Thin film Bragg deflection gratings for integrated optics

Pun, Edwin Yue Bun (1983) Thin film Bragg deflection gratings for integrated optics. PhD thesis, University of Glasgow.

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Two types of single mode thin film Bragg deflectors were investigated : periodic index waveguides (with the guiding layers posessing parallel boundaries) by diffusion through a metallic grating mask; and periodic thickness (corrugated) waveguides by forming photoresist gratings on top of the waveguides (four-layered devices), or with the relief patterns ion-beam etched (using argon ions) directly onto the surfaces of the guides (three-layered devices). The periodic index waveguides were fabricated by silver-sodium ion exchange in glass substrates, and proton-exchange or titanium indiffusion in LiNbO3 substrates. The metallic grating masks (periodicity = 3mum) on the surfaces of the guides prior to diffusion were formed by conventional photolithographic technique, followed by vacuum deposition of the metal and liftoff. Sputtered homogeneous 7059 glass waveguides on standard Fisher microscope slides were used in the manufacture of corrugated waveguides. Submicron gratings (periodicity = 0.3mum) were fabricated by holographic exposure of the photoresist-coated samples. General waveguide theory of homogeneous and inhomogeneous dielectric slab waveguides is presented, using both geometrical (or ray) optics and electromagnetic theory. Properties of periodic waveguides are also described, and the Bragg deflectors fabricated are analysed using one-dimensional coupled mode formalism and two-dimensional coupled mode formalism. The theory predicts two unique properties of oblique incidence not found in normal incidence: mode conversion (TE/TM) and variation of power across the width of the diffracted and transmitted beam. The fabrication techniques for both types of deflectors, including the three diffusion processes and the interferometric technique for producing submicron gratings, are discussed in some details. The waveguide and grating parameters were measured using standard techniques, and the grating groove depths were deduced from diffraction efficiency measurements. The periodicity is determined from the measurement of the autocollimation angle of the first diffracted order. The Bragg devices were tested using a visible HeNe laser. For periodic index waveguides, the experimental results are in good agreement with theoretical calculations using onedimensional coupled mode formalism. However, mode conversion (TE-TM), a unique property in oblique incidence of guided wave onto a grating element, was not observed because of the long periodicity (small Bragg angle) of the devices. Furthermore, a two-step diffusion process, using the first diffusion to form the waveguide and a second diffusion to produce the grating structure, was also demonstrated. This gives further flexibility to the diffusion technique, and the parameters in the first and second diffusion steps can be chosen independently for optimised performance. For the corrugated waveguides, the experimental results agree better with calculations using the two-dimensional coupled mode formalism. The two unique properties of oblique incidence: mode conversion (TE-TM) and non-uniform intensity distribution across the width of both the diffracted and transmitted beams, were observed. It is confirmed experimentally that for right angle deflection, the TE-TE coupling is zero. When the diffraction efficiency is high, the diffracted beam 'breaks up'. The results from this investigation suggest that the periodic index waveguides fabricated have distinct advantages over the corrugated counterparts in terms of lower scattering loss, ease of fabrication and mechanical ruggedness. The two unique properties-of oblique incidence may pose a problem-in certain applications, such as beam expanders and multiplexer/demulplexer. 'Breaking up' of the beams and crosstalk introduced by mode conversion must be taken into account when designing these devices.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: J Lamb
Keywords: Optics
Date of Award: 1983
Depositing User: Enlighten Team
Unique ID: glathesis:1983-72026
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 17 May 2019 13:20
Last Modified: 17 May 2019 13:20

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