Manufacturing of fibre preforms with granulated oxides: Influence of the grain size

Transcription

1 PART III 1

2 Manufacturing of fibre preforms with granulated oxides: Influence of the grain size R. Scheidegger, L. Di Labio, W. Lüthy, T. Feurer Institute of Applied Physics, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland Abstract Manufacturing of fibre preforms with the use of granulated oxides is studied with respect to the influence of grain size on the homogeneity of the refractive index. Silica tubes are filled with granulated oxides (SiO 2, Al 2 O 3 and Yb 2 O 3 ) of different particle sizes. The filled tubes are mounted in the drawing furnace. After evacuation and preheating to about 1400 C for one hour, drops are produced at a drawing temperature of 1900 C. From each drop a slice of 0.8 mm thickness is cut and analyzed in a Mach-Zehnder interferometer. The dependence of the spatial homogeneity of the refractive index on the particle sizes is described. 2

3 Introduction There is much interest in precise doping of silica fibres. The more large cores are envisaged for high power applications the more precision is required to realize the small refractive index steps that still allow for transversal single mode operation of the fibre. In fibre manufacturing by modified chemical vapour deposition (MCVD) or outside vapour deposition (OVD) [1] the index is enhanced with e.g. germanium oxide simultaneously with silica and is as precise as the flow control of the apparatus allows. A common method of rare earth doping is solution dipping [1]. The concentration of dopant then depends on the porosity of the SiO 2 soot, and parameters such as exposure time and concentration of the solution. The concentration of rare earths has a considerable influence on the refractive index. As an example Al 2 O 3 enhances the index stronger than GeO 2 [2] but 0.7 at.% Yb 2 O 3 leads to an index step that is equal to that induced by 5 at.% Al 2 O 3 [3]. Therefore a uncertainty in the treatment of the preform and especially the porosity of the soot can considerably alter the refractive index. A more precise method is potentially the sol-gel technology where the composition of the liquid phase can be fixed with great precision [4]. It is not clear, however, what precision can be reached when the preform is manufactured with the method of granulated dry oxides [5]. As in the case of sol-gel technology the compounds can be weighted and combined with great precision leading to a very precise average refractive index. The mixing of the oxide grains and their size can lead to local inhomogeneities that modulate the refractive index in an undesired way. In the present paper we report on experiments with different grain size of SiO 2, Al 2 O 3 and Yb 2 O 3. The variations in the refractive index are measured in a Mach-Zehnder interferometer. 3

4 Experiments and discussion Manufacturing of the preforms and production of the drop in the drawing tower is described in detail in [3]. From the drop with a centre of vitrified powder and a cladding of perfectly vitreous glass from the tube a thin slice is sawed with a slowspeed diamond saw (Buehler Isomet). Few samples are polished for analysis in the Mach-Zehnder interferometer. To save time, most samples, however, are used as cut and interferometry is performed in a bath of index matching oil. The manufactured samples are summarized in Tab.1. Sample Thickness SiO 2 Al 2 O 3 Yb 2 O μm μm 10 μm (10 at.%) 1-3 μm (0.7 at.%) μm μm μm (5±1) μm μm μm 10 μm (10 at.%) μm (5±1) μm 10 μm (10 at.%) μm (5±1) μm 10 μm (10 at.%) 1-3 μm (0.7 at.%) Tab. 1: Sample list. The experimental arrangement is shown in Fig. 1: Fig. 1: Experimental arrangement for the measurement of the spatial homogeneity of the refractive index. 4

5 The beam of a HeNe laser is expanded with a 25 x telescope to a diameter of 20 mm. The beam is used to illuminate a Mach-Zehnder interferometer. In one branch of the interferometer a cell is introduced that contains the sample immersed in index matching oil. The cell is built with two 30 mm diameter by 10 mm thickness BK7 glass plates of λ/20 flatness glued to an aluminum spacer. Index matching oil of n oil (@ 633 nm) = is used to match silica of n SiO2 = (at 633 nm). Index matching (Fig. 2) allows to compensate the surface roughness of the samples after sawing and to avoid cumbersome polishing. Fig. 2: The index matching cell. A photograph of the interferogram of the sample 1 is shown in Fig. 3: Fig. 3: Interferogram of sample 1. This sample has a thickness of 350 mm and is additionally polished mechanically. Despite of the small thickness of the sample the optical path length varies in the order of one wavelength in the distorted zone. In this case the index varies according to 5

6 λ Δ(n L) =Δn L =λ Δ n = = = (1) L 350 With a variation of the index of over a length of 0.35 mm it is absolutely excluded to reach a precision of over m lengths as it was envisaged in [3]. In a first experiment the influence of the grain size on the vitrification of pure SiO 2 grains is studied. To this end two preforms are drawn to produce samples 2 and 3 (c.f. Tab 1). The interferograms of the two samples are shown in Fig. 4. Fig. 4a: Sample 2 Fig. 4b: Sample 3 The interferograms of sample 2 ( μm grains) as well as sample 3 (5 μm grains) show good vitrification and only minor variations of the index as far as this can be read from an imperfect interferogram. The smooth distortion of the fringes can often be considerably reduced by better cleaning of the cell walls or by stirring of the index matching oil. It can be seen from Fig. 4 that vitrification of pure silica is not depending on grain size. In a second experiment a mixture of SiO 2 with 10 at.% Al 2 O 3 is tested. 6

7 Fig. 5a: Sample 4 Fig. 5b: Sample 5 Figs. 5a and 5b show the homogeneity of an Al 3+ :silica glass sample. Sample 5a with large SiO2 and small Al2O3 grains is completely distorted. After the transition from undoped SiO2 to doped SiO2 circular fringes are visible that show a gradual slope of the refractive index instead of a sharp index step. In the centre of the doped zone no fringes can be seen. That shows that the fluctuations of the refractive index exceed one wavelength. Sample 5 (Fig. 5b) shows a relatively undistorted fringe pattern indicating absence of index fluctuations. Further the figure shows a different fringe pattern in the sample with respect to the background. This is because of the different refractive index of the Al 3+ :silica glass sample that is not index matched as good as pure silica glass. The interferogram of a Yb 3+ :Al 3+ :silica glass sample is shown in Fig. 6. Fig. 6: Interferogram of sample 6. This sample with 5 μm grains of SiO 2, 10 μm grains of Al 2 O 3 and 1-3 μm grains of Yb 2 O 3 is free of fluctuations of the index. The results of Figs. 5b and 6 clearly prove that the use of similar small grain size is helpful in producing glass without fluctuations of the refractive index. 7

8 We do not want to conceal that the use of micron size grains leads to new problems in the handling of the preforms. The density of the powder is much smaller than the vitrified material. In the drawing furnace it is therefore necessary that new powder flows from the top continuously into the molten zone. In contrast to sub mm grains the fine powder will not flow as constantly and tends to block the tube. The generated void allows the tube to locally collapse and upon subsequent deformation it may be blocked in the drawing furnace. Conclusion Manufacturing of fibre preforms with the use of granulated oxides has been studied with respect to the influence of grain size on the homogeneity of the refractive index. Silica tubes were filled with granulated oxides (SiO 2, Al 2 O 3 and Yb 2 O 3 ) of different particle sizes. The filled tubes are mounted in the drawing furnace. After evacuation and preheating to about 1400 C for one hour, drops are produced at a drawing temperature of 1900 C. From each drop a slice of 0.8 mm thickness is cut and analyzed in a Mach-Zehnder interferometer. It could be shown that large SiO 2 grains mixed with small Al 2 O 3 grains lead to a very inhomogeneous mixture giving rise to serious fluctuations of the refractive index. The mixture of micron-size SiO 2 grains with micron-size Al 2 O 3, and also with micron-size Yb 2 O 3 grains, on the other hand, leads to very homogeneous material. Acknowledgments We thank V. Romano, C. Pedrido, S. Scheidegger, M. Mühlheim and M. Neff for stimulating discussions and their help in the drawing tower. 8

9 References [1] S. P. Craig-Ryan and B. J. Ainslie Glass structure and fabrication techniques in Optical Fibre Lasers and Amplifiers P.W. France editor, p. 61 ff ISBN , Blackie, Glasgow and London (1991) [2] K. Yoshida Preparation and preform fabrication of silica optical fibres in Glass and Rare Earth-Doped Glasses for Optical Fibres p. 55 D. Hewak editor, emis datareviews series N o. 22, ISBN X Inspec publication, London (1998) [3] R. Scheidegger, L. Di Labio, W. Lüthy, F. Sandoz, P. Roy, T. Feurer Design of a large-core Yb 3+ :Al 3+ :silica fibre and its manufacturing with granulated oxides IAP Scientific report ZD / ID 3006 (2007) [4] F. Wu, D. Machenwirth, E. Snitzer, G.H. Sigel Jr An efficient single-mode Nd 3+ fiber laser prepared by the sol-gel method J. Mater. Research 9 (10) (1994) [5] R. Renner-Erny, L. Di Labio, W. Lüthy A novel technique for active fibre production Optical Materials 29, , (2007) 9