Planarization of film deposition and improvement of channel structure for fabrication of anti-resonant reflecting optical waveguide type X-crossing vertical coupler filter

To realize a low-sidelobe narrow-band Anti-resonant reflecting optical waveguide (ARROW)-type vertical coupler filter, the dependence of the refractive index and thickness profiles of compound glass films, deposited by a magnetron RF sputtering technique, on the mixture ratio of Ar and O₂ gases was investigated. First, the depth profile of the refractive index of SiO₂ films was planarized using pure O₂ gas. Next, the lateral profiles of the refractive index and thickness of compound glass films were planarized by optimizing the gas mixture ratio of Ar and O₂. When the mixture ratio was Ar : O₂ = 1 : 3, the index variation of the Corning 7059 glass film was vastly improved from 0.3% to 0.05%, and the thickness variation was also improved from 9.0% to 4.0%. A similar improvement was also achieved for SK1 glass (from SCHOTT) and the Ta₂O₅-SiO₂ binary glass system. A low sidelobe ARROW-type vertical coupler filter (VCF) with Corning 7059 glass and SK1 glass cores was demonstrated, using these deposition techniques and a wet chemical etching technique. In addition, the planarization technique of stripe-lateral-confinement (SLC)-ARROW waveguide was developed using a lift-off technique. A narrow band ARROW-type vertical coupler filter with Ta₂O₅-SiO₂ cores was achieved by the lift-off and reactive ion etching (RIE) techniques.