## 5 Physical Layer: Five Decades of Evolution (from 45 Mb/s to 100+ Tb/s)
### Five Decades of Physical Layer Evolution in Backbone
#### 5.2.2 Main Achievements per Generation  
_First Decade (1970-1980)_  
The birth of fiber-optic communications is a direct result of the third Industrial Revolution. Fiber communications began officially with the advent of the first laser, which enabled light transmission within the optical fiber over km distances. The first commercial systems became available around 1980, after experimental trials that took place around 1977-1979. These systems were characterized by limited bit rates of about 45 Mb/s and a limited reach of up to 10 km, while they exploited the first transmission window and deployed multimode fiber (MMF) as a transmission medium. Compared with modern communications, the attainable bit rate of the first systems seems incredibly small; however, it opened new horizons in telecommunications, as it expanded the \(B\) times \(L\) product by about 2-3 orders of magnitude compared with co-axial cables, allowing to transport 700 telephone calls simultaneously within a single telecommunication channel. _Second Decade (1980-1990)_  
During the 1980s, new lasers were proposed in order to exploit the second and third transmission window whilst SMF was selected over MMF. In this way, the transmission reach could be extended to 100 km, as the impact of SMF's dispersion was significantly smaller than the impact of MMFs. In particular, the local dispersion parameter of a standard SMF yields around zero in 1300 nm and about 17 ps/nm/km around 1550 nm, making the impact of dispersion insignificant for distances smaller than 100 km for bit rates up to 2.5 Gb/s. With the new developments on electronics, the bit rate was also able to reach 10 Gb/s, making the dispersion an important constraint for the third transmission window. A solution to this problem was the introduction of new fibers, which had appreciably lower local dispersion parameter compared with standard SMF. These fibers were the dispersion flattened fiber and dispersion shifted fiber, which allowed the light propagation to exceed 100 km with sufficient performance. _Third Decade (1990-2000)_  
The third decade was clearly the era of optical amplification and WDM. WDM allowed for the simultaneous transmission of multiple optical channels within a single optical fiber, allowing to outreach 1 Tb/s. At the same time, advanced modulation formats were  
Figure 31: Evolution of transported capacity per single channel for commercially available systems over the last 50 years (based on [82, 83, 84, 85, 86]). proposed that could attain greater spectral efficiency compared to NRZ, most notably DPSK and DQPSK, which could achieve data rates of 40 Gb/s per channel. Moreover, the use of optical amplifiers gave us the opportunity to overcome the attenuation problem effectively. At the same time, the introduction of dispersion compensation mechanisms, such as dispersion compensation fiber (DCF), allowed for the surmounting of the problem of dispersion. As a result, the transparent length could outdistance 1000 km, which is a tenfold increase compared with the previous decade and 100 km. _Fourth Decade (2000-2010)_  
The fourth decade was the decade of dynamic optical switching, the introduction of coherent communications and full C and L-band amplification. Dynamic optical switches allowed incoming traffic to be forwarded to the proper output links in a dynamic way, bypassing the need for opto-electronic conversion. This all-optical method led to significant cost savings and, at the same time, to a significant decrease in the overall end-to-end delay, allowing the underpinning of time sensitive services. On the other hand, coherent communications allowed for the concurrent exploitation of the amplitude and the phase of the optical carrier through the use of multi-level modulation formats, such as QPSK, 16 QAM, etc., in this way exceeding line rates of 100 Gb/s. In addition, coherent receivers eased the use of DSP modules, and now the dispersion problem could be solved more efficiently compared with DCFs, which inevitably allowed an amount of residual dispersion.