A simple process is presented for fabricating channel waveguides by a thallium‐ion diffusion technique. The thallium diffusion coefficient is estimated to be 2×10−12cm2/sec at 400 °C. Effective mode index measurements show a complementary error function index profile in the waveguide. Waveguide losses are measured to be −0.60 dB/cm. A passive component of optical communications systems, a star coupler, is fabricated using the thallium‐ion exchange process.