Label-free highly multimodal nonlinear endoscope

We demonstrate a 2 mm diameter highly multimodal nonlinear micro-endoscope allowing label-free imaging of biological tissues. The endoscope performs multiphoton fluorescence (3-photon, 2-photon), harmonic generation (second-SHG and third-THG) and coherent anti-Stokes Raman scattering (CARS) imaging over a field of view of 200 µm. The micro-endoscope is based on a double-clad antiresonant hollow core fiber featuring a high transmission window (850 nm to 1800 nm) that is functionalized with a short piece of graded-index (GRIN) fiber. When combined with a GRIN micro-objective, the micro-endoscope achieves a 1.1 µm point spread function (PSF). We demonstrate 3-photon, 2-photon, THG, SHG, and CARS high resolution images of unlabelled biological tissues.

Origin of spontaneous wave mixing processes in multimode GRIN fibers

We show that geometric parametric instability (GPI) in graded-index multimode fibers is strongly influenced by higher-order dispersion. By measuring the output spectrum for different core radii, we distinguish peaks generated by GPI from other coexisting parametric processes using phase-matching arguments and numerical simulations. We highlight for the first time a non-degenerate GPI process involving two pumps at different wavelengths.

Double clad tubular anti-resonant hollow core fiber for nonlinear microendoscopy

We report the fabrication and characterization of the first double clad tubular anti-resonant hollow core fiber. It allows to deliver ultrashort pulses without temporal nor spectral distortions in the 700-1000 nm wavelength range and to efficiently collect scattered light in a high numerical aperture double clad. The output fiber mode is shaped with a silica microsphere generating a photonic nanojet, making it well suitable for nonlinear microendoscopy application. Additionally, we provide an open access software allowing to find optimal drawing parameters for the fabrication of tubular hollow core fibers.

Collision between a dark soliton and a linear wave in an optical fiber

We report an experimental observation of the collision between a linear wave propagating in the anomalous dispersion region of an optical fiber and a dark soliton located in the normal dispersion region. This interaction results in the emission of a new frequency component whose wavelength can be predicted using phase-matching arguments. The measured efficiency of this process shows a strong dependency with the soliton grayness and the linear wave wavelength, and is in a good agreement with theory and numerical simulations.

Two-dimensional nonlinear modes and frequency combs in bottle microresonators

We theoretically investigate frequency comb generation in a bottle microresonator accounting for the azimuthal and axial degrees of freedom. We first identify a discrete set of the axial nonlinear modes of a bottle microresonator that appear as tilted resonances bifurcating from the spectrum of linear axial modes. We then study azimuthal modulational instability of these modes and show that families of two-dimensional (2D) soliton states localized both azimuthally and axially bifurcate from them at critical pump frequencies. Depending on detuning, 2D solitons can be stable, form persistent breathers or chaotic spatio-temporal patterns, or exhibit collapse-like evolution.

Grayness-dependent emission of dispersive waves from dark solitons in optical fibers

We report the experimental observation of dispersive wave emission from gray solitons propagating in the normal dispersion region of an optical fiber. Besides observing for the first time, to the best of our knowledge, the emission of a dispersive wave from an isolated dark soliton, we show that the dispersive wave frequency and amplitude strongly depend on soliton grayness. This process can be explained by the higher-order dispersion contribution into the phase-matching condition and the grayness of the soliton. Numerical simulations and theoretical predictions are in good agreement with the experiments.

Towards two-photon excited endogenous fluorescence lifetime imaging microendoscopy

In situ fluorescence lifetime imaging microscopy (FLIM) in an endoscopic configuration of the endogenous biomarker nicotinamide adenine dinucleotide (NADH) has a great potential for malignant tissue diagnosis. Moreover, two-photon nonlinear excitation provides intrinsic optical sectioning along with enhanced imaging depth. We demonstrate, for the first time to our knowledge, nonlinear endogenous FLIM in a fibered microscope with proximal detection, applied to NADH in cultured cells, as a first step to a nonlinear endomicroscope, using a double-clad microstructured fiber with convenient fiber length (> 3 m) and excitation pulse duration (≈50 fs). Fluorescence photons are collected by the fiber inner cladding and we show that its contribution to the impulse response function (IRF), which originates from its intermodal and chromatic dispersions, is small (< 600 ps) and stable for lengths up to 8 m and allows for short lifetime measurements. We use the phasor representation as a quick visualization tool adapted to the endoscopy speed requirements.

Modulation instability in the weak normal dispersion region of passive fiber ring cavities

We report the experimental observation of modulation instability in the weak normal dispersion region of a passive fiber ring cavity. We show that the fourth-order dispersion strongly modifies the dynamics of the cavity through the generation of new instability bands. Experimental results are in excellent agreement with theoretical predictions and numerical simulations.

Longitudinal soliton tunneling in optical fiber

We report the observation of the longitudinal soliton tunneling effect in axially varying optical fibers. A fundamental soliton, initially propagating in the anomalous dispersion region of a fiber, can pass through a normal dispersion barrier without being substantially affected. We perform experimental studies by means of spectral and temporal characterizations that show the evidence of the longitudinal soliton tunneling process. Our results are well supported by numerical simulations using the generalized nonlinear Schrödinger equation.

Spectral wings of the fiber supercontinuum and the dark-bright soliton interaction

We present experimental and numerical data on the supercontinuum generation in an optical fiber pumped in the normal dispersion range where the seeded dark and the spontaneously generated bright solitons contribute to the spectral broadening. We report the dispersive radiation arising from the interaction of the bright and dark solitons. This radiation consists of the two weak dispersing pulses that continuously shift their frequencies and shape the short and long wavelength wings of the supercontinuum spectrum.

Dynamics of Turing and Faraday instabilities in a longitudinally modulated fiber-ring cavity

We experimentally investigate the round-trip-to-round-trip dynamics of the modulation instability spectrum in a passive fiber-ring cavity presenting an inhomogeneous dispersion profile. By implementing a real-time spectroscopy technique, we are able to record successive single-shot spectra, which display the evolution of the system toward a stationary state. We find that the two instability regimes (Turing and Faraday) that compete in this kind of inhomogeneous cavity not only differ by their characteristic frequency but also by their dynamical behavior. The dynamic transition between those two regimes of instability is also presented.

Spectral and fluorescence lifetime endoscopic system using a double-clad photonic crystal fiber

We present a customized small-core double-clad photonic crystal fiber for spectral and fluorescence lifetime measurements of human samples. In this Letter, the new fiber has been characterized on different fluorophores and samples of human brain tumor; a comparison to a bi-fiber homemade system and a commercial fiber probe was made.

Heteroclinic Structure of Parametric Resonance in the Nonlinear Schrödinger Equation

We show that the nonlinear stage of modulational instability induced by parametric driving in the defocusing nonlinear Schrödinger equation can be accurately described by combining mode truncation and averaging methods, valid in the strong driving regime. The resulting integrable oscillator reveals a complex hidden heteroclinic structure of the instability. A remarkable consequence, validated by the numerical integration of the original model, is the existence of breather solutions separating different Fermi-Pasta-Ulam recurrent regimes. Our theory also shows that optimal parametric amplification unexpectedly occurs outside the bandwidth of the resonance (or Arnold tongues) arising from the linearized Floquet analysis.

Characterization of fiber ultrashort pulse delivery for nonlinear endomicroscopy

In this work, we present a detailed characterization of a small-core double-clad photonic crystal fiber, dedicated and approved for in vivo nonlinear imaging endomicroscopy. A numerical and experimental study has been performed to characterize the excitation and collection efficiencies through a 5 m-long optical fiber, including the pulse duration and spectral shape. This was first done without any distal optics, and then the performances of the system were studied by using two kinds of GRIN lenses at the fiber output. These results are compared to published data using commercial double clad fibers and GRIN lenses.

Emission of dispersive waves from a train of dark solitons in optical fibers

We report the experimental observation of multiple dispersive waves (DWs) emitted in the anomalous dispersion region of an optical fiber from a train of dark solitons. Each DW can be associated to one dark soliton of the train, using phase-matching arguments involving higher-order dispersion and soliton velocity. For a large number of dark solitons (>10), we observe the formation of a continuum associated with the efficient emission of DWs.

Shock wave generation triggered by a weak background in optical fibers

We experimentally report the observation of dispersive shock waves from a short pulse superimposed onto a small continuous wave background in optical fibers. We show that the background allows us to strongly enhance the extension and contrast of the oscillatory wave train inherent to the dispersive shock. More than seven periods of oscillations with high contrast are observed experimentally and confirmed with numerical simulations. The dynamics of the process are simply explained from spectro-temporal representations.

Solitonization of a dispersive wave

We report the observation of a nonlinear propagation scenario in which a dispersive wave is transformed into a fundamental soliton in an axially varying optical fiber. The dispersive wave is initially emitted in the normal dispersion region and the fiber properties change longitudinally so that the dispersion becomes anomalous at the dispersive wave wavelength, which allows it to be transformed into a soliton. The solitonic nature of the field is demonstrated by solving the direct Zakharov-Shabat scattering problem. Experimental characterization performed in spectral and temporal domains show evidence of the solitonization process in an axially varying photonic crystal fiber.

Optimal frequency conversion in the nonlinear stage of modulation instability

We investigate multi-wave mixing associated with the strongly pump depleted regime of induced modulation instability (MI) in optical fibers. For a complete transfer of pump power into the sideband modes, we theoretically and experimentally demonstrate that it is necessary to use a much lower seeding modulation frequency than the peak MI gain value. Our experiment shows that, at such optimal modulation frequency, a record 95 % of the output pump power is frequency converted into the comb of sidebands, in good quantitative agreement with analytical predictions based on the simplest exact breather solution of the nonlinear Schrodinger ¨ equation.

Enhanced structural sensitivity of hybrid-mode acoustic phonons in axially-varying photonic crystal fiber

We report the observation of anti-crossings between hybrid-mode acoustic phonons in an axially-varying photonic crystal fiber. Our experimental results are analyzed using an electrostriction theory which reveals strong coupling between longitudinal and shear components of elastic wave. These anti-crossings are highly sensitive to the transverse fiber structure and thus could be potentially used for ultra-sensitive sensors and new opto-acoustic devices.

Bouncing of a dispersive wave in a solitonic cage

We report the experimental observation of a weak dispersive wave trapped within a cage formed by two solitons in an optical fiber. We show that the dispersive wave bouncing is accompanied by a back-and-forth wavelength conversion of the probe to an idler wave. Additionally, we observed the destruction of the soliton cage when the dispersive wave power is increased, leading to the collision of the solitons.

Observation of the stepwise blue shift of a dispersive wave preceding its trapping by a soliton

The trapping of a weak dispersive wave by an intense soliton is a complex process occurring at the early stage of supercontinuum generation. It is theoretically predicted to arise from multiple soliton-dispersive wave interactions, producing a stepwise frequency blue shift of the dispersive wave. We report here the first experimental evidence of this frequency blue shift using a tapered fiber which acts as a prism, allowing to disperse the blue spectral components in order to identify unambiguously each soliton-dispersive wave collision.

A two-stage photonic crystal fiber / silicon photonic wire short-wave infrared wavelength converter/amplifier based on a 1064 nm pump source

We demonstrate a two-stage wavelength converter that uses compact near-infrared sources to amplify and convert short-wave infrared signals. The first stage consists of a photonic crystal fiber wavelength converter pumped by a Q-switched 1064 nm pump source, while the second stage consists of a silicon photonic wire waveguide wavelength converter. The system enables on-chip amplification and conversion of up to 30 dB . We demonstrate amplification in a broad wavelength range around 2344 nm using temporally long pulses (>300ps).

Soliton annihilation into a polychromatic dispersive wave

We investigate the propagation of a soliton in an axially varying optical fiber with a progressive change from anomalous to normal dispersion regimes. Spectral and temporal measurements provide evidence for a complete annihilation of the soliton, which explodes into a polychromatic dispersive wave. This interpretation is confirmed by numerical solution of the generalized nonlinear Schrödinger equation.

Emission of multiple dispersive waves from a single Raman-shifting soliton in an axially-varying optical fiber

We provide the experimental demonstration of the generation of multiple dispersive waves from a single soliton propagating in the vicinity of the first zero-dispersion wavelength of an axially-varying optical fiber. The fiber is designed such that the Raman-shifting soliton successively hits three times the longitudinally evolving zero-dispersion wavelength, which results in the emission of three distinct dispersive waves at different fiber lengths. These results illustrate how suitably controlled axially-varying fibers allow to tailor the soliton dynamics in a very accurate way.

Experimental dynamics of Akhmediev breathers in a dispersion varying optical fiber

We investigate experimentally the dynamics of Akhmediev breathers in an optical fiber with a longitudinally tailored dispersion that allows to nearly freeze the breather evolution near their point of maximal compression. Our results are in good agreement with numerical simulations.

Control of the soliton self-frequency shift dynamics using topographic optical fibers

We demonstrate that the dynamics of the soliton self-frequency shift can be accurately controlled by using tapered optical fibers with optimized longitudinal profile shape (that we term topographic fibers). The tapering profiles tailored for a targeted soliton spectral trajectory through dispersion and nonlinearity management are determined by an inverse algorithm. This control is demonstrated experimentally with topographic photonic crystal fibers fabricated directly on a drawing tower.

Real time noise and wavelength correlations in octave-spanning supercontinuum generation

We use dispersive Fourier transformation to measure shot-to-shot spectral instabilities in femtosecond supercontinuum generation. We study both the onset phase of supercontinuum generation with distinct dispersive wave generation, as well as a highly-unstable supercontinuum regime spanning an octave in bandwidth. Wavelength correlation maps allow interactions between separated spectral components to be identified, even when such interactions are not apparent in shot-to-shot or average measurements. Experimental results are interpreted using numerical simulations. Our results show the clear advantages of dispersive Fourier transformation for studying spectral noise during supercontinuum generation.

Simultaneous scalar and cross-phase modulation instabilities in highly birefringent photonic crystal fiber

We report the experimental observation of scalar and cross-phase modulation instabilities by pumping a highly birefringent photonic crystal fiber in the normal dispersion regime at 45° to its principal polarization axes. Five sideband pairs (two scalar and three vector ones) are observed simultaneously in the spontaneous regime, four of which have a large frequency shift from the pump, in the range 79-93 THz. These results are in excellent agreement with phase-matching arguments and numerical simulations.

20 THz-bandwidth continuous-wave fiber optical parametric amplifier operating at 1 µm using a dispersion-stabilized photonic crystal fiber

We report the experimental demonstration of a continuous-wave all-fiber optical parametric amplifier in the 1 µm band with a record bandwidth of more than 20 THz and a peak gain of almost 40 dB. This is achieved by using a photonic crystal fiber with a high figure of merit and strongly reduced longitudinal dispersion fluctuations. Due to their unique bandwidth and gain characteristics, fiber parametric amplifiers at 1 µm provide an interesting alternative to solid-state or ytterbium-doped fiber amplifiers for ultrafast optical pulse and signal processing.

Active reduction of fluctuations in fourth-order modulation instability

We experimentally study the fluctuation properties of a scalar fourth-order modulation instability (MI) process obtained by pumping a photonic crystal fiber in the normal dispersion region. We observe large wavelength-dependent pulse-to-pulse fluctuations that cannot be significantly reduced by stimulating the process with a single seed. Their reduction requires two seeds slightly detuned from the maximum gain frequency in order to also stimulate the second-order MI process cascaded from the fourth-order one. This concept is validated by experiments and numerical simulations.

Synchronously pumped photonic crystal fiber-based optical parametric oscillator

We report the development of a compact, tunable synchronously pumped photonic crystal fiber (PCF)-based optical parametric oscillator (FOPO). The oscillator is pumped using a gain-switched laser diode producing 220 ps pulses around 1062 nm, amplified in a ytterbium doped amplifier to peak powers of 3.5 kW. The FOPO produces anti-Stokes pulses at wavelengths between 757 and 773 nm, with durations of 150 ps at average output powers exceeding 290 mW. The output slope efficiency of the device varies with output wavelength from 1.9 to 6.0%.

Black-light continuum generation in a silica-core photonic crystal fiber

We report the observation of a broadband continuum spanning from 350 to 470 nm in the black-light region of the electromagnetic spectrum as a result of picosecond pumping a solid-core silica photonic crystal fiber at 355 nm. This was achieved despite strong absorption and a large normal dispersion of silica glass in the UV. Further investigations reveal that the continuum generation results from the interplay of intermodally phase-matched four-wave mixing and cascaded Raman scattering. We also discuss the main limitations in terms of bandwidth and power due to temporal walk-off, fiber absorption, and the photo darkening effect, and we suggest simple solutions.

Emergence of spectral incoherent solitons through supercontinuum generation in a photonic crystal fiber

We report an experimental and numerical study of the spontaneous emergence of spectral incoherent solitons through supercontinuum generation in a two zero-dispersion wavelengths photonic crystal fiber. By using a simple experimental setup, we show that the highly nonlinear regime of supercontinuum generation is characterized by the emergence of a spectral incoherent soliton in the low-frequency edge of the supercontinuum spectrum. We show that a transition occurs from the discrete spectral incoherent soliton to its continuous counterpart as the power of the laser is increased. Contrary to conventional solitons, spectral incoherent solitons do not exhibit a confinement in the space-time domain, but solely in the frequency domain. These incoherent structures owe their existence to the noninstantaneous nature of the nonlinear Raman effect and, more specifically, to the causality property underlying the Raman response function.

Transient radiation-induced effects on solid core microstructured optical fibers

We report transient radiation-induced effects on solid core microstructured optical fibers (MOFs). The kinetics and levels of radiation-induced attenuation (RIA) in the visible and near-infrared part of the spectrum (600 nm-2000 nm) were characterized. It is found that the two tested MOFs, fabricated by the stack-and-draw technique, present a good radiation tolerance. Both have similar geometry but one has been made with pure-silica tubes and the other one with Fluorine-doped silica tubes. We compared their pulsed X-ray radiation sensitivities to those of different classes of conventional optical fibers with pure-silica-cores or cores doped with Phosphorus or Germanium. The pulsed radiation sensitivity of MOFs seems to be mainly governed by the glass composition whereas their particular structure does not contribute significantly. Similarly for doped silica fibers, the measured spectral dependence of RIA for the MOFs cannot be correctly reproduced with the various absorption bands associated with the Si-related defects identified in the literature. However, our analysis confirms the preponderant role of self-trapped holes with their visible and infrared absorption bands in the transient behaviors of pure-silica of F-doped fibers. The results of this study showed that pure-silica or fluorine-doped MOFs, which offers specific advantages compared to conventional fibers, are promising for use in harsh environments due to their radiation tolerance.

Symmetry-breaking dynamics of the modulational instability spectrum

We demonstrate in an optical fiber that third-order dispersion yields an unexpected symmetry-breaking dynamics of the modulational instability spectrum. It is found in particular that this spectral asymmetry does not smoothly and monotonically increase when approaching the zero-dispersion wavelength. Instead, it exhibits several local extrema and it can even be reversed at a particular dispersion value. We interpret this behavior as resulting from interactions between dispersive waves and solitons generated from modulation instability.

Control of pulse-to-pulse fluctuations in visible supercontinuum

Long-pulse supercontinuum sources are initiated by modulation instability and consequently suffer from stochastic shot-to-shot variations of their spectral power density. In this paper, we provide a measurement of pulse-to-pulse fluctuations over the whole supercontinuum spectrum, and we show that their spectral dependence follows the group index curve of the fiber. Then, we demonstrate a significant reduction of supercontinuum pulse-to-pulse fluctuations in the visible by using a photonic crystal fiber with longitudinally tailored guidance properties. We finally show numerically that this new source would allow a significant improvement of the signal-to-noise ratio in fluorescence microscopy.

Long wavelength extension of CW-pumped supercontinuum through soliton-dispersive wave interactions

A supercontinuum spanning over 700 nm with an average spectral power of 1.7 mW/nm and flatness of 6 dB was produced in a solid core, double zero dispersion wavelength photonic crystal fiber pumped with a high power, continuous-wave ytterbium fiber laser. The spectrum displays a strong feature centered around 1980 nm. Through numerical simulations we demonstrate that this feature is initially generated through the shedding of Cherenkov radiation by solitons at the second zero dispersion wavelength, and then extended by a four-wave mixing process between this generated dispersive component and other solitons forming the continuum.

Significant reduction of power fluctuations at the long-wavelength edge of a supercontinuum generated in solid-core photonic bandgap fibers

We show that the infrared edge of supercontinua generated in solid core photonic bandgap fibers is characterized by a very different temporal behavior than the one obtained in standard fibers. In particular, pulse-to-pulse spectral power fluctuations are significantly reduced near the bandgap edge, and the statistical distribution is quasi-gaussian. The spectral dynamics of this process and statistical properties are investigated experimentally and confirmed by numerical simulations. The reduction of power fluctuations originates from the cancellation of the soliton self-frequency shift near the bandgap edge.

Photonic crystal fiber mapping using Brillouin echoes distributed sensing

In this paper we investigate the effect of microstructure irregularities and applied strain on backward Brillouin scattering by comparing two photonic crystal fibers drawn with different parameters in order to minimize diameter and microstructure fluctuations. We fully characterize their Brillouin properties including the gain spectrum and the critical power. Using Brillouin echoes distributed sensing with a high spatial resolution of 30 cm we are able to map the Brillouin frequency shift along the fiber and get an accurate estimation of the microstructure longitudinal fluctuations. Our results reveal a clear-cut difference of longitudinal homogeneity between the two fibers.

Efficient blue conversion from a 1064 nm microchip laser in long photonic crystal fiber tapers for fluorescence microscopy

Using a low-cost microchip laser and a long photonic crystal fiber taper, we report a supercontinuum source with a very efficient visible conversion, especially in the blue region (around 420 nm). About 30 % of the total average output power is located in the 350-600 nm band, which is of primary importance in a number of biophotonics applications such as flow cytometry or fluorescence imaging microscopy for instance. We successfully demonstrate the use of this visible-enhanced source for a three-color imaging of HeLa cells in wide-field microscopy.

Impact of the third-order dispersion on the modulation instability gain of pulsed signals

We demonstrate that modulation instability gain of time-localized signals (i.e., pulsed signals) depends strongly on the third-order dispersion, contrary to the well-known case of time-extended signals (cw signals). This surprising contribution of an odd dispersion term on this four-photon-mixing process is established analytically and confirmed by numerical simulations.

White-light cw-pumped supercontinuum generation in highly GeO(2)-doped-core photonic crystal fibers

GeO(2)-doped-core photonic crystal fibers show greatly enhanced Kerr and Raman responses with respect to pure silica without any significant modification of the group-velocity dispersion curve. We show that such fibers allow a significant improvement of cw-pumped supercontinuum generation. We report for the first time to our knowledge the generation of a white-light cw-pumped supercontinuum spanning from 470 nm to more than 1750 nm.

Control of supercontinuum generation and soliton self-frequency shift in solid-core photonic bandgap fibers

We experimentally investigate the nonlinear propagation of subnanosecond pulses in solid-core photonic bandgap fibers. By launching pulses with a few kilowatts peak power, a flat supercontinuum is generated. The long-wavelength edge of the supercontinuum can be controlled thanks to the original linear properties inherent to solid-core photonic bandgap fibers. This allows one to tailor the generated supercontinuum radiation and to keep it over a given spectral range of interest without any significant power loss.

Observation of extreme temporal events in CW-pumped supercontinuum

We study experimentally and numerically the temporal features of supercontinuum generated with a continuous-wave ytterbium-doped fiber laser. We show that the temporal output of the supercontinuum is characterized by strong and brief power fluctuations, i.e. so-called optical rogue waves. We demonstrate numerically that these rare and strong events that appear and disappear from nowhere result from solitonic collisions.

Experimental investigation of combined four-wave mixing and Raman effect in the normal dispersion regime of a photonic crystal fiber

We study the effect of stimulated Raman scattering on four-wave mixing sidebands generated by pumping in the normal dispersion regime of a photonic crystal fiber. Q-switch nanosecond pulses at 1064 nm are used to generate signal and idler wavelengths by degenerate four-wave mixing. These three waves generate their own Raman Stokes orders, leading to a broadband supercontinuum.

Influence of electrode composition on the second-order nonlinearity profile in thermally poled silica glass

Silica glass plates (Infrasil from Heraeus) have been thermally poled with different types of electrodes (evaporated or pressed-on) having different chemical compositions (Au, Al, n-doped Si or p-doped Si). We show for the first time to our best knowledge that the profile of the induced second-order nonlinearity strongly depends on the composition of electrodes used for the thermal poling process.

Time evolution of the second-order nonlinear distribution of poled Infrasil samples during annealing experiments

The spatial distribution of the second-order nonlinearity induced in thermally poled Infrasil silica samples is recorded after thermal annealing experiments. Two regimes have been studied: short and long poling durations. For short poling durations, the observations are in good agreement with a model where only one ion type recombines inside the depletion region. The nonlinear distribution and erasure observed for the other case are well explained by considering the addition of another positive-charged ion injected during the poling process. This second ion acts as a barrier during thermal annealing and reduces the mobility of the first one.

Zero-dispersion wavelength decreasing photonic crystal fibers for ultraviolet-extended supercontinuum generation

We report the fabrication of photonic crystal fibers with a continuously-decreasing zero-dispersion wavelength along their length. These tapered fibers are designed to extend the generation of supercontinuum spectra from the visible into the ultraviolet. We report on their performance when pumped with both nanosecond and picosecond sources at 1.064 microm. The supercontinuum spectra have a spectral width (measured at the 10 dB points) extending from 0.372 microm to beyond 1.75 microm. In an optimal configuration a flat (3 dB) spectrum from 395 to 850 nm, with a minimum spectral power density of 2 mW/nm was achieved, with a total continuum output power of 3.5 W. We believe that the shortest wavelengths were generated by cascaded four-wave mixing: the continuous decrease of the zero dispersion wavelength along the fiber length enables the phase-matching condition to be satisfied for a wide range of wavelengths into the ultraviolet, while simultaneously increasing the nonlinear coefficient of the fiber.

Modeling of the chi(2) susceptibility time-evolution in thermally poled fused silica

The dynamics of the second-order nonlinearity induced in a thermally poled Infrasil silica glass is experimentally and theoretically studied. 200 mum and 500 mum-thick samples have been poled for different durations varying from 1 minute to 100 minutes. After the poling process, the magnitude and the spatial distribution of the induced chi((2)) susceptibility have been characterized accurately with the "layer peeling" method. A two-charge carrier model with an electric field dependant charge injection is used to explain the experimental time-evolution of the chi((2)) profiles. A good agreement between experimental results and simulations is reported.