Experimental and theoretical study of turning mirrors and beam splitters with optimized waveguide structures

Integrated waveguide turning mirrors (IWTM) based on total internal reflection are investigated theoretically and experimentally. The effects of the mirror surface displacement, the rotation, and the roughness are studied. The loss due to the various spectral components of the roughness is comprehensively analysed. A wide waveguide turning mirror is more influenced by the low-frequency component, which is the major component of the actually fabricated mirror surface, resulting in high radiation loss of an IWTM with long taper, contrary to expectation. The excess radiation loss is about 0.8 dB/mirror and 1.0 dB/mirror for quasi-TE and quasi-TM modes, respectively, in the optimum design, which could be reduced by further minimizing the displacement error of the mirror position. We also designed and fabricated a low-loss beam splitter with modal interference region, the measured excess losses being about 1.2 dB and 1.8 dB for quasi-TE and quasi-TM modes, respectively, which are believed to be the lowest in this kind of beam splitter.