Cyril Cambournac

Simulations and experiments on self-focusing conditions in nematic liquid-crystal planar cells

Owing to the nonlinear effect of optical field-induced director reorientation, self-focusing of an optical beam can occur in nematic liquid crystals and an almost diffraction-compensated propagation can be observed with milliwatts of light power and propagation lengths of several millimeters. This opens the way for applications in all-optical signal handling and reconfigurable optical interconnections. Self-focusing of an optical beam in nematic liquid-crystal cells has been studied experimentally and by means of numerical simulation. The relationships between bias voltage, cell thickness and required optical power have been examined, thus allowing the determination of the most favorable conditions for soliton-like beam propagation.

Measurement of the self-induced waveguide of a solitonlike optical beam in a nematic liquid crystal

Using phase-measurement interferometry, we observe the waveguide induced by a solitonlike optical beam sustained by the molecular reorientation-induced optical nonlinearity of a nematic liquid crystal in planar configuration. Our purpose in the study is to characterize the nonlocality of the optical response of nematic liquid crystals. A good agreement is obtained between the experiment and a full (2+1)-dimensional numerical simulation of the nonlinear optical beam propagation in the cell.

Nanophotonic Polarization Diversity Demultiplexer Chip

We demonstrate a very compact multifunctional photonic-crystal demultiplexer on high index contrast InP-membrane for coarse WDM applications. Polarization diversity is implemented using 2D-grating couplers. The performance of the device is evaluated using integrated p-i-n photodetectors. Polarization diversity from fiber to detector-without intermediate functional device-results in a minimal polarization dependent loss (PDL) of 0.2 dB. This value increases to 1.1 dB when including the photonic-crystal demultiplexer.

Slow-light regime and critical coupling in highly multimode corrugated waveguides

Large and periodically corrugated optical waveguide structures are shown to possess specific modal regimes of slow-light propagation that are easily attainable. The very multimode nature of the coupling is studied by employing coupled-mode theory and the plane-wave expansion method. Given a large enough light cone, associated with a surrounding medium with low enough refractive index, we notably identify a critical slowdown regime with an interesting bandwidth-slowdown product. Essential features of these original systems are further explored: the nature of the coupled modes, the role of gain, symmetry effects, polarization, and relation with photonic-crystal systems. Practical systems are introduced using finite-difference time-domain methods, which provides first-order rules for the use of the above phenomena and their implementation in devices.

Time dependence of soliton formation in planar cells of nematic liquid crystals

Spatial optical solitons can be observed in bulk nematic-liquid-crystal cells with an entrance window and a bias voltage applied over the cell to enhance the optical nonlinear effect of laser-induced molecular reorientation. The soliton-induced waveguide is observed in transmission by use of a second light source and crossed polarizers. This setup allows us to investigate the time dependence of the molecular reorientation that sustains the soliton-like beam propagation. Results from a numerical simulation are in good agreement with the experimental results, which confirms the validity of our model.

Photonic-Crystal Demultiplexer With Improved Crosstalk by Second-Order Cavity Filtering

In this paper, we implement on a silicon-on-insulator (SoI) platform a photonic-crystal demultiplexer that operates on the principle of mini-stopbands of broader waveguides. Previous InP-based membrane versions showed modest crosstalk at 10 nm spacing. Here, we implement a second cavity in the form of a second parallel waveguide almost identical to the core one. Simulations employing three-modes coupled-mode theory and finite-difference time-domain help optimizing the coupling. The experimental realization with deep ultraviolet exposure and grating couplers is the first of this demux strategy on SoI. It successfully shows crosstalk in the 15-20 dB range on a very small footprint.

Integration of grating couplers with a compact photonic crystal demultiplexer on an InP membrane

We demonstrate the integration of a 30% efficient grating coupler with a compact photonic crystal wavelength demultiplexer (DeMUX). The DeMUX has seven output channels that are spaced 10 nm apart and is aimed at coarse WDM applications. The integrated devices are realized on a high-index-contrast InP membrane using a simple benzocyclobutene wafer bonding technique. Cross talks of -10 to -12 dB for four channels 20 nm apart are obtained without optimization.

In-plane Littrow lasing of broad photonic crystal waveguides

Broad photonic crystal waveguides forming open resonators are shown to support an hitherto unnoticed lasing pattern. The feedback for lasing originates in Littrow-type reflections of higher-order modes from the waveguide boundaries. The authors employ plane wave and finite-difference time-domain simulations of bulk crystal and waveguide to substantiate the concept of a distributed Littrow reflector. Experimental results are reported for a 10-μm-wide photonic crystal waveguide deeply etched into InP substrate. In-plane lasing and low modal threshold gain due to longer path lengths are the key features of this resonator.

Multifunctional Photonic Crystal Compact Demux-Detector on InP

We demonstrate a very compact multifunctional photonic crystal device on InP- membrane. Grating-coupled fibers feed a multimode photonic crystal wedged waveguide accomplishing individually selectable coarse WDM demux within 20 μm per channel toward membrane integrated detectors.

Fast self-pulsing through nonlinear incoherent feedback

We consider a double-pass ring cavity with nonlinear incoherent optical feedback and analyze its response when it is driven by a continuous laser beam. This particular cavity is equivalent, in the temporal domain, to a simple spatial-pattern-generating system made from a Kerr slice and a feedback mirror. After formulating the evolution equations, we investigate the behavior of small-amplitude solutions and obtain an expression for the round-trip gains. We then explore the important effect of dispersion in the nonlinear medium. Finally, we show that stable modes are possible by solving numerically the full nonlinear equations.