Laser Phys. Lett. 6, No. 10, 707–710 (2009) / DOI 10.1002/lapl.200910063 707

Abstract: In this letter we report a diode-pumped single fre-quency Tm:LuAG laser at room temperature. One uncoated fusedsilica etalon (0.1 mm) and one uncoated YAG etalon (1 mm) areemployed to narrow the laser line-width. Single frequency laseris achieved by tuning the angle of the etalons. The oscillatingwavelength can be changed (from 2018 to 2030 nm) and at eachwavelength, single-frequency laser is achieved. The maximumsingle frequency laser power is up to 148.0 mW with the centralwavelength of 2026.4 nm. To our knowledge, this is the first timeto obtain single-frequency Tm:LuAG laser of up to 148.0 mWwith Fabry-Perot etalons in the cavity.

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Output power performance of multi-mode and single-mode atroom temperature

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Room temperature operation of single frequencyTm:LuAG laser end-pumped by laser-diodeC.T. Wu, ∗ Y.L. Ju, Q. Wang, Z.G. Wang, F. Chen, R.L. Zhou, and Y.Z. Wang

National Key Laboratory of Tunable Laser Technology, Harbin Institute of Technology, Harbin 150001, China

Received: 14 June 2009, Revised: 16 June 2009, Accepted: 19 June 2009Published online: 27 June 2009

Key words: solid-state lasers; diode pumping; Tm:LuAG

PACS: 42.55.Xi, 42.60.Pk

1. Introduction

The interest in all-solid-state laser operating in the eye-safespectral region near 2 μm is acknowledged for applica-tions such as gas sensing, wind measurement, and iatricalapplications [1–4]. A laser-diode-pumped thulium-dopedLu3Al5O12 (Tm:LuAG) laser has proven to be an efficientroom-temperature laser around 2 μm [5]. Tm:LuAG oper-ating on the 3F4 to 3H6 transition is isomorphic to YAG,which has the advantage of high heat conductivity similarto the YAG host [6]. As a quasi-three-level laser material,Tm:LuAG is more promising for it has lower populationdensity of the lower laser level. In addition, the emissionwavelength of Tm:LuAG (2.026 μm) is shifted slightly tolonger wavelength in comparison with YAG (2.013 μm)crystal [7]. The transmission through the atmosphere is

better at the longer wavelengths. In recent years, manyworkgroups have researched the laser characteristics ofTm:LuAG [8].

In 1995, a total optical-to-optical efficiency of 7.3%and an optical-to-optical differential efficiency of 23.6%were achieved by diode-pumped Tm:LuAG laser [5]. In2000, the maximal output 0.92 W was obtained fromdiode-pumped Tm:LuAG laser under the pump powerwas 10 W [9]. In 2007, the spectroscopic properties oftwo thulium-doped Lu3Al5O12 (Tm:LuAG) samples withTm3+ concentrations of 0.5 and 5 at.% were reported [10].In 2008, we used 3×3×7 mm3 Tm:LuAG crystal in aplano-concave cavity to observe its high power output.The maximum of 4.91 W and slope efficiency of 25.39%are obtained [11]. As the source of the LIDAR, the highpower Q-switched solid-state laser must be seeded by a

∗ Corresponding author: e-mail: bigsnow1@126.com

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708 C.T. Wu, Y.L. Ju, et al.: Room temperature operation of single frequency Tm:LuAG laser

Laser

4 W LDFiberCoupling optics

systemDichromatic

mirror

Outputcoupler

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Figure 1 (online color at www.lphys.org) Schematic of the ex-perimental setup

continuous wave single mode master oscillator. In 2004,diode pumped single mode Tm:LuAG and Tm,Ho:LuAGlasers were developed as master oscillators for an air-borne LIDAR system, 51 mW of output power with aTm(10%):LuAG laser was reached at room temperature byusing a new fourfold pump setup [8]. In 2008, we used aset of double cavity to achieve single-longitude-mode out-put. But the single-mode output power is only 17.1 mW[12].

This letter demonstrated a laser-diode-pumpedTm:LuAG laser with single longitudinal mode operation.The wavelength of the single longitude mode could betuned over a range from 2018 to 2030 nm by turningthe angle of the Fabry-Perot (F-P) etalons in the cavity.The maximum power and the slope efficiency of singlefrequency output are 148 mW and 5.6% respectively. Theoutput wavelength is 2026 nm.

2. Experimental setup

The schematic diagram of the experimental setup is shownin Fig. 1. To make the system simple and compact, aplano-concave resonator is employed. The cavity lengthis 30 mm. The Tm:LuAG crystal is pumped by a CW laserdiode with maximum output power of 4 W and centralwavelength of 790.0 nm. The output wavelength of the LDcan be tuned to coincide with the 788 nm Tm absorptionband by turning its working temperature. The LD output isshaped and focused by a series of convex lenses. The imag-ing into the crystal yields a pump waist diameter of nearly120 μm. The total transmission efficiency of the beam-reshaping system is about 92%. The mode matching be-tween pump mode and laser mode is optimized by chang-ing the pump beam waist radius and its location. The di-mension of Tm:LuAG crystal is 3×3×2 mm3, and the con-centration of Tm is 4%. The pumping side of the crystal iscoated with a high transmission at 788 nm (R < 0.5%) andhigh reflectivity at 2.02 μm (R > 99.9%). The opposite sur-face of the laser crystal is coated anti-reflection at 788 nm(R < 0.5%) and 2.02 μm (R < 0.1%). The temperature ofthe crystal is measured on the surface of the copper, whichis the holder of the crystal cooled by a TE-cooler (main-tained at 290 K). The radii of curvature and the transmis-sion at 2.02 μm of the output coupler are 100 mm and 2%respectively.

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Figure 2 (online color at www.lphys.org) Output spectrum ofthe free-running laser

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Figure 3 (online color at www.lphys.org) The wavelength ofTm:LuAG laser

3. Experimental results

In our experiment, the spectral output of the Tm:LuAGlaser is recorded with a Burleigh WA-650 spectrum ana-lyzer combined with a WA-1500 wavemeter (0.7 pm res-olution). The output spectrum of the free-running laseris shown in Fig. 2. The emission line is centered on2.023 μm. There are mainly seven peaks. To investigate itslongitudinal mode structure, the output of the laser is in-terrogated using a scanning F-P interferometer. The free-running laser typically ran on many longitudinal modes,the mode competition is quite intensely.

To achieve single frequency operation, one uncoatedfused silica etalon and one uncoated YAG etalon are usedto control and tune the laser wavelength by angle tuningthe etalons. The two etalons have thicknesses of 0.1 and1 mm respectively. The 1 mm etalon is placed in a standardmirror mount allowing a two-axis rotation of the etalonand is mounted at a fixed angle in the cavity. The 0.1 mmetalon is placed in a second two-axis mount allowing con-tinuous angle tuning by hand. The output laser radiation isdeparted to two parts. One part is going into the waveme-

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Laser Phys. Lett. 6, No. 10 (2009) 709

Output wavelength, Output powernm of single-mode, mW2018.5 342019.6 482020.7 732021.9 952022.9 352023.1 1332024.1 1452025.2 412026.4 1482027.6 972028.6 992029.8 882030.8 74

Table 1 Output wavelength and the maximum power of single-frequency

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Figure 4 (online color at www.lphys.org) F-P spectrum of thesingle frequency Tm:LuAG laser

ter to measure the output wavelength. The other part is go-ing through a diagnostic air-gap scanning F-P interferom-eter, which measures the longitude mode of the Tm:LuAGlaser. The collimated continuous output of the Tm:LuAGlaser is transmitted through the F-P interferometer, and thetransmitted intensity is detected by a 9-V biased InGaAsdetector. The electronic signal from the detector is fed intoa digital oscilloscope through a preamplifier.

Fixed 1 mm YAG etalon, changed the angle of the0.1 mm etalon, it is found that the wavelength could tuneover a range of approximation 13 nm. At each wave-length, single longitude mode can be obtained by tuningthe etalon elaborately. Fig. 3 is one typical output signalfrom wavemeter. Fig. 4 is a typical output signal from thescanning F-P interferometer for a free spectral range of3.75 GHz. The upper trace is the F-P ramp voltage and thelower trace is the voltage of the InGaAs detector measur-

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Figure 5 (online color at www.lphys.org) Output power perfor-mance of multi-mode and single-mode at room temperature

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Figure 6 (online color at www.lphys.org) Output from single-mode to multi-mode

ing the Tm:LuAG laser transmission through the F-P in-terferometer. As can be seen, the laser operated on a singlelongitudinal mode.

Table 1 shows the result of the effect from turning the0.1 mm F-P. The left is the output wavelength. And theright is the output power of single-frequency under the rel-ative wavelength.

Fig. 5 plots the multi-mode and single-mode outputpower as a function of pump power. The threshold of free-running lasing is 1.17-W. The high threshold is owe to themismatched between the pump wavelength and the cen-tral absorbed wavelength of Tm:LuAG. The output wave-length of the LD is changed by its working temperature.However, its working temperature cannot be high enoughto match the absorbed peak wavelength of Tm:LuAG atlow power. Under pump power of 3.43 W, the maximum

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710 C.T. Wu, Y.L. Ju, et al.: Room temperature operation of single frequency Tm:LuAG laser

power of 223 mW is achieved with the crystal temperatureat 290.0 K. A linear fit to the data yields a slope efficiencyof 10.9%. The optical-to-optical conversion efficiency atthe maximum power level is approximately 6.5%. Themaximum power of single frequency output is 148.0 mWwith the wavelength of 2.026 μm. As single frequency isachieved at high pump power, we reduced the pump powerstep by step. When the pump power is low down to 2.67 W,single mode changes to multi-mode, as shown in Fig. 6.The working temperature of the LD is increased from 28.2to 28.6◦C. Then the output of Tm:LuAG turned two modesto single mode. But the output power is quite nonlinear af-ter changing the temperature of LD. At this point, the out-put power of the single-mode is only 88.0 mW. It’s maybecause the mode had changed from one to another.

4. Conclusion

In summary, we have reported a diode end-pumpedTm:LuAG laser with maximal CW output power of223.0 mW. A single-mode output power of 148.0 mW isobtained by tuning the angle of 0.1 mm F-P etalon (1 mmYAG etalon was fixed), and the output center wavelengthis 2026.4 nm. With changing the angles of the two etalons,the oscillation wavelength shifts correspondingly. The os-cillating wavelength shifts from 2018.5 to 2030.8 nm. Inour future work, a 788 nm LD with wavelength controlledmore accurately is needed.

Acknowledgements This work is supported by the program ofExcellent Team in Harbin Institute of Technology.

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