Ion-exchanged waveguide lasers in Er^3+/Yb^3+ codoped silicate glass

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Ion-exchanged waveguide lasers in Er31yYb31

codoped silicate glass

Philip M. Peters, David S. Funk, Adele P. Peskin, David L. Veasey, Norman A. Sanford,Susan N. Houde-Walter, and Joseph S. Hayden

We investigated an Er31yYb31 codoped silicate glass as a host material for waveguide lasers operatingnear 1.5 mm. Spectroscopic properties of the glass are reported. Waveguide lasers were fabricated byK1-ion exchange from a nitrate melt. The waveguides support a single transverse mode at 1.5 mm. Aninvestigation of the laser performance as a function of the Yb:Er ratio was performed, indicating anoptimal ratio of approximately 5:1. Slope efficiencies of as great as 6.5% and output powers as high as19.6 mW at 1.54 mm were realized. The experimental results are compared with a waveguide lasermodel that is used to extract the Er31 upconversion coefficients and the Yb31–Er31 cross-relaxationcoefficients. The results indicate the possibility of obtaining high-performance waveguide lasers from adurable silicate host glass.

OCIS codes: 140.3500, 130.2790, 130.3120, 160.2750, 160.5690.

1. Introduction

Waveguide lasers and amplifiers in glasses codopedwith Er31 and Yb31 are promising candidates forompact multifunctional devices operating near 1.5m. The large gain bandwidth resulting from the

nhomogeneously broadened spectrum of the glassost makes these devices ideal as sources to be used

n wavelength division multiplexing applications.n addition, because of their short cavity lengths,hese waveguide lasers offer the possibility of mode-ocked operation with fundamental repetition ratesn the gigahertz regime by use of semiconductor sat-rable absorbers. Such lasers would be ideal asources for soliton communications systems. Otherpplications requiring an eye-safe wavelength, suchs remote sensing and range finding, could benefitrom compact, high-power, cw or Q-switchedaveguide laser sources based on Er31yYb31 codoped

glasses.

P. M. Peters, D. S. Funk, D. L. Veasey, and N. A. Sanford arewith the Optoelectronics Division and A. P. Peskin is with the HighPerformance Systems and Services Division of the National Insti-tute of Standards and Technology, 325 Broadway, Boulder, Colo-rado 80303. S. N. Houde-Walter is with The Institute of Optics,University of Rochester, Rochester, New York 14627. J. S. Hay-den is with the Research and Development—Materials Group,Schott Glass Technologies, Inc., 400 York Avenue, Duryea, Penn-sylvania 18642. The e-mail address for P. M. Peters [email protected].

Received 11 March 1999.

2

The performance of waveguide laser devices inglasses doped only with Er31 is limited, because ofthe weak pump absorption bands of the Er31 ion.1,2

The use of Yb31 as a sensitizing ion has led to signif-icantly improved performance.3–5 The 2F7y23

2F5y2absorption of Yb31 near 980 nm is approximately anorder of magnitude stronger than the 4I15y23

4I11y2absorption of Er31. The energy absorbed by theYb31 ions in a codoped glass is transferred to the Er31

ions, and lasing occurs on the 4I13y234I15y2 transition

of Er31. Owing to its simple energy-level structure,Yb31 does not suffer from ion–ion interaction pro-cesses, such as cross-relaxation and cooperative up-conversion, that lead to concentration quenching inions such as Nd31 and Er31. As a result the Yb31

concentration is limited only by the solubility of therare-earth ion in the glass host. That is not to say,however, that the Yb31 concentration should be madeas large as possible. Numerous factors contribute tothe determination of the ideal Yb31 concentration.For example, the concentration of the sensitizing ionis necessarily related to the device length. For agiven Yb31 concentration, a device shorter than theabsorption length will not make use of the availablepump light and will not operate as efficiently as pos-sible. A device that is too long will suffer from signalreabsorption by the three-level Er31 ion in the un-pumped region. Other concerns with increasingYb31 concentration include increased scatteringlosses in the waveguide and increased backtransferfrom Er31 to Yb31.

0 November 1999 y Vol. 38, No. 33 y APPLIED OPTICS 6879

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Table 1. Rare-Earth Ion Densities of Three Codoped Silicate Glasses

6

The choice of host glass is another important con-sideration in the development of waveguide laser de-vices. The major considerations include the opticalproperties of the active rare-earth ions in the host ofchoice as well as the suitability of the host for aparticular waveguide fabrication technique. Ion ex-change offers a relatively simple and low-cost methodfor forming low-loss optical waveguides. Phosphateglasses are often cited for having spectroscopic prop-erties that are superior to those of silicate glasses.6,7

However, issues relating to the chemical durability ofphosphate glasses make ion exchange more difficultin phosphates than in silicate glasses.8,9

In this paper we report on a comprehensive studyof an Er31yYb31 codoped silicate glass. Severalspectroscopic quantities are measured and reported.Waveguide lasers are fabricated in the silicate glassby a simple one-step ion-exchange process. The la-ser performance is investigated as a function of theratio of the number of Yb31 ions to Er31 ions. Foreach ion ratio the output coupler reflectance and de-vice length are optimized for maximum slope effi-ciency. Finally, the experimental results arecompared with a laser model that is used to estimateadditional glass spectroscopic quantities includingthe upconversion and the Yb31–Er31 energy-transferoefficients.

2. Experimental Details

A. Glass Samples

The host glass used in this study is IOG10, a com-mercially available laser glass.10 The glass is a

hosphorus-free, mixed-alkali, zinc-silicate glass.he particular melts used for these experiments wille referred to for convenience as glasses A, B, and C.ll three glasses were doped with 1-mass % Er2O3.

Glasses A, B, and C were codoped with 3-, 5-, and8-mass % Yb2O3, respectively. Er31 and Yb31 ion

ensities listed in Table 1 were calculated from theeasured glass densities and the batch constituents.dditional samples were doped with only Er31 or

Yb31 to measure the spectroscopic properties of theisolated ions in the silicate glass host.

B. Spectroscopic Characterization

Ground-state absorption was measured on 2-mm-thick samples with parallel polished faces. A com-mercial spectrophotometer was used to record theoptical density as a function of the wavelength. Tosubtract scattering and absorption losses that aredue to the host glass, low-order polynomials were

Used in Laser Experiments

GlassEr31 density~1020 cm23!

Yb31 density~1020 cm23!

A 0.85 2.47B 0.86 4.16C 0.88 6.83

880 APPLIED OPTICS y Vol. 38, No. 33 y 20 November 1999

fitted to the regions between the rare-earth absorp-tion bands and were then extrapolated under theabsorption peaks. This baseline was then sub-tracted from the raw data, resulting in a measure-ment of absorption that is due to the rare-earth ionsonly. The data were then converted to an absorptioncross section by use of the calculated ion densitiesfound in Table 1.

To determine the emission cross section for the4I13y23

4I15y2 transition of Er31 in this glass, we usedthe approximate McCumber method devised by Mi-niscalco and Quimby.11 The McCumber methoduses the measured absorption cross-section spectrumto derive the spectral shape of the emission crosssection. The approximate method employs mea-sured absorption and emission spectra to estimatethe Stark level spacing, the knowledge of which isnecessary for calculating the peak emission cross sec-tion. The technique is expected to predict peakemission cross-sectional values to within 20% of theactual values.

We acquired emission spectra by exciting the sam-ples with a cw Ti:sapphire laser operating at 979 nm.The infrared photoluminescence, collected with a mi-croscope objective, was focused on the entrance slit ofa 0.3-m-focal-length, single-grating monochromator~600 linesymm!. A slit width of 250 mm was used,giving a resolution of approximately 0.25 nm. A ger-manium detector was used along with a lock-in am-plifier to detect the photoluminescence signal.

We defined the spectral width by

Dl 5 * se~l!

se~lpeak!dl, (1)

where se is the calculated emission cross section andlpeak is the wavelength of peak emission. The radi-ative lifetime is also calculated from the derivedemission cross-sectional spectrum according to

1tr

5 8pcn2 * se~l!

l4 dl, (2)

where n is the refractive index of the glass. Sincethese quantities are calculated from the emissioncross-sectional spectrum, the maximum uncertaintyis approximately 25%.

To measure the excited-state lifetime of the 4I13y2level of Er31, the cw Ti:sapphire pump laser waspulsed with an acousto-optic modulator. The decay-ing signal was detected by an InGaAs p-i-n photode-tector with a 125-MHz bandwidth. Data wereacquired with a digital storage oscilloscope that wasused to average 128 successive decays. The re-sponse time ~90–10% fall time! of the system was 2.5ms. To determine the decay constant, a single expo-nential function was fitted to the decay data. Thereported lifetimes were obtained with approximately5 mW of pump power. No significant variation oflifetime with pump power was observed, indicatingthat cooperative upconversion makes a negligiblecontribution to the excited-state decay for such low

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pump powers. The uncertainty in lifetime measure-ments is estimated to be 5% or less.

A similar experimental arrangement was used tomeasure the excited-state lifetime of the 2F5y2 level ofYb31. Decay from this level was assumed to be purelyradiative in nature. Equation ~2! was then used toscale the measured Yb31 emission spectrum to obtainthe peak emission cross section for Yb31.

C. Waveguide Laser Fabrication and Testing

Waveguides were produced in the glass by K1 forNa1 ion exchange. A 150-nm-thick layer of alumi-num was deposited on 2-mm-thick glass substrates.Channels 3 mm in width were created in the masklayer by photolithography and wet etching. The ionexchange was carried out in a melt of 100% KNO3 for4 h at 400 °C. The remaining aluminum wastched away, and the waveguide end facets were cutnd polished.The dimensions of the optical modes of theaveguides at the signal wavelength were evaluatedy coupling a 1.5-mm LED into a waveguide while theutput of the waveguide was imaged onto an infraredamera. To estimate the waveguide loss, cutbackeasurements were performed on samples prepared

dentically to the laser samples. The loss was eval-ated at 860 nm, away from the broad Yb31 absorp-

tion spectrum and the Er31 absorption peak near 800nm.

Since the performance of a three-level waveguidelaser strongly depends on the length of the device,samples were fabricated with various lengths. Ini-tial device lengths for the three different hosts werechosen by the application of a waveguide laser modelthat uses several measured and estimated input pa-rameters.12 In glass A, lengths ranging from 1.8 to.9 cm were investigated. Lasers fabricated in glass

were between 1.2 and 2.0 cm long whereasaveguide lasers ranging from 1.2 to 1.7 cm in lengthere tested in glass C.For laser characterization, dielectric mirrors were

attached to the polished end facets of the waveguidelaser samples with index-matching fluid and held inplace by a small clip. The input mirror had a reflec-tance at 1536 nm of 99.9% and a transmittance at thepump wavelength of .90%. Output couplers withignal wavelength reflectances ranging from 60 to8% were used. All output couplers were also trans-issive at the pump wavelength. The lasers were

umped by a Ti:sapphire laser operating at 974.5 nm.ump light was coupled into the waveguides with a3 ~0.10 N.A.! microscope objective, and the output

signal light was collected by a 203 objective. Forignal power measurements the output from theaveguide was focused onto an InGaAs powermeter.or spectral measurements a spectrum analyzer with.2-nm resolution was used.

3. Results

A. Spectroscopic Properties

Figure 1 shows a comparison of the absorption cross

2

sections of the 4I15y234I11y2 transition of Er31 and

the 2F7y232F5y2 transition of Yb31 in IOG10 glass.

The peak of the Yb31 absorption cross section is 12 310221 cm2 at 975 nm whereas the peak of the Er31

absorption cross section is only 1.0 3 10221 cm2 at980 nm. The laser transition is the 4I13y23

4I15y2emission of Er31. The absorption and emission cross-sectional spectra for this transition are shown in Fig.2. Relevant spectroscopic quantities for Er31 andYb31 in IOG10 silicate glass are summarized in Table2.

B. Waveguide Characteristics

The waveguides supported a single transverse modeat the signal wavelength. The approximate dimen-sions of the signal mode are 20.5 mm wide by 11.5 mm

eep, measured at the 1ye points of the intensityistribution. Although the waveguides supported

Fig. 1. Comparison of Er31 and Yb31 absorption cross sectionsnear 980 nm in IOG10 glass.

Fig. 2. Comparison of Er31 absorption and emission cross sec-ions in IOG10 glass.


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Table 2. Various Spectroscopic Quantitites for Er31 and Yb31 in IOG10 Silciate Glass

6

multiple transverse modes at the pump wavelengthof 975 nm, an examination of the pump mode whilethe device was lasing showed that only the lowest-order pump mode appeared to be excited by the Ti:sapphire pump laser. The result is excellent overlapbetween the pump and the signal modes, which isindicated in Fig. 3. The pump mode measured 15.2mm wide by 7.0 mm deep. Uncertainty in the mode

imension measurements is estimated to be 10%.Results of the loss measurements are reported in

able 3. The quantity indicated as measured loss ishe average loss at 860 nm of multiple waveguides onsingle chip whereas the reported uncertainty is the

Spectroscopic Quantity

Peak Er31 4I15y2 34I11y2 absorption wavelength

Measured peak Er31 absorption cross sectionPeak Yb31 2F7y2 3

2F5y2 absorption wavelengthMeasured peak Yb31 absorption cross sectionMeasured 2F5y2 excited-state lifetimeCalculated peak Yb31 2F5y2 emission cross sectionPeak Er31 4I13y2 3

4I15y2 emission wavelengthEmission spectral widthCalculated peak Er31 4I13y2 emission cross sectionMeasured peak Er31 4I15y2 3

4I13y2 absorption crosCalculated radiative lifetime of Er31 4I13y2

Measured lifetime of Er31 4I13y2 level

Fig. 3. Near-field profile across the width of a K1-ion-exchangedwaveguide showing the overlap of the 1.5-mm signal ~dotted curve!and the 0.975-mm pump ~solid curve! modes.

Table 3. Waveguide Loss as Measured at 860 nm with the CutbackMethod and Corrected for the Nonzero Yb31 Absorption at 860

Glass~Yb31:Er31!

Measured Loss~dBycm!

Corrected~dBycm!

A ~3:1! 0.25 6 0.11 0.17B ~5:1! 0.32 6 0.08 0.20C ~8:1! 0.66 6 0.13 0.45


tandard deviation of the mean. The loss measure-ents were also corrected for a small, but nonzero,b31 absorption coefficient at 860 nm. The

waveguide loss increases as a function of the totalrare-earth content of the glass. To estimate thewaveguide loss at 1.5 mm, the loss measured at 860nm was scaled by l-4. This estimate predicts theosses at 1.5 mm to be less than 0.08 dBycm for eachlass, thus indicating the potential for low-lossaveguides with the K1-ion-exchange technique.

However, it should be noted that other loss mecha-nisms, such as loss due to roughness of the glasssurface, are likely to dominate at longer wavelengths.The waveguide losses at 1.5 mm are further examinedbelow as a fitting parameter in comparing the exper-imental laser data with a laser simulation.

C. Laser Characteristics

Qualitatively similar spectral characteristics wereobserved in all the laser devices tested. Close tothreshold, lasing was normally observed near 1536nm whereas at higher pump powers the devices lasedsimultaneously at both 1536 and 1545 nm. A typicallaser spectrum under high pump power is shown inFig. 4. The 3-dB full width of each laser peak is lessthan 0.5 nm.

To compare the laser performance for each of thethree laser host glasses, it was necessary to simulta-neously optimize the device length as well as theoutput coupler reflectance. The laser performancewas compared on the basis of slope efficiency as wellas launched pump power threshold. The launchedpump power greatly depends on the coupling effi-ciency. To measure the coupling efficiency, the de-vice was mounted into the laser test bed and thedevice throughput was maximized and measured.The appropriate loss figure listed in Table 3 was as-sumed, and the measurement was corrected for thetransmittance of the input and the output optics.Coupling efficiencies were measured for each lasersample and were typically between 55% and 70%.We determined the reported slope efficiencies by fit-ting lines to the laser data. The reported launchedpump power thresholds were determined from a lin-

Measured Value

980 nm1.0 3 10221 cm2

975 nm12 3 10221 cm2

1.3 ms19 3 10221 cm2

1536 nm32 nm

5.8 3 10221 cm2

tion 5.7 3 10221 cm2

17.8 ms10.2 ms

s sec

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ear fit to the experimental points for pump powersclose to threshold.

Figure 5 shows the laser output power near 1536nm versus launched pump power at 974.5 nm for a1.80-cm-long device fabricated in glass A with variousoutput couplers. When lasing is performed with a98% reflecting mirror as output coupler, the devicehas a slope efficiency of 1.3% and a threshold of32-mW launched pump power. The slope efficiencyincreases with the output coupler throughput until amaximum slope efficiency of 5.2% is achieved with an80% reflector. The slope efficiency then falls off withmore transmissive output couplers. The launchedpump power threshold generally increases with thethroughput of the output coupler. Although only 32mW is required for lasing with a 98% reflector used asthe output coupler, 148 mW of pump power is neces-

Fig. 4. Typical laser spectrum from Er31yYb31 codoped silicatelass waveguide laser with high pump power.

Fig. 5. Signal power versus launched pump power for a 1.80-cm-long laser fabricated from glass A by use of output couplers withdifferent reflectances.

2

sary for observing lasing with an output coupler witha reflectance of 60%.

A similar investigation of the laser performance asa function of output coupler reflectance was also com-pleted for the other two glass hosts, which containgreater concentrations of Yb31. While the 80% re-flecting output coupler gave optimal performance forlasers fabricated in glass A, lasers fabricated in bothof the other glasses achieved superior performancewith a 70% reflecting output coupler. Figure 6shows the laser slope efficiency for devices in eachglass as a function of the output coupler reflectance.

We were able to reproduce this behavior of thelasers as a function of the output coupler reflectance,using an extensive waveguide laser model that hasbeen described in detail elsewhere.12 Measuredspectroscopic properties ~see Table 2! were input tothe model, and other quantities were used as fittingparameters. The fitting parameters included thewaveguide loss at the signal wavelength a1.5, theupconversion coefficient Cup, and the Yb31–Er31

cross-relaxation coefficient Ccr, which describes theenergy-transfer efficiency from Yb31 to Er31. The up-conversion coefficient was further restricted to be thesame for all three glasses, since it should depend onlyon the Er31 concentration and not on the Yb31 con-centration. The experimentally determined peakcross section for stimulated emission from the 4I13y2level of Er31 se~Er! had a major effect on the resultsof the simulation, and as a result it was allowed tovary within the 20% error bars typical for the methodused in its calculation. Comparisons of slope effi-ciency as well as pump power at threshold are shownas a function of the output coupler reflectance in Fig.7 for the 1.80-cm-long device fabricated from glass A.Qualitatively similar data are obtained from themodel for the other two glasses. The fitting param-eters used in the simulation of the experimental re-sults are listed in Table 4.

Fig. 6. Slope efficiency versus output coupler reflectance for la-sers fabricated in silicate glasses with different Yb31:Er31 ratios.


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Relevant experimental data for the laser producedin each glass that exhibited the highest slope effi-ciency can be found in Table 5. Signal power versuspump power data for these devices are plotted in Fig.8. The best overall performance, on the basis ofslope efficiency, was realized with a 1.68-cm-long de-vice fabricated from glass B. The maximum slopeefficiency from this device was 6.5% when a 70%reflector was used as the output coupler. The devicelased with a threshold pump power of 87 mW andachieved 19.6 mW of output power near 1536 nm with400 mW of input pump power at 974.5 nm.

Fig. 7. Slope efficiency and threshold pump power as a function ofoutput coupler reflectance for a 1.80-cm laser fabricated from glassA. Circles and crosses, experimental data; curves, model simula-tions with the parameters in Tables 2 and 4.

Table 4. Fitting Parameters Used to Model the Waveguide LaserPerformance as a Function of Rare-Earth Dopant Ratio

Glass~Yb31:Er31!

a1.5

~dBycm!Ccr

~cm3 s21!Cup

~cm3 s21!se ~Er31!

~cm2!

A ~3:1! 0.04 2.4 3 10217 2.0 3 10218 5.3 3 10221

B ~5:1! 0.19 5.0 3 10217 2.0 3 10218 5.7 3 10221

C ~8:1! 0.28 6.0 3 10217 2.0 3 10218 5.3 3 10221

Table 5. Performance Data for Highest Slope Efficiency DevicesFabricated in IOG10 Silicate Glass with Different Yb31:Er31 Dopant

Ratios

Glass~Yb31:Er31!

DeviceLength

~cm!

OutputCoupler

Reflectance~%!

SlopeEfficiency

~%!

LaunchedPump

Threshold~mW!

Outputwith

400-mWPump~mW!

A ~3:1!~3:1! 2.94 80 5.5 96 17.0B ~5:1! 1.68 70 6.5 87 19.6C ~8:1! 1.42 70 5.0 241 8.1


4. Discussion

The dimensions of the optical mode reported aboveare large in comparison with the 3-mm apertureshrough which the ion exchange was performed.his results from the fact that the host glass is aixed-alkali silicate containing a significant amount

f potassium. Consequently, potassium-for-sodiumon exchange in this glass leads to only a smallhange in the index of refraction, and the opticalode is not tightly confined. The change in the in-

ex of refraction resulting from the ion exchange wasstimated from the waveguide mode indices at 632.8m to be no more than 0.005. The potential conse-uence of the large optical mode is significant absorp-ion in the tails of the signal mode. However, thearge modal cross section has several benefits. Thearge signal mode allows for the incorporation of areater number of active rare-earth ions in the laseravity, and therefore the potential for high outputower before saturation is reached. The large pumpode allows for easy coupling of pump light into theaveguide. In addition, as a result of the small in-ex change, fewer pump modes are supported by theaveguide. This lack of mode structure leads to im-roved overlap between the pump and the signalodes.Er31yYb31 codoped glass lasers use energy trans-

fer between Yb31 and Er31 to obtain efficient pumpabsorption near 980 nm and lasing at 1.5 mm. As aesult, an important quantity to characterize for aaser host is the energy-transfer probability betweenhe two ions. Unfortunately, because of the largeumber of interactions involved, it is difficult to mea-ure this quantity directly. By modeling the exper-mental laser results, we are able to give estimates forhe cross-relaxation coefficient. As indicated in Ta-le 4, the cross-relaxation coefficient increases withhe Yb31 concentration from 2.4 3 10217 cm3 s21 for

glass A with a Yb31 concentration of 2.47 3 1020 cm23

Fig. 8. Output signal power versus launched pump power fordevices with optimized output coupler and length in glasses withdifferent Yb31:Er31 ratios.

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to 6.0 3 10 cm s for glass C with a Yb con-centration of 6.83 3 1020 cm23. These numbers,along with the best-fit upconversion coefficient of2.0 3 10218 cm3 s21, are in agreement with previ-ously reported data for glasses with comparable rare-earth concentrations.13,14 Studies have suggestedthat, for Yb31 concentrations below 5 3 1020 cm23, atrong linear or quadratic dependence of Ccr on NYb

can be expected whereas for higher Yb31 concentra-tions the dependence is slower.13,15

The simulation gives good agreement with the ex-perimental data, despite some approximations. Ab-sorption and emission cross sections for the peakEr31 emission at 1536 nm are input to the model.

owever, at high pump powers, these devices lase notnly at 1536 nm but also on the 1545-nm transition.n addition, the backtransfer process from Er31 to

Yb31 is not explicitly included in the model. Thegreement between experiment and theory, however,uggests that this host glass is not plagued by a sig-ificant amount of backtransfer. Finally, althougheasurements indicate that the majority of the pump

ower is carried by the fundamental pump mode,ome power is undoubtedly carried in higher-orderump modes. These higher-order modes, which areot accounted for in the model, will have significantlyeduced overlap with the signal mode.

One of our research goals was to investigate exper-imentally the optimal ratio of sensitizing Yb31 ions tolasing Er31 ions. The data above indicate that, asthe Yb31:Er31 ratio is increased from 3:1 to 5:1,

igher slope efficiency is possible. The pump powerequired for reaching threshold is essentially theame for both ion ratios as a result of the longerength of the device fabricated in the 3:1 glass. Inddition, in glass B, doped at a ratio of 5 Yb31 per

Er31, it is possible to attain ;10% more output powerwith 400 mW of pump power launched into thewaveguide. When the dopant ratio is increased to8:1, no further improvement in slope efficiency is re-alized, and there is a substantial penalty in the pumppower required for reaching threshold. Therefore aYb31:Er31 dopant ratio of approximately 5:1 appearso give optimal performance.

Strictly speaking, this result applies only to thearticular host glass studied here, since the spectro-copic properties of rare-earth ions are host depen-ent. However, the variation of spectroscopicuantities for erbium-doped oxide glasses is typicallyfactor of only 2 or less.16 As a result, repeating the

study with different glass hosts may lead to a refine-ment of the 5:1 optimal ratio, but 5:1 is likely to be agood starting point for such an investigation.

These experiments were conducted in glass hostsdoped with 1% Er2O3 by mass. For hosts with dif-ferent Er31 concentrations the important quantity isnot likely to be the exact ratio of Yb31:Er31 but ratherthe average separation between Yb31 and Er31 ions.

or glass B, with a total rare-earth density of 5.02 3020 cm23, the ytterbium and the erbium ions are

separated by 0.78 nm, assuming a homogeneous dis-tribution of rare-earth ions.

2

The output power obtainable in these devices waslimited only by the available pump power and was farfrom any significant saturation behavior. This wasconfirmed by measurements of absorbed pumppower. The measured absorption coefficient in glassB is 3.4 cm21 at 974.5 nm. This suggests an unsat-urated absorption of greater than 99% in the 1.68-cm-long device. Measurements of the transmittedpump light performed while the device was lasingindicate a pump absorption of approximately 96%with 300 mW launched into the waveguide. Withsuch high pump absorption it is likely that the Er31

population on the end of the device farthest from thepump source is inverted due to signal reabsorptionand not to energy transfer from Yb31.

Finally, as indicated above, the relatively largetransverse dimensions of the pump mode resulted inhigh coupling efficiency of the pump light into thewaveguide. As a result, the requirements for inci-

ent pump power may be reduced. For example, the.68-cm-long glass B device had a measured pump-oupling coefficient of 67%, resulting in a slope effi-iency versus incident pump power of 4.5% with the0% reflecting output coupler. By comparison, Ag1-

ion-exchanged waveguides fabricated in our labora-tory,17 which typically have mode dimensions at 1.5mm of approximately one third of those reported here,and waveguides produced by Tl1 ion exchange,4 haveeach had coupling coefficients of only ;35%.

5. Conclusions

We have investigated a chemically durable,phosphorus-free, mixed-alkali silicate glass, whichhas been codoped with Yb31 and Er31, as a host glassfor waveguide lasers operating at 1.5 mm.

aveguides were produced in this glass by a simplene-step K1-ion exchange. The use of K1-ion ex-

change with a mixed-alkali ~potassium-containing!host glass has resulted in waveguides with relativelylarge mode volumes for both signal and pump, as wellas excellent overlap between the signal and the pumpmodes, owing to efficient coupling into the lowest-order pump mode.

The laser performance of codoped IOG10 glass wasmeasured as a function of the ratio of Yb31 to Er31

ions. The device length and the output coupler re-flectance were simultaneously optimized. The ex-perimental results indicate that superiorperformance is achieved with a ratio of approxi-mately five Yb31 ions per Er31 ion. Output powersof as high as 20 mW have been realized fromwaveguide lasers fabricated in this silicate glass.The laser performance was compared with theoreti-cal results from a laser model that is used to extractinformation about the cross-relaxation and upconver-sion coefficients.

The results suggest at least two directions for fu-ture research. First, the long measured excited-state lifetime of erbium in this glass ~10.2 ms!ndicates that the glass does not suffer from concen-ration quenching at the current Er31 level. Higher

Er31 concentrations may lead to improved device per-


erbium13–ytterbium13 codoped glass waveguide laser,” IEEE

1

1

6

formance if the long excited-state lifetime can bemaintained. An investigation of more heavily dopedglasses is being conducted. Second, the adjustmentof the pre-ion-exchange alkali content of the hostglass has presented itself as a means of tailoring themode profile of the waveguide while maintain-ing single-transverse-mode performance. Such amethod may allow for matching of the waveguidemode to a desired pump input andyor signal outputmode. We are studying the effect of the alkali con-tent of the base glass on the mode diameter, as wellas the pump-signal modal overlap, with the goal ofreducing the required pump power while maintain-ing the achievable output power.

P. M. Peters acknowledges the support of the U.S.Air Force Research Laboratory under the PalaceKnight Internship Program during the time this re-search was conducted. Research at the Universityof Rochester was supported by National ScienceFoundation grant DMR-9612267.

References and Note1. T. Feuchter, E. K. Mwarania, J. Wang, L. Reekie, and J. S.

Wilkinson, “Erbium-doped ion-exchanged waveguide lasers inBK-7 glass,” IEEE Photon. Technol. Lett. 4, 542–544 ~1992!.

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Ion-exchanged waveguide lasers in Er^3+/Yb^3+ codoped silicate glass

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