Gil M. Fernandes

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.

Tunable Mode Conversion Using Acoustic Waves in Optical Microwires

We propose a tunable inline mode converter based on the acousto-optic effect in optical microwires. A theoretical model is developed to analyze the interaction between the acoustic waves and the optical modal coupling in microwires. We show that adjusting the acoustic wave frequency we can switch between different optical modes. The conversion efficiency can be adjusted through the acoustic wave amplitude.

Switching in multicore fibers using flexural acoustic waves.

We propose an in-line wavelength selective core switch for multicore fiber (MCF) transmission systems, based on the acousto-optic effect. A theoretical model addressing the interaction between flexural acoustic waves and the optical signal in MCFs is developed. We show that an optical signal propagating in a particular core can be switched to any other core or distributed over all the cores. By tuning the acoustic wave amplitude, we can adjust the amount of optical power transferred between the cores.

Halting the fuse discharge propagation Using optical fiber microwires

We report and analyze the halting of the fuse effect propagation in optical fiber microwires. The increase of the mode field diameter in the tapered region decreases the optical intensity resulting in the extinction of the fuse effect. This fiber element presents a low insertion loss and can be introduced in the optical network in order to protect the active equipment from the damage caused by the fuse effect.

Theoretical Analysis of Multimodal Four-Wave Mixing in Optical Microwires

Optical fiber microwires (OFMs) are nonlinear optical waveguides that support several spatial modes. The multimodal generalized nonlinear Schrödinger equation (MM-GNLSE) is deduced taking into account the linear and nonlinear modal coupling. A detailed theoretical description of four-wave mixing (FWM) considering the modal coupling is developed. Both, the intramode and the intermode phase-matching conditions is calculated for an optical microwire in a strong guiding regime. Finally, the FWM dynamics is studied and the amplitude evolution of the pump beams, the signal and the idler are analyzed.

Continuous wave supercontinuum generation pumped in the normal group velocity dispersion regime on a highly nonlinear fiber

We have obtained spectral broadening by pumping a nonmicrostructured highly nonlinear fiber with a continuous wave signal from a Raman fiber laser. The experiment was simulated using a generalized Schrodinger equation containing the actual Raman response of the fiber as calculated from the experimental Raman gain. A different input-noise model, that reproduces well the power spectral density of the laser, was used and compared with others previously proposed.

Continuous wave supercontinuum generation aided by a weaker pulse laser

The supercontinum generation has been achieved mainly by two different approaches, namely, with femtosecond intense pulses or using a continuous wave laser or larger pulses centered on the anomalous dispersion region. In order to improve temporal coherence, it has been suggested the introduction of a pulse seed or the propagation of both a large pulse pump and a small weaker continuous wave to control the soliton fission. Here we propose supercontinuum generation using a hybrid input, we pump with a continuous laser and copropagate a picosecond signal. We compare the bandwidth of the supercontinuum using only the continuous pump or the hybrid setup. Simulations of the generalized Schrodinger equation, using an adequate input-noise model to reproduce the spectrum of the continuous signal, are performed in order to investigate the supercontinuum generation in the optical communication window under different dispersion regimes.

Dispersion engineered high index contrast silica microfibers

We report simulation results regarding the dispersion control in silica based microfibers, with subwavelength core radii. The results cover cylindrical waveguides obtained by tapering down a standard silica based optical fiber. As a strongly index mismatched clad layer, we study a polymer/resin with an refractive index tunable around 1.38 (through temperature or UV irradiation) or composed of the surrounding air. By analyzing a two layer vector model, we show how the dispersive properties of the waveguide, namely the zero dispersion wavelength and its slope, can be effectively engineered.

Advanced digital signal processing techniques based on Stokes space analysis for high-capacity coherent optical systems

Stokes space based digital signal processing (DSP) techniques can improve polarization demultiplexing, polarization dependent losses compensation, and other polarization-related tasks in coherent receivers, particularly in terms of convergence speed and transparency to higher-level M-ary modulated signals. This paper reports recent advances on Stokes space based DSP for coherent optical communications. We study and discuss the performance the polarization demultiplexing algorithm in terms of convergence speed and computational complexity, and present a new space-demultiplexing algorithm based on signal analysis in higher-order Poincaré spheres for spatial division multiplexing transmission systems. Moreover, practical issues regarding the real-time implementation on FPGAs of Stokes space based algorithms for flexible optical communications are also investigated and discussed.

MIMO processing based on higher-order Poincaré spheres

A multi-input multi-output (MIMO) algorithm based on higher-order Poincaré spheres is demonstrated for space-division multiplexing (SDM) systems. The MIMO algorithm is modulation format agnostic, robust to frequency offset and does not require training sequences. In this approach, the space-multiplexed signal is decomposed in sets of two tributary signals, with each set represented in a higher-order Poincaré sphere. For any arbitrary complex modulation format, the samples of two tributaries can be represented in a given higher-order Poincaré sphere with a symmetry plane. The crosstalk along propagation changes the spatial orientation of this plane and, therefore, it can be compensated by computing and realigning the best fit plane. We show how the transmitted signal can be successfully recovered using this procedure for all possible combinations of tributaries. Moreover, we analyze the convergence speed for the MIMO technique considering several optical-to-noise ratios.