Opt Quant ElectronDOI 10.1007/s11082-014-9902-6

High power Ho:YAG laser pumped by two orthogonallypolarized Tm:YLF lasers

Bao-Quan Yao · Ying-Jie Shen · Liu Han ·Chuan-Peng Qian · Xiao-Ming Duan ·You-Lun Ju · Yue-Zhu Wang

Received: 14 October 2013 / Accepted: 18 February 2014© Springer Science+Business Media New York 2014

Abstract High efficient and high power 2µm room temperature Ho:YAG laser resonantlydual-end-pumped by two orthogonally polarized diode-pumped Tm:YLF lasers at 1.91µmwas demonstrated. A maximum continuous wave output power of 61.9 W was generated,corresponding to the slope efficiency of 63.9 % with respect to the incident pump powerand optical-to-optical conversion efficiency of 59.9 %. At 20 kHz of the pulsed repetitionfrequency, we achieved a less than 25 ns FWHM pulse, with the maximum output energyper pulse of 2.84 mJ as well as a peak power of approximately 113.6 kW. A beam quality ofM2 <1.6 was achieved.

Keywords Solid states · Orthogonally polarized · Ho:YAG · Dual-end-pumped

1 Introduction

High energy and high power 2µm lasers have proven important for a variety of applications,including remote sending, wind lidar, medical as well as frequency conversion by opticalparametric oscillators (Taczak and Killinger 1998; Kang et al. 2006; Budni et al. 2000). Inparticular, Ho:YAG is in this respect an attractive candidate for Q-switched operation due tothe long upper laser level lifetime. Diode-pumped Tm-doped bulk lasers (Budni et al. 1998,2000) and Tm-doped fiber lasers (Lippert et al. 2006; Philipp Koopmann et al. 2011) turn outto be attractive sources for direct pumping of Ho:YAG laser (Budni et al. 2003). Due to theexcellent beam quality of the pump sources, the very low quantum defect in the Ho:YAG andcompact resonator configuration, up to 80 % of slope efficiency using Tm-doped fiber lasersfor in-band pumping of Ho:YAG are reported (Mu et al. 2009; Shen et al. 2004). Recentresults of published by Lamrini et al. show that up to 55 W of output power as well as 30 mJof output energy was obtained from a Ho:YAG laser directly pumped by a GaSb-based laser

B.-Q. Yao · Y.-J. Shen (B) · L. Han · C.-P. Qian · X.-M. Duan · Y.-L. Ju · Y.-Z. WangNational Key Laboratory of Tunable Laser Technology, Harbin Institute of Technology,Harbin 15001, Chinae-mail: yingjieyj@163.com

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Fig. 1 Diagram of the experimental setup

diode stack at 1.9 µm (Lamrini et al. 2012; Samir Lamrini et al. 2012). However, these diodestack lasers were not commercial available to us.

The thermal distribution in the laser crystal is more evenly spread out in dual-end-pumpedstructure compared with the single-end-pumped one (Yao et al. 2005). Owing to the smallertemperature gradient and longer thermal focal length, with dual-end pumping, higher pumppower can be focused into the laser crystal, and hence higher output power will be obtained inthe same cavity. Directly resonant pumping Ho lasers based on YAG (Zakharov et al. 2010),YAP (Duan et al. 2009), LuAG (Duan et al. 2009) have been demonstrated.

In this letter we report a high power and high efficiency CW and Q-switched operation ofHo:YAG laser which was resonantly dual-end-pumped by two orthogonally polarized diode-pumed Tm:YLF lasers at 1,908 nm. The Ho:YAG laser yielded 61.9 W of a maximum CWoutput at 2,097 nm for an incident pump power of 103.4 W, which corresponded to a slopeefficiency of 63.9 % and an optical-to- optical conversion efficiency of 59.9 %. In the activeQ-switch regime, up to 57.9 W of laser output at 25 kHz and 56.9 W at 20 kHz were achieved,corresponding to slope efficiency of 60.0 and 59.6 %, respectively.

2 Experimental setup details

The schematic diagram of the double-end-pumped Ho:YAG laser was shown in Fig. 1. Twodiode-pumped Tm:YLF lasers with two Tm:YLF rods in a single cavity were employed asthe pump source of the Ho:YAG laser. Each of them produced up to 55 W laser output witha slope efficiency of 37.5 % and an optical to optical conversion efficiency of 31.1 %. TheTm laser was pumped by four 50 W laser diodes, and each of the two Tm: YLF rods wasend pumped by two laser diodes. The beam quality factor of the Tm laser at the highestoutput power was less than 2. By using the volume Bragg grating (VBG) combined with0.5 mm thickness uncoated etalon, the emission wavelength of the Tm:YLF laser was stabi-lized at 1,908 nm owing to that the peak absorption line of the Ho:YAG are near 1,908 nm(Duan et al. 2009), and the central wavelength shift of it was less than 0.3 nm. Two thin filmpolarizers (TFPs) were employed in the experiment to avoid the Tm:YLF laser from beinginfluenced by each other. The Tm pump beam radiation was focused by a telescopic lens sys-tem to a 1/e2 diameter of 460µm inside the Ho:YAG rod. Double end pumping of this rod wasdone in an L-shaped configuration into which a Q-switched incorporated. With this arrange-ment, shown in Fig. 1, a plane mirror (M1) with high transmission (T>99.8 %) at 1.9µmand high reflectivity (R>99.5 %) at 2.1µm, a 45◦ dichroic mirror (M2) with high transmis-sion (T>99.6 %) at 1.9µm and high reflectivity (R>99.7 %) at 2.1µm were employed. The

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Two orthogonally polarized Tm:YLF lasers

Fig. 2 Output powers as afunction of total incident pumppower for Ho:YAG laser underCW and Q-switched mode

output coupler (M3) used in our experiment is a curvature radius of 200 mm as well as atransmission of 51 % at 2.1 µm. The laser was Q-switched by a fused silica acoustic opticalmodulator (AOM) (50DMA05-A, Gooch & Housego) with Brewster-angle end faces andan acoustic aperture of 1.8 mm. The acousto-optic Q-switch material with 30 mm of crystallength is crystal quartz with 99.6 % transmission for 2.1 µm operation. The radio frequency(RF) was 40.7 MHz, and the RF power was 25 W.

The Ho:YAG crystal dimension is 5 mm in diameter and 50 mm in length with a Holmiumdopant concentration of 0.5 at.%. The low doping concentration of Ho3+ is employed todiminish the upconversion losses (Lippert et al. 2006). The absorption coefficient of theHo crystal was 0.47 cm−1 at the pump power of 2.6 W. Both end faces of crystal wereantireflection-coated at the pump and laser wavelengths. The crystal was wrapped in indiumfoil and press-fitted into a copper heat sink. The heat deposited was radially removed by thewater-cooled heat sink. In this case, the Ho:YAG crystal was hold at 18 ◦C. The physicallength of resonator was 133 mm resulting in an estimated resonant mode size of 522µmin the crystal. A 100µm YAG uncoated etalon was inserted into the resonator to force theHo:YAG laser emitting at single wavelength.

3 Results and discussion

The laser operation was achieved at both CW and Q-switched modes. The slope efficiencies asa function of the incident pump power were shown in Fig. 2. Under CW mode, the maximumoutput power of 61.9 W was obtained in relation to incident pump power of 103.4 W, corre-sponding to a slope efficiency of 63.9 %. When the Ho laser was operated at approximatelysixteen times above threshold, the optical-to-optical efficiency was over 59.9 %. When thelaser runs in Q-switch mode, also shown in Fig. 2, greater than 57.9 W with a slope efficiencyof 60.0 % at 25 kHz and 56.9 W with a slope efficiency of 59.6 % at 20 kHz were achieved,respectively. In addition, the output power increased almost linearly with the incident pumppower and no slope efficiency saturation is observed, which indicates that the Ho crystal ismore suitable for being pumped by much higher power.

Figure 3 shows the performance achieved during a Q-switched operation. At 20 kHz of thepulsed repetition frequency, we achieved the maximum output energy per pulse of 2.84 mJ,with a less than 25 ns FWHM pulse as well as a peak power of approximately 113.6 kW. Thepulse duration shows a steady decrease with the incident pump power up to values of 45 W.

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Fig. 3 Pulse width and pulse energy versus total incident pump power under Q-switched mode. The inset isthe oscilloscope trace for the pulse with 2.84 mJ energy

Fig. 4 The output spectra of theHo:YAG laser under Q-switchedmode

Using a 300 mm WDM1-3 monochromator with a 600 lines/mm grating blazed for2.0 µm, we scan across the spectrum (0.1 nm resolution) of Ho laser at Q-switched operation.An InGaAs detector with a SRS830 lock-in amplifier for signal extraction was employed inour experiment. As shown in Fig. 4, the output spectra of Q-switched Ho laser with etalon andwithout etalon are all measured. Without etalon the Ho laser in Q-switched operation showstwo simultaneously lasing emission, 2,090 and 2,097 nm. A 0.1 mm thick YAG uncoatedplate was employed in the laser resonator as a low finesse etalon to select one of the twootherwise simultaneously lasing emission. We selected the 2,090 nm laser line because thegain cross section of Ho:YAG shows a maximum at 2,090 nm for high inversion levels andin this case the Ho laser produced higher output power.

To determine the beam quality factor M2, We routed the Ho laser radiation though a150 mm focal length lens, as shown in Fig. 5, and measured the 1/e2 beam radius along thepropagation direction at different CW output powers by using 90/10 knife-edge technique.By fitting Gaussian beam standard expression to these data, the fit yields M2 = 1.50 ±0.02, M2 = 1.53 ± 0.02 and M2 = 1.59 ± 0.02, which corresponding the output power of12, 31 and 61 W. From the data we note that the beam quality factor of Ho laser was influencedweakly with the increased output power. Based on the above investigation, a beam quality ofM2 < 1.6 was achieved.

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Fig. 5 M2 measurement ofHo:YAG laser under differentoutput powers

4 Conclusion

In conclusion, we have reported a high efficient and high power Ho:YAG laser by doubleend pumping using two orthogonally polarized Tm:YLF lasers. Up to 61.9 W of CW outputpower was produced by 103.4 W of incident pump power with a slope efficiency of about63.9 %. When the laser runs in Q-switch mode, up to 57.9 W at 25 kHz and 56.9 W at 20 kHzwere achieved, corresponding to the slope efficiency of 60.0 and 59.6 %, respectively. Inaddition, the slope efficiency shows no saturation at the highest available incident pumppower, suggesting that there is room for further power scaling of the laser by increasing theincident pump power. A beam quality of M2 <1.6 was achieved.

Acknowledgments This work was supported by National Natural Science Foundation of China (60878011and 61308009) and Fundamental Research funds for the Central Universities (Grant No. HIT. NSRIF.2014044).

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