Fabrication of Annealed Proton-Exchanged Waveguides for Vertical Integration



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There is a drive for improving the surface uniformity of optical waveguide devices in the photonics lab. This report focuses on the exploration of annealed proton exchange (APE) waveguide fabrication on lithium niobate crystal as a method of producing optical waveguides. These waveguides aim to have little variation in step height or surface roughness in the transition area from the waveguide location to that of the bulk crystal, providing a uniform surface amenable to vertical device integration. This is a substantial improvement over the titanium diffused waveguide process, which can have surface variations in excess of 100nm. It is anticipated that the smoother surface will enable light to couple more easily into photonic devices, such as ring resonators, as compared to the current Ti diffused waveguide process.

This work explores the design and fabrication aspects of annealed proton exchange waveguides. A review of literature on modeling hydrogen diffusion into lithium niobate is presented, as well as computer models for simulating the bidimensional fractional hydrogen proton concentration distribution. This is used to determine the change in refractive index of the waveguide needed to simulate the mode propagation and profile in the device.

Fabrication processes involved in proton exchange waveguide formation are outlined, and measurements for working devices are presented. Best case loss for current devices are 0.5 dB/cm. These samples exhibit smooth surfaces with only ?60A in variation of surface uniformity. Concluding remarks present ideas to further the work by lowering propagation losses, improving mode matching to single mode fiber, and improving the consistency of fabrication conditions.