Journal of Non-Crystalline Solids 377 (2013) 100–104

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Journal of Non-Crystalline Solids

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Spectroscopic investigation of Nd3+ single doped and Eu3+/Nd3+ co-dopedphosphate glass for solar pumped lasers

Nadia G. Boetti a,⁎, Davide Negro a, Joris Lousteau a, Francesca S. Freyria a, Barbara Bonelli a,Silvio Abrate b, Daniel Milanese a

a PhotonLab, Dipartimento di Scienza Applicata e Tecnologia, Istituto di Ingegneria e Fisica dei Materiali, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129, Torino, Italyb PhotonLab, Istituto Superiore Mario Boella, Via P.C. Boggio, 61, 10138, Torino, Italy

⁎ Corresponding author. Tel.: +39 0112276312; fax:E-mail address: nadia.boetti@polito.it (N.G. Boetti).

0022-3093/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.jnoncrysol.2013.01.004

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 September 2012Received in revised form 28 November 2012Available online 31 January 2013

Keywords:Solar pumped laser;Phosphate glass;Eu3+ Nd3+ co-doping;Energy transfer;Spectroscopic properties

We report on the fabrication and characterization of Nd3+ doped phosphate glass to be used as active laser me-dium for developing solar pumpedfiber laser emitting at 1.06 μm. Several Nd3+-doped phosphate glass sampleswere fabricated, with a host composition of (mol%) 65P2O5:17Li2O:3Al2O3:4B2O3:5BaO:6La2O3 and with a con-centration of Nd3+ up to 10 mol%. In order to exploit the unabsorbed solar energy by Nd3+ ions, Eu3+

co-doping of Nd3+-doped phosphate glasses was also investigated to asses a potential increase in the overallpump power conversion efficiency. Physical and thermal properties of single doped and co-doped sampleswere measured and their spectroscopic properties are discussed. The effect of Nd3+ doping concentration onemission spectra and lifetimes was investigated in single doped samples in order to study the concentrationquenching effect on luminescence performance. The shape of the fluorescence spectrum did not change by in-creasing the Nd3+ doping level while the lifetime of the Nd3+:4F3/2 was found to decrease. The following char-acteristic value parameters were calculated: radiative lifetime τ0=367 μs and quenching concentration N0=8.47 E+20 ions/cm3.The Eu3+ co-doping was found to give a limited increase to the glass pump power absorption limited to theUV range. Although energy transfer from Eu3+ to Nd3+occured, a large Eu3+ concentration dependentquenching of Nd3+

fluorescence was observed. This latter process decreases the radiative quantum efficiencyof Nd3+:4F3/2 emitting level and hence limits the attractiveness of Eu3+ sensitization for Nd3+ laser action.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

The idea of directly converting incoherent broadband solar radia-tion into narrow-band and coherent laser light, is almost as old as thelaser itself [1–3]. The first demonstration of a solar pumped laser(SPL) was obtained in 1963 using Dy3+ ions as active element in aCaF2 matrix [1]. Soon after, a 1 W emission from a sun pumped con-tinuous wave Nd3+:YAG laser was reported [3]. From then on, thechoice of Nd3+: YAG technology for the realization of SPL has becomepractically a standard: YAG as host material because it is a stable com-pound,mechanically robust and it is able to accept trivalent laser activa-tor ions such as rare earth (RE) elements; Nd3+ as an emitting ion,because it presents a very strong absorption band where sun spectraldensity is almost maximum.

After these first reports several improvements have been reported.Among them, the largest output power of 500 W was achieved in1993 by side-pumping a Nd3+:YAG rod with light concentrated from

+39 0112276299.

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a 660 m2 collecting mirror system [4]. However, in this work the col-lection efficiency, i.e. the ratio of laser power to primary mirror area,was very low, around 0.76 W/m2. The record collection efficiencyreported with a Nd3+:YAG rod was 6.7 W/m2 [5]. In recent years afurther increase of solar laser power efficiency was obtained usingan alternative solar power collection means, i.e. a Fresnel lens track-ing automatically the sun trajectory, and by using as active medium achromium (Cr3+) co-doped Nd3+:YAG ceramic in order to enhancethe absorption of solar radiation [6,7].

Despite the long history of the research on SPLs as novel renew-able energy source, in order to spread this technology worldwide,reliability, cost performance and optical-to-optical conversion effi-ciency of the system must be improved considerably. We believethat this can be achieved by utilizing Nd3+-doped pumped fiberlasers made of phosphate glass materials. It is necessary for fiberlasers to make use of glass material, which can be easily drawn intoan optical fiber. The phosphate glass system has been chosenbecause it possesses a large glass formation region, a high solubilityfor rare earth oxides, good thermo-mechanical and chemical proper-ties and no evidence of photodarkening even at high populationinversion [8,9]. All these features make phosphate glass a very

Table 1Molar and ionic concentration of dopants in the manufactured phosphate glasssamples.

Glass label Dopant concentration

[mol%] [×1020 ions/cm3]

Nd3+ Eu3+ Nd3+ Eu3+

Nd1 0.1 0.134Nd2 1 1.32Nd3 2 2.66Nd4 4 5.33Nd5 6 7.72Nd6 10 13.3Eu7 4 5.3EuNd8 2 4 2.65 5.29EuNd9 4 4 5.29 5.29EuNd10 6 4 7.93 5.29EuNd11 2 8 2.64 10.5

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potential host material for compact high-power and single-frequencyfiber lasers.

Concerning the doping ion, the choice of Nd3+ as a doping ion hasbeen driven by the overlapping ratio between the standard solar spec-trum and the ion absorption bands that was calculated to be 0.14 [10].Moreover Nd3+ is one of the most important RE activators for crystal-line and bulk glass lasers because of the power and efficiency availablefrom the transition at around 1064 nm. ANd3+-doped laser operated atthis wavelength behaves like a four-level laser system, so a positive in-ternal gain is possible even for a very small pump power and, therefore,a very low threshold can be achieved.

The use of an optical fiber configuration as active medium of thesolar laser will exploit the benefit of the optical confinement providedby a waveguide structure resulting in a low threshold and a high op-tical conversion efficiency.

Another attractive feature of fiber lasers includes outstandingheat-dissipating capability thanks to their high surface-to-volumeratio. By contrast, thermal effects that can degrade the beam qualityand fracture the laser rod are key limitations of bulk lasers and re-quire efficient and expensive cooling system to avoid heat loads.Moreover, fiber laser generates a diffraction-limited beam qualitythat allows strong focusing and maximizes the output intensity. Anovelty of the proposed solar laser system is the side pumping ofthe optical fiber by concentrated solar light. An optical fiber is verythin and flexible, thus it can be coiled in a desired area. In this waya perfect match between solar concentrator image in the focal planand pumping absorbing area could be achieved thus resulting in ahigher conversion efficiency.

The idea of a solar pumped fiber laserwas proposed for the first timein 1997 [11] and recently the first realization under natural sunlight hasbeen reported [12]. However in this work the use of an end-pumpingconfiguration results in a very low output power of less than 1 mW.Moreover the chosen host material, fluoride glass, suffers from poorchemical durability and temperature stability and requires stringentmanufacturing process in order to avoid crystallization.

In order to increase the SPL efficiency, Eu3+ co-doping of Nd3+-dopedphosphate glasses has been evaluated, because it potentially enablesan increase in the overall pump power conversion efficiency becauseof the transfer of the unused solar energy in the UV region by radia-tive sensitization to wavelengths absorbable by the laser material.Previous works [13–15] on Eu3+/Nd3+ co-doping led to differentconclusion concerning the increase of the Nd3+

fluorescence due tothe energy transfer between Eu3+ and Nd3+.

In this work we present a preliminary study towards the realizationof a solar pumpedfiber laser, focused on the evaluation of the activema-terial performances.We report physical and thermal properties of Nd3+

single doped and Eu3+/Nd3+ co-doped phosphate glass samples andtheir spectroscopic investigation.

2. Experimental

Glass samples (30 g batch) used in this research were preparedby conventional melt-quenching technique using chemicals of99+% purity. The host glass with the composition of 65P2O5:17Li2O:3Al2O3:4B2O3:5BaO:6La2O3 (mol%) was ad hoc developed for this re-search in order to have a stable and robust glass, able to incorporatehigh amount of RE and suitable for fiber drawing.

A first group of glasses, named for short Nd1÷Nd6, was obtaineddoping the host material with an increasing level of Nd2O3 (rangingfrom 0.05 to 5 mol%) added in substitution of La2O3. A second groupof glasses, named for short Eu7, EuNd8÷EuNd11, was manufacturedin order to evaluate the effect of Eu3+ co-doping. They were obtainedby doping the host glass with different amounts of Eu2O3 (2 and4 mol%) and Nd2O3 (ranging from 0 to 2 mol%), both added in substitu-tion of La2O3. The full list of glass samples manufactured for this re-search and their doping concentration is reported in Table 1.

After weighting and mixing, the batched chemicals were meltedwithin a chamber furnace at a temperature of 1500 °C, under driedair atmosphere with a water level lower than 2 ppmv. The meltwas cast into a brass mold preheated at 480 °C and annealed at thesame temperature for 10 h. The obtained glasses were cut and opti-cally polished with a diamond paste to 2 mm thick samples for opti-cal and spectroscopic characterization.

Density of samples was measured by Archimedes method usingdistilled water as an immersion fluid. The dopant ion concentrationswere calculated from measured sample densities and their initialcompositions.

Thermal analysis was performed on fabricated glasses using aNetzsch DTA 404 PC Eos differential thermal analyzer up to 1400 °Cunder Ar flow with a heat rate of 10 °C/min in sealed Pt pans usingtypically 30 mg samples. Thermal analysis was carried out in orderto measure the characteristic temperatures Tg (glass transition tem-perature) and Tx (onset crystallization temperature). An error of±3 °C was observed in measuring the characteristic temperatures.

The refractive index of the glasses was measured at 5 differentwavelengths by prism coupling technique (Metricon, model 2010).Five scans were performed for each wavelength, the average valueof these five measurements representing the refractive index. Esti-mated error on the measurement was 0.001.

The absorption spectra were measured at room temperature forwavelength ranging from 350 to 3000 nm using a double beam scan-ning spectrophotometer (Varian Cary 500).

Fluorescence spectra of Nd3+ single doped samples (Nd1÷Nd6)were acquired using a Jobin Yvon iHR320 spectrometer equippedwith a Thorlabs PDA10CS and standard lock-in technique. The sam-ples were excited using two different excitation sources: a mono-chromatic light at the wavelengths of 785 nm, emitted by a singlemode fiber pigtailed laser diode (Axcel B1-785-1400-15A) and a broad-band visible light emitted by a supercontinuum source (Coheras SuperKExtreme), in order to simulate the broadband solar light.

The fluorescence lifetime of Nd3+:4F3/2 level, in single doped andco-doped samples, was obtained by exciting the samples with lightpulses of 785 nm single mode fiber pigtailed laser diode, recordingthe signal by a digital oscilloscope (Tektronix TDS350) and fitting thedecay traces by single exponential. Fluorescence spectra and lifetimemeasurements were collected in both wavelength regions by excitingthe samples at the very edge in order to minimize reabsorption.

Emission spectra of samples Eu7 and EuNd8÷EuNd11 wereobtained using a Perkin Elmer LS 55 fluorescence spectrometerequipped with a Xenon lamp as excitation source. The signal wasdetectedwith a red-sensitive R928 photomultiplier. For measurements,excitation and emission wavelengths were set at 390 nm and between

Fig. 1. Absorption cross-section spectrum for sample EuNd9. The inhomogeneouslybroadened bands due to the Nd3+ ion are assigned (in black) to the transitions fromthe ground state 4I9/2 to the excited states of Nd3+ ion. The inhomogeneously broad-ened bands due to the Eu3+ ion are circled in red and are assigned (in red) to the tran-sitions from the ground state 7F0 to the excited states of Eu3+ ions. In green, forreference, the solar spectral irradiance at sea level is reported (ASTM G173-03).

102 N.G. Boetti et al. / Journal of Non-Crystalline Solids 377 (2013) 100–104

550 and 750 nm respectively and excitation and emission slit widthswere 10 nm.

All measurements were performed at room temperature.

3. Results and discussion

3.1. Physical and thermal properties

Table 2 reports the typical physical property values of themanufactured glass samples. These values did not change by chang-ing the doping level of the glasses, due to the fact that the doping ox-ides, Nd2O3 and Eu2O3, were both added in substitution of La2O3 inthe host matrix.

The measured characteristic temperatures, Tg and Tx, indicate satis-fying thermal properties of the glasses. Their measurement allowedassessing the corresponding glass stability parameter ΔT=Tx−Tgthat is commonly used as an estimate of the glass stability and of glassability to be processed in fiber form without crystallization. Theobtained value is around ΔT=207±6 °C, which suggests that theseglasses are suitable for performing fabrication and crystal-free fiberdrawing.

Concerning the linear thermal expansion coefficient, although thisvalue remains an order of magnitude higher than silica glass, it is none-theless suitable for practical applications, in particular in the form ofthin glass fibers.

3.2. Absorption

UV–VIS–NIR spectroscopy was carried out on all prepared samplesand absorption spectra were recorded.

Fig. 1 shows the normalized absorption spectra obtained for theco-doped glass sample EuNd9. Most of the inhomogeneouslybroadened bands are due to the Nd3+ ion and they are assigned tothe transitions from the ground state 4I9/2 to the excited states ofNd3+ ion (see for reference Fig. 2).

The exceptions are the bands circled in red, that are due to Eu3+ ion;they are assigned to the transitions from the ground state 7F0 to the ex-cited states of Eu3+ ions (see for reference Fig. 2). Other Eu3+ absorp-tion bands usually observed in glasses [16] are not visible because ofthe strong absorption bands of Nd3+ ions at the same wavelengths.

In Fig. 1, for reference, the solar spectral irradiance at sea level is alsoreported. Asmentioned above, Nd3+presents several strong absorptionbands in the visible, where the sun has its maximum emission, makingit a suitable doping ion for the development of a SPL. The Eu3+ ion ab-sorption bands are, as expected,mostly located in theUV spectral range,but they are small if compared to the ones measured for Nd3+, so theircontributions to the total absorption of the glass are not so relevant.

3.3. Fluorescence

Fluorescence spectra of the Nd3+ single doped glass samples,Nd1÷Nd6, were measured in the wavelength range 1000÷1500 nmunder excitation at 785 nm. A typical spectrum is shown in Fig. 3. Theemission spectrum consists of two large and non-symmetricbands: a main peak centered around 1055 nm assigned to thetransition 4F3/2→ 4I11/2 and a less intense peak centered around

Table 2Typical physical property values of the manufactured glass samples.

Density 2.88±0.05 g/cm3

Glass transition temperature (Tg) 495±3 °CCrystallization temperature (Tx) 702±3 °CGlass stability parameter (ΔT=Tx−Tg) 207±6 °CCoefficient of thermal expansion 10 E-6 °C−1

Refractive index at 633 nm 1.551±0.001

1325 nm corresponding to the 4F3/2→ 4I13/2 transition of Nd3+ ion(see Fig. 2).

Further analysis on the fluorescence spectra indicated that theshape of the main emission peak did not change with an increasingNd3+ doping level. Fig. 4 reports the peak height of the 1055 nmemission with respect to the Nd3+ ion concentration. It can be ob-served that the peak emission intensity increases steadily with therising doping concentration, due to the increasing absorption ofpump source at the beginning stage. But when Nd3+ concentrationbecomes higher than 7.7×1020 ions/cm3 (sample Nd5) the absorp-tion saturation of pump source is reached and the rate of the increaseof emission intensity starts decreasing. Moreover, for high Nd3+ ionconcentration, unwanted energy transfer between Nd3+ ions occurs,hence decreasing the fluorescence intensity.

Themain emission peak at 1055 nmwas alsomeasured exciting theNd3+ single doped samples with a broadband visible light emitted by asupercontinuum source. As expected, the spectra obtainedwith the twodifferent excitation sources are the same [17], in spite of the distinct ex-citation paths. This observation demonstrates the validity of using thesingle frequency source as excitation light for characterizing our sam-ples and then extrapolates those results to the case of a polychromaticpump source, like the sun.

Eu3+ ion fluorescence was measured under the excitation at390 nm either in a single doped sample (Eu7) or in the presence ofNd3+ (EuNd8÷EuNd11). Fig. 5 reports the emission spectra of samples

Fig. 2. Schematic energy level diagram of Eu3+ and Nd3+ ions.

Fig. 3. Emission spectra of the sample Nd5 (6 mol% Nd3+ concentration) under excita-tion at 785 nm.

Fig. 5. Fluorescence spectra of samples doped with a fixed amount of Eu3+ (2 mol%concentration) and an increasing concentration of Nd3+, ranging from 0 (blackcurve) to 6 mol% (green curve).

103N.G. Boetti et al. / Journal of Non-Crystalline Solids 377 (2013) 100–104

doped with a 2 mol% fixed amount of Eu3+ and an increasing concen-tration of Nd3+, ranging from 0 (black curve) to 6 mol% (green curve).

Eu3+fluorescence decreasedwith the increasing of Nd3+ content

due to the energy transfer between those ions. The decrease is higherfor the peak at 593 nm compared to the one at 618 nm, due to theoverlapping with the strong Nd3+ absorption band centered at thiswavelength. Possible energy transfer between ions could occurbetween the level Eu3+:5D0 and Nd3+: 2G7/2, 4G5/2 or betweenEu3+:5D0 and Nd3+:4F5/2, 2H9/2 (see Fig. 2).

3.4. Time resolved fluorescence

The fluorescence lifetime determines the decay rate for radiationfrom a particular transition. It is an essential parameter for the charac-terization of the emission properties of RE ions in a host medium, andtherefore its suitability for active optical devices. Moreover, the fluores-cence lifetime is one of the requested parameter for laser design.

For a Nd3+-doped laser operated at 1.06 μm, the important pa-rameter is the 4F3/2 lifetime. A longer lifetime of level 4F3/2 benefitsthe population inversion between this level (the upper laser level)and the 4I11/2 one (the lower laser level).

The decay curves of the Nd3+:4F3/2 were measured under 785 nmexcitation for all single doped and co-doped samples and the fluores-cence decay time are listed in Table 3.

Considering the single doped samples, Nd1÷Nd6, it can be ob-served that the lifetime of the Nd3+:4F3/2 state decreases with the

Fig. 4. Dependence of the peak height of the Nd3+:4F3/2→4I11/2 transition on Nd3+ ionconcentration.

increasing doping concentration, because of the unwanted energytransfer between ions at high concentration level. This concentrationdependence, referred to as concentration quenching, has importantimplications for the performance of active devices because it resultsin the loss of excitation, and thus an efficiency reduction. This effectcould be described by the empirical formula proposed by Auzel et al.[18]:

τ Nð Þ ¼ τ0

1þ 92π

NN0

� �2

where τ is themeasured lifetime at a given Nd3+ ion concentration (N),τ0 is the lifetime in the limit of “zero” concentration, i.e. the radiativelifetime and N0 is the quenching concentration.

The experimental data were fitted by the above formula and goodagreement (R=0.988) with the fit curve for the whole range of con-centration was found (Fig. 6). This indicates that the decrease of theNd3+:4F3/2 lifetime with the increasing doping concentration is cor-rectly described by the expression corresponding to the case of diffu-sion limited situation [18] in which the quenching rate is proportionalto the square of concentration. The following values of fitting parame-ters were obtained: τ0=367 μs and N0=8.47 E+20 ions/cm3. Theseresults agree with the values reported in literature [19,20].

Considering samples Nd3, EuNd8 and EuNd11, they have a fixedNd3+ concentration of 2 mol% and an increasing Eu3+ concentra-tion, ranging from 0 to 8 mol%. In these samples, the lifetime oflevel Nd3+:4F3/2 has a sharp decrease with the increasing Eu3+ con-centration, reducing from 327 μs in the absence of Eu3+ to 50 μs inthe co-doped sample EuNd11 (8 mol% of Eu3+). This result indicatesthat there is an Eu3+ concentration dependent quenching of Nd3+

Table 3Fluorescence decay time of the 4F3/2 state of Nd3+ ion in single doped and co-dopedsamples under laser excitation at 785 nm.

Glass label Dopant concentration Nd3+:4F3/2 lifetime

[mol%] [μs]±15 μs

Nd3+ Eu3+

Nd1 0.1 365Nd2 1 358Nd3 2 327Nd4 4 212Nd5 6 177Nd6 10 92EuNd8 2 4 103EuNd11 2 8 50

Fig. 6. Experimental and fitted values of Nd3+:4F3/2 excited state lifetime in singledoped samples Nd1÷Nd6.

104 N.G. Boetti et al. / Journal of Non-Crystalline Solids 377 (2013) 100–104

fluorescence due to unwanted interactions between the two ionspecies, as already observed in [14] and as indicated in Fig. 2.

Lower lifetime means higher decay rate from the Nd3+:4F3/2 emit-ting level due to non-radiative decay process towards the Eu3+ energylevel. We can estimate the efficiency of this energy transfer using thefollowing formula [21]:

P ¼ 1− ττ0

� �

where τ0 and τ are the lifetime Nd3+:4F3/2 in absence and presence ofEu3+ co-doping. The calculated efficiency of the transfer for theco-doped samplewith the higher amount of Eu3+ ions (EuNd11) is 85%.

These non-radiative decay processes will reduce the radiative quan-tum efficiency of the Nd3+:4F3/2 emitting level and hence will limit theusefulness of Eu3+ sensitization for Nd3+ laser action.

4. Conclusion

This paper reports the fabrication and characterization of a novelNd3+ doped phosphate glass system proposed for the development ofa solar pumped fiber laser. All prepared glass samples were homoge-neous and presented a good thermal stability and thus are suitable forfiber drawing.

In the Nd3+ single doped glass a strong emission at 1.06 μm wasmeasured for each sample by using two different excitation sources: alaser at 785 nm and a polychromatic light, confirming the possibilityto pump these glasses by concentrated sunlight.

The effect of Nd3+ doping concentration on emission spectra andlifetimes was investigated in order to study the concentrationquenching effect on luminescence performance. The shape of thefluorescence spectrum did not show any changes by increasing the

Nd3+ doping level while the lifetime of the Nd3+:4F3/2 was foundto decrease. The following characteristic value parameters werecalculated: radiative lifetime τ0=367 μs and quenching concentra-tion N0=8.47 E+20 ions/cm3.

In order to increase the efficiency of the active material for SPL,Eu3+ co-doping of Nd3+-doped phosphate glass was evaluated. Theco-doping gave a limited increase to the glass pump power absorp-tion limited to the UV range. Although energy transfer from Eu3+ toNd3+occured, a large Eu3+ concentration dependent quenching ofNd3+

fluorescence was observed. This latter process decreases theradiative quantum efficiency of Nd3+:4F3/2 emitting level and hencelimits the attractiveness of Eu3+ sensitization for Nd3+ laser action.

The next step in this researchwill be the design, based on these pre-liminary results, of the optical fiber to be pumped with concentratedsolar light. The choice of the most suitable Nd3+ doping level will bedriven not only by the concentration quenching effect, but also by themaximum energy available for pumping. Thus, it will depend on the di-mension and efficiency of the solar element collection system and theoptical pumping scheme.

Acknowledgments

The authors acknowledge the support of the Regione Piemontethrough the Converging Technologies “Hipernano” research project.

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