We have reported continuous-wave (CW) and Q-switched operations of a polycrystalline ceramic Ho:LuAG laser in band pumped by a Tm:fiber laser at the wavelength of 1907 nm. By using an output coupler of 20% transmission, maximum continuous-wave output power of 2.87 W for 9.72 W of incident pump power was achieved, corresponding to a slope efficiency of 31.9%. Shortest pulse duration of 21.0 ns with peak power of 28.2 kW has been obtained at 500 Hz pulse repetition frequency (PRF) under 5.65 W of incident pump power.
© 2014 Optical Society of America
Pulsed lasers with eye-safe wavelengths in the 2.1 μm spectral region based on a holmium-doped laser crystal, e.g. Ho:YAG, Ho:YLF or Ho:LuAG, have found wide applications in range finding, remote sensing, LIDAR, medicine and generation of 3-5 µm mid-infrared lasers via nonlinear frequency conversion [1–3]. With the advent of 1.9 µm high power lasers such as thulium fiber lasers , Tm:YLF solid state lasers  and semiconductor InGaAsP/GaSb diode lasers , resonantly pumped singly Ho3+-doped crystal lasers have obviated the energy-transfer loss between Tm3+, Ho3+ ions in Tm, Ho-codoped materials and demonstrated high efficiency and power scalability. Of ten garnet host materials, LuAG has a large manifold splitting and results in a low thermal occupation factor for the lower laser level and in favorable branching ratios for 2.1 μm lasing at room temperature. Work has shown that Ho:LuAG has a slope efficiency of 82% with respect to the absorbed pump power , higher than that of Ho:YAG and Ho:YLF.
In recent years, Ho3+-doped transparent ceramics have drawn much attentions as new laser gain media because they have advantages over single crystals as rapid and large volume fabrication, furthermore, materials that are difficult to grow into single crystals by the conventional Czochralski method (e.g., Y2O3 and Lu2O3) can be fabricated in ceramic form [8–13]. Resonantly pumped Ho3+-doped ceramic lasers at ~2.1 μm spectral region have been reported [14–18]. First resonantly diode pumped laser operation based on Ho3+-doped Y2O3 was demonstrated at 2.119 μm with a slope efficiency of 35% with respect to the absorbed pump power and 2.5 W of CW output at 77 K . By using Tm:fiber pump source at 1907 nm, the Ho:YAG ceramic laser has produced maximum output power of 21 W, corresponding to a slope efficiency of 63.6% and an optical conversion efficiency of 61.1% . Q-switch operations for Ho:YAG ceramics have also been reported. With multilayer-graphene as saturable absorber, stable pulses of 28-64 kHz repetition rate and 2.6-9 μs pulse widths were generated and maximum average power of 264 mW with 9.3 μJ pulse energy was obtained . In the actively Q-switched operation, pulse energy of 10.2 mJ at a PRF of 100 Hz was obtained and the wavelength of Ho:YAG ceramic laser was tuned from 2090.70 to 2098.10 nm . As regards to Q-switched Ho:LuAG solid-state lasers, only operations of single crystal samples have been reported, up to 9.9 W of average output power has been obtained so far .
Ho3+-doped LuAG transparent polycrystalline ceramic were fabricated and characterised successfully for the first time in 2013 . In this paper, we describe a high efficiency cw and Q-switched operation of Ho:LuAG ceramic laser, which was in-band pumped (5I8→5I7) by a cladding-pumped Tm fiber laser at ~1907 nm. With an output coupler of 20% transmission and 1.0 at.% Ho3+-doped LuAG ceramic sample, the laser yielded a maximum cw output power of 2.87 W at 2124.5 nm for the incident pump power of 9.72 W, corresponding to a slope efficiency of 31.9%. In Q-switched operation, we obtained a peak power of 28.2 kW at 500 Hz pulse repetition frequency with a pulse duration of 21.0 ns, this pulse width is currently considered the shortest pulse duration reported for a resonantly pumped actively Q-switched Ho:LuAG laser.
2. Experimental setup
A 1.0 at.% Ho3+-doped Ho:LuAG ceramic (fabricated at Jiangsu Normal University) was used as the laser gain medium. It was 13.2 mm in length and 3 × 3 mm2 in cross section with both end facets antireflection-coated in the 1800-2150 nm wavelength range. The crystal was wrapped with indium foil (0.1 mm in thickness) and mounted on a water-cooled copper heat-sink maintained at ~15°C to ensure efficient heat removal. Single-pass absorption characteristic at 1907 nm was measured under non-lasing condition (i.e., without the existence of a laser resonator), the absorption was 77% for small signal pumping while it decreased to 48% for maximum incident pump power of 2.92 W due to the strong ground-state bleaching (see Fig. 1). Unabsorbed pump light after the first-pass was reflected back into the ceramic media by the output coupler to increase the overall absorption.
Laser configuration used in our experiment is shown schematically in Fig. 2. A simple two-mirror resonator was adopted, it comprised a plane pump input coupler (IC) with high reflectivity (R>99.8%) at the lasing wavelength (2050-2250 nm) and high transmission (>95%) at 1850-1960 nm, and a concave output coupler (OC) of 100 mm radius-of-curvature (ROC). The output coupler had a transmission of either 6 or 20% at 2000-2250 nm and high reflectivity at pump wavelength to improve the total pump absorption efficiency. In cw operation, the physical cavity length is about 18 mm, resulting in an estimated resonant mode size of ~300 μm in the gain medium with a resonator analysis. The fiber pump laser used in our experiments was constructed in-house and comprised ~4 m of double clad fiber with a 25 μm diameter Tm-doped alumino-silicate core (0.17 NA) and 350 μm diameter pure silica inner-cladding of 0.46 NA. Operating wavelength of the Tm:fiber laser was tuned to match the absorption peak of Ho:LuAG at ~1907 nm. The beam propagation factor (M2) of the laser output was measured with a beam profiler (Nanoscan, Photon Inc) to be ~2.6. The pump source was collimated by a 30 mm focal length plano-convex lens and subsequently focused to a beam of ~230 μm diameter at the center of Ho:LuAG using a 200 mm focal length lens, resulting a confocal parameter of ~30 mm inside the Ho:LuAG ceramic.
For Q-switching, a 60 mm long acousto-optic Q-switch (Gooch & Housego Ltd) with an acoustic aperture of 2.0 mm in the central component was inserted into the resonator and the physical cavity length was increased to 80 mm. The acousto-optic material is a crystal quartz with 99.6% transmission at 2.1 μm spectral region. It is driven with 100 W radio-frequency (RF) power at frequency of 27.12 MHz. Its diffraction efficiency is greater than 85%, which is adequate to hold on the intra-cavity laser actions.
3. Results and discussion
A comparative study was first conducted to evaluate the laser performance of the Ho:LuAG ceramic sample with output couplers of 6 and 20% transmissions (Toc). The output power as a function of incident pump power is shown in Fig. 3. It can be seen that better performance in terms of maximum output power and slope efficiency is achieved for the 20% output coupler. The laser reached threshold at ~0.65 W of incident pump power and yielded 2.87 W of output power for 9.72 W of incident pump power at 1907 nm, corresponding to a slope efficiency with respect to incident pump power of 31.9% and optical-to-optical conversion efficiency of 30%. For the output coupler of Toc = 6% transmission, a lower output power of 1.81 W and slope efficiency of 19.8% was obtained.
Spectrum of the Ho:LuAG ceramic laser was analyzed using an optical spectrum analyzer (AQ6375, Yokogawa) with a resolution of 0.1 nm (as shown in the insert of Fig. 3). Emission wavelength of the laser output was measured to be 2124.5 nm for the output coupler of 6%, while shifted to 2100.7 nm for the output coupler TOC = 20%. This may be attributed to the fact that lasing at 2124.5 nm benefits from a more four-level character requiring smaller quantity of excited Ho3+ ions to reach transparency than that requires for 2100.7 nm transition. Compared to 6% transmission OC, both the oscillating thresholds for 2100.7 nm and 2124.5 nm wavelength increased with 20% transmission OC. With the accumulation of population inversion, the 2100.7 nm transition reaches threshold first and laser oscillation starts at this wavelength due to its relatively larger emission cross-section. Simultaneously, dual-wavelength lasing at both 2100.7 nm and 2124.5 nm should be achievable by choosing an output coupler of suitable transmission.
An acousto-optic Q-switch was inserted into the cavity following the Ho:LuAG gain medium for Q-switched operation. For this cavity geometry, beam spot radius in the center of the rod increased from ~150 μm to ~170 μm. The OC of 20% transmission used in cw operation was employed to output pulsed laser radiation. Pulse width was detected using a fast 2 μm InGaAs PIN detector (Newport, 818-BB-51) and then recorded with a 1 GHz bandwidth oscilloscope (Tektronix, DPO 7104C). Pulse energy as a function of incident pump power for the Q-switched Ho:LuAG ceramic laser at PRFs of 500 Hz, 1 kHz, 2 kHz and 6 kHz showed a monotonic tendency over the threshold until the maximum pump power of 6.23 W in our experiment (see Fig. 4). Single pulse energies increased from 0.09 to 0.46 mJ for different PRFs from 6 kHz to 1 kHz under the same incident pump power of 6.23 W. The maximum pulse energy of 0.60 mJ was obtained under pump power of 5.65 W at 500 Hz PRF.
Figure 5 illustrates the dependence of peak power and pulse width on PRF under 5.65 W of incident pump power. That shows a dramatic monotonous decrease in peak power from 28.2 to 0.2 kW and a linearly increase in pulse width from 21.0 to 246 ns when PRF is increased from 500 Hz to 10 kHz. The shortest pulse duration of 21.0 ns was obtained at 500 Hz PRF, corresponding to a peak power of 28.2 kW. To the best of our knowledge, this pulse width is the shortest for in-band pumped Q-switched Ho:LuAG lasers. Twin peaks emitting at 2100 nm and 2094 nm simultaneously have always been observed for our Q-switched Ho:LuAG ceramic laser under different pump powers and PRFs. A typical spectrum profile is shown in the insert of Fig. 5.
A moderate amplitude fluctuation has been inspected if the average pulse energy is below 0.15 mJ, the pulse train becomes more and more stable when the pulse energy is increased, stable pulse train with slight pulse jitter can be attained over 0.30 mJ of pulse energy. The pulse to pulse amplitude fluctuation has been estimated to be less than 4%. The pulse train with 21 ns average pulse duration at 500 Hz PRF for pump power of 5.65 W has been illustrated in Fig. 6.
The beam quality of the output beam was measured at 500 Hz PRF under 5.65 W of pump power by a beam profiler (NanoScan, Photon Inc.). Figure 7 depicts the beam radii along the axis and the inset shows the profile of the laser beam near the focus. A slight shift of beam waist along the two perpendicular directions can be seen in the figure. By fitting the measured data with a hyperbolic curve, the M2 factors were calculated to be 1.23 and 1.20 in x and y directions, respectively.
In summary, we have demonstrated highly efficient continuous wave and actively Q-switched polycrystalline Ho:LuAG ceramic laser resonantly pumped by a Tm:fiber laser at room temperature. 2.87 W of output power at 2100.7 nm has been obtained with a near diffraction-limited beam quality under 9.72 W of incident pump power, corresponding to a slope efficiency of 31.9% with respect to the incident pump power. The shortest pulse duration of 21.0 ns is achieved at 500 Hz PRF. This is believed to be first report of Q-switched Ho:LuAG ceramic laser.
This work is supported by the National Natural Science Foundation of China (NSFC 61177045 and 11274144), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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