Optical Detection & Regeneration

Before reading this you may find the following tutorials useful:
Laser Basics, Optical Amplification, Erbium Doped-Fiber Amplifiers (EDFAs)

Optical networks are basically flashes of laser light traveling through optical fibers representing information. We have covered in detail the different protocols that are used to convert data into this digital information, and then how these 1s and 0s can be transmitted by lasers through optical fibers. This beginners' guide aims to close the loop in explaining how these light pulses can be converted back into useful information at the receiving end.

The first step is to convert the light back into electricity using what is known as a photodetector or receiver. Creating the light involved the use of a semiconductor device to convert electricity to light; semiconductors can be used to reverse the process.

If you remember back to the Laser Basics tutorial, light is created in a laser by photons (particles of light) stimulating electrons to fall from high energy states to low energy states and in doing so giving out further photons. In the case of a photodetector (a light detector), the key is to encourage incident photons to cause electrons to rise from their low energy state to their high-energy state – thereby creating an electrical current representing the light information.

Electron energy states in a laser and a detector The most basic form of detector is known as a “p-i-n photodiode.” In such a device, for every single photon incident, a single electron will rise to its excited state. This will be satisfactory for most short-range and low bit-rate systems. However, if a signal has been weakened significantly, then a more advanced type of detector may be required to detect it.

One option would be to use an “avalanche photodiode” (APD), which is a discrete semiconductor device like the p-i-n photodiode. It differs, however, in that for every incident photon, it can generate several excited electrons – as many as 100. Therefore the signal is boosted many times over and so lower optical powers can be detected successfully. An APD operates at a much higher voltage than a p-i-n and is designed so that a photon moves an electron to its excited state with enough energy to cause further electrons to be excited also. These extra electrons can themselves cause further electrons to rise to their excited states, and so there is a chain reaction process – or an “avalanche.”

Another way to improve the detection of a signal is through the use of Erbium Doped-Fiber Amplifiers (EDFAs). An EDFA can be added before a semiconductor detector in order to give a power boost to the signal optically before the detector converts it to electricity. This is usually performed in conjunction with a p-i-n photodiode to form what is known as a pre-amplifier.

Suitable detectors can be selected by looking at various characteristics that are quoted in data sheets. Importantly, the detector’s sensitivity must be suitable for the optical powers expected to reach it; it must be able to cope with the bit rate of the signal; and it must also be sensitive to the specific wavelengths being used.

So now that the light has been converted back into electricity, sense has to be made of it. The data left the source as very clearly defined 1s and 0s. However, after travelling through many kilometers of optical fiber these clear pulses will have altered quite significantly. They will have lost power, picked up noise, and also moved out of synchronization with each other.

The reduction in power can be compensated by electrical amplification processes to boost the electrical signal. This is known as re-amplification. To correct for the noise now present in the signals, re-shaping takes place. Again, this is performed by electronics and serves to clean up the smeared and noisy signals into crisp and clear electrical 1s and 0s. Finally comes the process of re-timing. The time in between bits was rigid at the source, but at the receiving end this is no longer the case; and so electronics are again needed to slightly adjust the 1s and 0s so they are equally spaced in time and match the bit rate of the system.

These three processes are collectively referred to as 3R regeneration (re-amplification, re-shaping, and re-timing). References to 1R regeneration will usually be made about a re-amplification process used in isolation. “Optical 1R regeneration” means that the re-amplification takes place entirely in the optical domain, i.e., without any conversion of the light back into electricity. This can take place in the middle of a system span by using such devices as EDFAs. It is also worth noting that a great deal of research is taking place into creating an all-optical 3R regenerator that can perform all the 3R functions but without the signal needing to be converted to electricity beforehand. It is hoped that this will work at a significantly lower cost than the electrical version.

So now our optical signal has been converted to a noisy electrical signal, and we have used electrical 3R regeneration to clean it up. The result is a clear set of electrical 1s and 0s that can now be converted back into useful information by the protocol being used.

Key Points