Photonics is the field of optics that studies the possibility of controlling flows of photons, and to realize devices similar to electronic ones but using photons instead of electrons. In the field of telecommunications, the term is used to indicate the set of technologies used for the generation, transmission, detection and processing of modulated signals on optical carriers.
The term “photonics” has come into use in the field of telecommunications as a result of technical developments in the field of optoelectronics and for the reliability of some particular techniques, increasingly innovative. In the beginning, the optical systems, while carrying out the transmission of the signal on optical fiber, used the same electronic systems to perform the necessary operations, passing continuously from optical signal to electrical signal and vice versa as often as necessary.
Initially, almost totally optical devices were developed, which made it possible to dispense with such double conversion, and the term photonics began to be used in the field of telecommunications, in a parallel sense to the term electronics. The photonic systems applied to telecommunications, therefore, can be distinguished in transmission systems and switching systems.
The first applications in this field can be traced back to the mid-1970s, with the testing of optical fibers as broadband transmission channels. However, in the first applications, the signal was kept in optical form only for transmission purposes, while it was converted into electrical form every time it was necessary to carry out amplification, processing or switching operations on it; these operations on the electrical signal were performed according to the usual electronic techniques, in the analog or numerical field. Subsequently, thanks to the extremely rapid development of optoelectronics, various innovative technologies have been developed, by means of which the signal on optical carrier remains in that form in increasingly large sections of the connection, limiting the number of electro-optical conversions, with great advantages in terms of performance, reliability and compactness of the systems.
Technical development is also linked to the development of various elements that, similarly to resistors, inductances and capacitances in use in electrical systems, are called optical or optoelectronic components. Leaving aside lasers, optical detectors and amplifiers, it is appropriate to consider those components known as passive optical components, including fiber optic joints. These components have proven to be crucial for the commissioning of fiber transmission routes.
Photonic optical transmission systems differ from other telecommunications systems in that they use optical fibers as the transmission medium. This implies that the carrier frequency is extremely high (around 105 GHz) and therefore the bandwidth of the transmittable signal can be much higher than that usable in all other cases. An optical transmission system consists of a laser transmitter modulated by the electrical signal, a transmitting medium and a receiver that allows the conversion of the optical signal back into an electrical signal. The transmitting medium consists of a single mode optical fiber and optical amplifiers. In an amplified optical transmission line, the main limitations in performance are due to a combination of 4 main effects:
- chromatic dispersion, consisting in the fact that the various spectral components of the signal travel at slightly different speeds;
- polarization dispersion, due to the different speeds at which the polarized components of the signal propagate;
- non-linear effects present in the transmission of the signal in the fiber, which in turn are due to three physical effects, known as the Brillouin effect, Raman effect and Kerr effect;
- optical noise generated in the amplifiers. In the numerical transmission of pulses on the fiber, the above effects act simultaneously and therefore give rise in general to distortions (distortion of the signal spectrum, widening of the pulse duration, etc.).
Photonic techniques allow to realize complex communication systems, completely optical. This applies not only to transmission, but also to switching systems in interconnected fiber optic networks. The key element in systems of this type is represented by the optical nodes, in which various types of architectures are realized in which the routing of communications can be done with spatial switching matrices or with frequency switching systems. Optical distribution networks allow to use, at the user level, transmission channels with a much wider bandwidth than the one used so far, and have unquestionable advantages over distribution networks on twisted pair or coaxial cable. In fact, telematics type services, i.e. integrated audio and video services, have been standardized in relation to distribution networks of this type (telematics).