We report on efficient actively Q-switched Ho:YAP laser double-pass pumped by a 1.91-μm laser. At room temperature, when the incident pump power was 20.9 W, a maximum average output power of 10.9W at 2118 nm was obtained at the repetition rate of 10 kHz, and this corresponds to a conversion efficiency of 52.2% and a slope efficiency of 63.5%. Moreover, a maximum pulse energy of ~1.1 mJ and a minimum pulse width of 31 ns were achieved, with the peak power of 35.5 kW.
©2009 Optical Society of America
Solid-state 2-μm lasers are useful for a variety of scientific and technical applications. Also, they are efficient pump sources for mid-infrared optical parametric oscillators[1,2]. The rare-earth-ion thulium (Tm3+) and holmium (Ho3+) co-doping materials, which work as quasi-three-level laser systems, are commonly used to obtain the 2-μm laser. However, in order to achieve a high power laser output, they need to operate at 77 K[3–6]. At present, 2-μm lasers based on holmium (Ho) ions have become prominent in these applications with the development of the 1.9-μm laser as an efficient pump source. Direct (in-band) pumping Ho 5I7 manifold offers the advantages of high quantum efficiency, minimal heating due to low quantum defect between pump and laser of ~10%, and reduced upconversion losses caused by Tm sensitized. Budni et al. reported 18.8 W of continuous wave (CW) Ho:YAG output with an M 2 ~ 1.48 pumped by a Tm:YLF laser, corresponding to a Tm-to-Ho optical conversion efficiency of 56%. Schellhorn reported a CW output power of 9.4 W Ho:YAG thin-disc laser with an M 2~ 5.2 pumped by a diode-pumped Tm:YLF laser, corresponding to a slope efficiency of 40% and an optical-to-optical efficiency of 36% with respect to the incident pump power. Lippert et al. reported 9.8 W of Ho:YAG output at a 20 kHz pulse repetition rate with M 2 < 1.1 pumped by a 15W thulium-doped fiber laser emitting at 1907 nm. Dergachev et al. reported 25 W of CW Ho:YLF output pumped by a 60W thulium-doped fiber laser emitting at 1940 nm, corresponding to optical-to-optical efficiency of 42%. In addition, Hart et al. demonstrated a room-temperature high efficient Ho: LuAG laser operating at 2.1 μm .
Among rare-earth doped host materials, Yttrium aluminium oxide (YAlO3, YAP) was chosen for study as a promising efficient singly doped laser material. YAP is a biaxial crystal possessing orthorhombic structure. Its natural birefringence combined with good thermal and mechanical properties similar to those of YAG. The spectral characteristics and laser actions of Ho:YAP was investigated, CW output power of 5.5W with 47% slope efficiency relative to the incident pump power of the 1 at.% Ho:YAP crystal was obtained .
In this paper, we present an efficient Q-switched Ho:YAP laser double-pass pumped by a diode-pumped Tm:YLF laser at room temperature. A slope efficiency of 65.3% and a conversion efficiency of 53.6% were obtained with CW output power of 11.2 W under the incident pump power of 20.9 W. With the same incident pump power, a maximum average output power of 10.9W was obtained at the repetition rate of 10 kHz, and this corresponds to a conversion efficiency of 52.2% and a slope efficiency of 63.5%.
2. Experimental setup
The experimental configuration is shown in the Fig.1. A diode-pumped Tm:YLF laser with emission wavelength of 1.91 μm is utilized as a pump source of Ho:YAP laser, since other high power lasers coinciding with the absorption peaks of Ho:YAP are not available to us. Tm:YLF crystal for the experiment is a-cut with dimensions of 3×3×12 mm3, and the doped concentration is 4 at. %. The pump source of Tm:YLF laser is a 60 W laser diode (MIF4S22-793.3-60C-H200H, DILAS) coupled by a fiber with core-diameter of 400 μm and numerical aperture of 0.22. By use of dual-end-pumped configuration, the pump waist is imaged to 360 μm, which is positioned ~ 4mm inside the Tm:YLF crystal. Considering the transmission losses, nearly 90% of the pump power input to the Tm:YLF crystal. The Tm:YLF laser resonator is folded with a physical cavity length of 130 mm. The output coupler coated with 22% transmittance at 1.91 μm is a plano-concave mirror with radius of curvature of 300 mm. A quartz etalon (0.1 mm in thickness) is used to tune the Tm:YLF lasing at 1.91μm.
Compared with our previous work, we used Ho crystal with lower dopand concentration in this paper. Low Ho3+ concentrations result in lower rates of ETU (energy transfer upconversion), which has significant impact on the laser performance , as well as reducing thermal loading. YAP crystal with 0.5 at. % Ho3+ concentration is grown by the Czochralski technique. The Ho:YAP crystal for the experiment is 35mm in length and 4×4 mm2 in cross section. The laser-through faces of crystal are polished plane and coated at both 1.91 μm and 2.12 μm with reflectivity < 0.5%. The absorption coefficient of Ho:YAP at 1.91 μm is about 0.36 cm-1, implying nearly 92% double-pass pump absorption by the crystal. The crystal is wrapped in indium foil and clamped in a copper crystal-holder held at a temperature of 15°C with a thermoelectric cooler.
The Ho:YAP laser resonator is folded with a physical cavity length of 100 mm. The flat mirror is high reflectivity (R >99%) in the wavelength range 1.9-2.2 μm. Flat 45° dichroic mirror is high reflection (R>99.5%) at 2.12 μm and high transmission (T>97%) at the p-polarized in the wavelength range 1.9-1.92μm. The output coupler coated with 52% transmittance at 2.12 μm is a plano-concave mirror with radius of curvature of 120 mm. The calculated TEM00 beam diameter is about 400 μm in the Ho:YAP crystal. The pump spot at the input surface of the Ho:YAP crystal is approximately 450 μm in diameter. As a result, the good overlap of the pump-to-Ho-resonator mode is achieved. Considering the transmission losses, nearly 95% of the Tm power is injected into the Ho:YAP crystal. Q-switching experiments are achieved with a 46 mm long fused silica acousto-optical (A-O) Q-switch (QS041-10M-HI7, Gooch & Housego). Its maximum RF power is 50 W and the repetition rate could be tuned continuously from 1 kHz to 50 kHz. To prevent Tm:YLF laser from being influenced by the feedback, a diaphragm is placed into the pump path, and the Ho resonator’s axis is misaligned from the pump axis by approximately 10 mrad.
3. Results and discussion
The power meter used in the experiment was Coherent PM30. Under incident diode power of 55.1 W, the Tm:YLF laser output power of 22.3 W was achieved with M 2 ~1.2 when the crystal temperature was 18 °C. The output power of Ho:YAP laser as a function of the 1.91-μm pump power is illustrated in Fig. 2. The maximum CW output power was 11.2 W under the incident pump power of 20.9 W, corresponding to a Tm-to-Ho conversion efficiency of 53.6% and a slope efficiency of 65.3%. An estimate of the slope efficiency relative to the absorbed power (approximately 87%, relative to the output power of the Tm laser) yielded a value of 75.1%. Operating at a CW pumped and repetitively Q-switched mode, the Ho:YAP laser achieved 10.9W average output power at repetition rate of 10 kHz, with corresponding to a conversion efficiency of 52.2% and a slope efficiency of 63.5%. To determine the beam quality factor M 2, we routed the Ho:YAP laser radiation through a 200mm focal-length lens and measured the beam radius along the propagation direction at full Q-switched laser power. By fitting Gaussian beam standard expression to these data, we estimate the beam quality to be M 2 ~1.44. Compared to Tm, Ho-codoped laser, this gives several advantages. First, with a low quantum defect, the laser’s slope efficiency could potentially be increased. Second, reduced the heat deposited in the laser medium, leading to increased beam quality and tolerance to high pump intensities. Third, reduced cooperative upconversion losses between Tm and Ho ions result in a significant increase in the effective upper level lifetime.
The output wavelength of Ho:YAP laser was recorded with a spectrum analyzer (WA-650, EXFO) combined to a wavemeter (WA-1500, EXFO). As is shown in Fig.3, the output laser wavelength was centered at 2118.25 nm with FWHM of about 0.5 nm. According to energy levels of Ho:YAP at 15 K, 2118 nm laser transfers from the 5187 cm-1 level of 5I7 manifold to 465 cm-1 level of the 5I8 manifold (illustrated in Fig.3). This indicates Ho:YAP laser is a quasi-two-level system . The laser was pumped at 1.91μm and lased at 2118 nm, which yielded a very low quantum defect of ~11%. The slope efficiency relative to the absorbed power achieved represents value of quantum efficiency greater than 80%. One possible reason why the limit was not achieved is the photon avalanche upconversion process in Ho:YAP. The results of Ref.  revealed that the dominant green fluorescence in Ho:YAP crystal utilized a 584.3nm excitation source. In the experiment, we measured the upconversion spectrum of 2% and 1% Ho3+ doped YAP crystal excited by a Tm:YLF laser from 400nm to 1000nm at room temperature. The spectrometer used was Ocean Optics HR-4000CG-UV-NIR. As is shown in Fig.4, the spectrum exhibited low intensity signal centered around 545nm corresponding to the 5S2→5I8 transition, and intense emissions centered around 657 and 893nm corresponding to the 5F5→5I8 and 5I5→5I8 transitions, respectively. The upconversion excitation mechanism concerning the Ho3+ excited-state levels is accomplished through a two/three-photon absorption, succeeded by cross-relaxation and excited-state absorption . The 1.91μm pump photon nonresonantly excites Ho3+ ions from the 5I7 upper-level to the 5I5 excited-state level which generates emission around 893nm. In this process, the participation of two/three host-phonons compensates for the energy mismatch of 850 cm-1 between the 5I7→5I5 transition and the pump photon energy. A second pump photon nonresonantly excites Ho3+ ions from the 5I5 to the 5F5 excited-state level which generates the red emission around 657nm. Simultaneously, a 2118nm photon promotes some Ho3+ ions from the 5F5 level to the 5F3 excited-state level, then relaxing to the 5S2 level which generates the green emission around 545nm. In the experiment, we observed that the upconversion fluorescence intensity becomes weaker with the decrease of Ho3+ concentration.
The Q-switched laser pulse was detected by using an InGaAs photodiode and recorded with an oscilloscope (wavejet 332, Lecroy). At the fixed repetition rate of 10 kHz, the dependence of laser pulse width on incident pump power was measured and is shown in Fig. 5. The pulse width shortens sharply when the incident pump power increases. As a result, the pulse width was 31 ns at PRF of 10 kHz when the incident pump power was 20.9 W, the profile of which can be seen from Fig.6. Furthermore, at the fixed incident pump power of 15.5W, we measured the dependence of laser pulse width on repetition rates. The pulse width increased greatly from 37 ns at 10 kHz to 133 ns at 50 kHz. Correspondingly, the peak power of the laser output decreased from 20.5 kW to 1.14 kW.
The dependence of the output power (Q-switched at 10 kHz) of the Ho:YAP laser on the temperature of the crystal-holder at an incident pump power of 15.5W has been measured as shown in Fig. 7. The average output power decreased from 8.1 W to 6.6 W as the crystal-holder temperature was increased from 5 °C to 30 °C. A simple linear fit to the data yields a slope of Ho:YAP laser output versus a crystal-holder temperature of -59mW/°C, indicating that the 1.91μm-pumped Ho:YAP laser possesses a low sensitivity of output over room temperature.
To summarize, we have demonstrated a room temperature efficient CW and Q-switched Ho:YAP laser double-pass pumped by a diode-pumped Tm:YLF laser. In CW mode, the maximum output power of 11.2 W and a slope efficiency of 65.3 % with respect to the incident pump power were achieved. In Q-switched mode with high PRF, we achieved the greater than 1 mJ energy per pulse at 10 kHz, and 10.9 W average output power with pulse width of 31 ns. The laser operated at a single mode (TEM00) with a beam quality factor of M 2 ~ 1.44, which was demonstrated by a knife-edge method.
This work is supported by the National Natural Science Foundation of China (Grant No. 60878011).
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