Impairments HOW LIGHT IS TRANSMITTED THROUGH AN OPTICAL FIBER

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. 194 OPTICAL FIBERS AND COMPONENTS Table 8.2 ITU-T DWDM grid. Channel code λ nm Channel code λ nm Channel code λ nm Channel code λ nm 18 1563.05 30 1553.33 42 1543.73 54 1534.25 19 1562.23 31 1552.53 43 1542.94 55 1533.47 20 1561.42 32 1551.72 44 1542.14 56 1532.68 21 1560.61 33 1590.12 45 1541.35 57 1531.90 22 1559.80 34 1550.12 46 1540.56 58 1531.12 23 1558.98 35 1549.32 47 1539.77 59 1530.33 24 1558.17 36 1548.52 48 1538.98 60 1529.55 25 1557.36 37 1547.72 49 1538.19 61 1528.77 26 1556.56 38 1546.92 50 1537.40 62 1527.99 27 1555.75 39 1546.12 51 1536.61 – – 28 1554.94 40 1545.32 52 1535.82 – – 29 1554.13 41 1544.53 53 1535.04 – – ITU-T proposed a set of closely spaced wavelengths in the 1550 nm window. The reason that the 1550 nm window was chosen is due to the fact that it has the smallest amount of attenuation and it also lies in the band where the Erbium-doped fiber amplifier see Section 8.3.3 operates. The ITU-T proposed guard is 0.8 nm or 100 GHz and the grid is centered in 1552.52 nm or 193.1 THz. The ITU-T grid is given in Table 8.2. The ITU-T grid is not always followed, since there are many proprietary solutions. Note that this book uses the acronym WDM.

8.3.2 Photo-detectors and Optical Receivers

Let us consider the receiving side of the WDM link see Figure 8.16. The WDM optical signal is demultiplexed into the W different wavelengths, and each wavelength is directed to a receiver Rx. The demultiplexer is a 1-to-N splitter see Section 8.3.3. Each receiver consists of a photodetector, an amplifier, and a signal-processing circuit. The photode- tector senses an optical signal and produces an electrical signal that contains the same information as in the optical signal. The electrical signal is subsequently amplified so that it can be processed electronically by the signal-processing circuit.

8.3.3 Optical Amplifiers

The optical signal looses its power as it propagates through an optical fiber, and after some distance it becomes too weak to be detected Section 8.2.2. Optical amplification is used to restore the strength of the signal. Optical amplification can be used as power amplifiers, in-line amplifiers, and preamplifiers, in a WDM link Figure 8.16. The optical signal can be boosted after it leaves the multiplexer, and before it enters the receiver. In- line amplifiers are used for very long transmission WDM links, and they typically boost the power of the signal to compensate for what was lost prior to entering the optical amplifier. Prior to optical amplifiers, the optical signal was regenerated by first converting it into an electrical signal, then apply 1R or 2R or 3R regeneration, and then converting the COMPONENTS 195 regenerated signal back into the optical domain. In 1R, the electrical signal is simply re-amplified, in 2R, the signal is re-amplified and re-shaped, and in 3R, the signal is re- amplified, re-shaped, and re-timed. In order to re-amplify or re-shape an electrical signal, we do not need to have knowledge of its bit-rate and frame format. However, for re-timing knowledge of both the bit-rate and frame format is necessary. Optical amplification, as in the 1R scheme, can be done without knowledge of the bit rate and the framing format, and it can be applied simultaneously to the combined signal of all of the wavelengths in a WDM link. Currently, re-shaping and re-timing cannot be done in the optical domain. There are several different types of optical amplifiers, such as the Erbium-doped fiber amplifier EDFA, the semiconductor optical amplifier SOA and the Raman amplifier. Below, we describe the Erbium-doped fiber amplifier, a key technology that enabled the deployment of WDM systems. The SOA is mostly used in optical cross-connects OXCs and is described in Section 8.3.5. The Erbium-doped fiber amplifier EDFA The EDFA consists of a length of silica fiber whose core is doped with Erbium, a rare earth element. As shown in Figure 8.18, a laser is emitted into the fiber and is combined through a coupler see Section 8.3.4 with the signal that needs to be amplified. This laser operates at 980 nm or 1480 nm, and the signal to be amplified is in the 1550 nm window. The signal from the laser pumps the doped fiber and induces a stimulated emission of the electrons in the fiber. That is, electrons are induced to transmit from a higher energy level to a lower energy level, which causes the emission of photons, and which in turn amplifies the incoming signal. An isolator is used at the input andor output to prevent reflections into the amplifier. In practice, EDFAs are more complex than the one shown in Figure 8.18; the two-stage EDFA shown in Figure 8.19 is much more common. In the first stage, a co-directional Laser 850 nm Signal to be amplified 1550 nm Coupler Erbium- doped fiber Isolator Isolator Figure 8.18 The Erbium-doped fiber amplifier. Signal to be amplified 1550 nm Isolator Isolator Coupler Coupler Erbium- doped fiber Laser 850 Laser 850 Figure 8.19 A two-stage EDFA. 196 OPTICAL FIBERS AND COMPONENTS laser pumps into the coupler in the same direction as the signal to be amplified, and in the second stage, a counter-directional laser pumps into the coupler in the opposite direction of the signal to be amplified. Counter-directional pumping gives higher gain, but co-directional pumping gives better noise performance.

8.3.4 The 2 × 2 Coupler

The 2 × 2 coupler is a basic device in optical networks, and it can be constructed in variety of different ways. A common construction is the fused-fiber coupler. This is fabricated by twisting together, melting, and pulling two single-mode fibers so that they get fused together over a uniform section of length. Each input and output fiber has a long tapered section see Figure 8.20. Let us assume that an input light is applied to input 1 of fiber 1. As the input light propagates through the fiber 1 tapered region into the coupling region, an increasing portion of the input electric field propagates outside of the fiber 1 core and is coupled into fiber 2. A negligible amount of the incoming optical power is reflected back into the fibers. In view of this, this type of coupler is known as a directional coupler. The optical power coupled from one fiber to the other can be varied by varying the length of the coupling region, the size of the reduced radius of the core in the coupling region, and the difference in the radii of the two fibers in the coupling region. There is always some power loss when the light goes through the coupler. A more versatile 2 × 2 coupler is the waveguide coupler see Figure 8.21. A waveguide is a medium that confines and guides a propagating electromagnetic wave. A Coupling region Tapered region Tapered region Input 1 Output 1 Output 2 Fiber 1 Fiber 2 Input 2 Figure 8.20 A fused-fiber 2 × 2 coupler. Coupling region Input 1 Output 1 Output 2 Input 2 Figure 8.21 A 2 × 2 waveguide coupler.