In this paper, we propose for the first time a high-coupling-efficiency micron-size mode-converter between slot-waveguides and channel-waveguides. Fig.
2005 Conference on Lasers and Electro-Optics Europe
Compact slot-waveguide/channel-waveguide mode-converter J. Blasco and C.A. Barrios Nanophotonics Technology Center, Universidad Politecnica de Valencia, Camino de Vera s/n, Edificio IDI-S, 46022 Valencia (Spain) jablasol@doctor. upv. es
Abstract: A compact structure for highly efficient optical coupling to/from slot-waveguide from/to channel-waveguide is presented. The mode-converter permits an achievement of coupling efficiency as high as 99.3% for a 3-jim-long coupler. A new type of integrated optical waveguide called slot-waveguide has been recently demonstrated [1]. By using the electric-field (E-field) discontinuity at the interface between high-index-contrast materials (e.g. Si/SiO2), this structure enables the guidance and confinement of light inside a nanometre-size area of low-index material, with high E-field amplitude and optical intensity in that region. This property allows highly efficient interaction between fields and active materials, which may lead to all-optical switching [2] and ultra-sensitive sensors on integrated photonics. The use of a high-index-contrast system enables the implementation of highly integrated photonic circuits. Integration of conventional channel-waveguides, for low loss optical interconnects, and slot-waveguides, for the implementation of active devices, on a single Si chip makes necessary the use of a compact optical mode-converter for efficient optical coupling between both types of waveguides. In this paper, we propose for the first time a high-coupling-efficiency micron-size mode-converter between slot-waveguides and channel-waveguides.
Fig. Ia shows a schematic diagram ofthe proposed structure. Si/SiO2 material system is assumed. A single-mode channel-waveguide is tapered to form one strip of the slot-waveguide. The taper delocalizes the propagating mode, which is funnelled into the slot-waveguide by using the other strip ofthe slot-waveguide. Thus, mode and effective index matching is achieved. The structure was modelled by using the beam propagation method and finite-difference time-domain (FDTD) method. Fig. lb shows the coupling efficiency for the quasi-TE mode (major E-field along the xaxis) of the structure for different coupler lengths (1c) and an operation wavelength of 1.55 jim. It is seen that the coupling efficiency saturates for coupler lengths > 3 jm. A spectral analysis ofthe proposed structure indicates a very low wavelength dependence since there is no interferometric effect involved. The coupling efficiency varies less than 1% over a wavelength span of more than 200 nm. The studied structure has been shown to be advantageous as compared to other coupling geometries.
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Fig. 1. a) Schematic diagram of a compact slot/strip-waveguide mode-converter structure. b) Coupling efficiency for the quasi-TE mode as a function of the taper length (1c) for 1.55-jim wavelength. 1. V. Almeida, Q. Xu, C.A. Barrios and M. Lipson, Optics Letters, vol. 29, no. 1l, p. 1209, 2004. 2. C.A. Barrios, Electronics Letters, vol. 40, no. 14, p. 862, 2004. 0-7803-8974-3/05/$20.00 ©2005 IEEE
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