Agilent 5991-1809EN Essentials of Coherent Optical Data Transmission - Application Note c20140919 [15] free download

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Keysight Technologies Essentials of Coherent Optical Data Transmission The Concept of Complex Optical Modulation for More Efficient Data Transfer Application Note Introduction Data centers are being built across the globe enabling medium and smaller size enterprises to also store and analyze big collections of structured and unstructured data in the cloud, in order to optimize the supply chain, marketing activities and more. The storage and analysis capacities are in place but the more critical question is if the infrastructure outside of the data centers can keep up the pace. The explosively growing amount of data is becoming an enormous challenge for our backbone networks. If they don't want to become the bottleneck of the future, the spectral efficiency needs to be increased in fiber optical networks. Today, fiber optical infrastructure and signal concepts need to support data rates of 100 Gbit/s, soon 400 Gbit/s and even higher. This is a problem for traditionally applied data coding schemes. 03 | Keysight | Essentials of Coherent Optical Data Transmission - Application Note The Beginning Optical data transport started out like the electronic with the simplest and therefore cheapest digital coding schemes, which are `return-to-zero' (RZ) or `non-return-to-zero' (NRZ) on-off-keying (OOK). The signal here is ideally a rectangular sequence of ones (power-on) and zeros (power-off). This concept faced a limit when transfer rates reached for 40 Gb/s. At 40 Gbit/ s and above, an additional limiting factor comes into the game. Due to the high clock rate, the bandwidth occupied by the signal gets larger than the channel bandwidth of a 50 GHz ITU channel. As can be seen in Figure 1, spectrally broadened channels start to overlap with the neighboring channel and the signals are shaped by the wavelength filters, resulting in crosstalk and degradation of the modulated information. At the latest then, we have to turn our back on OOK and move to more complex modulation schemes, like differential quadrature phase shift keying (DQPSK) for example. Complex modulation reduces the required bandwidth, depending on the symbol clock rate, and higher data rates can be transmitted again in the 50 GHz- ITU channel as illustrated in Figure 1 on the example of DQPSK. RZ or NRZ modulation (10 Gb/s) RZ or NRZ modulation (100 Gb/s) Channel interference Simple Simple hardware hard