A novel design for a polymer-based singlemode beam splitter with a refractive index difference of (Delta) n equals 0.01 has been developed to grant the functionality at higher tolerances of (Delta) n(+/- 0.002). This was done by interpreting simulations carried out using the beam propagation method. For efficiently increasing the fiber to waveguide coupling a double-sinus s-bend taper could be identified for adapting the beam shapes of the standard singlemode fiber to the waveguide. Before splitting the beam is led through a 'rib mode stripper' which allows the suppression of higher modes and causes only a negligible disturbance of the fundamental model. For realizing this component, a number of commercially available polymers for optical waveguide components has been analyzed under the aspect of temperature stability of refractive indices. For that purpose a heatable Abbe refractometer has been used. Core and substrate materials are showing different thermal behaviors. Therefore, we could prove that for waveguide designs requiring the standard singlemode fiber refractive index difference of (Delta) n equals 0.004, the necessary stability of (Delta) n may only be achieved within a typical temperature interval of about 10K. Only one optical core material enabled a temperature range of about 95K, still somewhat below the requirements of the Bellcore- specifications.
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