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Tunable lasers based on diode pumped Tm-doped vanadatesTm:YVO4, Tm:GdVO4, and Tm:LuVO4

Jan Šulca, Petr Korandaa, Pavel Černýa, Helena Jelínkováa, Yoshiharu Uratab

Mikio Higuchic, Witold Ryba-Romanowskid, Rados�law Lisieckid

Piotr Solarzd, Grazina Dominiak-Dzikd, Marcin Sobczyke

aCzech Technical University, Faculty of Nuclear Sciences and Physical EngineeringBřehová 7, 115 19 Prague 1, Czech Republic

bMegaopto Co., Ltd, RIKEN Cooperation Center W414, 2-1 HirosawaWako, Saitama 351-0106, Japan

cGraduate School of Engineering, Hokkaido University, N13, W8Kita-ku, Sapporo 060-8628, Japan

dInstitute of Low Temperature and Structure Research, Polish Academy of SciencesOkólna 2, 50 422 Wroc�law, Poland

eUniversity of Wroc�law, Faculty of Chemistry, ul. F. Joliot-Curie 14, 50-383 Wroc�law, Poland

ABSTRACT

Thulium doped vanadates Tm:YVO4 (5 at.% Tm/Y, grown by the Czochralski technique), Tm:GdVO4 (2 and6 at.% Tm/Gd, grown by the floating-zone technique), and Tm:LuVO4 (3 at.% Tm/Y, grown by the floating-zonetechnique) were investigated as an active medium for diode pumped tunable laser operating around 1.9µm. Forthulium laser tuning single 1.5mm thick Brewster-angled birefringent quartz plate (Lyot filter) was placed insimple 80mm long linear quasi-hemispherical resonator. For thulium doped vanadates pumping a fibre-coupled(core diameter 400µm) temperature-tuned laser diode operating in range from 799 up to 810 nm was used(max available power 20W). All tested crystals were investigated under CW and pulsed pumping. Under pulsedpumping (4% duty-cycle, reduced heat generation) lasing and laser tuning was demonstrated with all availa-ble samples. Lasers were tunable in following wavelength ranges: Tm:YVO4 5 at.% Tm/Y (1841 – 1927 nm),Tm:GdVO4 2 at.% Tm/Gd (1830 – 1982 nm), 6 at.% Tm/Gd (1850 – 2010 nm), and Tm:LuVO4 3 at.% Tm/Lu(1860 – 1940 nm). Under CW pumping only Tm:GdVO4 crystal was lasing (lasing of Tm:YVO4 and Tm:LuVO4was not reached under elevated pumping duty factor). Using Tm:GdVO4 (2 at.% Tm/Gd) the power up to 2.6Wand slope efficiency ∼ 30% (with respect to absorbed power at 808 nm under lasing condition) was obtained atwavelength 1.91µm. Tunable operation with greater that 1W output and 130 nm tuning range (1842 – 1972 nm)was demonstrated for Tm:GdVO4 (2 at.% Tm/Gd) pumped at 802 nm.

Keywords: Tm:YVO4, Tm:GdVO4, Tm:LuVO4, tunable laser, Lyot filter.

Further author information: (Send correspondence to J.Š.)J.Š.: E-mail: [email protected], Tel.: +420 224 358 672, Fax: +420 221 912 252P.K.: E-mail: [email protected], Tel.: +420 224 358 672, Fax: +420 221 912 252P.Č.: E-mail: [email protected], Tel.: Fax:H.J.: E-mail: [email protected], Tel.: +420 224 358 538, Fax: +420 221 912 252Y.U.: E-mail: [email protected], Tel.: +81 48 469 3377, Fax: +81 48 469 3332M.H.: E-mail: [email protected], Tel.:, Fax:W.R-R.: E-mail: [email protected], Tel.: +48 71 34 350 21, Fax: +48 71 44 10 29R.L.: E-mail: [email protected], Fax: +48 71 344 10 29P.S.: E-mail: [email protected], Tel.: , Fax:G.D-D.: E-mail: [email protected], Tel.: , Fax:M.S.: E-mail: [email protected], Tel.: +048 71 375 7333, Fax:

Solid State Lasers XVII: Technology and Devicesedited by W. Andrew Clarkson, Norman Hodgson, Ramesh K. Shori

Proc. of SPIE Vol. 6871, 68711V, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.763011

Proc. of SPIE Vol. 6871 68711V-1

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1. INTRODUCTION

Much effort has been focused on research and development of thulium-doped laser materials in the last years.1

There are several reasons for this growing interest, one of them being commercial availability of high-powerInGaAs laser diodes at the wavelength around 800 nm, which can be used to pump these lasers with highefficiency. Secondly, Tm-based lasers are useful as pump sources for optical parametrical oscillators and solid-state lasers further in the mid-infrared region, which have a growing number of applications in spectroscopy,medicine, defense, and basic research. Thirdly, Tm-based solid state lasers can be operated with some tuningcapability around the central wavelength in the 1.8 − 2.1µm range,1 where several molecular absorption linesare present, making them attractive for applications such as high resolution spectroscopy, atmospheric remotesensing, laser radar, and laser microsurgery.

Efficient high power diode-pumped operation with thulium doping was previously attained in traditionallyused hosts such as YAG, YLF, and YAP.2,3 However, these suffer from low absorption of pump radiation thatpeaks around 785 − 795 nm and therefore special laser diodes are required for pumping. Also, the absorptionbands of these crystals are relatively narrow putting more demand on diode wavelength control.

Promising materials for thulium doping were found in vanadates: YVO4, GdVO4, and LuVO4.4–7 Thesematerials, similarly as with Nd-doping, exhibit extremely high absorption and strong emission compared toYAG and other hosts. The absorption band of Tm-doped vanadates is over 10 nm broad and extends beyond810 nm and therefore common high power diodes for Nd-doped lasers are suitable also for these materials.Furthermore, the vanadate crystals are naturally birefringent and laser output is linearly polarized along theextraordinary π-direction. The natural birefringence dominates the thermally induced birefringence effects, whichavoids undesirable thermal effects on laser performance and improves beam quality.8

Recent developments of Tm3+-doped vanadates concentrate on improving the efficiency and tuning range.Till now tunable operation using Ti:sapphire pumped Czochralski grown Tm:GdVO4 and Tm:LuVO4 crystalswas demonstrated. With Tm:GdVO4 the tuning was obtained from 1875 nm up to 1995 nm with the maximumpower 60mW at 1.94µm.9 Using Tm:LuVO4 crystal, it was achieved the tunable operation from 1839 nm to1952 nm with the maximum power 150mW at 1.93µm.7

This work is a follow-up to our previous study of floating-zone grown Tm-doped GdVO4 and LuVO4 crys-tals.10,11 We have evaluated continuous tuning of these crystals together with Czochralski grown Tm:YVO4under diode pumping.

2. MATERIALS AND METHODS

2.1 Tm-doped crystals samples

Several samples of thulium doped YVO4, GdVO4, and LuVO4 were investigated. Parameters of these sampleswere resumed in Table 1.

Table 1. Parameters of tested crystal samples (AR – antireflection coatings for pumping and laser radiation).

Crystal Tm3+ doping Length Coatings Face Crystal growing

Tm:YVO4 + CaO 5 at.% 2mm no 3× 5mm Czochralski

Tm:GdVO4 2 at.% 3mm AR 3× 5mm Floating-zone

Tm:GdVO4 6 at.% 3mm no 3× 5mm Floating-zone

Tm:LuVO4 3 at.% 2mm no 3× 5mm Floating-zone

The YVO4 crystal containing 5 at.% of Tm3++ 0.5 at.% of CaO was grown by the Czochralski method. Thesample has been oriented by the X-ray technique and cut and polished to parallelepipeds of a few millimetersedge. The Tm:GdVO4 samples were grown by the infrared lamp heated floating zone method. This method waspreviously found to be beneficial for the growth of Nd-doped GdVO4 crystal.12,13 The growth direction wasalong the [110] axis. For laser experiment, samples of 3-mm thickness with Tm3+ concentrations of 2 and 6 at.%



were cut along the a axis and the facets were optically polished. The Tm:LuVO4 crystal with concentrations of3 at.% was grown also by the infrared lamp heated floating zone method. The growth direction was along the[110] axis. For the laser investigation, samples of thickness 2mm were cut perpendicularly to the axis of growthand optically polished. Except of Tm:GdVO4 with 2 at.% of Tm3+, no antireflection (AR) coating was providedon these samples. The Tm:GdVO4 crystal with 2 at.% had broadband AR coatings at 800 nm and 1900 nm.

The detailed comparative spectroscopic study of the tested vanadate samples was published previously.14 Herewe present only the room-temperature polarization resolved absorption and emission cross-section spectra, whichare relevant to laser performance – see Figure 1. The absorption spectra were obtained using a Varian model 5UV-VIS-NIR spectrophotometer. The emission spectra were calculated using so-called reciprocity method. Thespectra consist of poorly resolved bands resulting from the superposition of lines associated with transitionsbetween individual crystal field levels of the two multiplets involved, additionally broadened by electron-phononinteraction. Strong homogeneous broadening of absorption and emission bands is advantageous for laser diodeexcitation and for tunable laser operation. Similarly as with Nd3+ doping, a strong anisotropy of absorption inπ and σ polarizations is present for all vanadates. The peak absorption cross-section for π-polarized radiationreaches 6.2 × 10−20 cm2 at 797.8 nm in case of Tm:LuVO4 crystal. This is the highest value in the family ofthulium-doped vanadate materials. This material possesses also the strongest anisotropy. To have possibility tocompare parameters of investigated materials, the summary of vanadates properties is presented in Table 2.

2.2 Untuned laser setup

Fundamental arrangement of untuned laser system based on longitudinally diode pumped Tm-doped vanadatesis presented in Figure 2. This setup was used to evaluate the performance of the tested laser crystals. The end-pumped crystal samples were clamped in a water-cooled copper block (water temperature ∼ 18 ◦C). Indium foil

Table 2. Comparison of Tm:YVO4, Tm:GdVO4, and Tm:LuVO4 crystals properties. Spectroscopic data were obtainedfrom Ref.14 Fluorescence decay time was measured directly using samples described in Table 1 and therefore reabsorptioneffects could have influenced the lifetime observation.

Crystal Tm:YVO4 Tm:GdVO4 Tm:LuVO4

Refractive index ne 1.9315 1.9616 2.24917

(λ ∼ 2µm) no 2.1415 2.1716 2.03117

Density [g/cm3] 4.22,8 4.3315 5.488 6.06,18 6.219

Thermal conductivity [Wm−1K−1] axis a 5.1,20 7.2,21 9.022 8.6,22 9.7,23 10.121 6.14,18 7.9619

axis c 5.2,20,24 9.4,21 12.022 10.4,22 11.4,21 12.324 9.7719

Lattice constants [nm] axis a 0.71220 0.72121,25 0.70317

[nm] axis b, c 0.62920 0.63521,25 0.62317

Melting point [ ◦C ] 1825, 181020 178026 1800

Thermal expansion [10−6K−1] axis a 1.69,22 4.4320 1.1422 1.519

[10−6K−1] axis c 8.19,22 11.420 7.8922 9.119

Specific heat [Jkg−1K−1] 56022 42922 390,18 44219

dno/dT [10−6K−1] axis a 7.92,22 8.58 10.122 —

dne/dT [10−6K−1] axis c 2.7,27 2.98 4.7,27 13.822 —

Absorption peak [nm] E||c 789.4 799.1 797.8

Abs. cross-section [10−20cm2] E||c 3.0 4.0 6.2

Eimss. cross-section peak [nm] E||c 1804 1807 1797

Eimss. cross-section [10−20cm2] E||c 2.7 2.0 3.1

Fluor. decay time (Tm3+) [µs] 1400 (5 at.%) 2500 (2 at.%) 560 (3 at.%)

1300 (6 at.%)

Tm3+ density (1 at.%) [1020 cm−3] 1.25 1.21 1.29



Figure 1. Polarization-resolved room temperature absorption (σa) and stimulated emission cross-section (σem) spectraof Tm:YVO4 (a), Tm:GdVO4 (b), and Tm:LuVO4 (c) crystals. Light was polarized perpendicular to the optical axis (πspectrum) or parallel to the optical axis (σ spectrum) of the crystal.

was used to achieve as good as possible thermal contact to the sample. Due to the samples geometry, the heatremoval was taking place from the top and bottom of the crystal, only. A fiber coupled laser diode (HLU20F400– LIMO Laser Systems) was used to pump the crystal. The laser diode was operating in pulsed (pulse length4ms) or in CW regime. The diode’s heat sink was kept at a constant temperature 11 ◦C. The pumping radiationwavelength was in the range from 800 nm to 804 nm depending on the diode current. The pump power availablewas up to 20W. The 400-µm fiber tip was imaged into the crystal by means of two achromatic doublet lenses(Thorlabs, Inc., AC508-075-B) with the focal length f = 75mm which resulted in a pumping beam waist diameterof ∼ 260µm inside the active medium. The pumping beam waist was ∼ 4mm long.Up to 50mm long semi-hemispherical laser resonator consisted of two mirrors. The rear resonator mirror used

for pumping had antireflection coatings for pumping radiation and high reflectivity for laser emission at 2µmrange. As an output coupler it was used the curved mirror (curvature radius 150mm) with reflectivity 97.5% at2µm range (mirror with the reflectivity 96% was used width CW pumped Tm:GdVO4 laser).



Figure 2. Longitudinally diode pumped untuned laser based on Tm-doped vanadates.

2.3 Tunable laser setup

Tuning of the longitudinally pumped thulium laser was accomplished by using a birefringent filter (single 1.5mmthick quartz plate) placed inside the optical resonator at the Brewster angle (∼ 56.7 ◦).1 This tuning element waschosen because of its high damage resistance in comparison with blazed diffractive gratings, low insertion losses,and relative simplicity of the overall arrangement. The 80mm long semi-hemispherical laser resonator consistedof two mirrors. The birefringence plate was placed between the output coupler and the laser active medium –see Figure 3. The laser emission tuning was reached by rotating the plate in its plane. The birefringence plate atthe Brewster angle inside the resonator ensured the horizontal polarization of the output radiation.

Figure 3. Longitudinally diode pumped tuned Tm-doped vanadate laser – top view.

2.4 Measuring instruments

The time development of Tm3+ laser output radiation was observed by an InAs/InAsSbP photodiode (mo-del PD36-05, IBSG Co, Ltd.), which covers spectral range from 800 nm up to 3800 nm. With this photodiodeTektronix oscilloscope TDS3052B (500MHz, 5GS/s) was used. The laser output mean power was measured byMolectron energy/power meter EMP2000 with the PowerMax probe PM3 or PM10. The laser output wavelengthwas monitored using a single grating monochromator (Oriel 77250) and the above mentioned photodiode.

3. RESULTS AND DISCUSSION

All tested Tm-doped vanadate crystal samples were investigated under pulsed pumping with low duty-cycle (pulsewidth 4ms, pulse repetition rate 10Hz) and under CW pumping. Under pulsed pumping the lasing was reachedwith all crystals. Under CW pumping only Tm:GdVO4 crystals were operating. The results are summarized infollowing sections.

3.1 Pulsed pumping of Tm-doped vanadates

The laser diode was working in the pulsed regime (4ms pulse length, 10Hz pulse repetition rate). The duty-cycle4% ensure low thermal load even under maximum diode pumping power amplitude 20W. Under such pumpingall samples were lasing. At first, laser crystals were characterized in the short resonator without tuning elementand the laser input-output characteristic was measured for each crystal sample. Next, the resonator was extendedto allow an insertion of a tuning element. The laser emission was tuned rotating the birefringent plate in its plane.The laser output power and emission wavelength was measured in dependence upon the angle of rotation. Resultsof these measurements are plotted in Figures 4-6.



Figure 4. (a) – Tm:YVO4 laser performance in untuned regime with short resonator, (b) - output power as a function ofthe emission wavelength for the 5 at.% doped Tm:YVO4 sample in tunable resonator.

Figure 5. (a) – laser performance of 2 at.% and 6 at.% doped Tm:GdVO4 in untuned regime with short resonator, (b) -output power as a function of the emission wavelength for the Tm:GdVO4 samples in tunable resonator.

Figure 6. (a) – Tm:LuVO4 laser performance in untuned regime with short resonator, (b) - output power as a function ofthe emission wavelength for the 3 at.% doped Tm:LuVO4 sample in tunable resonator.

For all samples the laser threshold was approximately 20mJ of absorbed power. The slope efficiency up to40% was reached in case of the Tm:GdVO4 with 6 at.% of Tm3+. Tm:GdVO4 material also offers broadest andsmoothest tuning curve which in dependence on Tm3+ concentration covers range from 1830 nm up to 2010 nm.



3.2 CW pumping of Tm-doped vanadates

Under CW pumping only Tm:GdVO4 crystals were operating. With other Tm-doped vanadates the lasing wasnot reached for duty-cycle higher than ∼ 60%. Unfortunately, the Tm:GdVO4 sample with 6 at.% Tm/Gd wasdamaged under maximum CW pumping power and only the results obtained with 2 at.% doped Tm:GdVO4sample are presented.

Figure 7. (a) – Tm:GdVO4 (2 at.% Tm/Gd) laser performance in untuned regime with short resonator, (b) – output poweras a function of the emission for the 2 at.% doped Tm:GdVO4 sample.

Firstly, the 2 at.% doped crystal was performed in 36mm long untuned laser resonator with output couplerreflectivity 96%. The laser was optimized in the CW mode and up to 2.6W of output was obtained for maximumpumping power. The output was limited only by the pump power available (see Figure 7a). The slope efficiencywith respect to the absorbed pump power was 30.8%.

In the next step the laser resonator was elongated to 100mm and the birefringent filter was placed in thelaser cavity to tune the wavelength. In this measurement, the output coupler reflectivity and curvature radiuswas 97.5% and 300mm, respectively. To avoid any possible damage to the sample, the pump power was keptbelow maximum at a constant level corresponding to ∼ 7W of absorbed power. The laser could be continuouslytuned from 1840 nm up to 1970 nm with a maximum at 1920 nm, where over 1 W output was attained – seeFigure 7b. Somewhat lower output power in comparison with the short resonator can be explained by changedoutput characteristics with the 98% reflectivity mirror, diffraction losses introduced by a stronger influence ofthe thermal lens, poorer matching of the pump beam to the cavity mode, and insertion losses of the filter.

4. CONCLUSION

Using Tm-doped vanadates Tm:YVO4, Tm:GdVO4, and Tm:LuVO4 under diode pumping tunable laser gene-ration was investigated. Tunability range 1830−2010 nm was obtained using Lyot filter. All results are summarizedin Table 3. Powerful tunable operation of CW pumped Tm:GdVO4 was demonstrated and over 1W of outputwas measured and 130 nm of continuous tunability was obtained.

Table 3. Tunability of lasers based on diode pumped Tm-doped vanadates. PU – pulsed pumping regime, CW – continuouswave regime, FWHM – full spectral width in half of maximum output power/energy.

Laser crystal Regime Range FWHM Maximum

Tm:YVO4 (5 at.%) PU 1841− 1927 nm 64 nm 7.3mJ @ 1905 nm

Tm:GdVO4 (2 at.%) PU 1830− 1982 nm 107 nm 8.5mJ @ 1910 nm

Tm:GdVO4 (6 at.%) PU 1852− 2010 nm 74 nm 8.0mJ @ 1945 nm

Tm:LuVO4 (3 at.%) PU 1860− 1937 nm 54 nm 3.8mJ @ 1930 nm

Tm:GdVO4 (2 at.%) CW 1842− 1972 nm 88 nm 1W @ 1920 nm



In case of Tm:YVO4 and Tm:LuVO4 probably thermally related issues prevented achieving continuous wavelaser action for this moment in current setup. However, we believe that an optimized cooling geometry combinedwith an improvement in crystal quality and AR coatings will allow reaching the CW operation in the near future.The new Tm:LuVO4 material can be proposed as attractive for laser formats requiring high absorption such asmicrochip or thin disk setups.

The tunable version of investigated Tm-doped vanadate lasers can be proposed as attractive for many im-portant applications which utilize continuous or discrete selection of laser lines in the 1.9µm wavelength region.Even though in the current arrangement the emitted radiation is relatively broadband (up to several nanome-ters linewidth was estimated in our measurement), there are many straightforward methods how to narrow thespectral line. Either more tuning elements with different free spectral ranges can be inserted into the cavity ora narrow-band intra-cavity grating can be used for wavelength selection.

The output spectrum of realized Tm-doped vanadate lasers is well-matched to the strong absorption lines ofHolmium in the 1900− 1920 nm range. Pumping of Cr2+-doped chalcogenide materials, where pump absorptiondecreases strongly towards 2µm, is also an attractive possibility.

5. ACKNOWLEDGEMENT

Research has been supported by Grant of the Czech Ministry of Education MSM684 0770 022 “Laser Systems,radiation and modern optical applications”.

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