a high speed driven laser diode. Furthermore, the optical receiver has to be mentioned; it consists of a photodiode and a low noise, high bit rate, front-end amplifier. Yet, in the focus of the considerations, you will find the optical fiber as the dominant element in optical communication systems. Different fiber types are presented, and their properties explained. The joint action of these three basic components can lead to fiber-optic systems, mainly applied to data communication. The systems can operate as transmission links with bit rates up to 40 Gbit/s. But communication systems are also used for recent application areas in the MBit/s region, e.g. in aviation, automobile, and maritime industry. Therefore—besides pure glass fibers—polymer optical fibers (POF) and polymer-cladded silica (PCS) fibers have to be taken into account. Moreover, even different physical layers like optical wireless and visible light communication can be a solution.
TopIntroduction
Since the beginning of the sixties, there has been a light source which yields a completely different behavior compared to the sources we had before: This light source is the LASER. The first realized laser was the bulk-optic ruby laser (Maiman 1960). Only a short time after this very important achievement, diode lasers for usage as optical transmitters had already been developed (see Figure 1) (Quist et al. 1962). Parallel to that accomplishment in the early seventies, researchers and engineers accomplished the first optical glass fiber with sufficient low attenuation to transmit electromagnetic waves in the near infrared region (Kapron, et al. 1970).
Figure 1. Basic arrangement of a fiber-optic system
The photodiode as detector already worked (Adams & Day 1876), and thus, systems using optoelectric (O/E) and electrooptic (E/O) components for transmitters and receivers as well as a fiber in the center of the arrangement could be developed. The main fields of application of such systems are found in the area of fiber-optic transmission and fiber-optic sensors (see Figure 2).
Figure 2. Application of fiber-optic systems
However, the first optical transmission is much older. Indians, for instance, already knew communication by smoke signals a long time ago (see Figure 3, A. Marstaller, personal communication, 1990). Furthermore, it was a very sophisticated and modern system because it already was a digital system, consisting of “binary 1” and “binary 0” (smoke/no smoke).
Figure 3. Digital optical transmission by use of smoke signals
Charles Kao (Kao & Hockham 1966) and Manfred Börner (Börner 1967) can be regarded as the inventors of fiber-optic transmission systems. Nowadays, their invention would not be very spectacular: Take a light source as transmitter, an optical fiber as transmission medium, and a photodiode as detector (see Figure 1)! Yet, in 1963, it was a revolution because the attenuation of optical glass was in the order of 1000 dB/km, and therefore totally unrealistic for usage in practical systems. Today’s fibers achieve attenuation below 0.2 dB/km, which means that after 100 km, there is still more than 1% of light at the end of the fiber. This low value of attenuation is one of the most attractive advantages of fiber-optic systems compared to conventional electrical ones (see Figure 4). In addition, low weight, small size, insensitivity to electromagnetic interference (EMI), electrical insulation, and low crosstalk must be mentioned. Apart from low attenuation, the enormous achievable bandwidth must be pointed out. That leads to a high transmission capacity in terms of the product of fiber bandwidth and length. One of the most important goals is to maximize this product for every kind of data transmission. Figure 4 depicts the attenuation behavior. In particular, we observe independence from modulation frequency of fiber-optic systems in contrast to electrical ones, which suffer from the skin effect.
Figure 4. Attenuation of coaxial cables and optical fibers
TopOptical Fibers
The most important demands on optical fibers are a proper wave guiding, low loss of optical power and low distortion of the transmitted optical signals. The principle of operation of guiding a light wave can be explained by Snell’s law (see Figure 5).
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