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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
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 reamplified, 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 and/or 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
Figure 8.18 The Erbium-doped fiber amplifier.
Figure 8.19 A two-stage EDFA.
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 x 2 Coupler
The 2 x 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 x 2 coupler is the waveguide coupler (see Figure 8.21). A waveguide is a medium that confines and guides a propagating electromagnetic wave. A
Figure 8.20 A fused-fiber 2 x 2 coupler.
Figure 8.21 A 2 x 2 waveguide coupler.
waveguide coupler has two identical parallel guides in the coupling region. (Alternatively, one guide might be wider than the other.) As in the fused-fiber coupler, part of the light going down one guide is coupled onto the other guide. The degree of interaction between the two guides can be varied through the width of the guide, the gap between the two guides, and the refractive index between the two guides.
Couplers are reciprocal devices. That is, they work exactly in the same way if their inputs and outputs are reversed.
A 2 x 2 coupler is called a 3-dB coupler when the optical power of an input light applied to, say input 1 of fiber 1, is evenly divided between output 1 and output 2. If we only launch a light to the one of the two inputs of a 3-dB coupler, say input 1, then the coupler acts as a splitter. If we launch a light to input 1 and a light to input 2 of a 3-dB coupler, then the two lights will be coupled together and the resulting light will be evenly divided between outputs 1 and 2. In this case, if we ignore output 2, the 3-dB coupler acts as a combiner.