## 5 Physical Layer: Five Decades of Evolution (from 45 Mb/s to 100+ Tb/s)  
### Two Key Factors of Evolution  
The main objective of optical transmission is to maximize the capacity (\(B\)) times transparent length (\(L\)) product. These are the two main factors which mark the passage from a technological era to the next one. For this purpose, the overall capacity transmitted within a single optical fiber can be increased in three ways: (a) employing a higher number of channels, (b) increasing the line rate per channel, and (c) packing the channels more densely. The first option requires the existence of lasers and amplifiers that can emit and amplify light, respectively, in a number of different wavelengths. The second option requires the use of higher modulation formats that can increase the spectral efficiency or the number of transmitted bits per one symbol and per one Hz. This designates a migration from OOK modulation schemes towards more advanced modulation formats such as differential phase shift keying (DPSK), differential quadrature phase-shift keying (DQPSK), etc., to even polarization multiplexed (PM)-MQAM and probabilistic shaping. During this migration, the cost of the employed transceivers has to be quantified and contrasted with the potential gains on the line rate, as the increase on modulation format comes with an increased cost and system complexity both in the transmitting part, e.g., due to the use of additional MZMs, and in the receiving end, e.g., due to the use of additional components such as PINs, LOs, optical hybrids, etc. The technology that will prevail is based mainly on the metric cost/power consumption per bit that needs to be minimized. Toward this target, oDAC-based transceivers such as those presented in Section 4.1.10 are a very cost-effective option. The third solution allows us to exploit the available spectrum more \"wisely\". In particular, it manifests a migration from, e.g., NRZ, which has a sinc-shaped spectrum, towards more spectrum-wise formats, such as coherent-OFDM or Nyquist-WDM. In the latter cases, an almost rectangular-shaped spectrum can be transmitted, allowing, in the ideal case, the channel bandwidth to equal channel spacing and, in this way, increasing the number of transmitted channels. The second critical factor that can accelerate the network evolution is the transparent length. In order to increase its value, we have to concurrently combat the main physical layer effects, which are fiber attenuation, fiber dispersion, fiber nonlinearity and amplified spontaneous emission (ASE) noise. Fiber attenuation may induce a severe power depletion, which can impact the system's performance. The overall loss can be mitigated by using optical amplifiers periodically within the link. Next, fiber dispersion leads to a pulse broadening, which, in turn, causes inter-symbol interference (ISI), resulting in unacceptable BER values. The impact of fiber dispersion can be counterbalanced (a) partially, e.g., by using transmission fibers with low local dispersion, such as dispersion shifted fiber (DSF) and dispersion flattened fiber (DFF), or (b) fully, e.g., by using DCMs periodically within the link or employing electronic dispersion compensation by means of DSP modules. Further, fiber nonlinearity and ASE noise are considered antagonistic effects, as in low transmitted power, the ASE noise dominates over fiber nonlinearity whilst in increased optical power, the nonlinearity dominates over the ASE noise. As a consequence, a quest for finding the optical power that maximizes the overall transmission performance is usually a system vital procedure. A well-known method for finding the optimum power within an optical network can be found in [81]. To better understand the impact of fiber parameters on physical layer performance, Figure 28 illustrates the attenuation and dispersion parameters for the low loss frequencies of a G.652D fiber (\(\alpha<\) 0.4 dB/km).