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We report a full investigation of continuous-wave diode-pumped Nd:LuAG single crystal lasers on 4F3/2→4I13/2 transition around 1.3 and 1.4 μm. In free-running mode, a maximum output power of 4.18 W is achieved for a simultaneous dual-wavelength laser operation at 1321 and 1338 nm, which represents the highest output power for Nd:LuAG laser material at a 1.3 μm emission band. Three single wavelength lasers at 1340, 1332 and 1322 nm, as well as a dual-wavelength laser at 1334 and 1338 nm, are also generated with maximum output powers of 2.39, 2.63, 2.24 and 1.23 W, respectively, with the aid of a glass etalon. Moreover, a single wavelength laser at 1353 nm is also obtained with a maximum output power of 1.53 W. Furthermore, in free-running mode, simultaneous tri-wavelength eye-safe lasers at 1419, 1432 and 1442 nm are attained with a maximum output power of 1.83 W, and a 1.08 W single wavelength laser at 1419 nm is also yielded. Most of the 1.3 μm lasers and all of the 1.4 μm lasers are demonstrated for the first time to our knowledge. Thus, this work indicates that Nd:LuAG crystal is a very promising laser gain medium for high-power continuous-wave infrared laser generation.
© 2016 Optical Society of America
1. Introduction
During the past decades, many laser host materials have been developed. Among these hosts, YAG, a garnet host material, is still a top-class host material because of its excellent mechanical, thermal and optical properties. In recent years, by substituting completely or partially the Y3+ in YAG, some novel garnet laser materials have been developed, such as LuYAG [1–3], GdYAG [2,4], GdLuAG [2,5] and LuAG [6–10]. Of these various garnet hosts, LuAG was chosen for study in this work as a promising efficient laser material owing to the following reasons. LuAG has a large manifold splitting, leading to a low thermal occupation factor for the lower laser level [11]. Moreover, LuAG is, in general, a robust crystal with physical properties comparable to YAG. However, the properties of the LuAG crystal do not change appreciably with the addition of dopant ions because Lu is denser than Y. Furthermore, LuAG is harder than YAG and has a higher melting point, which is believed to have a higher damage threshold [12]. In addition, measurements showed that the specific-heat curve for LuAG is lower than that of YAG because the atomic mass of Lu is larger than that of Y [13]. In fact, LuAG has attracted the most attention and it has already been developed into various infrared lasers from about 1 μm to 2 μm including Nd3+ [6], Yb3+ [7], Er3+ [8], Tm3+ [9], Ho3+ [10]. In terms of laser emissions at infrared spectral region, Nd3+ provides the richest and also the most efficient laser sources including 0.94, 1.06, 1.12, 1.34, 1.44 and even 1.83 μm [14].
Nd:LuAG laser at 1064 nm has been first reported by Xu et al. [6] in continuous-wave mode, since then Nd:LuAG lasers have been developed gradually. For instance, in 2011, Di et al. reported a continuous wave Nd:LuAG crystal laser still at 1064 nm and then passively Q-switched it with Cr:YAG to achieve a 12 ns pulsed laser [15]. In 2012, Xu et al. demonstrated a 5.4 ps pulsed laser in the crystal using a SESAM saturable absorber [16]. In 2014, Chen et al. obtained 78 ns Q-switching from this crystal using multi-walled carbon nanotube [17]. However, these reports were only concerning Nd:LuAG laser emission at 1064 nm line. Until 2015, Liu et al. [18] reported continuous-wave and Q-switched laser operation on 4F3/2→4I13/2, but only lasing at around 1338 nm. Almost at the same time, Ye et al. [19] reported Nd:LuAG ceramic lasers at 1064 nm and 1123 nm.
In this work, we conducted a full investigation into 4F3/2→4I13/2 transition. Using Nd:LuAG single crystal as gain medium, we obtain not only various 1.3 μm laser emissions at 1319, 1322, 1332, 1338, 1340 and 1353 nm but also 1.4 μm laser emissions at 1419, 1432 and 1442 nm. The 1.4 μm laser emissions can be used for surgery and blood coagulation because water and collagen absorption strongly increase at these wavelengths. In 2009, Tark et al. [20] presented the first report on 1444 nm Nd:YAG laser using for performing lipolysis. The research showed that the 1444 nm laser have great advantages over the 1064 nm laser in lipolytic features because it can be absorbed strongly by fat. In fact, considering the thermal effect on the water absorption band [21], these developed 1.4 μm Nd:LuAG laser emissions with shorter wavelengths than 1444 nm could be more suitable wavelengths in medicine.
2. Experimental details
The pump source was a fiber-coupled 808 nm AlGaAs diode laser with maximum output power of about 25 W, core diameter of 400 μm and N.A. of 0.22. The pump beam coupling system consisted of two doublet lenses both with focal lengths of 40 mm. The laser resonator was a simple and compact two-mirror plane-concave cavity. For completely investigating the laser performances at various emission lines around 1.3 and 1.4 μm related to transitions between different Stark energy levels, three input mirrors (IMs) and five output couplers (OCs) were used during the laser experiments (see Fig. 1 for transmissions of all these mirrors). Further descriptions on the dependence between these mirrors and laser results can be found in next section.
The available laser crystal was a Nd:LuAG single crystal with dimensions of 3 × 3 × 8 mm3 (8 mm in thickness) and nominal doping concentration of about 0.3%. Thus, after single pass, the crystal absorbed about 57.3% of the incident pump power onto it. The laser crystal was wrapped with indium foil and then mounted inside a copper block connecting with a circular water chiller. The chiller was temperature set at 10°C during the laser experiments. For wavelength tuning to other emission lines, an 80-μm-thin BK7 uncoated glass plate was used to play a role as Fabry-Perot etalon.
3. Results and discussion
We present a full emission spectrum of Nd:LuAG single crystal from 0.9 μm to the considered 1.4 μm emission in Fig. 2, since no emission spectrum of Nd:LuAG single crystal longer than 1.4 μm has been reported at present. According to Fig. 2, within the considered 1.3 and 1.4 μm spectral regions, the emission peaks locate at ~1320, ~1332, ~1334, ~1338, ~1340, ~1350, ~1353, ~1419, ~1433, and ~1442 nm. Moreover, evidently, to operate the ~1.4 μm lasers, it is necessary to suppress these ~1.3 μm emission lines possessing higher emission intensity. In the same way, to operate the ~1.3 μm lasers, these high-gain emission lines at about 0.94, 1.06 μm and even the 1.12 μm emission bands should be restrained.
Fig. 2 Emission spectrum of Nd:LuAG single crystal from 900 to 1460 nm. Inset: emission spectrum with respect to the 1.3 and 1.4 μm spectral regions.
3.1 laser operation at 1.3 μm
1.3 μm laser experiments were first carried out using IM1, OC1 and OC2 (see Fig. 1(a) for transmissions of IM1, OC1 and OC2). The IM1 has high transmission of about 93% at pump wavelength and high reflection of about 99.7% at wavelength shorter than 1338 nm corresponding to the emission peak. For longer emission peak at 1353 nm, the transmission increased to 4.1%. The OC1 and OC2 both have partial transmissions of about 2.0% and 3.8%, respectively, at 1.3 μm. Moreover, from Fig. 1(a), one can see that the transmissions arising from IM1 and OC1 or OC2 are quite high at the 0.94, 1.06 and 1.12 μm, which ensures complete suppression of these potential high-gain emissions. Thus, under this situation, a maximum output power of 4.18 W (measured by a Coherent PowerMax PM30 power meter) was obtained with threshold absorbed pump power of 0.8 W by using the OC2 (see in Fig. 3(a)). Linearly fitting the data leads to a slope efficiency of about 34.3%. In addition, using OC1, the maximum output power and slope efficiency respectively decreased to 2.92 W and 23.7%.
Fig. 3 (a) Output powers versus absorbed powers for Nd:LuAG 1.3 μm laser in free-running mode, (b) single wavelength laser spectrum at 1338 nm at low pump powers, (c) dual-wavelength laser at 1321 and 1338 nm with the increasing of the pump powers and (d) dual-wavelength laser at the same two lines with maximum pump power.
The laser spectra were shown in Fig. 3(b)-3(d) for laser experiment using the OC2. At low pump power (below 2.54 W of absorbed power), only the 1338 nm laser can be observed (see Fig. 3(b)). Further increasing the pump power, lasing at 1321 nm can also be observed (see Fig. 3(c), obtained at absorbed power of 2.95 W). Finally, dual-wavelength laser at the 1321 and 1338 nm was achieved although the 1338 nm laser still dominated (see Fig. 3(d), obtained at absorbed power of 13.65 W). Recently, Lin et al. [22] reported a similar dual-wavelength laser behavior in Nd:YAG laser. Using the so-called direct pumping at 885 nm, the authors achieved a slope efficiency of 35.01%, just comparable to ours in this work. Hence, our results indicate that the Nd:LuAG crystal is an excellent laser material with low threshold and high efficiency, like Nd:YAG. In addition, using OC1, we found a similar lasing behavior to that of using OC2. At maximum output power of 4.18 W, using an infrared camera (Electrophysics, MicronViewer 7290A), we measured the laser beam quality with M2 factors of 3.5 and 3.7 in x and y directions. The power stability was estimated to be about 4.3% (RMS) in one hour.
An etalon is one of the most typical and representative intracavity optics for wavelength tuning. It is cost-effective. Its insertion into laser cavity does not destroy the compactness of laser system. In addition, an etalon with suitable thickness is efficient in generating almost all potential emission lines having relatively low gains [23,24]. Other methods, e.g. specific coating [21] and intracavity selective absorber [25], can only make one lasing possible, in general.
Using IM1 and OC2, at the same time inserting the 80-μm-thin uncoated glass etalon into laser cavity, single wavelength lasers at 1340 nm (see Fig. 4(b) for laser spectrum), 1332 nm (see Fig. 4(c) for laser spectrum) and 1322 nm (see Fig. 4(d) for laser spectrum), as well as dual-wavelength laser at 1334 and 1338 nm (see Fig. 4(e) for laser spectrum) were all generated. The corresponding laser output powers with respect to the four cases are shown in Fig. 4(a). For the 1340 nm laser, a maximum output power up to 2.39 W and slope efficiency of 22.2% were obtained. For the 1332 nm laser, the maximum output power reached 2.63 W with slope efficiency of about 22.7%. For the 1322 nm laser, the maximum output power was 2.23 W and the corresponding slope efficiency was about 20.9%. Finally, for the 1334 and 1338 nm dual-wavelength laser, the maximum output power was 1.23 W with slope efficiency of about 10.5%. Note that the optimization criterion for the dual-wavelength laser was to generate two laser wavelengths with comparable intensities, as shown in Fig. 4(e). For all these four cases of lasers obtained by using the etalon, the laser beam qualities in both directions were measured to be around 3.0-4.0, close to the highest 4.18 W dual-wavelength laser at 1321 and 1338 nm. The power stabilities, for the three single wavelength lasers, were all less than 3.5%, i.e. showing better stability than that of the 4.18-W laser. However, the dual wavelength laser at 1334 and 1338 nm still had relatively strong mode competition with stability of about 4.4%.
Fig. 4 (a) Laser output powers versus absorbed powers for single wavelength lasers at 1340, 1332, 1322 nm and dual-wavelength laser at 1334 and 1338 nm, as well as corresponding laser spectra at (b) 1340 nm, (c) 1332 nm, (d) 1322 nm and (e) 1334 and 1338 nm.
Because of the relatively high transmission of about 4.1% for the IM1, the present laser cavity did not support laser emission at the longer 1353 nm wavelength despite with the aid of the glass etalon. Therefore, we made effort to lase the 1353 nm laser by employing the second set of laser mirrors (see Fig. 1(b) for transmissions of the IM2 and OC3). The IM2 has a little bit reduced transmission by 3.8% at pump wavelength compared with IM1, and a high-reflection transmission extended to include the 1353 nm. However, in order to suppress other high-gain emission lines at the 1338 and 1319 nm, the OC3 has transmissions of 10.8% at the 1338 nm and 58.6% at the 1319 nm. At the desired 1353 nm, its transmission is 2.1%. Thus, the 1353 nm was successfully achieved with maximum output power of 1.53 W and slope efficiency of 13.5% (see Fig. 5(a) for output powers and Fig. 5(b) for laser spectrum). The laser M2 factor was measured to be about 3.2 and 3.3 in x and y directions, respectively. The power stability was estimated to be about 3.7% (RMS) in one hour.
Fig. 5 (a) Output powers versus absorbed powers of single wavelength 1353-nm laser and (b) Laser spectrum of the single wavelength 1353 nm emission.
3.2 laser operation at 1.4 μm
1.4 μm laser experiments were fulfilled using laser mirrors with transmissions as shown in Fig. 1(c). The IM3 has high transmissions of 86.7%, 92.3% and 50.9% respectively at pump wavelength, 1064 nm and 1338 nm, as well as high reflection of 99.8% at these considered ~1.4 μm laser emissions. The IM3 ensured only laser emissions at ~1.4 μm can be realized. The OC4 has transmissions of 1.9%, 2.2% and 2.4% at 1418, 1432 and 1442 nm. For OC5, the transmissions are 5.6%, 20.9% and 54.8% at these three wavelengths.
Figure 6(a) plots the output powers characteristics obtained by using OC4 and OC5. The maximum output power of 1.83 W was achieved with the OC4 and the corresponding slope efficiency was about 19.2% (optical-to-optical efficiency of about 14.4%). In this case, the laser operated at simultaneous tri-wavelength mode, i.e. 1418, 1432 and 1442 nm as shown in Fig. 6(b) for laser spectrum obtained at maximum output power, from which we can find that the 1418 and 1442 nm lasers displayed a comparable intensity and the 1432 nm laser spectrum intensity is weaker and narrower than that of the 1418 and 1442 nm lasers. In fact, during the laser experiment, the 1418 and 1442 nm lasers lased almost together. In other words, these two wavelengths have similar laser threshold. When the absorbed pump power increased to 10.32 W with corresponding output power of 1.26 W, the 1432 nm laser started to lase. For the tri-wavelength laser, the laser beam quality was measured to be about 3.8 in both directions and the power stability was about 4.8% (RMS) in one hour. We compared the present results of the 1.4 μm Nd:LuAG lasers with that reported in Ref [21]. First, our laser threshold of the 1.4 μm was far less than that reported in Ref [21]. despite similar parameters of the pump source. Second, for both 808 nm pumping, our laser optical efficiency was comparable to theirs. Since Fig. 6(a) shows no output power saturation, therefore, third, power scaling to more than that in Ref [21]. has been greatly expected by using new pump source with higher power.
Fig. 6 (a) Output powers versus absorbed powers of 1.4 μm lasers and laser spectra of (b) tri-wavelength lasing at 1418, 1432 and 1442 nm, and (c) single wavelength lasing at 1418 nm
Using the OC5, single wavelength laser at 1418 nm can be obtained (see Fig. 6(c) for laser spectrum) with a maximum output power of 1.08 W and slope efficiency of about 10.8%. Obviously, no lasing at 1442 nm line should be attributed to the high transmission of the OC5. For the single wavelength laser, the laser beam quality was 3.4 and 3.1 in x and y directions. The power stability was estimated to be about 3.2% (RMS) in one hour.
Finally, it should be pointed out that, during the laser experiments, polarization characteristics of the output lasers were individually measured by recording the output power variations with the rotation of a Glan-Taylor polarizer. The measuring results indicated that all these output lasers were partially polarized more or less. Since the LuAG host, like YAG, is isotropic material, the non-zero polarizability could be explained by thermal induced birefringence inside the isotropic Nd:LuAG crystal, which was indeed often observed for isotropic laser gain medium.
4. Conclusion
In this work, we showed a complete emission spectrum of Nd:LuAG crystal from 0.9 μm to 1.4 μm. We conducted a full investigation into continuous-wave Nd:LuAG lasers around 1.3 and 1.4 μm spectral regions corresponding to 4F3/2→4I13/2 transition. In free-running laser mode, for 1.3 μm emission band, a dual-wavelength laser at 1321 and 1338 nm was attained with maximum output power of 4.18 W and slope efficiency of 34.3% although the 1338 nm laser dominated the two lasers. By introducing a glass etalon into the laser cavity, three single wavelength lasers at 1340, 1332 and 1322 nm were generated with maximum output powers of 2.39, 2.63 and 2.24 W, respectively. Moreover, dual-wavelength laser at 1334 and 1338 nm with comparable intensities can also be achieved with maximum output power of 1.23 W. Furthermore, in free-running mode with IM2 and OC3, single wavelength laser at 1353 nm was obtained with maximum output power of 1.53 W and slope efficiency of 13.5%. 1.4 μm lasers were also demonstrated by using IM3, OC4 and OC5. Using IM3 and OC4, we obtained a simultaneous tri-wavelength laser at 1419, 1432 and 1442 nm with maximum output power of 1.83 W and slope efficiency 19.2%. Using IM3 and OC5, a single wavelength laser at 1419 nm was achieved with maximum output power of 1.08 W and slope efficiency of about 10.8%.
Funding
National Natural Science Foundation of China (61575164); the Specialized Research Fund for the Doctoral Program of Higher Education (20130121120043); Natural Science Foundation of Fujian Province of China (2014J01251).
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