Elsevier

Journal of Non-Crystalline Solids

Volume 353, Issue 29, 15 September 2007, Pages 2767-2773
Journal of Non-Crystalline Solids

Improvement of the Tm3+:3H4 level lifetime in silica optical fibers by lowering the local phonon energy

https://doi.org/10.1016/j.jnoncrysol.2007.05.025Get rights and content

Abstract

The role of some glass network modifiers on the quantum efficiency of the near-infrared fluorescence from the 3H4 level of Tm3+ ion in silica-based doped fibers is studied. Modifications of the core composition affect the spectroscopic properties of Tm3+ ion. Adding 17.4 mol% of AlO3/2 to the core glass caused an increase of the 3H4 level lifetime up to 50 μs, 3.6 times higher than in pure silica glass. The quantum efficiency was increased from 2% to approximately 8%. On the opposite, 8 mol% of PO5/2 in the core glass made the lifetime decrease down to 9 μs. These changes of Tm3+ optical properties are assigned to the change of the local phonon energy to which they are submitted by modifiers located in the vicinity of the doping sites. Some qualitative predictions of the maximum achievable quantum efficiency are possible using a simple microscopic model to calculate the non-radiative de-excitation rates.

Introduction

Thulium doped fibers have been widely studied in the past few years. Because of Tm3+ ion rich energy diagram, lasing action and amplification at multiple infrared and visible wavelengths are allowed. Thanks to the possible stimulated emission peaking at 1.47 μm (3H43F4, see Fig. 1), discovered by Antipenko et al. [1], one of the most exciting possibilities of Tm3+ ion is amplifying optical signal in the S-band (1.47–1.52 μm), in order to increase the available bandwidth for future optical communications. Unfortunately, the upper 3H4 level of this transition is very close to the next lower 3H5 level so non-radiative de-excitations (NRD) are likely to happen in high phonon energy glass host, causing detrimental gain quenching.

In standard silica-based optical fibers the maximum phonon energy is high: Ep  1100 cm−1. Then only three vibrational quanta (phonons) are needed to bridge the energy gap between the 3H4 and 3H5 multiplets (ΔE  3700 cm−1). Consequently the measured 3H4 level lifetime τ is 14 μs only [2], much shorter than the purely radiative lifetime τrad = 650 μs [3], and the quantum efficiency is only ∼2%. Therefore efficient host materials for implementing Tm3+ ion in the S-band have been restricted to non-silicate glasses with low phonon energy. In fluoride glass host (Ep  580 cm−1) five phonons are needed to bridge ΔE, the probability of NRD is very low and the de-excitation is almost purely radiative. As a consequence, the 3H4 level lifetime almost equals the radiative lifetime, τ  τrad = 1.3 ms [3] and the quantum efficiency approaches 100%. This is why the first thulium doped fiber amplifier (TDFA) was realized in 1995 by Komukai et al. in fluoride glass [4]. There has been many investigations for optimizing pumping schemes with newly available pump sources [5], [6]. Recently, TDFAs made of other low phonon glasses have been investigated. In tellurite glass (Ep  700 cm−1) amplification in the S-band is possible thanks to the 90% quantum efficiency and the 310 μs lifetime [7], [8]. TDFAs using multicomponent silicate glasses were also realized [9], [10] but small information about the glass composition and preparation of the samples was given in the cited references. The potential of alkaline-earth aluminate glass (Ep  780 cm−1) for TDFA was studied [11]: S-band quantum efficiency of 35% and 3H4 level lifetime of around 230 μs were reported. Note that using low phonon energy matrix like oxyfluoride glass ceramic (Ep  300 cm−1) is possible [12], [13]. Unless they show good amplification results, all those fiber glasses or ceramics cannot be easily used in telecommunication networks based on silica fibers, because of their poor chemical durability, high production price, low reliability. As they are not fusion-spliceable to silica fibers one must use other techniques such as connectors or glued butt-coupling that are more lossy and easily damaged under high pumping level. For those reasons, it would still be very interesting to demonstrate a TDFA based on silica glass.

To address this problem, we propose to study the effect of some modifications of Tm3+ ion local environment. Keeping the overall fiber composition as close as possible to that of a standard silica fiber, we expect to control the rare-earth spectroscopic properties by codoping with selected modifying oxides. We have studied the incorporation of modifying elements compatible with a standard fabrication technique, such as modified chemical vapor deposition (MCVD). GeO2 is widely used in silica fiber fabrication to increase the core refractive index. AlO3/2 is a promising candidate because it is known to improve the spectroscopic properties of Er3+ ion for C-band amplification [14]. It is also used to reduce quenching effect through clustering in highly rare-earth-doped silica [15]. Both oxides have a lower maximum phonon energy than silica. We use high phonon energy PO5/2 as opposite demonstration.

In this article, we describe the samples preparation and compositional characterization, then the absorption, fluorescence and fluorescence decay measurement techniques. We use measured lifetimes to derive the quantum efficiency of Tm3+ ion. Then we present the results and discuss the effects of the core composition on the spectroscopic properties of thulium in the fibers. In particular, we show that codoping with phosphorus or aluminum results in the strong modification of the local environment of thulium, whereas germanium codoping has a lesser impact. Aluminum is the most interesting modifier in the series studied here, as a four-times increase in the Tm3+ ion quantum efficiency is observed when codoping with less than 10 mol% of Al2O3. These results are in qualitative agreement with a simple model implementing multiphonon NRD empirical theory, which shows that the striking differences between codopants lie in the local phonon energy alteration they induce to Tm3+ ion.

Section snippets

Sample preparation

The Tm3+-doped optical fibers were drawn from silica-based preforms using a drawing tower. The preform samples were prepared from pure silica tube by classical MCVD method allowing phosphorus and/or germanium to be incorporated in the glass [16]. Doping with thulium and/or aluminum was performed using the so called ‘solution doping technique’ [17]. The core silica layer was deposited at lower temperature than the preceding cladding layers, so that it was left porous. Then the substrate tube was

Effect of core composition on absorption

The room temperature GSA spectra from the fundamental Tm3+:3H6 multiplet of Tm(Al), Tm(Ge) and Tm(P) fiber samples are shown in Fig. 3. The GSA to the 3H4 level is shown in Fig. 3(a). The Tm(Al) samples 3H4 spectra are very similar, except for small changes on the wings of the band. For the seek of clarity we show only one GSA spectrum from the Tm(Al) sample series. More importantly the 3H4 peak wavelength is the same for all Tm(Al) samples, well within the spectral resolution of the

Discussion

From this point, we calculate WNRD along two possible microscopic models of which we will discuss the validity. In the first model it is assumed that the glass modifiers are homogeneously dispersed in the glass. As all Tm3+ ions sustain the same local vibrational environment, the local environment is equivalent to pure silica in terms of phonon transfer and propagation properties. Variations within sample types are only characterized by ΔE changes. In this case, WNRD and τcalc are calculated

Conclusion

We have studied the effect of modifying the composition of Tm3+-doped silica-based optical fibers on the 3H4 level lifetime. Al and P doping are efficient to increase and decrease the lifetime and quantum efficiency, respectively. An addition of 17.4 mol% of AlO3/2 to silica glass increases the thulium 3H4 level lifetime by 3.6 to above 50 μs, whereas 8 mol% of PO5/2 reduces it to 9 μs only. Ge doping has a benefic effect, but is less efficient than Al. This behavior versus core composition is

Acknowledgments

BF acknowledges support from the French Ministry of Research and Technology for his PhD scholarship. Laboratoire de Physique de la Matière Condensée is part of the CNRS-GIS GRIFON multisite optical fiber technological consortium. Electron probe microanalysis measurements were performed at Institute of Photonics and Electronics, Academy of Sciences of the Czech Republic, Prague.

References (32)

  • B.G. Aitken et al.

    J. Non-Cryst. Solids

    (2004)
  • D.A. Simpson et al.

    J. Non-Cryst. Solids

    (2006)
  • M. Grinberg et al.

    Opt. Comm.

    (1998)
  • J. Wang et al.

    J. Non-Cryst. Solids

    (1993)
  • A. Monteil et al.

    J. Non-Cryst. Solids

    (2004)
  • B.M. Antipenko et al.

    Sov. J. Quant. Electron.

    (1983)
  • S.D. Jackson et al.

    J. Light. Technol.

    (1999)
  • M.J.F. Digonnet

    Rare Earth Doped Fiber Lasers and Amplifiers

    (1993)
  • T. Komukai et al.

    IEEE J. Quant. Electron.

    (1995)
  • J.F. Martins-Filho et al.

    IEEE Photon. Technol. Lett.

    (2003)
  • F. Roy et al.

    Electron. Lett.

    (2001)
  • M. Naftaly et al.

    Appl. Opt.

    (2000)
  • R. Caponi, A. Potenza, B. Sordo, E.M. Taylor, L.N. Ng, J. Nilsson, F. Poli, in: ECOC’03, Tu3.75,...
  • B.N. Samson, N.J. Traynor, D.T. Walton, A.J.G. Ellison, J.D. Minelly, J.P. Trentelman, in: OAA Top. Meet. OSA’00, 44...
  • B. Cole, M.L. Dennis, in: OFC’01, TuQ3-1,...
  • M.J. Dejneka

    MRS Bulletin

    (1998)
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