Multi-mode and Single-mode Optical Fibers

COMPONENTS 191 travels along two polarization planes which are vertical to each other. In an ideal circularly symmetric fiber the light traveling on each polarized plane has the same speed with the light traveling on the other plane. However, when the core of the fiber is not round, the light traveling along one plane might travel either slower or faster than the light polarized along the other plane. This difference in speed will cause the pulse to break.

8.2.3 Types of Fibers

The multi-mode fiber has been used extensively in LANs and, more recently, in 1-Gigabit and 10-Gigabit Ethernet. A vast majority of the installed multi-mode fiber has a core diameter of 62.5 µm and operates in the region of 850 nm and 1300 nm. It provides speeds up to 100 Mpbs. A small percentage of multi-mode fiber adheres to an earlier standard which has a core diameter of 50 µm and operates in both regions of 850 nm and 1300 nm. Single-mode fiber is used for long-distance telephony, CATV, and packet-switching net- works. The following are various different types of single-mode fiber, classified according to their dispersion loss. 1. Standard single-mode fiber SSMF: Most of the installed fiber falls in this category. It was designed to support early long-haul transmission systems, and it has zero dispersion at 1310 nm. 2. Non-zero dispersion fiber NZDF: This fiber has zero dispersion near 1450 nm. 3. Negative dispersion fiber NDF: This type of fiber has a negative dispersion in the region 1300 to 1600 nm. 4. Low water peak fiber LWPF: As shown in Figure 8.14, there is a peak in the atten- uation 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.

8.3 COMPONENTS

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, 192 OPTICAL FIBERS AND COMPONENTS a key technology that enabled the deployment of WDM systems. In Section 8.3.4, we will describe the 2 × 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.

8.3.1 Lasers

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 λ i , i = 1, 2, . . . , W . The output of a trans- mitter 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. Wavelength demultiplexer Wavelength multiplexer In-line amplification Optical fiber Optical fiber l 1 l w l 1 l w Tx Power amplifier Tx Rx Rx Pre- amplifier • • • • • • Figure 8.16 A WDM point-to-point link. Left facet Right facet Figure 8.17 The Fabry-Perot resonator cavity. COMPONENTS 193 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. If we are to make the length of the cavity very small, then only one resonant wavelength occurs. It turns out that such a cavity can be done on a semiconductor substrate. In this case the cavity is vertical with one mirror on the top surface and the other in the bottom surface. This type of laser is called a vertical cavity surface emitting laser VCEL. Many VCELs can be fabricated in a two-dimensional array. Tunable lasers Tunable lasers are important to optical networks, as will be seen in the two subsequent chapters. Also, it is more convenient to manufacture and stock tunable lasers, than make different lasers for specific wavelengths. Several different types of tunable lasers exist, varying from slow tunability to fast tunability. Modulation Modulation is the addition of information on a light stream. This can be realized using the on-off keying OOK scheme. In this scheme, the light stream is turned on or off depending whether we want to modulate a 1 or a 0. OOK can be done using direct or external modulation. In direct modulation, the light drive current into the semiconductor laser is set above threshold for 1 and below it for a 0. As a result, the presence of high or low power is interpreted as a 1 or 0, respectively. Direct modulation is easy and inexpensive to implement. However, the resulting pulses become chirped; that is, the carrier frequency of the transmitted pulse varies in time. External modulation is achieved by placing an external modulator in front of a laser. The laser continuously transmits but the modulator either lets the light through or stops it accordingly if it is a 1 or a 0. Thus, the presence of light is interpreted as a 1 and no light is interpreted as a 0. An external modulator minimizes chirp. Dense WDM DWDM In the literature, the term dense WDM DWDM is often used. This term does not imply a different technology to that used for WDM. In fact, the two terms are used interchangeably. Strictly speaking, DWDM refers to the wavelength spacing proposed in the ITU-T G.692 standard. Originally, the wavelengths were separated by wide bands which were several tens or hundreds of nanometers. These band became very narrow as technology improved.