Lutz Leine

Evolution of nonlinear guided optical fields down a dielectric film with a nonlinear cladding

We present a comprehensive, numerically oriented investigation of spatially nonstationary, TE-polarized optical field distributions guided by a thin film that can support both TE and TE waves. The film is asymmetrically bounded by a Kerr-like nonlinear cladding that may exhibit linear material losses. All field distributions have been generated by exciting different guided waves at one of the film end faces and tracing them down the film with the help of an extended propagating beam method. The long-distance asymptotic behavior is of particular interest. The numerical-stability analysis of the dispersion curve for nonlinear guided waves (NGW’s) indicates that the positively sloped TE0 branches are stable; this is not strictly true for the TE1 branch. Unstable NGW’s display a rich set of nonstationary phenomena, including soliton formation and nonlinear beating effects. Characteristic transformation lengths depend strongly on the definite, unstable initial NGW profile. The main effect of cladding losses is that the field tries to avoid the lossy region, leading to a considerable loss reduction. This property may destroy characteristic NGW features for propagation lengths approaching the absorption length of the lossy cladding. Phase-locked single-soliton and multisoliton emission can be induced by launching linear guided modes with the appropriate power flow through one of the film end faces.

Strongly nonlinear effects with weak nonlinearities in smart waveguides

Abstract A properly tailored semiconductor multifilm configuration may be viewed as an effective waveguide with arbitrarily small refractive index differences. For the first time we report that strongly nonlinear effects as the existence of nonlinear guided waves can occur in materials with weak, off-resonant Kerr nonlinearities. This result is expected to offer the opportunity to observe nonlinear guided waves experimentally.

Propagation phenomena of nonlinear film guided waves in a configuration with material losses: a numerical analysis

The propagation of TE-polarized nonlinear guided waves excited at different points of the nonlinear dispersion curve has been examined along a thin film bounded by two identical self-focusing media with linear absorption. The evolution of the fields indicates that the stable branches of the nonlinear dispersion curve can be passed through when the total energy flow decreases because of absorption. The evolution of nonstationary waves initially excited at unstable branches of the dispersion curve shows significant dependence on the magnitude of the absorption.

Antiresonant waveguide add/drop filter using Fabry-Pérot interference

Abstract We describe planar waveguide structures that accomplish the Fabry-Perot filtering functionality by using the wavelength selective coupling between antiresonant optical waveguides. By using different cladding layer thicknesses wavelength selectivity is achieved despite the fact that the coupled waveguides have equal core thicknesses, in contrast with filters based on conventional waveguides. For the polymer-based configuration investigated here a resolution of 1.2 nm was obtained with a free spectral range of 12 nm. We also describe a dual output filter which could drop or add two different wavelengths at once in a wavelength division multiplexing (WDM) system.

The effect of bandwidth limited amplification on soliton interaction

Abstract A detailed investigation of the effect of bandwidth limited amplification on the propagation of a single soliton as well as on the interaction process in soliton trains is performed both numerically and in the framework of a perturbation approach. Two major effects of the bandwidth limited amplification may be identified, namely the creation and amplification of low-frequency radiation and the shift of the frequencies of the pulses towards the centre of the filter curve. It is shown that the initial separation and the strength of bandwidth limited amplification determine which effect prevails. Furthermore, we show that the perturbation approach describes reasonably the latter effect whereas the amplification of the low frequency components may lead to soliton instability invalidating perturbation arguments. Eventually, it turns out that the results obtained for two-soliton interaction cannot be extended straightforwardly towards N -soliton interaction.

Waveguide element for highly effective dispersion compensation in fiber transmission lines

We show that dispersion compensation over 70 km of a standard optical fiber at 20 Gbit/s can be achieved by using a strip waveguide configuration which consists of two dissimilar guides and is only a few centimeters long. Moreover, any required amount of third-order dispersion can also be compensated for.

Soliton Switching in Semiconductor Directional Couplers

There is an urgent need in all-optical signal processing to implement fast switching devices with high switching contrast. Nonlinear directional couplers (NLDC) have attracted a great deal of interest as basic all-optical elements because the output may be routed between the two channels as a function of the input power launched into one channel. The stationary response of that device has been discussed in detail for numerous situations (for a summary, see e.g. the review paper1 and the references therein). That approach holds also in the nonstationary regime provided that the pulses are sufficiently long and/or the waveguides are short. In that case the dispersion of the group velocity (GVD) may be neglected. The time in the reference frame of the pulse enters the coupled mode equations merely as a parameter. This has the consequence that the coupling behavior (efficiency and coupling length) depends on the instantaneous intensity of the pulse at a certain time leading to the detrimental pulse break up2 and thus to an incomplete switching between both channels. This phenomenon limits considerably the opportunity of cascading different switching elements.

Remote Coupling in a Rib Arrow Coupler

Almost a decade ago a novel guiding scheme which relies on resonance effects such as one finds in Fabry-Perot resonators could be identified1. Those so-called Anti-Resonant-Reflecting-Optical-Waveguides (ARROWs) did not attract a considerable attention among the waveguide community until recently. They exhibit several striking peculiarities as quasi-monomode behavior, particular dispersion relations that can be exploited to implement attractive devices as e.g. remote couplers2 ,remote switches3, polarizers4, polarization splitters and extremely narrow-band wavelength filters.

Soliton switching in properly tailored nonlinear directional couplers

For the first time, we show that soliton switching in semiconductor nonlinear directional couplers without the detrimental pulse break up can be achieved by properly tailoring the waveguide configuration. In particular, we exploit the unusual dispersion characteristic of Anti-Resonant-Reflecting-Optical Waveguides to overturn the normal group velocity dispersion in semiconductors below half-the-band-gap. The coupled set of modified Nonlinear Schrodinger equations is derived and solved by using the Beam Propagation Method.

Design and fabrication of rib ARROW couplers

In the conventional waveguiding concept the coupling distance between two waveguides is limited in order to ensure a sufficient overlap of the evanescent tails of the modal field profiles. The Anti-Resonant Reflecting Optical Waveguide (ARROW) scheme provides modal fields exhibiting an oscillating nature in the coupling region too, thus allowing for remote coupling. The design of low-loss and loss-free rib ARROW couplers is demonstrated. In order to give precise predictions of the coupling length, the computation of the propagation constants demands full vectorial numerical methods for solving the wave equation. In a SiOxN1-x configuration the coupling over a spacing distance up to 93 micrometers between two rib ARROWs has been demonstrated experimentally. Experimental and theoretical results are compared and discussed.