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4. Low water peak fiber (LWPF): As shown in Figure 8.14, there is a peak in the attenuation curve at 1385 nm, known as the water peak. With this new type of fiber this peak is eliminated, which allows the use of this region.
Single-mode and multi-mode fibers are costly and require a skilled technician to install them. Plastic optical fibers (POF), on the other hand, are inexpensive and can be easily installed by an untrained person. First introduced in 1960s, POFs perform well over distances of less than 30 meters. The core of a plastic optical fiber is made of a general-purpose resin called PMMA; the cladding is made of fluorinated polymers. The core has a very large diameter - about 96% of the cladding’s diameter. Plastic optic fibers are used in digital home appliance interfaces; home networks; and mobile environments, such as automobiles.
In the rest of this chapter, we describe some of the components used in WDM optical networks. In the previous section, we talked about launching a light into an optical fiber. In Section 8.3.1, we will see how this light is generated using a laser, and how a data stream is modulated onto the light stream. Curiously, laser is not a word! Rather, it is an acronym derived from the name of the underlying technique: light amplification by stimulated emission of radiation. In the same section, we will also discuss the concept of dense WDM and the wavelength grid proposed in the ITU-T G.692 standard. In Section 8.3.2, we briefly discuss photo-detectors and optical receivers. In Section 8.3.3, we discuss optical amplifiers and in particular we describe the Erbium-doped fiber amplifier (EDFA),
OPTICAL FIBERS AND COMPONENTS
a key technology that enabled the deployment of WDM systems. In Section 8.3.4, we will describe the 2 x 2 coupler and the star-coupler, and last but not least, in Section 8.3.5, we will describe various technologies used for optical cross-connects (OXC).
Let us consider the transmitting side of the WDM link with W wavelengths shown in Figure 8.1, and reproduced again here in Figure 8.16. There are W different transmitters, each transmitting at a different wavelength Xi7 i = 1, 2,... ,W. The output of a transmitter is modulated by a data stream, and the W modulated outputs are all multiplexed onto the same fiber using an N-to-1 combiner (see Section 8.3.4).
A transmitter is typically a laser, although a light-emitting diode (LED) can also be used. There are different types of lasers, of which the semiconductor laser is the most commonly used laser in optical communication systems. Semiconductor lasers are very compact and can be fabricated in large quantities.
A laser is a device that produces a very strong and concentrated beam. It consists of an energy source which is applied to a lasing material, a substance that emits light in all directions and it can be of gas, solid, or semiconducting material. The light produced by the lasing material is enhanced using a device such as the Fabry-Perot resonator cavity. This cavity consists of two partially reflecting parallel flat mirrors, known as facets. These mirrors are used to create an optical feedback which causes the cavity to oscillate with a positive gain that compensates for any optical losses. Light hits the right facet and part of it leaves the cavity through the right facet and part of it is reflected (see Figure 8.17). Part of the reflected light is reflected back by the left facet towards the right facet, and again part of it exits through the right-facet and so on.
Tx Aw Power fiber In-line fiber Pre- 'Kw
/ amplifier amplification amplifier N Rx
Figure 8.16 A WDM point-to-point link.
Left facet Right facet
Figure 8.17 The Fabry-Perot resonator cavity.
Consider a wavelength for which the cavity length (i.e., the distance between the two mirrors) is an integral multiple of half the wavelength. That is, the round trip through the cavity is an integral multiple of the wavelength. For such a wavelength, all of the light waves transmitted through the right facet are in phase; therefore, they reinforce each other. Such a wavelength is called a resonant wavelength of the cavity.
Since there are many resonant wavelengths, the resulting output consists of many wavelengths spread over a few nm, with a gap between two adjacent wavelengths of 100 GHz to 200 GHz. However, it is desirable that only a single wavelength comes out from the laser. This can be done by using a filtering mechanism that selects the desired wavelength and provides loss to the other wavelengths. Specifically, another cavity can be used after the primary cavity where gain occurs. Using reflective facets in the second cavity, the laser can oscillate only at those wavelength resonant for both cavities.