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Diode-end-pumped continuous-wave and acousto-optics Q-switched Tm:LiLuF4 slab lasers were demonstrated. The a-cut Tm:LiLuF4 slab with doping concentrations of 2 at.% was pumped by fast-axis collimated laser diodes at room temperature. The maximum continuous-wave output power of 10.4 W was obtained while the absorbed pump power was 31.6 W and the cavity length was 30 mm. For Q-switched operation, we got the maximum pulse energy of 8 mJ with pulse width of 315.2 ns at 1 KHz pulse repetition frequency.
©2009 Optical Society of America
1. Introduction
2-µm lasers have been an important orientation of laser developing for years because of their many significant applications. Firstly, as an eye-safe waveband, 2-µm lasers can be a kind of good surgery light source in medicine. Secondly, 2-µm lasers have huge application prospects in remote sensing and optical communications fields, especially in coherent Doppler LIDAR and water vapor parabolic surface differential absorption laser radar system. Moreover, 2-µm solid-state lasers with high peak power and short pulse are effective pump sources for 3~5-µm optical parametric oscillators (OPOs) [1–4]. Thulium tervalent ions have long fluorescence lifetimes, high quantum efficiency and the absorption of thulium tervalent ions is near 790 nm which matches well with the commercially laser diodes [5–6]. Various 2-µm solid-state lasers based on Tm-doping crystal, such as Tm:YAG, Tm:YLF, Tm:KLu(WO4)2 and Tm: YAP have been studied and reported [7–13].
With similar pyramidal system structure to LiYF4,Tm:LiLuF4 crystal also has long upper level lifetime and lower pumping threshold [14–15]. Tm:LiLuF4 has broad emission band which is favorable to obtain tunable laser output. Coluccelli has reported a 1.1 W continuous-wave (CW) output power while the output laser wavelength ranges from 1.82 to 2.06 µm with the doping concentration of 12 at.% [16–17].
In this paper, we report a high power diode-end-pumped CW and acousto-optics Q-switched Tm:LiLuF4 slab lasers with 2 at.% doping concentration. The a-cut Tm:LiLuF4 slab is end-pumped by a fast-axis collimated laser diode (LD) which center wavelength is near 792 nm at 297 K. To get good thermal removing, the Tm:LiLuF4 crystal has been designed to thin slab structure. At room temperature, the maximum continuous-wave output power is 10.4 W with 31.6 W absorbed pump power. For Q-switched operation, the maximum pulse energy of 8 mJ at 1 KHz repetition frequency is obtained.
2. Spectroscopy
The Tm:LiLuF4 crystal is grown at Laser and Optoelectronic Functional Material R&D Center, Shanghai Institute of Optics and Fine Mechanics, China by the Czochralski technique. The cross relaxation induced by the similarity of the energy level difference between 3H4→3F4 and 3H6→3F4 enable the Tm: LiLuF4 lasers to leave two Tm3+ ions in the upper laser level 3F4 for every ion originally excited, so the Tm:LiLuF4 lasers has high quantum efficiency theoretically. Fig. 1 shows the unpolarized absorption spectrum of Tm:LiLuF4 crystal between 660 nm and 820 nm at room temperature (Measured by JASCO V-570 UV/VIS/NIR spectrophotometer). Taken into account the match to commercial laser diode (LD), the LD with 792 nm centre wavelength is used as pump source in our experiment. Fig. 2 shows the fluorescence spectrum from 1400 to 2200 nm of Tm:LiLuF4 crystal at room temperature (Measured by Nikon G250 spectrometer). The broad emission band with FWHM of 220 ns is favorable to obtain tunable laser output.
Fig .1. Unpolarized absorption spectrum of the 2 at.% and a-cut Tm:LiuiF4 crystal at room temperature
Fig .2. Unpolarized fluorescence spectrum of the 2 at.% and a-cut Tm:LiuiF4 crystal at room temperature
3. Laser experiments and results
Table. 1 Pshows the properties of the a-cut Tm:LiLuF4 crystal used in our experiments. In order to gain high power laser output, we lengthen the slab by choosing a 2 at.% doping concentration Tm:LiLuF4 crystal which doping concentration is much lower than the Tm:LiLuF4 crystal in Ref.17. And 2 at.% doping concentration has been proved to be a proper doping concentration in high power Tm:YLF laser [9]. The cross emission section and energy level lifetime have great influence in laser’s performance. Like Tm:YAG and Tm:YLF, the Tm:LiLuF4 also has long lifetime of 3F4 energy level which is favorable to get high laser energy output [18–19].
Table 1. Properties of the a-cut Tm: LiLuF4 crystal used in the laser experiments
The Tm:LiLuF4 slab crystal had a dimension of 1.5×6×18 mm whose two end facets (1.5×6 mm) were AR-coated at both 790±15 nm and 1920±50 nm. The two large surfaces (6×18 mm) were wrapped in indium foil and cooled by a copper micro channel heat-sink. The temperature of cooling water for LD and crystal are both set at 290 K in our experiment. Fig. 3 shows the scheme of the Tm: LiLuF4 crystal slab laser. The pump beam is shaped by one plano-concave cylindrical lens whose radius of curvature is 28 mm and two plano-convex spherical lenses whose radius of curvature are both 22 mm. Both surfaces of these lenses are AR-coated at 790±15 nm. The size of pump beam at rear end facet of Tm:LiLuF4 slab is about 0.5×1.5 mm. The rear mirror M1 is AR-coated at 790±15 nm and HR-coated at 1920±50 nm, and the mirror M2 is a plane-concave output coupler with different reflectivity at 1920±50 nm and different radius of curvature.
The absorbed pump power is added from 3.25 W to 31.6 W while the incident LD power increases from 4.7 W to 57.6 W as shown in Fig. 4. Correspondingly, the absorption coefficient is changed from 0.65 cm-1 to 0.44 cm-1. The variety of the absorption coefficient is mainly caused by the centre wavelength red-shift from 788.5 nm to 794.5 nm as the incident LD power increasing from 4.7 W to 57.6 W.
Fig. 4. Absorption efficiency of the 2 at.% and a-cut Tm:LiuiF4 crystal pumped by fast-axis collimated laser diodes at room temperature
For continuous operation, a compact plane-concave cavity with 30 mm length and the Tm: LiLuF4 crystal is in the centre of the cavity. Figure 5 shows the continuous-wave output power versus absorbed pump power with 95%, 90%, 85% and 80% reflectivity of output coupler. The maximum output power of 10.4 W and 40.4% slope efficiency was obtained with the 90% reflectivity of output coupler. With 95%, 85% and 80% reflectivity of output coupler, the maximum output power are 9.4 W, 9 W and 8 W respectively. The output laser beam is s-polarized which vibration direction is vertical to the plane of incidence.
Fig. 5. Continuous-wave output power versus absorbed pump power with different reflectivity of output coupler and radius of curvature
Figure 6 shows the laser spectrum of Tm:LiLuF4 laser with the 90% reflectivity of output coupler and 31.6 W absorbed pump power. The central laser wavelength of Tm: LiLuF4 laser is 1916 nm with the spectral line width of 4 nm. As shown in Fig. 6, we can find the second peak of laser spectrum is 1922 nm and the spectral line width is also 4 nm.
For Q-switched operation, infrared Fused Silica acousto-optic Q-switch (Gooch & Housego, QS027-4M-AP1) is inserted between the output coupler and Tm:LiLuF4 crystal. The reflectivity of output coupler is 90% and the cavity length is lengthened to 95 mm while the positions of mirror M1 and Tm:LiLuF4 crystal are fixed. Figure 7 shows the average output power width with 1, 5, 10 and 50 KHz repetition frequency. At 50 KHz repetition frequency, the maximum output power is 8.8 W with 36.9% slope efficiency. We can find that the average output powers don’t drop obviously while the pulse repetition frequency (PRF) is changed from 50 KHz to 1 KHz. The main reason is that the long upper-level lifetime of Tm:LiLuF4 crystal enables the laser to work at low PRF and the output laser performance would not be affected greatly. So the energy per pulse increase greatly as the dropping of PRF as shown in Fig. 8. The max energy per pulse of 8 mJ is obtained at 1 KHz and 31.6 W absorbed pump power while the energy per pulse is only 0.176 mJ at 50 KHz PRF with the same pump power. The pulse width is reduced from 1102 ns to 315.2 ns while the repetition frequency is changed from 50 KHz to 1 KHz with the same absorbed pump power of 31.6 W. Correspondingly, the peak power is changed from 0.16 KW to 25.4 KW Correspondingly. In order to gain maximum output energy, the time for Q-switch switching from high-loss to low-loss must less than the pulse build-up time [20–21]. The open duration of the acousto-optic Q-switch is 5 µs and the rise time is 109 ns. As shown in Fig. 9, the fast dropping at pulse trail maybe due to that the Q-switch has closed before the pulse tails emerge completely. And the slight impedance mismatch between the detector and oscilloscope may also influence the pulse shape.
Fig. 8. Pulse width and energy/pulse at different repetition frequency with absorbed pump power of 31.6 W
Fig. 9. Q-switched pulse with a FWHM of 315.2 ns at a repetition frequency of 1 kHz and absorbed pump power of 31.6 W
4. Conclusion
In conclusion, a a-cut Tm:LiLuF4 slab laser with doping concentration of 2 at.% was demonstrated. At room temperature, the continuous-wave output power of 10.4 W at 1916 nm was achieved with a slope efficiency of 40.4%. With Q-switched mode, the maximum pulse energy of 8 mJ with pulse width of 315.2 ns, corresponding to the peak power of 25.4 KW, was obtained at 1 KHz repetition frequency. The experiment result shows that Tm:LiLuF4 crystal will be a suitable material to get high pulse energy output because of the long upper-level lifetime.
The authors acknowledge researchers from Laser & Optoelectronic Functional Material R&D Center, Shanghai Institute of Optics and Fine Mechanics, China for their support with the Tm:LiLuF4 crystal.
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