Scholarly article on topic 'CW laser operation around 2- μm in (Tm,Y b):KLu(WO4)2'

CW laser operation around 2- μm in (Tm,Y b):KLu(WO4)2 Academic research paper on "Physical sciences"

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{"Diode pumped solid state lasers" / "Double tungstate crystals" / "Thulium lasers"}

Abstract of research paper on Physical sciences, author of scientific article — Martha Segura, Xavier Mateos, Maria Cinta Pujol, Joan Josep Carvajal, Valentin Petrov, et al.

Abstract Laser generation in continuous wave (CW) regime at 1.94-μm from (Tm,Yb) codoped system has been investigated in two different hosts: KLu(WO4)2 and KY (WO4)2. The high quality crystals were grown by the Top-Seeded Solution Growth Slow Cooling (TSSG-SC) method with doping levels of 2.5 at. %Tm and 5 at. %Yb. The active media were pumped with a diode laser at 980nm. We demonstrated the superior performance of KLu(WO4)2 compared to that of KY (WO4)2 and improved the results already obtained in the literature. The maximum laser output power reached was 157mW for (Tm,Yb): KLu(WO4)2 and 123mW for (Tm,Y b):KY (WO4)2.

Academic research paper on topic "CW laser operation around 2- μm in (Tm,Y b):KLu(WO4)2"

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Physics Procedía 8 (2010) 157-161

VI Encuentro Franco-Español de Química y Física del Estado Sólido VIeme Rencontre Franco-Espagnole sur la Chimie et la Physique de l'État Solide

CW laser operation around 2-^m in (Tm,Yb):KLu(WO4)2

Martha Seguraa, Xavier Mateosa*, Maria Cinta Pujola, Joan Josep Carvajala, Valentin Petrovb, Magdalena Aguilóa and Francesc Díaza

aFisica i Cristal.lografía de Materials i Nanomaterials (FiCMA-FiCNA), Universitat Rovira i Virgili (URV), c/Marcel.líDomingo s/n, 43007

Tarragona, Spain.

bMax-Born-Institute for nonlinear optics and ultrafast spectroscopy, 2A Max-Born-Street, D-12489 Berlin, Germany


Laser generation in continuous wave (CW) regime at 1.94-^m from (Tm,Yb) codoped system has been investigated in two different hosts: KLu(WO4)2 and KY(WO4)2. The high quality crystals were grown by the Top-Seeded Solution Growth Slow Cooling (TSSG-SC) method with doping levels of 2.5 at. %Tm and 5 at. %Yb. The active media were pumped with a diode laser at 980 nm. We demonstrated the superior performance of KLu(WO4)2 compared to that of KY(WO4)2 and improved the results already obtained in the literature. The maximum laser output power reached was 157 mW for (Tm,Yb):KLu(WO4)2 and 123 mW for (Tm,Yb):KY(WO4)2.

© 2010 Published by Elsevier Ltd.

Keywords: Diode pumped solid state lasers; double tungstate crystals; thulium lasers.

1. Introduction

The laser emission around 2-prn based on the 3F4 ^ 3H6 transition of thulium (Tm) has become interesting for medical applications and atmospheric sensing, mainly due to the strong absorption of water around this wavelength, as well as for pumping OPO's for conversion in the mid-IR [1]. The 2-pm laser emission can easily be achieved by pumping directly around 800 nm with AlGaAs diode lasers in the case of single doped Tm laser crystals or glasses, or by pumping around 980 nm with InGaAs diode lasers when using a sensitizer ion such as Yb3+. The laser

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1875-3892 © 2010 Published by Elsevier Ltd. doi:10.1016/j.phpro.2010.10.027

operation with single doped Tm3+ has been successfully demonstrated in a wide variety of hosts such as YAG (Y3AI5O12) [2], YLiF4 [3], YVO4 [4] and the double tungstates KRE(WO4)2 (Hereafter, KREW; RE=Y, Gd, Lu) [5,6]. On the other hand, there are many works focused on the Yb3+ as sensitizer ion for Tm3+ particularly due to the high absorption cross section of Yb3+ and the effective energy transfer from Yb3+ to Tm3+ [7, 8, 9].

The high absorption and emission cross sections for the rare earth ions and also the possibility of high concentration doping levels make the double tungstates very attractive materials to be used as laser host. For instance the maximum values of the absorption cross section of the Yb doped KYW and KLuW single crystals are 11.7 x 10-20 cm2 [10] and 11.8 x 10-20 cm2 [11] respectively. These values are considerably large compared to other commonly used hosts such as YAG, YAlO3, YLF [12] or YVO [13]. Regarding the emission cross section of thulium in KREW's, it has been found that 1.15x10-20 cm2 at 1910 nm for E//Nm, and 1.20x10-20 cm2 at 1950 nm for E//Nm are the maximum values for KYW [14] and KLuW [11], respectively. The laser action of single doped Tm and Yb ions has been successfully achieved in both hosts with high efficiencies. The laser emission at 1910 nm of (Tm,Yb) codoped system has been achieved in KYW in [9], but to the best of our knowledge, lasing of (Tm,Yb) codoped KLuW single crystals has not yet been demonstrated.

The Yb3+ ion used as sensitizer is of great advantage since it has only two manifolds, the ground state 2F7/2 and the excited state 2F5/2. Once the ion is excited it only can decay to the ground state or transfer part of its energy to another ion if the material is codoped and if the levels are reasonably resonant in energy.

The population of the 3F4 level of Tm3+ via energy transfer from Yb3+ can be described using the energy levels scheme in figure 1. The 980 nm pump light is absorbed by Yb3+ ions, a part of this energy is transferred (Tj) from the 2F5/2 level of Yb3+ to the 3H5 level of Tm3+ that decays via non-radiative process to the 3F4 level. Part of these electrons in 3F4 level could decay to the ground level, another could absorb energy (T2) and get up to 3F2,3 levels, decaying to 3H4 level via non-radiative process. Finally, the 3F4 level can be populated via cross relaxation process such as R1 and R2 in figure 1. Another processes in the (Tm,Yb) system, such as T3 process, give rise to the population of the :G4level that decays emitting at 480 nm (:G4^ 3H6) and 650 nm (:G4 ^ 3F4).

In this work, we report on the laser oscillation of (Tm,Yb) codoped KLuW and KYW single crystals grown by the Top Seeded Solution Growth Slow Cooling (TSSG-SC) method. The obtained crystals (2.5 at. % Tm and 5 at. % Yb) were cut according to the principal optical axes for better laser performance and placed in a hemispherical resonator for the laser experiments.




1.9 fm

■ 1G,

1.9 fm

Figure 1. Energy levels scheme indicating the energy transfer processes in the (Tm,Yb) system. 2. Active media

For laser experiments, we used two different hosts from monoclinic double tungstates family: KY(WO4)2 (KYW) and KLu(WO4)2 (KLuW) doped with Tm and Yb. The cell parameters of KYW, in the C2/c space group, are a = 10.631 A, b = 10.345 A, c = 7.555 A, p = 130.75° and Z = 4 [15], similarly, the cell parameters for KLuW are a = 10.576 A, b = 10.214 A, c = 7.487 A, p = 130.68° and Z = 4 [16]. For both hosts, the principal optical axis

480 nm

650 nm

980 nm

Np is parallel to b crystallographic direction, while Nm and Ng lie in the a-c crystallographic plane. The principal optical axis Ng is at 18.5° clockwise with respect to c crystallographic direction.

The (Tm,Yb)-doped crystals were grown by the Top-Seeded Solution Growth Slow-Cooling (TSSG-SC) method described in detail in [17]. The crystals grew from a KLuW (or KYW) seed oriented along b crystallographic direction. The doping levels were 2.5 at. % Tm and 5 at. % Yb. After growth, the chemical composition of the crystals was KLuo.9i9Tmo.o3oYbo.o5i(WO4)2 and KYo.^Tmo.o^Ybo.os^WO^ measured by Electron-Probe Micro-Analysis (EPMA).

For the laser experiments, we constructed a hemispherical resonator consisting of a planar mirror with antireflecting coating (AR) for the pump wavelength (77o-io5o nm) and highly reflecting (HR) coating for the laser wavelength (18oo-2o75 nm). As output coupler we tested different mirrors with several transmissions at the laser wavelength (182o-2o5o nm) Toc = 1.5%, 3%, 5% and 9% and different radius of curvature Roc = 25, 5o and 75 mm. The pump source was a fiber-coupled (NA = o.22, core diameter = 2oo цm) high power InGaAs diode laser delivering up to 5o W emitting in the 976-98o nm range depending on the current level. The active media were cut for propagation along the Ng direction with dimensions 3 x 3 x 1.6 mm3 along Np x Nm x Ng. The uncoated high optical quality polished samples were mounted in a water cooled copper holder for heat dissipation. The normal incident pump beam was focused to a 2oo цm spot diameter onto the crystal with a lens of 2o mm focal length.

3. Laser experiments results

The input-output characteristics of (Tm,Yb):KLuW and (Tm,Yb):KYW for several Toc and Roc are summarized in table 1, and the results for Roc= 5o mm are shown in fig. 2 as representative of the whole results obtained in the present work.

Table 1. Maximum output powers and their corresponding incident powers and laser wavelengths of the 2.5 at.% Tm, 5 at.% Yb:KLuW (left) and KYW (right) depending on the T„. and R^..

(TmYb):KLuW Roc(mm) (TmYb):KYW Roc(mm)

Toc(%) 25 5o 75 25 5o 75

pout=153 Pout=157 Pout=14o Pout=122 Pout=123 Pout=126 mW

1.5 mW mW mW mW mW Pin=5.2 W

Pin=5.2 W Pin=5.2 W Pin=5.2 W Pin=4.9 W Pin=4.7 W ^=3.1%

^=3.6% ^=3.8% ^=3.4% ^=3.o% ^=3.5%

Pout=143 Pout=148 Pout=121 Pout=1o3 Pout=94 mW Pout=1o7 mW

3 mW mW mW mW Pin=4.4 W Pin=4.9 W

Pin=4.4 W Pin=4.9 W Pin=4.9 W Pin=4.9 W ^=3.1% ^=3.2%

^=4.3% ^=4.o% ^=3.3% ^=2.6%

Pout=118 Pout=121 Pout=1o9 P0ut=87.1m Pout=8o mW Pout=83 mW

5 mW mW mW W Pin=4.4 W Pin=4.9 W

Pin=4.9 W Pin=4.7 W Pin=4.9 W Pin=4.7 W ^=3.o% ^=2.8%

^=3.4% ^=3.7% ^=3.2% ^=2.8%

9 Pout=73.5 Pout=75.3m Pout=74.8m

mW W W

Pin=4.7 W Pin=4.9 W Pin=4.9 W

^=2.7% ^=2.6% ^=2.5%

The results, in terms of wavelength and slope efficiency, are very similar for different RoC in KLuW and KYW. However, the best efficiency for KLuW was achieved with Toc = 3% and R^ = 25 mm while for KYW the highest efficiency was obtained with Toc = 1.5% and Roc = 50 mm. In general, saturation of the output power was observed for pump powers higher than 5 W in the case of (Tm,Yb):KLuW and 4 W for (Tm,Yb):KYW. This effect is mainly due to thermal load because when a chopper is used (duty cycle of 50%) the linear dependence is maintained for higher powers. In any case, no cracking of the crystals was observed. The laser wavelengths were, in general, longer for (Tm,Yb):KLuW than for (Tm,Yb):KYW in agreement with the maximum values of emission cross section for Tm in these hosts, though there was a broader emission in the laser wavelength for Toc = 1.5% and Roc = 25 mm for (Tm,Yb):KLuW, and Toc = 1.5%, R^ = 50 mm for (Tm,Yb):KYW.

Figure 2. CW laser operation of 2.5% Tm, 5% Yb in a) KLuW, and b) KYW with several output couplers and R^ = 50 mm.

The very low efficiencies were produced from the non-optimized mode matching of the pump and resonator modes. In the work of Batay et al. [9] a beam spot of 80 ^m has been used to obtain an efficiency of 19% with respect to the absorbed power. Here we estimate an absorption of 75% and the spot size was 200 ^m, so that the bleaching conditions are different and cannot be compared. Moreover, using high power levels the crystal is more likely to suffer thermal effects, as can be seen in figure 2(b) where the thermal effects are characterized by the saturation of the power. This could be avoided with either a reduction of the Yb doping level or an increase of the Tm doping level to optimize the energy transfer from Yb to Tm.

The high laser thresholds are adscribed to the non-perfect resonant energy transfer from Yb to Tm and to the 3-level nature of the Tm ion with partial population of the ground at room temperature. The maximum power for (Tm,Yb):KLuW was 157 mW achieved with Toc=1.5% and Roc=50 mm, while for (Tm,Yb):KYW was 123 mW with the same Toc and Roc. The latter value is twice than that obtained in [9] also for (Tm,Yb):KYW with an optimum doping level, 6 at. % Tm, 5 at. % Yb, the doping concentration in our case is 2.5 at. % Tm, 5 at. % Yb. In [9], the authors established that low doping levels of Tm, like 3 at. % did not generate laser radiation, probably due to the non-effective cross relaxation mechanism.

R =50 mm

R—=50 mm

4. Conclusions

In summary, we have analysed the laser operation around 2 ^m from the 3F4 ^ 3H4 transition, under diode pumping at 980 nm, obtained in 2.5 at. % Tm, 5 at. % Yb codoped system in two similar hosts: KLuW and KYW. We conclude that the doping level of 2.5 at. % Tm is not enough to populate efficiently the 3F4 energy level of Tm via energy transfer of Yb, this would mean that even higher doping levels of Tm in (Tm,Yb):KLuW could increase the results enhancing the cross relaxation process of Tm.


This work was supported by the Spanish Government under projects MAT2oo8-o6729-Co2-o2/NAN, MAT2oo8-o4o46-E/MAT, TEC2o1o-21574-Co2-o2, Pio9/9o527, DE2oo9-ooo2 and the Catalan Authority under project 2oo9SGR235. M. Segura thanks the Catalan Government for the funds provided through the fellowship 2oo9FI_B oo43o. J. J. Carvajal is supported by the Education and Science Ministry of Spain and European Social Fund under the Ramon y Cajal program, RYC2oo6 - 858.


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