SPIE Proceedings [SPIE Symposium on Integrated Optics - San Jose, CA (Saturday 20 January 2001)] Rare-Earth-Doped Materials and Devices V - Erbium-activated silica-titania planar waveguides

Erbium-activated silica-titania planar waveguides preparedby rf-sputtering

Sabina Ronchjn*a, Alessandro Chiaseraa, Maurizio Montagnaa, Raffae!la Ro1W', Cristiana Tosell&',Stefano Pelli', Giancarlo C. Righinib, Rogeria R. Goncalvesc, Sidney Ribeiro', Carlo De Bernardi',

Fabio Pozzi", Claire Duvergere, Romina Bel1i, Maurizio

aJJMDjp di Fisica, Università di Trento, via Sommarive 14, 1-38050 Povo, Trento, ItalybOptoelectroni and Photonics Dept. IROE-CNR, via Panciatichi 64, 1-50127 Firenze, Italy

Cltitt of Chemistry- UNESP C. P. 3 55, CEP 1 4801-970, Araraquara-SP, BrazildAgilent Technologies Torino Technology Centre, via G. Reiss Romoli 274, 10148 Torino, ItalyeLab. des Fluorures UPRES A 6010 CNRS, Université du Maine, Av. Messiaen, 7201 7 Le Mans,

Cedex, FranceDip. di Ingegneria Meccanica e Strutturale, Università di Trento, via Mesiano 44, 1-38050 Povo,

Trento, ItalyCentro Fisica Stati Aggregati, via Sommarive, 14 1-38050 Povo, Trento, Italy

ABSTRACT

Erbium-activated silica-titania plaiiar waveguides were prepared by rf-spuuenng technique. The films were deposited bothon v-Si02 and silica-on-silicon substrates obtained by plasma-enhanced chemical vapor deposition. The refractive index, thethickness and the total attenuation coefficient of the waveguides were measured by prism coupling technique. Scanningelectron microscopy was used to analyze the morphology of both substrates and waveguiding film. Energy DispersiveSpectrometry was performed in order to obtain a compositional analysis. Roughness measurements were carried out bymeans of a stylus profilometer. After thermal annealing at 600°C for 6 hours the waveguides exhibited several well confinedTE and TM propagating modes at 633 nm and one mode at 1550 nm. The attenuation coefficient at 1550 nm was 0.9 and0.7 dB/cm for the films deposited on silica-on-silicon and the v-Si02 substrates, respectively. Structural information aboutthe deposited films were obtained by waveguide Raman spectroscopy. Waveguide luminescence spectroscopy was used tostudy the '13/2 '15/2 transition of Er ion. The emission at 1530 mu was observed at room temperature upon continuouswave excitation at 514.5 nm. A lifetime of 3.7 ms for the metastable '13/2 level was measured.

Keywords: rf-spuuenng, erbium, Si02-Ti02, planar waveguides, optical properties, Raman spectroscopy,photoluniinescence.

1. INTRODUCTION

Integrated optics is becoming increasingly important in modem telecommunication technology. In particular, erbium-activated amplifiers, operating at the third telecommunication window near 1 .5 im, are attractive due to their small size andpotential integration as loss-compensating components with other optical devices, such as passive splitters, switches, ormultiplexers on a single chip.16Silicate-based glasses offer a solubility for rare-earth ions of about 6 x 1020 cm3, are transparent in the NIR-visible regionand are compatible with integrated optics technology. In particular, the Si02-Ti02 binary system is of significanttechnological importance because it offers the possibility of producing planar waveguides with a controlled refractive indexdepending on the Ti02/Si02 molar ratio and represents a good compromise between high performance and low cost.5' 8The silica-on-silicon (SOS) format for integrated optics is favored for a variety of functions in optical communicationsystems. The common trend is to use silica-on-silicon substrates produced by plasma-enhanced chemical vapor deposition(PECVD) technique. The sputtering process has been successfully used for producing waveguiding films and recently, we

*

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Rare-Earth-Doped Materials and Devices V, Shibin Jiang, Editor,Proceedings of SPIE Vol. 4282 (2001) © 2001 SPIE · 0277-786X/01/$15.00 31

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have shown that radio-frequency (if) sputtering is a suitable technique for preparing titania-silica planar waveguidesactivated by high contents of rart91In this study we present the results obtained for SiO2-TiO2:Er3 planar waveguides deposited by ri-sputtering on v-Si02 andSOS substrates.

2. EXPERIMENTAL

The SOS substrates were prepared by PECVD with a deposition rate of 105 nm/mm. The silica layers, deposited on Siwafer, have a constant thickness of 3 .4 pm. On these PECVD substrates, a subsequent deposition of the erbium-activatedsilica-titama film was performed by spuuenng a target disk of silica (100 mm diameter) on which 8 disks of Ti02 (10 mmdiameter) and a metallic erbium wire (2 mm diameter, 26 mm total length) were put. The deposition time was of 3 h 30' inorder to have the appropriate thickness for supporting one mode at 1.5 rim. The residual pressure, before deposition, wasabout 2x107 mbar. During the deposition process the substrates were not heated. The sputtering was carried out with an Argas at a pressure of 7x103 mbar and the applied rf power was 150 W with a reflected power of 18 W. The same parameterswere employed for the deposition of the waveguide on v-Si02 substrates.It should be noted that the as prepared waveguides did not propagate the light because of non-stoicliiometnc structure ofSiox with x<2 and a thermal annealing in air for 6 h at 600° C was necessary to achieve light propagation.10' 11 In order toprevent cracking, microfractures or bad cohesion between the waveguidling film and the substrate, the SOS substrates wereannealed at 600°C before the film deposition.'1

Scanning electron microscopy (SEM) was used to analyze the morphology ofboth the substrates and the waveguiding films.The surface of the films was analyzed by a JEOL-JSM 6300 apparatus at 15 KV by covering the films with a 20 nm goldlayer. Energy Dispersive Spectrometry (EDS) was performed in order to achieve a compositional analysis, by using a NoranInstruments mod. Voyager apparatus.

Roughness measurements have been carried out by means of a Tencor P-1O stylus profilometer. Scan area was lOOxlOOim2, with 0.01 m x 1 pm lateral resolution in the two directions in the scan plane, and 30 A vertical resolution, limited bybackground vibration noise. An estimate of the roughness for the investigate films was obtained by averaging on differentlinear scans picked up from this area. In order to obtain information about a larger surface, another scan area of 600x600m2, with 0.2 m x 1 m lateral resolution, was studied.

The refractive index and the thickness of the substrates and the waveguides were measured for both transverse electric (TB)and transverse magnetic (TM) polarization by an m-line apparatus.'2 Two He-Ne lasers, operating at 632.8 and 543.5 nm,and a diode laser operating at 1550 mu were employed. The resolution in the determination of the angles synchronous to thepropagation modes was 0.0075°.

In order to measure propagation losses, the light intensity scauered out of the waveguide plane, which is proportional to theguided intensity, was recorded by a video camera (visible range) and by a fiber probe scanning down the length of thepropagating mode (NIR region). The losses were evaluated by fluing the intensity to an exponential decay function,assuming a homogeneous distribution of the scattering centers in the film.'3 The measurements were performed by excitingthe transverse electric TB0 mode of the waveguide with a He-Ne laser and diode lasers operating at 632.8 and 1550 nm,respectively.

The waveguiding configuration was used for both Ranian and photoluminescence measurements. The Raman spectra werecollected in \V polarization by exciting the TE0 mode with an Ar laser, operating at 457.9 nm and detecting the scatteredlight from the front of the waveguide. The signal was selected by a double monochromator and analyzed by a photon-counting system.

Photoluminescence spectroscopy, in the region of the I,3/2 — transition of Er3 ions, was performed using the 514.5nm line of an Ar laser as the excitation source. The luminescence was dispersed by a 320 mm single-grating monochromatorwith a resolution of 2 nm. The light was detected using a InGaAs photodiode and lock-in technique. Decay curves wereobtained recording the signal by a digital oscilloscope.

Proc. SPIE Vol. 428232


3. RESULTS AND DISCUSSION

The optical parameters of two erbium-activated Si02-Ti02 planar waveguides deposited on SOS (Wi) and v-SiO2 (W2)substrates are reported in Table 1 . The Wi and W2 waveguides are obtained by using the same target composition and thesame deposition time. The waveguides support three TE and TM modes at 632.8 nm, four at 543.5 nm and one mode at1550 nm.

substrate SOS v-5i02number of modes at 632.8 nm 3 3number of modes at 543.5 nrn 4 4number of modes at 1550 nm 1 1

ns,sate at 632.8 iim 1.4522 0.0005 1.4575 + 0.0005

nsubsfrate at 543.5 mu 1.4550 0.0005 1.4603 0.0005

11substrate at 1550 nm 1.4400 0.0005 1.4450 0.0005

substrate thickness Qim) 3.4 0.1

nfilm at 632.8 nrn 1.503 0.002 1.508 0.002

nat543.5nm 1.508±0.002 1.515±0.002nfilm at 1550 nm 1.468 0.002 1.466 0.002

film thickness ( m) 2.5 0.1 2.5 0.1

tttenuation coefficient at 632.8 urn (dB/cm) 1.4 0.3 0.7 0.1

tttenuation coefficient at 1550 nrn (dB/cm) 0.9 0.1 0.7 + 0.1

Table 1. Optical parameters obtained by modal and losses measurements of two Si02-Ti02:Er + planarwaveguides.

element 0 Si Ti Erconcentration (at. %) 65.2 30.6 3.5 0.7Table 2. Atomic concentration of the components of the W2 waveguide obtained by EDS measurement. The estimatederroris 10%.

Table 2 reports the oxygen, silicon, titanium and erbium atomic concentration obtained by EDS for the W2 waveguide.

Figure 1 shows the SEM micrographies of SOS (a) and v-Si02 (c) substrates and of the planar waveguides Wi (b) and W2(d) deposited by rf-spuuering on the substrates (a) and (c), respectively. The surface morphology of the waveguiding filmreproduces that of the substrate. The micrography of the Wi (Figurel(b)) shows structures with an average size of somehundred of urn. However, from the surface profile shown in Figure 2, an average roughness of only 80 A is estimated for theWi film. Measurements performed on a larger area show that this roughness is constant along the film surface.

The total loss of a planar waveguide consists of absorption and scattering contributions. The scattering optical loss measuredfor an amorphous waveguide is the sum of two contributions: volume scattering, due to local fluctuation in the refractiveindex resulting from density and compositional variations, and surface scattering due to surface roughness.14 An attenuationcoefficient of 0.7 dB/cm, both at 633 urn and at 1550 nm was measured for the W2 waveguide deposited on v-SiO2 (seeTable 1). The waveguide deposited on SOS substrate has an attenuation coefficient of 1.4 dB/cm at 633 urn and 0.9 dB/cmat 1550 urn. This is an indication that the scattering losses, due to surface roughness of the film which reflects that of thesubstrate (Figure 1(a), (b)) are not negligible. Since the scattering losses depend on the fourth power of the frequency, bycomparing the losses at 1550 and 633 urn it results that the total loss at 1550 urn is dominated by erbium absorption.

Figure 3 shows the refractive index profile of the Wi waveguide reconstructed from the effective indices at 543.5 urn by aninverse Wentzel-Kramers-Brillouin method.15 From the equivalence of the refractive index profiles obtained for TE and TMmodes it appears that the birefringence in this waveguide is negligible. The uniform refractive index along the thickness isan indication of the homogeneity of the film.

Waveguide labelingWi W2

Proc. SPIE Vol. 4282 33


Figure 1. SEM micrographies of the SOS substrate (a), SiO2- TiO2:Er3 film deposited on SOS (b), v-SiO2 substrate (c),SiO2- TiO2:Er3 film deposited on v-Si02 (d).

300

250

200

150

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Figure 2. Roughness measurement of the Wi waveguide.

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Figure 3. Refractive index profiles of the Wi planar waveguide reconstructed from modal measurements at 543.5 am forthe TE (solid line) and TM (dashed line) polarization. The effective indices of the TE (0) and TM (LI) modes arereported.

U)

00a,a)

a)(0

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Depth [p.m]

Figure 4. Calculated squared electric field profiles of the TE0 mode at 632.8 nm (a) and 1550 am (b) across the layeredstructure, cladding of air (c), waveguiding film (w) and Si02 substrate (s), of the Wi planar waveguide.

Figure 4 shows the squared electric field profiles of the TE0 mode of the Wi waveguide, calculated at 632.8 and 1550 nrnby using the parameters obtained by the rn-line measurements. The ratio of the integrated intensity, i.e. the ratio between thefield intensity in the waveguiding film and the total intensity, which includes also the squared evanescent fields, is 0.99 and

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.

I III

0.1

0.010 2 4 6 8 10 12 14

Time [ms]

Figure 7. Room temperature luminescence decay curves from the '13/2 state of Er3 ion in Wi sample obtained afterpumping at 514.5 nm with an excitation power of 60 mW. The straight line is the fit by an exponential decay with alifetime t=3.7ms.

Figure 7 shows the '13/2 —÷ '15/2 luminescence decay profile of Er3 ion in Wi sample obtained after pumping at 514.5 urnwith an excitation power of 60 mW. The decay exhibits a single exponential profile with a lifetime of 3.7 ms and no



changes in the decay curve and in the shape of the I13/2 —* '15/2 transition were observed for powers up to 300 mW. Themeasured lifetime of 3 .7 ms seems to be short if compared to that of 14 ms calculated for silicate glasses on the basis oftheJudd-Ofelt theory.2° Since, at our knowledge, no values of the radiative lifetime for the metastable 113/2level in 5i02-Ti02host are available, it is hazardous to estimate the quantum efficiency. We can compare the measured lifetime with themeasured values reported for erbium-activated silica-titania planar waveguides produced by sol-gel route. Orignac et al.5reported a measured lifetime of about 3 ms for the fluorescence lifetime of the I13/2 state in a planar waveguide ofcomposition 8OSiO-2OTiO2-5Al2O3 activated by 0.7 at. % of erbium. Yeatman et al.8 measured a lifetime of about 8 ms instrip-loaded waveguides ofcomposition 935i02-7TiO2-5A1203 for concentrations ranging from 0. 1 to 0.5 mol % Er/(Si+Ti).It is possible that the shorter lifetime measured for the silica-titania waveguides is due to larger distortions of the Er3 site insilica-titania with respect to silica. However, we can not exclude that energy transfer processes could contribute to reducethe lifetime. In fact recent modeling by molecular dynamics simulations on Erbium activated silica-titania glass (8.5 at. %Ti/Si, 2.3 % Er/Si) have shown that more than 55 % ofEr ions belong to pairs or clusters ofhigher size.22

4. SUMMARY

Erbium-activated silica-titama planar waveguides were deposited by the rf-spuuering technique on v-Si02 and SOSsubstrate produced by PECVD. After thermal annealing in air for 6 h at 600° light propagation occurs in all waveguides.The waveguides show four well confined TB and TM propagating modes at 543.5 nm and one mode at 1.5 m.Propagation losses of 0.7 dB/cm and 0.9 dB/cm, were measured at 1550 nm for the silica-titama waveguides deposited on v-SiO2 and SOS substrates, respectively. The scattering losses, due to the surface roughness of the film which reflects that ofthe substrate, are not negligible in the case of the waveguide deposited on SOS substrates. By comparing the losses at 1550and 633 inn it results that the total loss at 1550 nm is dominated by erbium absorption.

The reconstructed refractive index profiles and the Raman measurements indicate that the films are homogeneous with anetwork consisting of intermingled silica-titama units.

Emission in the third teleconunumcation window with a spectral width of 45 urn and a lifetime of 3.7 ms has been observed.

The results indicate that the rf-sputtering technique can be a practicable route to prepare optically active planar waveguidesin the silica-on-silicon format for optical amplification. In order to perform the optical gain measurements by pumping at980 urn, the production of channel waveguides is in progress.

ACWNOWLEDGMENTS

This research was partially supported by the "Progetto Finalizzato MADESS ir' CNR Project, a MURST-Cofin 99, and aFrench-Italian Program Galileo 98-2000. Stefano Pelli acknowledges the grant by MADESS II for his PhD fellowship at theUniversity of Firenze, Electronic Engineering Department.

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SPIE Proceedings [SPIE Symposium on Integrated Optics - San Jose, CA (Saturday 20 January 2001)] Rare-Earth-Doped Materials and Devices V - Erbium-activated silica-titania planar waveguides