Investigation on roughness-induced scattering loss of small-core polymer waveguides for single-mode optical interconnect applications.

We investigated the roughness-induced scattering loss (LossR) of small-core polymer waveguides fabricated using the photolithography method, both theoretically and experimentally. The dependence of LossR on the roughness parameter, waveguide dimension, operation wavelength, refractive index difference and distribution, polarization sensitivity, sidewall angle, and bending radius were studied. The surface roughness of both the sidewall and the top/bottom of the fabricated waveguides were measured using laser confocal microscope, and the results showed that the averaged sidewall roughness was approximately 60 nm, which is 3 times that of the top/bottom surface. As a result, the sidewall roughness-induced LossR is 9 times that induced by the top/bottom roughness. The calculated value of LossR agrees well with the measured value. LossR increases rapidly with the decrease in the waveguide width, especially when the waveguide width is reduced below 10 µm, at which the LossR is approximately 0.3 dB/cm. On the other hand, the dependence of LossR on the waveguide height is negligible. Our results provide guidance for developing single-mode polymer waveguides with low loss for optical interconnect applications.