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Diode-pumped continuous-wave Nd:LuAG ceramic lasers at the 1.4-μm eye-safe spectral region are reported. A simultaneous dual-wavelength laser at 1418 nm and 1442 nm is achieved with a maximum output power of 1.37 W and a slope efficiency of 13.5% using a plane-concave cavity configuration. Laser generation at another emission line of 1432 nm is also realized with a maximum output power of 0.82 W with the help of an intracavity etalon. To improve the laser beam quality, a parallel plane laser cavity operating still at 1418 nm and 1442 nm is adopted in free-running mode.
© 2017 Optical Society of America
1. Introduction
During the past years, eye-safe lasers, operating at about ~1.4 and ~1.9 μm, are of great interest because such lasers can be absorbed greatly by water [1, 2], which leads to the lasers cannot reach the retina. Therefore, relevant applications can be found including laser radar, material processing (plastic welding), atmospheric measurements, biomedical and medical applications (such as laser angioplasty, ophthalmic procedures, laser lithotripsy, and laser surgeries). The eye-safe advantage makes the use of these laser less demanding in terms of safety installation. The ~1.9 μm laser sources based on Tm3+ and Ho3+ lasers are typical quasi-three level system exhibiting unavoidable reabsorption losses [3–8]. Recently, based on the true four-level scheme and therefore without any reabsorption problem, eye-safe laser source at around 1.83 μm have been reported using Nd ions as gain medium [9–11]. However, achieving 1.83 μm Nd3+ lasers with high efficiency are very difficult because of very weak emission cross section for the 1.83 μm transition. Comparatively, high-efficiency Nd3+ lasers at 1.4 μm can be generated because of higher emission cross section [12–17].
On the other hand, compared with crystal, ceramic laser materials have attracted a lot of attention during the past decade because of their several advantages. First, ceramic is easy to grow within a few days, however several weeks for growing crystals using the Czochralski method. Second, ceramics are easy to grow with large size and high doping concentration. Third, in general, growing ceramics is less expensive because it is not necessary to use crucible. However, crystals have to be grown in an expensive iridium crucible. At present, Nd3+ ceramic laser materials have been widely investigated with the advancement of ceramic growth techniques. However, Nd3+ ceramic lasers operating at the important 1.4 μm eye-safe spectral region are very limited. For instance, in 2013, Zhang et al. [16] reported the first Nd:YAG ceramic laser at 1442.8 nm. Two years later, based on the previous research, researchers from the same group Q-switched the Nd:YAG ceramic laser using a graphene as saturable absorber [17].
However, it is worth to mention that these above mentioned Nd3+-based eye-safe laser at 1.4 μm have few concerned dual-wavelength lasing behavior. In this work, using a Nd:LuAG ceramic as gain medium, we have presented the first demonstrations of continuous-wave eye-safe lasers at 1.4 μm, to the best of our knowledge. Dual-wavelength laser at 1418 nm and 1442 nm was achieved in free-running mode. In addition, a single-wavelength laser at 1432 nm has also been realized under the help of an intracavity etalon.
2. Laser experimental details
The diode-end-pumped Nd:LuAG ceramic laser system is specifically described as follows. The pump source was a fiber-coupled near infrared diode laser with wavelength of about 808 nm and maximum output power of 26 W. The fiber core diameter is 200 μm in radius and the fiber numerical aperture is 0.22. Two positive lenses, both with focal lengths of 50 mm and anti-reflection coating, were used as coupling optics to first collimate the pump beam and then to focus it into the laser ceramic passing through an input mirror. The flat input mirror was coated with high transmission of about 94% at pumping wavelength and high reflection of 99.8% at laser wavelengths. Moreover, in order to suppress the 1.06 μm, 1.1 μm and 1.3 μm emission lines, the input mirror also has high transmissions of about 85%, 82% and 38% respectively at these three potential emissions. Two output mirrors were used during the laser experiments: a 50 mm (curvature radius) mirror and another one flat mirror, both with partial transmission of about 2.2% at laser wavelengths. For the plane-concave laser cavity, the final cavity length was optimized to be about 43-mm-long. However, for the plane-parallel laser cavity, the cavity length was configured to be about 20 mm. The laser experimental configuration is shown in Fig. 1.
The laser gain medium, Nd:LuAG ceramic with cross section of 3 × 3 mm2, length of 4 mm and doping concentration of 0.4%, was closely placed to the input mirror. Under this situation, the laser material absorbed about 59% of the incident pump power. In addition, a water cooling system was used to remove the thermal load of the Nd:LuAG ceramic by setting the water temperature at 16°C. A 100-μm glass etalon, inserting between the laser gain medium and output mirror, was used to select the intracavity mode.
3. Results and discussion
Figure 2 shows the absorption spectrum at the concerned wavelength to the used 808-nm pump source. The intense absorption peak locates at about 808.6 nm with a FWHM of about 5.7 nm, which is advantageous for AlGaAs diode pumping because the broad absorption is less demanding for pumping wavelength. Figure 3 shows the fluorescence spectrum from 920 nm to 1500 nm, which contains three main transition bands, i.e. from upper level 4F3/2 to three lower level 4I9/2, 4I11/2 and 4I13/2, respectively. Obviously, compared to the ~0.94 μm, ~1.06 μm, ~1.1 μm and ~1.3 μm emission lines, the investigated ~1.4 μm lines exhibit the weakest gain properties, which restricts the high-efficiency laser generation in the specific spectral domain on the one hand. However, on the other hand, as one of the most efficient measures for laser generation at the eye-safe spectral region, these ~1.4 μm lines in Nd:LuAG ceramic have sufficient separation from those high-gain lines (~80 nm from the closest ~1.34 μm lines), which makes it possible to lase them simply by coating on end-face mirrors, as will be presented in the following.
Fig. 3 Fluorescence of Nd:LuAG ceramic from ~920 nm to ~1500 nm containing three main transition bands, i.e. from upper level 4F3/2 to three lower level 4I9/2, 4I11/2 and 4I13/2, respectively.
Laser experiments were then implemented using the curved output mirror at first. At laser threshold, only single wavelength lasing at 1418 nm was observed. However, above the threshold, with the increase of the pump power, emission at 1442 nm can also be monitored. Finally, a simultaneous dual-wavelength laser was achieved with maximum output power of 1.37 W. Laser output powers and laser spectrum were recorded in Fig. 4. From Fig. 4(a), one can see that the laser threshold was about 4.03 W in absorbed pump power and the laser slope efficiency was fitted to be about 13.5%. From Fig. 4(b), one can see an obvious dual-wavelength emission at 1418 nm and 1442 nm with comparable optical intensities. We compared our results with that obtained by Lee et al. [12] in 2012. Using a 808 nm pump source, the authors demonstrated a 4.2 W Nd:YAG crystal laser at 1415 nm with absorbed pump power of 28 W. However, when the absorbed pump power was at the same level to ours, i.e. about 14.5 W, their output power was not better than ours. Our present laser has not shown output power saturation and the maximum output power was only limited by the pump source. This indicated a potential power scaling simply by using a pump source with higher pump power.
Fig. 4 Laser output power characteristic of dual-wavelength Nd:LuAG ceramic laser at 1418 nm and 1442 nm (a) and achievement of laser spectrum (b) obtained with a free-run plane-concave cavity.
It should be pointed out that, at present, Nd3+-based dual-wavelength eye-safe laser has few been reported. Recently, Guo et al. [17] reported a dual-wavelength Q-switched Nd:YAG ceramic laser at 1413 nm and 1443 nm. However, when the Nd:YAG ceramic operated in continuous-wave mode, only 1443 nm wavelength was observed. We also noticed that, very recently, Lin et al. [18] reported a simultaneous tri-wavelength laser emission at 1418, 1432 and 1442nm in a diode-pumped Nd:LuAG single crystal with a maximum output power of 1.83W. As the authors described, the 1432 nm laser appeared only when the laser was operated at high pump power. Instead of single crystal, in this work, we used Nd:LuAG ceramic with more potential advantages to operate a dual-wavelength laser. Therefore, by improving the quality of the Nd:LuAG ceramic and by optimizing the laser system, e.g. the transmission of the output coupler, higher output power at the eye-safe spectral domain could be expectable in the near future. In addition, in our work, we have not achieved 1432 nm laser without the help of the etalon, even at high pump power. The different laser behaviors (3 wavelengths versus 2 wavelengths) should mainly originate from the discrepancy of the emission intensity ratio of the three lines, 1418, 1432 and 1442 nm, in Nd:LuAG crystal and ceramic. As shown in Fig. 3 for ceramic, the 1432 nm emission line has lower intensity than 1418 and 1442 nm lines. However, for crystal [18], the 1432 nm line has more comparable intensity to the two neighboring lines. In general, if no special care is paid, more lasing wavelengths simultaneously lead to worse power stability because of mode competition. This could explain why we obtained better power stability of about 3.4% (RMS) in one hour than that obtained in [18], which gave a stability of 4.8%.
Furthermore, the output beam quality of the dual-wavelength laser was measured by recording laser beam radii at different distances. By fitting these data, we deduced the beam propagation factors M2 to be about 2.93 and 3.16 in x and y directions. In addition, stability of the maximum output power was also measured to be about 3.4% (RMS) in one hour. Note that the achieved dual-wavelength laser is not only applicable for some potential applications in medicine because of their eye-safe advantage, but also potentially useful in terahertz wave generation via difference frequency. The present dual-wavelength laser could produce 3.48 THz wave since the two peaks were at about 1418.96 nm and 1442.74 nm.
According to Fig. 3, it is clear that there is still one emission line at about 1432 nm cannot be generated naturally with the present plane-concave cavity. As a consequence, an etalon was inserted into the cavity to introduce additional loss for the two relatively intense emission lines as 1418 nm and 1442 nm. When the etalon was tilted to a suitable angle of about 7.5o, only lasing at 1432 nm can be observed, i.e. the 1418 nm and 1442 nm lines have been completely suppressed. Figure 5(a) shows the 1432 nm laser having a maximum output power of 0.82 W with slope efficiency of about 9.1%. The stability of the maximum output power was also measured to be about 2.6% in one hour. The improved stability should be firstly attributed to the fact that, not like dual-wavelength laser, for single-wavelength laser no intense mode competition existed. Moreover, secondly, the insertion of the etalon should also be a positive factor to improve the stability because it narrowed the linewidth of the laser emission. Figure 5(b) shows the laser spectrum peaking at 1432.11 nm. M2 factors in x and y directions, in this case, were measured to be about 2.65 and 3.33. The present single-wavelength 1432 nm laser beam showed more elliptic than the above reported dual-wavelength laser, probably due to the insertion of the tilted etalon.
Fig. 5 Laser output power characteristic (a) and laser spectrum (b) of single-wavelength Nd:LuAG ceramic laser at 1432 nm.
Excellent laser beam quality is in general required for various practical applications. In order to further optimize the output laser beam quality, we therefore re-configured the laser cavity by replacing the 50 mm curved output mirror with a flat mirror, and at the same time we shortened the laser cavity to about 20 mm in physical length, at which we obtained the best laser performance. Note that the two mirrors have the same coating. Figure 6(a) shows the output power characteristic of the flat-flat laser cavity with maximum output power of 1.15 W and slope efficiency of about 10.9%. Moreover, the laser threshold was found to a little increase to 4.53 W probably because of higher intracavity deflection loss arising from the plane-parallel cavity itself. The same, we measured the stability of the maximum output power to be about 3.9%. The laser spectrum in Fig. 6(b) shows that, under this situation, the laser cavity still lased at two wavelengths, i.e. the 1418 nm and the 1442 nm, although the 1418 nm laser exhibited higher intensity than that of the 1442 nm laser. Therefore, taking into account the worse stability of the output power, mode competition under this situation must be more intense. The laser beam quality was finally measured and shown in Fig. 7. Under this situation, the M2 factors in x and y directions were fitted to be about 1.36 and 1.24, which indicated a TEM00 mode with very good beam quality close to diffraction limit.
Fig. 6 Laser output power characteristic of dual-wavelength Nd:LuAG ceramic laser at 1418 nm and 1442 nm (a) and laser spectrum (b) obtained with a free-run parallel plane cavity.
4. Conclusions
In summary, continuous-wave laser operation of a diode-pumped Nd:LuAG ceramic in eye-safe spectral region have been demonstrated. Using a plane-concave laser cavity, a simultaneous dual-wavelength laser at 1418 nm and 1442 nm was achieved with maximum output power of 1.37 W and slope efficiency of 13.5%. The M2 factor of the dual-wavelength laser was measured to be about 2.93 and 3.16 in x and y directions. Using a parallel plane laser cavity for optimizing the laser beam quality, although the maximum output power and slope efficiency of the dual-wavelength laser decreased to 1.15 W and 10.9%, the beam quality was improved to 1.36 and 1.24 of the M2 factors in x and y directions. Using an etalon to select the intracavity mode, a single-wavelength laser at 1432 nm was also realized with maximum output power of 0.82 W. Thus, in this work, three main emission peaks for Nd:LuAG ceramic have been lased.
Further investigation into the Nd:LuAG ceramic will be focused on the improvement of the material quality and then on power scaling of the eye-safe Nd:LuAG ceramic with better laser efficiency. Passively Q-switched the Nd:LuAG ceramic lasers at eye-safe spectral region is also very attractive and will be our next investigation.
Acknowledgments
The authors appreciate the assistance of Mr. Luhao in the sintering of Nd:LuAG ceramics.
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