Efficient laser operation of Nd:Lu2O3 ceramic fabricated by Spark Plasma Sintering (SPS) was demonstrated. Transparent Nd:Lu2O3 ceramic was successfully fabricated by Spark Plasma Sintering and its laser experiment was done. On the 4F3/2 to 4I11/2 transition, the obtained maximum output is 1.25W at the absorbed pump power of 4.15W with a slope efficiency of 38% and two spectral lines at 1076.7nm and 1080.8nm oscillated simultaneously. The slope efficiency of 38% is near two times higher than the previously demonstrated SPSed Nd:Lu2O3 ceramic lasers. On the 4F3/2 to 4I13/2 transition, the laser operated at the wavelength of 1359.7nm and the maximum output of 200mW was obtained at the absorbed pump power of 2.7W.
© 2016 Optical Society of America
Rare-earth doped transparent ceramic laser materials have been intensively studied in the past decade [1–4]. Compared with the rare-earth doped single crystals, the ceramic laser materials have the advantages of short fabrication time, feasibility of large size and high dopant concentration, composite structure and multiple functionality, and so on. So far, different ceramic fabrication methods have been successfully used to sinter laser graded transparent ceramics, which includes vacuum sintering , hot isostatic pressing sintering  and Spark sintering method , etc. And various rare earth doped garnets [5, 6, 8], sesquioxide [9, 10], and fluoride  ceramics were successfully fabricated.
For high power solid-state lasers, the thermal conductivity of the laser gain media is of crucial importance, where gain media with as high thermal conductivity as possible are highly preferred. Extensive studies have shown that sesquioxides have high thermal conductivity, whose values are even higher than that of the YAG , the currently predominant high power solid-state laser gain host. However, it is extremely difficult to grow high quality large size crystalline sesquioxides with the conventional crystal growth techniques [12, 13] because sesquoxides have a melting point of above 2400°C . Considering the ceramic materials could be sintered at temperatures much lower than their corresponding melting point, researchers began to use ceramic processing methods to fabricate high quality sesquioxide laser gain media [9, 10, 14, 15].
An attractive feature of the Nd3+ doped sesquioxides is that they have many emission lines around 0.9μm, 1.08μm and 1.4μm . Therefore,Nd3+ doped sesquioxide lasers was proposed to be used as laser source for the atmosphere detection lidar [17, 18]. Moreover, the emission cross section ratio between 0.9 μm and 1.08 μm in the Nd3+ doped sesquioxides are larger than that in Nd:YAG [17, 19], which is advantageous for limiting the amplified spontaneous emission (ASE) effects . Thirdly, Nd3+ doped sesquioxide lasers are quite easy to operate in the dual wavelength regime when the transition 4F3/2-4I11/2 is used to lase , which has potential applications in the generation of coherent terahertz radiation by difference frequency and the generation of ultrahigh repetition rate pulse by optical beating.
In this paper, the authors report on a high efficient Nd:Lu2O3 ceramic laser in which the gain medium is fabricated using the spark plasma sintering (SPS) technique. Compared with other transparent laser ceramic fabrication techniques, the SPS technique has a unique characteristic that almost full densification of the ceramic could be realized within a few minutes . Hence, limited grain growth and uniform grain-size distribution could be obtained with SPS technique. The first Nd:Lu2O3 ceramic laser was reported by J. Lu et al in 2002 , in which the Nd:Lu2O3 ceramic was fabricated by nonpress vacuum sintering method and the laser delivered 10 mW output. An et al successfully fabricated high quality transparent Nd:Lu2O3 ceramic using the SPS method for the first time . With the SPSed Nd:Lu2O3 ceramic, laser operation was also demonstrated afterwards [24–26]. However, either the output power or the optical efficiency was quite low in the previous demonstrations. We realized watt-level and efficient laser output with Nd:Lu2O3 ceramic fabricated by SPS. On the 4F3/2 to 4I11/2 transition, maximum output of 1.25W was obtained with a slope efficiency of 38%. Laser operation on the 4F3/2 to 4I13/2 transition was also demonstrated and maximum output of 200mW was obtained. Our experimental laser results indicate high optical quality Nd:Lu2O3 ceramics could be fabricated by the SPS method and have good potential as gain medium for high power lasers.
2. Spectral properties of SPSed Nd:Lu2O3 ceramic sample
SPSed Nd:Lu2O3 ceramic with 1.0at.% concentration for laser experiments were fabricated. In the previous studies, An et al succeeded in fabricating Nd:Lu2O3 ceramic by the SPS approach using 0.2at.% LiF as the sintering aids . In this work, the similar SPS sintering procedure was adopted. Considering the melting point of LiF (1121 K), the calcination temperature for powder mixtures was decreased from 1273K to 1073K. By using this approach, highly transparent Nd:Lu2O3 ceramics were fabricated. Both sides of the as-sintered specimens were mirror-polished with diamond slurry. The inset in Fig. 1 is a photo of an as-sintered Nd:Lu2O3 ceramic. The in-line transmittance of a polished specimen Nd:Lu2O3 ceramics was measured to be 81.7% at 1078nm, which is very close to its theoretical limit. The absorption cross section spectrum of a polished specimen is shown in Fig. 1. There are several absorption bands in the vicinity of 800nm. The absorption peak at 806nm was used to absorb the pump energy in our experiment. The absorption cross-sections at 806nm and 822nm are 1.31 × 10−20 cm2 and 1.67 × 10−20 cm2, respectively.
The fluorescence spectrum from 850 to 1500nm of the SPSed Nd:Lu2O3 ceramic was excited by a 807nm LD and is shown in Fig. 2. The three remarkable groups of fluorescence emission lines are corresponding to the transitions of 4F3/2-4I9/2, 4F3/2-4I11/2, and 4F3/2-4I13/2,respectively. The two strongest emission peaks located at 1076nm and 1080nm are closely spaced and of the similar intensities, which would easily cause dual wavelength laser operation if no wavelength selective element is used in the laser cavity. The emission spectra in this paper and in Ref . are almost the same. In Ref , the detailed optical spectroscopic properties of SPSed Nd:Lu2O3 ceramics has already been reported and the emission cross sections at 1076nm and 1080nm for annealed 1.0at% Nd:Lu2O3 ceramics were calculated to 6.3x10−20 cm2 and about 6.0x10−20 cm2, respectively. The fluorescence decay curve at 1076nm is shown in the inset in Fig. 2. If the curve is fitted with a first-order exponential formula, the decay time was found to be 231μs.
3. Laser experimental setup
The schematic of the Nd:Lu2O3 ceramic laser setup is shown in Fig. 3. A three-mirror V-shaped cavity was employed. The mirror M1 is a planar one and acts as an input mirror. The mirror M2 with radius of curvature of 300mm acts as a folding mirror. The mirror OC is a planar output coupler (OC). The distances from M1 to M2 and from M2 to OC are 151 mm and 300mm, respectively. The folding angle is about 5 degree. The fundamental mode radius of laser beam in the ceramic is calculated to be 72μm. Laser performances on two transitions of 4F3/2-4I11/2 and 4F3/2 -4I13/2 were both tested, so two sets of mirrors were used. For the 4F3/2-4I11/2 transition, an input mirror coated HT for 807nm and HR for 1080nm, a folding mirror coated HR for 1080nm and three different output couplers were used. For the 4F3/2-4I13/2 transition, an input mirror coated HT for 806nm and HR for 1360nm, a folding mirror coated HR for 1360nm and two different output couplers were used. The pump source was a fiber coupled laser diode (LIMO, GmbH). The coupling fiber had a core diameter of 100μm and a numerical aperture of 0.22. The pump beam was first collimated by a doublet with a focal length of 80mm and then focused to the gain medium by a second doublet with a focal length of 100mm. The spot size of the re-focused pump beam was measured with the knife-edge method to be ~130μm. The Nd:Lu2O3 ceramic sample has a cross section of 3x3mm2 and a length of 1mm. The end-facets of the ceramic were polished and AR coated for 807nm and 1000-1100nm. The ceramic was wrapped with indium film and mounted into a water cooled copper whose temperature was kept at 17 °C. An optical spectrum analyzer was used to record the laser wavelengths.
4. Experimental results and discussion
Figure 4 shows the continuous-wave performance of the Nd:Lu2O3 ceramic laser on the 4F3/2 - 4I11/2 transition. The Nd:Lu2O3 ceramic absorbed about 40% of the pump radiation. The maximum of the applied incident pump power was 10.37W. When an OC with 1.6% transmission was used, a maximum output power of 1.25W under an absorbed pump power of 4.15W was obtained, with an optical conversion efficiency of 30% and a slope efficiency of 38%. When another two OCs with 5% and 9% transmission were used, the obtained maximum output power was 1.1W and 0.67W, and the corresponding slope efficiencies were 36% and 28%, respectively. The maximum optical-to-optical efficiency with respect to the incident power is about 12.1% and the output was un-polarized. The laser spectra are also shown in Fig. 4. For the three different OCs, the three output spectra were all recorded under the maximum pump. The laser emitted two wavelengths, one at 1076.7nm and the other at 1080.8nm and the intensity ratios between the 1080.8nm line and the 1076.7nm line were 0.22, 0.18 and 0.14, respectively. Because the emission cross sections at 1076nm and 1080nm are much larger than the emission cross sections at other positions, so spectral lines at 1076nm and 1080nm will oscillate first and suppress other spectral lines. Similarly, the 1076nm spectral line will oscillate first and suppress the 1080nm spectral line because the former is slightly stronger than the latter. The same dual-wavelength emission phenomena have been observed in previous Nd:Lu2O3 ceramic lasers [19, 22, 25]. As the pump increases, the 1076nm and the 1080nm wavelengths will oscillates simultaneously, which can be traced back to the comparable emission cross sections of the two spectral lines.
It is noticeable that as the pump power increased, the laser output exhibited obvious saturation for the three OCs, and the higher the OC’s transmission is, the earlier the saturation occurs. The reason is believed to be due to the thermal lensing effect. As the pump increased, the gradually intensifying thermal lensing effect made the match between the laser mode and pump mode in the ceramic worse. Consequently, the energy extraction from the pump radiation became harder and the conversion efficiency decreased accordingly. Moreover, more heat would be deposited in the ceramic for the OC with higher transmission because of its lower efficiency, so the saturation occurs earlier. With the method presented by F. Song et al , we measured the focal length of the thermal lensing in gain medium. The OC with 1.6% transmission was placed after the gain medium to form a plano-plano cavity. Under the pump power of 10W, the focal length was found to be about 9cm. Under the maximum output power of the OCs with 1.6% transmission, the beam quality was also determined and the M2 parameters for tangential direction and sagittal direction were 2.75 and 2.70, respectively.
The left side of Fig. 5 shows Determination of the round-trip cavity loss. A method proposed by Findlay and clay was used to determine the total round-trip cavity loss . Five different OCs with different transmissions was used. Besides the above-mentioned three OCs, another two with T = 0.8% and T = 3.5% was added to ensure the accuracy as high as possible. For the five OCs with T = 0.8%, 1.6%, 3.5%, 5%, 9%, the corresponding incident pump thresholds are 0.138W, 0.24W, 0.46W, 0.67W and 1.22W, respectively. The total round-trip cavity loss was estimated to be 0.0024. Assuming that the loss was totally owing to the gain ceramic, we can get a loss coefficient of about 0.012/cm. In Fig. 4, one can see that the lasing efficiency decreased with the OC transmission increasing. The optimal transmission was also determined at a fixed incident pump power of 3W and shown in Fig. 5 (right side). It was found between 1.6% and 3.5%. The optimal transmission is subject to the pump intensity and the gain length. For our Nd:Lu2O3 laser, the pump intensity and gain length were both limited, especially the 1mm length of the gain medium, which is much shorter than commonly used bulk gain media. In Fig. 4, the slope efficiencies for T = 1.6% and T = 5% were close to each other, but the slope efficiency for T = 9% decreased fast. The reason is that the laser slope efficiency decreases fast if the output coupling is far from the optimal value.
Figure 6 shows the laser operation on the transition from 4F3/2 to 4I13/2 energy levels. It should be noted that the AR coatings on the two end facets of the ceramic have only transmission of 86% for the 1360nm wavelength. Despite of the fact that the coatings were not optimized, laser oscillation was still successfully achieved on the transition from 4F3/2 to 4I13/2. Two different OCs, one with 4% transmission and the other with 8%, were used as the output coupler, respectively. As plotted in Fig. 6, the laser thresholds were 0.33W and 0.59W (absorbed pump power), and at an absorbed pump power of 2.7W the obtained maximum outputs were 200mW and 105mW, respectively. The slope efficiencies were 10% and 5%, respectively. When the pump power was further increased, the output power decreased. The decrease in output should be attributed to the thermal lensing effect. Comparing the output curves in Fig. 4 and Fig. 6, one can see that much more heat would be deposited in the ceramic for laser operation on the 4F3/2-4I13/2 transition, so the thermal lensing effect was much severer for the latter. For the OC with transmission of 8%, the laser didn’t operate any more when the absorbed pump power reached 4.1W, which is because the intensive thermal lensing effect had made the cavity unstable. We tested the influence of thermal load in the ceramic on the cavity stability by stopping the circulation of the cooling water. The test point is marked in Fig. 6 with a green rectangle. After the circulation of the cooling water had been stopped, the laser output continuously decreased until the laser became extinct. Then the cooling was resumed, the laser oscillated again and the output is back to the origin. The laser spectrum is also shown in Fig. 6. The laser oscillated at the wavelength of 1359.7nm. The low output and efficiency for the 1359nm laser should be mainly contributed to the loss brought by the coatings on the ceramic. If the ceramic is AR coated for 1359nm laser, near watt-level output could be expected.
All the previous Nd:Lu2O3 laser results are summarized in Table 1. From the table, one can see that the obtained maximum output power from Nd:Lu2O3 is 4W, with a corresponding slope efficiency of 16.6% , and the obtained maximum slope efficiency is 21.4% with a corresponding maximum output of 3.52W . In our experiment, we demonstrated a high efficient Nd:Lu2O3 Laser, of which the maximum slope efficiency of 38% is much higher than the one of any other Nd:Lu2O3 Laser. Moreover, the laser performance has shown significant improvement in terms of both output power and efficiency when compared with all other SPSed Nd:Lu2O3 ceramic Lasers. The slope efficiency of 38% is near two times higher than the ones of the previously demonstrated SPSed Nd:Lu2O3 ceramic lasers. Output power is currently limited by the severe thermal lensing effect. In order to obtain higher output, more intensive cooling and/or a small ratio of cross-section to thickness of the laser medium is required. In view of the limited thickness of ceramics by SPS, thin disk geometry is quite suitable for the SPSed gain media. We believe much higher output power and higher efficiency could be realized if thin disk geometry is adopted for SPSed Nd:Lu2O3 ceramic lasers.
In conclusion, high efficient laser operation based on the SPSed Nd:Lu2O3 ceramic was demonstrated. On the transition from 4F3/2 to 4I11/2, two spectral lines at 1076.7nm and 1080.8nm oscillated simultaneously. When an OC with T = 1.6% was used, a maximum output of 1.25W was obtained under an absorbed pump power of 4.15W, with a corresponding slope efficiency of 38%. This value is near two times higher than the previously demonstrated SPSed Nd:Lu2O3 ceramic lasers. On the transition from 4F3/2 to 4I13/2, the laser operated at the wavelength of 1359.7nm. The maximum output of 200mW was obtained at an absorbed pump power of 2.7W. In our experiment, the thermal lensing effect was quite severe because the large ratio of cross-section to thickness of the laser medium, which was not favorable to the heat diffusion. Our results showed that the high optical quality Nd:Lu2O3 ceramic could also be fabricated by Spark Plasma Sintering and have good potential as gain medium for high power lasers.
National Natural Science Foundation of China (NFSC) (61405126, 61505147); The Program of Fundamental Research of Shenzhen Science and Technology Plan (JCYJ20140828163633984); The Science and Technology Planning Project of Guangdong Province (2016B050501005).
We thank Dr. Liqiong An with University of Shanghai Maritime University for her kind help and discussions.
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