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Millimeter-sized whispering gallery mode resonators produced from bulk crystalline substrates show great potential as optoelectronics components but are still delicate to produce. We report here on an improved manufacturing technique based on femtosecond laser ablation, which allowed us to obtain state-of-the-art performance (quality factor around 109) in calcium fluoride with only half the processing time required before. Our results are supported by optical profilometric and cavity ringdown measurements, and could be extended to a variety of other substrates.

Recent years have seen renewed interest in the study of nonlinear fibre laser and propagation dynamics through the use of real-time measurement techniques for non-repetitive ultrafast optical signals. In this paper we review our recent work in this field using dispersive Fourier Transform and Time Lens techniques.

International audience; We investigate high-speed glass cleaving with ultrafast laser beams with engineered transverse intensity profile. We achieve accuracy of ~ 1 µm at 25 mm/s and drastically enhance cleavability compared to standard Bessel beams.

Femtosecond pulses provide an extreme degree of confinement of light matter-interactions in high-bandgap materials because of the nonlinear nature of ionization. It was recognized very early on that a highly focused single pulse of only nanojoule energy could generate spherical voids in fused silica and sapphire crystal as the nanometric scale plasma generated has energy sufficient to compress the material around it and to generate new material phases. But the volumes of the nanometric void and of the compressed material are extremely small. Here we use single femtosecond pulses shaped into high-angle Bessel beams at microjoule energy, allowing for the creation of very high 100:1 aspect ratio voids in sapphire crystal, which is one of the hardest materials, twice as dense as glass. The void volume is 2 orders of magnitude higher than those created with Gaussian beams. Femtosecond and picosecond illumination regimes yield qualitatively different damage morphologies. These results open novel perspectives for laser processing and new materials synthesis by laser-induced compression.

Femtosecond Bessel pulses with a needle-like intensity distribution were focused inside sapphire crystal to create voids and the shock-wave affected volume which is by more than two orders of magnitude larger as compared with that made by the Gaussian pulse.

The influence of sliding filters on the phase jitter of soliton pulses propagating in single-channel constant-dispersion systems is studied using the perturbation theory. Although sliding filters allow the reduction of the amplified radiative background, they introduce coupling between the soliton amplitude and frequency, and between its timing and phase, which result in a growth of the phase jitter as the cube of the propagation distance. Comparisons of the analytical results with Monte Carlo simulations validate the perturbation theory.

International audience; We investigate a method to measure ultrafast laser ablation threshold with respect to spot size. We use structured complex beams to generate a pattern of craters in CVD graphene with a single laser pulse. A direct comparison between beam profile and SEM characterization allows us to determine the dependence of ablation probability on spot-size, for crater diameters ranging between 700 nm and 2.5 μm. We report a drastic decrease of ablation probability when the crater diameter is below 1 μm which we interpret in terms of free-carrier diffusion.

International audience; We investigate the early stage of propagation of Bessel–Gauss vortex beams where a transition regime shows a progressive lateral expansion of the main intensity ring before reaching a diffraction-free regime. The eikonal equation is used to characterize the beam structure. The beam is featured by a family of hyperboloids with variable waists, generating a tapered tubular caustic. Our analytical results are in excellent agreement with numerical and experimental results. We show the transition regime can be well eliminated by using hollow input beams.

International audience; We numerically investigate the spatio-temporal structure of Bessel beams generated with Spatial Light Modulators (SLMs). Grating-like phase masks enable the spatial filtering of undesired diffraction orders produced by SLMs. Pulse front tilt and temporal broadening effects are investigated. In addition, we explore the influence of phase wrapping and show the spatio-temporal structure of SLM-generated femtosecond Bessel beams is similar to Bessel X-pulses at short propagation distance, and to subluminal Pulsed Bessel Beams at long propagation distance.

We present a frequency-resolved optical gating geometry permitting the characterization of 1-mW average power 2.5-ps pulses over the whole C-band at 10-GHz repetition rate. This represents an order of magnitude improvement in sensitivity over other reported self-referenced characterization techniques. Measurements of pulses from an actively mode-locked fiber laser are reported.

International audience; We report the fabrication of micro and nano-disks in single layer chemical vapor deposition graphene on glass substrate using femtosecond laser ablation with vortex Bessel beams. The fabricated graphene disks with diameters ranging from 650 nm to 4 μm were characterized by spatially resolved micro-Raman spectroscopy. The variation of ablation threshold was investigated as a function of the number of pulses showing an incubation effect. A very high degree of size control of the fabricated graphene disks is enabled using a sequence of femtosecond pulses with different vortex orders.

Summary form only given. Femtosecond laser micro and nano-processing is a versatile material processing tool which has opened up a broad range of technologies and applications. However, in the particular context of the fabrication of deep trenches and channels, precise control of profile of the ablated structure is extremely challenging. We have recently developed a novel approach to fabricate controlled high aspect structures using novel Bessel and Airy beams, and this contribution will review our recent work in this field.

International audience; We use caustic beam shaping on 100 fs pulses to experimentally generate nonparaxial accelerating beams along a 60° circular arc, moving laterally by 14 µm over a 28 µm propagation length. This is the highest degree of transverse acceleration reported to our knowledge. Using diffraction integral theory and numerical beam propagation simulations, we show that circular acceleration trajectories represent a unique class of nonparaxial diffraction-free beam profile which also preserves the femtosecond temporal structure in the vicinity of the caustic.

International audience; We report femtosecond laser micromachining of micron-size curved structures using tailored accelerating beams. We report surface curvatures as small as 70 μm in both diamond and silicon, which demonstrates the wide applicability of the technique to materials that are optically transparent or opaque at the pump laser wavelength. We also report the machining of curved trenches in silicon. Our results are consistent with an ablation-threshold model based on calculated local beam intensity, and we also observe asymmetric debris deposition which is interpreted in terms of the optical properties of the incident accelerating beam.

International audience; We review our recent results applying the stable nonlinear propagation of nondiffracting Bessel beams to femtosecond laser ablation in dielectric materials. We describe our spatial light modulator setup that allows for flexible and reconfigurable beam profile synthesis, we interpret the physics in terms of the formation of a steady plasma channel, and we present results of both multishot and single shot machining.

International audience; We generate arbitrary convex accelerating beams by direct application of an appropriate spatial phase profile on an incident Gaussian beam. The spatial phase calculation exploits the geometrical properties of optical caustics and the Legendre transform. Using this technique, accelerating sheet caustic beams with parabolic profiles (i.e. Airy beams), as well as quartic and logarithmic profiles are experimentally synthesized from an incident Gaussian beam, and we show compatibility with material processing applications using an imaging system to reduce the main intensity lobe at the caustic to sub-10 micron transverse dimension. By applying additional and rotational spatial phase, we generate caustic-bounded sheet and volume beams, which both show evidence of the recently predicted effect of abrupt autofocussing. In addition, an engineered accelerating profile with femtosecond pulses is applied to generate a curved zone of refractive index modification in glass. These latter results provide proof of principle demonstration of how this technique may yield new degrees of freedom in both nonlinear optics and femtosecond micromachining.

Ultrashort laser material processing is a versatile technology that allows material structuring at nanometer scales [1]. In the context of the fabrication of nanofluidics components or photonics crystals, high aspect ratio nanochannel in dielectrics is required. Deep nano-craters have been produced with single shot from gaussian beams by taking advantage of geometrical aberrations and nonlinear propagation to elongate the beam focus [2]. However, the distortion of gaussian beams during nonlinear propagation in dielectrics produces irregular structures with strongly interdependant channel diameter and length.

International audience; Ultrafast laser pulses are a powerful tool to process dielectrics. Here, we review our recent work concerning high aspect ratio micro and nanochannel processing in glass. We show how femtosecond Bessel beams overcome many of the difficulties associated with Gaussian beams. We report on single shot processing of nanochannels with aspect ratio up to 100. Underlying physical phenomena are discussed.

International audience; We report high aspect ratio nanochannel fabrication in glass using single-shot femtosecond Bessel beams of sub-3 μJ pulse energies at 800 nm. We obtain near-parallel nanochannels with diameters in the range 200–800 nm, and aspect ratios that can exceed 100. An array of 230 nm diameter channels with 1.6 μm pitch illustrates the reproducibility of this approach and the potential for writing periodic structures. We also report proof-of-principle machining of a through-channel of 400 nm diameter in a 43 μm thick membrane. These results represent a significant advance of femtosecond laser ablation technology into the nanometric regime.

The use of nondiffracting femtosecond Bessel beams has recently been shown to be a powerful new approach for writing nanometer and micrometer surface features in glass. In this paper we review our recent experimental results in this area. The advantages of Bessel beams in yielding non-critical sample positioning and for fabricating high aspect ratio microchannels will be discussed in detail, and the underlying physical mechanisms involving plasma channel formation and filament formation will also be considered.

International audience; Femtosecond Bessel beams are demonstrated for high aspect ratio machining in the filamentation regime with water assistance. Taper-free microchannels and micro-trenches are demonstrated with aspect ratio up to 40 and diameters down to 2 µm.

International audience; The novel propagation characteristics of Bessel beams have been widely applied to optical manipulation and harmonic generation, and have provided new perspectives on fundamentals of ultrashort laser pulse propagation in nonlinear media. Fully exploiting their many unique properties, however, requires the development of techniques for the generation of high quality Bessel beams with flexible adjustment of the beam parameters. Moreover, long working distances are needed to produce Bessel beams inside bulk samples. In this paper, we report on the development of a novel spatial light modulator based setup that combines the properties of parameter flexibility, long working distance, high throughput and operation on micron-scale. We report both on the general characterization of the beam properties as well as a specific application in surface nanoprocessing.

Also selected for Virtual Journal of Ultrafast Science and Highlighted Nature Photonics; International audience; We present a systematic study of femtosecond laser microchannel machining in glass using nondiffracting Bessel beams. In particular, our results identify a source and focusing parameter working window where high aspect ratio taper-free microchannels can be reproducibly produced without sample translation. With appropriate source parameters, we machine channels of 2 μm diameter and with aspect ratios up to 40. We propose the filamentation stability of the Bessel beam propagation as the critical factor underlying the controlled and reproducible results that have been obtained

During nanopatterning of material surfaces by ultrashort laser pulses, the nano-damage depends critically on the laser fluence compared to the fluence threshold. The positioning of the sample surface with respect to the focal point is a strongly demanding factor when patterning large surfaces. We demonstrate the application of nondiffracting Bessel beams to reproducible nanometric-scale feature femtosecond laser patterning. A specific setup producing femtosecond pulse zero-order Bessel beam with a central spot radius of 360 nm was developed. We exhibit femtosecond laser processing of near identical nanocraters in glass over a longitudinal positioning range of more than 20 µm. The use of Bessel beams significantly reduces constraints on critical sample positioning in the nanoscale writing regime, enabling the use of femtosecond laser pulses for fast inscription of nanometer-scale features over large sample areas.

International audience; We propose a new optoelectronic oscillator (OEO) architecture, able to generate at the same time ultra-low jitter optical pulses and ultra-low phase noise microwaves. This new system relies on the single-loop OEO, with two modulation branches: an intensity modulation for the formation of sinusoidal pre-pulses, and a phase modulation to chirp them. The train of chirped pre-pulses is then strongly amplified by an erbium-doped fiber amplifier and subsequently launched into a single-mode optical fiber. Owing to the combined effects of dispersion and self-phase modulation, the pre-pulses are optimally compressed with a compression factor which may be as high as 20, following a scenario referred to as time-lens soliton-assisted compression.

International audience; A numerical study of soliton spectral tunnelling in an index-guiding photonic crystal fibre with a sub-wavelength central air core defect is presented. Specifically, for a fibre design where two regions of anomalous dispersion bracket a 90 nm wide region of normal dispersion, simulations show tunnelling from 1300 to 1475 nm across the forbidden normal dispersion regime, with near unity efficiency. In addition, this effect can be exploited to obtain over a factor of two nonlinear pulse compression by interrupting propagation at the distance at which soliton tunnelling occurs.

International audience; Optimised compression of femtosecond fibre laser pulses in highly nonlinear fibre is demonstrated. Specifically reported are compressed pulse durations below 30 fs using a simple setup where a single sub-10 cm segment of highly nonlinear fibre is directly spliced to the output pigtail of a commercial femtosecond fibre laser. The compressed pulse quality is confirmed through frequency resolved optical gating measurements.

Nonlinear pulse compression techniques are of course very well-known, exploiting the initial spectral broadening and temporal compression phase of higher-order soliton evolution in the anomalous dispersion regime of an optical fiber. However, the presence of effects such as Raman scattering, higher-order dispersion or input pulse noise can induce pulse break-up and instabilities through soliton-fission processes. Recent studies of soliton fission in the context of supercontinuum generation have provided improved insight into the way these processes can be avoided, allowing significant improvement in achievable compressed pulse quality and duration. In this paper we focus on providing an overview of a series of our own experiments around 1550 nm where we have used parabolic pulse similariton amplifiers to generate low noise pulses that have been linearly and nonlinearly compressed to the sub-30 fs regime using controlled compression in highly nonlinear fiber. In addition, we also describe recent results where comparable pulse durations have been obtained using sub-10 cm lengths of highly nonlinear fiber directly spliced to the output pigtail of a commercial femtosecond source.

We experimentally demonstrate parabolic pulse generation around 1550 nm in dispersion decreasing comb-like profiled fiber. Frequency resolved optical gating is used to confirm the output pulse parabolic intensity profile and linear chirp.

In this paper, we demonstrate simultaneous time-lens and soliton compression effects in standard single mode fiber by a simple amplification stage of the signal prior to the injection in the compression fiber. Our technique allows the generation of 3.2 ps pulses at a 10 GHz repetition rate without using special components or additional phase aberration correction, at a low driving voltage. The output pulses are characterized by a sensitive implementation of frequency-resolved optical gating, which allows the retrieval of both the intensity and phase profiles of the pulses.

We have developed a variant of frequency resolved optical gating (FROG) using fiber four-wave mixing that has allowed the characterization of milliwatt peak power picosecond pulses in a compact and all-fiber connectorized configuration. In this communication, we extend the application range of this technique to a cross-correlation geometry using two synchronous sources at distinct wavelengths.

This paper investigates electro-optic mapping for physical layer encryption. Preliminary results show that discrete time chaos can be obtained experimentally, which could be implemented in the future for application to optical packet data encryption at the physical layer. A modified architecture is also proposed to implement a chaotic phase modulation of the optical pulses.

International audience; Parabolic pulses from a similariton amplifier at 1550nm are compressed using a two-stage fiber compressor employing linear compression in photonic bandgap fiber and soliton effect compression in highly nonlinear fiber. Precise optimization of the compressor design using frequency resolved optical gating allows the generation of minimally chirped pulses as short as 20fs. This duration corresponds to only four optical cycles of the underlying electric field at this wavelength

Parabolic pulse generation around 1550 nm in dispersion decreasing comb-like profiled fibre is experimentally demonstrated. A six-segment comb profile based on two types of highly nonlinear fibre with normal dispersion is used to reshape 190 fs pulses into 540 fs linearly chirped parabolic pulses after a propagation distance of only 1.7 m. Frequency resolved optical gating is used to measure the output pulse intensity and chirp, and the experimental results are in good quantitative agreement with numerical simulations.

An optoelectronic nonlinear delay oscillator seeded by a pulsed laser source is used to experimentally demonstrate a new transition scenario for the general class of delay differential dynamics, from continuous to discrete time behavior. This transition scenario differs from the singular limit map, or adiabatic approximation model that is usually considered. The transition from the map to the flow is observed when increasing the pulse repetition rate. The mechanism of this transition opens the way to new interpretations of the general properties of delay differential dynamics, which are universal features of many other scientific domains. We anticipate that the nonlinear delay oscillator architecture presented here will have significant applications in chaotic communication systems.

Chaos-based secure communication is intended to operate as an additional encryption mean at the physical layer, with the unique capability for operating at several 10s of Gb/s. Different chaos generators have been recently explored for optical telecommunications, among which three setups were particularly interesting in terms of encryption speed capability: the all-optical external cavity semiconductor laser, the direct optoelectronic feedback semiconductor laser, and the nonlinear electrooptic feedback. The latter solution was mainly investigated by our group during the last 5 years. We report its numerous advantages in terms of chaos complexity (attractor dimension as high as several hundreds have been obtained), architecture reliability (different chaos generator have been performed experimentally), encryption bandwidth capability (more than 30GHz chaotic spectrum has been observed), stability, and decoding quality. The potential for high speed operation was recently demonstrated with an electrooptic chaos generator at 3Gb/s, with BER as low as 10 -9 .

Summary form only given. We report a new FROG geometry using wavelength conversion in optical fiber which has been used to characterize 1 mW peak power, 20 ps pulses from an unamplified semiconductor laser. We use a novel configuration whereby a pulsed pump is mixed with a CW signal to generate an idler We thus obtain an effective second-order nonlinear response that can be used to generate a FROG trace.

We describe a frequency resolved optical gating geometry based on four-wave mixing in a semiconductor optical amplifier, exhibiting high sensitivity and broadband tunability as compared to previously reported fiber setups. Experimental results are discussed.

We investigate femtosecond pulse propagation in air-silica microstructure fiber (ASMF), reporting the generation of femtosecond Raman soliton pulses tunable over 200 nm in the near infrared. For sufficiently broad spectral content, SRS transfers energy from the higher frequency spectral components to lower frequencies, resulting in a continuous self-frequency shift to longer wavelengths. Furthermore, numerical solutions to the nonlinear Schrodinger equation (NLSE), using the measured intensity and phase of the injected pulse as initial condition for the split-step Fourier method (SSFM), predict accurately the observed spectra and the presence of the Raman soliton.

We investigate femtosecond pulse propagation in photonic crystal fiber, reporting the generation of tunable femtosecond soliton pulses. For sufficiently broad spectral content, stimulated Raman scattering transfers energy from the higher frequency spectral components to lower frequencies, resulting in a continuous self-frequency shift to longer wavelengths. Power dependent spectral analysis reveals a well-formed soliton at peak powers exceeding 100 W. Background-free intensity autocorrelation measurements confirm soliton formation with a duration of < 90 fs and with an energy conversion efficiency of 60%. Numerical solutions were performed based on a generalized nonlinear Schrodinger equation that included the effects of dispersion, self-steepening, optical shock formation, self-phase modulation and stimulated Raman scattering. The resulting spectra from the simulations are in excellent agreement with the measured spectra, and are consistent with the intensity autocorrelation measurements.

Frequency resolved optical gating using a novel fiber-based wavelength conversion geometry is used to characterize the intensity and phase evolution of milliwatt peak power pulses propagating over 300 km in an optical fiber recirculation loop.

International audience; We describe experimental studies of the dynamical behavior of a recently proposed electro-optic discrete time nonlinear delay oscillator. With appropriate choice of the oscillator loop parameters and external forcing of the dynamics using a pulsed laser source, the system allows for the physical realization of a high dimensional mathematical nonlinear mapping. The dynamical features observed with this new class of discrete time delay oscillator differ significantly from those observed with similar continuous time nonlinear delay feedback oscillators and reveal the intrinsic discrete time nature of the dynamics. We also discuss specific applications to chaos communications using regularly clocked binary data.

The influence of in-line filters on the phase jitter of chirped optical pulses propagating in arbitrary dispersion-managed systems is studied with a semianalytic moment method. Because of its stabilizing effect on the amplitude, filtering reduces the nonlinear phase jitter that accumulates through self-phase modulation. As in the case of constant-dispersion soliton links, we observe that the phase variance grows only linearly with distance in the presence of filtering. Phase jitter reduction is observed and accurately predicted by the moment method in two dispersion-managed systems with different levels of nonlinearity and filter strength.

Optical phase jitter limits the performance of amplified differential-phase-shift-keyed optical communication systems. We propose an approach to evaluate the phase jitter for arbitrary pulses in dispersion-managed links based on the moment method. This calculation requires only the knowledge of the unperturbed optical signal, therefore avoiding computationally intensive Monte Carlo simulations. We apply this method to a dispersion-managed soliton system and a quasi-linear dispersion-compensated channel and demonstrate its validity by comparing the obtained results with Monte Carlo simulations.

Complete intensity and chirp characterisation of 15 ps pulses at a wavelength of 1557 nm propagating over a distance of 308 km in a dispersion-shifted fibre recirculation loop is reported. A novel frequency-resolved optical gating configuration based on wavelength conversion allows pulse characterisation at the 10 mW peak power level.

Femtosecond Raman soliton generation, tunable from 850–1050 nm, has been demonstrated by pumping untapered photonic crystal fibre with 110 fs pulses from a modelocked Ti:sapphire laser. Experimental results are in excellent agreement with the results of numerical simulations using an extended nonlinear Schrodinger equation.