R. Schiek

Nonlinear refraction caused by cascaded second-order nonlinearity in optical waveguide structures

In crystals that lack a center of inversion, in addition to the third-order nonlinearity a cascaded χ(2):χ(2) second-order nonlinearity contributes to the nonlinear refraction. A coupled-mode theory in the frequency domain is used to investigate theoretically the application of this new nonlinear index of refraction for realizing intensity-dependent temporal and spatial light-field evolution in waveguide structures. The significant modification and the immense enhancement of the effects of purely third-order optical nonlinearity by second-order down-mixing of the driven second-harmonic field with the incoming light at the fundamental are discussed.

Second-harmonic generation and cascaded nonlinearity in titanium-indiffused lithium niobate channel waveguides

We summarize and discuss the results of our second-harmonic generation experiments in titanium-indiffused lithium niobate optical channel waveguides. The wave-vector mismatch in the nonlinear wave interaction was varied with temperature tuning around the second-harmonic resonances. Fundamental depletion and second-harmonic tuning curves show a strong power dependence, which is an indication of an intensity-dependent wave-vector modification of the interacting modes. This nonlinear refractive effect (which is called cascaded nonlinearity) is characterized with interferometric measurements of the resulting nonlinear phase shift of the fundamental. Large nonlinear phase shifts (>2π) appear in regions of low fundamental depletion (<10%) because of a nonuniform wave-vector mismatch along the waveguide. At resonance a maximum fundamental depletion of more than 90% was observed. All the measured results are explained well theoretically with a coupled-mode model that has proved to be a reliable design tool for fabricating waveguide devices for applications of the cascaded nonlinearity.

All-optical switching in the directional coupler caused by nonlinear refraction due to cascaded second-order nonlinearity

The intensity-dependent switching characteristic of the nonlinear coupler is simulated. Here, for the first time, a cascaded χ(2) : χ(2)second-order nonlinearity in addition to the direct χ(3) third-order nonlinearity is used as the switching nonlinearity. The nonlinear refraction due to the cascade process can produce much larger effects than the electronic third-order nonlinearity which yields a reduced switching power of the nonlinear coupler. The description of the dynamics of the switching process suggests that switching of subpicosecond pulses is realizable. The resonance of the cascaded nonlinearity is primarily the resonance of second harmonic generation. Thus, the strength of the cascaded nonlinearity can simply be optimized corresponding to the required bandwidth by adjusting the phase-mismatch.

OBSERVATION OF ONE- AND TWO-DIMENSIONAL DISCRETE SURFACE SPATIAL SOLITONS

The recent theoretical predictions and experimental observations of discrete surface solitons propagating along the interface between a one- or two-dimensional continuous medium and a one- or two-dimensional waveguide array are reviewed. These discrete solitons were found in second order (periodically poled lithium niobate) and third order nonlinear media, including AlGaAs, photorefractive media and glass, respectively.

Bandstructure measurement in nonlinear optical waveguide arrays

We demonstrate a technique for measuring the linear bandstructure of periodic systems with quadratic nonlinearity. The phase-matching constraints of parametric optical frequency mixing are employed to probe the longitudinal wavevectors of the eigenmodes. The method is verified experimentally by measuring the diffraction relation of eigenmodes in periodically poled titanium-indiffused lithium niobate waveguide arrays with second-harmonic generation.

Thin film devices for all-optical switching and processing via quadratic non-linearities

Quadratic non-linear optical effects, specifically the phase-shift that a fundamental frequency wave experiences through cascading during second-harmonic-generation and the successive down-conversion, are a viable route to all-optical signal processing in integrated optics geometries. We review the most relevant device concepts and their potential applications, with examples of experimental results and prospects for implementation in non-linear thin films.

Cascaded second-order nonlinearity in nonuniformly temperature tuned LiNbO/sub 3/ channel waveguide

We have demonstrated by changing the temperature scan speed the possibility of achieving different temperature profiles, therefore different wave-vector mismatch along the LiNbO/sub 3/ optical channel waveguide. We can control the sign of the induced large nonlinear phase shift in the small fundamental depletion region, which is very useful in applications. We also tried to adjust the scan speed to obtain the uniform temperature profile. Theoretical explanations of the experimental results have been given.

Nonlinear effects in PPLN waveguide resonators

Resonator bistability in Ti:PPLN waveguide was tested experimentally. Spectrum changes were measured at different input powers and compared with theory. Parameters necessary for bistable operation are accessible via temperature and wavelength tuning.

All-Optical Wavelength Conversion, Parametric Amplification, Multiplexing, and Switching in Integrated PPLN-Devices

Recent progress achieved with integrated optical devices exploiting quasi-phasematched second order nonlinear interactions in periodically poled Ti:LiNbO3 waveguide structures is reported. Wavelength conversion, parametric amplification, multiplexing, and switching are demonstrated as examples of ultra-fast all-optical signal processing in the 1.5-mum wavelength range for transparent optical networks

Direct measurement of the diffraction relation in waveguide arrays

High quality one-dimensional waveguide arrays were fabricated and characterized. Second-harmonic generation in the array waveguides proved to be an ideal tool for a direct measurement of the discrete diffraction relation.