A fabrication process for PFCB waveguide air-trench bends on silicon substrate with electron beam lithography (EBL) and autoalignment has been developed and a high efficiency air-trench bend (97.2% for TE polarization and 96.2% for TM polarization) has been demonstrated. The simulation predicts an optical efficiency of 98.5% for TE and 98.2% for TM when the Goos Hanchen shift is properly accounted for. We have adapted this process to making air trench bends in PFCB waveguides on polyimide substrates. Compared to silicon substrates, the polyimide substrates have the advantage of less...