We report on diode-pumped continuous-wave Tm:LuAG ceramic laser with a maximum output power of 2.64 W and slope efficiency of about 35.6%. Using a Cr:ZnSe saturable absorber, we also operate the Tm:LuAG ceramic laser in passively Q-switched regime. A maximum average output power up to 1.06 W with a slope efficiency of 16.1% is achieved with a V-shaped laser resonator. The narrowest pulse width is measured to be about 277 ns and the maximum pulse energy is about 81.4 μJ.
© 2017 Optical Society of America
During the past decade, researches on Tm3+ lasers are of great interest in view of a wide variety of emerging scientific and technological applications, such as remote sensing of atmospheric species, laser radar systems, medical surgery, plastic processing and pump sources for mid-infrared OPOs. In the field of solid state lasers, the development of Tm3+ lasers should be attributed to the development of Tm3+ laser materials, to great extent. At present, laser crystal materials have still been attached great importance. However, a rising research interest has also been focused onto laser ceramic materials because growing large-size and highly doped ceramics is relatively easy compared to growing crystals. Moreover, in general, growing ceramics is less expensive because it is not necessary to use crucible, which is necessary for growing crystals. Since the development of Tm:YAG ceramic lasers [1–3], several Tm3+ ceramic lasers have been subsequently explored, such as Tm:Lu2O3 , Tm:Y2O3 , Tm:LuScO3 , Tm:CaF2 and Tm,Ho:CaF2 .
LuAG host has been proved to be a promising laser material because of its excellent properties , e.g. comparable physical properties to YAG and higher damage threshold than YAG. During the past years, lasers based on LuAG doped with Nd3+ [8–10], Yb3+ [11,12], Er3+  and Ho3+  rare-earth ions have been developed greatly. In terms of Tm3+:LuAG, the longer operating wavelength of Tm:LuAG laser than that of Tm:YAG has been proposed to be of advantage because the transmission through the atmosphere is better at longer wavelengths . In fact, after the early report of the Tm:LuAG crystal laser , researchers have still made efforts to further study this crystal. For instance, a diode-pumped Tm:LuAG crystal laser was reported with maximum output power of 4.91 W and slope efficiency of 25.39% . In 2015, Wu et al.  reported a passive Q-switching of Tm:LuAG crystal laser with pulse width of 63.3ns and pulse energy of 5.51 μJ by using a Bi-doped GaAs saturable absorber. In 2016, researchers from same group reported a passive Q-switching of the crystal by WS2 saturable absorber with a pulse width 660 ns . At present, Tm:LuAG ceramic laser was only once reported very recently by Wang et al. , who obtained a maximum output power of 830 mW in continuous-wave mode and a pulse width of 2.7 ps in mode-locked mode. However, for achieving large single pulse energy, Q-switched laser operation is still an important method. Moreover, ns pulse by Q-switching has different applications that ps and fs pulses by mode locking cannot provide. At present, no Q-switched Tm:LuAG ceramic laser has been reported. On the other hand, improving the output power of continuous-wave Tm:LuAG ceramic laser to the level of several watts is also necessary for applications.
In this paper, using a 790-nm pump source, continuous-wave (with relatively high output power) and passively Q-switched (using a Cr:ZnSe saturable absorber) Tm:LuAG ceramic lasers have been investigated, respectively using a two-mirror linear cavity and a three-mirror V-shaped folded cavity.
2. Experimental setup
The laser experimental setups of the diode-pumped Tm:LuAG ceramic lasers in continuous-wave and Q-switched regimes are schematically shown in Fig. 1(a) and 1(b). The pump source was a fiber-coupled diode laser emitting at 790 nm. The core diameter is about 105 μm with numerical aperture of 0.22. The pump beam was collimated and then focused into the laser gain medium via two doublet lenses with focal lengths of 30 and 50 mm. For continuous-wave laser operation, the laser resonator consisted of a simple and compact two-mirror configuration, i.e. a plane-parallel cavity. The flat input mirror (IM) has a high transmission of about 94% at pumping wavelength and high reflection of more than 99.8% at laser wavelength. Two output couplers (OCs) with partial transmissions of 10.3% and 8.1% were used to couple the cavity mode out of the laser resonator. The physical length of the two-mirror laser cavity was about 45 mm. For Q-switched laser operation, a V-type laser resonator was employed to form a three-mirror laser configuration with a newly inserted folded mirror (FM). The FM has a high transmission at pumping wavelength and high reflection (more than 99.8%) at laser wavelength. The curvature radius of the FM is 100 mm. The physical length of the three-mirror cavity was about 230 mm with arm lengths of about 95 mm for IM-FM and 135 mm for FM-OC. The folded angle was mounted as small (about 15°) as possible to reduce the astigmatism.
The laser gain medium was an end-face polished and uncoated Tm:LuAG ceramic with dopant concentration of 6at% and dimensions of 3 × 3 × 6 mm3 (6 mm in length), which was wrapped with indium foil and mounted inside a copper block for thermal mitigation. The copper block was connected to a water-cooled chiller with temperature set at 16°C. During Q-switched laser experiments, an anti-reflection coated Cr:ZnSe crystal was used as saturable absorber with initial transmission of about 95%. The Cr:ZnSe crystal was sandwiched between two pieces of copper plates without additional cooling.
3. Results and discussion
3.1 Spectral properties
The Tm:LuAG ceramic was fabricated by using a solid-state reactive sintering method using high-purity powders of Lu2O3, α-Al2O3 and Tm2O3 as starting materials. A 6at%-doped Tm:LuAG ceramic sample with thickness of 2 mm was used to measure the absorption and fluorescence spectra. Figure 2(a) shows the room temperature absorption spectrum from 400 to 2000 nm. Five bands are associated with Tm3+ transitions from the 3H6 ground state to 1G4, 3F3 + 3F2, 3H4, 3H5, 3F4 excited states, respectively. The absorption peak lies at about 682 nm corresponding to 3H6 → 3F3 + 3F2 absorption. In terms of 3H6 → 3H4 absorption, a two-peak structure with very close intensities can be observed with peak at 784 nm and neighbouring sub-peak at 788 nm. The full widths at half-maximum (FWHM) of this absorption band is about 15 nm, which is very similar to that reported in Ref . This broad absorption makes our commercially available 790-nm AlGaAs laser diode effective as a pump source. Figure 2(b) shows the fluorescence spectrum of the Tm:LuAG ceramic sample from about 1400 to 2200 nm corresponding to 3F4→ 3H6 transition. Although the fluorescence peak is at about 1773 nm, it is not capable of lasing because of the strong reabsorption effect, as shown in Fig. 2(a). Operating short-wavelength Tm3+ lasers need to manage the intracavity round-trip loss and to tune the lasing wavelength with the aid of prisms , filter , or by applying particularly coated cavity mirrors . Besides the 1773 nm line, there are several obvious fluorescence peaks at 1792, 1873 and 2016 nm with considerable intensities. Among them, the 2016 nm line is very likely to lase thanks to its weakest reabsorption.
3.2 Laser results and discussion
Figure 3(a) shows the output power characteristics of Tm:LuAG ceramic lasers using the two-mirror plane-parallel cavity. Using the 10.3% transmittive OC, a maximum output power reached 2.41 W with threshold (absorbed power) of 1.28 W and slope efficiency of about 35%. Replacing the 10.3% OC with the 8.1% one, we achieved a maximum output power of 2.64 W with a reduced threshold to 0.84 W and a slope efficiency of about 35.6%. The laser spectra were measured using an optical spectrum analyzer (Ocean Optics, NIRQuest) with a precision of about 3.2 nm, as shown in Fig. 3(b). For both OCs, the laser spectra show the same peak wavelengths at about 2016 nm.
It should be pointed out that, from the output power curves, the output powers show good linearities, i.e. no indication of power saturation. Therefore, the present maximum output powers were mainly restricted by the available absorbed power resulting from the weak absorption because of mismatching between the pumping wavelength and absorption peak of the Tm:LuAG ceramic, as shown in Fig. 2. We estimated the round-trip loss L based on Findlay-Clay analysis by using the following equation,Eq. (1), we can readily calculate the round-trip loss to be about 3.8%. The relatively large loss indicates that the laser performance could be improved by optimizing the quality of the Tm:LuAG ceramic. Under the present laser configuration, the cavity mode size of waist inside the Tm:LuAG ceramic was estimated to be about 110 μm. Thus, using the similar calculating process to , we estimated that the overlap efficiency between the pump beam and cavity mode was about 60%.
Before operating a passively Q-switched Tm:LuAG ceramic laser using the V-shaped laser cavity, continuous-wave laser operation was carried out without the insertion of the saturable absorber. Using the 8.1% OC, a maximum output power up to 2.46 W with slope efficiency of about 32.9% was achieved and the threshold was also increased to 1.0 W of absorbed power. Compared with the two-mirror cavity, the V-shaped cavity showed a power reduction of about 7%, which is understandable if considering a potential additional round-trip loss originating from the longer cavity and the imperfect coating of the FM. The output power and corresponding laser spectrum are shown in Fig. 4. Under current laser configuration, the waist size of the cavity mode inside the Tm:LuAG ceramic was estimated to be about 115 μm, i.e. almost the same to the above two-mirror cavity case. Since the present folded angle was small, the waist sizes between FM and OC were almost the same to be about 107 μm for sagittal and tangential beams.
According to the above estimations of the waist sizes, we then inserted the as-prepared Cr:ZnSe saturable absorber to where the waist was. By slightly adjusting the orientation and position of the Cr:ZnSe, stable Q-switched laser pulse was obtained with maximum average output power of 1.06 W and slope efficiency of about 16.1%, as shown in Fig. 4(a). Figure 4(b) shows the laser spectrum of the Q-switched laser with peak wavelength at about 2016 nm, namely very similar to the continuous-wave cases. The threshold of the Q-switched laser was about 1.79 W, higher (but not so higher) than the continuous-wave case. This indicated that the insertion loss and non-saturable loss of the Cr:ZnSe was not high. The typical Q-switched pulse trains is shown in Fig. 5(a) measured at maximum output power with a repetition rate of 13.02 kHz. Correspondingly, the pulse time duration was also measured to be about 282 ns, as shown in Fig. 5(b). Using an RF spectrum analyzer (Gwinstek GSP-930) with resolution bandwidth (RBW) of 100 Hz, the RF spectrum of the Q-switched laser was measured to have a signal-to-noise (SNR) of about 35.5 dB (see in Fig. 5(c)), which verifies a moderate stability of the Q-switched operation. Recently, we have operated a passively Q-switched Tm:CaGdAlO4 laser with the shortest pulse width of about 44 ns in a two-mirror linear cavity with length of 45 mm , which is far shorter than the presently obtained pulse width. It seems clear and understandable for the 282 ns pulse width if taking the present longer cavity into account. However, due to the usage of the folded cavity in this work and therefore no influence coming from the residual pump on the saturable absorber, the Q-switched Tm:LuAG ceramic laser can be operated at higher level of pump power.
To completely explore the laser performance of the Q-switched Tm:LuAG ceramic laser, evolutions of the pulse width and pulse repetition rate varying from the increasing of the absorbed powers are shown in Fig. 6(a) and 6(b). The pulse width narrowed to about 277 ns at an absorbed power of 3.68 W. After, the pulse width fluctuated at around 280 ns. Meanwhile, the pulse repetition rate monotonously increased to the maximum despite showing saturation phenomenon especially when the absorbed power exceeded about 7.5 W. Using these data recorded in Fig. 6(a) and 6(b), we can readily estimate the values of pulse energy and pulse peak power, as shown in Fig. 6(c) and 6(d). The maximum pulse energy reached 81.4 μJ and the maximum pulse peak power was about 288 W.
In conclusion, we demonstrated diode-pumped Tm:LuAG ceramic lasers in continuous-wave and passively Q-switched regimes. For continuous-wave operation, a maximum output power up to 2.64 W was achieved with slope efficiency of about 35.6%. Passively Q-switched laser operation was also obtained with a maximum average output power of 1.06 W and corresponding slope efficiency of about 16.1%. The narrowest pulse width was measured to be about 277 ns and the maximum pulse energy reached 81.4 μJ. This work has represented the first passive Q-switching of Tm:LuAG ceramic laser, to the best of our knowledge. In the following, we hope to scale the pulse energy to the level of mJ by optimizing the laser quality, narrowing the pulse width and at the same time by improving the maximum output power.
National Natural Science Foundation of China (61575164).
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