A novel technique has been developed for the deposition of low-loss planar glass waveguides by directly spin coating from the liquid, thus overcoming the problems of reproducing glass stoichiometry when depositing from the vapour

Brown, C.T.A.

Harwood, D.W.J.

Payne, D.N.

Shepherd, D.P.

Taylor, E.R.

English

Southampton (e-Prints Soton)

1 Optoelectronics Research Centre, University of Southampton, UKFigure 1: Cross-sectional SEM picture of a 40 micronthick AlF waveguide fabricated by spin coatingNovel fabrication technique for planar glass waveguidesD W J Harwood, E R Taylor, C T A Brown, D P Shepherd, D N PayneAbstract: A novel technique has been developed for the deposition of low-loss planar glass waveguides by directlyspin coating from the liquid, thus overcoming the problems of reproducing glass stoichiometry whendepositing from the vapour.IntroductionNovel glasses, e.g. fluoroaluminates,chalcogenides, are important for future opticalwaveguide devices, such as 1.3µm planaramplifiers and non-linear switches. However, theyare difficult to fabricate as thin films because oftheir tendency to crystallise and volatility.Reproducing the stoichiometry of multi-componentglasses in planar waveguide form has traditionallyproved difficult by conventional techniques, suchas sputtering, due to the different vapour pressuresof each element [1]. Pulsed laser deposition (PLD)can overcome this problem because of the highvaporisation rates provided by excitation atultraviolet wavelengths, but is often troubled bypoor surface topography [2]. By dissolving thebulk material in a solvent, spin or dip coating canbe used to retain the waveguide composition for alimited range of materials such as sol-gel glasses[3].At temperatures above the liquidus temperature,several soft glass families, such as the fluorides,sulphides and phosphates, have viscositiescomparable to those of spin coating solutions. Aneodymium-doped fluoroaluminate (AlF) glass,with a viscosity of  0.1 poise at 1000EC, hassuccessfully been spin coated into a waveguidegeometry (Figure 1) directly from its liquid phase.Laser operation around 1050nm has been observedin a 20 micron thick, 0.5% Nd-doped glasswaveguide fabricated using this technique.Performance is consistent with propagation lossesof 0.5dB/cm, which have been independentlymeasured by imaging the scattered light from thewaveguide surface.Fabrication of waveguidesNd-doped waveguides were fabricated using theapparatus shown in Figure 2. A 3 mm thick, 25mmdiameter  AlF glass substrate of refractive index1.417 was preheated to its glass transitiontemperature of 430EC in an upper annealingfurnace. In the lower furnace, a 30g melt of Nd-doped AlF glass of refractive index 1.432 washeated to 1000EC. When the temperatures of the two furnaces hadstabilised,  an Aerotech ATS0200 positioningsystem was used to transfer the molten glassbetween furnaces. Accurate control of the meltposition resulted in the surface of the substratebeing dipped a fraction of a millimetre into themolten glass. The computer was used tosynchronise the withdrawal and spinning of thesubstrate up to a maximum speed of 6000rpm. Theas-deposited waveguides were then annealed in theupper furnace at T = 430EC for three hours, g followed by a slow ramp down to roomtemperature at 30EC/hour.Characterisation of waveguidesScanning electron microscopy was used to examinethe cross-section of the films. The waveguidesexhibited excellent thickness uniformity(0.1µm/mm) over the  central area of the substrate,with some curvature of the surface observed at theedges due to re-melting of the underlying substrateglass. The high quenching rates achieved bycoating a thin layer of molten glass onto a muchlarger cooled substrate produce a crystal-freeglass/waveguide interface, as seen in SEMmicrographs (Figure 1).In order to measure the propagation losses, a 2mWHeNe laser was end-coupled into a 20 micron thick2 Optoelectronics Research Centre, University of Southampton, UKFigure 2: Apparatus for spin coating of waveguidesusing molten glass.Figure 3: Lasing spectrum for an Nd-dopedAlF glass waveguide illustrating operation onseveral lines.Figure 4: Output power at 1050nm as afunction of pump power at 800nm for an Nd-doped AlF glass waveguide laser.parallel end-polished sample. A silicon CCD formed between the surface of the mirrors and thecamera was then used to image the scattered back of their respective substrates. The maximumradiation from the waveguide surface. An slope efficiency was measured to be 10% withexponential fit to the scattered intensity along thelength of the waveguide gave a loss of ~0.5dB/cm.Laser operation around 1050nmA laser cavity was formed by the attachment ofplane mirrors to the polished, parallel end faces ofa 20 micron thick, 5 mm long waveguide. Thechopped output from a Ti:sapphire laser operatingaround 800nm was end-launched into thewaveguide using a 50mm focal length microscopeobjective. The input coupler was highly reflecting(HR) at 1050nm, and highly transmitting (HT) at800nm. A 3.5% output coupler at 1050nm wasused which was HR at 800nm in order to doublepass any unabsorbed pump. Laser output was observed at several wavelengthsbetween 1045nm and 1065nm, as shown in Figure3. The peaks correspond to the positions ofmaximum reflectivity in the Fabry-Perot cavityrespect to incident power for a correspondingthreshold of 170mW (Figure 4).The measured slope efficiency is consistent withthe waveguide losses described earlier. Theselosses are similar to the typical bulk losses of theglass of 0.3dB/cm and can be expected to improvewith higher-purity starting materials for the glass.ConclusionLaser operation around 1050nm has been observedfor the first time, to our knowledge, in a Nd-dopedplanar waveguide prepared by spin coating of afluoroaluminate glass directly from the liquidusphase. Losses in the waveguide were determined tobe of the order of 0.5dB/cm compared with typicalbulk losses of 0.3dB/cm.3 Optoelectronics Research Centre, University of Southampton, UKFurther work ReferencesFuture use of the technique will concentrate on the [1] G.R.J.Robertson et al. APPLIED OPTICS,fabrication of overclad waveguides. A two-stagespin coating process will be applied to both burythe waveguides, for a reduction in the propagationlosses, and to reflow the original guiding layer,thus reducing the guide dimensions to single mode.The Nd-fluoroaluminate glass composition hasbeen designed and optimized for use as a 1.3µmamplifier in the important secondtelecommunications window. Low loss waveguidesfabricated using this technique will lead to thefabrication of a low-cost integrated amplifier forthis region.AcknowledgementsThis work was funded by EPSRC with CASEsupport from GEC-Marconi Research.1991, 30, 3, pp. 276-278[2] C.N.Afonso et al. APPLIED SURFACESCIENCE, 1996, 96-98, pp.760-763[3] X.Orignac et al. APPLIED PHYSICSLETTERS, 1996, 69, 7, pp. 895-897

Novel fabrication technique for planar glass waveguides

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