Over the years, capacity has been improved through the combination of multiple mechanisms:
· Installation of additional fiber optics cables
· Increase of the baud rate for a given link
· Improvement of the transmission characteristics of the fiber by reducing or mitigating the effects of attenuation and dispersion
· Multiplex of multiple signals in a single fiber by assigning different wavelengths to them
· Increase of the number of wavelengths transported by a single fiber by reducing the distance between them
· Addition of FEC (Forward Error Correction) techniques to enable faster connections in in lossy or dispersive environments.
The above improvements have been applied over time to optical signals using the traditional OOK (On-Off Keying) direct modulation scheme. Information is coded by controlling two states of the optical transmitter. Ideally, in one of them full power is transmitted while in the other zero power should be transmitted so only one bit can be coded by each symbol.
Figure 1. Spectral efficiency of optical transmissions may be improved by modulating both the amplitude and the phase of an optical carrier, which requires coherent modulation and detection. In this WDM link, four different wavelengths share the same fiber in a standard ITU 50GHz grid. Wavelength 4 is carrying a 10Gb/s signal using the traditional intensity modulation (or On-Off Keying, OOK). Part of the optical power goes directly to the carrier and does not transport any information. Carrier 3 is modulated using a QPSK modulation so 2 bits are transported by each symbol, doubling the capacity of the OOK-modulated channel in the same bandwidth. Capacity may be increased through the usage of more complex modulations or baseband filtering. Wavelengths 1 and 2 transport 28 Gbaud signals with 2 and 5 bits per symbol respectively.
As bits are transmitted faster and faster, optical signals drift away from the ideal conditions and issues like bandwidth (both optical and electronic) and dispersion