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Proceedings Paper

Three-dimensional buried polymer waveguides via femtosecond direct laser writing with two-photon absorption
Author(s): Duc H. H. Nguyen; Kerstin Kaleta; Stefan Hengsbach; Ute Ostrzinski; Karl Pfeiffer; Uwe Hollenbach; Jürgen Mohr
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Paper Abstract

Integrated optics has emerged as a promising solution to the electronic interconnect bottleneck, enabling high bandwidth density and low power consumption. Recently, confining photochemical and physical reactions in a micro-volume has given an extra dimension to optical interconnection using glass or polymer. Three-dimensional waveguides can then connect, combine, or split the optical signal among any blocks in all dimensions. However, the refractive index increase is still a challenge to fabricate free-form, stable and single-mode three-dimensional buried waveguides.

This paper presents a new concept to tackle this challenge using the combination of femtosecond direct laser writing (FsDLW) in polymer and external diffusion of a gaseous monomer. FsDLW with two-photon absorption was used to initiate cross-linking following a programmed trajectory to form the waveguide core. A thermal treatment was then needed to complete cross-linking. Afterwards, a low-index monomer from a gas atmosphere was diffused into the uncross-linked cladding. Since this diffusion hardly occurred in the already cross-linked pattern, the subsequent UV flood exposure only cross-linked the diffused monomer with host oligomer in the cladding. This low-index monomer decreased the refractive index of the cladding and, therefore, created enough refractive index contrast for total internal reflection. Finally, the whole structure was hard-baked for polymerization and stabilization.

The peak refractive index change of 0.012 was revealed using refractive near field method. Measured near-field intensity at the end facet of waveguides showed single-mode Gaussian profiles. We further demonstrated how feature sizes can be linearly adjusted in the range of 5-12 μm by varying scanning speed and laser intensity. Moreover, changing the voxel shape by a field aperture in front of the objective was investigated. Our fabrication method requires only one layer of a single material without masks, contact or wet processing. Free-form waveguides with high index contrast have high potential to improve the density and flexibility of optical interconnects at board level. Some applications of this concept are three-dimensional arrays of optical waveguide network routers, optofans, pitch converters or splitters.

Paper Details

Date Published: 2 May 2014
PDF: 8 pages
Proc. SPIE 9130, Micro-Optics 2014, 91300N (2 May 2014); doi: 10.1117/12.2051326
Show Author Affiliations
Duc H. H. Nguyen, Karlsruher Institut für Technologie (Germany)
Kerstin Kaleta, Karlsruher Institut für Technologie (Germany)
Stefan Hengsbach, Karlsruher Institut für Technologie (Germany)
Ute Ostrzinski, micro resist technology GmbH (Germany)
Karl Pfeiffer, micro resist technology GmbH (Germany)
Uwe Hollenbach, Karlsruher Institut für Technologie (Germany)
Jürgen Mohr, Karlsruher Institut für Technologie (Germany)


Published in SPIE Proceedings Vol. 9130:
Micro-Optics 2014
Hugo Thienpont; Jürgen Mohr; Hans Zappe; Hirochika Nakajima, Editor(s)

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