High-efficiency resonantly pumped roomtemperature Ho:YVO4 laser

Gang Li, Bao-Quan Yao,* Pei-Bei Meng, You-Lun Ju, and Yue-Zhu WangNational Key Laboratory of Tunable Laser Technology, Harbin Institute of Technology, Harbin 150001, China

*Corresponding author: yaobq08@hit.edu.cn

Received June 13, 2011; revised June 28, 2011; accepted June 30, 2011;posted July 5, 2011 (Doc. ID 149132); published August 1, 2011

An efficient 2 μm room temperature Ho:YVO4 laser resonantly pumped by a 1:94 μm Tm:YAP laser is demonstrated,for the first time to our knowledge, in this Letter. Up to 8:58W of laser output at 2053nm is obtained with the optical-to-optical conversion efficiency as much as 41.2%. The output beam had quality of M2 factor with Mx2 of 3.58and My2 of 1.76 at 8W output level. In addition, we also obtained 4:18W laser output at 2066nm and 7:04W at2040nm from the Ho:YVO4 laser with conversion efficiency of 27.8% and 33.8%, respectively. © 2011 OpticalSociety of AmericaOCIS codes: 140.3480, 140.3580, 140.5680.

Solid-state lasers emitting around 2 μm have wide appli-cations in optical metrology, wind lidar, atmosphericsounding, free-space communication, medicine, etc.Diode-pumped Tm, Ho codoped laser materials, whichbenefit from the two-for-one process pumped around800 nm [1,2], have been widely investigated to generatethe 2 μm laser radiation. However, the quasi-three-levelTm, Ho codoped lasers rely on energy-transfer processesand, consequently, on that more channels for radiativeand nonradiative losses are present, which will result inlarge heat loading of the laser crystal for room tempera-ture operation [3]. In order to achieve high power andhigh brightness laser output, they need to be operatedunder liquid N2 temperature [4,5]. Conversely, direct re-sonant pumping of singly Ho-doped lasers such as YAG[6], LuAG [3], YAP [7], YLF [8], etc., to obtain 2 μm laserradiation allows for the advantages of high conversionefficiency, minimal thermal loading due to low quantumdefect between the pump and laser, and room tempera-ture operation availability.The tetragonal uniaxial yttrium vanadate YVO4 crystal

belongs to a space group of D4h with unit cell parametersa ¼ b ¼ 0:712 nm and c ¼ 0:629 nm [9] and has beendemonstrated to be an excellent laser host for Nd3þ ions[10,11]. It is sufficiently hard (Mohs hardness 5), non-hygroscopic, resistive to chemicals, and, with Ho ionsdoped in it, named Ho:YVO4, has been shown to bean effective solid-state saturable-absorber Q switch forflash-lamp-pumped 2 μm Tm, Cr:Y3Al5O12 lasers [12].The spectral characteristics of Ho:YVO4, including ab-sorption and emission spectra, around 2 μm have beenpreviously reported by Gołvab et al. [13] and recentlyby our work group [4]. The large emission cross section(2:4 × 10−20 cm2 at 2052 nm and 2:7 × 10−20 cm2 at2040 nm for Π polarization, 0:7 × 10−20 cm2 at 2066 nmand 1:3 × 10−20 cm2 at 2008 nm for σ polarization, respec-tively [4]) and long fluorescence decay lifetime of Ho 5I7manifold (∼4ms) make the Ho:YVO4 potentially an ex-cellent laser host for 2 μm laser radiation. In addition, theelative large absorption cross section at 1:94 μm, which isabout 0:9 × 10−20 cm2 for σ polarization [4], makes theHo:YVO4 able to be efficiently resonant pumped by1:94 μm Tm:YLF or Tm:YAP lasers [14]. Our previous ex-periment has revealed that the Tm, Ho codoped YVO4

can be efficiently operated under liquid N2 temperature[5,15]. However, there is little report on singly Ho-dopedYVO4 lasers. Most recently, Newburgh and Dubinskii firstreported a resonantly pumped Ho:YVO4 laser at 77K, andthey obtained 1:6W laser output at 2054 nm [16].

This Letter, to the best of our knowledge, is the firsttime to report a room temperature Ho:YVO4 laser reso-nantly pumped by a 1:94 μm Tm:YAP laser. With 30mmlong (3mm × 3mm × 30mm) and 0:5 at:% Ho-dopedYVO4 crystal, up to 8:58W laser output at 2053 nm wasobtained under 20:8W total incident pump power, whichcorresponded to optical-to-optical conversion efficiencyof 41.2%. The beam quality of the M2 factor was deter-mined to be with Mx2 of 3.58 and My2 of 1.76 at 8W out-put level. The Ho:YVO4 laser can oscillate at 2066, 2053,or 2040 nm by choosing different output mirror transmis-sions or with a Fabry–Perot (FP) etalon inserted in thelaser resonator.

The experimental setup of the resonantly pumpedroom temperature Ho:YVO4 laser is shown in Fig. 1. Thepump source was a 1:94 μm Tm:YAP laser with two Tm:YAP rod series connected in a single cavity, which coulddeliver up to 21W laser output with slope efficiency of30.8% and optical-to-optical conversion efficiency of23.8%. The volume Bragg grating (VBG) combined witha 0:5mm thickness YAG FP etalon were used to stabilizethe Tm:YAP laser output at 1:94 μm, and the centralwavelength shift of it was less than 0:6nm [14]. The3mm×3mm×30mm with 0:5 at:% Ho3þ active Ho:YVO4

Fig. 1. (Color online) Experimental setup of the Ho:YVO4laser. OC, output coupler; LD, laser diode.

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crystal was grown by the Czochralski method and a cutalong the growth direction. The two end faces ofthe crystal were antireflection coated at both 1:94 μm(R < 0:5%) and 2 μm (R < 0:3%). The crystal waswrapped in indium foil and clamped in a copper heatsink, which was kept at 288K by thermoelectric control-ler. A simple L-shaped resonator was used for theHo:YVO4 laser with cavity length of about 85mm. Thecavity mirror M1 was flat with high reflectivity at both1:94 μm and 2 μm. The flat 45° dichroic mirror M2 hadhigh transmission at 1:94 μm and high reflectivity at 2 μm.The output mirror M3 was plane-concave with radius ofcurvature of 100mm, and the transmission at the laserwavelength was in the range of 5% to 55% in our experi-ment. This laser resonator could keep stable with thethermal focal length of Ho:YVO4 larger than 63mm, andthe calculated TEM00 laser mode radius within the crystalwas about 182 μm if not taking the thermal lens effect intoaccount. The 1:94 μm pump beam was focused to radiusof about 180 μm inside the Ho:YVO4 crystal, resulting ina Rayleigh length zr (zr ¼ πw2

pn=λpM2) more than40mm inside the crystal with refraction index n ¼ 1:92and M2 ¼ 2:5. In addition, an iris was inserted betweenthe Tm:YAP and Ho:YVO4 laser, and the Ho:YVO4 laserresonator mirrors were wedged slightly to prevent thebackward-propagating pump beam from re-entering theTm:YAP laser.To give a rough estimation of the oscillated wavelength

of the Ho:YVO4 laser, we plotted the gain cross section,which is shown in Fig. 2, based on our previous ab-sorption and emission cross section measurements [4].The gain cross section can be expressed as σgðλlÞ ¼PσemðλIÞ − ð1 − PÞσabsðλIÞ, where P is the population in-version parameter defined as the ratio of the numberof active ions in the excited state to the total numberof active ions [9]. One can see from Fig. 2 that, with Pin the range of 0.3 to 0.5, the 2053 nm wavelength withπ polarization is preferred to oscillate. For the purposeof achieving 2040 nm wavelength oscillation, a larger po-pulation inversion parameter of P is required. For the Pless than 0.3, the oscillated wavelength is unclear fromFig. 2 due to our spectral measurement errors. However,Gołvab’s measurement revealed that the 2066 nm wave-length output with σ polarization may appear with P lessthan 0.3 [13]. This indicates that the output wavelength ofthe Ho:YVO4 laser operated under room temperaturemay be simply artificially selected by the use of differentoutput mirror transmissions.In our experiment, we used five different output mirror

transmissions to evaluate the Ho:YVO4 laser perfor-

mance, which were 7.3%, 15.7%, 19.6%, 26.8%, and 51.1%,all of which were obtained from direct experimentalmeasurement. The pump absorption coefficient underlow incident pump power (less than 1W) was experi-mentally measured to be about 0:33 cm−1 for σ polariza-tion and 0:22 cm−1 for π polarization, and the a axis ofHo:YVO4 crystal was adjusted to be parallel to the polar-ization of the 1:94 μm pump beam to maximize the ab-sorption efficiency. For the transmission of 7.3%, theoutput wavelength was centered at 2066 nm as shownin Fig. 3(a), and the maximum output power was 4:18Was shown in Fig. 4, which was limited by the low damagethreshold of the 45° dichroic mirror M2 due to the highcirculated power density in the laser resonator. For theother four different output mirror transmissions, the out-put wavelengths were all centered at 2053 nm as shownin Fig. 3(b), and the best laser performance was achievedwith the transmission of 26.8%. With the output mirrortransmission of 26.8%, we obtained up to 8:58W laseroutput at 2053 nm, corresponding to optical-to-opticalconversion efficiency of 41.2%, which we believe is thefirst and highest output power obtained from a room tem-perature Ho:YVO4 laser up to the present. The laser out-put beam had an elliptical shape, and the quality of M2

factor was determined to be with Mx2 of 3.58 and My2

of 1.76 by the 90=10 knife edge technique, respectively.Figure 5 shows the dependence of output power on the

crystal-holder temperature at an incident pump power of17W for the Ho:YVO4 laser with output mirror transmis-sion of 26.8%. The output power decreased from 7.17 to6:4W as the temperature increased from 10 °C to 25 °C,and the central wavelength did not change, which wasalso at 2053 nm.A linear fit to the experimental data showsa slope of −56mW=°C for the output versus temperature,

1.90 1.95 2.00 2.05 2.10-0.75

-0.50

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0.00

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Gai

n c

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0.2 0.25 0.3 0.35 0.4 0.45 0.5

π

1.90 1.95 2.00 2.05 2.10

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0.0

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0.20.250.30.350.40.450.5

σ

Fig. 2. (Color online) 5I7 − 5I8 gain cross section of Ho:YVO4at 300K with different population inversion levels.

Fig. 3. (Color online) Output spectra of the Ho:YVO4 laser,(a) T ¼ 7:3% and (b) other output mirror transmissions usedin the experiment.

0 2 4 6 8 10 12 14 16 18 20 220

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T=7.3%T=15.7%T=19.6%T=26.8%T=51.1%T=26.8%

and with FP inserted

Fig. 4. (Color online) Output performance for the Ho:YVO4laser under different output mirror transmissions (for T ¼26:8% and with FP etalon inserted in the resonator, the outputwavelength was centered at 2040nm).

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indicating that the 1:94 μm pumped Ho:YVO4 laser pos-sesses a low temperature sensitivity of output over roomtemperature.The Ho:YVO4 laser output wavelength is expected to

shift to 2040 nm as shown in Fig. 2 (left) by use of highertransmission of output mirror. To obtain 2040 nm wave-length oscillation, a 0:05mm thickness YAG FP etalonwas inserted into the resonator when the output mirrortransmission was 51.1% since higher transmission of theoutput mirror was unavailable during the experiment.The output performance at 2040 nm is also shown inFig. 4, and as shown in Fig. 4, we obtained up to 7:04Wlaser output at 2040 nm, which corresponded to conver-sion efficiency of 33.8%. To give the readers more directinsight of the room temperature Ho:YVO4 laser, Table 1summarizes the main results of the laser performance wehave obtained.One can see that there was a sharp bend for the

Ho:YVO4 laser output at a pump power of ∼8W in Fig. 4for all output mirror transmissions. Two possible reasonsmay cause this phenomenon in our experiment. The firstone is the pump power and wavelength fluctuation of theTm:YAP laser, which may result in different absorbedpump power. The Tm:YAP laser had a sharp fluctuationaround the output power of 8W (fluctuated in the rangeof 7.8 to 9:2W, but at the other output power point, it wasquite stable), and the wavelength, which was measuredby an EXFOWA-650 spectrum analyzer combined with anEXFO WA-1500, also had a fluctuation in the range of1940.05 to 1940:34 nm (at the other output power point,the wavelength was also stable). The second one may beattributed to the mode overlap between the pump and thelaser. Our previous investigation of the Tm:YAP laser re-vealed that the Tm:YAP had a strong thermal lens and thefocused pump diameter will increase as the laser diodepump power increases [14]. This increased pump beamradius along with the laser mode, which was determinedby the laser resonator geometric structure and thermallens in the Ho:YVO4 crystal, would influence the modeoverlap and thus influence the laser output performance.Considering this, further investigation of this Ho:YVO4laser is needed.In conclusion, we have demonstrated, for the first time

to our knowledge, an efficient 1:94 μm Tm:YAP laser

resonantly pumped Ho:YVO4 laser operated at room tem-perature. As much as 8:58W of laser output at 2053 nmwas obtained with the output mirror transmission of26.8% under total incident pump power of 20:8W, andthe beam quality of the M2 factor was determined to bewith Mx2 of 3.58 and My2 of 1.76 at the 8W output level.It is found that the Ho:YVO4 laser output possesses a lowtemperature sensitivity, decreasing about 56mW as thecrystal-holder temperature increases every 1 °C overthe temperature range of 10 °C to 25 °C. In addition, wealso obtained 4:18W laser output at 2066 nm and 7:04Wat 2040 nm from the Ho:YVO4 laser with conversion effi-ciency of 27.8% and 33.8% respectively.

This work is supported by the National NaturalScience Foundation of China (NSFC) (60878011 and61078008) and Program for New Century ExcellentTalents in University (NCET-10-0067).

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8 10 12 14 16 18 20 22 24 260

2

4

6

8

Out

put p

ower

(W

)

Temperature (°C)

Experimental Data

-56 mW/oCP

in=17 W

T=26.8%

Fig. 5. (Color online) Dependence of the output power oncrystal-holder temperature for the Ho:YVO4 laser. Pin, total in-cident pump power; T , transmission.

Table 1. Laser Performance of the Ho:YVO4 Lasera

T λl (nm) Pout (W) Pol. Con. Eff.

7.3% 2066 4.18 a axis 27.8%26.8% 2053 8.58 c axis 41.2%51.1%

(with FP inserted)2040 7.04 c axis 33.8%

aT , output mirror transmission; λl, laser wavelength; Pout, maximumoutput power; Pol., polarization; Con. Eff., conversion efficiency.

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