Integrated optical amplifiers and microspherical lasers
based on erbium-doped oxide glasses

Giancarlo C. Righini, Carole Arnaud, Simone Berneschi, Marco Bettinelli, Massimo Brenci, Alessandro Chiasera, Patrice Feron, Maurizio Ferrari, Maurizio Montagna, Gualtiero Nunzi-Conti, et al.

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To cite this version:

Giancarlo C. Righini, Carole Arnaud, Simone Berneschi, Marco Bettinelli, Massimo Brenci, et al. Integrated optical amplifiers and microspherical lasers based on erbium-doped oxide glasses. 2nd International Workshop on Photonic and Electronic Materials, 2005, San Sebastian, Spain. pp.1711-1717, 10.1016/j.optmat.2004.11.041. hal-00167949

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Integrated optical amplifiers and microspherical lasers based
on erbium-doped oxide glasses

G.C. Righini a,*, C. Arnaud b, S. Berneschi a,c, M. Bettinelli d, M. Brenci a, A. Chiasera f,
P. Feron b, M. Ferrari f, M. Montagna e, G. Nunzi Conti a, S. Pelli a, H. Portales e,
C. Siligardi g, A. Speghini d, L. Zampedri f

a Optoelectronics & Photonics Department, Nello Carrara Institute of Applied Physics, IFAC-CNR, Via Panciatichi 64, 50127 Firenze, Italy

b Laboratoire d'Optronique (CNRS-UMR 6082-FOTON), ENSSAT, 6 rue de Kerampont, 22300 Lannion, France

c Dipartimento di Elettronica Informatica e Sistemistica (DEIS), Bologna University, Viale Risorgimento 2, 40100 Bologna, Italy

d Scientific and Technological Department, Verona University, Strada le Grazie 15, 37134 Verona, Italy

e Dipartimento di Fisica, Università di Trento and INFM, via Sommarive 14, 38050 Povo, Trento, Italy

f CNR-IFN, Istituto di Fotonica e Nanotecnologie, CSMFO group, via Sommarive 14, 38050 Povo, Trento, Italy

g Dipartimento Ingegneria dei Materiali e dell'Ambiente (DIMA), Modena and Reggio Emilia University, Via Vignolese 905, 41100 Modena, Italy

Er³⁺-doped glasses have been a subject of great interest in the recent years for their application in the areas of guided wave optical amplifiers and lasers. Oxide-glass matrices, in particular, offer the advantage of relatively simple fabrication processes—both for bulk glasses and optical fibers—and have demonstrated quite good properties in the 1.5 µm wavelength band. Here we present some results we have obtained in the development of different silica-based glasses and in their application to the production of integrated optical amplifiers and microspherical lasers. As to the former application, two classes of silica glasses have been produced and investigated, namely a melted soda-lime-alumino-silicate glass and a sol-gel silica-hafnia glass. Both of them exhibit a wide emission bandwidth and seem quite promising for broadband optical amplifiers. We also demonstrated low-threshold microlasers, based on whispering gallery mode spherical resonators, fabricated in different oxide glasses.

Keywords: Silica glass; Er³⁺-doped glass; Optical amplifier; Microsphere; Microlaser; Integrated optics

**1. Introduction**

The possibility of confining light inside guiding films
and structures has opened new perspectives in the very
broad area of passive and active optical components
for either telecom or sensing systems. The aim of this
paper is to discuss some advances in guided-wave struc-

tures for light amplification and lasing, based on oxide
glasses. In particular, reference will be made to (a)
planar optical waveguides, fabricated in erbium-doped
silica glasses, that appear quite promising for the
development of broadband integrated optical amplifiers,
and (b) dielectric spherical microresonators, where the
light is guided through whispering-gallery modes
(WGMs), that allowed us to demonstrate low-threshold
microlasers.

For what concerns the development of Er³⁺-doped
waveguide amplifiers (EDWAs), we have focused our

<footnote>* Corresponding author. Tel.: +39 055 4235239; fax: +39 055 4235350.</footnote>

<footnote>E-mail address: g.c.righini@ifac.cnr.it (G.C. Righini).</footnote>
attention on silica-based glasses in virtue of their good
chemical durability and adaptability to different wave-
guide fabrication process. A net optical gain of 1.5 dB/
cm was demonstrated in Er³⁺/Yb³⁺ co-doped soda-
lime-silicate glasses [1,2]. Their performance, however,
was limited by the relatively narrow fluorescence band-
width, the full width half maximum (FWHM) value
being around 17 nm. We have therefore searched for
novel materials that could offer a larger bandwidth: as
it is well known that introduction of aluminum in a sil-
icate glass may lead to increase the emission bandwidth
[3,4], a first approach has been that of co-doping our
soda-lime-silicate glass matrix with different quantities
of aluminum oxide (up to about 20 mol%) and to inves-
tigate the effect of aluminum content on the optical
properties of the glass and in particular on the emission
bandwidth of the ⁴I₁₃/₂ → ⁴I₁₅/₂ Er³⁺ ions transition at
1550 nm. A second approach has concerned the use of
sol-gel-derived silica-hafnia films, that represent an
innovative binary glass system with valuable optical,
spectroscopic and structural characteristics [5]. Here
we report on the spectroscopic properties of these two
glass systems, and on the characteristics of the corre-
sponding optical waveguides.

As to the second issue, it is well known that in dielec-
tric spheres light can be guided through high-Q whisper-
ing-gallery-modes with an unique combination of strong
temporal and spatial confinement of light. Glass micro-
spheres are of interest for a large number of applica-
tions, such as cavity quantum electrodynamics [6],
nonlinear optics [7], photonics [8] and chemical or bio-
logical sensing [9]. In particular, since the studies on
morphology-dependent-resonances (MDRs) and lasing
effects in droplets during the 1980s [10], rare earth doped
glass microsphere lasers have become the subject of
many investigations and have been demonstrated as
potentially compact laser sources [11–13]. Here we
briefly report both on the spectroscopic characterization
of an Er³⁺ doped microsphere in a modified-silicate
glass, and on lasing action in that glass and in an
Er³⁺/Yb³⁺ co-doped phosphate glass.

**2. Experimental**

2.1. *Sol–gel silica–hafnia films*

By using the sol-gel process, we produced and subse-
quently tested several films of composition (100 – x)
SiO₂–xHfO₂ (x = 10, 20, 30, 40), with the aim of assess-
ing the role of hafnia concentration on the structural
and optical properties of these waveguides. Two differ-
ent sets of films were produced, containing 0.01 and
0.3 mol% Er³⁺ ions, respectively. All the films were
deposited on cleaned pure-silica substrates by dip-coat-
ing. After each layer's deposition, the sample was

annealed in air for 50 s at 900 °C. After a 10-dip
cycle, the film was heated for 2 min at 900 °C. Finally,
the waveguides were subject to a further annealing
at 900 °C, whose duration was different for each
waveguide; for instance, for the samples containing
0.3 mol% Er³⁺ the annealing time was varying from
30 h (x = 10) down to 5 min (x = 40). The necessary
number of dips for the corresponding samples was 50
and 25. More details on the fabrication process are
available in previous papers [5,14].

Modal characterization was performed by the well
known dark-line spectroscopy; one TE and one TM
mode at 632.8 nm were supported by each waveguide.

Photoluminescence measurements in the region of the
⁴I₁₃/₂ → ⁴I₁₅/₂ transition and decay curves from the ⁴I₁₃/₂
level were obtained with the experimental set-up de-
scribed elsewhere [15]. All the spectroscopic measure-
ments were, thus, made in guided-wave configuration,
using prism coupling and as excitation source either
the 980 nm line of a Ti:sapphire laser or the 514.5 nm
line of an argon laser.

2.2. Soda-lime–alumino-silicate glasses and ion-
exchanged waveguides

These glasses were prepared by conventional melt-
ing process and had a basic composition of the
type Na₂O-CaO-Al₂O₃-SiO₂ with small percentages
of P₂O₅ and K₂O. The batches were prepared by using
reagent grade SiO₂, Na₂CO₃, K₂CO₃, Al₂O₃, CaCO₃,
(NH₄)₃PO₄, and Er₂O₃ as starting materials. Melting
occurred in an electrically heated furnace within Pt cru-
cibles, following the same heating cycle: from 20 to
1000 °C at 10 °C/min, with a 24 h soaking time at
1000 °C, from 1000 to 1550 °C at 20 °C/min and finally
1 h of soaking time at the maximum temperature of
1550 °C. Eventually, the melt was quenched in a graph-
ite mould to obtain small pieces of glass having a bar
form; each bar was then cut to 1 mm thickness and
optically polished on both faces. Room temperature
absorption spectra were measured in the visible region
using a Kontron UVICON 941 Plus double beam spec-
trophotometer (spectral bandwidth of 0.5 nm) and in
the near infrared region with a Nicolet Magna 760
FTIR spectrometer (spectral bandwidth of 2 cm⁻¹) or
a double beam Perkin-Elmer spectrophotometer with
a resolution of 1 nm. The fluorescence spectra of Er³⁺
ions were detected using a 976 nm laser diode as exci-
tation source: luminescence was dispersed using a
monochromator with a resolution of 1 nm and detected
with an InGaAs photodiode and lock-in technique.
Lifetime τ of the ⁴I₁₃/₂ level was also measured after
excitation at 976 nm.

As to the waveguide fabrication process in these
glasses, our attention was focused on ion-exchange; in-
deed, we had designed a soda-lime-silicate glass just in
order to take advantage of a large content of Na⁺ ions. Thus, planar optical waveguides were produced by using the Ag⁺ ↔ Na⁺ exchange process in a dilute AgNO₃ solution (AgNO₃:NaNO₃ = 0.5:99.5 mol%) at 325 °C. Exchange times were chosen in such a way to obtain multimode waveguides, in order to use with good accuracy the inverse WKB (IWKB) method for the determination of the refractive index profile resulting from the exchange process. Modal characterization at various wavelengths was performed by dark line spectroscopy using a semi-automatic system (COMPASSO) developed at IFAC [16]. The same device had also been used to measure the refractive indices of the bulk glasses.

### 2.3. Er-doped glass microspherical resonators

Different kinds of glasses were used to produce microspheres: three experimental modified-silica glass samples, doped with 0.2, 0.5 and 1.5 mol% of erbium, respectively [17], as well as a commercial Er³⁺/Yb³⁺ co-doped phosphate glass (Schott IOG-2), which contains 2 wt% of Er₂O₃ and 3 wt% of Yb₂O₃. The refractive indices at 632.8 nm of the former three glasses were equal to 1.5604, 1.5607, and 1.5871, respectively.

Each glass was ground, and microspheres were prepared from the powders by using a microwave plasma torch. Powders were axially injected and melted when passing through the plasma flame, superficial tension forces giving them their spherical shape. Free spheres were collected a few ten centimeters down. The diameter of the spheres depends essentially on the powder size and may vary from 10 to 200 µm [18]. By visual analysis with a microscope, the microspheres with diameter in the range 50–100 µm were selected and then glued to the stretched tip of an optical fibre (~20–30 µm in diameter), which in turn was mounted on a submicrometer translation stage.

Photoluminescence spectroscopy in the 1.5 µm region was performed using the 980.8 nm line of a Ti:sapphire laser as excitation source. The luminescence was dispersed by a 320 nm single-grating monochromator with a resolution of 2 nm. The light was detected using a Si/InGaAs two-colors photodiode and standard lock-in technique. Lifetime decay curves were obtained recording the signal by a digital oscilloscope. For both the precursor bulk samples and the microspheres the standard bulk measurement configuration has been used.

A single half-fibre-taper was used to couple the pump light from a pigtailed laser diode into the WGMs of the microsphere and—at the same time—to allow us to couple the fluorescence or laser light out of the microsphere. The half-taper was obtained using a fusion splicer by heating and stretching a standard telecommunication fibre until breaking. The drawn length was typically

850 µm, and the taper end was reduced down to 1.5 µm in diameter.

## 3. Results and discussion

### 3.1. Sol-gel silica-hafnia waveguides for EDWAs

Table 1 summarizes some optical and spectroscopic parameters of the fabricated silica-hafnia planar waveguides, together with their compositions. In all samples, birefringence is not negligible, as indicated by the significant difference between the refractive indices measured for TE and TM polarizations. Propagation losses were also measured in various samples by using the technique based on the collection of the light scattered out of the plane of the waveguide, and an average value was ≈0.9 dB/cm at 632.8 nm.

The photoluminescence (PL) spectra relative to the <sup>4</sup>I<sub>13/2</sub> → <sup>4</sup>I<sub>15/2</sub> transition of the Er<sup>3+</sup> ions for all the waveguides, obtained upon excitation at 514.5 nm and 980 nm, exhibit a main emission peak at 1.53 µm and a broad FWHM spectral width of about 50 ± 2 nm. Fig. 1 shows the emission spectra of the samples doped with 0.3 mol% Er<sup>3+</sup> for excitation at 514.5 nm; the shape and the bandwidth of these spectra, however, do not change with the excitation wavelength, indicating that site selection is negligible. The similarity of PL spectra for all waveguides seems to indicate that a small inhomogeneous broadening is present independently on HfO<sub>2</sub> content.

The measured lifetime of the <sup>4</sup>I<sub>13/2</sub> metastable state, also reported in Table 1, decreases with the increasing of the HfO<sub>2</sub> molar concentration. At very low Er<sup>3+</sup> concentration, the energy transfer processes should be very improbable. Moreover, considering that in completely densified silicate glasses the multiphonon decay does not significantly affect the <sup>4</sup>I<sub>13/2</sub> level lifetime, we can assume that the lifetimes measured at 0.01 mol% Er<sup>3+</sup> doping level are very close to the radiative ones. Under this hypothesis, we can estimate that a quantum efficiency between 84% and 88% is obtained in all the samples doped with 0.3 mol% of erbium.

### 3.2. Soda-lime-alumino-silicate glasses and ion-exchanged waveguides

The main oxides' concentrations of the six produced soda-lime-alumino-silicate glasses are reported in Table 2, together with the emission bandwidth B, the measured τ<sub>meas</sub> and the calculated radiative τ<sub>rad</sub> lifetimes of the <sup>4</sup>I<sub>13/2</sub> metastable state. The data in the table clearly indicate an increase of the fluorescence bandwidth with the increase of alumina content.

This is also shown in Fig. 2, where the FWHM bandwidth of both the absorption and the emission peak
<table><caption>Table 1
Compositional, optical and spectroscopic parameters of the Er³⁺-doped (100-x) SiO₂-xHfO₂ planar waveguides</caption><thead><tr><th rowspan="2">Waveguide label</th><th rowspan="2">SiO<sub>2</sub>:HfO<sub>2</sub> molar ratio</th><th rowspan="2">Erbium content (mol%)</th><th colspan="2">n at 632.8 nm ± 0.005</th><th rowspan="2">Thickness (μm) ± 0.05</th><th rowspan="2"><sup>4</sup>I<sub>13/2</sub> lifetime (ms) ± 0.5</th></tr><tr><th>TE</th><th>TM</th></tr></thead><tbody><tr><td>SH10</td><td>90:10</td><td>0.3</td><td>1.470</td><td>1.468</td><td>2.05</td><td>7.1</td></tr><tr><td>SH20</td><td>80:20</td><td>0.3</td><td>1.543</td><td>1.537</td><td>1.27</td><td>6.5</td></tr><tr><td>SH30</td><td>70:30</td><td>0.3</td><td>1.600</td><td>1.588</td><td>1.10</td><td>6.0</td></tr><tr><td>SH40</td><td>60:40</td><td>0.3</td><td>1.663</td><td>1.646</td><td>0.96</td><td>5.8</td></tr><tr><td>SH10a</td><td>90:10</td><td>0.01</td><td>1.494</td><td>1.494</td><td>1.16</td><td>8.5</td></tr><tr><td>SH20a</td><td>80:20</td><td>0.01</td><td>1.562</td><td>1.562</td><td>0.69</td><td>7.5</td></tr><tr><td>SH30a</td><td>70:30</td><td>0.01</td><td>1.605</td><td>1.601</td><td>0.69</td><td>7.0</td></tr><tr><td>SH40a</td><td>60:40</td><td>0.01</td><td>1.664</td><td>1.655</td><td>0.53</td><td>6.6</td></tr></tbody></table>

<figure><img src="image_3.png" /><figcaption>Fig. 1. Photoluminescence spectra relative to the <sup>4</sup>I<sub>13/2</sub> → <sup>4</sup>I<sub>15/2</sub> transition of the Er³⁺ ions for the (a) SH40, (b) SH30, (c) SH20, and (d) SH10 waveguides, obtained upon excitation at 514.5 nm.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 2. Effective bandwidths (absorption and emission) as a function of Al₂O₃ concentration. The line is only traced as an help to eyes.</figcaption></figure>

<table><caption>Table 2
Main chemical components, FWHM emission bandwidth, measured and calculated lifetimes of the <sup>4</sup>I<sub>13/2</sub> metastable state of the six aluminosilicate glasses we had produced</caption><thead><tr><th rowspan="2">Sample</th><th colspan="5">Main oxides content (mol%)</th><th>B</th><th>τ<sub>meas.</sub></th><th>τ<sub>rad.</sub></th></tr><tr><th>SiO<sub>2</sub></th><th>Na<sub>2</sub>O</th><th>Al<sub>2</sub>O<sub>3</sub></th><th>CaO</th><th>Er<sub>2</sub>O<sub>3</sub></th><th>(±1 nm)</th><th>(±0.2 ms)</th><th>(±0.2 ms)</th></tr></thead><tbody><tr><td>AL1</td><td>72.64</td><td>13.94</td><td>0.99</td><td>10.96</td><td>0.47</td><td>17</td><td>7.0</td><td>12.6</td></tr><tr><td>AL5</td><td>69.84</td><td>13.40</td><td>4.81</td><td>10.54</td><td>0.45</td><td>19</td><td>8.0</td><td>11.4</td></tr><tr><td>AL10</td><td>66.64</td><td>12.79</td><td>9.17</td><td>10.05</td><td>0.43</td><td>20</td><td>7.8</td><td>10.3</td></tr><tr><td>AL15</td><td>63.71</td><td>12.23</td><td>13.16</td><td>9.61</td><td>0.41</td><td>25</td><td>5.1</td><td>8.76</td></tr><tr><td>AL20</td><td>61.14</td><td>11.73</td><td>16.67</td><td>9.23</td><td>0.39</td><td>30</td><td>6.2</td><td>8.43</td></tr><tr><td>AL25</td><td>58.70</td><td>11.26</td><td>20</td><td>8.86</td><td>0.39</td><td>33</td><td>5.1</td><td>8.18</td></tr></tbody></table>

around 1.5 µm is plotted as a function of the alumina content. One can notice that the slope of increase of the emission bandwidth changes significantly from the region where the alumina content is lower than 9 mol% to the region where the content is equal to or higher than 13 mol%. Correspondingly, an abrupt change in the physical (density) and optical (bulk refractive index) properties of the glass was observed as well. These facts suggest that the behaviour of this class of

glasses has a drastic change when the concentration of aluminium ions becomes larger than that of sodium ions.

The measured lifetimes were compared with the radiative lifetimes calculated using Judd-Ofelt (J-O) analysis; in particular, the phenomenological intensity parameters Ωᵢ were estimated and are reported in Table 3. The values of the J-O parameters appear to be higher, especially for the samples at high alumina content, than
<table><caption>Table 3
Judd-Ofelt intensity parameters ($\Omega_i$), with their uncertainties, calculated for the six soda-alumino-silicate glasses</caption><thead><tr><th>Glass</th><th>Ω<sub>2</sub> (pm<sup>2</sup>)</th><th>Ω<sub>4</sub> (pm<sup>2</sup>)</th><th>Ω<sub>6</sub> (pm<sup>2</sup>)</th></tr></thead><tbody><tr><td>AL1</td><td>5.24 ± 0.18</td><td>1.11 ± 0.25</td><td>0.574 ± 0.088</td></tr><tr><td>AL5</td><td>5.43 ± 0.17</td><td>1.15 ± 0.24</td><td>0.691 ± 0.086</td></tr><tr><td>AL10</td><td>6.12 ± 0.19</td><td>1.38 ± 0.26</td><td>0.813 ± 0.093</td></tr><tr><td>AL15</td><td>6.47 ± 0.21</td><td>1.60 ± 0.28</td><td>0.98 ± 0.10</td></tr><tr><td>AL20</td><td>7.15 ± 0.25</td><td>1.95 ± 0.34</td><td>1.01 ± 0.12</td></tr><tr><td>AL25</td><td>7.89 ± 0.21</td><td>1.83 ± 0.28</td><td>1.06 ± 0.10</td></tr></tbody></table>

those found for other silicate glasses [19]. Moreover, a monotonic and remarkable increase of all the J–O parameters is observed while increasing the amount of alumina in the host glass; it is in agreement with the results reported by Tanabe [4]. These values were then used to calculate the emission probabilities between the energy levels of erbium in the soda-lime–alumino-silicate host matrix, and the radiative lifetime $\tau_{rad}$ of this emission. As an example, a quantum efficiency near 75% is estimated for the AL20 sample, which has a high $Al_2O_3$ concentration (16.67 mol%).

The presence of guided modes in the $Ag^+$-exchanged glasses was checked by using an SF6 prism to couple light into the waveguides at the wavelength of 635 nm. The effective indices of TE modes were thus measured. It appeared that, under the same ion-exchange process conditions, the number of modes supported by each waveguide was depending on the alumina content. In order to derive accurate values for the ion-exchange parameters, namely the diffusion coefficient of incoming ions $D_{Ag}^+$ and the diffusion depth $d$, in the six glasses, we found out that the best fit for the measured effective indices was an analytical function given by a linear combination between erfc and exponential functions [20]:

$$n(x) = n_{\text{sub}} + \Delta n \left\{ H * \text{erfc} \left( \frac{x}{2\sqrt{Dt}} \right) + (1 - H) * \exp \left( - \left( \frac{x}{2\sqrt{2Dt}} \right)^3 \right) \right\} \quad (1)$$

where $x$ is the depth coordinate, $n_{sub}$ is the index of the glass substrate, $n_s = n_{sub} + \Delta n$ is the surface index, $d$ is the effective depth of the waveguide, and $D = D_{Ag}$. In an analogous way, $D_{Na}$ indicates the effective diffusion

coefficient for the $Na^+$ ions. The value of $D$ is related to the diffusion time $t$ by the equation $d = (Dt)^{1/2}$; it also depends on the molten salt solution, the glass and the temperature. It is possible to relate the fitting value $H$ to the Stewart coefficient $\alpha = 1 - D_{Ag}/D_{Na}$ [20]. Table 4 lists the number of TE modes and the values of $D_{Ag}$ $\Delta n$, $H$ and $\alpha$ calculated for the six doped glasses after the ion-exchange. There is a marked decrease of $\Delta n$ in this class of glasses with the increase of Al content, while the diffusion coefficient $D_{Ag}$ does not present a monotonic behaviour, but exhibits a maximum for the AL10 sample, that has a sodium content slightly higher than aluminium content. Fig. 3 shows the behaviour of the diffusion coefficient and the index change versus the atomic concentrations of Al and Na ($C_{Al}$ and $C_{Na}$, respectively). It confirms that there is a change in the diffusion process of the $Ag^+$ ions when the value of $C_{Al}$ approaches that of $C_{Na}$ (‘breakpoint zone’). As a consequence, the reconstructed index profiles of the waveguides, which are shown in Fig. 4, have different shapes when $C_{Al} < 10$ mol%, while they are very similar for values of $C_{Al} > 10$ mol%.

### 3.3. Er-doped glass microspherical lasers

Spectroscopic characterization of $Er^{3+}$-doped modified-silica glass microspheres indicated that both the

<figure><img src="image_5.png" /><figcaption>Fig. 3. Index change $\Delta n$ after ion-exchange and diffusion coefficient of $Ag^+$ ions versus the atomic concentrations of Al and Na. The lines are only traced as an help to eyes.</figcaption></figure>

<caption>Table 4</caption>

Ion-exchange process parameters and fitted values of $D_{Ag}$, $\Delta n$, $H$ and $\alpha$ obtained using Eq. (1) as index profile for the slab waveguides fabricated in six glasses

<table><thead><tr><th>Sample</th><th>Melt salt and temperature</th><th>Exchange time (min)</th><th>Modes at 635 nm</th><th>Δn</th><th>D (10<sup>-3</sup> μm<sup>2</sup>/s)</th><th>H</th><th>α</th></tr></thead><tbody><tr><td>AL1</td><td>AgNO<sub>3</sub>:NaNO<sub>3</sub>(0.5:99.5 mol%) 325 °C</td><td>90</td><td>11</td><td>0.067</td><td>1.42</td><td>0.25</td><td>0.94</td></tr><tr><td>AL5</td><td></td><td></td><td>11</td><td>0.050</td><td>1.87</td><td>0.24</td><td>0.93</td></tr><tr><td>AL10</td><td></td><td></td><td>13</td><td>0.032</td><td>4.25</td><td>0.36</td><td>0.89</td></tr><tr><td>AL15</td><td></td><td></td><td>10</td><td>0.034</td><td>2.57</td><td>0.50</td><td>0.83</td></tr><tr><td>AL20</td><td></td><td></td><td>8</td><td>0.029</td><td>2.07</td><td>0.53</td><td>0.81</td></tr><tr><td>AL25</td><td></td><td></td><td>7</td><td>0.029</td><td>1.72</td><td>0.58</td><td>0.78</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>Fig. 4. Refractive index profiles of the waveguides produced by ion-exchange in the six aluminosilicate glasses. The profiles were numerically reconstructed by using the function given in Eq. (1).</figcaption></figure>

luminescence spectrum and the lifetime changed when passing from the bulk glass to the microsphere [21]. In fact, the emission bandwidth is larger for microspheres than for the precursor glass, demonstrating that the site-to-site inhomogeneities are effective in the microspheres. This is probably due to the fabrication process, but, due the small number of samples, so far it has been impossible to determine the specific reason of the induced inhomogeneity. Lifetimes measurements gave further information about the site-to-site inhomogeneities; the decay curves of the erbium in various microspheres all showed a faster relaxation than in the corresponding bulk glasses, indicating that energy transfer mechanism among active ions was not negligible. This behaviour is evident in Fig. 5, which compares the measured lifetimes of the bulk and microsphere glass doped with 0.5 mol% of erbium oxide.

Single line laser action upon pumping at 1480 nm was obtained by using a 0.5 mol% Er-doped microsphere of about 80 µm in diameter. The laser spectrum with emis-

<figure><img src="image_7.png" /><figcaption>Fig. 5. Luminescence decay curve of the ⁴I₁₃/₂ level of Er³⁺ ion in the bulk sample (squares) and in the microsphere (triangles) for the same modified silicate glass (0.5 mol% of erbium oxide).</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 6. Lasing spectrum of a microsphere of diameter 80 µm in the modified silicate glass.</figcaption></figure>

sion at 1568.5 µm is shown in Fig. 6. The pump threshold was 15 mW (measured at the input of the tapered fiber).

As to the phosphate IOG-2 microspheres, lasing action was obtained quite easily for any sphere diameter on the range 50–100 µm, as reported in a previous paper [13].

4. Conclusions

The results we have obtained so far indicate that there are several classes of rare-earth-doped oxide glasses that are worth being investigated in greater detail with the aim of producing high-performance broad-band integrated optical amplifiers. One of these glasses is represented by SiO₂–HfO₂ binary systems, where Er³⁺-doped sol–gel planar waveguides have shown high quantum efficiency and ≈50 nm FWHM bandwidth around 1.53 µm. Work is now in progress for the fabrication of channel waveguides and their characterization.

A second class is constituted by soda-lime-alumino-silicate glasses, that combine the very good chemical durability of silicates with the better spectroscopic properties induced by the presence of a relatively high percentage of aluminium. An increase up to 60% of the effective fluorescence bandwidth was observed varying alumina content from 1% to 20% molar, and an high value of quantum efficiency was calculated for these glasses. Ag⁺ ⇌ Na⁺ ion-exchanged waveguides with good characteristics were also successfully produced. Both spectroscopic and ion-exchange results have shown that there is a significant change in the behaviour of the glass when the aluminum atomic concentration is of the same order as that of sodium: these aspects are now being investigated in greater detail. Moreover, integrated optical amplifiers in these glasses are being produced and tested.

Microspherical WGM resonators were produced in other Er³⁺-doped oxide glasses (a modified silicate and a phosphate), with the aim of producing low-threshold
microlasers. Preliminary studies have shown that the transformation of a glass into a microsphere could actually change the environment of erbium ions, and therefore its spectroscopic properties. This issue is being further investigated. Lasing action was obtained in both the investigated glasses.

## Acknowledgements

This work was partially supported by MIUR, Italy, through the FIRB project 'Sistemi Miniaturizzati per Elettronica e Fotonica' (RBNE012N3X-005), and by the bilateral collaboration program between CNRS, France and CNR, Italy (FOTON-IFAC). The authors are grateful to B. Gaillard-Allemand and V. Benoit (Cristallerie de Baccarat, France) for providing the samples of the modified silicate glass and to Mr. Roberto Calzolai and Mr. Franco Cosi (IFAC-CNR) for their technical assistance.

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