*DOI: http://dx.doi.org/10.5772/intechopen.88354 All Optical Signal Processing Technologies in Optical Fiber Communication*

wavebands, sub wavelength channels) and provide on-demand bandwidth in a scalable and reconfigurable fashion [20].

The graph (**Figure 2**) shows that the evolution of transmission capacity x fiber link has been growing over the years. The optical fiber bandwidth utilization approaches its peak limit quickly. Given the potential for such capacity crunch, the research community has concentrated on finding alternatives that make the most of the scarce network resources and meet the consistently expanding traffic requests [21]. In such context, adaptability or reconfigurable capability of networks will become more and more critical and hence spectrum efficient optical networking techniques have been introduced as a way to offer efficient utilization of the available optical resources. The place to start is with the transport network, which forms the foundation of elastic networking.

Currently, all deployed optical transport technologies are mainly based on a fixed grid 50 GHz or 100 GHz/frequency grid standardized by ITU-T and the same modulation technology has been used for optical signals at the same bitrate regardless of the transmission distance. In this scenario, the system is reaches its limits in terms of both capacity and flexibility [10, 23, 24] as higher capacities per fiber have been achieved by improving the spectral efficiency (SE) through increasing the bitrate per channel while keeping or even narrowing the channel

**Figure 3.**

*Roadmap toward elastic spectrum by all-optical signal processing based on transmission distance and user traffic volume [27].*

spacing. In order to fully realize the vision of elastic optical networking, network operators have migrated to flexible transport technology solutions capable of supporting grid spacing flexibility. Opportunities for exploiting underutilized spectral resources are shown in **Figure 3**. The essence of elastic optical path network yields highly-efficient optical path accommodation. The resulting spectral savings is achieved by taking advantage of the spectral resources that had not yet been fully utilized, thus results in an increase in network capacity [25]. Let us consider an example in which mixed-rate with different modulation format traffic is transported in a single fiber. For spectrum A, four 100-Gb/s optical channels headed for the same destination. These can be combined them into a tightly spaced 400-Gb/s superchannel and transported as a single entity by eliminating the unnecessary spacing between the channels. Spectrum B shows the ITU-T fixed grid with excess channel spacing. In the next step (spectrum C) implement a flexible grid by using the all optical processing technique i.e. defragmentation. For client traffic that does not fill the entire capacity of a wavelength, the elastic optical path network provides the right sized intermediate bandwidth [26] by format conversion through adaptive modulation represented in spectrum D. This makes the unused client bandwidth available for use. The wavelengths routed in the same direction, grooming is performed as illustrated in spectrum E. Finally in spectrum F, for shorter optical paths, which suffer from less SNR degradation, employment of more spectrally efficient modulation format, such as 16QAM or DP-QPSK, further combined with elastic channel spacing, where the required minimum guard band for wavelength routing is assigned between channels is performed [25]. In this way, elastic optical path networks accommodate a wide range of traffic in a highly spectrally efficient manner [26].
