Thermal stabilities of three-cavity narrow-bandpass (NB) filters with high-index half-wave spacers and 78–102 layers of Ta2O5 and SiO2 prepared by reactive ion-assisted bipolar direct-current (dc) magnetron sputtering of tantalum and silicon targets, respectively, were investigated. Pure argon and pure oxygen were used as the sputtering gas and the reactant, respectively. The oxygen gas was introduced and ionized through the ion gun and toward the unheated BK7 glass substrate. The refractive indices of single-layer Ta2O5 and SiO2 films were 2.1 and 1.45, respectively, at 1550 nm, which were comparable with those of films prepared by other ion-assisted coating techniques. The moisture-resistant properties of the films were excellent as evidenced from the water-immersion test, implying that the packing density of the films was close to that of their bulk materials. The temperature-dependant wavelength shifts of the NB filters were <3 × 10-3 nm/°C at temperatures of <75 °C, indicating that the temperature-induced wavelength shift of the filter was <0.15 nm when the temperatures were raised from room temperature to 75 °C, which was compliant with Bellcore GR-1209-CORE generic requirements of NB filters used for optical-fiber communication systems.
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