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We numerically study the behavior of a semiconductor ring laser subject to bidirectional delayed optical feedback, when the isolated laser is in the quasi-unidirectional regime. The optical feedback, provided by two external reflectors located in front of the ring output waveguides, can modify the laser regime produced by the cross-saturation between the clockwise and the counter-clockwise mode. Thus, the system exhibits new different regimes, most of which are asymmetric and bidirectional, with alternating dominant mode. Two of these regimes are of special interest in view of applications, because the laser switching period, between the clockwise and the counter-clockwise mode, is linearly related to the time of flight from the laser to one or both reflectors. In these operating conditions, the laser is thus suitable to implement a telemeter. A convenient electrical output signal is obtained by a photodiode located behind one (partially reflecting) fixed mirror, or by measuring the voltage drop across the laser junction. Simulations are performed by mathematical models based on rate-equations, assuming typical literature parameters for a 1 mW ring laser. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper describes and compares three optical methods for performing spot measurements on micromachined mirrors, designed for photonic switching in fiberoptic networks. For static characterization, two spot-position detection systems, one based on a vidicon camera and the other based on a bidimensional silicon position sensitive detector (PSD), are illustrated, tested, and compared. Moreover, the dynamic behavior has been monitored with the PSD-based detection arrangement and with a semiconductor laser feedback interferometer. Advantages and drawbacks of these methods are highlighted. Testing is reported on torsional, silicon micromachined mirrors, with a single degree of freedom. High dc voltage for static measurements, and sinusoidal or white-noise or step excitation for dynamic characterization, have been used for mirror driving. [ABSTRACT FROM PUBLISHER]

Feedback interferometry is a useful tool to characterize micromachined devices. We consider a silicon vibrating gyroscope, in which the angular rotation is transduced into the vibration amplitude of a small suspended mass. Measurements of the mass displacement at submicrometer resolution are reported on a 400×400 μm sensor, using an 800-nm 20-mW laser diode. The resonance curves of the device have been determined for different values of pressure and other parameters, which allows us to tune the resonance frequency and maximize the Q factor. Hysteresis and other nonlinear phenomena on specific samples also have been detected. The proposed method provides a direct inspection tool and represents a practical alternative to the standard electrical measurements [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this letter, we numerically investigate the statistical properties of the optical chaos hiding a digital message in secure transmission schemes. It is found that even if chaos statistics are usually not Gaussian, the statistics of the residual chaos, over the extracted message, are nearly Gaussian. Thus, assessing the bit-error rate from the Q parameter, as it is often done in the numerical analysis of such schemes, is a viable approach. [ABSTRACT FROM PUBLISHER]

A numerical analysis of an optical chaotic transmission system, based on the synchronization of two chaotic lasers, in a master-slave closed loop configuration is presented. At the transmitter, the master chaotic wave is superposed on the information message; at the receiver, the message is recovered by subtracting the synchronized slave chaotic wave from the received signal. The performances are analyzed in terms of the Q-factor, considering two different message modulation formats: the nonreturn-to-zero and the Manchester coding. The Manchester coding shows enhanced performances due to the shift of the signal spectrum to higher frequencies. [ABSTRACT FROM PUBLISHER]

Wavelength conversion in the transmission of a message masked by optical chaos is experimentally demonstrated. In our setup, chaos is generated by a distributed-feedback laser subject to delayed optical feedback, and hides a message by additive chaos masking. The optical wavelength is converted, along the transmission line, by four-wave mixing in a semiconductor optical amplifier. At the receiver, the message is extracted by master-slave synchronization. Our experiments demonstrate that secure communications based on chaos are compatible with channel switching as required in reconfigurable optical networks. [ABSTRACT FROM PUBLISHER]

We present a numerical and experimental evaluation of message encryption by phase modulation, using a chaotic optical carrier generated by a laser subject to delayed optical feedback. This method offers better security than the conventional amplitude masking, where the signal is simply added to the chaotic waveform [ABSTRACT FROM PUBLISHER]

We propose a new telemeter scheme for absolute distance measurements, based on a semiconductor ring laser, working in the bistability regime. The optical feedback provided by two external reflectors (a fixed one at short distance, and a moveable one defining the measuring arm) generates commutations of the propagation direction (clockwise, counter-clockwise) inside the ring laser, the period of which is linearly related to the distance of the measure arm reflector. A convenient electrical output signal can be easily obtained by a photodiode located behind the (partially reflecting) fixed mirror. This telemeter, which combines time-of-flight and optical injection, is very simple to implement, since, in addition to the laser, it only requires mirrors and collimation or focusing optics. Also electronic driving and processing are straightforward. Differently from most time-of-flight telemeters, this scheme does not require special provisions or processing to tackle the ambiguity problem. Simulations are performed by mathematical models based on rate-equations. This telemeter has been evaluated in the range 10 cm–32 m of round trip distance, with a fixed arm of 10 μm–10 cm, assuming typical literature parameters for a 1 mW ring laser. [ABSTRACT FROM AUTHOR]