Optical waveguides and lasers in improved gallium lanthanum sulphide glass
University of Southampton, Department of Electronics and Computer Science,
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A number of developmental stages are still required to advance and mature optical waveguide technology in non-silica glasses. The primary stage includes raw material purification and improving quality and thermal stability of an optical glass for waveguide fabrication processes. Further stages can include design, application and integration of these waveguides with other photonic devices. Gallium lanthanum sulphide (Ga:La:S) chalcogenide glass (ChG), first discovered in 1976, is a material proposed as an optical waveguide for use in the infrared (IR). Interest in this glass system has been maintained, over the years, primarily due to its exceptional and unusual optical properties. The aim of this project is to advance the current state of art for Ga:La:S glass by demonstrating working solutions for fibre and planar waveguides. Chapter 1 of this thesis provides a general overview of current glass technology and the motivations of this project.
The optical glass system under study has yet to attain acceptable stability for fibre production and as such investigation into fundamental manufacturing steps is still required. Raw material purity is an important aspect, of fabricating practical optical glasses, and directly affects performance. Chapter 2 of this thesis describes the purification and synthesis processes performed to produce raw materials with purity far superior to similar products available commercially. Each powdered precursor synthesised in our labs and used in fabrication of Ga:La:S based optical glasses has a transition metal impurity content of less than 1 parts-per-million (ppm wt%). The water content, OH-, of these fabricated glasses has been reduced to < 2 ppm. The primary concern when fabricating Ga:La:S based optical fibre is crystallisation. Optimising the composition to obtain a glass suitable for fibre fabrication is significant in providing thermal stability for fibre drawing. Chapter 3 describes some of the steps taken towards the fabrication and improvement of Ga:La:S based glasses for waveguide technology. The invention of a new variant in the Ga:La:S family of glasses provides key enhancements over existing Ga:La:S and Ga:La:S:O glasses. The hybrid oxy-chalcohalide glass, Ga:La:S:O:F, contains compounds of sulphide, oxide and fluoride as constituents. This new glass type provides significant thermal stability, in the context of fibre drawing. Fibre drawn from a single piece of polished Ga:La:S:O:F glass had attenuation at 1.5 and 4.0 µm of 3.3 and 2.1 dB m-1 respectively. The reduction of the OH- absorption at 2.9 µm to < 1 ppm in Ga:La:S:O:F glass, can potentially allow development of planar waveguide devices for the mid-IR. A range of extremely stable compositions for Ga:La:S, Ga:La:S:O and Ga:La:S:O:F glasses was also identified. These glasses were amorphous upon slow cooling in the furnace (8 oC min-1) indicating danced thermal stability against crystallisation.
In Chapter 4 and 5, the fabrication and characterisation of channel waveguides is discussed. Photoinduced changes were introduced by directly writing waveguides into Ga:La:S glass through exposure to short wavelength light (l = 244 nm). Focused fluence of 1.5 - 150 J/cm2 from a continuous wave laser operating at 244 mn was applied, inducing photocompaction and photochemical changes. These passive channel waveguides were spatially single-mode and bad Dn ~ +10-3. The first chalcogenide channel waveguide laser in Nd3+-Ga:La:S glass was also demonstrated. Maximum laser output (l = 1075 nm) of 8.6 mW for an absorbed laser pump power of 89 mW and slope efficiency of 17% was achieved with measured device attenuation of < 0.5 dB cm-1. Discussed in Chapter 6 is the first demonstration of the hotdip spin coating process used to fabricate thin films of a ChG (Ga:La:S). This promising technique is presented as an enhancement to waveguide development. In addition, buried (50 µm) channel waveguides were directly written into the spun thin film using a pulsed laser source (l = 830 nm). These buried channel waveguides had a measured attenuation of < 1 dB cm-1.
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