1982 J. Opt. Soc. Am. B/Vol. 19, No. 9 /September 2002 Coleman et al.

Spectroscopic and energy-transfer parametersfor Er3¿-doped and

Er3¿, Pr3¿-codoped GeGaS glasses

Daniel J. Coleman, Paul Golding, and Terence A. King

Laser Photonics Group, Department of Physics and Astronomy, Schuster Laboratory, University of Manchester,Manchester M13 9PL, UK

Stuart D. Jackson

Optical Fibre Technology Centre, University of Sydney, Australian Photonics CRC, 206 National Innovation Centre,Australian Technology Park, Eveleigh, 1430 Australia

Received December 20, 2001; revised manuscript received April 25, 2002

The absorption and emission spectra and the rate parameters for the important energy-exchange processesrelevant to the 4I11/2 → 4I13/2 laser transition in Er31-doped and Er31, Pr31-codoped GeGaS glasses are pre-sented. The rate parameters are determined after optical excitation with a tunable pulsed optical parametricoscillator that excites the 4I11/2 and 4I13/2 energy levels directly. For the Er31 singly doped samples, theenergy-transfer upconversion (ETU) macroscopic rate parameters for the upper and lower laser levels weremeasured for a range of Er31 concentrations. In correspondence with ZBLAN and the fluoride crystals, the(4I13/2 , 4I13/2) → (4I9/2 , 4I15/2) ETU process of the lower laser level is measured to be stronger than the corre-sponding ETU process of the upper laser level. Such a condition enables energy recycling to occur. A higherrate of energy transfer from the 4I13/2 level of Er31 to Pr31 deactivator is measured as compared with the cor-responding energy transfer from 4I11/2 level to both the Pr31 deactivator and OH impurity. With these favor-able energy-transfer conditions, the prospect for the future development of highly efficient ;3-mm fiber laserswith GeGaS glass as the host material is excellent. © 2002 Optical Society of America

OCIS codes: 160.5690, 300.6280, 140.3510.

1. INTRODUCTIONThe sulfides of the period four elements Ga, Ge, and Asare known to form stable chalcogenide glasses in both bi-nary and ternary compositions.1–3 Gallium sulfide,Ga2S3 , forms a wurtzitelike compound upon fast quench-ing from the melt; however, the addition of either GeS2 ,As2S3 , or La2S3 in proper proportions allows genuine bi-nary glasses to form after quenching. In particular, theuse of La2S3 (to form GaLaS glass) allows for relativelyhigh concentrations of rare-earth elements to be incorpo-rated into the glass composition.4 An alternative binary-glass composition, GeS2 –Ga2S3 (GeGaS) glass is unusualin that GeS2 is the glass former, not Ga2S3 , as is the casewith the GaLaS binary glasses. Although the maximumrare-earth-ion solubility is currently lower in GeGaSglass as compared with GaLaS glass, for fiber-laser appli-cations in which the interaction length is large, concen-trations of the rare-earth ion up to 1 mol. % that can beprovided by GeGaS glass are sufficient. As a result, anumber of rare-earth ions, such as Pr31,5 Dy31,6 Tm31,7

and Tm31, and Ho31,8 have been doped into GeGaS glassand studied spectroscopically with fiber-laser and ampli-fier applications in mind.

Due to the strong linear absorption at wavelengths of;3 mm in most biological tissues, high-power 3-mm er-bium lasers have been applied to the medical field, e.g., asefficient cutting tools for surgery. Pulsed lasers operat-

0740-3224/2002/091982-08$15.00 ©

ing in this wavelength region have been widelystudied,9–12 and several demonstrations of diode-pumpedcw 3-mm crystal-based lasers have displayed high poweroutput.13–16 The recent demonstrations of the high-power cw erbium-doped ZBLAN fiber laser17–19 and theanalysis of the interaction of this radiation with biologicaltissues20 highlight the excellent potential for fiber lasersfor this particular application. Mid-infrared (mid-IR) fi-ber lasers, in general, require host materials with rela-tively low phonon energies in order to minimize the rateof nonradiative decay between the closely spaced elec-tronic energy levels, therefore maximizing the quantumefficiency relevant to mid-IR emission.

Currently, fluoride glass is the only glass material thathas been used for the development of fiber lasers withoutput wavelengths longer than ;2 mm. The sulfideglasses,21,22 however, are an alternative glass for this ap-plication, as they have very low phonon energies (;350–400 cm21) and relatively high refractive indices(2.1–2.6).23 The resultant higher quantum efficiency andchemical and environmental durability of sulfide glass ascompared with fluoride glass means that the range of ap-plications is potentially extended. Optical amplifica-tion,24 as well as laser operation in both bulk25 and fiber26

geometries, has recently been demonstrated from laserswith sulfide glass as the host material. The sulfideglasses are particularly useful because the visible-to-

2002 Optical Society of America

Coleman et al. Vol. 19, No. 9 /September 2002 /J. Opt. Soc. Am. B 1983

near-infrared absorption is lower as compared with theselenide and telluride glasses, making optical excitationpossible with standard laser sources.

In this investigation, we are interested in measuringthe spectroscopic properties of Er31-doped and Er31,Pr31-codoped GeGaS glass in order to have an informedcomparison between the popular low-phonon-energyglasses. We have measured the absorption and emissionspectra, metastable lifetimes, oscillator strengths, Judd–Ofelt parameters, and the macroscopic ion–ion interac-tion parameters. We compare these measurements withprevious measurements relating to ZBLAN glass27 in or-der to ascertain the appropriateness of this material to beused in high-power midinfrared fiber-laser applications.

2. SPECTROSCOPY AND EXPERIMENTALDETAILSThe important electronic energy levels and electron en-ergy exchanges relevant to the 2.7-mm-laser transition oferbium ions in the presence of praseodymium ions areshown in Fig. 1. The energy-exchanges processes havebeen explained in detail elsewhere.28–30 Briefly, cw op-eration is possible mainly due to the Stark splitting of the4I13/2 lower laser level and the energy-transfer upconver-sion (ETU2) process that depopulates the 4I13/2 level; seeFig. 1. The latter phenomenon and the second(4I11/2 , 4I11/2) → (4F7/2 , 4I15/2) process, ETU1, have ratesdepending on the square of the excited-state density, andtherefore they occur most strongly in highly dopedsamples and at high pump intensities. In codoped sys-tems, deactivation of the 4I13/2 level of Er31 by way oftransferring energy to the 3F3 and 3F4 levels of nearbyPr31 ions (the ET2 process) is also efficient, see Fig. 1.27,31

Indeed, energy transfer to the 1G4 level of Pr31 (the ET1process) and detrimental energy transfer to OH impuri-

Fig. 1. Partial energy-level diagram of Er31 and Pr31 indicatingthe pump wavelengths of the 4I9/2 and 4I11/2 levels, the 2.7-mmlaser transition, the ETU processes from the upper and lower la-ser levels, and energy-transfer process from Er31 to Pr31.

ties is also possible from the upper laser level and reducesthe quantum efficiency of the laser transition.

The precise sulfide-glass composition under investiga-tion in this study is Ge24Ga10S66 . The method used tofabricate the glasses has been described previously.32

The selection of this binary-glass composition relates toselecting the Ga content in order to maximize the solubil-ity of the rare-earth dopant. In a recent study,33 it wasobserved that the concentration of Ga needs to be approxi-mately an order of magnitude higher than the concentra-tion of the rare-earth ion to be doped into the GeGaS glasscomposition. One set of samples was prepared with Er31

concentrations of 0.875, 0.5, 0.125, and 0.05 mol.%, and afurther set of Er31, Pr31-codoped samples was preparedwith the following Er31/Pr31 concentrations: 0.875/0.125, 0.875/0.05, and 0.875/0.025 mol.%. The finishedsamples were disk-shaped, 10 mm in diameter, and ap-proximately 5 mm in thickness. The circular surfaces ofeach sample were shaped and polished for the experi-ments.

The optical absorption spectrum was measured in thewavelength regions 300 nm to 3 mm and 2.5 to 20 mm byuse of a Hitachi 3501 photospectrometer and a Fourier-transform spectrometer, respectively. The emission spec-trum at ;2.7 mm was measured after optical excitation ofthe 4I9/2 level with a cw Ar1 pumped Ti:sapphire laser op-erating at a wavelength of 804 nm. The power of thebeam incident on the sample was approximately 500 mW.The fluorescence due to the 4I11/2 → 4I13/2 transition wasfocused into a 0.5-m computer-controlled monochromator(Acton SpectraPro 500i) and detected with a liquid-nitrogen-cooled, amplified InSb photodiode (Judson J10)in combination with a lock-in amplifier. The scatteredpump light and fluorescence at wavelengths shorter than;1.8 mm were filtered out with a germanium long-passfilter, and the resultant spectrum was corrected for thespectral response of the InSb photodiode.

The temporal decays of the 4I11/2 and 4I13/2 levels weredetermined by measuring the luminescence decay follow-ing direct pulsed excitation of these levels at 980 nm and1533 nm, respectively. The experimental arrangement isshown in Fig. 2 and is similar to the experimental setupused for the spectroscopic analysis of Er31-doped andEr31, Pr31-codoped fluoride glass.27 The excitation

Fig. 2. Experimental arrangement for the measurement of theluminescent decay from the 4I13/2 and 4I11/2 energy levels ofEr31.

1984 J. Opt. Soc. Am. B/Vol. 19, No. 9 /September 2002 Coleman et al.

source was a KTP optical parametric oscillator pumped bya frequency-doubled, 10-Hz, Q-switched Nd:YAG laser at532 nm. A signal-tuning range of 770 nm to 990 nm withthe associated idler wavelength of 1700 nm to 1160 nmwas attainable, thereby providing independent excitationof the 4I11/2 and 4I13/2 energy levels with a maximum out-put of 3 mJ and a pulse length of approximately 6 ns.The short pulse length allowed each level to be excited ona time scale much smaller than its luminescent lifetime.The 4I11/2 level was pumped on the long-wavelength sideof the absorption peak to minimize excited-state absorp-tion (ESA) from the 4I11/2 level to the 4F7/2 level.34

The pump beam was focused into the sample with an8-cm focal-length lens to give a beam waist of approxi-mately 400 mm. A 200-mm pinhole was used to aperturethe beam so that the excitation was approximately uni-form across the width of the beam. To avoid reabsorptioneffects that would give spuriously long lifetimemeasurements,35 the samples were positioned so that thebeam passed very close to the edge of the sample fromwhich the luminescence was collected. The luminescencefrom the samples was imaged onto a 30-cm monochro-mator, and the intensity was measured with a silicon pho-todiode (IPL 10530DAL) and an InGaAs photodiode(Hamamatsu G5746-01) in the 980-nm and 1500-nmwavelength regions, respectively. The signals were aver-aged over 512 pulses on a digital-storage oscilloscope(Hewlett-Packard 54522A). Decay curves were mea-sured over a range of pump energies in order to vary thestrength of the energy-transfer processes. For the life-time measurements the pump power was kept low tominimize interionic interaction.

3. RESULTSA. Absorption SpectraThe Er31 ground-state absorption cross sections mea-sured for the GeGaS glass under consideration is shownin Fig. 3(a). The Urbach edge (starting at approximately433 nm) has been removed from the spectrum in order toisolate the six ground-state absorption bands. The oscil-lator strengths of these transitions are presented in Table1 as well as the results from previously reported measure-ments taken from Er31-doped GeGaS33 and GaLaS36

glasses. Note that the GeGaS composition used in theprevious study was slightly different (being Ge25Ga5S70)from the GeGaS composition used in this study. The cal-culated oscillator strengths in our Ge24Ga10S66 glass aresomewhat lower than those determined for bothGe25Ga5S70 and GaLaS glasses. The absorption peaksoccurred at the same wavelengths in each of the threecompositions, indicating that the degree of bonding cova-lency did not significantly change between the differentsulfide compositions. The two important pump bands forthe 2.7-mm-laser transition are shown on an expandedscale in Fig. 3(b). Since GeGaS glass is more covalentthan ZBLAN glass, the absorption peaks are slightly red-shifted with respect to the equivalent peaks inEr31-doped ZBLAN.37 For instance, the 4I9/2 absorptionpeak occurring at a wavelength of 804 nm overlaps wellwith the output of AlGaAs diodes, usually operating at810 nm.

The Judd–Ofelt intensity parameters shown in Table 2were calculated for Er31 in GeGaS from the measured os-cillator strengths.38,39 The reduced matrix elementswere taken from Ref. 40. The refractive index was mea-sured to be 2.15 at 632.8 nm with an ellipsometer, and

Fig. 3. Measured ground-state absorption spectrum ofEr31-doped GeGaS glass for (a) the range 500 to 1650 nm and (b)for the two important pump bands relevant to the 2.7-mm lasertransition.

Table 1. Oscillator Strenghts in Er31-DopedSulfide Glasses

TransitionWavelength

[nm]

Oscillator Strength, f 3 1028

Ge24G10S66 Ge25G5S7033 GaLaS36

4I15/2 → 4I13/2 1537 6 0.5 219 6 5 252 2814I15/2 → 4I11/2 984 6 0.5 88 6 3 94 1104I15/2 → 4I9/2 804 6 0.5 67.4 6 2 78.0 844I15/2 → 4F9/2 658 6 0.5 345 6 6 436 4624I15/2 → 4S3/2 547 6 0.5 78 6 3 76 1304I15/2 → 2H11/2 526 6 0.5 1414 6 30 1860 –

Table 2. Judd–Ofelt Intensity Parameters inEr31-Doped Sulfide Glasses

GlassComposition V2 @10220 cm2# V4 @10220 cm2# V6 @10220 cm2#

Ge24Ga10S66 5.54 6 1.0 1.86 6 0.2 0.93 6 0.1Ge25Ga5S70

33 7.28 2.54 0.99GaLaS36 6.54 2.00 0.97

Coleman et al. Vol. 19, No. 9 /September 2002 /J. Opt. Soc. Am. B 1985

Table 3. Calculated and Measured Luminescent Lifetimes in Er31-Doped GeGaS Glass

Level a Level b l [nm]Aed

[s21]Amd[s21]

Anr[s21] h [%] trad@ms# tmeasured@ms#

4I13/24I15/2 1538 233 126 0.01 100 2.8 2.9 6 0.2

4I11/24I13/2 2728 37 20 52.3 154I11/2 981 327 0 0.00 85 2.3 1.8 6 0.1

this value was used for all six absorption transitions usedin the Judd–Ofelt analysis. The rms value fit was calcu-lated to be 1.8%.

The radiative lifetimes shown in Table 3 were calcu-lated from the Judd–Ofelt intensity parameters. Themultiphonon decay rates were calculated with the equa-tion

Wmp 5 Wo exp~2aDE !, (1)

where Wo and a are 2.6 3 106 s21 and 2.95 3 10 2 3 cm,respectively, for GeGaS.7 The predicted value of the4I13/2 level lifetime, t(Er:4I13/2), of 2.8 ms agrees wellwith the experimentally measured value of 2.96 0.2 ms. The calculated value for the 4I11/2 level life-time, t(Er:4I11/2), is longer than the measured value,which may suggest that this lifetime has been slightlyquenched by energy transfer to OH impurity in the host.

B. Emission SpectraThe luminescence from the 4I11/2 → 4I13/2 transition foran Er31-doping concentration 0.875 mol.% is shown inFig. 4. The fluorescence emission peaks at ;2740 nmand has a FWHM of ;90 nm. The characteristics of theemission spectrum for GeGaS glass is very similar to theequivalent emission spectrum in GaLaS glass.36 Theemission peak, however, occurs at the slightly shorterwavelength of 2730 nm in GaLaS glass. In Er31-dopedZBLAN the wavelength of the peak in the emission spec-trum occurs at the shorter wavelength of 2720 nm.41

C. Laser Level LifetimesThe measured values for t(Er:4I13/2) and t(Er:4I11/2) as afunction of Er31 concentration, NEr , are presented in Fig.5. t(Er:4I13/2) is affected by concentration quenching; byextrapolating to zero concentration, the intrinsic lifetimecan be determined to be 2.9 6 0.2 ms. The luminescentdecay curves were initially nonexponential as a result ofthe ETU2 process; however, after approximately 0.5 ms,the decay settles into a simple exponential, and this latterportion of the data was used to determine t(Er:4I13/2).The temporal decays for the 4I11/2 level, however, were af-fected by ETU1 and energy transfer to OH, and they re-mained significantly nonexponential for the whole of thedecay. As we show later, primarily the ETU1 process af-fects the initial part of the decay, and primarily energytransfer to OH affects the latter part of the decay. Towithin experimental error, it can be observed from Fig. 5that Dt(Er:4I13/2)/DNEr ' Dt(Er:4I11/2)/DNEr . Sincethe decays were not exponential, they were normalizedand fitted to the equation42

I~t ! 5 I0 expS 2t

t02 gt1/2D , (2)

where I0 is the initial intensity, t0 is the intrinsic lifetime,and g is an empirical parameter describing the degree ofnonexponential behavior. The values of g were 1, 3, 2.5,and 6 for the Er31-doping concentrations 0.875, 0.50,0.125, and 0.025 mol.%, respectively. This suggests thatenergy transfer to OH has less effect on the decay athigher rare-earth doping concentrations where ETU1 isstronger. The intrinsic value for t(Er:4I11/2) was deter-mined to be 1.8 6 0.1 ms by extrapolating the measure-ments to zero Er31 concentration. The measured valuesfor t(Er:4I13/2) and t(Er:4I11/2) for several chalcogenideglass hosts are presented in Table 4. The value fort(Er:4I13/2) measured in this study is significantly shortercompared with the results of previous work.33,36,43,44 Inthese other studies, a chopped Ti:sapphire beam was usedto pump the 4I11/2 level rather than direct pulse pumpingof the 4I13/2 level. If there is a significant population gen-

Fig. 4. Measured emission spectrum of the 4I11/2 ← 4I13/2 tran-sition in Er31-doped GeGaS glass.

Fig. 5. Measured values for t(Er:4I13/2) and t(Er:4I11/2) inEr31-doped GeGaS glass for a range of Er31-doping concentra-tions.

1986 J. Opt. Soc. Am. B/Vol. 19, No. 9 /September 2002 Coleman et al.

erated in the 4I11/2 level, then feeding from this level mayresult in an increase in the measured value fort(Er:4I13/2).

The measured values for t(Er:4I13/2) and t(Er:4I11/2)for the Er31, Pr31-codoped samples are presented in Fig.6. The effect of energy transfer from the 4I13/2 level ofEr31 to the 3F3 , 3F4 energy levels of Pr31 results in astrong quenching of the luminescent lifetime of the 4I13/2energy level of Er31. A Pr31 concentration of 0.025mol.% is sufficient to make t(Er:4I13/2) , t(Er:4I11/2).As mentioned above, t(Er:4I11/2) is quenched by energytransfer to both OH and to the 1G4 level of Pr31. Thedecays were nonexponential and were therefore fitted byuse of Eq. (2). The quenching of the 4I11/2 level due toenergy transfer to both Pr31 and OH was smaller relativeto the quenching of the 4I13/2 level from energy transfer toPr31 alone for Pr31 concentrations greater than 0.25mol.%. Although not supported by direct evidence, wecan deduce that the value for ET1 is probably less thanthe value for ET2 within the range of the Er31 and thePr31 concentrations .0.25 mol.% used in this study. Therate of energy transfer, REr-Pr , from the Er31 ions to thePr31 ions can be described by the equation27

REr-Pr 5 WEr-Pr N~Er:4I13/2!N~Pr31!, (3)

where WEr-Pr is the macroscopic energy-transfer param-eter, N(Er:4I13/2) is the excited-state population density ofthe 4I13/2 level, and N(Pr31) is the concentration of Pr31

ions. The measured WEr-Pr values for the ET2 process inGeGaS glass are presented in Fig. 7. Within the experi-mental error of the measurements, we can state that theWEr-Pr values are independent of the Pr31 concentration

Fig. 6. Measured values for t(Er:4I13/2) and t(Er:4I13/2) withPr31-codoping in Er31-doped GeGaS glass.

Table 4. Measured Luminescent Lifetimes of the4I13Õ2 and 4I11Õ2 Levels in Er31-Doped Sulfide

Glasses

Glass Composition t(Er:4I13/2) [ms] t(Er:4I11/2) [ms]

GeGaS 2.9 6 0.2 1.8 6 0.1GeGaS33 5.5 2.32GaGeAsS43 5.0 1.7GeGaSbS44 4.0 1.7GaLaS36 2.3 6 0.1 1.23 6 0.04

within the range of deactivator concentrations used inthis study. The WEr-Pr values have increasingly large un-certainties because the Pr31 ion more strongly quenchesthe intensity of the luminescence from the samples mak-ing the measured decays particularly susceptible to noise.A small uncertainty in the measured lifetimes translatesinto relatively large uncertainties for the values of theWEr-Pr parameters.

D. Energy-Transfer UpconversionIf we neglect repopulation processes, the population of the4I13/2 and 4I11/2 (in the absence of energy transfer to OH)energy levels can be described by the equation27

dN

dT5 2

N

t2 2WETU N2, (4)

where t is the intrinsic lifetime, N is the excited-statepopulation density, and WETU is the macroscopic param-eter describing the rate of ETU. The WETU parameter forthe 4I13/2 and 4I11/2 levels is shown in Fig. 8 as deter-mined from the measured luminescent decays with themethod set out in references.27,45 The WETU values in-

Fig. 7. Calculated macroscopic energy-transfer parameterWEr-Pr relevant to energy transfer from the Er31:4I13/2 level to thePr31:3F4 and Pr31:3F3 levels (ET2) in Er31 Pr31-codoped GeGaSglass.

Fig. 8. Calculated macroscopic parameter WETU for ETU fromthe 4I13/2 and 4I11/2 levels of Er31-doped GeGaS glass for a rangeof Er31-doping concentrations.

Coleman et al. Vol. 19, No. 9 /September 2002 /J. Opt. Soc. Am. B 1987

crease with Er31 concentration, and there is no evidenceof any significant effect on the values for WETU due to en-ergy transfer to OH.

4. DISCUSSIONThe use of chalcogenides, in particular the sulfide glasses,for fiber-laser applications is an open question because ofthe difficulties arising in relation to the purity of thestarting materials and the complexity of the fiber drawingprocess for soft glasses. Alternative low-phonon glasseshave been studied46–48; however, the use of sulfide glassshows more promise with laser operation at a wavelengthof 1 mm, recently demonstrated for a Nd31-doped GaLaSglass in both bulk and fibers geometries.24,25 Althoughmuch of the work has centered around the spectroscopicanalysis of rare-earth-doped sulfide glass materials,therefore paving the way for effectively designed systems,a great deal of work is necessary to fabricate chalcogenideglass fibers to the level of maturity of, e.g., fluoride glassfibers.

The absorption due to the fundamental stretchingmode of the OH impurities and the SH bond is shown inFig. 9. Since the fundamental SH vibrational mode has amuch lower energy than the electronic transitions of theEr31 ions of present interest, the presence of SH has littleeffect on the spectroscopic properties of the Er31-dopedglasses in this study. OH impurities, however, are par-ticularly relevant to the energy transitions of Er31 ions.The fundamental stretching mode of the OH impuritypeaks at 3420 cm21 (2.92 mm) in GeGaS with a FWHM of;530 cm21; see the inset to Fig. 9. The wavelength atthe peak of the absorption and FWHM of the fundamentalOH absorption band for a number of glasses are pre-sented in Table 5. The broadening of the fundamentalOH absorption peak in GeGaS is comparable to that offluoride glass, 410 cm21,49 but much smaller than in tel-lurite glasses, 900 cm21.49 The particularly broad ab-sorption peak in tellurite glasses is explained by the pres-ence of multiple bonding sites within the glass structure.The OH impurities bond into different sites and experi-ence slightly different local electric fields and thereforehave a range of absorption wavelengths. The relativenarrowness of the OH absorption peak in GeGaS glasssuggests that the OH impurities are bonded into a com-paratively large number of similar sites.

The peak absorption coefficient (at 2.92 mm) for OH ineach singly doped sample is displayed in Fig. 10(a). Ingeneral, it can be observed that the absorption and henceconcentration of OH increases as the rare-earth-ion dop-ing level increases. This suggests that either most of theOH enters the glass through the Er2S3 starting materialor, at higher Er31 doping levels, the glass is more suscep-tible to the incorporation of OH impurities during the fab-rication of the glass. The OH absorption coefficientshown in Fig. 10(b) shows that the samples doped withboth Er31 and Pr31 have lower OH concentrations thanthe singly doped sample with the same Er31 concentra-tion. Significantly reducing the level of OH impurity in-GeGaS glasses is very important so that glasses of highoptical quality are produced.

One interesting point to note is the similarity betweenthe change in the measured OH concentrations shown inFig. 10(a) and the change in t(Er:4I11/2) shown in Fig. 5.

Fig. 9. Measured absorption spectrum of Er31-doped GeGaSglass in the range 2.5 mm to 11 mm. The Er31 concentration is0.5 mol.%. The inset shows the corresponding absorption spec-trum in the vicinity of the fundamental stretching mode of OH.

Fig. 10. Measured values of the OH absorption peak in (a) thesingle Er31-doped samples and (b) the Er31, Pr31-codopedsamples. Er31 concentration is 0.875 mol.%.

Table 5. Position and Width of the FundamentalOH Absorption Peak in a Number of Er31-Doped

Glasses

GlassPeak Absorption

Wave Number [cm21] FMHW [cm21]

GeGaS 3420 (2.92 mm) 530 6 50Fluoride 3480 (2.87 mm)37 41049

Tellurite 2980 (3.356 mm)37 90049

Silica 3680 (2.72 mm)37 13049

1988 J. Opt. Soc. Am. B/Vol. 19, No. 9 /September 2002 Coleman et al.

The results in Fig. 10(a) indicate that a higher level of OHis present in the 0.05 mol.% Er31 sample as comparedwith the 0.125 mol.% Er31 sample. This fact results in asmaller t(Er:4I11/2) for the 0.05 mol.% Er31 sample ascompared with the 0.125 mol.% Er31 sample; see Fig. 5.In addition, the OH concentration levels in the 0.05 mol.%and 0.5 mol.% Er31 samples are similar, and an equiva-lence in t(Er:4I11/2) is also measured; see Fig. 5. Theseresults indicate that the nonexponential contribution tothe decay of the 4I11/2 level relates to ETU1 in the initialpart of the decay and to energy transfer to OH in the lat-ter part of the decay. This is expected since ETU1 is pro-portional to the square of the population in the 4I11/2level, whereas energy transfer to OH is directly propor-tional to the population in the 4I11/2 level.

Codoping Er31-doped GeGaS glass with Pr31 ions has astrong quenching effect on the lower laser level of the 2.7-mm-laser transition and is likely to be beneficial to laseroperation because the corresponding quenching effect onthe upper laser level is significantly smaller. The macro-scopic energy-transfer parameters, WEr-Pr , for GeGaSglass are smaller (by a factor of approximately 3) than thecorresponding parameters for ZBLAN.26 TheEr31-doping concentrations used in the ZBLAN studywere much higher (14 3 1020 cm23), and it was estab-lished that the energy-transfer rate was strongly depen-dent on donor ion concentration, but almost independentof acceptor ion concentration. Within the limits of the ex-perimental uncertainties, the WEr-Pr values measured inour GeGaS glass samples are also independent of acceptorion concentration.

The main advantages of GeGaS glass over ZBLANglass are essentially its lower phonon energy and higherrefractive index. Difficulties with fiber drawing andoverall glass purity have so far prevented it from beingused for widespread fiber-laser applications, particularlyin the mid-IR. GeGaS glass has lower rare-earth-ionsolubility than ZBLAN, and it is unlikely that ETU fromthe lower laser level will be strong enough to improve theefficiency of the laser transition significantly. If this isthe case, then deactivation of the lower laser levelthrough Pr31 codoping will probably be the most usefulway of deactivating the electron population in the 4I13/2level. Considerable research has already been carriedout on rare-earth-doped GaLaS glasses. GeGaS glass50

has a lower phonon energy than GaLaS36 (approximately350 cm21 compared with 425 cm21), which will result in acomparatively higher quantum efficiency for mid-IR fluo-rescence transitions. GeLaS glass has a higher refrac-tive index than GeGaS glass; therefore it will have higherradiative transition rates. Both of these sulfide glasseshave good potential as hosts for 2.7-mm fiber lasers, andtheir utility will depend on how easily they can be drawninto rare-earth-doped fibers. The measurement of theexcited-state absorption spectra relevant to the 4I11/2 and4I13/2 energy levels is also necessary to more completelycharacterize these materials.

5. CONCLUSIONThe absorption and emission properties of Er31-doped Ge-GaS glass have been investigated to evaluate its potential

as a host material for fiber lasers emitting on the 4I11/2→ 4I13/2 laser transition at wavelengths ;2.7 mm. Themeasured oscillator strengths and luminescent lifetimeswere comparable to those measured in previous studies ofGaLaS glass. The emission from the 4I11/2 → 4I13/2 tran-sition was measured to peak at 2740 nm in GeGaS glass.The macroscopic parameters describing the rate of ETUfrom the 4I13/2 and 4I11/2 levels were measured for a rangeof Er31 concentrations; the rate of ETU of the 4I13/2 levelwas greater than the corresponding rate of ETU of the4I11/2 level. The effect on t(Er:4I13/2) and t(Er:4I11/2) ofcodoping with Pr31 deactivator ions was also investi-gated. The degree of lifetime quenching of the laser lev-els of Er31 in the presence of Pr31 deactivator ions wasmeasured to be larger for the lower laser level, as com-pared with the lifetime quenching of the upper laser level,despite the presence of OH impurities that also act toquench t(Er:4I11/2).

ACKNOWLEDGMENTSThe GeGaS glass samples for the experiments were fabri-cated by Se Ho Park, Yong Beom Shin, and Jong Heo atthe Department of Materials Science and Engineering,Pohang University of Science and Technology. The Hita-chi 3501 photospectrometer was operated by StephenBeesley at Pilkington PLC. The authors acknowledgethe financial support from the Engineering and PhysicalSciences Research Council and the Australian ResearchCouncil.

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