The rapid increasing of internet services requires communication capacity of optical fibre networks. Such a task can be carried out by Er3+-doped fibre amplifiers, which allow to overcome limits of unrelayed communication distances. The development of efficient numerical codes provides an accurate understanding of the optical amplifier behaviour and reliable qualitative and quantitative predictions of the amplifier performance in a large variety of configurations. Therefore, the design and optimization of the optical fibre can benefit of this important tool. This chapter proposes an approach based on the Particle Swarm Optimization (PSO) for the optimal design and the characterization of a photonic crystal fibre amplifier. Such approach is employed to find the optimal parameters maximizing the gain of the amplifier. The comparison with respect to a conventional algorithm shows that the proposed solution provides accurate results. Subsequently, the presented method is used to study the amplifier behaviour by evaluating the curves of optimal fibre length, erbium concentration, gain, and pumping configuration. Finally, the PSO based algorithm is exploited to determine the upconversion parameters corresponding to a desired value of gain. This application is particularly intriguing since it allows recovery of the values of parameters of the optical amplifier, which cannot be directly measured.
TopBackground
In the 90s the global breakthrough of Internet changed the telecommunications bandwidth requirements permanently. In fact, the rapid increasing of internet services demanded communication capacity which has been more than doubled every year. Such need can be satisfied by optical fibre networks (Azadeh, 2009). Therefore, great efforts have been devoted to the development of dense wavelength division multiplexing technology, which allows the simultaneous transmission of many channels with different wavelengths on a single optical fibre.
Optical amplifier is an essential device which enables to compensate the transmission losses, overcoming the limit of the unrelayed communication distance of the system. Rare-earth doped optical fibre amplifiers represent the most widely used devices to obtain optical amplification. In particular, Er3+-doped fibre amplifiers (EDFAs) are used to realize high-speed, large-capacity and long-haul optical communication systems without the use of optoelectronic and electro-optical conversions of signals. EDFAs allow the exploitation of an uniform dopant concentration and the enhancement of heat dissipation due to the long interaction length. Moreover, several characteristics make EDFAs attractive: their high gain, wide optical bandwidth, high output saturation, near quantum-limited noise, low insertion losses, high reliability and compactness, polarization-independence, immunity to saturation-induced and to crosstalk, possibility of choosing the pumping laser diode at 980 nm or 1480 nm wavelengths. Although this kind of technology is mature and widely employed, further researches are needed to obtain amplifiers with higher efficiency, higher output power and shorter active fibre length. In detail, these devices should be characterized by high power conversion efficiency and high gain coefficient due to two main reasons. Remote-pumping in long transmission links can be carried out without repeaters, even if low pump power levels are collected, as it takes place when the amplification of extremely weak signals is requested in the field of optical fibre sensors/monitoring. Moreover, the advantages can be extended to other application fields where high power optical fibre amplifiers are largely used: booster amplifiers for long haul repeaterless optical links, 1×N loss-less splitters, CATV distribution architectures, key photonic device for highly-distributed data networks, fibre optic gyroscopes, high-speed intersatellite links and deep-space optical communications (Girard, 2009, Rochat, 2001, Wright, 2005).
The optimization of fibre transversal section is crucial to improve the amplifier performance in terms of gain, noise figure and output power characteristics as well as device compactness and pump power consumption. In fact, in the rare earth doped devices the fibre geometry influences the pump intensity, the overlap of the pump and the signal propagation modes with the doped core. Sophisticated design methods and fabrication techniques have been developed to construct single-mode optical fibre amplifiers. To this aim, a fine control of refractive index profile of both core and cladding as well as more design flexibility of fibre cross section is needed. The conventional optical fibre is not able to satisfy these requirements, but the photonic crystal fibre (PCF) technology seems to be an attractive solution.