Acousto-Optic Filter Device Using Proton-Exchanged Waveguides

Duffy, Joseph Francis (1986) Acousto-Optic Filter Device Using Proton-Exchanged Waveguides. PhD thesis, University of Glasgow.

Abstract

This thesis describes an investigation of the acoustic and optical properties of waveguides fabricated by proton-exchange from dilute melts. Initially a narrow-band acousto-optic filter, based on the Integrated Optical Spectrum Analyser, is investigated. The device is proposed to detect and separate two acoustic frequencies and it was envisaged that the angular separation of the acoustically diffracted optical beams could be performed within a channel waveguide structure. However, analysis has indicated the impracticality of the device when operational requirements are considered. The dilute melt proton-exchange process for waveguide fabrication on LiNbO3 substrates is characterised. The waveguides have step-index profiles and are stable immediately after exchange. The waveguide depth increases as a t function of exchange time, so a temperature dependent diffusion coefficient can be assigned and this coefficient fits an Arrhenius law. In comparison with pure melt proton-exchange waveguides, dilute melt waveguides exhibit less in-plane scattering and Rutherford Back-Scattering Spectrometry (RBS) measurements indicate a significantly lower level of latice disorder. Close agreement is found between optical measurements of waveguide depth (assuming a step-index profile) and RBS data for a range of melt dilutions, thus supporting the step-index model as an appropriate profile for the refractive index. Multi-mode waveguides are fabricated on Y-cut LiNbO3 substrates without surface damage, though short post annealing is necessary to relieve induced strain. Planar optical waveguides are fabricated on X-cut and Z-cut LiTaO3 substrates from dilute melts without the need for prolonged post annealing. High quality waveguides with low in-plane scattering levels are produced. The refractive index change is similar to that achieved with titanium indiffused LiNbO3 and RBS measurements indicate a region of lattice disorder similar to that observed with LiNbO3. A line-focus-beam Scanning Acoustic Microscope is used to investigate the acoustical properties of dilute melt proton-exchanged LiNbO3 waveguides. Substantial changes (as much as 5%) in the surface acoustic wave (SAW) propagation velocity due to proton-exchange are observed on all three major-axis crystal cuts, with both increases and decreases in velocity occurring, depending on propagation direction. Changes in acoustic attenuation are also observed. A simple model is used to calculate the 'true' velocity change for a proton-exchange region without the infulence of a virgin LiNbO3 base and the acoustic anisotropy exhibited by the X-and Y-cut LiNbO3 appears greater than that of virgin LiNbO3. A theoretical analysis based on the quasi-static approximation is used to determine the overlap integral between the acoustic and optical waves in proton-exchange waveguides. The overlap integral falls with variations in index profile from step-index to graded-index and the domination of the acousto-optic interaction by the electro-optic effect is emphasized. The analysis indicates that shallow (~3mum) step-index waveguides are required for maximum interaction efficiency and that prolonged annealing results in reduced interaction efficiencies. Acousto-optic devices are fabricated on dilute melt proton-exchanged LiNbO3 and LiTaO3 waveguides using simple, non-optimal, electrode structures. For 7. 5 minutes exchange in 0.5% dilute melt on X-cut LiNbO3 at 220

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Electrical engineering
• Date of Award: 1986
• Depositing User: Enlighten Team
• Unique ID: glathesis:1986-77455
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 14 Jan 2020 09:08
• Last Modified: 14 Jan 2020 09:08
• URI: https://theses.gla.ac.uk/id/eprint/77455