In principle our second waveguide fabrication technique in Lithiumniobate, reverse proton exchange consists of three single steps: proton exchange, annealing and the burying of the waveguide. All of these are independent diffusion processes, where the final state of hydrogen concentration after one step is the initial state for the next step. The most important technological steps are shown in Fig. 3.
1.2.1. Proton Exchange For the exchange of lithium against hydrogen ions the Ti- masked samples are immersed into a melt of a weak organic acid. Due to its low melting point and vapour pressure, most commonly benzoic acid is used as the proton source. Inside the mask windows an ion exchange is performed at about 170°C for several hours leading to channels of increased extraordinary refractive index (thus guiding only one polarization).
The as-exchanged regions of the substrate undergo structural phase transitions and the nonlinear relation between proton-concentration and index increase results in a step-like index profile and waveguides of high loss without any second-order optical nonlinearity.
1.2.2. Annealing Annealing reduces the proton concentration by diffusion deeper into the substrate. In this way structural phase transitions are reversed back to the so called low loss α-phase in which the optical nonlinearity is fully recovered and the relation between proton concentration and index increase is linear leading to a Gaussian-like profile. The annealing process is carried out at about 330°C for several hours.
The remaining strongly asymmetric concentration profile still results in asymmetric mode intensity distributions of non ideal overlap for nonlinear frequency conversion. To improve this overlap, to further reduce scattering losses and to optimize fiber-chip coupling a third step has to be carried out to achieve a buried almost symmetric waveguide.
1.2.3. Burying the waveguide In the regime of low proton concentration a symmetric proton concentration profile would lead to a symmetric profile of the extraordinary refractive index. When we drive the maximum concentration of protons deeper into the substrate this symmetrization becomes feasible.
Reversing the ion exchangeclose to the substrate means to out-diffuse protons and to in-diffuse lithium ions again. This can be achieved by immersing the sample into alithium-rich eutectic melt of LiNO3, KNO3 and NaNO3. The high gradient of proton concentration at the surface yields a stronger out-diffusion rather than a diffusion deeper into the substrate. As a result the maximum of the concentration profile moves into the substrate.
In Fig. 4 the three steps for the fabrication of RPE waveguides are shown schematically in terms of concentration profiles.