Low-Loss Silica-Based SiO2-Ta2O5 Waveguides With Extremely High Δ Fabricated Using Sputtered Thin Films

Low-loss silica-based SiO
₂
-Ta
₂
O
₅
waveguides with an extremely high refractive index difference (Δ) are realized, while avoiding Ta
₂
O
₅
crystallization. First, we determined in detail the thermal conditions needed for thermal treatment in a film-refinement process after the deposition of extremely high-Δ SiO
₂
-Ta
₂
O
₅
thin films to suppress Ta
₂
O
₅
crystallization. Moreover, we confirmed that the crystallization of Ta
₂
O
₅
was prompted mainly by the diffusion of boron (B) and phosphorus (P) dopants from the overcladding layer into the SiO
₂
-Ta
₂
O
₅
core region of the waveguide during the embedding process. By suppressing the diffusion of the B and P dopants from the overcladding layer using the double-layer cladding technique, we successfully fabricated extremely high-Δ waveguides and achieved a propagation loss of as low as 0.06 dB/cm. Extremely high-Δ waveguides appear to be a promising technology with which to realize high-density optically integrated and low-cost optical devices.