| Summary: | Abstract The terpolymer (PFS–GMA–TFEMA) was prepared and coated on silica waveguide for polymeric optical overclad. In our study, athermal characteristics were realised by adjusting the coefficient of negative thermal expansion of the synthesised terpolymer overclad, and the positive thermo‐optic coefficient of the silica thin film. The measured values were −1.11 × 10–4 /K and 0.79 × 10–5 /K (dn/dT). The thermal properties according to the temperature change indicated that the wavelength had shifted by about 0.15 nm from 20 to 90°C, with little change over 50°C. When the temperature increased, the wavelength shift of the silica/terpolymer hybrid arrayed waveguide grating was −0.002 nm/°C, compared to the 0.01 nm/°C of silica arrayed waveguide grating. Terpolymer overclad arrayed waveguide grating showed a centre wavelength loss of 5.334 dB, loss uniformity of 0.864 dB, a polarisation dependent loss of 0.162 dB, and a non‐adjacent crosstalk of −31.67 dB.
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