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## 4.3 Spectral Efficiency

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We saw in Chapter 2 that the ultimate bandwidth available in silica optical fiber is about 400 nm from 1.2 μm to 1.6 μm, or about 50 THz. The natural question that arises is, therefore, what is the total capacity at which signals can be transmitted over optical fiber?

There are a few different ways to look at this question. The *spectral efficiency* of a digital signal is defined as the ratio of the bit rate to the bandwidth used by the signal. The spectral efficiency depends on the type of modulation and coding scheme used. Today’s systems primarily use on-off keying of digital data and in theory can achieve a spectral efficiency of 1 b/s/Hz. In practice, the spectral efficiency of these systems is more like 0.4 b/s/Hz. Using this number, we see that the maximum capacity of optical fiber is about 20 Tb/s. The spectral efficiency can be improved by using more sophisticated modulation and coding schemes, leading to higher channel capacities than the number above. As spectral efficiency becomes increasingly important, such new schemes are being invented, typically based on proven elect....