Optical Communication Systems

This final chapter of the fundamentals tutorial series brings together everything we have learned thus far. To recap:

• In chapter 1, we introduced fibre optics and the role it plays in connecting the world
• In chapter 2, we dissected a fibre to see what it was made of. We also explored some of the most important light and fibre characteristics.
• In chapter 3, we made the distinction between single mode and multimode fibres.
• In chapter 4, we took a look at the many different types of connectors.
• In chapter 5, we shed some light on optical sources: both lasers and LEDs.
• Finally, in chapter 6, we saw what characteristics define optical detectors and which ones are most popular.

An optical communication system encompasses all these concepts and represents how they interact with one another. It entails turning an electrical signal into an optical one, transmitting one or many signals along an optical path, amplifying the signal along the way, and finally receiving and interpreting the data at the other end. The way we do this has evolved over time in order to support different technologies and applications (such as in metro regional and long-haul networks). The modern systems are much more complex than their predecessors. First, we’ll take a look at single channel systems(Figure 1), which enable the transmission of a single signal over long distances. Then, we’ll look at how multiplexing revolutionized this process and enables us to now send multiple signals over a lone single mode long-distance fibre.

Single channel systems

Figure 1: Single channel systems

Single channel systems were developed as the first means to transmit a signal across a single data stream in optical form. The defining characteristic of this model is that only one signal can be transmitted over a single wavelength at a time. First, an optical transmitter transforms an electrical signal into an optical one and sends it along the optical path. An optical receiver on the other end takes the signal and transforms it into an electrical one. it is imperative that the signal will be boosted along the way or else it would not be able to travel such long distances. This is where amplifiers come in.

As we saw in chapter 2, a signal naturally weakens as it travels along a fibre. Since the form of an optical signal is predetermined and predictable, it can be regenerated and reconstructed into its original shape whenever necessary – depending on the loss across the system. Amplifiers are placed along the optical path at specific intervals when the transmission is weak but still understandable. They simply boost the signal from its weakened state back to its original size and shape, thus allowing the signal to travel hundreds, even thousands of kilometres to reach its destination. Amplifiers are crucial components in long haul and submarine systems which span incredibly long distances.

Figure 2: Amplifier 

Wavelength Division Multiplexing (WDM) Systems

Figure 3: WDM System

Over time, we figured out how to combine many signals into one stream so that we can transmit more data at once along an optical path. This is made possible through a process called multiplexing. There are different types of multiplexors which all revolve around the same principle: combining several signals into a single transmission path. The type we will focus on today is Wavelength Division Multiplexing (Figure 3). This multiplexor – sometimes referred to simply as a “mux” – selectively combines many signals across different wavelengths and transmits them all simultaneously across a single optical path. At the receiving end of the optical system, a demultiplexer – or demux – takes the signal and splits it back into its original constituents. Multiplexors are extremely handy because they expand the network capacity without adding any more fibre to the mix.
There are many different types of wavelength division multiplexing, but the two most common are: coarse (CWDM) and dense (DWDM). The pair is similar, but differ in the range of the optical spectrum they cover as well as the spacing between the channels in this range. In general, CWDM is cheaper but uses less precise lasers. The channels are more spaced out. DWDM is more expensive but allows for many more channels within the same fibre. Multiplexing is a topic so broad that it could be the subject of an entire chapter on its own. Discussions of components, applications, wavelengths, equipment, and diagrams will be kept for another time.

Thanks for keeping up with our tutorial chapters. This concludes the Fundamentals series, which focused on the basic concepts in fibre optics communication. If you haven’t already done so, check out the video versions of the chapters for a more engaging experience. Stay tuned for the next series in the LambdaGain Learning Centre!