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International audience; Background: Liquid filled hollow core fiber Raman wavelength converters are devices enabling to red-shift the wavelength of laser beams delivered by either fixed wavelength or tunable lasers. Most of the reported converters operate in a pulsed regime with pulse durations comprised between a few hundreds of picoseconds and up to a few nanoseconds, and energy of a few micro-Joules. However, and depending on the pulse duration, the optimization of Raman converters turns to be rather challenging since the desired forward Raman scattering may compete with counter-propagating nonlinear effects like Raman and Brillouin back-scatterings. The Brillouin back-scattering is especially hard to cancel since the Brillouin gains are usually larger than the Raman gains. Techniques have already been investigated but are not fully satisfactory. Our aim in this paper is to propose a technique to optimize the Raman forward scattering and to minimize these back-scattering effects.Methods: We compare the interaction lengths in the forward and backward directions in the above-mentioned temporal regimes. If the fiber length is higher that the backward effective length of interaction then the backward effects should decrease at the benefit of the forward Raman scattering. Numerical simulations enable to estimate more precisely how long the fiber should be to get large Raman conversion efficiency and negligible Brillouin scattering.Results and discussion: To validate this technique we build two identical Raman converters differing only by the fiber lengths (0.5 m and 1.5 m). We present the experimental setup. The experimental results are confronted to the numerical simulations. As expected increasing the fiber length strongly decreases the Brillouin scattering.Conclusions: We have proposed a simple and efficient solution to minimize Brillouin back-scattering in Raman wavelength converters by optimizing the fiber length. The experimental demonstration was conducted with low energy (≈ 6 μJ) but the energies should scale with the fiber effective area if more energy is needed, without affecting the conversion efficiencies.

We demonstrated a class of highly nonlinear hybrid waveguide structures based on infiltration of As2S3 chalcogenide glass into silicon slot waveguides. The nonlinear properties of the hybrid waveguides were precisely quantified via a bidirectional top-hat D-scan method, enabling a direct comparison between properties measured using different device geometries. We experimentally demonstrate hybrid As2S3-Si slot waveguides with a two-photon absorption (TPA) figure of merit exceeding 2 at near infrared wavelengths. These waveguides largely satisfy the critical criterion for efficient nonlinear integrated photonics (FOMTPAwg>1), allowing phase shifts greater than π with minimal overall losses. These results pave the way for efficient and robust ultrafast all-optical devices and circuits in large-scale silicon photonics technology.

International audience; We report on the linear and nonlinear measurements of GeSbS chalcogenide glasses in waveguide configuration. Single-mode waveguides and ring resonators have been characterized in linear and nonlinear optical regimes around 1.58 µm wavelength demonstrating the interest of using this material for ultrahigh-bandwidth optical communications systems.

The characterization of a third order nonlinear integrated waveguide is reported for the first time by means of a top-hat Dispersive-Scan (D-Scan) technique, a temporal analog of the top-hat Z-Scan. With a single laser beam, and by carrying two counter-directional nonlinear transmissions to assess the input and output coupling efficiencies, a novel procedure is described leading to an accurate measurement of the TPA figure of merit, the effective Two-Photon Absorption (TPA) and optical Kerr (including the sign) coefficients. The technique is validated in a silicon strip waveguide for which the effective nonlinear coefficients are measured with an accuracy of $\pm 10~\%$, Comment: 5 pages, 4 figures

International audience; Fibered sources of photon pairs can be easily integrated into future quantum communication networks. However, in silica-core fibres, the broadband spectrum of spontaneous Raman scattering strongly contributes to uncorrelated noise photons degrading the quality of the source. To overcome the problem of the silica Raman scattering, a new architecture was recently proposed, with hollow-core photonic crystal fibers (LF-HCPF) whose core and cladding are filled with a non-linear liquid. As opposed to the Raman scattering spectrum of silica, which is broadband and continuous, the Raman scattering spectrum of liquids is composed of narrow lines. Thus, generating photon pairs between these narrow lines permits to avoid the spectral overlap between the SFWM pairs and the Raman photons. A near infra-red LF-HCPCF photon pair source has been demonstrated, featuring a three order of magnitude suppression of the Raman noise [1]. We have also demonstrated the first non-linear LF-PCF with a transmission band and a zero-dispersion-wavelength (ZDW) in the telecom wavelength range combined with a non-linearity of the same order of magnitude as the one of silica, as a first step towards a Raman-free photon pair source in the telecom band [2]. Injecting light on both sides of this fiber in a Sagnac loop configuration can then be used to obtain entanglement.

The difficulty in characterising nonlinear integrated structures lies in assessing the real power injected into the guided mode. Nonlinear characterisations of nano-waveguides currently reported in literature rarely give a precise determination of the coupling efficiency, while it directly affects the nonlinear response. Using a single beam top-hat Dispersive-Scan (D-Scan) method, a temporal analogue of the top-hat Z-Scan [1], we show the first nonlinear waveguide characterisation that simultaneously and accurately measures the input (k in ) and output (k out ) coupling efficiencies, the effective Two-Photon Absorption (γ TPA ) and Kerr (γ) nonlinear parameters. Like in Z-Scan, the D-Scan technique [2] advantageously gives access to the sign of the Kerr coefficient.

Raman wavelength converters based on hollow core microstructured fibres filled with gases or liquids have been widely studied in the last few decades [1, 2]. In these converters the wavelength of a laser pump beam is red-shifted by stimulated Raman scattering in the material filling the core of the fibre, giving rise to new discrete wavelengths that can be used in many domains of applications (biophotonics, environment …). The possibility to strongly confine the pump light over long distances is an asset to get a high conversion efficiency. However if no particular care is taken unwanted non linear effects can occur at the expense of the stimulated forward Raman scattering. These effects are the Raman and Brillouin back-scatterings. In this work we are interested in the optimization of liquid-core Raman converters operating with pulses of a few nanoseconds or less. In these temporal regimes competition between the abovementioned effects are known to be very strong. We validate here a solution to minimize Raman and Brillouin back-scatterings by an appropriate choice of the fibre length. The effective length of interaction L eff in the backward direction is estimated to be cΔt / 2n, where At is the pulse width duration and n the refractive index. In the forward direction, as the dispersion is negligible, this effective length is the length of the fibre L. In liquids, as backward and forward Raman gains are nearly the same, L has just to be slightly higher than L eff to avoid Raman back-scattering. In most liquids and for long pulses Brillouin gain is much higher than Raman gain (typically one order of magnitude). To decrease Brillouin scattering L has to be subsequently higher than Leff. We demonstrate experimentally these trends by building two Raman converters differing only by their lengths. Our initial aim was to realize a Raman converter emitting in the near IR range. The pump source is a microlaser at 532 nm delivering 900 ps pulses at a repetition rate of 4.5 kHz. We choose ethanol as the Raman liquid. When pumped at 532 nm, the first Stokes order of ethanol is at 630 nm and the second Stokes order is at 772 nm, i.e the near IR wavelength we want to generate. A Kagome fibre was used to realize the two converters. When empty this fibre guides in the near IR range over a 1 μm wide transmission band. When filled with ethanol this transmission band is blue-shifted and then covers the pump and the Stokes wavelengths. In our configuration L eff =0.1 m. The lengths of the two converters are 0.5 m and 1.5 m. The results are shown in figure 1. As expected we observed no Raman backscattering in both cases and Brillouin effect is strongly attenuated for the longer fibre. For instance at the critical point (when the output Raman Stokes energy equals the transmitted pump energy), the Brillouin energy is 66% of the forward Raman energy for the 0.5 m fibre and is only 9% for the 1.5 m fibre. As a consequence, the output energy at 772 nm is highly improved and we obtain an output energy of 0.9 μJ at 772 nm for an injected energy of 5.8 μJ.

International audience

We report on the first third order nonlinear experimental characterization of Ge-rich Si 1.x Ge x waveguides, with Germanium concentrations x ranging from 0.7 to 0.9. The experimental values will be compared with theoretical models. These results will provide helpful insights to assist the design of nonlinear integrated optical based devices in both the near- and mid-IR wavelength ranges.

International audience; The effective Kerr and two photon absorption (TPA) nonlinear parameters of a nano-waveguide are characterized by means of a bi-directional top hat D-Scan technique. This approach requires a single beam and allows the measurement of the coupling efficiencies.

International audience; We measure the span-to-span noise covariance as indicator of Gaussian noise model accuracy over dispersion-managed systems. We found that covariance shouldn’t exceed 1/3 of variance, which occurs once residual dispersion per span exceeds 60ps/nm.

The χ(3) nonlinear properties of slotted hybrid silicon photonic waveguides infiltered with third-order nonlinear materials (NM) are studied by calculating the effective nonlinear susceptibilities associated to the silicon and cladding material, respectively. Optimization of the geometry of silicon-slotted waveguides is performed to engineer the nonlinear figure of merit (FOMTPA) of the guided mode up to 4.25, which is related to the balance between the Kerr and the TPA effects. The importance of engineering the nonlinear FOMTPA is illustrated by simulating the self-phase modulation effect in slotted hybrid silicon photonic waveguides.

The χ(3) nonlinear properties of slotted hybrid silicon photonic waveguides infiltrated with third-order nonlinear materials (NM) are studied by modelling the self-phase modulation (SPM). The broadness and intensity of SPM spectrum are investigated through nonlinear FOM TPA optimization to inhibit the detrimental two-photo absorption in slot waveguides, which demonstrates slot waveguides with higher FOM TPA could perform as wide but much more intense broadened spectrum than those with low FOM TPA , even though high FOM TPA slot waveguides intrinsically have larger effective mode areas, namely weaker Kerr effects. This study will draw attention to future nonlinear optimization of hybrid silicon waveguides to make balance between effective mode areas related Kerr effects and FOM TPA related TPA inhibition.

The χ(3) nonlinear properties of slotted crystalline silicon photonic waveguides filled with third-order nonlinear materials (NM) are studied by calculating the effective nonlinear susceptibilities associated to the silicon and cladding material, respectively. The adopted approach circumvents the assumptions that the introduced NM dominates the nonlinear behavior of the slotted waveguide and that strong light confinement in the slot allows neglecting the two-photon absorption (TPA) process in the silicon rails. Optimization of the geometry of silicon-slotted waveguides is performed on the basis of the nonlinear figure of merit (FOM(TPA)) of the guided mode, which is related to the balance between the Kerr and the TPA effects, allowing to reach a FOM(TPA)=4.25. The obtained results reveal the importance of properly choosing the waveguide width of the silicon rails in order to minimize the TPA effect even by tolerating a reduced overall nonlinearity.

Third-order nonlinear effects in silicon photonics are traditionally spoiled by free carrier effects. Optimization of the geometry of silicon slotted waveguides has been performed on the basis of the FOMTPA of the guided mode and has revealed the importance of properly choosing the waveguide width of the silicon rails in order to minimize TPA.

International audience; The large field enhancement that can be achieved in high quality factor and small mode volume photonic crystal microcavities leads to strengthened nonlinear interactions. However, the frequency shift dynamics of the cavity resonance under a pulsed excitation, which is driven by nonlinear refractive index change, tends to limit the coupling efficiency between the pulse and the cavity. As a consequence, the cavity enhancement effect cannot last for the entire pulse duration, limiting the interaction between the pulse and the intra-cavity material. In order to preserve the benefit of light localization throughout the pulsed excitation, we report the first experimental demonstration of coherent excitation of a nonlinear microcavity, leading to an enhanced intra-cavity nonlinear interaction. We investigate the nonlinear behavior of a Silicon-based microcavity subject to tailored positively chirped pulses, enabling to increase the free carrier density generated by two-photon absorption by up to a factor of 2.5 compared with a Fourier-transform limited pulse excitation of equal energy. It is accompanied by an extended frequency blue-shift of the cavity resonance reaching 19 times the linear cavity bandwidth. This experimental result highlights the interest in using coherent excitation to control intra-cavity light-matter interactions and nonlinear dynamics of microcavity-based optical devices.

L'insertion d'un cristal photorefractif a l'interieur de la cavite d'un laser permet de fortement reduire le nombre de modes laser. Les modes enregistrent dans le cristal un hologramme qui modifie en retour leurs pertes. Il en resulte une tres forte selection entre les modes et qui, sous certaines conditions, force le laser a osciller sur un seul mode. L'hologramme etant systematiquement adapte a la structure de modes emis par le laser, on parle alors de cavites laser auto-organisables. Nous presentons les resultats qui ont ete obtenus sur le filtrage de modes longitudinaux dans des lasers de type quatre niveaux fonctionnant en regimes continu et impulsionnel, ainsi que dans des lasers a semi-conducteurs. En outre, nous montrons que l'insertion d'un tel cristal peut avoir comme application la stabilisation d'un mode. Le laser est alors monomode, l'inscription d'un hologramme par ce mode diminue ses pertes et etend son domaine de stabilite.

International audience; nonlinear properties of slotted silicon photonic waveguides filled with third-order nonlinear materials (NM, DDMEBT polymer) are quantitatively studied by separately calculating the effective nonlinearity susceptibilities associated to the silicon and cladding material, respectively. Optimization of the silicon slotted waveguide geometry is performed and focused on the optimization of optical power confinement in the high FOMTPA cladding material and of Aeff(NM)/Aeff(Si). The simulated nonlinear wave evolution results show the importance of properly choosing the silicon rail and slot widths in order to minimize the influence of the two-absorption process and associated free carrier effects (free carrier absorption, free carrier refraction).

International audience; Coherent excitation of a nonlinear semiconductor microcavity is theoretically reported. It intends to counterbalance the frequency drift of the cavity resonance driven by the nonlinear refractive effects, which causes a limitation in the energy coupling efficiency of an input pulse into the cavity resonance. We show that exciting such a nonlinear microcavity with tailored chirped pulses allows to maintain the benefit of light localization and to further enhance light-matter interactions, opening the way to the realization of highly efficient nonlinear devices.

International audience; Two-photon counting (TPC) interferometry has been realized by measuring the electrical current due to two-photon absorption in the space charge layer of a semiconductor detector located at the output port of an interferometer. We apply this technique to study the correlation properties of twin beams issued from parametric fluorescence. We describe in details how the different second-order correlation functions (interbeam, intrabeam) can be extracted at the femtosecond timescale from raw data. The values of these correlation functions determined by our experiments are in excellent agreement with theory. More precisely, extrabunching in twin beams is unambiguously demonstrated and theoretically described using two models: a comprehensive multimode quantum optics model and a simpler classical stochastic approach. Given the high brightness of our twin-beam source, both theories yield similar results. Finally, convenient analytical expressions of the correlation functions were derived from both theories, expressions in which we have been able to relate specific terms to accidental and exact coincidences between photons. Two-photon interferometry thus determines to which extent twin photons are twin. This technique should become a useful tool for future quantum optics developments.

International audience; We report our recent efforts for the design and fabrication of comb slot photonic crystal waveguides in silicon on insulator photonics. The main issues related to dispersion engineering, investigation of slow mode optical losses, and modal confinement metrics, have been investigated in various geometries. Nonlinear χ (3) optical properties have then been explored using a pump probe picosecond experiment and the obtained trends and results are discussed. OCIS codes: (130.5296) Photonic crystal waveguides, (130.4310) Nonlinear 1. Introduction Silicon photonics has received a strong interest in the last years, and the main efforts of the recent period have been focused on high-frequency active devices such as optical modulators and integrated detectors. To go beyond the data rates practically achievable using optoelectronic components and to reduce the energy ber bit of on-chip optical data transmissions, which appears to be a key metrics for future applications, an interesting opportunity is to exploit nonlinear optics to achieve all-optical signal processing. The main drawback of silicon is this purpose is the absence of second order nonlinear effects (χ (2)) and the strong free-carrier phenomena (Free Carrier Dispersion: FCR, Free Carrier Absorption) accompanying the Kerr effect when using χ (3)-processes. To address this issue and improve the functionality of photonic devices, the introduction of a slot in the silicon waveguide provides a way to confine light in the low index material due to the strong discontinuity at the index contrast interface [1]. Chosen according to its second and third order non-linear susceptibilities χ (2) and χ (3) , the low index material can dramatically enhance the non-linear effects in the photonic crystal waveguides. χ (2) electro-optic modulation [2] in slotted photonic crystals and χ (3) Four-Wave Mixing in slot waveguides [3] have been for example successfully demonstrated. But slotted waveguides can be also very useful for sensing thanks to the infiltration, because the analytes can interact with the extrema of the electric field, not only its evanescent part [4].

Correlations of twin beams generated by parametric down-conversion are quantitatively determined by two-photon counting interferometery. Compared with incoherent light, photon extrabunching at the fs scale is unambiguously and precisely measured.

For many years twin beams originating from parametric down-converted light beams have aroused great interest and attention in the photonics community. One particular aspect of the twin beams is their peculiar intensity correlation functions, which are related to the coincidence rate of photon pairs. Here we take advantage of the huge bandwidth offered by two-photon absorption in a semiconductor to quantitatively determine correlation functions of twin beams generated by spontaneous parametric down-conversion. Compared with classical incoherent sources, photon extrabunching is unambiguously and precisely measured, originating from exact coincidence between down-converted pairs of photons, travelling in unison. These results strongly establish that two-photon counting in semiconductors is a powerful tool for the absolute measurement of light beam photon correlations at ultrashort timescales., The second order correlation function g(2) is used to test quantum correlation properties of light. Here, two-photon counting is used to measure g(2) and an extrabunching effect is demonstrated, providing evidence that two-photon counting is an appropriate method for measuring light beam photon correlations.

Slow-mode micro-structured waveguides enable a large enhancement of optical nonlinearities. Using a generalization of the local field factor theory of nonlinear homogeneous media, we review the impact of the group velocity reduction on the hierarchy of the semi-conductor nonlinearities. As an illustration, we experimentally and theoretically reported an intrinsic limitation concerning the Raman amplification of short pulses in silicon nanowires through the self-phase modulation induced by both the optical Kerr effect and free-carrier refraction. Hence, we provide a fundamental insight on the nonlinear interaction of short pulses for the design of future slow-mode devices.

We consider the Raman amplification problem for silicon waveguides in the regime in which both the pump and signal pulses are relatively short but wide enough that their duration exceeds the phonon lifetime (about 3 ps in silicon). We use the coupled pump-signal equations for numerical simulations that include all competing nonlinear effects such as self- and cross-phase modulations, two-photon and free-carrier absorptions, and changes in the refractive index induced by the free carriers. However, numerical simulations do not provide much physical insight. For this reason, we also develop an approximate analytic approach for solving the Raman amplification problem. We introduce the concept of an effective Raman gain and show analytically how it depends on the pump bandwidth. As the pump spectrum broadens inside the silicon waveguide, the effective Raman gain is reduced considerably. We obtain an analytical form of the nonlinear phase accumulated during propagation inside a silicon waveguide and use it to calculate the total spectral broadening experienced by a pump pulse. Using this result, we can predict changes in the effective Raman gain as a function of pump pulse energy. A comparison of our predictions with the recent experimental data shows that our model is reasonable and captures the essential physics.

International audience; We study the photon correlation properties of broadband parametric down-converted light. The measurement of the photon correlation is carried out thanks to a modified Hanbury Brown-Twiss interferometer based on two photon absorption in GaAs detector. Since this method is not affected by the phase matching conditions of the detecting apparatus (so called “final state post-selection”), the detection bandwidth can be extremely large. This is illustrated by studying, with the same apparatus, the degree of second order coherence of parametric light in both degenerate and non-degenerate cases. We show that our experiment is able to determine the coherent as well as the incoherent contributions to the degree of second order coherence of parametric light with a time resolution in the fs range scale.

In recent years, a significant amount of attention has been drawn towards all-optical semiconductor switching. To that respect, exploitation of light localization, in slow light photonic crystal waveguides (PCWs) or in microcavities, is of considerable interest because it enables enhancement of optical nonlinearities. Recently, we observed experimental and theoretical enhancement of optical Kerr effect, two-photon absorption (TPA) and free-carrier index changes (FCI) in GaAs PCWs, which demonstrated the important role of group velocity [1].

5 pages; International audience; An optical parametric oscillator for the infrared region pumped by a picosecond Ti:Sapphire laser is demonstrated. Fourier transform limited pulses of 15 and 10 ps, for signal and idler wavelengths respectively, have been obtained using a periodically poled stoechiometric lithium tantalate nonlinear crystal, without any spectral filtering. A complete experimental study of the influence of the cavity length detuning on the spectral and temporal dynamic of the output radiation is discussed.

International audience; Brightness improvement of high-power semiconductor lasers can be achieved by placing a broad-area laser diode, used as a pure optical amplifier, in an extended cavity. We report the modal behavior of an extended cavity which contains a photorefractive crystal. The dynamic holographic medium that plays the role of a spatial and spectral filter acts on the transverse and axial structure of the extended cavity modes and improves the single-mode operating range of the laser. An enhancement of the brightness in single-mode operation by a factor of seven has been experimentally observed thanks to the intracavity photorefractive crystal.

International audience; Previous works on photorefractive self-organizing laser cavities were about lasers that oscillate, prior the self-organization process occurs, on a set of axial modes sharing the same transverse structure. In a welldesigned broad-area laser diode extended cavity, we theoretically and experimentally demonstrate that the insertion of a photorefractive crystal can also affect the transverse modal structure to force the laser, initially oscillating on several transverse modes, to oscillate on a single transverse and axial mode. This spatial self-organization process leads to an enhancement of the single mode operating range of the laser.

We describe an experiment that allows advanced master students in optical science and engineering to understand noise measurements in optical amplifiers (e.g. Erbium Doped Fiber Amplifiers). Noise and signal are studied then measured in both electrical and optical domains. The degradation of the signal to noise ratio through EDFA's along a real or simulated long-haul optical link can also be studied.

Many industrial applications require high power semiconductor laser sources emitting beams of good quality. However, the emission of a free running high-power broad-area semiconductor laser contains many lateral modes that explains its poor beam quality and low brightness. One of the techniques to improve beam quality consists in placing a broad-area laser diode, used as a pure optical amplifier, in an extended cavity. Such a technique has proved its efficiency to produce a nearly diffraction limited beam at least for low pumping current level. For higher pumping currents, its spatial quality is deteriorated by the oscillation of higher order extended cavity modes. Using numerical simulations, we demonstrate that the insertion of a photorefractive crystal inside a broad-area laser diode extended cavity should extend the laser operating single mode range.

Single longitudinal-mode laser systems that use semiconductor lasers are nowadays really efficient. Nevertheless they often remain very costly since they require high-end technology in order to keep single-mode emission. We previously showed that inserting a photorefractive crystal inside a laser cavity induces a mutual adaptation of the laser modes to the induced hologram: the crystal acts as a spectral filter that adapts itself in order to modify the gain versus losses balance so that only one cavity-mode oscillates. We built such a compact monolithic cavity using a red laser diode and a BaTiO3:Co crystal. This cavity does not comprise any spectral filter but the photorefractive crystal and without thermal control nor additional anti-reflection coating of the laser diode. We will present the spectral behavior of the self-adaptive cavity when changing the operating point of the laser and specify characteristics such as intensity and wavelength variations over time, which are important regarding to applications.

Many industrial applications require high power semiconductor laser sources emitting beams of good quality. However, the emission of a free running high-power broad-area semiconductor laser contains many lateral modes that explains its poor beam quality and low brightness. One of the techniques to improve beam quality consists in placing a broad-area laser diode, used as a pure optical amplifier, in an extended cavity. Such a technique has proved its efficiency to produce a nearly diffraction limited beam at least for low pumping current level. For higher pumping currents, its spatial quality is de-teriorated by the oscillation of higher order extended cavity modes. Using numerical simulations, we demonstrate that the insertion of a photorefractive crystal inside a broad-area laser diode extended cavity should extend the laser operating single mode range.

International audience; Inserting a photorefractive crystal inside a laser cavity leads to striking behaviors. The oscillating modes record a dynamic hologram in the crystal, which in turn acts as a spectral filter for these modes. For a correctly designed system, this mutual adaptation spontaneously forces the laser to oscillate on a single longitudinal mode. We describe this self-adaptive process, and we illustrate its operation in the case of two different lasers.

International audience; A photorefractive crystal inserted inside an extended cavity laser diode is an adaptive frequency filter. Without any other selective element, the mutual adaptation between the modes and the dynamic photorefractive hologram leads to a single-longitudinal-mode oscillation. We demonstrated this adaptive process with a 810-nm diffraction limited laser diode and a BaTiO3 photorefractive crystal. We analyzed the transient spectra from the initially multimode oscillation until the single-mode steady state.

International audience; The reduction of the number of oscillating modes by a self-adapted Fabry–Perot (FP) filter was first demonstrated in a dye laser in 1987 [Opt. Lett. 12 (1987) 117]. A renewal of interest in this technique arose recently with the observation of a dramatic reduction of the number of modes in a pulsed Sapphire:Ti laser [Appl. Phys B 69 (1999) 155]. A self-adapted Fabry–Perot filter is made of a photorefractive crystal inserted inside a linear laser cavity between the amplifier and the output coupler. This coupler acts as one mirror of this Fabry–Perot while the Bragg reflection grating recorded by the standing waves inside the crystal plays the role of the second mirror. The dynamic properties of the photorefractive effect make this Fabry–Perot continuously adapted to the oscillating modes. Its spectral reflectivity reinforces the competition between these modes which, in some cases, leads to a single-mode operation. We analyze the laser mode competition taking into account the laser gain properties and the wavelength dependence of the cavity losses induced by the self-adapted Fabry–Perot.

Dramatic reduction of the number of oscillating modes in CW and pulsed lasers has been observed using a photorefractive crystal placed inside the cavity. Reflective Bragg gratings recorded by the standing waves inside the crystal act with the output coupler as a self-adapted Fabry-Perot filter. Its spectral characteristics enhance mode competitions which can lead to a single mode operation.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Summary form only given. Two interfering beams inside a photorefractive crystal record a dynamic index grating. For a single mode laser, diffraction of the recording beams onto the induced grating leads to the well known two beam coupling phenomenon: energy is transferred from one beam to the other one. If the laser runs on several longitudinal modes, the energy exchange differs from mode to mode. The photorefractive crystal thus acts as a frequency filter whose spectral transmission depends on the spectral modal structure. We describe the main characteristics of several photorefractive self-adaptive filters based on either two or four wave mixings. We also show how such filters can modify the laser transverse structure.

A novel extended cavity arrangement for a laser diode is proposed. It uses a dynamic holographic filter made of a photorefractive crystal and a distant plane mirror. Single-mode operation has been achieved using a 810 nm diffraction limited laser diode. The time evolution of the laser spectrum from the initial multimode operation until the final single-mode steady state is presented. The holographic filter increases the current range over which the laser is single-mode.

Experimental results on the realization and characterization of microspherical lasers at 1.56 micrometers with Er-doped ZBLAN glasses are presented. The lasers are excited through an evanescent wave coupling. Thresholds of 600 (mu) W have been obtained. Multimode operation has been observed with linewidths of the order of 250 KHz.

Experimental results on the realization of microlasers at 1.05 micrometers with Nd-doped ZBLAN spheres are described. A microwave plasma torch process was developed to produce spheres with very low ellipticities. Laser oscillation was obtained using properties of whispering gallery modes. These modes were excited with an evanescence wave coupling which provided a low threshold and an oscillation with only few modes.

Light can be confined efficiently in the high-Q, small-volume whispering-gallery-modes observed in silica microspheres. By coupling these microspheres to eroded optical fibers and fiber tips, direct mapping of the whispering-gallery modes has been achieved and the mode numbers have been assessed. The properties of these modes have allowed us to obtain laser action with very low thresholds in Nd-doped silica microspheres. Further projects in the field of non-linear optics and cavity quantum electrodynamics are described.

We use scanning near-field optical microscopy to study the spatial structure of electromagnetic resonance modes in a high-Q dielectric microcavity of the whispering-gallery mode type. Direct observation of the spatial mode structure on the microcavity surface allows us to estimate the mode numbers of individual resonances. In this way we are able to identify those modes having the most suitable characteristics (high Q, low mode volume) for use in proposed experiments with these microcavities. Furthermore we believe that our experiments demonstrating the ability to manipulate microscopic objects in the mode volume of a high-Q microcavity suggest one way in which the interaction between these modes and atoms or molecules could be studied.

International audience; We demonstrate the efficient excitation of high-Q whispering-gallery modes in near-spherical fused-silica microparticles in the size range 60–450 μm by the use of an eroded monomode optical fiber. When the sphere is placed in the evanescent field of the guided fiber mode, light is resonantly coupled from the fiber into the microparticle. We report a broadening of resonance modes and a shift of the resonance central frequency as the coupling strength is increased by reduction of the gap between the sphere and the fiber.

International audience; We demonstrate the use of a near-field probe to map the angular dependence of high-Q whispering-gallery modes in fused-silica microspheres. The mapping is performed by placing a micrometer-sized tip formed on the end of a monomode fiber into the evanescent field at the microsphere surface, causing light to be coupled from the microsphere resonance into the fiber guided mode. The light output of the fiber is then measured while the tip is moved to different points on the microsphere surface. We have used this method to investigate the lifting of spherical degeneracy in the system.

Exciting work is being done in quantum information theory and the detection of low light levels.

International audience; We demonstrated the use of a near-field probe to map the evanescent field of an optical standing wave in a fused-silica whispering-gallery mode microresonator. The periodicity of the observed standing wave allows us to estimate accurately the radial mode number of the whispering-gallery mode resonance that is being excited. We find that the use of a fiber half-coupler to excite these resonances in fused-silica microspheres results in only the lowest radial mode numbers’ being strongly excited, as predicted.

Previous works on photorefractive self-organizing laser cavities were about lasers that oscillate, prior the self-organization process occurs, on a set of axial modes sharing the same transverse structure. We have now demonstrated that the insertion of a photorefractive crystal can also create a dynamic spatial filter. In a well-designed extended cavity broad-area laser diode, such a filter can affect the transverse modal structure to force a laser, initially oscillating on several transverse modes, to oscillate on a single transverse and longitudinal mode. A large increase of the single-mode operating range of the laser has been observed.

We report the first third order nonlinear characterization of Ge-rich Sij.xGex waveguides, with Germanium concentrations ranging from 0.7 to 0.9. A bi-directional top hat D-Scan method was used to determine the waveguide nonlinear parameters and to deduce the Kerr nonlinear refractive index and the two-photon absorption coefficient at the wavelength of 1.58 μm.