Proton Exchange in Lithium Niobate and Lithium Tantalate.
The fabrication and characterization of waveguides and devices using the proton exchange (PE) process in lithium niobate and lithium tantalate was studied. Applications for common integrated optical devices using this process were theoretically investigated. These applications included radar signal processing and beam steering, active polarization control and coherent detection. Emphasis was placed on the use of optical directional couplers. Investigations were carried out to further understand the PE process in lithium niobate. A comprehensive literature survey was performed to clarify what is already known about the process and to point out the areas where further research is needed. Optical characterization of planar and channel waveguides was carried out. A parameter space study to determine the fabrication conditions necessary to produce low loss, single mode channel waveguides in x-cut lithium niobate was undertaken. Planar waveguides were characterized by measuring the mode propagation constants. From these measurements, the depth and refractive index increase of the waveguides were calculated. These results were used to fabricate optical directional couplers. The optical directional couplers were characterized as a function of the annealing time. It was demonstrated that the devices could be "fine tuned" to virtually any desired output power ratio. The PE process in lithium tantalate was studied due to its higher optical damage threshold. Secondary ion mass spectrometry was used to understand the underlying process. From these measurements, exchange and anneal diffusion coefficients were calculated. Infrared absorption and optical characterization were performed on x and z -cut planar waveguides. The change in refractive index due to the PE process was estimated from these measurements. Refractive index profiles on x-cut samples that had been annealed were also obtained. Channel waveguide parameter space studies were carried out on both x and z-cut crystal orientations. The mode nearfield size, insertion loss, polarization extinction ratio and optical damage threshold were measured for channel waveguides. Directional couplers were fabricated using PE in lithium tantalate. By monitoring the device performance as a function of time, an index instability was discovered. The nature and magnitude of this instability were investigated theoretically and experimentally. Possible explanations and suggestions for avoiding this instability are presented.
- Pub Date:
- January 1991
- Physics: Optics; Engineering: Materials Science