Mário F. S. Ferreira

Analysis of the gain and noise characteristics of fibre Brillouin amplifiers

A theoretical analysis is performed of the transfer characteristics, gain, amplified spontaneous emission noise, on/off ratio and nonlinear phase shift of a fibre Brillouin amplifier, taking into account a possible signal detuning from the line centre and the nonlinear pump power depletion effect. Among other features, it is shown that the amplified spontaneous emission noise is reduced at higher signal levels and that detuning the signal reduces the amplifier gain in the linear regime and increases the spontaneous noise power in the saturation regime. As a consequence, the on/off ratio is significantly degraded by such an effect.

Broadband polarization pulling using Raman amplification

The Raman gain based polarization pulling process in a copropagating scheme is investigated. We map the degree of polarization, the angle between the signal and pump output Stokes vectors, the mean signal gain and its standard deviation considering the entire Raman gain bandwidth. We show that, in the undepleted regime (signal input power ~ 1 μW), the degree of polarization is proportional to the pump power and changes with the signal wavelength, following the Raman gain shape. In the depleted regime (signal input power ≳ 1 mW), the highest values for the degree of polarization are no more observed for the highest pump powers. Indeed, we show that exists an optimum pump power leading to a maximum degree of polarization.

Soliton explosion control by higher-order effects

We numerically study the impact of self-frequency shift, self-steepening, and third-order dispersion on the erupting soliton solutions of the quintic complex Ginzburg-Landau equation. We find that the pulse explosions can be completely eliminated if these higher-order effects are properly conjugated two by two. In particular, we observe that positive third-order dispersion can compensate the self-frequency shift effect, whereas negative third-order dispersion can compensate the self-steepening effect. A stable propagation of a fixed-shape pulse is found under the simultaneous presence of the three higher-order effects.

Analysis of femtosecond optical soliton amplification in fiber amplifiers

Abstract Adiabatic soliton trapping is observed due to the frequency-dependent gain in fiber amplifiers. The soliton pulse width is practically unaffected by the nonresonance of the carrier frequency, while the soliton self-frequency shift (SSFS) becomes very sensitive to this nonresonance. It is shown that the effective component of the gain spectrum for the SSFS compensation is the linearly frequency dependent gain with a positive slope, and an optimal compensation condition is identified. The stability of the steady-state solution for the perturbed nonlinear Schrodinger equation is also studied using the phase-plane formalism.

Stable soliton propagation in a system with spectral filtering and nonlinear gain

Stable soliton propagation in a system with linear and nonlinear gain and spectral filtering is investigated. Different types of exact analytical solutions of the cubic and the quintic complex Ginzburg-Landau equation (CGLE) are reviewed. The conditions to achieve stable soliton propagation are analyzed within the domain of validity of soliton perturbation theory. We derive an analytical expression defining the region in the parameter space where stable pulselike solutions exist, which agrees with the numerical results obtained by other authors. An analytical expression for the soliton amplitude corresponding to the quintic CGLE is also obtained. We show that the minimum value of this amplitude depends only on the ratio between the linear gain and the quintic gain saturating term.Stable soliton propagation in a system with linear and nonlinear gain and spectral filtering is investigated. Different types of exact analytical solutions of the cubic and the quintic complex Ginzburg-Landau equation (CGLE) are reviewed. The conditions to achieve stable soliton propagation are analyzed within the domain of validity of soliton perturbation theory. We derive an analytical expression defining the region in the parameter space where stable pulselike solutions exist, which agrees with the numerical results obtained by other authors. An analytical expression for the soliton amplitude corresponding to the quintic CGLE is also obtained. We show that the minimum value of this amplitude depends only on the ratio between the linear gain and the quintic gain saturating term.

Nonlinear effects in optical fibers: limitations and benefits

Nonlinear effects in optical fibers impose different limitations on the communications link, and an understanding of such effects is almost a prerequisite for actual lightwave-system designers. On the other hand, they offer a variety of possibilities for all-optical signal processing, amplification and regeneration. The nonlinear effects are enhanced dramatically and new phenomena are observed in the so called photonic crystal fibers. In this paper we review the effects - both detrimental and potentially beneficial - of optical nonlinearities in conventional and in photonic crystal fibers.

Optical Solitons in Fibers for Communication Systems

Abstract A brief history of attempts to use solitons for optical fiber communications and of the technical developments toward making soliton transmission practical is reviewed. The main problems affecting long-distance soliton transmission, as well as techniques to solve them, are described. Recent developments concerning the use of dispersion management and some of the remarkable properties of dispersion managed solitons are also discussed.

QBER Estimation in QKD Systems With Polarization Encoding

A model for quantum BER estimation in polarization encoded quantum key distribution systems is presented. Both TDM and WDM based polarization control schemes are analyzed. It is shown that TDM presents some important advantages when compared with the WDM control scheme. In WDM, the polarization decorrelation between the reference and data signals is an intrinsic and very limitative impairment. This effect has a contribution to quantum BER that increases with the propagation distance, and is highly dependent on the fiber polarization mode dispersion. In the TDM control scheme, the polarization decorrelation is less critical and other issues, like the single photon detector and feedback polarization control system performance tend to dominate. We show that for long distances the fiber losses represent the main contribution to the total quantum BER. Nevertheless, for distances shorter than 70 km and frequencies higher than 5 MHz the after pulse detections provide an important contribution to the total quantum BER.

Uniform Polarization Scattering With Fiber-Coil-Based Polarization Controllers

A detailed study of fiber-coil-based polarization controllers (PCs) is performed. First, a method to deterministically calculate the PC configuration in order to transform between any two states of polarization is presented. In a second stage, the case in which the configuration angles are randomly changed is studied. The cases of a single PC and of the system obtained with the concatenation of several PCs are analyzed. For both cases, a general expression for the variance of the Stokes parameters is obtained. Using this expression, it is demonstrated that it is possible to achieve uniform polarization scattering using a concatenation of fiber-coil-based PCs. Finally, it is shown that fiber-coil-based PCs can be used to emulate both first- and second-order polarization-mode dispersions

Timing jitter in soliton transmission with up- and down-sliding-frequency guiding filters

We derive an analytical expression for the variance of the timing jitter of a soliton transmission system using sliding-frequency guiding filters, taking into account the third-order filter term. An improved analytical result for the upper limit of the sliding rate for stable soliton propagation is also obtained. We show that the variance of timing jitter is significantly increased by the sliding action. As a consequence of the third-order filter contribution, the timing jitter is lower in a system with down-sliding compared with the up-sliding regime at the same sliding rate.