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RARE-EARTH-DOPED NIOBIUM PHOSPHATE GLASSES FOR INTEGRATED OPTICAL AMPLIFIERS

S. Berneschi, M. Brenci, G. Nunzi Conti, S. Pelli, S. Sebastiani, G.C. RighiniIstituto di Fisica Applicata Nello Carrara, IFAC-CNR, via Panciatichi 64, Firenze, Italy

RARE-EARTH-DOPED NIOBIUM PHOSPHATE GLASSES FOR INTEGRATED OPTICAL AMPLIFIERS

S. Berneschi, M. Brenci, G. Nunzi Conti, S. Pelli, S. Sebastiani, G.C. RighiniIstituto di Fisica Applicata Nello Carrara, IFAC-CNR, via Panciatichi 64, Firenze, Italy

AIM OF THE RESEARCH: To investigate optical and spectroscopic properties of different types of oxide glasses doped with rare earths, and to develop the fabrication processes for the implementation of integrated optical amplifiers and lasers to be used in optical communication systems as well as in medical systems and instrumentation.

1. INTRODUCTION In recent years much work has been devoted to the development of novel glasses with advanced functionality, suitable for integrated optical circuits and particularly for optical amplification. 1,2 In this paper we report about a family of sodium-niobium phosphate glasses which we have recently started to synthesize and investigate. 3 It was already known that good-quality optical waveguides can be produced in this type of glass by ion-exchange technique.3 Further, the presence of niobium in the matrix gives these glasses the potential of exhibiting electro-optical and electro-chromic properties. 4

We have studied in particular two sodium-niobium phosphate glasses, doped with Er 3+ and Sm 3+ , which have intense emission in the near-infrared region and in the visible region, respectively. Samples with different rare-earth concentrations were synthesized, and their spectroscopic properties are shown in the following.

2. EXPERIMENTAL The major component of the glass matrix is sodium phosphate (65 mol%), together with Nb2O5 and Ga2O3. In a few samples a part of NaPO3 is substituted by NaF. Different quantities of Er2O3 or Sm2O3 were then added to the same glass matrix. The glasses were produced by melting a small quantity (each charge was 150 g) of the component oxides at 1300 °C for 2 hours in platinum crucible in air atmosphere, melt stirring being applied for better homogeneity of the glass. Air atmosphere was chosen so as to maintain an oxidizing environment when processing the glass batch and therefore to prevent the appearance of reduced forms of niobium. The resulting glass was then poured out on a brass plate and thereafter annealed at the glass transition temperature for 2 hours. The compositions of the different samples are shown in Table I.

Referring in particular to sample PER1, that has an Er concentration of 5.5 10 20 atoms/cm 3, starting from the absorption spectrum the Judd-Ofelt parameters were calculated, which resulted to be: 2 = 4.39 10 -20 cm 2, 4 = 1.31 10 -20 cm 2, 6 = 1.11 10 -20 cm 2.

The theoretical value of radiative lifetime of the metastable 4I13/2 level of the Er 3+ ion, still computed in the framework of the Judd-Ofelt parameterization, was equal to 6.73 ms. The experimentally measured lifetime, using the 980 nm pump radiation (with 15 Hz chopping frequency and 1:7 duty cycle, in order to guarantee that Er relax after each excitation pulse) was equal to 4.46 ms, so indicating a quantum efficiency slightly higher than 60%.For Sm-doped samples, modeling is much more complicated due to the very numerous possible transitions. Calculations of Judd-Ofelt parameters for that sample, assuming a refractive index n = 1.64 at 635 nm, gave the following results: 2 = 3.95 10 -20 cm 2, 4 = 3.41 10 -20 cm 2, 6 = 3.30 10 -20 cm 2.

Optical waveguides were produced in all the samples by Ag+-Na+ion exchange, using eutectic nitrate mixtures: 0.5 AgNO3-49.75 NaNO3-49.75 KNO3 (% mol). Single-mode optical waveguides at = 1550 nmwere produced with an exchange temperature of 280°C and an exchange time of 180 sec. Single-mode at = 635 nm optical waveguides can be obtained with an exchange time of 30 sec. Propagation losses, unfortunately, are quite high (around 3 dB/cm at 633 nm) in these first samples, and an investigation of the reasons is under way.

5. CONCLUSIONS Several glass samples have been fabricated by conventional melting technique, all based on the NaPO3 - Nb2O5 - Ga2O3 matrix, and Er2O3 or Sm2O3 doping. Preliminary results of spectroscopic and modal characterization of these glasses confirm their suitability for integrated optics. Optical amplification in the near-infrared or in the visible region (for Er 3+ or Sm 3+ -doped glasses, respectively) appears possible. Single- and multi-mode waveguides were produced, and the full characterization of all the produced samples is in progress.

ACKNOWLEDGEMENTS The work on integrated optical amplifiers is made possible by the collaboration with several groups with experience in material science: Marco Bettinelli and Adolfo Speghini (Verona University), Andrey Lipovskii and Dimitry Tagantsev (St. Petersburg), Angela Seddon and Victor Tikhomirov (Nottingham University). The collaboration with the group of Maurizio Montagna (Trento University) and Maurizio Ferrari (IFN CNR, Trento Section) is also gratefully acknowledged.

ACKNOWLEDGEMENTS The work on integrated optical amplifiers is made possible by the collaboration with several groups with experience in material science: Marco Bettinelli and Adolfo Speghini (Verona University), Andrey Lipovskii and Dimitry Tagantsev (St. Petersburg), Angela Seddon and Victor Tikhomirov (Nottingham University). The collaboration with the group of Maurizio Montagna (Trento University) and Maurizio Ferrari (IFN CNR, Trento Section) is also gratefully acknowledged.

REFERENCES1. G.C. Righini, M. Brenci, M.A.Forastiere, S.Pelli, G.Ricci, “Rare-earth-doped glasses and ion-exchange integrated optical amplifiers and lasers”, Philosophical Magazine B, vol. 82, n. 6, p. 721-734 (2002)2. G.C. Righini, S. Pelli, M. Fossi, M. Brenci, A.A. Lipovskii, E.V. Kolobkova, A. Speghini, M. Bettinelli, in “Rare-Earth- Doped Materials and Devices V” , S. Jiang, Ed., Proc. SPIE Vol. 4282, 210-215 (2001).3. N.V. Nikonorov, E.V. Kolobkova, M.B. Zakhvatova, Glass.Phys.Chem., 19, 48-51 (1993).4. G.O.Karapetyan, A.A. Lipovskii, V.V.Loboda, L.V.Maksimov, D.V. Svistunov, D.K.Tagantsev, B.V.Tatarintsev, A.A.Vetrov, Proc. SPIE, Vol. 4353, 23-28 (2001).

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PER1-1 PER2-1 PER3-1 PER2F-1

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3. GLASS COMPOSITION

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Erbium doped glasses

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p = 472 nm

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p = 496 nm

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Samarium doped glasses1400 1450 1500 1550 1600 1650 1700

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oxidePER1 PER2 PER3 PER2F PSM1 PSM2 PSM3 PSM2F

NaPO3mol

%65 65 65 62 65 65 65 62

Nb2O5 20 20 20 20 20 20 20 20

Ga2O3 15 15 15 15 15 15 15 15

NaF 3 3

Er2O3w% 5 7,5 10 7,5

Sm2O3 5 7,5 10 7,5