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Low-loss air trench structures that permit reduced-size bends in low (Δ = 0.7-7%) and medium (Δ = 7-20%) index contrast waveguides are demonstrated. Local high index contrast at bends is achieved by introducing air trenches with "cladding tapers" that provide an adiabatic mode transition between low and high index contrast regions. We have fabricated and measured the performance of bends and T-splitters in silica low index contrast waveguides. Complimentary metal-oxide-semiconductor (CMOS) compatible processes are effective in processing, and measured losses are low and consistent with theoretical simulations.