We present our results on optical waveguides formed by thermal diffusion of ions in glass. It was found that the peak of the ion-exchanged region can be shifted into the substrate interior by limiting the diffusion process. We also found that low loss films (<0.1 dB/cm) can be fabricated using this process and that the modal losses in these films do not agree with those losses predicted by existing theories. Also, the ion-exchange process has proved to be a simple means for fabricating tapered-edge couplers.
Optical mode measurements have been used to determine the anisotropic diffusion coefficient and surface index changes for planar Ti : LiNbO3 waveguides at 0.633 μm. Measured values for the diffusion coefficient at 1000 °C are 𝒟y=9.4×10−13 and 𝒟z=1.4×10−12 cm2/sec. The ratio of the extraordinary to the ordinary surface index change (Δne/Δn0) was found to vary between 1.3 and 1.8, depending on diffusion temperature and Ti film thickness. Ti and Li concentration profiles were measured by secondary-ion mass spectrometry. Additional high concentration peaks, 0.2–0.3 μm wide for both ions, were found superimposed on otherwise well-behaved diffusion profiles. This observation is interpreted to result from a tendency towards Li-Ti-O compound (Li2TiO3 or Li2Ti3O7) formation at the diffusion temperature in a dilute mixture with LiNbO3. Such compound formation has the effect of impeding the Ti diffusion into the LiNbO3 substrate. Electron microprobe measurements were used to measure lateral diffusion from channel waveguides. Under nominally identical conditions, lateral diffusion coefficients varying by as much as a factor of 3 have been observed. The minimum lateral diffusion coefficients observed at 1000 °C were 𝒟y=9.7×10−13 and 𝒟z=1.4×10−12 cm2/sec.
A planar optical 3-dB coupler is proposed that uses only a single region of mode conversion followed by a region of adiabatic modal evolution. These two regions are provided by an overlay with fast and slow tapers, which makes the device synchronous at a single point. Device length is noncritical. The device was fabricated with sputtered barium silicate and fused silica films and tested experimentally. Maximum power transfer of 40% was limited by a fabrication problem with the fast taper, but agreement with theory was obtained. An approximate coupled mode representation of the coupling between local normal modes is used to estimate power transfer at an abrupt transition and to provide design requirements for the fast and slow tapers. The theoretical concepts developed are applied to describe the operation of conventional directional 3-dB couplers and channel modal evolution 3-dB couplers which use branching waveguides.
A Mach-Zehnder interferometric waveguide modulator is demonstrated which simultaneously modulates both TE and TM modes. Two sets of electrodes are utilized to provide approximate independent control of vertical and horizontal field components across the channel waveguides. In Z-X–cut LiNbO3 this is shown to provide two polarization-independent points of operation. With Ti-diffused waveguides operated at 0.633 μm we obtained 90–95% modulation with applied voltages of 1.3 and 44 V and also with −8 and −36 V. Electrode design and alignment are shown to be important factors in device operation.
The symmetry of the Vi center was studied using a technique of bleaching with polarized light. The Vi band in KG and KBr produced by x-rays and 2-Mev electron bombardment was examined. In addition, the effect of bleaching with polarized light on the phosphorescent emission of KG was observed. It is concluded that the center responsible for the Vi absorption band in KG and KBr has cubic symmetry insofar as its optical absorption is concerned. The phosphorescent emission of KG became anisotropic under polarized bleaching light, indicating the presence of a center of noncubic symmetry. This second center has a [Oil] axis of symmetry.
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