Low-loss waveguides written with a femtosecond laser for flexible interconnection in a planar light-wave circuit

We describe a low-loss single-mode waveguide in planar light-wave circuit (PLC) glass doped with boron and phosphorus, which is more difficult to write than pure-silica glass. The written waveguide has a rectangular core, a symmetric near-field pattern, and a propagation loss of $0.35\mathrm{dB}∕\mathrm{cm}$. The loss that originates from the mode-field mismatch between the mode-field diameters of the written and the PLC waveguides is less than $0.1\mathrm{dB}∕\text{point}$. In addition, we successfully connected PLC waveguides with a $500\text{-}\mu \mathrm{m}$-long waveguide written with a laser. The laser-written waveguide can flexibly connect PLC waveguides with a low coupling loss.