Silvia M. Pietralunga

Polarization Stabilization in Optical Communications Systems

The control of the state of polarization (SOP) of light remains one of the open issues in optical communications. In particular, the achievement of a stabilization of the SOP can find many applications in advanced optical communication systems: from the mitigation of polarization-mode dispersion to the development of novel multilevel modulation formats. In this paper, theoretical and experimental aspects of polarization stabilization are dealt with, and a novel algorithm to overcome the issues related to the practical availability of finite-range birefringent components and to solve the requirement for endless stabilization is also presented. A complete analysis of the control algorithm, based on the Jones matrix formalism, is also presented. The practical implementation of the polarization stabilizer is discussed, and experimental demonstrations based on liquid crystal and magnetooptical retarders are shown

Demonstration of net gain at 1550nm in an erbium-doped polymersingle mode rib waveguide

A polymer-based waveguide optical amplifier doped with Er3+ ions has been modeled, fabricated, and characterized. Propagation losses have been measured using both cutback and Fabry-Perot methods, resulting in similar, reasonable loss values that do not exceed 1.31dBcm−1 at 1540nm. By accounting for overall propagation loss, a net gain of 1.34dB at 1540nm is demonstrated for a 1.6cm long single mode waveguide amplifier.

Titania inverse opals for infrared optical applications

Photonic crystals have gathered great importance in recent years. In particular macroporous materials (inverse opals) show interesting properties as photonic crystals. Ordered macroporous titanium dioxide (TiO2) is made using polystyrene spheres as a template. Titania is chosen for its high refractive index (>2.5). Following an already known technique [E.G. Judith, J. Wijnhoven, W.L. Vos, Science 281 (1998) 802; B.T. Holland, C.F. Blanford, A. Stein, Science 281 (1998) 538; B.T. Holland, C.F. Blanford, T. Do, A. Stein, Chem. Mater. 11 (1999) 795] large-scale order in macroporous TiO2 is obtained both using the 778 and the 3190 nm beads as documented by optical microscope and SEM images. These structures would lead to applications in the mid-infrared range.

Enhancement of PMMA nonlinear optical properties by means of a quinoid molecule

Abstract In this paper we present the study of the linear and nonlinear optical properties of organic films based on mixing a polymeric matrix (PMMA) and a highly nonlinear organic material recently synthesised and characterised, namely (5,5 ′ -Bis(3,5-di- tert -butyl-4-oxo-2,5-cyclohexadien-1-ylidene)-5,5 ′ -dihydro-2,2 ′ -bithiophene), with the prospective of reaching a good compromise between good processability and high nonlinear optical (NLO) properties of the solid solution. Linear optical characterisation is performed by means of variable angle spectroscopic ellipsometry in the spectral range between 260 and 1700 nm. Third order NLO properties of the samples have been investigated by third harmonic generation at fundamental wavelength of 1500 nm. Different guest/host concentrations have been studied. Due to the high solubility of the guest into the polymeric matrix, we have been able to reach concentration up to 50% in weight, reaching an enhancement of the NLO properties of PMMA by three order of magnitudes.

Sputtered stoichiometric TeO2 glass films: Dispersion of linear and nonlinear optical properties

We report on the optical characterization of TeO2 amorphous films deposited by radio-frequency (rf) reactive sputtering. X-ray diffraction, electronic microscopy, and Rutherford backscattering analysis have been performed in order to assess the structural and compositional properties of the samples. The linear optical characterization has been performed by variable angle spectroscopic ellipsometry at wavelengths between 260 and 1700 nm, leading to determination of the dispersion of the complex refractive index for sputtered tellurium dioxide. In the spectral range of transparency, between 1700 and 500 nm, refractive index values ranging from 2.05 to 2.16 have been obtained. The dispersion of the complex third-order nonlinear optical coefficient of the sample has been evaluated in the near-infrared spectral range by third harmonic generation measurements. The out-of-resonance value of the χ(3) coefficient for TeO2 is in the range between 1.2×10−12 and 1.4×10−12 esu. The conjugation of optical transparency,...

CdTe:In MONOCRYSTAL MODULES FOR ALL-OPTICAL PROCESSING

The aim of this work is to present the application of photoconductive properties of electro-optical n-doped CdTe:In single crystals to all-optical processing of signals at λ=1550 nm. The mechanism exploits the photogeneration of charge carriers from deep intragap energy levels and their trap-limited drift process under the action of an externally applied cw electric field. An internal space-charge counterfield results that locally shields the electro-optic effect. Different types of free-space architectural elementary modules in CdTe:In have been implemented. A non-coherent wavelength converter, a switching device, a sampler and a time-to-space converter are presented. The nanosecond regime has been reached.

Time-resolved photocurrent and electric field measurements in high resistivity CdTe

We studied current and electric field transients in high resistivity CdTe:In crystals by means of nanosecond time-resolved photocurrent and electro-optic sampling measurements. Electron and hole dynamics have been investigated for different excitation regimes (incident photon fluence and wavelength). A numerical solution for a drift-diffusion model in presence of several defect centers is given which allows to interpret experimental results. Current decay can be described by two time constants of about 10 and 100 ns, the first one being predominant for low incident photon energy (<1 eV). By comparison with the numerical model, the fast and slow time constants are related to hole and electron trapping, respectively. Electric field decays in a millisecond time scale. This is related to charge trapped in a deep recombination center located at 0.75 eV from the conduction band.

Tungsten oxide nanowires grown on amorphous-like tungsten films.

Tungsten oxide nanowires have been synthesized by vacuum annealing in the range 500-710 °C from amorphous-like tungsten films, deposited on a Si(100) substrate by pulsed laser deposition (PLD) in the presence of a He background pressure. The oxygen required for the nanowires formation is already adsorbed in the W matrix before annealing, its amount depending on deposition parameters. Nanowire crystalline phase and stoichiometry depend on annealing temperature, ranging from W18O49-Magneli phase to monoclinic WO3. Sufficiently long annealing induces the formation of micrometer-long nanowires, up to 3.6 μm with an aspect ratio up to 90. Oxide nanowire growth appears to be triggered by the crystallization of the underlying amorphous W film, promoting their synthesis at low temperatures.

Direct observation of local birefringence and axis rotation in spun fiber with centimetric resolution

By means of the centimetric resolution of the developed cut-back method, we have directly observed a rotation of the birefringence axis and a sinusoidal evolution of the local retardation along spun fiber samples, confirming theoretical predictions regarding how the spinning process acts in reducing the internal fiber birefringence.

Modification of local stress-induced birefringence in low-PMD spun fibers evaluated by high-resolution optical tomography

Photoelastic tomography is exploited to investigate the effect of the spinning process on residual stress development in spun fibers. High-spatial-resolution measurements of stress profiles in spun and corresponding unspun fibers are compared. The resulting differences correspond to a reduction of the intrinsic linear birefringence as a consequence of an applied constant spinning.