University of Twente Student Theses


Experimental demonstration of reduced bend losses in low-contrast polymer hybrid waveguides

Meent, W.E. van de (2015) Experimental demonstration of reduced bend losses in low-contrast polymer hybrid waveguides.

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Abstract:In the pursuit of smaller and smaller optical devices, efficient waveguides are desired for signal transmission and sensing applications. New waveguide designs should meet economic criteria as well as technical requirements: they have to be energy-efficient and processable on micrometer scale. Polymer waveguides have been proposed as a good candidate for integrated optical circuitry, because they are easy to fabricate at low cost. Also polymer materials span a wide range of refractive index values, and they exhibit a large transparency window. But unfortunately such waveguides show relatively high radiation losses in sharp bends, which until now has limited their use in practical applications. The high losses in bends are due to the low refractive index contrast between the polymer and its surroundings. The smallest bend radius that still exhibits sufficiently low losses is in the order of tens of microns. Because of the otherwise promising properties of polymer waveguides, a practical way to reduce their bend losses is highly desirable. Several mechanisms contribute to losses in waveguide bends: straight-to-bend and bend-tobend mode transition losses, propagation losses, and losses due to radiation into the substrate or cladding. Pure bend losses are defined as the sum of propagation and radiation loss. With numerical simulations the Optical Sciences group at the University of Twente proved that adding a thin metal layer underneath the core of a polymer waveguide results in better confinement of the bend mode, and therefore decreases the bend losses for both polarizations. By the introduction of this metal layer the optimum radius of curvature was reduced to only a few microns. In this work, the performance of fabricated waveguides with a metallic layer underneath (metallic waveguides) was characterized in comparison to their dielectric counterparts. TEpolarized light from a laser diode emitting at 1550 nm was coupled to the waveguides by focusing on the entrance face of the waveguides, and the output power was measured. From in- and output power measurements the total system loss was calculated. Indeed, the total system losses in sharply bent metallic waveguides with radius of curvature R = 4 and R = 7 mm were significantly lower than in the non-metallic waveguides. The output power was measured for identical waveguides with increasing number of 90◦ bends. The increase in loss per two extra bends was equal to the sum of two total bend losses plus one bend-to-bend loss. Even though many of the waveguides were damaged and demonstrated unexpected extra losses, still the metallic waveguides exhibited much lower losses than the non-metallic ones. However, the obtained loss factors were 2 to 10 times smaller than was predicted by numerical simulations. It is expected that this was due to differences in thickness and width between the fabricated waveguides and the simulated structure. Further research may focus on the determination of the pure contribution of total bend loss, by characterizing waveguides that are corner-shaped. Also it is desirable to enhance the quality of individual waveguides for more accurate measurements. This can be done by fabricating new samples as well as polishing the rough end facets of the waveguides.
Item Type:Essay (Bachelor)
Faculty:TNW: Science and Technology
Subject:33 physics
Programme:Applied Physics BSc (56962)
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