Photonics Global Conference Singapore December 15, 2012 PaperID: c12a774 Rare earth doped optical fiber fabrication by the granulated silica method Manuel Ryser Dereje Etissa, Soenke Pilz, Valerio Romano Optical Fibers and Fiber Lasers Institute of Applied Physics University of Bern Switzerland

Fiberproduction & limitations Pedrazza: Sol-gel funktioniert > 100nm Schichten ergeben gute Homogenität > kein Vorteil gegenüber MCVD > MCVD ähnlich als Nasschemische Beschichtung Granulated-silica method: our proposal to overcome limitations > first mentioned by John Ballato and Elias Snitzer, “Fabrication of fibers with high rare-earth concentrations for Faraday isolator applications,” Applied Optics, vol. 34, no. 30, p. 6848, Oct > Silitec SA, Switzerland holds a patent for producing claddings out of sand not very flexible: > difficult to fabricate large homogeneous cores > relatively big technical effort/time consuming gases soot deposition sintered glass layer highest quality, very low scattering losses Chemical Vapor Deposition methods (MCVD, VAD, OVD, PCVD, IMCVD) Sol-Gel Technology  simple technology  wet chemistry  OH Granulated Oxides  rapid manufacturing, low cost  possible scattering losses John Ballato und Elias Snitzer [Applied Phyiscs 34 (30), 6848 (1995)] approach is very flexible concerning core sizes large number of designs realizable (multicore, PCF) highly doped fibers possible scattering losses → our focus is on light sources and not telecommunication for undoped fibers, for doped fibers) we want to push this approach to its limits

Granulated silica-method rapid preform/fiber prototyping

Granulated silica method proof of principle 100µm small silica tube (3x5mm) large silica tube (16x19mm) (improve solubility) (reduce photodarkening) The melting points of Al 2 O 3 and RE 2 O 3 are between 2054 °C and 2435 °C. At the drawing temperature of ~1850 °C no melting occurs!  soluble in SiO 2 of low viscosity ??? R. Renner-Erny, L. Di Labio, and W. Lüthy, “A novel technique for active fibre production,” Optical Materials, vol. 29, no. 8, pp. 919–922, Apr

Test of rare-earth-oxides solubility All investigated rare-earth-oxides are soluble. Lifetimes are as expected.

Optional vitrification step reducing bubbles and thus scattering losses 1.A narrow pure silica tube embedded with a larger silica tube 2.Doped silica sand is introduced into the narrow tube 3.Pure silica sand is introduced into the interspace between the two tubes 4.The narrow silica tube is retired 5.The structure is evacuated and heated with a cleaning gas treatment Sand is vitrified, all solid preform is obtained → to vacuum pump important parameter: grain-size > should be more or less of same size and small: better diffusion and less refractive index-fluctuations > small (<100µm) → evacuation difficult

Granulated silica method improving homogeneity Multi-mode fibre produced without melting and milling (strongly over-exposed) Multi-mode fibre produced with melting and milling applied twice. The white lines indicate the position of the fibre. Green fluorescence from the 4 S 3/2 → 4 I 15/3 transition is excited by energy transfer up-conversion (ETU) pumping with a diode laser operated at 975 nm. → solution to increase homogeneity: Sol-Gel material

Combination of methods Sol-gel and granulated silica method > production of multicomponent glass powders by sol-gel > eventual homogenization of dopant by CO 2 laser re-melting and milling (several times) > assembling of preform > eventual pre-vitrification > drawing flexibility of dopant content - any water or ethanol soluble dopant can be dissolved homogeneously flexibility of choosing processing temperatures (200°C °C) very cost-effective wet chemical process → OH groups (large SiOH 950nm, 1240nm, 1390nm) rapid prototyping and manufacturing possible scattering losses inhomogeneous distributed dopants very cost-effective produce granulate: all grains are homogeneously doped! TEOS (Tetraethyl orthosilicate)

From Sol-gel to granulate porosites diappear OH content is lowered

Test for crystalline silica X-Ray Diffraction Measurement a) crystalline c) amorphous > quenching the Sol-Gel material after heating at 1600 Fiber from Sol-gel graunlate Fiber directly from granulates > crystalline material was amorphized after fiber drawing process amorphous b) before drawing

Elemental distribution by Electron Probe Microanalysis precursor materials still in the fiber? core of the fiber, diameter approx. 65µm Average elements distribution along the active fiber core. Al 2 O 2 Yb 2 O 2 P2O2P2O2

Refractive index profilometry refractive index profiles of Yb +3, Al +3, P +5 doped fibers ∆n = 5.3 * σ = 4 * N A = 0.12

Fiber fabrication progress overview produced by combining Sol-gel and granulates mixing > fiber-loss: piecewise (up to 50cm) (MCVD nm) > refractive index profile is smooth > no crystallization > precursor materials from sol are in the optical fiber > big core, flexible geometries, high dopant concentration leakage channel fiber with 80  m core → single mode! → high-power fiberlaser and amplifiers

Examples rapid preform/fiber prototyping

Ø Cladd = 180 µm Ø Core = 6.5 µm NA = at. % Nd at. % Ho at. % Er at. % Tm at. % Yb 3+ 7 at. % Al 3+ superbroadband fluorescence source Loredana Di Labio, Willy Lüthy, Valerio Romano, Frédéric Sandoz, and Thomas Feurer, "Superbroadband fluorescence fiber fabricated with granulated oxides," Opt. Lett. 33, (2008) Measured broadband power: mW pump (804nm) (~15W /cm 2 ) xxxx comparison?? 365 nm – 2300 nm Ground State Absorption (GSA) Excited State Absorption (ESA) / Energy Transfer Up-Conversion (ETU)

superbroadband double clad fluorescence source Same idea as previous fiber but: - Double Clad for higher pumping - Addition of Bismuth to fill „dip“ between 1100 nm and 1330nm Carmem Barbosa, Martin Neff, Valerio Romano, 2010, „NIR CW broadband source“, to be published

superbroadband double clad fluorescence source

7-core fiber 18 L. Di Labio, W. Lüthy, V. Romano, L. Di Labio, F. Sandoz, and T. Feurer, “Superbroadband fluorescence fiber fabricated with granulated oxides,” Applied Optics, vol. 33, no. 10, p. 1581, Mar

7-core fiber 19 L. Di Labio, W. Lüthy, V. Romano, L. Di Labio, F. Sandoz, and T. Feurer, “Superbroadband fluorescence fiber fabricated with granulated oxides,” Applied Optics, vol. 33, no. 10, p. 1581, Mar nm – 2300 nm

Summary optical fiber fabrication with the granulated silica method > all investigated rare-earths are soluble > high dopant concentrations possible (typical at.%, up to 10 at. % without quenching > improvement of homogeneity: by evacuation, vitrification, iterative milling and melting > large core sizes possible > allows almost arbitrary arrangement of core/cladding Sol-gel combined with granulated silica method > goal: further improvement of homogeneity > Sol-gel allows to produce (already) homogeneously doped grains as starting material results from ongoing-work: > we developed methods to produce granulate from Sol-gel > no crystalline silica in the fiber > precursor materials are still in the fiber! > smooth index profile > losses are tolerable: for the use in fiber amplifiers

Summary examples > single mode superbroadband fluorescence source > double clad superbroadband fluorescence source > 7-core fiber, each core doped with another rare earth

Acknowledgments Dereje Etissa, Theo Schmidt, Soenke Pilz, Valerio Romano, Thomas Feurer Martin Neff, Urs Pedrazza, Loredana Di Labio, Carmem Barbosa Ruth Renner-Erny Willy Lüthy, H.P. Weber Carlos Pedrido, Philippe Hamel, Frederic Sandoz