Nelson J. Muga

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.

Digital PDL Compensation in 3D Stokes Space

A Stokes space based technique for digital polarization-dependent losses (PDL) monitoring and compensation is proposed. The channel PDL monitoring is carried out with a resolution of 0.15 dB by computing the mean point of the signal samples represented in the three-dimensional Stokes space. The presented compensation technique is based on rotations and translations of the signal samples in the Stokes space, and provides a PDL compensation up to 2.5 dB. Error vector magnitude results show a gain of about 3 dB (assuming a PDL value equal to 1.5 dB) to a Stokes space based polarization demultiplexing technique without PDL compensation.

Adaptive 3-D Stokes Space-Based Polarization Demultiplexing Algorithm

We propose a novel polarization demultiplexing technique for complex modulated signals using the Stokes space polarization representation of the signal samples. Polarization demultiplexing of an endless time-varying SOP signal is achieved through the adaptive estimation of the inverse rotation matrix, without the calculation of a best fitting plane. Simulation results show the effectiveness of the proposed technique, with a high convergence speed, and a good response to time-varying SOP rotations. The performance of the method is assessed through EVM calculations for frequency rotations up to 15 μrad/symbol, showing a maximum penalty of 1 dB. Moreover, the technique can incorporate an additional stage for PDL compensation without penalties in terms of EVM results and algorithm complexity.

Effective Nonlinear Parameter Measurement Using FWM in Optical Fibers in a Low Power Regime

The four-wave mixing (FWM) process in a low power regime is studied both theoretically and experimentally. The coupled-equations for the complex amplitudes are derived and solved. The proposed model is compared with experimental data. Results shows the need of considering both nonlinear and polarization dependent effects in order to obtain an accurate description of the four-wave mixing process in a low power regime. The effective nonlinear parameter is experimentally measured in a dispersion-shifted fiber, and the transition region between an almost co-polarized situation to a decorrelated state of polarization is observed.

Influence of the Stimulated Raman Scattering on the Four-Wave Mixing Process in Birefringent Fibers

The influence of the stimulated Raman scattering in the four-wave mixing process in high birefringent fibers is theoretically analyzed. We consider the dual pump configuration in both co-polarized and orthogonal polarization schemes and treat simultaneously the stimulated Raman scattering and the four-wave mixing processes. The obtained results are valid even when the shift between the pumps and signal lies inside the Raman band. Results show that when the phase-matching condition is achieved the generation of the idler wave is mostly dependent on the real part of the fiber nonlinear response function, whereas the amplification of the signal wave is mostly dependent on the imaginary part. Results also show that when the phase-matching condition is satisfied the optical power evolution of the signal and idler waves for small frequency detunings are mostly described by the anisotropic Raman response of the fiber, whereas for high frequency detunings the isotropic response dominates. Our analysis also shows that the stimulated Raman scattering can increase the efficiency of the four-wave mixing process, for certain frequency detunings.

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

Polarization Mode Dispersion in High-Speed Optical Communication Systems

Abstract We review the research progress concerning some fundamental issues related to polarization-mode dispersion (PMD) in high-speed fiber-optic transmission systems. We pay particular attention to issues such as the PMD-induced pulse broadening, PMD measurement and emulation, as well as PMD compensation. An electrical equalization technique based on a transversal filter and an optical technique based on a nonlinear chirped fiber Bragg gratings for PMD compensation will be discussed.

Experimental Assessment of the Adaptive Stokes Space-Based Polarization Demultiplexing for Optical Metro and Access Networks

We experimentally analyze an adaptive polarization-demultiplexing (PolDemux) technique based on the representation of the state of polarization of the signal in the Stokes space. The performance and convergence speed of the Stokes PolDemux are compared with the constant modulus algorithm, for dual polarization QPSK and 16QAM modulation formats in a system with a maximum polarization rotation of 6 kHz. Moreover, the applicability of the adaptive Stokes algorithm is tested in an ultradense wavelength-division multiplexing scenario over 80 km of standard single-mode fiber. The BER results show a very small power penalty confirming the suitability of the adaptive Stokes method in systems where the accumulated chromatic and polarization-mode dispersion are small, such as high capacity optical metro and access networks scenarios.

Exchange bias of MnPt/CoFe films prepared by ion beam deposition

We report on exchange bias of Mn100−xPtx/Co90Fe10 bottom-pinned bilayers prepared by ion beam deposition. The Pt content in the film was varied 6 at. % with x between 46 and 52 at. % by changing the angle of substrate relative to the target in a range of 40°. Exchange coupling for a Mn100−xPtx(20 nm)/Co90Fe10(5 nm) bilayer was found to be maximum (Jex=0.34 erg/cm2) at a composition with x=50 at. %. Structure and magnetic properties of the bilayers deposited on Ta, Ta/Ru, and Ta/Ni81Fe19 seed layers were studied by vibrating sample magnetometry, x-ray diffraction, Rutherford backscattering spectroscopy, and atomic force microscopy. Exchange bias and coercive field can be tuned as functions of the seed layer. The effect of different annealing conditions on exchange bias is discussed to improve thermal stability of the bilayers. Maximum exchange coupling is obtained at an annealing temperature TA⩾280 °C, while the blocking temperature TB keeps increasing with annealing up to TA=420 °C.