We present the room temperature Q-switched performances of a Ho:LuAG laser operated at 2.1µm. At the repetition rate of 10kHz, the maximum average output power of 9.9W with a slope efficiency of 69.9% relative to absorbed pump power was obtained in Ho:LuAG laser. Also, the minimum pulse width of 33.0 ns was obtained, corresponding to the peak power was 30.0kW.
© 2009 Optical Society of America
Q-switched 2-µm lasers have applications in remote sensing, and as pump sources for mid-infrared (mid-IR) optical parametric oscillators [1,2]. In recent years, with the development of 1.9-µm laser as an efficient pump source, 2-µm lasers based on holmium (Ho) ions have become prominent in these applications. Due to inherent low quantum defect between pump and laser, resonantly pumped Ho lasers have several advantages, including high quantum efficiency, minimal heating, as well as reduced upconversion losses compared to Tm-sensitized material. Moreover, due to a large emission cross-section and a good energy storage capability, the Ho system is suited for generation of ns-class pulses with high peak power by Q-switching technique. At present, Ho lasers based on YAG[3-6], YAP[7,8] and YLF[9,10] host materials have been investigated for generation of high power or high pulse energy 2-µm laser.
Lutetium Aluminum Garnet (Lu3Al5O12, LuAG) is a garnet isostructure similar to YAG, and its excellent mechanical properties are also similar to those of YAG . The LuAG (LuLF) has a higher crystal field than YAG (YLF), resulting in a large manifold splitting and a low thermal occupation for the lower laser level. And the slight molar mass difference of 5.8% between Ho and Lu (46.7% between Y and Ho) makes a weak decrease in thermal conductivity of LuAG with holmium doping . It’s for these reasons that the Tm-sensitized Ho doped LuAG (LuLF) lasers have much higher 2 µm laser efficiency compared to YAG (YLF) [14,15]. Moreover, previous works demonstrated that the substitution of Lu3+ ions for Y3+ ions in several structures provides Lu compounds with preferable conditions for trivalent lanthanide lasant doping [16-19]. Therefore, LuAG is a promising singly doped host material for Ho3+ ions to produce 2 µm radiation.
The pluse-pumped Tm,Ho:LuAG lasers have been investigated by V. Sudesh et al.  and N. P. Barnes et al. . V. Kushawaha et al. reported continuous wave (CW) output power of 1.38W from diode-pumped Tm,Ho:LuAG laser at 77K . K. Scholle et al. reported 18mW of single-longitudinal-mode output from a Tm,Ho:LuAG laser at room temperature. For Ho:LuAG crystal, it’s detailed spectroscopic characteristics have been investigated by B.M. Walsh et al.  and D. N. Patel et al. . D. W. Hart et al. demonstrated 10 mJ of output energy from a free-running Ho:LuAG laser end-pumped by a pulsed Co:MgF2 laser . N. P. Barnes et al. reported pluse-pumped Ho:LuAG laser with slope efficiency of 24% relative to incident pump power . However, there is less report on the Q-switched performances of Ho3+ ions doped in LuAG.
In this paper, we investigated the room temperature CW and actively Q-switched performances of a Ho:LuAG laser. A slope efficiency of 72.4% relative to absorbed pump power was obtained with maximum CW output power of 10.2 W under the absorbed pump power of 15.8 W. With the same pump power, a maximum average output power of 9.9W was obtained at the repetition rate of 10 kHz, and this corresponds to a slope efficiency of 69.9% relative to absorbed pump power. Simultaneously, the minimum pulse width of 33.0 ns was obtained, corresponding to a peak power of 30.0kW.
2. Experimental setup
The experimental configuration of Ho:LuAG laser is shown in the Fig. 1. According to room temperature absorption spectrum of the Ho 5I7 manifold in LuAG, the absorption peaks near 1.9µm are located at 1.88 and 1.91 µm, so we chose the diode-pumped Tm:YLF laser with emission wavelength of 1.91 µm as a pump source of Ho:LuAG laser. The Tm laser had an output power of ~22 W with M 2 of ~1.3. The Ho:LuAG crystal with 0.8 at. % Ho3+ concentration was grown by a standard Czochralski technique, owing to a higher concentration would increase the up-conversion rate significantly. The Ho:LuAG crystal for the experiment was 5 mm in diameter and 25 mm in length. Both end faces of the crystal were antireflection coated for the laser wavelength around 2.1 µm and the pump wavelength around 1.91 µm. The single-pass absorption of the crystal was measured to be 61.5%, implying nearly 85.2% double-pass pump absorption by the laser crystal. The Ho:LuAG crystal was wrapped in indium foil and clamped in a copper heatsink held at a temperature of 15°C with a thermoelectric cooler.
The Ho:LuAG laser resonator was folded with a physical cavity length of about 80 mm. Flat 45° dichroic mirror M1 is high reflective (R>99.5%) at 2.1µm and high transmission (T~97%) for p-polarized in the wavelength range 1.9–1.92µm. Considering the transmission losses, nearly 92.6% of the Tm pump power was input to the Ho:LuAG crystal. The flat mirror M2 have high reflectivity (R>99%) in the wavelength range 1.9–2.2 µm. The output coupler M3 is a plano-concave mirror with radius of curvature of 100mm. The calculated TEM00 beam diameter was about 360 µm in the Ho:LuAG crystal. By use of a 200 mm focal length mode-matching lens, we measured the pump spot by the knife-edge technique at the input surface of the Ho:LuAG crystal, which was approximately 400 µm in diameter, and the diameter of pump beam was nearly unchanged over the length of the Ho crystal. As a result the good overlap of the pump-to-Ho-resonator mode was achieved. A 30-mm-long Brewster-cut acousto-optic Q-switch (Gooch & Housego Ltd.) with an acoustic aperture of 1.8mm is the central component for repetitively Q-switched operation. The Q-switch material is crystal quartz with 99.6% transmission for 2.1 µm operation. The rated radio frequency power is 20W with radio frequency of 40.7 MHz. The modulation loss (with vertical polarization) is greater than 45%, which is adequate to hold off the intracavity laser actions. In order to prevent the influence on Tm:YLF laser by the feedback, a diaphragm was placed into the pump path, and the axis of Ho resonator was misaligned from the pump axis by several milliradian.
3. Experimental results and discussion
In order to reduce intracavity energy fluence two output couplers with relative high transmittance of 29% and 51% were used in this study. The output power of Ho:LuAG laser in the experiment was measured using Coherent PM30. Firstly, we investigated the CW output performance of the laser, as shown in Fig. 2a). At the absorbed pump power of 15.8W, the maximum output powers of 10.2 W and 9.3W were obtained for T=29% and T=51% respectively, which indicated conversion efficiencies of 64.6% and 58.9%. The corresponding slope efficiencies were 72.4% and 68.2%, respectively. In Q-switched regime, the output performance of Q-switched Ho:LuAG laser was investigated at PRF of 10 kHz. The average output power of Ho:LuAG laser as a function of the absorbed pump power is shown in Fig. 2b). For T=29%, the laser achieved 9.9W average output power under the absorbed pump power of 15.8 W, corresponding to a conversion efficiency of 62.7% and a slope efficiency of 69.9%. For T=51%, the laser achieved 9.0 W average output power under the absorbed pump power of 15.8 W, corresponding to a conversion efficiency of 57.0% and a slope efficiency of 65.7%. These results indicate that the difference of slope efficiencies with that two transmittances was small. Under maximum output power for T=29%, the output beam radius was measured by a 90/10 knife-edge technique at several positions which pass through a imaging waist formed by a 200mm focus lens, as shown in Fig. 3. By fitting Gaussian beam standard expression to these data, we estimate the beam quality to be M 2~1.4. The inset of Fig. 3 is the measured beam intensity distribution by Spiricon Pyrocaml pyroelectric camera.
Figure 4 shows the dependence of average output power of the Ho:LuAG laser with several different temperatures of the crystal-holder at an absorbed pump power of 10.3W. The output power decreases quite slightly as the temperature is increased, from 6.5 W at a crystal-holder temperature of 6°C to 6.1 W at a temperature of 28°C, which means a approximately 6.2% change in output within the temperature range measured. A simple linear fit to the data yields a slope of -17.4mW/°C, indicating that the 1.91µm-pumped Ho:LuAG laser possesses a very low sensitivity of output over room temperature.
The Q-switched laser pulse was detected by an InGaAs photodiode and recorded with a 350MHz digital oscilloscope (wavejet 332, Lecroy). At the fixed repetition rate of 10kHz, the dependence of laser pulse width on absorbed pump power was measured and is shown in Fig. 5. From the Fig. 5, the difference of pulse widths in two cases is slight, and the pulse width shortens sharply when the absorbed pump power increases. The minimum pulse width was 33ns at PRF of 10kHz when the absorbed pump power was 15.8 W, the corresponding profile of which can be seen from Fig. 6. The maximum energies per pulse of 0.99 mJ and 0.9mJ, corresponding to the peak powers of approximately 30.0 kW and 27.2 kW were obtained for T=29% and T=51%, respectively. Furthermore, at transmittance of 29% we measured the dependence of laser pulse widths on different repetition rates when absorbed pump power was 10.3W, as shown in Fig. 7. The pulse width increased greatly from 42 ns to 122 ns as the repetition rate was tuned from 10 kHz to 50 kHz. Correspondingly, the peak power of the laser output decreased from 14.8 kW to 1.0kW.
The output wavelength of Ho:LuAG laser was recorded with a spectrum analyzer (WA-650, EXFO) combined to a wavemeter (WA-1500, EXFO), as shown in Fig. 8. The output wavelength was 2100.7 nm with FWHM of about 0.25 nm for T=29% and 2094.5 nm with FWHM of about 0.6 nm for T=51%, respectively. According to Stark-split ground and first excited states in Ho3+:LuAG at 8 K , 2100 nm laser transfers from the 5231 cm-1 level of 5 I 7 manifold to 469 cm-1 level of the ground-state 5 I 8 manifold, and 2094 nm laser transfers from the 5244 cm-1 level of 5 I 7 manifold to 469 cm-1 level of the ground-state 5 I 8 manifold. In addition, no visible wavelength shifts were observed under different pump power levels with both the two output couplers used in the experiment. Also it was found that the output wavelength of the laser crystal was not sensitive to temperature changes.
In conclusion, we have demonstrated up to 9.9W average output power of a Q-switched Ho:LuAG laser at PRF of 10 kHz using in-band pumping with a diode-pumped Tm:YLF emitting at 1.91µm. With double-pass pump absorption, the slope efficiencies of 72.4% and 69.9% with respect to the absorbed pump power were obtained for CW mode and Q-switched mode respectively at an output coupler transmittance of 29%. The laser operated at a TEM00 mode with a beam quality factor of M 2~1.4, which was testified by a 90/10 knife-edge method. Also, the minimum pulse width of 33.0 ns was obtained, corresponding to a peak power of 30.0kW. In order to effectively pump optical parametric oscillators generating mid-IR radiation, the single-doped Ho lasers must have the characteristics of high peak power and short pulse width [2,10,26]. Our experimental results indicated that Q-switched Ho:LuAG laser will provide an excellent pump source for mid-IR optical parametric oscillators.
This work was supported by National Natural Science Foundation of China under Grant No. 60878011, and also supported by Acknowledge Innovation Program of Chinese Academy of Sciences.
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