Study of Ag+/Na+ ion-exchange diffusion on germanate glasses: Realization of single-mode waveguides at the wavelength of 1.55 μm

Ion-exchange technology on glass has been successfully used for more than 20 years to realize dependable and low-cost passive and active integrated optic devices on silicate or phosphate glasses for the optical telecommunication systems operating around λ = 1.5 μm. However, the recent developments of integrated optics instruments for astronomical interferometers or biological sensors have required an increase of the devices operation range towards the mid-infrared. For these reasons, we present in this paper the development and the realization of single-mode waveguides by means of a binary ion-exchange on a germanate glass (BGA-G115 form Kigre Inc.) which transparency window reaches λ = 4 μm. A complete study of the silver ion diffusion in this glass matrix has been performed allowing the determination of silver and sodium ion diffusion coefficients as well as the value of the maximum refractive index change. Using these data, simulations were carried-out showing that an ion-exchange of 90 min in a 0.03AgNO3–0.97NaNO3 molten salt at a temperature of 330 °C would allow the realization of single-mode channel waveguides at λ = 1.55 μm. To demonstrate this on BGA-G115, a specific technological process based on the deposition of a polysilicon masking layer has been implemented. Single-mode channel waveguides, with a 2.5 μm diffusion window width, have thus been realized and characterized at the wavelength of 1.55 μm. Modal size has been measured to be 10 ± 1 μm × 7 ± 1 μm for propagation losses of 1.2 ± 0.5 dB/cm for a 2 ± 0.1 cm long device.