The typical optical carrier frequencies are ~ 200THz. The microwave carrier frequencies are ~1GHz. This means that optical communication systems have a larger capacity to carry information.
Figure 1 depicts a block diagram of an optical communication system. Optical communication systems can be classified by two categories – guided optical systems and unguided optical systems. For guided optical systems the beam, emitted by the transmitter, is spatially confined.
In practical terms these are optical fibres. And usually they are called fibre-optic communication systems. For the unguided optical systems the emitted beam spreads in space. However, there is no deterioration in the free-space communications above the atmosphere. So the free-space communications can be used for certain applications, and for terrestrial propagation, fibre optics are the best solution.
A fibre-optic communication system can be used in any application where transfer of the information is required from one place to another. Fibre-optic communication systems are mostly used in telecommunications. The telecommunication application can be divided into long-haul and short-haul, depending on the distance the signal was transmitted.
Each next generation of optical systems operate on higher bit rates and over longer distances, than the previous one. Long-haul optical communication systems are very good at transferring information over very long distances ~ up to 2,000km (using the transmitter amplifiers). Short-haul optical systems are mostly used for local data transmission – up to 10km. The most popular networks are multi-channel ones. And the best solution for wideband distribution requirements are fibre optics.
The Figure 2 describes a fibre optic communication system. There are several specifying components in contrary to Figure 1 – the communication channels are the optical fibre cable, and the optical transmitter and optical receiver. Let’s consider in detail how this system works.
The role of an optical fibre cable is to transfer the signal from transmitter to receiver without dissipation. The lightwave systems use fibre as communication channels because of the very small losses ~1% per 100km.
Fibre optic channels also lead to fibre dispersion, or broadening of optical pulses. If the transmitting signal is widened according to the bit slot, then the signal degrades significantly, which makes original signal recovering impossible. In case of multi–mode fibres the signal spreads even more rapidly, which is why for optical communication the best solution is single-wire fibre.
Let’s consider the optical transmitter and optical receiver. The Optical transmitter is the device that converts electrical signal into an optical signal, launching the last one into the fibre channel. The optical transmitter consists of an optical source, a modulator and a channel coupler.
The optical source is usually the semiconductor laser or a light emitting diode (LED). The optical signal is generated by modulating the carrier wave. The modulator is used to dispence the wave. The optical source can be modulated by varying the injection current.
Also the channel coupler is used here to focus the optical signal to the entrance of the optical fibre with maximum efficiency. The input power is determined by the formula below and is susceptible to non-linear effects: . Figure 3 shows the optical transiever from Harting. Below we will find out the optical transcievers parameters in details.
An Optical receiver is a device that converts the optical signal on the output of an optical fibre back to the electrical signal. Figure 4-1.12 depicts the scheme of the optical receiver. The optical coupler focuses the signal into the photodiode of the photodetector. A demodulator is needed to modulate the format of the signal. Figure 5 depicts the block diagram of an Analog Devices optical receiver.
Optical receivers are characterised by bit error rate (BER), the number of errors made per second and the receiver sensitivity, the minimum optical power needed to keep BER at the level of 10-9. Here we discussed the basic terms around what is a lightwave system and for optical fibres. In the next module we will discuss aspects of fibre optics more precisely.