High power and highly efficient operation of a Tm:YAG laser in-band pumped at 1617 nm

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2013 Laser Phys. Lett. 10 075801

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IOP PUBLISHING LASER PHYSICS LETTERS

Laser Phys. Lett. 10 (2013) 075801 (3pp) doi:10.1088/1612-2011/10/7/075801

LETTER

High power and highly efficient operationof a Tm:YAG laser in-band pumped at1617 nmJ J Sha1, D Y Shen1, T Zhao2 and X F Yang2

1 Key Laboratory of Micro and Nano Photonic Structures (Ministry of Education), Department ofOptical Science and Engineering, Fudan University, Shanghai 200433, People’s Republic of China2 School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116,People’s Republic of China

E-mail: shendy@fudan.edu.cn

Received 11 February 2012, in final form 24 August 2012Accepted for publication 10 October 2012Published 14 May 2013Online at stacks.iop.org/LPL/10/075801

AbstractWe report on highly efficient operation of a Tm:YAG laser resonantly pumped (3H6→

3F4) byan Er:YAG ceramic laser at 1617 nm. The lasing characteristics of 6 at.% Tm3+-dopedTm:YAG crystal were evaluated using output couplers of 6%, 10% and 20% transmission. Amaximum continuous wave (cw) output power of 5.6 W at 2015 nm was obtained with a 10%transmission output coupler under 10.6 W of pump power, corresponding to a slope efficiencyof 60.4% with respect to incident pump power.

(Some figures may appear in colour only in the online journal)

1. Introduction

Solid-state lasers operating in the eye-safe 2 µm spectralregion are of interest for a variety of applications includingmedicine, remote sensing and mid-infrared generation viapumping of optical parameter oscillators. Tm3+-dopedyttrium aluminum garnet (Tm:YAG) crystals are commonlyused as the gain media of 2 µm lasers due to theirunique thermal–mechanical properties. The small emissioncross-section (∼5 × 10−21 cm2) and the quasi-three-levelnature of Tm3+ ions at room temperature mean that anintense pump is needed to overcome the high laser threshold,and optimization of the Tm3+-doping concentration, thelaser rod dimensions and the pumping geometry for sucha laser system is also necessary [1, 2]. Conventionally,Tm:YAG lasers are pumped by commercially available highpower laser diodes at ∼0.8 µm. Efficient laser operationhas been demonstrated with slope efficiencies well abovethe Stokes limit (39%) owing to the fortuitous ‘two-for-one’cross-relaxation process [3–8]. With this pump scheme, high

Tm3+-doping concentration is generally required to achievehigh quantum efficiency as a result of the cross-relaxationprocess (i.e., 3H4 +

3H6 →3F4 +

3F4). High Tm3+-doping,however, leads to severe reabsorption losses and high heatloading densities in the laser crystal, which can furtherincrease the upconversion of the 3F4 energy level, preventingthe laser from power scaling and brightness improvement.

A promising way to alleviate the detrimental thermaleffects in high power Tm3+-doped solid-state lasers is topump Tm3+ ions directly to the upper laser level (3H6 →3F4) (as shown in figure 1) [9]. In-band pumped Tm- orTm, Ho-doped solid-state lasers have been demonstratedby several groups [10–12]. Cornacchia et al reported aroom temperature Ho, Tm:YLF laser in-band pumped bya Co:MgF2 laser at 1682 nm; ∼0.26 W of output powerwas obtained with a slope efficiency of 59% [10]. A singlyTm3+-doped KY(WO4)2 crystal laser under diode pumping at1.75 µm was demonstrated with 86 mW of output power anda slope efficiency of 28% [11]. ∼0.46 W of output power anda slope efficiency of 42% were obtained from a Tm:YLF laser

11612-2011/13/075801+03$33.00 c© 2013 Astro Ltd Printed in the UK & the USA

Laser Phys. Lett. 10 (2013) 075801 J J Sha et al

Figure 1. Stark energy-level diagram of Tm:YAG crystal.

in-band pumped at 1678 nm using a Raman shifted Er-fiberlaser [12]. Efficient operation of a polycrystalline Tm:YAGceramic laser in-band pumped at 1617 nm was very recentlydemonstrated [13].

In this letter, we report on output characteristics of aTm:YAG crystal laser resonantly pumped by a high powerEr:YAG ceramic laser at 1617 nm. The lasing characteristicsof the Tm:YAG were investigated with output couplers of 6%,10% and 20% transmission. Up to 5.6 W of output power at2015 nm was achieved under 10.6 W of pump power witha 10% transmission output coupler, corresponding to a slopeefficiency of 60.4% with respect to incident pump power.

2. Experiment and results

The pump source of the Tm:YAG laser used in ourexperiments was a homemade high power Er:YAG ceramiclaser operating at 1617 nm [14]. The resonator of the ceramicEr:YAG laser comprised a plane rear mirror with hightransmission at the pump wavelength of ∼1532 nm and highreflectivity for 1617 nm laser light, and a 100 mm radius ofcurvature concave output mirror with 15% transmission atthe lasing wavelength. The Er:YAG laser pump source coulddeliver up to 13 W of diffraction-limited output at 1617 nmwith an bandwidth (FWHM) of <0.5 nm. A Tm:YAG singlecrystal of 6 at.% Tm3+-doping concentration was used asthe gain medium. The crystal was 3 mm in diameter and15 mm in length with both end faces antireflection coated at∼1617 and ∼2015 nm. To allow for efficient heat removal,the sample was wrapped with indium foil and mountedin a water-cooled copper heat-sink with the cooling watermaintained at a temperature of ∼15 ◦C. The experimentalarrangement of the Tm:YAG laser cavity is schematicallyshown in figure 2. A simple two-mirror linear resonator was

Figure 2. Schematic diagram of the Tm:YAG laser: 1—Er:YAGceramic laser at 1617 nm, 2—collimating and focusing lenses,3—input coupler, 4—Tm:YAG crystal rod, 5—output coupler.

Figure 3. Laser output power as a function of incident pump powerfor different transmission of output couplers.

employed; it comprised a plane input coupling mirror (IC)with high reflectivity (R > 99%) at the lasing wavelength of1800–2100 nm and high transmission (>96%) at the pumplight, and a 100 mm radius of curvature concave outputcoupler (OC) of 6%, 10% and 20% transmission at thelasing wavelength and with high reflectivity (>97%) at thepump light. The physical length of the laser resonator was∼20 mm, resulting in a TEM00 mode radius of ∼147 µm inthe crystal. The output beam of the pump laser was collimatedand focused into the Tm:YAG crystal with a beam radiusof ∼150 µm using two 100 mm focal length plano-convexlenses, leading to a confocal length of ∼79 mm.

The absorption peak of the 3H6 →3F4 transition is at

∼1622 nm. The 1617 nm pump light was located at the wingof this absorption peak, and the off-peak pump configurationfacilitated reduction of heat loading density in the lasergain medium. The single-pass small-signal absorption of the1617 nm pump light in the Tm:YAG crystal was measuredto be ∼94% under non-lasing (i.e., without the existence of alaser resonator) and low pump density conditions, where therewas negligible ground-state bleaching. The pump absorptionunder cw lasing conditions should be close to this value dueto the relatively low lasing threshold in our experiments.

The output power as a function of the 1617 nm incidentpump power is shown in figure 3. It is obvious that thebest performance in terms of maximum output power andslope efficiency is obtained using the 10% transmission

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Laser Phys. Lett. 10 (2013) 075801 J J Sha et al

Figure 4. Laser output spectrum of the Tm:YAG laser at 2015 nm(solid curve) and fluorescence spectrum of the Tm:YAG ceramic atroom temperature (dotted line).

output coupler. The Tm:YAG laser reached threshold at anincident pump power of ∼0.85 W and yielded a maximumcw output power of 5.6 W under 10.6 W of pump power ina near-diffraction-limited beam with a measured M2 factorof ∼1.3 (NanoScan, Photon Inc.), corresponding to a slopeefficiency with respect to incident pump power of 60.4%and an optical conversion efficiency of 52.7%. It is worthnoting that the output power was essentially linear withrespect to the incident pump power at even the highestpower level, suggesting that there was room for further powerscaling in output power by simply increasing the incidentpump power. The emission spectra were analyzed using a0.55 m monochromator of 0.05 nm specified resolution at435.8 nm (Omni-λ5005, Zolix), and the lasing wavelengthwas confirmed to be 2015 nm with a spectral bandwidth(FWHM) of 0.5 nm (shown in figure 4). The thresholdincreased to 1.74 W for the 20% output coupler, while theslope efficiency and maximum output power decreased to50.7% and 4.46 W, respectively. This may be attributed toan enhanced upconversion loss (3F4 →

3H5,3F4 →

3H4).High output coupling loss results in not only high lasingthreshold and excitation density in the upper manifold(3F4) but also elevated energy transfer upconversion and,hence, degradation in laser performance [15], especiallywhen operated in the low repetition-rate Q-switched regimeand with high inversion densities. With our high powerand near-diffraction-limited 1617 nm pump source, furtherimprovement in laser performance should be achievable withTm:YAG crystal of lower Tm3+ concentration and, hence,reduced upconversion losses, reabsorption losses and thermalload densities.

3. Conclusion

In conclusion, we have reported a Tm:YAG crystal laserresonantly pumped by a ceramic Er:YAG laser at 1617 nm.The output characteristics were investigated with outputcouplers of 6%, 10% and 20% transmission. The best laserperformance in terms of output power and slope efficiencywas obtained with a 10% output coupler, and the laser yielded5.6 W of output power at 2015 nm for 10.6 W of incidentpump power, corresponding to a slope efficiency of 60.4%with respect to the incident pump power. It is believedthat improved laser performance should be achievable withlower Tm3+-doping concentration samples and reducedupconversion parameters and thermal load densities.

Acknowledgments

This work is supported by the National Science Foundationof China (NSFC) under contract numbers 61078035 and61177045.

References

[1] Payne S A et al 1997 IEEE J. Sel. Top. Quantum Electron. 3 71[2] Schellhorn M 2008 Appl. Phys. B 91 71[3] Honea E C, Beach R J, Sutton S B, Speth J A, Mitchell S C,

Skikdmore J A, Emanuel M A and Payne S A 1997 IEEE J.Quantum Electron. 33 1592

[4] Lai K S, Phua P B, Wu R F, Lim Y L, Lau E, Toh S W,Toh B T and Chng A 2000 Opt. Lett. 25 1591

[5] Yokozawa T and Hara H 1996 Appl. Opt. 35 1424[6] Bollig C, Clarkson W A, Hayward R A and Hanna D C 1998

Opt. Commun. 154 35[7] Li C, Song J, Shen D, Kim N S and Ueda K 1999 Opt. Express

4 12[8] Gao C, Gao M, Zhang Y, Lin Z and Zhu L 2009 Opt. Lett.

34 3029[9] Payne S A, Smith L K, Kway W L, Tassano J B and

Krupe W F 1992 J. Phys.: Condens. Matter 4 8525[10] Cornacchia F, Dilieto A, Maroni P, Toncelli A, Tonelli M,

Sorokin E and Sorokina I 2001 Appl. Phys. B 73 191[11] Troshin A E, Kisel V E, Yasukevich A S, Kuleshov N V,

Pavlyuk A A, Dunina E B and Kornienko A A 2007 Appl.Phys. B 86 287

[12] Kalachev Y L, Mihailov V A, Podreshetnikov V V andShcherbakov I A 2011 Opt. Commun. 284 3357

[13] Wang Y, Shen D Y, Chen H, Zhang J, Qin X P, Tang D Y,Yang X F and Zhao T 2011 Opt. Lett. 36 4485

[14] Zhang C, Shen D Y, Wang Y, Qian L J, Zhang J, Qin X P,Tang D Y, Yang X F and Zhao T 2011 Opt. Lett. 36 4767

[15] Rustad G and Stenersen K 1996 IEEE J. Quantum Electron.32 1645

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