PROTON EXCHANGE

Proton exchange is a commonly used technique when making optical waveguides in lithium niobate. It is known as APE (Annealed Exchanged Proton). The process substitutes Li+ lithium ions on the surface of the wafer by H+ protons contained in an acid bath. Defining the geometrical patterns of the waveguides in photosensitive resin by masking and photolithography makes up the first step in fabrication. Once the patterns in resin have become established, the wafer is immersed in a heated acid bath for a fixed length of time. The temperature and length of exchange are determinant factors in obtaining the index profile of the waveguide. In our case the proton exchange is carried out in a bath of benzoic acid. The substitution of L+ ions by H+ ions only takes place on the surface of the wafer and only on zones that are not masked by resin. Locally, the lithium niobate undergoes an increase in the refractive index according to the extraordinary axis and a decrease in refractive index according to the ordinary axes. Annealing then enables the protons concentrated on the surface to be diffused in order to obtain a stable waveguide profile. The length of annealing time again plays a major role. In the end, the waveguide will have undergone a positive variation of the extraordinary refractive index while the ordinary refractive index will be slightly under the ordinary index of the substrate. A direct consequence of this is that APE technology can only guide one single polarization, as according to the ordinary axes, guide conditions are not combined (the exchange index being lower than the substrate index).

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