We have previously demonstrated high efficiency small-area 45° single air interface waveguide bends in a perfluorocyclobutyl (PFCB) material system [Opt. Express 12, 5314 (2004)]. In this paper we show how the loss per bend can be decreased through improved bend interface position accuracy and sidewall smoothness. This is achieved with electron-beam lithography (EBL) in a scanning electron microscope (SEM) at the cost of increased fabrication complexity compared to our previous work based on a UV contact mask aligner. Using the EBL-based fabrication process, the measured loss per bend decreases from 0.33 dB/bend to 0.124 dB/bend (97.2% bend efficiency) for TE polarization (electric field in plane) and from 0.30 dB/bend to 0.166 dB/bend (96.2% bend efficiency) for TM polarization (electric field out of plane). Since the alignment accuracy and patterning capability within a single exposure field for our low-end electron-beam lithography approach is comparable to what is achievable in high-end stepper tools, the significance of this work is that very low loss air trench bends in low refractive index and low refractive index contrast waveguide materials should be achievable using a conventional high volume microfabrication toolset.
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