John Donne stated in 1623 that “No man is an island, entire of itself…” in Devotions Upon Emergent Occasions, Meditation XVII. Human beings do not thrive when isolated and, thus, his sermon underscores the immense importance of communication. It is also no surprise that optical communication dates back to antiquity, from fire and smoke signals to signalling lamps, flags, and semaphores.
Modern optical communications emerged with the development of both a powerful coherent optical source that could be modulated (lasers ) and a suitable transmission medium (optical fibres ). Expressed in terms of analogue bandwidth, a 1 nm waveband translates to a bandwidth of 178 GHz at 1300 nm and 133 GHz at 1500 nm. Thus, optical fibres have a total usable bandwidth of approximately 30 THz. Assuming the ubiquitous on-off keying format which has a theoretical bandwidth efficiency of 1 bps/Hz, one can expect a 30 Tbps digital bandwidth if fibre nonidealities are ignored.
Given the immense potential of optical fibres, it comes as no surprise that they are predominantly replacing copper as the transmission medium of choice, vastly increasing single-link bandwidth in the process. As shown in Fig. 1, the past decade has witnessed a networking paradigm shift from connection-oriented communication to high-bandwidth IP-centric, packet-switched data traffic. All this traffic is driven by the influx of high-bandwidth applications  which have caused an insatiable demand for increased data rates in optical long-haul communications. 
The availability of such high-bandwidth applications relies heavily upon the ability to transport data in a fast and reliable manner without significantly increasing operating and ownership costs. Consequently, researchers are being forced to create high-speed networks capable of supporting the varied bit rates, protocols, and formats required by these applications in a highly scalable manner. As modern networks continue to evolve in both size and complexity, new technologies have emerged to facilitate the most basic networking functions and to efficiently utilise the potential of optical fibres for routing, switching and multiplexing.
Figure 1: Forecasted growth of global IP traffic. Data source is Cisco report,” Cisco Visual Networking Index: Forecast and Methodology 2013–2018.” See reference .
One can define network transparency based on the parameters of the physical layer (e.g., bandwidth, signal-to-noise ratio). It can also be the measurement of the signals remaining in the optical domain, as opposed to those interchanging between the optical and electronic domains. Transparency can also mean the type of signals that the system supports, including modulation formats and bit rates. Given all these considerations, a transparent, all-optical network (AON) is commonly defined as one where the signal remains in the optical domain throughout the network. Transparent networks are attractive due to their flexibility and higher data rate. In contrast, a network is considered opaque if it requires its constituent nodes to be aware of the underlying packet format and bit rate.
The lack of transparency is a pressing concern in current networks, as the need to handle data streams in the electrical domain engenders a large optical-electronic bandwidth mismatch.  The bandwidth on a single wavelength is 10 Gbps (OC-192/STM-64) today and is likely to exceed 100 Gbps (OC-3072/STM-1024) in the near future. Electronics will be hard pressed to keep pace with the optical data rate as it spirals upwards, especially since device dimensions are fast approaching the quantum limit.  Additionally, high-speed electronics require prohibitively expensive infrastructure upgrades. Any network upgrade requires the replacement of all legacy equipment (a “forklift upgrade”), which involves the massive overhaul of the existing infrastructure. AONs, however, avoid this problem as the data rate is only limited by the end-station capabilities. Thus, connection upgrades do not require changes in the core, enabling metro operators to scale their networks to meet customer requirements and enhance their services more easily.
The advancement of device implementation technologies makes it possible to design AONs in which optical signals on an arriving wavelength can be switched to an output link of the same wavelength without conversion to the electronic domain. Signals on these AONs can be of different bit rates and formats, as they are never terminated inside the core network. This bit rate, format, and protocol transparency are vitally important in next-generation optical networks.
next; switching technologies