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Characteristics of Tm:YAG ceramic for high efficient 2-μm lasers are analyzed. Efficient diode end-pumped continuous-wave and Q-switched Tm:YAG ceramic lasers are demonstrated. At the absorbed pump power of 53.2W, the maximum continuous wave (cw) output power of 17.2 W around 2016 nm was obtained with the output transmission of 5%. The optical conversion efficiency is 32.3%, corresponding to a slope efficiency of 36.5%. For Q-switched operation, the shortest width of 69 ns was achieved with the pulse repetition frequency of 500 Hz and single pulse energy of 20.4 mJ, which indicates excellent energy storage capability of the Tm:YAG ceramic.
©2011 Optical Society of America
1. Introduction
Transparent laser ceramics have attracted more and more interest as an additional substitute for single crystals, due to many significant advantages such as higher concentration, multi-functional structure and lower cost when compared with single crystal materials [1–6]. For 1-μm wavelength lasers, ceramics has been successfully demonstrated with the same efficiency as that of single crystals [7–13]. However, only a few researchers have been aware of great advantages of ceramics for 2-μm wavelength lasers, which have many important applications [14,15], in remote sensing, LIDAR, medical treatment, etc. In this paper, we highlight the benefits of ceramic in developing high efficient 2-μm lasers and demonstrate an efficient short-pulse Tm3+-doped YAG ceramic slab lasers. The performance of actively Q-switched Tm:YAG ceramic laser is investigated for the first time to the best of our knowledge. When the absorbed pump power is 53.2 W, a minimum pulse width of 69 ns is achieved with the repetition rate of 500 Hz, which is much narrower than that reported from single crystal lasers [16]. The short pulse width indicates excellent energy storage capability of Tm:YAG ceramic. At the meantime, we obtain 17.2 W cw output power from this laser, corresponding to a slope efficiency of 36.5%, which is much higher than that reported previously by our group [17].
2. Characteristics of high efficient 2-μm ceramic
Transparent ceramics have additional remarkable characteristics that make them promising candidates for efficient laser operation in the 2 μm wavelength range. It is well known that the scattering loss in ceramic is main resistance to high efficient laser operation [18]. The scattering sources in ceramics have two different typical sizes. The sizes of pores and color centers are usually around several micrometers. The scattering losses, described by the Rayleigh scattering equation, are inverse proportional to the four orders of the laser wavelength. Consequently, the losses from these scattering sources at 2-μm wavelength are 16 times smaller than that at 1-μm wavelength. The shortcoming of ceramic with large scattering loss is relieved in the 2-μm wavelength range. In practice, laser operation in ceramics has been firstly demonstrated in the Mid-IR wavelength range [19]. On the other hand, the grain boundaries in the ceramics can be as fine as hundreds nanometers [20]. Whether for 1-μm or 2-μm wavelength laser, the scattering losses from the grain boundaries are negligible. However, UV light will be scattered by the grain boundaries. The UV light comes from the up-conversion process in 2-μm solid-state lasers, in which the population is excited to upper levels and emitted the photon at UV band. The up-conversion process consumes the population inversion and degrades the laser efficiency. When the UV light is scattered, the rate of up-conversion may be reduced, resulting in high efficiencies of 2-μm ceramic lasers.
As shown in Fig. 1 , the Mid-IR transmission spectrum of YAG ceramic extends to the longer wavelength range, in compared with YAG single crystal. It is known that the longest transmission wavelength corresponds to the lowest phonon energy [21]. The longer transmission range indicates lower phonon energy in the YAG ceramics. The lower phonon energy, then, leads to smaller non-radiative transition rate. There are two advantages from the smaller non-radiative transition rate. One is higher laser efficiency due to the increased branching ratio to the radiative transition. The other is longer lifetime of the upper level. The lifetime of Tm:YAG single crystal was reported to be 10~11 ms by different groups [22,23]. We measured the lifetime of Tm:YAG ceramic and single crystal in the same condition. The lifetime of Tm:YAG ceramic (10.5 ms) was 5% longer than that of Tm:YAG single crystal in our measurements. Generally speaking, the laser material with a longer lifetime has the better capability of energy storage. Longer lifetime is benefit to generate the large pulse energy at low repetition rate.
Another influence to the laser efficiency is the variance of doping concentration around grain boundaries in the ceramic. Near the grain boundary, the doping concentration is slightly higher than that in the center of grain [24]. Thus, the cross relaxation of Tm3+-ion is increased near the grain boundary, resulting in high quantum efficiency due to the “1-for-2” energy transform process of Tm3+-ions.
Unlike Nd:YAG ceramic, the emission spectrum of Tm:YAG ceramic is not exactly the same as that of single crystal. Figure 2 compares the emission spectrum of Tm:YAG ceramic and Tm:YAG crystal. The spectral peaks at 2015 nm, 1967 nm, 1888 nm are in the same positions for both ceramic and single crystals, but the spectral peaks for the Tm:YAG ceramic are slightly wider. It is a general phenomenon in ceramics because the crystal field and then the emission spectrum vary from site to site inside the grains. The assembly of these spectra blurs the fine spectral structure and leads to a wider linewidth, which is similar to that in the mixed crystal [25].
The spectral peak at 1792 nm in the Tm:YAG ceramic is a valley in the spectral of Tm:YAG single crystal. Furthermore, the peak at 1888 nm is higher in Tm:AYG ceramic than that in Tm:YAG single crystal. It makes the laser emission around 1800 nm more possible in Tm:YAG ceramic than that in Tm:YAG single crystal.
3. Experimental setup and results
The schematic diagram of the experimental setup is depicted in Fig. 3 . The Tm:YAG ceramic slab sample used in the experiment is 1 mm thick, 5 mm wide and 6 mm long with Tm3+-ion doping concentration of 6 at.%. Both sides of the ceramic are antireflection (AR) coated at 780 ± 15 nm and 2000 ± 50 nm. To dissipate the heat effectively, the ceramic is wrapped in indium foil and water-cooled by a copper micro-channel heat-sink and the temperature of cooling water is kept at 15°C in our experiment. A fast-axis collimated laser diode array (LDA) with the central laser wavelength of 782 nm is used and its maximum pump power is 80 W. The pump light is focused into the ceramic by one plano-concave cylindrical lens with the radius of 28-mm and plano-convex spherical lenses with the radius of 22 mm. The focused pump beam in the laser medium has a dimension ~1.5 mm × 0.5 mm. The compact cavity consists of two mirrors. The rear flat mirror M1 is AR coated at 780 ± 15 nm and high reflection coated at 2000 ± 50 nm. The output coupler M2 is also flat mirror with transmission of 5% at 2000 ± 50 nm. A beam splitter M3 is placed behind the output coupler, making it possible to measure the output power and the pulses simultaneously. In our experiment, the cw laser operation is tested firstly with the cavity length of 25 mm and the pulsed laser operation is carried out by inserting an AO Q-switcher (Gooch & Housego, QS027-4M-AP1) into the cavity with the cavity length increasing to 80 mm.
The cw and Q-switched laser characteristics with the transmission of 5% for output coupler are illustrated in Fig. 4a . The maximum output power of 17.2 W is achieved at the absorbed pump power of 53.2 W, giving the optical conversion efficiency of 32.3% and the slope efficiency of 36.5%. The central laser wavelength (Fig. 4b) of Tm:YAG ceramic is 2016 nm and 2006 nm, with a full width at half-maximum (FWHM) of about 5 nm, respectively. As far as we know, up to the present, it’s the highest power reported for Tm:YAG ceramic. The threshold pump power is 6.1 W. For the Q-switched operation, the maximum average output power is 11.8 W, 11.32 W and 10.2 W, corresponding to the pulse repetition frequency (PRF) of 5 kHz, 1 kHz and 500 Hz.
Fig. 4 (a) The output power versus the incident absorbed pump power. (b) Laser spectrum of Tm:YAG ceramic.
Figure 5 shows the pulse width and pulse energy versus the absorbed pump power. It can be found that, the pulse width became shorter gradually with the increasing of the absorbed pump power while the PRF was changed from 500 Hz to 5 kHz. Under the highest absorbed pump power of 53.2 W, the minimum pulse width of 69 ns was obtained with the PRF of 500 Hz. It is so far the shortest pulse width acquired for the Tm:YAG ceramic or single laser crystal to the best of our knowledge, resulting from better energy storage of Tm:YAG ceramic. Meanwhile, the largest single energy of 20.4-mJ was gained in the same condition. Figure 6 depicts a typical pulse profile with the pulse width of 69 ns. This verifies the excellent characteristics of Tm:YAG ceramic for the actively Q-switched operation.
4. Conclusion
In conclusion, we have successfully demonstrated an efficient diode-end-pumped cw and actively Q-switched polycrystalline Tm:YAG ceramic laser. An output power of 17.2-W near 2016-nm is obtained at the absorbed pump power of 53.2-W, corresponding to a slope efficiency of 36.5%. A shortest pulse width of 69 ns is achieved under the PRF of 500-Hz, and the corresponding single pulse energy is 20.4 mJ. The characteristics of Tm:YAG ceramic are analyzed for high efficiency of 2-μm laser operation. The results reveal that Tm:YAG ceramic is an excellent laser medium for high efficient, short pulsed 2-μm laser operation.
Acknowledgments
The works is supported by the Natural Science Foundation of Shanghai, China, under contract 09ZR135100.
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