Nicolas Dubreuil

Light localization induced enhancement of third order nonlinearities in a GaAs photonic crystal waveguide

Nonlinear propagation experiments in GaAs photonic crystal waveguides (PCW) were performed, which exhibit a large enhancement of third order nonlinearities, due to light propagation in a slow mode regime, such as two-photon absorption (TPA), optical Kerr effect and refractive index changes due to free-carriers generated by TPA. A theoretical model has been established that shows a very good quantitative agreement with experimental data and demonstrates the important role that the group velocity plays. These observations give a strong insight into the use of PCWs for optical switching devices.

Laser diode made single-mode by a self-adaptive photorefractive filter

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.

Enhanced nonlinear interaction in a microcavity under coherent excitation

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.

Modeling of laser mode self-adapted filtering by photorefractive Fabry–Perot interferometers

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.

Saturation of the Raman amplification by self-phase modulation in silicon nanowaveguides

We experimentally show that the self-phase modulation of picosecond pump pulses, induced by both the optical Kerr effect and free-carrier refraction, has a detrimental effect on the maximum on-off Raman gain achievable in silicon on insulator nanowaveguides, causing it to saturate. A simple calculation of the Raman gain coefficient from the measured broadened output pump spectra perfectly matches the saturated behavior of the amplified Raman signal observed experimentally at different input pump powers.

Self-organization of laser cavities using dynamic holograms

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.

Self-induced transverse mode selection in a photorefractive extended cavity laser diode

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 well-designed 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.

Bi-directional top hat D-Scan for characterization of third order nonlinear waveguides

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.

Nonlinear Properties of Ge-rich Si 1−x Ge x Materials with Different Ge Concentrations

Silicon photonics is a large volume and large scale integration platform for applications from long-haul optical telecommunications to intra-chip interconnects. Extension to the mid-IR wavelength range is now largely investigated, mainly driven by absorption spectroscopy applications. Germanium (Ge) is particularly compelling as it has a broad transparency window up to 15 µm and a much higher third-order nonlinear coefficient than silicon which is very promising for the demonstration of efficient non-linear optics based active devices. Si1−xGex alloys have been recently studied due to their ability to fine-tune the bandgap and refractive index. The material nonlinearities are very sensitive to any modification of the energy bands, so Si1−xGex alloys are particularly interesting for nonlinear device engineering. We report on the first third order nonlinear experimental characterization of Ge-rich Si1−xGex waveguides, with Ge concentrations x ranging from 0.7 to 0.9. The characterization performed at 1580 nm is compared with theoretical models and a discussion about the prediction of the nonlinear properties in the mid-IR is introduced. 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.

Bi-directional top-hat D-Scan: single beam accurate characterization of nonlinear waveguides

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 counterdirectional nonlinear transmissions to assess the input and output coupling efficiencies, a novel procedure is described leading to 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 ±10%.