In this paper a simple theoretical model is presented where the energy conservation principle is used. The model is based
on semi-analytical equations describing the behaviour of an erbium-doped photonic crystal fibre (PCF) inside a ring
laser. These semi-analytical equations allow the characterisation of the erbium-doped PCF. Spectral absorption and
emission coefficients can be determined through the measurement of the gain in the PCF as a function of pump power
attenuation for several fibre lengths by means of a linear fitting. These coefficients are proportional to the erbium
concentration and to the corresponding absorption or emission cross section. So if the concentration is known the erbium
cross sections can be immediately determined.
The model was successfully checked by means of two different home-made erbium doped PCFs. Once the fibres were
characterised the values of the spectral absorption and emission coefficients were used to simulate the behaviour of a
back propagating ring laser made of each fibre. Passive losses of the components in the cavity were previously
calibrated. A good agreement was found between simulated and experimental values of efficiency, pump power
threshold and output laser power for a wide set of experimental situations (several values of the input pump power,
output coupling factor, laser wavelength and fibre length).
We report on a comparison of characterization techniques for high concentration erbium-doped photonic crystal fibres
(PCFs). A highly erbium-doped-silica PCF was fabricated and an amplifier based on the PCF was built. Then,
measurements on the amplifier output optical powers were carried out. To model the amplifier, three different
formalisms were assumed for the Er3+-ion upconversion mechanism and the numerical results were fitted to the
experimental ones. The sets of best-fit parameters are compared and the use of these techniques for active PCF
characterisation is discussed.
An erbium-doped photonic crystal fiber laser has been designed, constructed and characterized in order to examine the
feasibility of this kind of devices for secure communications applications based on two identical chaotic lasers. Inclusion
of a tailored photonic crystal fiber as active medium improves considerably the security of the device because it allows
customization of the mode transversal profile, very influential on the laser dynamics and virtually impossible to be
cloned by undesired listeners. The laser design has been facilitated by the combination of characterization procedures
and models developed by us, which allow prediction of the most suitable laser features (losses, length of active fiber,
etc.) to a given purpose (in our case, a laser that emits chaotically for a wide assortment of pump modulation conditions).
The chaotic signals obtained have been characterized by means of topological analysis techniques. The underlying
chaotic attractors found present topological structures belonging to classes of which very scarce experimental results
have been reported. This fact is interesting from the point of view of the study of nonlinear systems and, besides, it is
promising for secure communications: the stranger the signals, the more difficult for an eavesdropper to synthesize
another system with similar dynamics.
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