Er3+- and Tm3+-Containing Ultra-Transparent Oxyfluoride-Based Glass Ceramics for Wavelength Division Multiplexing Optical Amplifiers

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Glass Physics and Chemistry, Vol. 31, No. 3, 2005, pp. 377–381.

1 INTRODUCTION

Interest in oxyfluoride glasses, which have been well known for many years, has grown after the demon- stration in the literature of the possibility of obtaining so-called rare-earth-containing ultratransparent glass ceramics (GC); this refers to glass-ceramic materials, with active ions embedded in the crystalline phase, that present a transparency comparable to that of the precursor glass at all wavelengths [1–4]. This class of materials is of great importance in photonics, because it com- bines the properties of the crystal as a rare-earth environment with the mechanical properties of an oxide glass. Moreover, in the case for oxyfluoride glass ceramics where the nucleated crystalline phase is fluoride, they can provide a low-phonon-energy host. This fact plays an important role for the emitting levels of the rare-earth ions that suffer competitive nonradiative relaxation, as is the case for Tm³⁺ [5]. For wavelength division multiplexing (WDM), both Er³⁺ and Tm³⁺ are of high interest because their complementary emissions (⁴I_13/2 ⁴I_15/2 centered at 1.53 µm for erbium and

3H₄ ³F₄ centered at 1.47 µm for thulium) could allow for the production of an optical amplifier in both the S and C telecommunication bands. High material

1This article was submitted by the authors in English.

transparency is required for optical devices. Great care has to be taken in the ceramming process in order to attain the optimal crystal structure without reducing the transparency, which is strongly dependent on the size and distribution of nanocrystals.

EXPERIMENTAL TECHNIQUE

The precursor oxyfluoride glass 32SiO₂ · 9AlO_1.5 · 31.5CdF₂ · 18.5PbF₂ · 5.5ZnF₂ and the resulting Er³⁺- and Tm³⁺-doped ultratransparent glass ceramics were fabricated using the procedures described in [6, 7].

The glass transition and the crystallization tempera- tures were determined from the differential thermal analysis (DTA) curves. On the basis of DTA data, the precursor glass (PG) doped with thulium was submitted to heat treatment at 440°C for 5 h (GCTm), while, in the case of erbium, the precursor glass was heat treated at 390°C for 3 h (GCErA) and at 440°C for 5 h (GCErB).

The DTA data were collected from 40-mg bulk samples using Perkin–Elmer DTA-7 equipment at 10 K min^–1 in a flowing Ar gas atmosphere. The baselines, mea- sured by running empty crucibles as the reference and sample, were subtracted from the DTA curves.

The absorption spectra in the ultraviolet, visible, and near-infrared (UV–VIS–NIR) regions were mea-

Er

³⁺

- and Tm

³⁺

-Containing Ultra-Transparent

Oxyfluoride-Based Glass Ceramics for Wavelength Division Multiplexing Optical Amplifiers

¹

**C. Tosello, M. Montagna, M. Mattarelli, M. Ferrari, S. Chaussedent, A. Monteil*, V. K. Tikhomirov, and A. B. Seddon

* Dipartimento di Fisica, Università di Trento, CSMFO group and INFM, Via Sommarive 14, Trento, I-38050 Italy

** CNR-IFN, Institute of Photonics and Nanotechnologies, CSMFO group, Via Sommarive 14, Trento, I-38050 Italy

***POMA, UMR CNRS 6136 Université d’Angers, 2 Bld. Lavoisier, Angers cédex 01, 49045 France

****Centre for Advanced Materials, Wolfson Building, University of Nottingham, Nottingham, NG7 2RD UK Abstract—Ultratransparent glass ceramics activated with thulium and erbium ions were fabricated. The precursor oxyfluoride glass was 32SiO₂ · 9AlO_1.5 · 31.5CdF₂ · 18.5PbF₂ · 5.5ZnF₂ · 3.5ReO_1.5 with Re = Tm or Er.

Upon heat treatment, Er³⁺ and Tm³⁺ nucleate the growth of nanocrystalline β-PbF₂, which acts as a host for rare-earth ions. The spectroscopic properties of the thulium and erbium ions, studied via UV–VIS–NIR absorption and luminescence spectroscopy, show that most of the active ions are embedded in the crystalline phase.

From the absorption measurement, we extracted the cross sections of Tm³⁺ ions embedded in the crystalline and glassy phases. The comparison between the experimental lifetimes before and after thermal treatment gave as a result that the ceramming process increases the quantum efficiency of the Tm³⁺ electronic transitions. In the case of Er³⁺-activated glass ceramics, the ⁴I_13/2 ⁴I_15/2 telecom transition broadens and flattens.

PROCEEDINGS OF THE TOPICAL MEETING

OF THE EUROPEAN CERAMIC SOCIETY “NANOPARTICLES, NANOSTRUCTURES, AND NANOCOMPOSITES”

(St. Petersburg, Russia, July 5–7, 2004)

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378 TOSELLO et al.

sured at room temperature by a double-beam spectrom- eter with a resolution of 0.5 nm.

Continuous-wave photoluminescence (PL) mea- surements were performed using the 980-(for erbium- doped samples) and 800-nm lines (for thulium-doped samples) of a titanium–sapphire laser as an excitation source and dispersing the luminescence light with a 320-mm single-grating monochromator with a resolution of 2 nm. The light was detected using an InGaAs photodiode or a PbS detector (for wavelengths higher than 1.6 µm) and the lock-in technique.

Decay curves were obtained by mechanically chop- ping the exciting beam and recording the signal with a digital oscilloscope. The lifetime is defined as the 1/e decay time of the fluorescence intensity. All measure- ments were performed at room temperature.

RESULTS AND DISCUSSION

The differential thermal analysis of our precursor doped and undoped glasses indicated that the Er³⁺ and Tm³⁺ dopants are nucleating agents. This behavior is quite similar to that in the erbium-doped oxyfluoride

glass, where the nanocrystalline PbF₂ : Er³⁺ phase grows and the crystallization peak shifts to lower temperature values as the ErF₃ concentration increases [8].

Figure 1 shows the DTA data of Tm³⁺-doped oxyfluoride glass. The first crystallization peak is found at about 437°C, corresponding to the growth of the nanocrystalline PbF₂ : Tm³⁺ phase nucleated by Tm³⁺

dopants. As was previously reported [7–9], the size of the nanocrystals (free cubic PbF₂ hosting Er³⁺ or Tm³⁺

dopants) depends on the time and temperature of heat treatment. In particular, for the higher temperature heat treatments, we have the formation of glass ceramics wherein the nanocrystals have an average size of about 12 nm for erbium-doped and about 15 nm for thulium- doped samples. However, if a minor heat treatment—as in GCErA—is carried out, we find smaller nanocrystals with sizes of about 2–3 nm.

Figure 2 shows the emission spectra of the Er³⁺

dopants (⁴I_13/2 ⁴I_15/2 transition) in the precursor glass and in the GCErA and GCErB glass ceramics.

In GCErA, the maximum emission of Er³⁺ is red- shifted to 1544 nm and that of absorption is blue-shifted to 1505 nm as compared with the precursor glass. The FWHM of the emission band of Er³⁺ at 1.54 µm in GCErA is 81 ± 2 nm, and the lifetime of the lasing level is 8.8 ± 0.2 ms. Moreover, the emission spectrum of Er³⁺ in this sample is rather flat between 1.53 and 1.56 µm within the C band, which is the band most often employed in modern WDM systems. The lifetime of the lasing level of Er³⁺ in GCErB is 6.4 ms, which is slightly lower than for Er³⁺ in the precursor glass (8.0 ms). The bandwidths and the lifetimes for the glasses and glass ceramics are reported in Table 1.

350 –0.2

400 450 500

0 0.2 0.4 0.6 0.8

300

Temperature, °C

∆T (endo down), °C

T_c

T_g

Fig. 1. DTA curves of the precursor glass 32SiO₂ · 12.5AlO_1.5 · 31.5CdF₂ · 18.5PbF₂ · 5.5ZnF₂: x(TmF₃) at x = 0 (dashed line) and x = 3.5 (solid line). AlO_1.5 was sub- stituted when doping.

1450 1500 1550 1600 1650 1400

Intensity, arb. units

Wavelength, nm GCErA

Glass

GCErB

Fig. 2. Normalized photoluminescence spectra of Er³⁺

(⁴I_15/2 ⁴I_13/2 transition) in the precursor oxyfluoride glass (thick solid line) and glass ceramic heat-treated at 390°C for 3 h (dashed line) and at 440°C for 5 h (dotted line). Excitation was at 980 nm, 50 mW. Spectral resolution is 0.5 nm.

Table 1. Experimental lifetime and bandwidths of the ⁴I_13/2 level of the Er³⁺ ion in glass and glass ceramics

Sample Experimental lifetime

of the ⁴I_13/2 level, ms Bandwidth, nm

Glass 8.0 66

GCErA 8.8 81

GCErB 6.4 75

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Er³⁺- AND Tm³⁺-CONTAINING ULTRA-TRANSPARENT OXYFLUORIDE-BASED GLASS 379

In the glass ceramic with smaller crystals, the largest bandwidth occurs. The appearance of a strong crystalline peak at about 1506 nm, together with the main glass peak at 1540 nm, broadens the emission and absorption spectra. Since the cross section of the rare- earth ions in crystals is higher [3], a small quantity of active ions embedded in the crystal—as in GCErA—

equals the contribution of the ions in the glassy phase, and, in fact, the peaks at 1506 and 1540 nm have roughly the same intensity. In GCErB, where most of the active ions are in a crystalline environment, the line width is reduced because of the decrease in the glass peak.

Figure 3 shows the absorption spectra for the Tm³⁺- doped PG and GC. Crystal-like peaks appear as a con- sequence of the ceramming process. The final states of the transitions ³H₆ ^{2S + 1}L_J are labeled according to [5].

From the absorption spectra, we can separate the contribution of Tm³⁺ ions in a crystalline environment from those remaining in the glass by assuming that the absorption cross section of the Tm³⁺ ions in the residual glass does not change with respect to the PG. This pro- cedure is reported in Fig. 4. First, we subtract a nearly constant background, which is due to all the contribu- tions other than Tm³⁺ absorption, and the Urbach tail of the low-wavelength range, which is approximated by an exponential curve. In order to estimate the absorption spectrum due to the crystalline phase, we subtract from the glass ceramic spectra the spectrum of the precursor glass multiplied by a factor A, which indicates

the amount of ions in the glass phase. The value of A = 0.43 was chosen as the limiting value, i.e., that for which the difference “crystalline” absorption spectrum is zero in the minimum at about 687 nm. With this pro- cedure we can estimate that at least 57% of the thulium ions in thulium-doped GC have a crystalline environment.

In Fig. 5 the photoluminescence spectra of Tm³⁺- doped glass (solid line) and glass ceramic (dashed line) upon an 800-nm excitation are reported. The appearance of very definite and narrowed Stark emission lines (at 1640 and 1210 nm) is characteristic of the embedding of thulium ions in nanocrystals. As reported in Table 2, the lifetimes show an important increase after ceramming. The increase in the lifetime is due to a change in the environment for the rare-earth ions, which changes from oxyfluoride (maximum phonon energy ~1100 cm^–1) to pure fluoride (β-PbF₂ nanocrystals, with a maximum phonon energy of about 500 cm^–1).

This fact allows for a reduction of the multiphonon 5

0 500

Absolute coefficient, cm^–1

Wavelength, nm

1000 1500 2000

10 15

PG GCTm

3F4 3H4

3H5

1G4 1D2

3F2–³F3

Fig. 3. Room-temperature absorption spectra of precursor glass (PG) (dashed line) and Tm³⁺-doped glass ceramics (GCTm) (solid line). The final states of the transitions

3H₆ ^{2S + 1}L_J are labeled. The assignment of the levels is made following [5]. The zero absorption level (dotted line) has been translated for the glass ceramic in order to make comparison between the spectra easier.

680

0 ~

1100 1200 1300

640 2 4 6 8

Wavelength, nm Absolute coefficient, cm^–1

GCTm A × Pg GCTm–A × Pg

~

Fig. 4. Separation between the contribution to the absorp- tion coefficient due to the ions in the crystalline phase and those in the residual glassy phase for two significant Tm³⁺

transitions in the glass ceramic heat treated at 440°C for 5 h (GCTm). We required that the absorption coefficient of crystal be zero at 687 nm. The value of A (fraction of ions still in the glassy phase) is 0.43 for GCTm.

Table 2. Experimental lifetime of the three first excited levels of the Tm³⁺ ion in precursor glass (PG) and glass ceramic (GC)

Level Experimental lifetime, ms

PG GC

3H₄ <0.5 1.6

3H₅ 1.5 6.0

3F₄ 2.5 10.0

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decay rate and an overall increase of the quantum efficiency. The transition ³H₄ ³F₄ at about 1.47 µm falls in the S telecom band. To exploit it for optical amplification, analogously to erbium upon pumping from the ground level, it is necessary to shorten the lifetime of the ³F₄ level, which, since it is longer than that of the ³H₄ level, prevents in such a material the formation of a population inversion. This can be accom- plished by co-doping with holmium, which allows for a faster depopulating of the ³F₄ level because it presents an electronic level at about the same energy as the ³F₄ level [10]. As an alternative, it is also possible to adopt a two-wavelength pumping scheme, with the first laser exciting the ions from the ground to a higher energy level and a second pump exciting the ions from the ³F₄ to the ³F₂–³F₃ levels [11].

It is interesting to compare the different features of the glass ceramic host required by erbium and thulium ions. The erbium ⁴I_13/2 ⁴I_15/2 transition is not strongly affected by the multiphonon relaxation. The erbium embedding in nanocrystals is, therefore, intended to have a crystal-like shape emission and, as we have observed, this can be exploited to increase the amplification band when the emissions of the ions in the crystal and glassy phase environments sum to give a broader band. For the ³H₄ ³F₄ transition of Tm³⁺

ions, multiphonon relaxation is critical and a fluoride environment strongly enhances the quantum efficiency.

The different requirements are reflected in the different ceramming processes that have to be chosen: (i) for the Er³⁺ glass ceramic, low-temperature heat treatment allows for the maximum widening of the emission band—raising the temperature would lead to a reduction of the bandwidth; (ii) for the Tm³⁺-doped glass

ceramic, a higher temperature heat treatment is prefer- able in order to have most of the ions in the crystal phase.

The possibility of fabricating a nanostructured oxyfluoride that presents emission in the S, C, and L bands make this system suitable for the development of WDM amplifiers.

CONCLUSIONS

Ultratransparent glass ceramics were fabricated by heat treatment of the precursor oxyfluoride glass 32SiO₂ · 9AlO_1.5 · 31.5CdF₂ · 18.5PbF₂ · 5.5ZnF₂ · 3.5ReF₃ mol %. The size of the nanocrystals increases with time and temperature of heat treatment and ranges from 2–3 nm to about 15 nm.

From the Tm³⁺-doped GC absorption spectra, the percentage of active ions in the crystal phase was esti- mated: in the glass ceramic with the higher temperature treatment, it is about 60%.

The embedding of erbium ions in the crystal phase has lead to a broadening of the emission band of the erbium ⁴I_13/2 ⁴I_15/2 transition. The widening is maximum when just a few of the ions have a crystalline environment, i.e., in the glass ceramic with small nanocrystals.

The β-PbF₂ nanocrystals provide a low-phonon- energy host for thulium ions. This allows for a high increase of the experimental lifetimes due to the reduction of the multiphonon decay rate.

Finally, the spectroscopic results indicate that these ultratransparent glass ceramics are a viable system for WDM application.

ACKNOWLEDGMENTS

The authors acknowledge the financial support of MIUR-COFIN 2002 “Nanostructured Materials for Integrated Optics,” MIUR-FIRB “Miniaturized Sys- tems for Electronics and Photonics” (RBNE012N3X- 005), PAT 2004-2006 FAPVU “Fabrication of Ultratransparent Glass-Ceramics-Based Planar Optical Amplifiers.”

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