H.L. Fragnito

Highly efficient generation of broadband cascaded four-wave mixing products

We propose a novel way to efficiently generate broadband cascaded Four-Wave Mixing (FWM) products. It consists of launching two strong pump waves near the zero-dispersion wavelength of a very short (of order a few meters) optical fiber. Simulations based on Split Step Fourier Method (SSFM) and experimental data demonstrate the efficiency of our new approach. Multiple FWM products have been investigated by using conventional fibers and ultra-flattened dispersion photonic crystal fibers (UFD-PCFs). Measured results present bandwidths of 300 nm with up to 118 FWM products. We have also demonstrated a flat bandwidth of 110 nm covering the C and L bands, with a small variation of only 1.2 dB between the powers of FWM products, has been achieved using highly nonlinear fibers (HNLFs). The use of UFD-PCFs has been shown interesting for improving the multiple FWM efficiency and reducing the separation between the pump wavelengths.

Spectral narrowing in the propagation of chirped pulses in single-mode fibers.

A theoretical study of the nonlinear propagation of picosecond chirped pulses in single-mode fibers is presented. We show that, under appropriate conditions, spectral narrowing-rather than broadening, as is generally believed-is induced, owing to the interplay of self-phase-modulation and dispersion. For downchirped pulses at a wavelength of 0.9 microm and a peak power as low as 0.1 W, substantial spectral narrowing occurs.

Experimental and numerical investigation of the SBS-threshold increase in an optical fiber by applying strain distributions

We study, experimentally and numerically, the increase of the stimulated Brillouin scattering (SBS) threshold in dispersion-shifted fibers (DSFs) by applying three different tensile-strain distributions. The best results are obtained with a 40-step stair-ramp distribution, for which we demonstrate a 8-dB SBS threshold increase in a 580-m DSF. The Brillouin frequency is observed to shift as a function of the strain at a rate of 0.464 GHz/%. We discuss the potentials and drawbacks of this technique for application in nonlinear devices.

Raman-like light scattering from acoustic phonons in photonic crystal fiber

Raman and Brillouin scattering are normally quite distinct processes that take place when light is resonantly scattered by, respectively, optical and acoustic phonons. We show how few-GHz acoustic phonons acquire many of the same characteristics as optical phonons when they are tightly trapped, transversely and close to modal cut-off, inside the wavelength-scale core of an air-glass photonic crystal fiber (PCF). The result is an optical scattering effect that closely resembles Raman scattering, though at much lower frequencies. We use photoacoustic techniques to probe the effect experimentally and finite element modelling to explain the results. We also show by numerical modelling that the cladding structure supports two phononic band gaps that contribute to the confinement of sound in the core.

Coherent control of ultra-high frequency acoustic resonances in photonic crystal fibers

Acoustic resonances trapped within the glass core (1 mum diameter) of a photonic crystal fibre are excited electrostrictively using laser pulses. Using pulse sequences they can be coherently controlled leading to a 100-fold increase in their amplitude.

Spectrally flat and broadband double-pumped fiber optical parametric amplifiers

We study theoretically and experimentally spectrally flat and broadband double-pumped fiber-optical parametric amplifiers (2P-FOPAs). Closed formulas are derived for the gain ripple in 2P-FOPAs as a function of the pump wavelength separation and power, and the fiber non-linearity and fourth order dispersion coefficients. The impact of longitudinal random variations of the zero dispersion wavelength (lambda(0)) on the gain flatness is investigated. Our theoretical findings are substantiated with experiments using conventional dispersion shifted fibers and highly nonlinear fibers (HNLFs). By using a HNLF having a low variation of lambda0 we demonstrate high gain and flat spectrum (25 +/- 1.5 dB) over 115 nm.

PROBING OF THE QUANTUM DOT SIZE DISTRIBUTION IN CDTE-DOPED GLASSES BY PHOTOLUMINESCENCE EXCITATION SPECTROSCOPY

We studied confinement effects in CdTe quantum dots by means of photoluminescence excitation spectroscopy. We show that by changing the detection energy we can resolve the spectrum of quantum dots of different sizes inside their much broader size distribution in CdTe‐doped glass. The spectra obtained show several well‐resolved lines. There is excellent agreement between the photoluminescence excitation spectra peak energies and calculations of the confined energy transitions based on a modified multiband envelope function model.

Simple method for measuring dispersion and nonlinear coefficient near the zero dispersion wavelength of optical fibers

We propose a fast and accurate method for measuring the fiber zero-dispersion wavelength, the dispersion slope and the nonlinear coefficient, which are the relevant parameters needed to evaluate the impact of fiber nonlinearity on transmission systems. The method is based on modulation instability sideband generation and uses a single continuous-wave tunable laser and a spectrum analyzer. For the dispersion parameters, our results are in very good agreement with those obtained from commercial instruments.

Modulation instability induced resonant four-wave mixing in WDM systems

We present an experimental observation of modulation instability (MI) impairments on a two-channel wavelength-division-multiplexed (WDM) system at 2.5 Gb/s per channel repeaterless link comprising 100 km of dispersion shifted fiber. When the channel spacing is close to the modulation instability frequency, channel depletion due to four-wave-mixing (FWM) interactions is resonantly enhanced and becomes the main source of system performance degradation. Numerical simulations agree well with the experiment and confirm that this effect dominates over the well known effect of MI amplification of noise. We discuss the influence of MI-induced resonant FWM as a function of channel spacing on WDM systems operating in the anomalous dispersion regime.

Double-pumped fiber optical parametric amplifier with flat gain over 47-nm bandwidth using a conventional dispersion-shifted fiber

We investigate the performance of conventional dispersion-shifted fibers (DSFs) as nonlinear medium in double-pumped fiber optical parametric amplifiers (2P-FOPAs). We report on high gain (37 dB) with low ripple (/spl plusmn/1.5 dB) over a 47-nm bandwidth using these fibers. We show that the 2P-FOPA average gain and the gain uniformity are limited by longitudinal variations of the zero dispersion wavelength and by polarization-mode dispersion. We demonstrate that 2P-FOPAs constructed with DSFs exhibit negligible crosstalk even for high gains (40 dB) and large output signal powers (12.5 dBm) in a ten-channel system.