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Optical Networks - _3_1 Couplers_35

# Optical Networks - _3_1 Couplers_35 - 114 Components Input...

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114 Components Input 1 Output 1 Output 2 Input 2 l (coupling length) Fibers or waveguides Figure 3.1 A directional coupler. The coupler is typically built by fusing two fibers together. It can also be built using waveguides in integrated optics. 3.1 Couplers A directional coupler is used to combine and split signals in an optical network. A 2 × 2 coupler consists of two input ports and two output ports, as is shown in Figure 3.1. The most commonly used couplers are made by fusing two fibers together in the middle—these are called fused fiber couplers. Couplers can also be fabricated using waveguides in integrated optics. A 2 × 2 coupler, shown in Figure 3.1, takes a fraction α of the power from input 1 and places it on output 1 and the remaining fraction 1 α on output 2. Similarly, a fraction 1 α of the power from input 2 is distributed to output 1 and the remaining power to output 2. We call α the coupling ratio. The coupler can be designed to be either wavelength selective or wavelength independent (sometimes called wavelength ﬂat) over a usefully wide range. In a wavelength-independent device, α is independent of the wavelength; in a wavelength- selective device, α depends on the wavelength. A coupler is a versatile device and has many applications in an optical network. The simplest application is to combine or split signals in the network. For example, a coupler can be used to distribute an input signal equally among two output ports if the coupling length, l in Figure 3.1, is adjusted such that half the power from each input appears at each output. Such a coupler is called a 3 dB coupler. An n × n star coupler is a natural generalization of the 3 dB 2 × 2 coupler. It is an n -input, n -output device with the property that the power from each input is divided equally among all the outputs. An n × n star coupler can be constructed by suitably interconnecting a number of 3 dB couplers, as shown in Figure 3.2. A star coupler is useful when

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Optical Networks - _3_1 Couplers_35 - 114 Components Input...

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