We report the demonstration of continue wave operation of diode end-pumped Er:Y2O3 and Er:Lu2O3 ceramic lasers operating at 2.7 μm at room temperature. The maximum output power of 320 mW and 611 mW was obtained from the Er:Y2O3 and Er:Lu2O3 ceramic lasers, with slope efficiency of 6.5% and 7.6%, respectively. Characteristics of Red-shift in lasing wavelength of the ceramic lasers was investigated and discussed. The study indicates that under 967 nm and 976 nm LD pumping, 15 at.% Er-doped Lu2O3 ceramic exhibit a better performance than that of Y2O3 at room temperature.
© 2014 Optical Society of America
Applications such as military countermeasures, remote sensing, atmosphere pollution monitoring, and medical applications can all benefit from the development of the 2.5–3 μm laser sources, especially in situations need strong absorption by water, and short penetration depths of a few micrometers into biological tissue. Compact diode-pumped solid-state lasers based on Er3+ doped garnets [1–7] and fluorides [8, 9] crystalline materials have been widely studied as 3 μm laser sources, because the energy spacing between its 4I11/2 and 4I13/2 manifolds facilitates lasing at wavelength 2.7–3 μm, and the high power ~970 nm InGaAs laser diodes (LD) pump resources are readily available at affordable prices. However, due to the unfavorable lifetime ratio and the relatively low stimulated-emission cross section (σ≈10−20–10−21 cm2), heavily doped Erbium ions are needed to help mitigating the “self-terminated” nature of the 4I11/2→4I13/2 transition via well known concentration dependent up-conversion process [4I13/2 + 4I13/2→4I11/2 + 4I9/2].
With the increase of Erbium ions concentration, the accompanying destructive effect, such as greatly decreased thermal conductivity and quite high absorption coefficient, must be considered in laser performance. Especially high phonon energy host materials, such as garnet, erbium ions at concentrations no less than 30 at.% are needed to compensate the nonradiative transition, of which, the gain is concentrated near the surface with a short pump absorption depth. For shallow pump absorption, the induced temperature gradient may lead to distortion of the laser mode and strong thermal lensing with pronounced spherical aberrations and, ultimately lead to bulk materials fracture in high-power end-pumped system. Diffusion-bonded absorption-free materials as heat sink at the pump facet [4, 5] were proposed to improve this issue. Nevertheless, thermal stresses at the pump interface between the two materials would limit the average output power of the diffusion-bonded setup .
Recently, cubic sesquioxides have attracted considerable attention due to their superior thermo-mechanical properties, which can be easily doped with rare-earth ions and exhibit large heat conductivity which exceed that of YAG by up to 50% . Furthermore, their relatively low maximum phonon energy of ~600 cm−1 (Y2O3 591 cm−1, Lu2O3 612 cm−1, Sc2O3 672 cm−1) compared to YAG (857 cm−1) is very meaningful to make efficient laser operation, which reduce the probability of non-radiative transitions of the laser ion and thus improves the quantum efficiency . With substantial progress in the growth of single crystal as well as fabrication of polycrystalline Er-doped sesquioxides laser materials, efficient Er sesquioxide lasers emitting at 3 μm range have been reported [12–14]. However, the growth of sesquioxide crystalline is very challenging due to the extremely high melting point of more than 2400°C. Compared with single crystals, rare-earth elements doped transparent ceramic as new laser gain medium have drawn great attention [15, 16], due to the rapid and larger volume fabrication, larger doping concentrations with controllable distribution in the volume of material, profile and sample structure, etc. Continuous wave (CW) power of 14 W has been recently obtained from Er-doped Y2O3 ceramic lasers with cryogenic cooling (77 K) , which presents the highest output power reported to data for a ceramic Er3+ laser operating in the 3 μm range.
In this paper, the first cw operation of LD pumped 2.7 μm Er:Lu2O3 and Er:Y2O3 ceramic lasers at room temperature were demonstrated. The maximum output power of 611 mW was achieved with Er:Lu2O3 ceramic lasers at absorbed pump power of 8.7 W. As for Er:Y2O3 ceramic, the maximum output power of 320 mW was achieved with absorbed pump power of 5.1 W under the same experimental conditions with Er:Lu2O3. The corresponding information on the lasing spectrum of Er:Lu2O3 and Er:Y2O3 ceramic lasers were investigated and discussed. Our study indicates that both Er:Lu2O3 and Er:Y2O3 ceramic are promising gain medium for the 3 μm lasers operating at room temperature and, Er:Lu2O3 exhibit a better laser performance under 967 nm and 976 nm LD pumping.
2. Experimental setup
To experimentally investigate the performance of Er:Y2O3 and Er:Lu2O3 ceramic lasers, a plane-plane resonator with a cavity length of about 10 mm was adopted and is schematically shown in Fig. 1. Fiber coupled laser diode (LD) with center wavelength at 967 nm and 976 nm were used as pumping sources in the experiment. The delivery fiber has a core diameter of 105 μm and numerical aperture (NA) is 0.22. In the laser cavity design, we mainly considered good mode matching between the pump beam and the laser mode. The pump light was re-imaged into the Er sesquioxide ceramic with a spot size of about 210 μm in diameter by a simple telescopic lens system. A flat mirror coated high transmission (HT, T > 85%) at pump wavelength and high reflection (HR, R > 99.8%) at 2.7 μm was used as pump input mirror. And the output coupler is also a plane mirror with transmittance of 1.5% at 2.7 μm and HR at 970 nm. The laser output power is measured by a thermopile power meter (OPHIR 30A-BB-18).
The ceramic used were fabricated using the solid-state reactive sintering under vacuum condition. According to the well examined Er:Y2O3 ceramic, with the doping concentration increasing, the phonon energy would decrease significantly  and energy recycling efficiency would be enhanced. Nevertheless, thermal conductivity would decrease dramatically as a result of the difference of ionic radii and bonding forces between the active ions and the ions be substituted. In order to comprehensively investigate the laser performance of Er:Y2O3 and Er:Lu2O3 in the eyes of power scaling, the ceramics used were both 15 at.% doped and dimensions of 2 × 3 × 6 mm3. End pumping was realized via the 2 × 3 mm2 face, which was cut and polished with flat and parallel but uncoated end-faces. For efficient heat removal, the ceramics was surrounded by indium foil and embedded in a water-cooled copper heat sink.
3. Experimental results and discussion
Figure 2 shows the output power obtained from the Er:Lu2O3 and Er:Y2O3 ceramic laser systems under the same pump and resonator conditions. The cooling temperature was kept at 278K for both laser systems. First, Laser performances of Er:Lu2O3 and Er:Y2O3 ceramic pumped by 976 nm LD were investigated. The maximum output power of 530 mW was obtained from Er:Lu2O3 with absorbed pump power at 7.5 W, corresponding to an optical-to-optical conversion efficiency of 7.0%. As for Er:Y2O3, the maximum output power of 270 mW was obtained with absorbed pump power at 5.2 W, corresponding an optical-to-optical conversion efficiency of 5.2%. The slope efficiency of Er:Lu2O3 and Er:Y2O3 ceramic lasers under 976 nm pumping was about 7.7% and 5.4%.
Further experiment was conducted with diode pumping at 967 nm. As for Er:Y2O3 ceramic systems, the maximum output power of 320mW was obtained with absorbed pump power at 5.1 W, and slope efficiency was about 6.5%. When the absorbed pump power reached 5.1 W, a roll over in the output power was observed. The laser beam quality factors of Er:Y2O3 at the maximum output powers was measured with the Slit-Based Beam Propagation Analyzer. The beam quality factors were calculated to be Mx2 = 1.79 and My2 = 1.64, respectively, and the corresponding laser beam profile is shown in the inset of Fig. 2. The value of slope efficiency (5.4% and 6.5%) is lower comparing with the early reported 2 at.% Erbium concentration doped Y2O3 ceramic  (slope efficiency of 15%), which can be attributed to the lower transmittance of the output coupler and non-optimized pumping wavelength.
For the Er:Lu2O3 ceramic laser system with 967 nm LD pumping, the slope efficiency was about 7.6%, and the output power saturated when the absorbed power was 8.7 W. A maximum output power of 611 mW was achieved, corresponding to an optical-to-optical conversion efficiency of 7.0%. The beam quality factors were also measured at an absorbed pump power of 5.1 W. The quality factors were calculated to be Mx2 = 1.31 and My2 = 1.21, respectively, and the laser beam profile is shown in the inset of Fig. 2. In all cases, Er:Lu2O3 ceramic lasers show an excellent Gaussian transverse profile. Beam quality factors were measured at the maximum absorbed pump power of 8.7 W, and calculated to be Mx2 = 2.58 and My2 = 2.44, respectively. Generally speaking, under 967 and 976 nm LD pumping, Er:Lu2O3 exhibit a better performance in the eyes of slope efficiency, maximum output powers, and conversion efficiency. But it still couldn’t ascertain that which laser system would be a superior laser source for 3 μm transitions at room temperature, due to the non-optimized pump wavelength and laser cavity [14, 18].
It is worth noting that CW output power of 611 mW can be compared to the early reported Er:Lu2O3 crystal lasers with the similar bulk dimension and resonator conditions . Meanwhile, the value of slope efficiency (7.6%) of Er:Lu2O3 ceramic is much higher than that Er:Lu2O3 crystal, which can be attributed to the better mode matching. Nevertheless, confocal parameter of the pump beam inside the ceramic was only 2.3 mm, due to the marginal quality of the pump resource. The relatively low slope efficiency can be explained by the currently nonoptimized ratio between the pumping beam and the laser resonator mode cross section, which was approximately 0.26. Therefore, further studies would still focus on the optimum the crystal size and cavity structure to improve laser performance of current experimental set-up. And, significant improvement of the Er:Lu2O3 ceramic lasers can be expected by optimizing the transmittance of the output coupler, together with reducing the intracavity losses by means of an antireflection-coated active medium.
Under 967 nm LD pumping, laser spectra of the sesquioxide ceramic lasers were analyzed using a 0.55 m monochromator (Omni-λ5005, Zolix) with a specified resolution of 0.05 nm at 435.8 nm. First, the Er:Lu2O3 ceramic laser was investigated. Under low pump power, only the 2714.8 nm laser emission was detected, while as the pump power was increased, simultaneous multi-wavelength emission of the ceramic laser was observed. The simultaneous dual-wavelength or three-wavelength emission is very useful for generation of THz emission and Doppler lidar . The simultaneous dual-wavelength emission at both 2714.8 nm and 2724.0 nm, 2722.4 nm and 2736.8 nm were achieved respectively. Even three-wavelength laser emissions at 2723.2 nm, 2736.2 nm, and 2737.8 nm were eventually achieved. However, at the maximum output power of 611 mW, only the dual wavelength 2736.2 nm and 2737.8 nm dominates most of the time, and 2736.2 nm always has a larger profile.
Lasing spectra of the two laser ceramics were further investigated. Emission lines of Er:Y2O3 ceramic lasers at stable output power of 45 mW, 150 mW, and 300 mW were recorded, respectively, as shown in Fig. 3. The simultaneous dual-wavelength emission at both 2707.8 nm and 2723.0 nm was achieved when the output power was 45 mW. Single wavelengths, 2724.4 and 2739.0 nm, were obtained when the output power were 150 mW and 300 mW, respectively. As a comparison, laser spectra of Er:Lu2O3 ceramic lasers at stable output powers of 25 mW, 160 mW, and 575 mW were recorded respectively, as shown in Fig. 4. The corresponding laser wavelengths were 2714.8 nm, 2722.6 nm, and 2736.2 nm, respectively. With slit width of 20 μm, bandwidth measurements made with the monochromator indicate that the FWHM of the laser are all around 0.3 nm. What deserves to be mentioned the most is that all the laser spectra exactly avoided the numerous water vapor absorption lines in this wavelength band . And, with the pump power increased, red-shift of the lasing wavelength is observed in the 3 μm erbium sesquioxide ceramic lasers.
Red-shifting behaviors have been reported in other erbium doped lasers [1, 21]. This phenomenon can be understood as follows. Before the laser emission, the lower multiplet is empty. Short wavelength with large emission cross section more intend to oscillate at the beginning of laser emission (four-level nature) . As for Er:Y2O3 ceramic, four peak emission cross sections pretty close to each other , transitions with smaller water absorption losses, 2707.8 nm and 2723.0 nm, would be more prefer for oscillation at the beginning. Once the lower multiplet is populated during laser emission, the laser is forced to operate at longer wavelength due to reabsorption losses build up at the short wavelength side of the fluorescence. As for the Er:Lu2O3 ceramic laser systems, all the laser spectra surrounded with fewer water absorption lines. Transitions with larger emission cross sections, 2714.8 nm, would be more prefer to oscillate at the beginning. With pump power increased, the ETU process and the nonradiative transition process [4I13/2→ 4I15/2] cannot deplete the 4I13/2 state so efficiently that large number residual populations would accumulate in the long-lived 4I13/2 lower laser level. Then the character of the lasing process is changed from four-level to quasi-three-level lasing, the reabsorption process would be enhanced and the laser is forced to oscillate at longer wave. According to the previous study , in the red-shifting regime, lasers were preferred oscillating at a wavelength with more favor Boltzmann factors ratios. In keeping with notation found in the Er:YAG , the 2706, 2723, 2725, 2739 and 2740 nm transitions of Er:Y2O3 correspond to the 6→5, 2→2, 4→4, 1→2 and 3→4 Stark transitions of 4I11/2→4I13/2 systems, respectively. And, the 2716, 2723, and 2739 nm transitions of Er:Lu2O3 correspond to the 1→1, 4→3, and 3→3 Stark transitions of 4I11/2→4I13/2 systems, respectively. Table 1 shows the Stark levels in the upper and lower laser level manifolds (denoted i and j, respectively) between which these transitions take place. The associated Boltzmann factors are denoted as αi and βj. Figure 5 and Fig. 6 shows the energy-level diagram of Er:Y2O3 and Er:Lu2O3, and labels the laser transition at 2.7 μm.
To investigate the quasi-three-level property when the excitation energy is accumulated in the long-lived 4I13/2 lower laser level, the laser operation of Er:Lu2O3 ceramic under different cooling temperatures was also undertaken . Laser performances were investigated with pumping wavelength at 967 nm. Figure 7 shows the output power of the laser as a function of the absorbed pump power when the cooling temperature of the ceramic mount was set as 278, 283, 290, and 298 K. It can be seen that the output power is increased with the reduction of the cooling temperature. The output power increased from 511 mW at 298K to 595 mW at 278K with the absorbed pump power of 8.1 W. This phenomenon may stem from the particular level structure and the long lifetime of the terminal level. The laser partially behaves like a quasi-three level system with strong thermal dependence. On the other hand, due to the smaller pump spot, and highly quantum defect, output power appeared roll over at absorbed pump power of 8.7 W, when the ceramic mount was set as 283, 290, and 298 K. However, the phenomenon didn’t appear at 278K, due to the high thermal conductivity of Er:Lu2O3, which indicates that a better cooling setup is very essential to heavily doped Er:Lu2O3 ceramic lasers.
With 967 and 976 nm LD pumping, a comparison of the laser performance of erbium doped transparent ceramics, Er:Y2O3 and Er:Lu2O3, both with 15 at.% doping, has been undertaken. True CW operation of the Er-doped sesquioxide ceramic was obtained for the first time at room temperature, with a short plane-parallel cavity. An output coupler with a 1.5% transmission was adopted for both Er:Y2O3 and Er:Lu2O3 ceramic lasers. The maximum output powers of 320 mW and 611 mW were obtained, respectively, with diode pumping wavelength at 967nm. The study indicates that under 967 and 976 nm LD pumping, Er:Lu2O3 exhibit a better performance compared to Er:Y2O3 system. Furthermore, the wavelength for both ceramics lasers were found to red-shift with the pump power increased, and the final and dominant wavelength determined to be 2736.2 nm and 2739.0 nm for the Er:Lu2O3 and Er:Y2O3 ceramic lasers, respectively.
This work is supported by the National Natural Science Foundation of China (Grant No. 61177045, 61308047, and 11274144), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant No. 13KJB510008), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
References and links
1. E. Arbabzadah, S. Chard, H. Amrania, C. Phillips, and M. Damzen, “Comparison of a diode pumped Er:YSGG and Er:YAG laser in the bounce geometry at the 3 μm transition,” Opt. Express 19(27), 25860–25865 (2011). [CrossRef] [PubMed]
2. J. Chen, D. Sun, J. Luo, H. Zhang, R. Dou, J. Xiao, Q. Zhang, and S. Yin, “Spectroscopic properties and diode end-pumped 2.79 μm laser performance of Er,Pr:GYSGG crystal,” Opt. Express 21(20), 23425–23432 (2013). [CrossRef] [PubMed]
3. C. Hagen, A. Heinrich, and B. Nussbaumer, “High power, diode pumped Er:YAG for dentistry,” Proc. SPIE 7884, 78840I (2011). [CrossRef]
4. B. J. Shen, H. X. Kang, D. L. Sun, Q. L. Zhang, S. T. Yin, P. Chen, and J. Liang, “Investigation of laser-diode end-pumped Er:YSGG/YSGG composite crystal lasers at 2.79 μm,” Laser Phys. Lett. 11(1), 015002 (2014). [CrossRef]
6. C. Ziolek, H. Ernst, G. F. Will, H. Lubatschowski, H. Welling, and W. Ertmer, “High-repetition-rate, high-average-power, diode-pumped 2.94-µm Er:YAG laser,” Opt. Lett. 26(9), 599–601 (2001). [CrossRef] [PubMed]
7. E. A. Arbabzadah, C. C. Phillips, and M. J. Damzen, “Free-running and Q-switched operation of a diode pumped Er:YSGG laser at the 3 μm transition,” Appl. Phys. B 111(2), 333–339 (2013). [CrossRef]
9. A. Dergachev and P. F. Moulton, “Tunable CW Er:YLF Diode-Pumped Laser,” in Advanced Solid-State Photonics, J. Zayhowski, ed., Vol. 83 of OSA Trends in Optics and Photonics (Optical Society of America, 2003), paper 3.
10. J. Sanghera, W. Kim, G. Villalobos, B. Shaw, C. Baker, J. Frantz, B. Sadowski, and I. Aggarwal, “Ceramic Laser Materials,” Materials 5(12), 258–277 (2012). [CrossRef]
11. S. Sharma, R. Shori, and J. K. Miller, “Spectroscopic properties of Er-sesquioxides,” Proc. SPIE 8235, 82350F (2012). [CrossRef]
12. T. Sanamyan, M. Kanskar, Y. Xiao, D. Kedlaya, and M. Dubinskii, “High power diode-pumped 2.7-μm Er3+:Y2O3 laser with nearly quantum defect-limited efficiency,” Opt. Express 19(S5Suppl 5), A1082–A1087 (2011). [CrossRef] [PubMed]
13. T. Sanamyan, J. Simmons, and M. Dubinskii, “Er3+-doped Y2O3 ceramic laser at 2.7 μm with direct diode pumping of the upper laser level,” Laser Phys. Lett. 7(3), 206–209 (2010). [CrossRef]
15. A. Ikesue and Y. L. Aung, “Ceramic laser materials,” Nat. Photonics 2(12), 721–727 (2008). [CrossRef]
16. N. L. Wang, X. Y. Zhang, and P. H. Wang, “Fabrication and spectroscopic characterization of Er3+:Lu2O3 transparent ceramics,” Mater. Lett. 94, 5–7 (2013). [CrossRef]
17. A. Joshi, The Er3+:Y2O3 Ceramic System, Ph.D. Thesis (University of California, 2012).
18. T. Li, K. Beil, C. Krankel, C. Brandt, and G. Huber, “Laser Performance of Highly Doped Er:Lu2O3 at 2.8 μm,” in Lasers, Sources, and Related Photonic Devices, OSA Technical Digest (CD) (Optical Society of America, 2012), paper AW5A.6.
19. W. L. Gao, J. Ma, G. Q. Xie, J. Zhang, D. W. Luo, H. Yang, D. Y. Tang, J. Ma, P. Yuan, and L. J. Qian, “Highly efficient 2 μm Tm:YAG ceramic laser,” Opt. Lett. 37(6), 1076–1078 (2012). [CrossRef] [PubMed]
20. L. S. Rothman, D. Jacquemart, A. Barbe, D. Chris Benner, M. Birk, L. R. Brown, M. R. Carleer, C. Chackerian Jr., K. Chance, L. H. Coudert, V. Dana, V. M. Devi, J.-M. Flaud, R. R. Gamache, A. Goldman, J.-M. Hartmann, K. W. Jucks, A. G. Maki, J.-Y. Mandin, S. T. Massie, J. Orphal, A. Perrin, C. P. Rinsland, M. A. H. Smith, J. Tennyson, R. N. Tolchenov, R. A. Toth, J. Vander Auwera, P. Varanasi, and G. Wagner, “The HITRAN 2004 molecular spectroscopic database,” J. Quantum Spectros. Rad. Trans. 96(2), 139–204 (2005).
22. M. Pollnan and S. D. Jackson, “Erbium 3 μm fiber lasers,” IEEE J. Sel. Top. Quantum Electron. 7(1), 30–40 (2001). [CrossRef]
23. V. Peters, Growth and Spectroscopy of Ytterbium-Doped Sesquioxides, dissertation (University of Hamburg, 2001).
24. D. Y. Shen, A. Abdolvand, L. J. Cooper, and W. A. Clarkson, “Efficient Ho:YAG laser pumped by a cladding pumped tunable Tm: silica-fibre laser,” Appl. Phys. B 79(5), 559–561 (2004). [CrossRef]