Close-loop three-laser scheme for chaos-encrypted message transmission

In this paper, we numerically evaluate private data transmission using a three-laser scheme, consisting of a pair of twin semiconductor lasers, driven to chaos by delayed optical feedback in a short cavity, and optically injected by a third chaotic laser which forces them to synchronize. This laser is selected with different internal parameters with respect to the twin pair, so that the emissions of the synchronized, matched lasers, are highly correlated, whereas their correlation with the driver is low. The digital message modulates the emission of the transmitter, as in a standard Chaos Modulation scheme. Message recovery is then obtained by subtracting, from the transmitted chaos-masked message, the chaos, locally generated by the synchronized receiver laser. Simulations have been performed with the Lang-Kobayashi model, and, in view of application to private transmission, we have investigated the effect of the parameter mismatch, between transmitter and receiver, on message recovery. A preliminary experimental evaluation has been also performed using specially designed InP integrated modules.     

Power efficiency of a semiconductor laser with an external cavity

Optical feedback modifies the power vs. current diagram of a laser diode as well as its spectrum. Though optimization of the spectral characteristics is usually the main goal in the design of an external cavity source, power efficiency is also important, especially with relatively high power devices, where temperature variations due to dissipation can have an impact on the wavelength stability and on the laser lifetime. A useful parameter to describe the total power efficiency of the stabilized laser, relative to that of the solitary laser, is proposed in this paper. The dependence of this parameter on the characteristics of the active device, and of the external cavity, is investigated.     

Thermodynamic phase noise in fibre interferometers

Phase noise due to thermodynamic fluctuations in the optical path length is evaluated in this paper for basic fibre interferometers. In Mach-Zehnder and Michelson interferometers, where the temperature phase fluctuation (TPF) is that intrinsic to the fibre, this noise has been reported to be comparable to shot noise and a possible limit to sensor sensitivity in practical cases. We show that in Sagnac interferometers, used in fibre gyro and in Faraday current sensors, the TPF noise is decreased with respect to that intrinsic to the fiber because propagation in the same optical path leads to a correlation of the phase fluctuations. In addition, we show that in Fabry-Perot and ring resonators, as multiple reflections increase the effective path length, TPF noise is enhanced and can be dominant over shot noise even for moderate fibre lengths.     

Combined reciprocal and non-reciprocal birefringence in optical monomode fibres

We analyse the overall birefringence which arises from the combined effects of Faraday rotation (non-reciprocal) and bending stress (reciprocal) applied to a monomode optical fibre loop. The theory is substantiated by numerical evaluations and appropriate experimental data are reported. These indicate how it is possible to build non-reciprocal birefringent components, i.e. rotators and isolators, by means of monomode coils.     

Chaos Encrypted Optical Communication System

Abstract

In this article, we report on the research activity that has been recently carried out in Italy on secure transmission by using chaotic carriers in the framework of a national and two international projects. Transmission of both analog and digital signals has been demonstrated, as well as theoretically and numerically investigated. Close-loop digital transmission over 100 km distance has been achieved for the first time.     

Squeezed states in direct and coherent detection

With reference to squeezed states, we analyse the noise performances of the basic detection schemes in fibre telecommunications and interferometry. We have developed a semiclassical approach that allows us to translate the quantum mechanical formalism into synthetic expressions and rules only involving directly measurable physical quantities. As a result, we find that in the telecommunication field squeezed states do not allow significant improvements in system performances: in practice, the signal-to-noise ratio cannot be increased in standard detection schemes, both direct and coherent, by squeezing either the signal (because of propagation losses) or by the local oscillator. On the contrary, a significant improvement can be obtained in interferometric measurements if squeezed vacuum is coupled into the normally unused beamsplitter port.