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The diode-end-pumped continuous-wave Nd:LuAG ceramic lasers at 1064 nm and 1123 nm with 4F3∕2→4I11∕2 transition are here presented for the first time. For the 1064 nm laser operation, the output has been optimized using output couplers with different transmissions. A maximum output power of 8.3 W was achieved under an incident pump power of 20.4 W with a conversion efficiency of 40.7%. Replacing the output coupler based on reasonable coating design for 1123 nm oscillation, a continuous-wave laser with single-wavelength at 1123 nm was obtained with the same ceramic. At an incident pumped power of 20.4 W, an output power of 3.5 W was achieved with a conversion efficiency of 17.2%.
© 2015 Optical Society of America
1. Introduction
In recent years, solid-state laser using transparent ceramic as the laser gain material has attracted increasing attention with the development of modern ceramics fabrication technology [1,2]. Compared with laser crystals, laser ceramics have advantage of low production cost, short production period, and flexible composite structures [3]. Since the first ceramic laser has been demonstrated by Ikesue et al. in 1995 [4], a number of ceramic laser gain materials have been successfully developed [5–9].
Lu3Al5O12 (LuAG), the same as Y3Al5O12 (YAG), belongs to the garnet family and owns similar physical and chemical properties. Since Xu et al. reported 1064 nm laser operation of a Nd:LuAG crystal in 2009 [10], the fabrication of Nd:LuAG and its laser operation has been widely studied [11–14]. Up to now, only the 1064 nm laser operation of the Nd:LuAG crystal has been reported. In 2011, Di et al. reported 2.18 W 1064 nm output power with the optical conversion efficiency of 47.2% [13]. Wagner et al. first fabricated the Nd:LuAG transparent ceramic in 2012 [15]. Nowadays, Nd:LuAG ceramics also attracted more attention to achieve high optical quality and optimized performance [15,16]. Recently, our group also fabricated the LuAG ceramic with different rare-earth ions doped [17,18].
In this paper, the laser operation of Nd:LuAG transparent ceramic was first demonstrated. The continuous-wave laser performances at 1064 and 1123 nm of Nd:LuAG ceramic with 4F3/2→4I11∕2 transition were successfully achieved. Maximum output power of 8.3 W at 1064 nm and 3.5 W at 1123 nm were obtained under incident pump power of 20.4 W.
2. Nd: LuAG ceramic and experimental setup
The Nd:LuAG ceramic used in our experiment was fabricated with the solid-state sintering method under high vacuum condition which is the same as that used to fabricate the Nd:YAG and Yb:LuAG ceramics. The absorption and fluorescence spectra of the Nd:LuAG are similar with that of Nd:YAG [10,19], which have been reported by Xu et al. Therefore, 4F3/2-4I11/2 transition of Nd:LuAG also emit around 1.0 and 1.1 μm as shown in Fig. 1.The strongest line with the wavelength of 1064 nm comes from the Stark transition R2-Y3. However, the longest wavelength for Stark transition R1-Y6 of 4F3/2-4I11/2 is 1123 nm. The effective stimulated emission cross section of 1123 nm is much smaller than that of 1064nm. So it is more difficult for the laser oscillating at 1123 nm than 1064 nm to get the high power laser output. In order to make 1123 nm laser operating at single wavelength, both the strongest line of 1064 nm and the similar intensity neighbour lines must be simultaneously suppressed, otherwise it may be make 1064 nm or 1.1 μm waveband multi-wavelength laser oscillating.
The experimental setup of diode-end-pumped Nd:LuAG ceramic with 4F3∕2→4I11∕2 transition was shown in Fig. 2.In this experiment, the pump source was a fiber-coupled laser diode array at 808 nm with a numerical aperture of 0.22 and a core diameter of 200 µm. The Nd:LuAG ceramic, 1.0-at. % Nd3+ doped, 3 × 3 × 5 mm3, was wrapped with indium foil and fixed on a copper holder with temperature controlled at 20°C using a thermo-electric cooler. The pump light was coupled into the ceramic Nd:LuAG with a spot diameter of about 320 µm by the two different plano-convex lenses with focal lengths of 50 and 80 mm. The resonant cavity was composed by the flat mirror M1 and M2 with the cavity length kept at about 30 mm. The input mirror M1 for both 1064 nm and 1123 nm operations were using the same mirror with high-transmission (HT)-coated at 808 nm (T>95%) and high-reflection (HR)-coated at 1050-1150 nm(R>99.9%). To get single continuous-wave lasers at stark transition of 4F3/2 → 4I11/2 with different wavelengths output, we only changed the output couplers with different coating.
3. Experimental results and discussion
Firstly, the continuous-wave laser operation at 1064 nm was investigated. Different output couplers with transmissions of T = 8%, T = 13%, T = 34%, T = 41%, and T = 66% were used to optimize the output power. The output power increases with the incident pump power with the results shown in Fig. 3. A maximum output power of 8.3 W was obtained at an incident power of 20.4 W when the output coupler with T = 34% was used, corresponding to the conversion efficiency of 41%. The output coupler with the transmission T = 13%, T = 34% and T = 41%, by which the output power of lasers all have exceeded 8 W, are also suitable for being used as the output couplers to get high power 1064 nm laser.
Figure 4 show the fitted slope efficiency of 1064 nm output using the output coupler with T = 34%. The slope efficiency is about 46.3% with respect to the incident pump power. The laser spectrum was measured by a monochromator with a resolution of 0.05 nm. In wavelength domain from 1000 to 1150 nm, only a single wavelength of laser operation at 1064 nm was detected.
Fig. 4 Output power at 1064nm versus incident pump power using the output coupler with T = 34%. Inset show the spectrum of the output light.
In order to investigate the oscillation of 1123 nm, we try to increase the transmission of the neighbour lines as well as 1064 nm strongest line. The only change in this experiment was replacement of the output coupler by the plane mirror with 4.4% transmission at 1123 nm with the transmission curve shown in Fig. 5.The reasonable coating was designed with the transmission monotonically increased from 1123 to 1064 nm. The transmission at 1064 nm for the output coupler reaches 95%. The single continuous-wave laser at 1123 nm was successfully obtained. The output power and laser spectra were shown in Fig. 6.The laser threshold was around 3.0 W. The maximum average power was approximately 3.5 W under the incident pump power of 20.4 W. The inset also shows only a single wavelength of laser operation at 1123 nm was detected. The slope efficiency is about 19.8% and the maximum optical-to-optical conversion efficiency is 17.2%. The conversion efficiencies of Nd:LuAG ceramic for both 1064nm and 1123nm operation is still lower than that using Nd:YAG crystal with similar experimental conditions [20]. We will try to improve the optical quality of this ceramic in the further work.
Fig. 6 Output power at 1123 nm versus incident pump power using the output coupler with T = 4.4%. Inset show the spectrum of the output light.
As a relative new laser material, the laser spectra of both 1064 and 1123 nm were compared with that of Nd:YAG crystal. Figure 7 show both lines obtained in the Nd:LuAG ceramic and Nd:YAG crystal. The results show the spectra almost the same for both samples. For Stark transition R2-Y3 of 4F3/2-4I11/2, the laser center wavelength is located at about 1064.4 nm with the line-width of about 0.3 nm. For Stark transition R1-Y6 of 4F3/2-4I11/2, the center wavelength is located at about 1123.0 nm with the line-width of about 0.4 nm.
4. Conclusion
In conclusion, we demonstrate diode-end-pumped continuous-wave Nd:LuAG ceramic lasers at 1064 nm and 1123 nm with 4F3∕2→4I11∕2 transition. For the 1064 nm laser operation, the output has been optimized using output couplers with different transmissions. A maximum output power of 8.3 W was achieved under an incident pump power of 20.4 W. The conversion efficiency and slope efficiency are about 40.7% and 46.3%, respectively. Replacing the output coupler based on reasonable coating design for 1123 nm oscillation, a continuous-wave laser with single-wavelength at 1123 nm was obtained with the same ceramic. At an incident pumped power of 20.4 W, an output power of 3.5 W was achieved with a conversion efficiency of 17.2% and slope efficiency of 19.8%. The laser output spectra were measured and shown similar to that of Nd:YAG crystal. The Nd:LuAG ceramic could be an excellent alternative material for high efficient diode-pumped continuous-wave lasers generation.
Acknowledgments
This work was supported by the natural Science Foundation of Zhejiang Province under Grants LQ13F050004, the technology Foundation for Selected Overseas Chinese Scholar, high-level talent innovation technology project fundation of Wenzhou, and the Priority Academic Program Development of Jiangsu Higher Education Institutions. One of us (A.A.Kaminskii) also thanks the Institute of Crystallography for many-years interest in the subject of laser ceramics.
References and links
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