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Er-dopedsilica-basedwaveguidespreparedbydifferenttechniques:RF-sputtering,sol-gelandion-exchange

ArticleinOpticalandQuantumElectronics·January2002

DOI:10.1023/A:1021338906917

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Invited paper

Er-doped silica-based waveguides prepared

by different techniques: RF-sputtering, sol-gel

and ion-exchangeq

G . C . R I G H I N I 1,* , S . P E L L I 1 , M . F E R R A R I 2 , C . A R M E L L I N I 2 ,

L . Z A M P E D R I 3 , C . T O S E L L O 3 , S . R O N C H I N 3 , R . R O L L I 3 ,

E . M O S E R 3 , M . M O N T A G N A 3 , A . C H I A S E R A 3

A N D S . J . L . R I B E I R O 4

1Optoelectronics and Photonics Department, IROE-CNR, 50127 Firenze, Italy2CNR-CeFSA, Centro Fisica Stati Aggregati, via Sommarive 14, 38050 Povo-Trento, Italy

(mferrari@science.unitn.it)3Dipartimento di Fisica and INFM, Universita di Trento, via Sommarive 14, 38050 Povo-Trento, Italy

(tosello@science.unitn.it)4 Instituto de Quimica, UNESP, Arraquara 14801-970, Brazil (sidney@iq.unesp.br)

(*author for correspondence: E-mail: righini@iroe.fi.cnr.it)

Abstract. Erbium-activated silica-based planar waveguides were prepared by three different technological

routes: RF-sputtering, sol–gel and ion exchange. Various parameters of preparation were varied in order

to optimize the waveguides for operation in the NIR region. Particular attention was devoted to the

minimization of the losses and the increase of the luminescence efficiency of the metastable 4I13/2 state of

the Er3+ ion. Waveguide properties were determined by m-line spectroscopy and loss measurements.

Waveguide Raman and luminescence spectroscopy were used to obtain information about the structure

of the prepared films and about the dynamical processes related to the luminescence of the Er3+ ions.

Key words: dip-coating, erbium, ion-exchange, luminescence spectroscopy, optical amplification, optical

waveguides, Raman spectroscopy, RF-sputtering, silicate glasses, upconversion

1. Introduction

The development of integrated optical amplifiers operating in the 1.55 lmband, based on glassy planar waveguides activated by Er3+ ions, is now a hottopic. Silica-based matrices are very attractive because of the good solubilityof rare-earth ions (~6 · 1020 cm)3), of their high optical quality, and chem-ical robustness. These factors make them very suitable for integrated opticsapplications, and various technologies are available for efficient waveguidefabrication (Orignac et al. 1999).

In order to assess some of these technologies, here we present optical andspectroscopic properties of Er3+-activated planar and channel waveguides

q This work was partially supported by the ‘‘Progetto Finalizzato MADESS II’’ CNR Project, a MURST-

Cofin 99 and a Galileo 98-2000 program.

Optical and Quantum Electronics 34: 1151–1166, 2002.

� 2002 Kluwer Academic Publishers. Printed in the Netherlands. 1151

prepared by three different technological routes: RF-sputtering (Tosello et al.1999), sol–gel (Ferrari et al. 2000) and ion-exchange (Pelli et al. 2000). In thecase of RF-sputtering and sol–gel techniques we fabricated planar wave-guides based on the SiO2–TiO2 binary system. This composition, in fact,allows one to easily tailor the waveguide refractive index by changing theTiO2/SiO2 molar ratio. In the third case, we have first designed a silicate glassmatrix suitable for ion exchange and we have then fabricated both planar andchannel waveguides by diluted silver–sodium ion exchange.

2. Waveguide fabrication

2.1. RF-SPUTTERING

SiO2–TiO2:Er3+ thin films were RF-sputtered on both vitreous (v)-SiO2 andsilica-on-silicon (SOS) substrates. The SOS substrates were prepared byplasma-enhanced chemical vapor deposition (PECVD): a silica layer, 3.4 lmthick, was deposited on top of Si wafer, at a deposition rate of 105 nm/min.On these SOS substrates, a subsequent deposition of the erbium-activatedsilica–titania film was performed by sputtering a target disk of silica (100 mmdiameter) on which eight disks of TiO2 (10 mm diameter) and a metallicerbium wire (2 mm diameter, 26 mm total length) were put. The residualpressure in the sputtering chamber, before deposition, was about2 · 10)7 mbar. Substrates were not heated during the process. However, inorder to prevent cracking, microfractures or bad cohesion between thewaveguiding film and the substrate, the SOS substrates were annealed at600�C before the film deposition (Tosello et al. in press). The sputtering wascarried out with an Ar gas at a pressure of 7 · 10)3 mbar and the applied RFpower was 150 W with a reflected power of 18 W. The deposition time was of3 h 30 m in order to have the appropriate thickness for supporting one modeat 1.5 lm. The same parameters were employed for film deposition onto v-SiO2 substrates.

It has to be remarked that the as-prepared films did not support anyguided mode because of non-stoichiometric structure of SiOx with x < 2; athermal annealing in air for 6 h at 600�C was necessary to achieve lightpropagation (Tosello et al. 1999, in press). Table 1 reports the oxygen, sili-con, titanium and erbium atomic concentration obtained by energy disper-sive spectrometry (EDS) for the waveguide deposited on the v-SiO2 substrate.

2.2. SOL–GEL

Figure 1 describes the sol–gel process for the preparation of the waveguides.The starting solution was obtained by mixing tetraethylorthosilicate (TEOS),

1152 G.C. RIGHINI ET AL.

ethanol, deionized water and hydrochloric acid as a catalyst. Titanium iso-propoxide (TiPOT) was used as TiO2 precursor. As TEOS and TiPOT havevery different hydrolysis rates, TEOS was first pre-hydrolyzed for 1 h at 65�Cbefore adding TiPOT solution. The final solution was let react under stirringfor 16 h at room temperature.

Silica–titania films were obtained by dip-coating, with a dipping rateof 40 mm/min, using cleaned pure v-SiO2 slabs (n ¼ 1.46 at 632.8 nm) as

Fig. 1. Flow diagram of the sol–gel process used for the preparation of erbium-activated SiO2–TiO2

planar waveguides.

Table 1. Atomic concentration of the components of the waveguide deposited on v-SiO2 substrate

obtained by EDS measurement

Element Concentration (at.%)

O 65.2

Si 30.6

Ti 3.5

Er 0.7

The estimated error is 10%.

Er-DOPED SILICA-BASED WAVEGUIDES 1153

substrates. After each deposition, the films were annealed in air at 700�C for2 min. A sufficient thickness to obtain a guiding layer was achieved by al-ternating a number of deposition and heat-treatment steps. A cycle of 10depositions was necessary to obtain monomode waveguides at 632.8 nm,with thickness of 0.5 lm.

2.3. ION-EXCHANGE

The base glass designed for ion-exchange was of soda-lime type, with app-roximate composition 73SiO2, 14Na2O, 11CaO, 1Al2O3, 0.4P2O5, 0.6K2O(mol%). Glass was produced by melting reagent-grade oxides (approximately650 g for each batch). Several samples were produced in different batches, byadding different quantities of Er2O3 and Yb2O3, with rare-earth concentra-tions ranging from 0.3 to 0.8 mol%. The samples were then cut in35 mm · 25 mm · 1 mm plates and optically polished on one side.

Planar and channel waveguides were subsequently obtained by immersingthe slides into a molten salt containing 0.5AgNO3 and 99.5NaNO3 mol% at325�C. For channel waveguides, an Al masking layer and conventionalphotolithography process were used. Their typical size was 7–13 lm widthand about 3 lm depth.

3. Results and discussion

3.1. OPTICAL PROPERTIES

The refractive index and the thickness of the various waveguides weremeasured by an m-line apparatus. The resolution in the determination ofthe angles synchronous to the propagation modes was 0.0075�, giving anerror on the effective index of about 0.0005. Total propagation losses weremeasured by collecting the light intensity scattered out of the waveguideplane. As usual, the measured intensity along the guided light streak was fitto an exponential decay function, assuming a homogeneous distributionof the scattering centers in the waveguide. In all cases only the fundamen-tal TE0 mode of the waveguide was excited. Scattered light was usuallycollected by a video camera; however, loss measurements at 1.5 lm werealso made by using a scanning optical fiber coupled to a Ge detector. In thelatter case, the light was injected in the waveguide from a laser emitting at1550 nm.

3.1.1. RF sputteringTable 2 reports the optical parameters measured for the planar waveguidesproduced by RF-sputtering.

1154 G.C. RIGHINI ET AL.

The loss values at the two wavelengths may appear not fully consistent.Two remarks, however, have to be done: (a) loss measurements at the twowavelengths are made by using techniques (video camera and scanning fiber,respectively) which have slightly different accuracy; (b) the absorption peakof erbium is at 1532 nm, so that the measurement at 1550 nm only partiallytakes into account erbium absorption. In any case, we can conclude thatscattering losses of these waveguides at 1.5 lm are very low.

The refractive index profile of a typical sputtered waveguide is shown inFig. 2. From the equivalence of the refractive index profiles obtained for TEand TM modes it appears that the birefringence in these waveguides isnegligible.

Fig. 3 shows the squared electric field profiles of the TE0 mode of the SOSwaveguide, calculated at 632.8 and 1550 nm by using the parameters ob-tained by the m-line measurements.

The ratio of the integrated intensity, i.e. the ratio between the field in-tensity in the guiding film and the total intensity, which includes also thesquared evanescent fields, is 0.99 and 0.89 at 632.8 and 1550 nm, respectively.

Table 2. Optical parameters of the SiO2–TiO2:Er3+ planar waveguides prepared by RF-sputtering

Substrate

SOS v-SiO2

Number of modes at 632.8 nm 3 3

Number of modes at 1550 nm 1 1

Film thickness (lm) 2.5 2.5

Refractive index at 632.8 nm 1.503 1.508

Attenuation coefficient at 632.8 nm (dB/cm) 1.4 0.7

Attenuation coefficient at 1550 nm (dB/cm) 0.9 0.7

Fig. 2. Refractive index profiles, of a SiO2–TiO2:Er3+ planar waveguide prepared by RF-sputtering on an

SOS substrate, reconstructed from modal measurements at 632.8 nm for (a) the TE and (b) TM polari-

zation. The effective indices of the TE (d) and TM (j) modes are reported.

Er-DOPED SILICA-BASED WAVEGUIDES 1155

These ratios indicate that an efficient injection at 1550 nm is possible for thiswaveguide.

3.1.2. Sol–gelSo far, by sol–gel route we produced SiO2–TiO2:Er3+ planar waveguideswith typical thickness of 0.5 lm only. These films therefore support only onepropagating mode at 632.8 nm: their attenuation coefficient is 0.5 dB/cm,indicating a quite good uniformity and overall optical quality. The refractiveindex is about 1.58 at 632.8 nm, and their confinement factor (still at632.8 nm) is about 0.85. The preparation of thicker waveguides, supporting apropagation mode at 1550 nm, is now in progress.

3.1.3. Ion-exchangeAs expected, ion-exchanged waveguides present a gradient-index profile;a typical curve, well approximated by an erfc function, is shown in Fig. 4.The index difference between surface index and substrate index, Dn, isaround 0.04.

Propagation loss of planar waveguides supporting a single mode at 1.5 lm(and three modes at 632.8 nm) was less than 0.4 dB/cm at 633 nm, around0.3 dB/cm at 840 nm, and in the range 3–4 dB/cm, depending on Er3+

concentration, at 1.5 lm. These losses were measured by using a vidiconcamera. Channel waveguides, fabricated by conventional photolithography,exhibit higher propagation losses. In order to decrease them, a purely thermalannealing was tested: it was found that a 50 min annealing at 320�C in airwas sufficient to lower losses down to 3 dB/cm at 1.5 lm (as in planarwaveguides) from a value higher than 8 dB/cm measured just after theexchange.

Fig. 3. Calculated squared electric field profiles of the TE0 mode at 632.8 nm (a) and 1550 nm (b) across

the layered structure, air clad (c), guiding film (w) and SiO2 substrate (s), of the SiO2–TiO2:Er3+ planar

waveguide prepared by RF-sputtering on an SOS substrate.

1156 G.C. RIGHINI ET AL.

3.2. SPECTROSCOPIC CHARACTERIZATION

A waveguide configuration was used for both Raman and photoluminescencemeasurements. Raman spectra were collected in VV polarization by excitingthe TE0 mode with an Ar laser, operating at 457.9 or 514.5 nm, and detectingthe scattered light, polarized perpendicular to the direction of propagation,from the front of the waveguide. The signal was selected by a doublemonochromator and analyzed by a photon-counting system.

Photoluminescence spectroscopy, in the region of the 4I13/2 fi 4I15/2 tran-sition of Er3+ ions, was performed using the 514.5 nm line of an Ar laser asthe excitation source. The luminescence was dispersed by a 320 mm single-grating monochromator with a resolution of 2 nm. The light was detectedusing a InGaAs photodiode and lock-in technique. Decay curves were ob-tained recording the signal by a digital oscilloscope.

3.2.1. RF sputteringThe Raman spectra of the v-SiO2 substrate (Fig. 5(a)) and of the silica–titania guiding film (Fig. 5(b)) deposited on that substrate were collected inVV polarization, in a 90� geometry, by exciting the TE0 mode with an Arlaser operating at 457.9 or 514.5 nm and detecting the scattered light, po-larized parallel to that of the exciting beam, from the front of the waveguide.The former figure clearly shows Raman bands at about 60, 440, 490, 600,800, 1060 and 1190 cm)1, characteristic of the silica network (Galeener 1979).The bands in the region above 200 cm)1 correspond to optical phonons, andthe band at 60 cm)1 is the boson peak, characteristic of the disordered glassystructure. The Raman spectrum of the silica–titania waveguide, in the latterfigure, shows bands at about 50, 440, 490, 600, 800, 920, 1090 and 1190 cm)1,respectively. This spectrum is similar to those reported for SiO2–TiO2 glasseswith titanium oxide content in the range 3–12 mol% (Bihuniak and Condrate1981; Best and Condrate 1985; Chmel et al. 1992).

Fig. 4. A typical reconstructed index profile: (a) TE and (b) TM polarization.

Er-DOPED SILICA-BASED WAVEGUIDES 1157

By comparing the Raman spectra of the silica–titania waveguide with thatof the fused silica the more evident differences are: (i) the appearance of theRaman bands at 920 cm�1 and at 1090 cm�1, due to the vibrations of mixedSi–O–Ti linkage (Bihuniak and Condrate 1981; Best and Condrate 1985;Chmel et al. 1992); (ii) a shift to lower frequencies of the boson peak and (iii)a decrease of the intensity of the defect bands D1 and D2 centered at 490 and600 cm�1. The strong intensity of the Si–O–Ti bands, compared to the in-tensity of the Si–O–Si band at 800 cm�1, is indicative of a good interminglingof the two components of the films. Raman structures due to crystallinephases of TiO2 are not observed.

The step-like refractive index profiles together with the Raman measure-ments suggest an essentially homogeneous structure of the waveguides, with anetwork consisting of intermingled TiO4 and SiO4 units.

The fluorescence spectrum of the 4I13/2 fi 4I15/2 transition of the Er3+ ionis characterized by a spectral width of about 45 nm (Fig. 6). The fluorescencedecay curve has single exponential profile; no changes in the decay curve andin the shape of the 4I13/2 fi 4I15/2 transition were observed for powers up to300 mW. The measured lifetime of 3.7 ms of the 4I13/2 metastable level seemsto be short if compared to that of 14 ms calculated for silicate glasses on thebasis of the Judd–Ofelt theory (Hehlen et al. 1997).

However, since, at our knowledge, no values of the radiative lifetime forthe metastable 4I13/2 in SiO2–TiO2 host are available in the literature, it ishazardous to estimate the quantum efficiency. We can just compare thislifetime with the measured values reported for erbium-activated silica–titaniaplanar waveguides produced by sol–gel route. Orignac et al. (1999) reporteda measured lifetime of about 3 ms for the fluorescence lifetime of the 4I13/2

Fig. 5. Raman spectra of a v-SiO2 substrate (a) and of the SiO2–TiO2:Er3+ planar waveguide prepared by

RF-sputtering on the v-SiO2 substrate (b), collected in the VV polarization by exciting at 457.9 nm. The

excitation of the waveguide is in the TE0 mode.

1158 G.C. RIGHINI ET AL.

state in a planar waveguide of composition 80SiO2–20TiO2–5Al2O3 activatedby 0.7 at.% of erbium. On the other hand, Yeatman et al. (1999) measured alifetime of about 8 ms in strip-loaded waveguides of composition 93SiO2–7TiO2–5Al2O3 with ratios Er/(Si + Ti) ranging from 0.1 to 0.5 mol%.

It is possible to attribute the shorter lifetimes measured in silica–titaniawaveguides to larger distortions of the Er3+ site in some silica–titania ma-trices than in others or in pure silica. However, we cannot exclude that energytransfer processes could also contribute to reduce the lifetime. In fact recentmodeling by molecular dynamics simulations on erbium-activated silica–ti-tania glass (8.5 at% Ti/Si, 2.3% Er/Si) have shown that more than 55% ofEr3+ ions belong to pairs or clusters of higher size (Chaussedent et al. 2000).

3.2.2. Sol–gelThe main factors affecting the performance of sol–gel-based waveguides arethe non-radiative relaxation channels due to rare-earth concentrationquenching and to vibration of the OH groups. The OAH stretching vibrationaffects the fluorescence decay at 1.5 lm because two OAH vibrations areenough to bridge the gap of about 6500 cm�1 between the ground state 4I15/2and the first excited state 4I13/2 of the Er3+ ion (Hehlen et al. 1997; Duvergeret al. 2001). Therefore, the sol–gel process has to be carefully controlled inorder to minimize the residual OH content and to obtain the largest dis-persion of Er3+ ions.

Fig. 7 shows the Raman spectra of some undoped 80SiO2–20TiO2 planarwaveguides, submitted to different annealing processes. The observed bandsare typical of the silica–titania binary system as already discussed in the caseof the planar waveguides prepared by RF-sputtering technique. However, in

Fig. 6. Room temperature photoluminescence spectrum of the 4I13/2 fi 4I15/2 transition of Er3+ ion,

obtained by exciting the TE0 mode at 514.5 nm. This spectrum refers to a SiO2–TiO2:Er3+ planar

waveguide prepared by RF-sputtering on SOS substrate.

Er-DOPED SILICA-BASED WAVEGUIDES 1159

the case of the sol–gel films we can note the additional presence of the broadband with the maximum at about 3560 cm�1, which is due to molecularwater and SiAOH groups (Duverger et al. 2001).

The Raman spectra of Fig. 7(b)–(d) were obtained for waveguides pro-cessed with the same intermediate thermal treatment of 700�C for 2 min atevery deposition but with different final thermal annealing. By increasing theannealing temperature, the intensity of the band assigned to the OH groupsdecreases. Furthermore, Raman structures appear in the region between 150and 350 cm�1. They are attributed to devitrification of the silica–titania film.Further increases of either the temperature or the annealing time produce anincrease of the intensity of the Raman peaks due to crystallization, and thetypical structures of the anatase phase appear. Therefore, high temperatureannealing lowers the OH content, but increases the losses by scattering fromthe titania nanoclusters. The best compromise was obtained for a finalthermal annealing of 2 min at 900�C and a molar ratio H2O/TEOS ¼ 2.

Earlier studies showed that the degree of heterocondensation, measured bythe number of Ti–O–Si linkages, is maximum for 20 mol% TiO2 corre-sponding to the minimum phase separation (Orignac and Almeida 1996;Almeida 1998). However, for 20TiO2–80SiO2 films, we were not able to avoiddevetrification after the thermal annealing necessary for full densification.Furthermore we observed that in presence of devitrification the decay of themetastable state 4I13/2 exhibits a non-single exponential profile with shortlifetime. Thus, in order to reduce the devitrification and increase the dis-persion of the erbium, we reduced the titania content without changing theother parameters of deposition and densification.

Fig. 7. Room temperature Raman spectra obtained in VV polarization by exciting at 514.5 nm the TE0

mode of undoped 80SiO2–20TiO2 planar waveguides. Spectra refer to waveguides produced by different

thermal treatments: (a) intermediate annealing (2 min at 700�C at every deposition); (b) intermediate

annealing plus final annealing at 800�C for 2 m 30 s; (c) intermediate annealing plus final annealing at

800�C for 5 m 30 s; (d) intermediate annealing plus final annealing at 900�C for 2 m.

1160 G.C. RIGHINI ET AL.

Fig. 8 shows the room temperature photo-luminescence spectrum corre-sponding to the 4I13/2 fi 4I15/2 transition of the Er3+ ions in a 93SiO2–7TiO2

planar waveguide activated by 1 mol% Er/(Si+Ti). The spectral width of theemission band, measured at 3 dB from the maximum of the intensity, was51±2 nm.

Fig. 9 shows the decay curve of the luminescence from the 4I13/2 metastablestate of the Er3+ ions in the same waveguide. The profile is single exponentialwith a lifetime of about 7 ms. This relatively long lifetime can be considered

Fig. 8. Room temperature luminescence spectrum of the 4I13/2 fi 4I15/2 transition of Er3+ ions for a

93SiO2–7TiO2 planar waveguide, prepared by sol–gel route, activated by 1 mol% Er/(Si+Ti). Excitation

at 514.5 nm in the TE0 mode.

Fig. 9. Room temperature decay of the luminescence from the 4I13/2 metastable state of the Er3+ ions for

a 93SiO2–7TiO2 planar waveguide activated by 1 mol% Er/(Si+Ti). Excitation at 514.5 nm in the TE0

mode.

Er-DOPED SILICA-BASED WAVEGUIDES 1161

good enough for devices development, and is comparable with the alreadymentioned value of 8 ms measured by Yeatman et al. who also demonstrateda signal enhancement up to 1 dB/cm (Yeatman et al. 1998, 1999). Even bycomparing our value of 7 ms with that of 14 ms calculated for silicate glasseson the base of the Judd–Ofelt theory (Hehlen et al. 1997), a quantum yield ofat least 50% can be inferred. We, however, as already discussed, estimate thatthe true quantum yield would be higher. In fact, the presence of the titania atthe local site of the Er3+ ion can increase the oscillator strength and sig-nificantly decrease the radiative lifetime. Further measurements are thereforenecessary to clarify this point.

Finally, we did not observe any change in the spectral shape of the4I13/2 fi 4I15/2 transition and in the decay profile of the luminescence fromthe 4I13/2 state by varying the excitation power up to 120 mW.

3.2.3. Ion-exchangeThe Raman spectra of the ion-exchanged planar waveguides showed that thesilver ions, which have higher mass and polarizability than sodium ions,modified the Raman activity of the vibrational modes and in particular theSiAO stretching modes (Montagna et al. 2000).

High-resolution spectra were taken in order to study in detail the lowfrequency part of the spectra. The low frequency vibrational band of thesilver nanoparticles was isolated from the Raman spectrum of the sur-rounding glass by a subtraction of the Raman spectrum of the substrate. Thisprocedure assumes that the Ag+ content of the exchanged glass does notdramatically influence the line shape and intensity of the boson peak, the lowfrequency contribution of Raman scattering of the glass. The result of thissubtraction is shown in Fig. 10 for the TE0 mode.

Fig. 10. Room temperature low frequency Raman spectrum for a silver-exchanged soda-lime waveguide

obtained by excitation of the TE0 mode at 514.5 nm.

1162 G.C. RIGHINI ET AL.

The band is attributed to l ¼ 2 spheroidal vibrations of a sphericalnanocluster (Ferrari et al. 1996). Its frequency is proportional to the inversecluster diameter:

x2 ¼ 0:85Vt

dc

where Vt is the transverse sound velocity, d is the diameter of the sphericalparticle and c is the speed of light. Its bandwidth is partly due to homoge-neous broadening, but mainly to the size dispersion of the nanoparticles(Montagna and Dusi 1995). Taking Vt ¼ 1660 m/s, a value averaged on thedifferent directions in the crystal (Fujii et al. 1991), from the TE0 spectrum ofFig. 10 we can deduce a mean cluster size of about 1.3 nm, with a large sizedispersion for the region near to the surface. Low-frequency Raman mea-surements were repeated by selectively exciting different TEm modes andconfirmed this finding. Such silver cluster size, fortunately, is not large en-ough to appreciably influence the optical properties of the waveguides in theNIR region.

Absorption spectra indicated, as expected, a significant increase of theabsorption at 980 nm in the Er/Yb-co-doped waveguides with respect to Er-doped ones. In both types of waveguides, however, the bandwidth of the4I13/2 fi 4I15/2 transition of the Er3+ ion was measured to be about 20 nm.The luminescence decay profiles were well fitted by a single exponentialcurve, and measured lifetimes ranged from 6 to 8 ms in all the samples,depending on rare-earth concentrations. The corresponding radiative life-times were in the range 12.2–14.5 ms, calculated for a glass co-doped with3 wt.% Er and 5 wt.% Yb and for a glass with 3 wt.% Er only, respectively.Thus, one can estimate a quantum yield between 50 and 55%.

Preliminary optical gain measurements were made in channel waveguidesby using an input signal of about 1 mW in the waveguide (much larger thanthe fraction of lW which is used in the small-signal-approximation mea-surements); as expected in this case, no net gain was detected, but signalenhancement of the order of 0.4 dB/cm in Er-doped waveguides and largerthan 2.5 dB/cm in Er3+/Yb3+ co-doped waveguides was measured, atpumping powers of about 60 mW. Very recent measurements in the small-signal-approximation, in fact, have proved that a net optical gain higher than1 dB/cm is reachable.

4. Conclusions

Erbium- and erbium/ytterbium-activated silica-based optical waveguideswere produced by three different technologies: RF-sputtering, sol–gel and

Er-DOPED SILICA-BASED WAVEGUIDES 1163

ion-exchange. The first two routes allowed us the deposition of the activelayers onto vitreous silica or SOS substrates, the latter one being moreconvenient for multi-functional optoelectronic integration. In our case ionexchange implied the use of rare-earth-doped bulk glass substrates, but this isnot a strict limitation. Fick et al. (2000), for instance, already demonstratedthe possibility of defining channel waveguides by ion-exchange in a Na-richsol–gel film.

The comparison among the three technological processes leads to thefollowing remarks:

(a) Silica–titania RF-sputtered waveguides: As-deposited films containing0.7 at.% erbium did not support guided light propagation, but after thermalannealing in air for 6 h at 600�C well confined TE and TM modes werevisible. Propagation losses of 0.7 and 0.9 dB/cm were measured at 1550 nmfor waveguides deposited on v-SiO2 and SOS substrates, respectively. Scat-tering losses were higher in the latter case, due to the surface roughness of thefilm which replicates that of the buffer layer. The reconstructed refractiveindex profiles and the Raman measurements indicated that the films arehomogeneous with a network consisting of intermingled silica–titania units.As to spectroscopic properties, on can note the wide fluorescence bandaround 1532 nm (45 nm FWHM), while the lifetime of 3.7 ms is not verylong, even if sufficient for optical amplification and lasing applications.

(b) Silica–titania sol–gel waveguides: Multi-layered films were necessary toobtain the proper thickness for an optical waveguide in the visible and NIR.Layers were deposited by dip-coating, with an intermediate annealing of2 min at 700�C at every deposition. The best compromise between highdensification of the film, i.e. low OH content, and low losses was obtained fora molar ratio H2O/TEOS ¼ 2 and a final thermal annealing of 2 min at900�C. After several tests, we found that a good composition was 93SiO2–7TiO2: in such a case, a guide activated by 10 000 ppm Er/(Si + Ti) exhib-ited quite good spectroscopic characteristics, namely a wide bandwidth of51 nm (FWHM) and a long lifetime of 7 ms.

(c) Ion-exchanged waveguides in silicate glasses: High-quality opticalwaveguides were produced by diluted silver ion-exchange in Er- and Er/Yb-doped silicate glasses. In planar waveguides losses were less than 0.4 dB/cmat 633 nm; in channel waveguides, on the other hand, a thermal annealing inair was necessary in order to keep losses lower than 3 dB/cm at 1532 nm(namely at the absorption peak of erbium). The fluorescence bandwith(FWHM) is around 20 nm, independently on the rare-earth concentration,while the lifetime of the excited state can vary between 6 and 8 ms in thedifferently doped samples.

As an overall assessment, we can conclude saying that all three techniqueshave a high potential for the development of integrated optical amplifiers and

1164 G.C. RIGHINI ET AL.

lasers, even if they can be considered to be at different stages: RF-sputteringseems to be the one that gives more lossy waveguides than the other ones(and, in our case at least, shorter lifetimes), while sol–gel technology is themost flexible one, but still requires deep investigation in order to exploit all itspotential. Ion-exchange, on the other hand, is the most mature technology,and its major disadvantage is in the limited capability of integration withsilicon-based devices.

Sputtered and sol–gel waveguides also seem more suitable for opticalamplifiers to be used in WDM systems, due to their wide bandwidth, whileion-exchanged ones may be more convenient for integrated optical lasers. Sofar, we have been able to test for optical gain only the ion-exchangedwaveguides, and a net optical gain higher than 1 dB/cm was measured veryrecently in Er/Yb-co-doped samples. Further tests are now in progress tomeasure gain properties of all the three types of waveguides.

Acknowledgments

The authors wish to thank Massimo Brenci (IROE) and Gualtiero NunziConti (Optical Sciences Center) for useful discussions. The technical help ofRoberto Calzolai (IROE) to cut and polish samples is also gratefully ac-knowledged.

References

Almeida, R.M. J. Sol–Gel Sci. Technol. 13 51, 1998.

Best, M.F and R.A. Condrate. J. Mat. Sci. Lett. 4 994, 1985.

Bihuniak, P.P and R.A. Condrate. J. Non-Cryst. Solids 44 331, 1981.

Chaussedent, S., C. Bernard, A. Monteil, N. Balu, J. Obriot, S. Ronchin, C. Tosello and M. Ferrari. In:

Rare-Earth Doped Materials and Devices IV, ed. S. Jiang, Proceedings of SPIE, SPIE, Bellingham,

Washington, USA, 3942 243, 2000.

Chmel, A., G.M. Eranosyan and A.A. Kharshak. J. Non-Cryst. Solids 146 213, 1992.

Duverger, C., M. Montagna, R. Rolli, S. Ronchin, L. Zampedri, M. Fossi, S. Pelli, G.C. Righini,

A. Monteil, A. Armellini and M. Ferrari. J. Non-Cryst. Solids 280 261, 2001.

M. Ferrari, et al. J. Sol–Gel Sci. Technol. 19 569, 2000.

Ferrari, M., F. Gonella, M. Montagna and C. Tosello. J. Appl. Phys. 79 2055, 1996.

Fick, J., A. Martucci and M. Guglielmi. J. Sol–Gel Science Technol. 19 573, 2000.

Fujii, M., T. Nagareda, S. Hayashi and K. Yamamoto. Phys. Rev. B 44 6243, 1991.

Galeener, F.L. Phys. Rev. B 19 4292, 1979.

Hehlen, M.P., N.J. Cockroft, T.R. Gosnell and A.J. Bruce. Phys. Rev. B. 56 9302, 1997.

Montagna. M and R. Dusi. Phys. Rev. B 52 10080, 1995.

Montagna, M., S. Ronchin, F. Rossi, C. Tosello, E. Moser, M. Ferrari and G.C. Righini. Proceedings of

SPIE 3942 224, 2000.

Orignac, X and R.M. Almeida. IEE Proc.-Optoelectron. 143 287, 1996.

Orignac, X., D. Barbier, X.M. Du, R.M. Almeida, O. McCarthy and E. Yeatman. Opt. Mater. 12 1, 1999

and references therein.

Er-DOPED SILICA-BASED WAVEGUIDES 1165

Pelli, S., M. Brenci, M. Fossi, G.C. Righini, C. Duverger, M. Montagna, R. Rolli and M. Ferrari. Rare-

Earth Doped Materials and Devices IV, ed. S. Jiang, Proceedings of SPIE, 3942 139, 2000.

Tosello, C., S. Ronchin, E. Moser, M. Montagna, P. Mazzoldi, F. Gonella, M. Ferrari, C. Duverger, R.

Belli and G. Battaglin, Phil. Mag. B 79, 2103, 1999.

Tosello, C., F. Rossi, S. Ronchin, R. Rolli, G.C. Righini, F. Pozzi, S. Pelli, M. Fossi, E. Moser, M.

Montagna, M. Ferrari, C. Duverger, A. Chiappini and C. De Bernardi. J. Non-Cryst. Solids (in press).

Yeatman, E.M., M.M. Ahmad, O. McCarthy, A. Vannucci, P. Gastaldo, D. Barbier, D. Mongardien and

C. Moronvalle. Opt. Commun. 164 19, 1999.

Yeatman, E.M., K. Pita, M.M. Ahmad, A. Vannucci and A. Fiorello. J. Sol–Gel. Sci. Technol. 13 517,

1998.

1166 G.C. RIGHINI ET AL.