2040 OPTICS LETTERS / Vol. 32, No. 14 / July 15, 2007

Tm-doped LiLuF4 crystal for efficient laser actionin the wavelength range from 1.82 to 2.06 �m

Nicola Coluccelli,1,* Gianluca Galzerano,1 Paolo Laporta,1 Francesco Cornacchia,2 Daniela Parisi,2 andMauro Tonelli2

1Istituto di Fotonica e Nanotecnologie–CNR and Dipartimento di Fisica del Politecnico di Milano,Piazza Leonardo da Vinci 32, Milano 20133, Italy

2National Enterprise for Nano Science and Technology–Istituto Nazionale per la Fisica della Materia, Dipartimentodi Fisica Università di Pisa, Largo P. Pontecorvo 3, 56127 Pisa, Italy

*Corresponding author: nicola.coluccelli@polimi.it

Received March 15, 2007; revised May 19, 2007; accepted May 19, 2007;posted May 22, 2009 (Doc. ID 81066); published July 9, 2007

Room-temperature continuous-wave laser action in a novel Tm-doped LiLuF4 crystal emitting at around1.9 �m is demonstrated. The crystal growth process, spectroscopic measurements, and laser performanceare presented for different Tm concentrations. An overall tunability extending from 1817 to 2056 nm, aslope efficiency of 46%, and a maximum output power in excess of 1.1 W are reported. © 2007 Optical So-ciety of America

OCIS codes: 140.3580, 140.3480, 140.3380, 140.3600, 140.5680.

In recent years, solid-state lasers based on Tm-dopedcrystalline hosts emitting at around 1.9 �m havebeen the object of growing scientific interest, thanksto a combination of specific features, which makesthem attractive for a wide range of applications. Tm-based solid-state lasers can be continuously tuned inthe wavelength range from 1.8 to 2 �m [1,2], whereseveral absorption lines of chemical compounds, suchas H2O, CO2, and NO2, are present, providing thenecessary tool for high-resolution molecular spectros-copy, atmospheric remote sensing, and medical sur-gery. Molecular absorption lines can also be used tostabilize the emission frequency of Tm lasers for de-velopment of optical frequency standards in thenear-IR spectral region. Moreover, direct pumping ofTm-doped active media by commercially availablehigh-power InGaAs laser diode at a wavelengtharound 790 nm allows the implementation of effi-cient, compact, and rugged laser sources.

In this Letter we report on crystal growth process,spectroscopic measurements, and laser performanceof a novel Tm-doped LiLuF4 crystal (Tm:LiLuF). In-vestigation of laser emission properties of Tm:LiLuFwas performed for different Tm3+ dopant ions concen-trations. A maximum output power of 1.15 W at3.2 W of incident pump power, a slope efficiency of46%, and a remarkable wavelength tuning range of230 nm, extending from 1826 to 2056 nm, are dem-onstrated with the optimized Tm concentration of12%. Focusing on tunability range, output power, andslope efficiency, the results obtained with Tm:LiLuFare comparable with the best results previously dem-onstrated with several diode-pumped Tm-doped crys-talline hosts [1–3]. Among fluoride hosts, LiLuF pro-vides a tuning range wider than that observed inCaF2 [4], KY3F10 [5], and YLiF4 [6] and approachesthe best result of 245 nm achieved with BaY2F8 [7].Tm:LiLuF has the advantages of larger emissioncross sections, lower phonon cutoff energy [8], andthe possibility to grow congruently, resulting in

higher optical quality of the grown crystal. The slope

0146-9592/07/142040-3/$15.00 ©

efficiency obtained with Tm:LiLuF represents, to theauthors’ knowledge, the highest value ever demon-strated with diode-pumped Tm-doped fluoride crys-tals.

The crystal growth apparatus consists of a home-made Czochralski furnace with conventional resis-tive heating; the vacuum system has an ultimatepressure limit better than 10−7 mbar, and the growthprocess was carried out in a high-purity (99.999%) ar-gon atmosphere. The 5N powders, as well as their pu-rification from OH− contamination, have been pro-vided by AC Materials (Tampa, Fl., USA). Thesamples were grown using LiF and LuF3 powders asraw material for the host and adding 8%, 12%, and16% mol of TmF3 powder to the melt.For all crystals the growing conditions were5 round/min for the rotation rate and 1 mm/h for thepulling rate, and the temperature of the melt was860°C. According to the measurements performed byinductively coupled plasma technique [9], we as-sumed the segregation coefficient to be close to 1. In-deed, the Tm3+ dopant ion substitutionally enters theLu3+ S4 site, having a coordination number equal to8, and the ionic radii of Tm3+ and Lu3+ are 1.13 and

Fig. 1. Room-temperature polarized cross sections of Tm3+

ions in LiLuF crystal: (a) absorption, (b) emission.

2007 Optical Society of America

July 15, 2007 / Vol. 32, No. 14 / OPTICS LETTERS 2041

1.11 Å, respectively. LiLuF has the same tetragonalstructure as scheelite, with lattice parametersa=5.167 Å and c=10.735 Å and symmetry groupI41/a. The single-crystalline character of the samplewas checked using a x-ray Laue technique that al-lowed us to identify the c-crystallographic axis and tocut oriented samples. The length of each sample hasbeen chosen as a function of Tm-doping concentra-tion. The crystal end faces �3 mm�3 mm� were pol-ished for laser quality.

For all the samples, room-temperature absorptionspectra in the 800 nm region (3H6→ 3H4 transition)and around 1.9 �m �3H6→ 3F4� were measured. Theresults closely match those reported in [10]. As an ex-ample, Fig. 1(a) shows the absorption cross sectionobtained with the 8% doped sample. The most in-tense absorption peak lies in the E �c polarization at779.8 nm with a FWHM of 7.6 nm, particularly suit-able for diode pumping. The polarized emission crosssections were evaluated starting from room-temperature fluorescence spectra. Figure 1(b) showsthe emission cross section of 8% Tm:LiLuF crystal asretrieved from the �−� method [11]. The maximumvalue of 0.4�10−20 cm2 was at a wavelength of1911 nm for E �a polarization and 0.31�10−20 cm2 at1886 nm for E �c polarization.

Figure 2 shows the three-mirror folded cavity andpump system configuration adopted during the laserexperiments. The pump source is a cw AlGaAs laserdiode with a linearly polarized emission tuned at ap-proximately 780 nm and a maximum output power of3.2 W. The pump beam is first collimated and thenfocused onto the active crystal by a pair ofantireflection-coated plano–convex lenses, F1 and F2,with focal lengths of 50 and 38 mm, respectively. The

Table 1. Tm:Li

Tm3+ IonConcentration

�ions/cm3�

OutputCoupler

(%)

ThresholdPump Power

(mW)

MOu

1.15�1021 (8%) 1 1065 206

1.72�1021 (12%) 1 965 180

2.30�1021 (16%) 1 1115 217

Fig. 2. Scheme of the Tm:LiLuF laser cavity and pumpsystem: LD, laser diode; F1, F2, F3: lenses; M1, M2, M3,mirrors; C, Tm-doped LiLuF crystal; LF, Lyot filter; OC,

output coupler.

Tm:LiLuF crystal, inserted in a copper holder, isplaced inside the resonator close to the plane high-reflectivity (99.9%) input mirror M1 and oriented atBrewster angle to minimize Fresnel losses of both thepump and laser beams originating from the air–crystal interfaces, exploiting the E �c polarization di-rection. Pump radiation transmitted by the activecrystal is backreflected for double pass pumping bylens F3 (75 mm focal length) and metallic mirror M3,improving laser pumping efficiency. A high-reflectivity spherical mirror M2, with 100 mm curva-ture radius, folds the cavity with an angle of approxi-mately 18°, depending on crystal length, to giveoptimum compensation of laser beam astigmatismintroduced by Brewster interfaces. Insertion of theLyot filter along the longest arm of the resonator al-lows for wavelength tuning of the laser emission.

Laser experiments were aimed to investigate theoutput power and tunability performance at roomtemperature of three Tm:LiLuF crystals with Tm-doping levels of 8%, 12%, and 16%, and differentlengths, which were, according to increasing absorp-tion, 2, 1.9, and 1.6 mm, respectively. The reason forthis analysis stems from the strong dependence of en-ergy transfer processes between lanthanide ions incrystal on doping level, which affects the dynamics oflevel populations and, consequently, laser perfor-mance. High doping level in principle allows one toefficiently populate the 3F4 manifold by means of the

3 3 3 3

performance

mumPower)

SlopeEfficiency

(%)

Pump PowerAbsorption

(%)

TuningRange(nm)

3 37 69 1828–20402 34 66 1830–19845 46 82 1826–20565 43 78 1817–19967 39 85 1851–20461 41 80 1826–1978

Fig. 3. Output power versus incident pump power ofTm:LiLuF laser with Tm3+ ion concentrations of 8% (tri-angles), 12% (circles), and 16% (squares). (a) 1% outputcoupler, (b) 5% output coupler.

LuF

axitput

(W

0.91.01.01.10.91.0

cross-relaxation process � H4 , H6�→ � F4 , F4�. How-

2042 OPTICS LETTERS / Vol. 32, No. 14 / July 15, 2007

ever, as the concentration increases, the reverse pro-cess �3F4 , 3F4�→ �3H4 , 3H6� becomes more importantand the laser efficiency is expected to decrease [10].

The output power of Tm:LiLuF laser as a functionof incident pump power is shown in Fig. 3 for 1% and5% output coupling. Table 1 summarizes the perfor-mance exhibited by Tm:LiLuF crystals with differentTm3+ ion concentrations. The best results were ob-tained with a 12% Tm-doping level. In this case, us-ing 5% output coupler, we obtained the highest out-put power of 1.15 W (at 3.2 W pump power). Using a1% output coupler we measured a slight increase ofthe slope efficiency (from 43% to 46%), however themaximum output power was lower (1.05 W at 3.2 Wpump power), due to a saturation of the input–outputcharacteristic for incident pump power higher than2.5 W. This behavior is ascribed to an increase ofthermal population of the lower laser level when thelaser is operated at high pump levels. In fact, satura-tion is significantly reduced when the crystal holderis temperature controlled by a thermoelectric coolerand maintained at around room temperature also athigh pumping rate.

Investigation of the emission tuning range ofTm:LiLuF crystals was performed by insertion of a2 mm thick birefringent quartz plate (Lyot filter), ori-ented at Brewster angle inside the resonator (see Fig.2). Figure 4 shows tunability curves demonstrated bythe three Tm:LiLuF crystals, measured with 1% out-put coupling and 2.5 W of incident pump power. Tun-ability measurements were also performed in thecase of 5% output coupling, and the corresponding re-

Fig. 4. Tunability curves of Tm:LiLuF laser measured

sults are reported in Table 1. Tm:LiLuF crystal with12% Tm concentration exhibited the widest tuningrange of 230 nm, from 1826 to 2056 nm, the shortesttuning wavelength, being probably limited by thefree spectral range of the Lyot filter ��250 nm� andthe finite bandwidth of the cavity mirrors. The pres-ence of spectral holes between 1.89 and 1.95 �m isdue to water vapor absorption.

In conclusion, Tm-doped fluoride LiLuF crystalswere grown, spectroscopically characterized, andcompared in terms of laser performance as a functionof Tm-doping level. LiLuF crystal doped with 12%Tm concentration showed the highest output powerof 1.15 W at 3.2 W of incident pump power when 5%output coupling was used, and a remarkable tuningrange of 230 nm, from 1826 to 2056 nm, with 1% out-put coupling.

The authors thank Ilaria Grassini for the technicalassistance and Hans P. Jenssen and Arlete Cassanhofor helpful discussions.

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with 1% output coupling for Tm3+-doping level of 8%, 12%,and 16%.