Sol±gel silica/titania-on-silicon Er/Yb-doped waveguidesfor optical ampli®cation at 1.5 lm

Xavier Orignac a,*, Denis Barbier a, Xin Min Du b, Rui M. Almeida b,Orla McCarthy c, Eric Yeatman c

a GeeO, 46 Avenue F�elix Viallet, 38031 Grenoble Cedex, Franceb IST, Rua Alves Redol, 9-3D, 1000 Lisboa, Portugal

c Imperial College of Science, Technology, and Medicine, Dept. of Electrical Engineering, Exhibition Road, London SW7, England

Received 15 June 1998; accepted 30 October 1998

Abstract

By combining the sol±gel method and the spin-coating technique, silica-on-silicon Er-doped glass optical planar

waveguides were fabricated. The active guiding layer consisted of SiO2±TiO2±Er2O3±Yb2O3±Al2O3 and its composition

was optimized by measuring the ¯uorescence lifetime s of the 4I13=2 metastable level of Er3� ions. The Er concentration

was chosen as the quenching concentration, which was found to be 1.4 ´ 1020 ions/cm3. The Yb concentration was

found to have little in¯uence on s, whereas a concentration of 1.5 ´ 1021 ions/cm3 of AlO3=2 was found to be enough to

maximize s. The 71aluminum concentration barely in¯uenced the ¯atness of the ampli®ed spontaneous emission

spectrum. Strip-loaded waveguides were designed by reactive ion etching of the cladding layer and their performance as

candidates for integrated optical ampli®ers was assessed. Propagation losses of 0.7 dB/cm and loss reduction of 2.7 dB

(for a 5.7 cm long waveguide) were measured. Ó 1999 Elsevier Science B.V. All rights reserved.

PACS: 42.82.Bq; 42.82.Et; 42.82.Gw; 81.20.Fw; 78.66.Jg; 68.55.Ln

Keywords: Integrated optics; Silica-on-silicon; Sol±gel; Optical ampli®er; Erbium; Quenching concentration

1. Introduction

The worldwide market for optical componentsis growing exponentially [1]. In particular, opticalampli®ers are strongly needed for the use of thelarge bandwidth that is allowed by optical ®bers.Erbium-Doped Fiber Ampli®ers (EDFAs) weretransferred from research [2,3] to industry in a very

short time and quickly spread in long distancetelecommunication networks [4]. However, theyare still rather space consuming and do not allowfor the integration of several functions on onechip. Hence, following an idea developed in 1969[5], numerous researchers are striving to conceiveintegrated optical ampli®ers. One of the majortechnological di�erences between ®bers and inte-grated ampli®ers is that several meters of opticalpath are available in ®bers, so that very low con-centrations of erbium (Er) can be used; on thecontrary, integrated ampli®ers should be as shortas possible (typically a few centimeters), so that

Optical Materials 12 (1999) 1±18

* Corresponding author. Address: IOT, Bruchsaler Strasse 22,

68753 Waghausel-Kirrlach, Germany. Tel.: +49-7254-925238;

fax: +49-7254-925210; e-mail: xo@iot.schott.de

0925-3467/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 5 - 3 4 6 7 ( 9 8 ) 0 0 0 7 6 - 7

high Er concentrations are required; however, inthe case of high concentrations, the Er3� ions arecloser together, so that deleterious non-radiativeenergy exchanges between neighboring ions cantake place. Consequently, a key parameter for thechoice of the material is its ability to isolate Er3�

ions from each other as much as possible; in otherwords, the quenching concentration, which is theEr3� concentration for which the ¯uorescencelifetime of the metastable level 4I13=2 is half of thatfound at very low concentrations, should be ashigh as possible.

Various technologies are available for the fab-rication of integrated optical ampli®ers: lithiumniobate [6±10], ion exchange [11±24], silicon-on-insulator [25±32] and silica-on-silicon [33]; inthis latter category, we ®nd thermal oxidation andnitridation [34±36], sputtering [37±40,175], ¯amehydrolysis deposition [41±46], chemical vapordeposition and plasma-enhanced chemical vapordeposition [47±54]. The sol±gel process [55] alsoallows for the fabrication of silica-on-siliconwaveguides [56±78] that can be easily doped withrare-earth elements [79±104,176,177]. It o�ersseveral advantages over the other competingtechnologies: it is potentially cheaper and simpler,especially for multicomponent glasses, whichmakes it a good candidate for low-cost massproduction; moreover, since it is a chemical pro-cess, the di�erent species are mixed at the mo-lecular scale, so that high homogeneity (andconsequently low losses and good dispersion ofEr3� ions) can be expected [67]. It has been shownthat sol±gel-derived glasses [105] and ®bers[106,107] actually accommodate dopant ions morereadily.

This work is a ®rst attempt to make Er-dopedintegrated optical ampli®ers by the sol±gel process.In the ®rst part, we show how we optimize thecomposition (Er3�, Yb3� and Al3� concentra-tions); this work was done with planar waveguides,since strip-loaded waveguides require more fabri-cation steps; the second part is devoted to thecharacterization of strip-loaded waveguides: in-sertion, coupling and propagation losses aremeasured; in the third part, the spectroscopicperformances of the ®nal components are pre-sented.

2. Experimental

2.1. Planar waveguides fabrication

The sol±gel process involves two main steps[55]: ®rst, the hydrolysis, in which a precursor(usually a metal alkoxide) is reacted with water;then, a condensation step, which turns the initialchemical solution into a sol. Later, solvent evap-oration will produce a gel. By depositing a fewdrops of the sol onto a silicon substrate andspinning it, evaporation is greatly accelerated anda thin solid gel ®lm is obtained in a matter of a fewseconds. Glass ®lm matrices can be obtained byproper heat-treatment of the gel ®lms. In order toform waveguides, layers of di�erent refractive in-dices, with controlled thicknesses, are needed, ac-cording to the mode cuto� relation [108]. Therefractive index can be easily tailored by addingtitania to a silica matrix [109±113]. For spectro-scopic characterizations, planar waveguides wereused. Silica-titania (80:20, mol%) active guidinglayers (refractive index � 1.6 @ 633 nm, thickness� 2 lm) were formed on silica substrates (refrac-tive index � 1.46 @ 633 nm, thickness � 2 mm,diameter� 3 cm). For the guiding layer, su�cientthickness can be obtained by alterning depositionand heat-treatment steps [114±116]. In the presentwork, the multilayer deposition process that weused was set up by Prof. R.R.A. Syms [71].

For the preparation of the active sol, TEOS ispre-hydrolyzed for 2 h, before TiPOT, Er(NO3)3,Yb(NO3)3, and Al(NO3)3 are added. The completechemical recipe is described in more detail [117].Aluminum is used for the dissolving of eventual Erclusters [118±120]. Ytterbium is used in order tocreate a strong absorption band at 980 nm, fore�cient pumping at this wavelength [121,122]. Thehydrolisis/condensation reaction is performed inacid conditions, in order to obtain dense ®lms witha polymeric structure [111].

The sol was spun for 30 s at 2500 rpm; theseconditions yield maximum thickness of a singlelayer without cracking [123]. The spinning atmo-sphere above the substrate was saturated withethanol, so as to reduce surface roughness andeventual scattering [124]. The spinning process hasbeen chosen for the ®lm deposition method since it

2 X. Orignac et al. / Optical Materials 12 (1999) 1±18

is simple and fast, thus compatible with massproduction; furthermore, modeling has shown,that it potentially gives a very good thicknessuniformity [125±128].

The optimum titania concentration for mini-mizing phase separation and improving the dis-solution of rare-earth ions was found to be 20mol% in previous works [129,130], and is used herefor the guiding layer. Each layer was heat-treatedat 900°C for 1 min.

2.2. Strip-loaded waveguides fabrication

In an attempt to make ampli®ers, ``rib'' (or``ridge'') waveguides are usually considered [33]: achannel waveguide is designed by etching the highindex guiding region, leading to rough core walls;roughness is then removed by a re¯ow step, beforethe channel is ®nally embedded into a low indexcladding material. In our case, however, it was notpossible to re¯ow the SiO2±TiO2 material beforemicrocrystallization appears.

Therefore, strip-loaded waveguides were fabri-cated. This geometry allows for the elimination ofboth re¯ow and burial steps. We ®rst depositedpure silica bu�er layers (refractive index� 1.46 @633 nm, thickness � 3 lm) on 10 cm diametersilicon wafers, then on silica-titania layers in thesame way as for the planar waveguides and ®nallypure silica capping layers (thickness � 0.75 lm).Channels were then patterned in the capping layerby reactive ion etching [131,132] with a Plasmalabequipment, using a mixture of CHF3, Ar and O2 asetching plasma. Waveguides were produced ac-cording to the method of [71], with a modi®edgeometry. Series of eight waveguides were pro-duced on each sample; all the waveguides wereidentical for a given series, whereas the width ofthe strip was di�erent from series to series, varyingfrom 6 to 9 lm. The resulting waveguides werefound to be monomode at 1.5 lm [133].

2.3. Fluorescence lifetime measurements

Short pieces of 2 mm long samples were cut inorder to eliminate e�ects due to ampli®ed spon-taneous emission [134] or reabsorption of the ¯u-

orescence [135]. The beam of a laser diodeemitting at 976 nm is passed through a microscopeobjective and focused on the input of the planarwaveguide, after being mechanically chopped at11 Hz. The resulting ¯uorescent beam is focusedon a photodetector by a second microscope ob-jective; an interference ®lter is placed in front ofthe photodetector in order to isolate the emissionat 1.5 lm (4I13=2 ® 4I15=2 transition of Er3� ions)from the residual pump and other eventual radi-ation. The ¯uorescence decay is monitored with anoscilloscope. The injected pump power was notdetermined (coupling e�ciency unknown), but thetotal pump power was varied between 20 and 100mW without appreciable change in the value ofthe ¯uorescence lifetime; this indicates an absenceof ASE or re-absorption processes or, moreprobably, that we are in a ``high pump'' regime[157].

The lifetime s is de®ned as the 1/e decay time ofthe ¯uorescence intensity. Due to the limited res-olution of the oscilloscope, there is a range of timevalues where the ¯uorescence intensity remainsequal to I(0)/e; this will be taken as the experi-mental error bar on s.

2.4. Loss measurements

2.4.1. Insertion lossLight emitted from a pigtailed laser diode is

coupled in one strip-loaded waveguide. The outputintensity is measured by a photodetector through amicroscope objective. The same experiment isperformed without the sample; the di�erence be-tween the two intensities represents the insertionlosses of the waveguide. The experiment was doneat 1.3 lm in order to avoid the strong absoprtiondue to Er3� ions at 1.5 lm.

2.4.2. Coupling lossWhen operating in a transmission line, ampli-

®ers are connected to input and output ®bers.Coupling losses C are given by the overlap integralbetween the ®ber and the waveguide mode, nor-malized to the product of the two integrated in-tensities, so that C� 1 if the two modes areperfectly identical

X. Orignac et al. / Optical Materials 12 (1999) 1±18 3

C �R R

Efib E�wgde dxdy��� ���2R R jEfibj2 dxdy

R R jEwgdej2dxdy; �1�

where summation is all over the plane, and Efib

and Ewgde are the amplitude mode pro®les of the®ber and the waveguide respectively. The use ofthis formula implies time-consuming calculations,so that a simpli®ed expression was used, based onGaussian ®eld approximation [136]

C � 4a2xy�a2 � x2��a2 � y2� ; �2�

where a is the ®ber mode radius, and x and y thehorizontal and vertical radius dimensions of thewaveguide mode, respectively. The losses expres-sed in dB are equal to 10 times (since C is a powerratio) the common logarithm of C. The experi-mental set-up for visualizing the ®ber and wave-guide modes is described in ref. [133]. Formula (2)is valid when the waveguide mode is ellipticallyGaussian and the ®ber mode is circularly Gauss-ian, which is a very good approximation to ourcase. It does not take into account other causes ofcoupling losses that can be found experimentally,such as misalignment of the ®bers or bad polishingof the waveguide edges; therefore, the couplinglosses we give can be underestimated.

2.4.3. Propagation lossPropagation losses are estimated as the di�er-

ence between insertion and coupling losses. Theymay be overestimated, given the above remarkconcerning coupling losses.

2.5. Absorption spectra

Light from a pigtailed white light source is in-jected into the waveguide and the output light isobserved with an optical spectrum analyzer. Thespectrum of the white light source is quite ¯at (lessthan 1% of relative intensity ¯uctuations) in theregion of interest (850±1050 nm and 1400±1650nm), so that the spectrum observed at the outputof the guide is very close to the absorption spec-trum of the material. This is only an approxima-tion however, since all the wavelengths arelaunched together in the sample; when we look at

the absorption at 1550 nm, the signal is superim-posed to ¯uorescence induced by, for instance, the980 nm component of the white light source.Nevertheless, the white light intensity launched inthe guide is very small (less than 2 nW/nm). Fi-nally, the coupling e�ciency between ®bers andwaveguide is wavelength-dependent, which slightlydegrades the accuracy of the absorption spectrummeasurements.

2.6. Gain measurement

Pump (980 nm) and signal (1534 nm) are com-bined in a ®ber multiplexer and injected into themicroguides. The ampli®ed signal is collected atthe output of the guide with a monomode ®berand is analyzed through an optical spectrum ana-lyzer. Two pumps and two multiplexers can alsobe used in order to add a contra-directionalpumping scheme to the original co-directional oneand increase the total injected pump power.

The net gain is de®ned as follows

GaindB � 10logSignalAmplified ÿASE

SignalInjected

!; �3�

where ASE is the Ampli®ed Spontaneous Emissionlevel, and SignalInjected is the signal level at the in-put of the guide, measured by putting the inputand the output ®bers together and by measuringthe level of the signal at the output of the second®ber. SignalAmplified is the signal at the output of thesecond ®ber.

The loss reduction is de®ned as the di�erencebetween the net gain and the insertion loss.

2.7. A few words about the concentrations

Since the starting precursors for the waveguidefabrication are TEOS (source for Si) and TiPOT(source for Ti), all the compositions were origi-nally de®ned in the ``atomic'' system 80SiO2±20TiO2±xErO3=2±yYbO3=2±zAlO3=2 for practicalreasons. Therefore, ratios add up to 100 + x + y+ z. For instance, the actual concentration of Erexpressed in at.% is

Cat � x100� x� y � z

at:%: �4�

4 X. Orignac et al. / Optical Materials 12 (1999) 1±18

Since ion concentrations are often given in ions/cm3, the following relation should be used for thesake of comparison

Cion � N � DM

� Cat; �5�where N is the Avogadro number, D the glassdensity and M the molar mass (in g/mol).

3. Optimization of the glass composition (planar

waveguides)

3.1. Erbium concentration

Three electronic levels are involved in the basicampli®cation process created by Er3� ions[135,137]: a photon with the 980 nm pump wave-length is absorbed, which promotes the excitationof an electron from the 4I15=2 fundamental level tothe 4I11=2 excited level; rapid de-excitation occursat the 4I13=2 metastable level, and a photon with the1.5 lm signal wavelength is spontaneously emit-ted, due to the ®nal electronic de-excitation backto the fundamental level. At low doping levels, thetotal number of available ions may be less than thenumber of incident photons: the ground state isquickly depleted, and signal ampli®cation is lim-ited. When one increases the Er3� concentration,other problems arise. However, the average dis-tance between neighboring ions diminishes con-versely, so that energy can be exchanged non-radiatively between them, due to electric andmagnetic multipolar coupling and S±S coupling[138±140]. The most important consequence is theAPTE e�ect [141] called homogenenous up-con-version, where two Er3� ions in the metastable4I13=2 state interact [135,142±146]; the donor iontransfers all its energy to the acceptor, leaving it-self in the ground state and the acceptor in thehigher energy 4I9=2 state; the acceptor then quicklydecays back to the 4I13=2 level. Other non-radiativeenergy exchange e�ects such as n-photon APTEe�ects, co-operative up-conversion processes andenergy migration [106,107,141,147,148] can takeplace and degrade the ampli®cation performance.All these processes result in a decrease of the ¯u-orescence lifetime of the metastable level 4I13=2 with

increasing Er concentration, obeying the followingempirical formula [149,150]

sobs � s0

1� q=Q� �p ; �6�

where sobs is the observed ¯uorescence lifetime, s0

the ideal ¯uorescence lifetime in the limit of zeroconcentration, q is the Er3� ion concentration, Q isthe quenching concentration and p a phenome-nological parameter characterizing the steepness ofthe corresponding quenching curve.

In the case of EDFAs, the Er3� concentration isnot a critical parameter: it is ®xed at very low value(q�Q), so that the energy exchange mechanismsbetween Er3� ions are negligible. Desired gain isachieved by setting the optimal ®ber length. In thecase of integrated waveguides, however, higherEr3� concentrations are required, since opticalpaths of a few centimeters, rather than meters, areused [151,152]. Therefore, the Er3� concentrationis designed as the quenching concentration Q,where s� s0/2, which represents a compromisebetween availability of active species and ampli®-cation degradation by energy exchanges. The ¯u-orescence lifetimes of eight samples with di�erentEr3� concentrations are presented in the quenchingcurve of Fig. 1. The ¯uorescence lifetime decreaseswith increasing concentrations, as expected. A

Fig. 1. Fluorescence lifetime of the 4I13=2 metastable state of

Er3� ions as a function of Er3� concentration. Compositions of

the samples are 80SiO2±20TiO2±xErO3=2±0.5YbO3=2±10AlO3=2,

with x� 0.1, 0.15, 0.2, 0.3, 0.4, 0.5, 1, 1.5 and 2. The black line

represents the result of the ®t of the data to Eq. (1).

X. Orignac et al. / Optical Materials 12 (1999) 1±18 5

value of 5.9 ms is reached for the sample dopedwith 0.1 at.%. When ®tted to Eq. (1), thequenching curve yields: s0� 6445 ls, Q� 0.62 at.%and p� 1.27. The optimum Er3� concentration istherefore 0.62 at.%, which corresponds to1.4 ´ 1020 ions/cm3 in our system.

When an isolated Er3� ion is considered in the4I13=2 metastable state, the ¯uorescence decay ispurely exponential, with a time constant s, ac-cording to Einstein's law: I(t)� I(0) exp(ÿt/s),where I(t) is the intensity of the light emitted at 1.5lm as a function of time. When several ions arepresent, the decay is no longer exponential, be-cause of the non-radiative energy exchange pro-cesses mentioned earlier, that introduce other timeconstants, dependent on the distances between theemitting ion and the neighboring ones. The ob-served lifetime results from the combination of allthe time constants of all the present ions. This isillustrated in Fig. 2, where the ¯uorescence decaysof samples doped with 0.15, 0.5 and 2 at.%, re-spectively, are shown; the non-exponentialityclearly increases with increasing doping levels.

Information about quenching concentrations inEr-doped glasses is relatively sparse. Some repre-sentative results are compiled in Table 1. Our sol±gel silicate glass exhibits a higher quenching con-centration than traditional melt-quenched silicate¯int and borosilicate glasses, and also than Al-Ge-doped silica ®ber. Two other silicate glasses have

higher quenching concentrations: the ®rst one is amagnesium-lithium silicate, claimed to be a refer-ence glass for ®ber fabrication [153], although thecomparison with an actual Al-Ge-doped silica ®-ber shows a higher quenching concentration forthe sol±gel material. The second one is a sodium-silicate. In both, there is a high concentration ofalkali ions. Energy exchange processes that lowerthe ¯uorescence lifetime are all the more e�cientwhen Er3� ions are closer to each other; this is evenmore true when rare-earth ions form microclus-ters. The formation of microclusters has been ex-plained as follows [118,119]: rare-earth cationssuch as Er3� have large cationic ®eld strength Z/r,where Z is the valence and r the ionic radius, sothat they require co-ordination of a su�cientlyhigh number of non-bridging oxygens for screen-ing the electric charge of the cation. However, Er3�

ions introduced into a rigid glass network cannotsu�ciently co-ordinate non-bridging oxygens andare in a high enthalpy state. Therefore, they tendto gather to form a cation-rich phase so as to re-duce excess enthalpy by sharing non-bridging ox-ygens among clustering cations. Since it is wellknown that alkali ions create non-bridging oxy-gens by acting as network modi®ers [162,163],their presence may increase the value of thequenching concentration. One sul®de-based glassalso exhibits a higher quenching concentration,but its low ¯uorescence lifetime value (s0� 2.2 ms)does not allow practical applications.

The ¯uorescence lifetime can also be lowered bya two phonon±OH decay mechanism, because theenergy of the 4I13=2 ® 4I15=2 transition correspondsto twice that of O±H stretching vibrations [159].This is actually observed (see Table 1) in sodium-silicate and phosphate glasses, where the quench-ing concentration increases after reduction of theOH content by drying/re-melting [159,161]. Sincethe sol±gel process is based on chemical reactionsincluding a hydrolysis step, they are known toretain more OH groups than traditional melt-quenched glasses [55]. We estimated the OH con-tent by a method based on infra-red spectroscopyand used by others [159,161]. Values as high as18 000 molar ppm (5000 weight ppm) were foundin some cases, which are quite high: only 400 and530 ppm were found in undried sodium-silicate

Fig. 2. Fluorescence decays of three samples with compositions

80SiO2±20TiO2±xErO3=2±0.5YbO3=2±10AlO3=2, with x� 0.15,

0.5 and 2.

6 X. Orignac et al. / Optical Materials 12 (1999) 1±18

and phosphate glasses respectively, [159,161],whereas the purest silicate glasses exhibit OHcontents as low as 100 ppm [164] and it is recom-mended not to exceed 200 ppm [165].

3.2. Ytterbium concentration

When only Er3� ions are present in shortwaveguides, the pumping e�ciency at 980 nm isnot very good, because the Er3� absorption cross-section at this wavelength is not very high. Thisproblem can be alleviated by the addition of Yb3�

[166±168]: pumping promotes an electron from the2F7=2 ground state to the 2F5=2 manifold; the ex-cited Yb3� ion then transfers its energy to the Er3�4I11=2 level; this transfer is very e�cient, since it isquasi-resonant; this is then followed by non-radi-ative decay to the Er3� metastable level 4I13=2.

Since the population of the 4I13=2 level is increasedby this mechanism, the ¯uorescence lifetime s isalso increased. Two deleterious processes can alsooccur: back energy transfer from Er3� to Yb3� ions[168], or double energy transfer, where a secondexcited Yb3� ion transfers its energy to the Er3�

ion and promotes one electron from the 4I11=2 tothe 4F7=2. In this latter case, the electron relaxesquasi-instantaneously to the 2H11=2 and 4S3=2 levels,and then back to the 4I15=2 ground state, by givinggreen emissions at 530 and 550 nm [169].

Based on these mechanisms, several trends maybe expected: with no Yb3� present, Er3� ions arenot screened on one another and energy transfersbetween them can lower the ¯uorescence lifetime.When the Yb3� concentration is increased, Er3�

ions start to ``see'' Yb3� ions and deleteriousEr3� M Er3� energy exchanges are progressively

Table 1

Quenching concentrations Q, ¯uorescence lifetimes of the metastable level 4I13=2 in the limit of zero Er3� concentration s0, and phe-

nomenological parameter p, for Er3� ions in various glass matrices. Data compiled from the literature. Corresponding references are

given in the last column. Values that are explicitly given in the references are in italics; other ones were derived by the authors, by ®tting

the data provided in the references to Eq. (1). When values of Q are not given in ions/cm3, data such as glass density were missing.

Concerning BGG 31, no data at low Er doping level are available

Type of glass Q s0 (ms) p Ref.

Sol-gel silicate 80SiO2±20TiO2±0.5Yb2O3±10Al2O3 planar waveguide (molar) 1.4 ´ 1020 ions/cm3 6.4 1.27 This

work

Silicate ¯int 66SiO2±13.9Na2O±22.5PbO±2.5BaO±0.3Al2O3 (weight) 5.6 ´ 1018 ions/cm3 7.5 0.99 [154]

Magnesium±Lithium±Silicate 57SiO2±14MgO±27LiO2±2Al2O3 (molar) 4.2 ´ 1020 ions/cm3 8.9 1.33 [155]

Al±Ge-doped silica ®ber 1 ´ 1020 ions/cm3 9.6 1.27 [156]

Al±Ge-doped silica ®ber 4.4 ´ 103 ppm 10.1 1.65 [157]

Borosilicate 7 ´ 1019 ions/cm3 9.9 1.55 [158]

Sodium-silicate Na2O±2SiO2 (melted for 20 h at 1300°C in air) 6.8 ´ 1020 ions/cm3 19.9 3.66 [159]

Sodium-silicate Na2O±2SiO2 (melted for 50 h at 1400°C in air) 1.4 ´ 1021 ions/cm3 19.7 28.4 [159]

Sodium-silicate Na2O±2SiO2 (re-melted for 50 h at 1200°C in a dry box) 2.1 ´ 1021 ions/cm3 20.1 2.8 [159]

Phosphate 20 � 1018±30 � 1018 ions/cm3 14 2 [160]

Phosphate 50P2O5±(26±30)Li2O±(3±3.5)Al2O3±16BaO (molar) undried 0.29 mol% 8.2 1.07 [161]

Phosphate 50P2O5±(26±30)Li2O±(3±3.5)Al2O3±16BaO (molar) dried 4.49 mol% 9.1 1.57 [161]

Phosphate (38±40)P2O5±(38±40)Na2O±(20±21.6)Al2O3 (molar) undried 0.72 mol% 10.7 1.06 [161]

Phosphate (38±40)P2O5±(38±40)Na2O±(20±21.6)Al2O3 (molar) dried 2.83 mol% 10.7 1.66 [161]

Phosphate 31P2O5±(42±44)±Na2O(20±22.6)Al2O3±2BaO (molar) undried 0.83 mol% 10.5 1.24 [161]

Phosphate 31P2O5±(42±44)Na2O±(20±22.6)Al2O3±2BaO (molar) dried 2.51 mol% 10.5 1.78 [161]

Phosphate 50P2O5±16.66MgO±33.33Li2O (weight) 1.9 ´ 1018 ions/cm3 4 1.26 [154]

Fluorophosphate 12.5MgF2±14.87CaF2±33.41BaF2±21.9AlF3±15.35NaPO3

(weight)

6 ´ 1018 ions/cm3 12.5 2.25 [154]

Aluminate 58.5CaO±27.5Al2O3±8.4MgO±5.6SiO2 (weight) 4 ´ 1018 ions/cm3 7.6 0.97 [154]

Germanate 70GeO2±24PbO±6PbF2 (weight) 5.2 ´ 1018 ions/cm3 7.3 1.74 [154]

Sulphate Ga2S3±GeS2±La2S3 3.2´1020 ions/cm3 2.2 1.29 [173]

Fisher-premium soda-lime 1020 ions/cm3 23.9 1 [174]

Alkali-borosilicate BGG 31 <5.8 ´ 1019 ions/cm3 >0.96 ? [173]

Alkali-borosilicate BGG 31 (Er implanted) ? >1.6 [174]

X. Orignac et al. / Optical Materials 12 (1999) 1±18 7

replaced by bene®cial Yb3� ® Er3� transfers(since Yb3� and Er3� possess similar valence andionic radius, they are expected to occupy similarsites in the glass network). When the Yb3� con-centration is further increased, the situation wheretwo Yb3� ions are in the vicinity of one Er3� ion ismore frequent, and deleterious double energytransfers occur.

Therefore, an optimum Yb3� concentration,maximizing the ¯uorescence lifetime, may be ex-pected. However, the lifetime was found to be in-dependent of Yb3� concentration. Similar resultswere found in borosilicate glasses, the eventuallifetime variations being explained by a variationin the water content [158]. This explanation couldhold for our samples. In any case, we cannot setthe best Yb3� concentration at this stage; thisproblem will be addressed by studying the in¯u-ence of the Yb3� concentration on the pumpinge�ciency directly in strip-loaded waveguides, aswill be shown in the second part.

3.3. Aluminum concentration

It is well known that aluminum co-doping canalleviate the problem of microclustering [118,119];AlO4=2 and AlO6=2 groups could act as solvationshells in the glass network for rare-earth ions [118].Therefore, the ¯uorescence lifetime is expected toincrease with increasing Al concentration, until allthe Er3� ions are solvated. This is con®rmed by theresults shown in Fig. 3: the lifetime reaches aplateau for Al/Er ratios of more than 15 ([Al]� 7.5at.% or 1.5 ´ 1021 ions/cm3); therefore, this is aminimum value. The Al concentration cannot beincreased too much, however, since homogeniza-tion of the solution was found to be di�cult forAl/Er ratios of more than 30.

In order to de®ne an optimal Al concentration,another parameter was considered: for wavelengthdivision multiplexing (WDM) applications, the¯uorescence spectrum must be as ¯at as possible.In Fig. 4, ¯uorescence spectra of samples withdi�erent Al contents are shown. As already notedin a previous work [170], all spectra are quite ¯atbetween 1535 and 1560 nm (variation is less than 1dB), which is promising for future WDM appli-cations. Since sol±gel derived glasses are in a

higher disordered state than traditional melt-quenched ones [55], Er3� ions may occupy moredi�erent sites in these glasses, which would lead toinhomogeneous broadening and ASE spectrum¯atness. Since all spectra in Fig. 4 are quite similarin ¯atness, we cannot conclude for the choice ofthe best Al concentration. Nevertheless, anotherfeature of Fig. 4 must be mentioned: the signal-to-noise ratio is similar in any spectrum with x com-prised between 0 and 7.5, whereas it improves withx P 10. Therefore, 10 at.% (2 ´ 1021 ions/cm3) willbe the chosen value.

Fig. 3. Fluorescence lifetime of the metastable level 4I13=2 of

Er3� ions as a function of Al concentration. Compositions of

the samples are 80SiO2±20TiO2±0.5ErO3=2±0.5YbO3=2±xAlO3=2,

where x� 0, 2, 5, 7, 10, 13, 15 and 20. The dotted line is only a

guide for the eyes.

Fig. 4. ASE spectra of samples with the compositions 80SiO2±

20TiO2±0.5ErO3=2±0.5YbO3=2±xAlO3=2, with x� 0, 5, 7, 10, 13,

15 and 20.

8 X. Orignac et al. / Optical Materials 12 (1999) 1±18

4. Strip-loaded waveguide characterization

4.1. Insertion loss

Insertion losses of all the waveguides availableon each sample were measured at 1300 nm. Resultsare compiled in Table 2. There are 2 or 3 series of8 waveguides on each sample. For a given series,all the waveguides possess the same opto-geomet-rical characteristics, whereas the width of the stripis 6, 7 or 8 lm, depending on the series. No ob-vious trend was observed.

4.2. Coupling loss

On each sample, the waveguide with the lowestinsertion loss was chosen: waveguide II.2 forsample D337, I.6 for sample D338 and I.1 forsample D339. Coupling losses were measured at1300 and 1550 nm. Results are shown in Table 3.Vertical con®nement is quite good, in agreementwith the high di�erence in refractive index (0.13)between the bu�er and the guiding layer. On theother hand, horizontal con®nement is not so good.As explained elsewhere [170], this comes from atechnological problem caused by a bad control onthe etching rate. Therefore, since the mode of a

standard telecommunication ®ber is round (``a''was measured to be 8 lm in our case), modemismatch is important and the resulting couplinglosses are high: the best ones are achieved withchannel I.1 on sample D339, with 1 dB of loss perface. This must be compared with the 0.1 dB ofloss per face that can be achieved with alternativetechnologies.

4.3. Propagation loss

Propagation losses were calculated at 1300 nmand are presented in Table 3. We have typicallosses around 1.5 dB/cm, with a best result at 0.7dB/cm. This is still too high for an e�cient op-tical ampli®er. Di�erent sources of losses can beidenti®ed: bulk scattering can be caused by phaseseparation and/or crystallization; surface scatter-ing by topographical non-uniformity. Phase sep-aration was already minimized in previous works[129,130]; crystallization could neither be detectedby X-ray di�raction (XRD) nor by Ramanspectroscopy in planar waveguides. Some crys-talline anatase phase could be detected, however,in strip-loaded waveguides by these two tech-niques. This di�erence can be explained as fol-lows: the silica capping layer requires a high

Table 2

Insertion losses at 1300 nm of series of eight waveguides patterned on samples D337, D338 and D339; the strip widths are 6, 7 and 8 lm

in series I, II and III respectively. Compositions are 80SiO2±20TiO2±xErO3=2±yYbO3=2±zAlO3=2, with x� 0.125, 0.25 and 0.0625,

y� 0.25, 0.25 and 0.1875, z� 2.5, 2.5 and 2.5 for D337, D338 and D339 respectively

Sample Average insertion loss @1300 nm (dB) Standard deviation (dB)

D337 (length� 5.7 cm) ÿ8.8 1.1

D338 (length� 5.8 cm) ÿ10.2 0.6

D339b (length� 5.7 cm) ÿ9.5 0.5

D339a (length� 2.1 cm) ÿ9.7 0.9

Table 3

Coupling and propagation losses at 1300 nm for strip-loaded waveguides with the lower insertion losses on samples D337, D338 and

D339.

Sample Waveguide number Horizontal radius

(lm)

Vertical radius

(lm)

Coupling losses

(dB)

Propagation losses

(dB/cm)

D337 II.2 5.5@1300 nm 0.9@1300 nm 3.8@1300 nm 0.7@1300 nm

4.5@1550 nm 0.9@1550 nm 3.7@1550 nm

D338 I.6 6.1@1300 nm 1.7@1300 nm 1.8@1300 nm 1.3@1300 nm

5.9@1550 nm 2.3@1550 nm 1.0@1550 nm

D339a I.1 8.6@1300 nm 1.0@1300 nm 4.5@1300 nm 1.6@1300 nm

7.8@1550 nm 1.0@1550 nm 4.1@1550 nm

X. Orignac et al. / Optical Materials 12 (1999) 1±18 9

temperature of densi®cation (1075°C), whereasonly 900°C is needed for the densi®cation of theguiding layer. Therefore, the crystalline phasemost probably appears in the guiding layer dur-ing the formation of the capping layer. Surfacescattering is expected to be very low, since thestrip-loaded structure itself avoids this problem,by etching the capping layer, rather than theguiding one; furthermore, the surface of the de-posited layer has been topographically ¯attenedby spinning under ethanol atmosphere [171].Future improvements will include the search fora capping layer with a refractive index similar topure silica, but with a much lower densi®cationtemperature.

5. Spectroscopic performance of the components

5.1. Fluorescence lifetime

Fluorescence lifetimes of samples D337 (80SiO2

±20TiO2±0.5ErO3=2±1YbO3=2±10AlO3=2), D338(80SiO2±20TiO2±1ErO3=2±1YbO3=2±10AlO3=2) andD339 (80SiO2±20TiO2±0.25ErO3=2±0.75YbO3=2±10AlO3=2), were measured. Results are given inTable 4. They agree fairly well with the results ofFig. 1, which support the good reproducibility ofour fabrication process. Light was ®rst coupledinto the strip-loaded structure and then in theplanar structure. Resulting ¯uorescence lifetimesare slightly longer in the planar structure, and wealso observed a weaker non-exponentiality in thedecays of the signal guided in the planar structure.This is probably due to the better con®nement inthe strip-loaded structure, leading to higher pumppower intensity, and thus higher occurrence of up-conversion processes. In any case, these values arestill too low for e�cient ampli®cation.

5.2. Absorption

The absorption spectrum of sample D339 cut at2.1 cm (sample with longer ¯uorescence lifetime) isshown in Fig. 5. There is a strong absorption peakcentered at 980 nm, typical of Yb3� absorption. Itpeaks 14 dB below the background level (dashedline); the contribution of Er3� to this absorptionpeak is negligible [172]. Therefore, the absorptiondue to Yb3� can be estimated to be 14 dB, for a 2.1cm long sample. In other words, the absorptionrate of the pump power is roughly 7 dB/cm, whichis much too high: only a small fraction of the Er3�

ions will be pumped; therefore, the Yb3� concen-tration (0.375 Yb2O3 in D339a) should be lowered.

In Fig. 6, we have presented the correspondinginsertion loss spectrum in the 1400±1650 nm re-gion. We can see that the background absorption(far from the absorption peak at 1530 nm) is aboutÿ11.5 dB. If we subtract twice the coupling losses(since there is coupling and de-coupling by theinput and the output ®bers) given in Table 3 (ÿ4.1dB) from this value, we obtain the total propaga-tion losses near 1550 nm: ÿ3.3 dB, for a 2.1 cm

Table 4

Fluorescence lifetimes measured in samples D337, D338 and D339 in the planar and in the strip-loaded structures

Sample Fluorescence lifetime Fluorescence lifetime

in the planar structure (ls) in the strip loaded structure (ls)

D337 3298±3479 2802±2993

D338 2054±2300 1591±2153

D339 4720±5443 4237±4550

Fig. 5. Absorption spectrum of sample D339 in the region 800±

1100 nm. Dashed line: background level.

10 X. Orignac et al. / Optical Materials 12 (1999) 1±18

long waveguide, which corresponds to 1.6 dB/cmof propagation losses. This value agrees remark-ably well with the value obtained at 1300 nm(Table 4), which means that our evaluation of thecoupling losses is quite accurate.

We can also extract from these spectra the ab-sorption due to Er at the peak wavelength. Theabsorption at 1530 nm peaks 2 dB below thebackground level, which corresponds to 1 dB/cm,for 0.125 mol% Er2O3.

5.3. Gain measurement

Since they are very lossy, the samples were cutat di�erent lengths for gain measurement. The re-sults are compiled in Table 5. They represent thevery ®rst gain measurement on integrated ampli-®ers made by the sol±gel process. In terms of netgain, we are still far from actual ampli®cation. Themain causes have been identi®ed: the ¯uorescence

lifetimes are still too low (best� 4.5 ms), which isdue to a combined e�ect of non-optimized com-position (Er3� ions in close vicinity) and high OHcontent; the propagation losses are still too high,due to microcrystallization in the guiding layer; thehorizontal con®nement is bad, causing high cou-pling losses.

In terms of internal gain, the best result is 2.7dB of loss reduction; this was obtained with D339,a sample of composition 80SiO2±20TiO2±0.0125Er2O3±0.375Yb2O3±5Al2O3, with a lengthof 5.7 cm and a total pump power of 100 mW. Wehave seen previously that at such an Er3� dopinglevel, the absorption due to Er3� at 1530 nm was 1dB/cm; this corresponds to a population inversionof ÿ1; there is a symmetry between the absorptionand emission spectra. Therefore, when the popula-tion is fully inverted (population inversion �+1),the emission spectrum peaks at 1 dB/cm and thecorresponding maximum theoretical loss reduction isthen 2 dB/cm ´ 5.7 cm� 11.4 dB. We have reached24% of the maximum theoretical loss reduction.

6. Conclusions

A ®rst step towards the fabrication of sol±gelderived Er-doped integrated optical ampli®ers wasaccomplished. Working in the SiO2±TiO2±Er2O3±Yb2O3±Al2O3 system, we found that the optimumcomposition was 80SiO2±20TiO2±0.31Er2O3±xYb2O3±5Al2O3, with x�0.375. For the ®rst time,the Er3� quenching concentration of a sol±gel de-rived glass was determined and found to be higherthan comparable traditional silicate glasses, asexpected. Typical propagation losses were 1.5 dB/cm

Table 5

Net gain and loss reduction measured in samples D337, D338 and D339 cut at di�erent lengths, with two pumping schemes

Sample Pumping scheme Signalamplified

(dB m)

ASE

(dB m)

Signalinjected

(dB m)

Net

gain (dB)

Internal

gain (dB)

D337 0.5Er/1Yb/10Al (5.7cm) co + contra directional 180 mW ÿ58.3 ÿ61.7 ÿ38.7 ÿ22.2 2.2

D337 (2.1cm) co + contra directional 56 mW ÿ50.3 ÿ61.1 ÿ36.7 ÿ14 1.5

D338 1Er/1Yb/10Al (5.8cm) co + contra directional 180 mW ÿ61.3 ÿ63.2 ÿ38.7 ÿ37.2 0.7

D339 0.25Er/0.75Yb/10Al (5.7cm) co-directional 50 mW ÿ19.4 ÿ68 ÿ1.7 ÿ17.7 0.6

D339 (5.7cm) co + contra directional 100 mW ÿ52.5 ÿ65.5 ÿ36.7 ÿ15.8 2.7

D339 (2.1cm) co-directional 50 mW ÿ47.8 ÿ68 ÿ35 ÿ12.8 1

D339 (2.1cm) co + contra directional 100 mW ÿ50.2 ÿ67 ÿ39.2 ÿ11 1.3

Fig. 6. Insertion losses spectrum of sample D339 in the region

1400±1650 nm.

X. Orignac et al. / Optical Materials 12 (1999) 1±18 11

and coupling losses were of the order of 1 to 4 dB/face. With such high losses, no net gain could beachieved. The best observed loss reduction was 2.7dB for a 5.7 cm-long sample, which represents 24%of the maximum theoretical gain.

The main problems were identi®ed: low ¯uo-rescence lifetime, high coupling losses and highpropagation losses. The ®rst problem is due to acombined e�ect of high energy exchanges betweenEr3� ions and high OH content and is inherent tothe sol±gel technology. Propagation losses arecaused by phase separation and microcrystalliza-tion in the guiding layer. But this does not seem tobe an intrinsic limitation of the sol±gel technology,since propagation losses as low as 0.03 dB/cm havebeen achieved in SiO2/TiO2 planar waveguidesmade in modi®ed sol±gel process [78]. Couplinglosses are due to poor horizontal con®nement,leading to high mode mismatch between the cou-pling ®bers and the waveguide itself; this is notrelated to the sol±gel technology.

As an example for comparison, alternative andmore mature technologies such has LiNbO3, ion-ex-change, sputtering, FHD and PECVD have led to netgain values of 13.8 dB [9], 11.6 dB [179], 10 dB [178],18.9 dB [180] and 5 dB [54], respectively. Despiteencouraging high quenching concentration values,the overall device performance is still mediocre.

Acknowledgements

This work was performed under the auspices ofthe European Community, CAPITAL project,ACTS program (#AC047).

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