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Proceedings Paper

Flat-top MZI filters: a novel robust design based on MMI splitters
Author(s): Matteo Cherchi; Mikko Harjanne; Sami Ylinen; Markku Kapulainen; Tapani Vehmas; Timo Aalto
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Paper Abstract

Multimode Interferometers (MMIs) are an attractive alternative to directional couplers, ensuring more relaxed tolerances to fabrication errors and broader operation bandwidth. The drawback is that only a limited discrete set of splitting ratios is achievable with MMIs of constant cross section. This issue clearly limits their use in flat-top interferometric filters, which design requires, in general, free choice of the splitting ratios. Here we show for the first time that it is possible to design 4-stage flat-top interferometers using only standard MMIs with 50:50 and 85:15 splitting ratios. The design approach is based on the representation of the system on the Bloch sphere. Flat-top interleavers with different free spectral ranges have been designed and fabricated on the silicon photonics platform of VTT, based on 3 μm thick rib and strip waveguides. Two different layouts have been explored: one where all components are collinear and a more compact one which elements have been folded in a spiral shape. All interleavers have been designed for TE polarization, and they work in a wavelength range comparable with the 100 nm bandwidth of the MMI splitters. Even though fabrication imperfections and non-ideal behaviour of both waveguide bends and MMIs led to reduced extinction compared to simulations, most devices show in-band extinction exceeding 15 dB. The in-band losses of the most central channels did not exceed 1.5 dB compared to the reference straight waveguide. The designed interleavers can be employed in cascaded configurations to achieve broadband and fabrication tolerant flat-top wavelength (de)multiplexers.

Paper Details

Date Published: 14 March 2016
PDF: 9 pages
Proc. SPIE 9752, Silicon Photonics XI, 975210 (14 March 2016); doi: 10.1117/12.2210937
Show Author Affiliations
Matteo Cherchi, VTT Technical Research Ctr. of Finland (Finland)
Mikko Harjanne, VTT Technical Research Ctr. of Finland (Finland)
Sami Ylinen, VTT Technical Research Ctr. of Finland (Finland)
Markku Kapulainen, VTT Technical Research Ctr. of Finland (Finland)
Tapani Vehmas, VTT Technical Research Ctr. of Finland (Finland)
Timo Aalto, VTT Technical Research Ctr. of Finland (Finland)

Published in SPIE Proceedings Vol. 9752:
Silicon Photonics XI
Graham T. Reed; Andrew P. Knights, Editor(s)

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