The spectra of Nd3+:Lu2O3 crystal have been examined at room temperature. Judd-Ofelt theory was applied to calculate the spectral parameters of the crystal. With a laser diode as pump source, a continuous-wave laser output power of 2.81 W is achieved, which is the highest value ever reported in this crystals to our knowledge, and its wavelength is also found to be dual-wavelength. Because of the emission cross-section at 1076 nm and 1080 nm are almost identical, laser oscillation for such two wavelengths can be obtained simultaneously. All the properties show that Nd:Lu2O3 is an excellent crystal for laser applications.
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Nowadays high-power lasers have been extensively utilized in many scientific and technological fields, such as military, industry, scientific research and medical treatment, due to their high efficiency and high power capability. Traditionally YAG crystals or ceramics doped with rare-earth (Re) ions have been identified as excellent materials for high-power lasers, since they possess high conductivity, excellent optical and laser properties . In recent years, high-temperature Re sesquioxides such as Lu2O3, Y2O3 and Sc2O3 have been suggested to be potential materials for high-power lasers, because those materials possess higher thermal conductivity and lower phonon energies comparing with YAG, which determines those materials can be used in much higher power lasers with more efficiency . Up to now, the research on the Re sesquioxides focused on the Yb doped ones. In 2009 and 2010, the high-power continuous-wave (cw) and femtosecond Yb:Lu2O3 lasers have been reported to be 149 W and 141 W, respectively [3,4], which has indentified its promising applications.
Besides the Yb doped materials, Nd doped crystals have also unique laser applications due to their abundant emission lines , such as about 0.9 μm, 1.08 μm and 1.4 μm, etc.. The Nd-doped sesquioxides has been proposed to be applied in Radar and eye-safe lasers [6,7]. However, the Nd-doped sesquioxides are rarely investigated especially of their spectra and lasers, no matter whether in the crystalline or in the ceramic form, and the output powers are still very low (about 700 mW for Nd:Sc2O3 crystal and 10mW for Nd:Lu2O3 ceramic) [6,8]. In this paper, we reported the measured spectra and watt-level laser output of Nd:Lu2O3 crystal at room temperature. The laser was found to be dual-wavelength at 1076 and 1080 nm oscillating simultaneously.
The Nd:Lu2O3 crystal was grown by optical floating zone method with Nd concentration of about 1 at.%. Samples used for spectral measurements were cut with dimensions of 4×4×1 mm3, where the two crystal faces of 4×4 mm2 were polished. The absorption spectrum was measured by using a V-570 JASCO spectrophotometer at room temperature, and the fluorescence spectrum excited by an 808 nm laser diode (LD) was done by a TRIAX550 spectrophotometer at room temperature.
Based on the plano-concave resonator configuration as shown in Fig. 1 , laser experiment was performed using a fiber-coupled LD with the central wavelength around 808 nm. The pump light is focused into the crystal and the spot radius is about 0.1mm in the crystal. M1 is a plano-concave mirror with a curvature radius of 150 mm, which is coated with high-transmission (HT) at 808 nm on the pump side while a HT coated at 808 nm and a high-reflective (HR) coated at 1.06 μm on the other side. M2 is a flat output coupler with a transmission efficiency of 1.6% at 1.06 μm. Sample was cut with the dimensions of 2×2×4 mm3, and the two 2×2 mm2 faces were polished and antireflection (AR) coated at 808nm and 1.06–1.08 μm. To remove the heat generated from laser crystal under high pump power levels, laser crystal was mounted in a copper block which was cooled by water.
3. Results and discussion
3.1 spectral properties
Room-temperature absorption spectrum in the wavelength range from 250 nm to 1000 nm is shown in Fig. 2 . There are many absorption bands around 358, 436, 482, 522, 542, 580, 694, 746, 806, 822 and 894 nm, which are assigned to spin- and electric- dipole-allowed transitions from the ground state (4I9/2) to 4D3/2, 2P1/2, 2G11/2 + 2P3/2 + 2D3/2 + 2G9/2, 4G9/2, 4G7/2, 4G5/2, 4F9/2, 4F7/2 + 4S3/2, 2H9/2, 4F5/2 and 4F3/2 energy levels, respectively . The full-width at half-maximum (FWHM) centered at about 806 nm is 6.1 nm, which is much larger than Nd:YAG (0.9 nm) and Nd:YVO4 (2.0 nm) . Because the wavelength of LD would change with its working temperature, it is suggested a larger FWHM is suitable for pumping by a LD.
Room-temperature fluorescence spectrum upon excitation at 808 nm is shown in Fig. 3 . The strongest emission peaks locating at 1076 and 1080 nm correspond to the 4F3/2→4I11/2 transition. The span of the emission peaks is caused by the Stark splitting in the crystal-field. The splitting of 4F3/2 level is 212 cm−1 for Nd:Lu2O3, which is a little wider than Nd:Y2O3 (196 cm−1), a difference which is assigned to the different crystal-field splitting .
The Judd-Ofelt (J-O) theory has been proven to be a valuable model in predicting rare-earth laser performance [10,11]. The important spectroscopic parameters can be obtained on the basis of the J-O theory. The refractive index n(λ) of Nd:Lu2O3 crystal has been calculated approximately from the following Sellmeier equation of Lu2O3 crystalline ceramics . All the values of the squares of the reduced matrix elements are cited from Carnall’s data . Three phenomenological intensity parameters are fitted to be Ω2 = 8.68 × 10−20, Ω4 = 2.06 × 10−20, Ω6 = 2.87×10−20 cm2, respectively. The calculated line intensities and optical parameters of the absorption spectrum are listed in Table 1 . The absorption cross-section of Nd:Lu2O3 at 806 nm is 1.97×10−20 cm2. The value is smaller than Nd:Y2O3 (about 2.5×10−20 cm2) and Nd:Sc2O3 (about 4×10−20 cm2) reported in the previous literature [6,7].
The fluorescence properties of the crystal are also calculated, and the results are listed in Table 2 . The information is summarized in Table 2 using the notation Dieke, namely, R = 4F3/2, X = 4I13/2, Y = 4I11/2 and Z = 4I9/2 with the Stark levels numbered upward . In this Table we show the center wavelength λ, the center energy in cm−1, the linewidth ΔE at FWHM, the peak cross-section σems, and the branching ratio β. The J-O theory determined radiative lifetime of the upper laser level 4F3/2 is 344μs and, when compared to the measured fluorescence lifetime of 286 μs, predicts a quantum efficiency of this level to be 0.83. We can see that the quantum efficiency of Nd:Lu2O3 is larger than that of Nd:YAG (about 0.63) , which shows that this crystal have the potential to obtain laser with high efficiency. Comparison of the stimulated emission cross-sections for the transitions of R1→Y1 and R1→Y2 are listed in Table 3 . In the table, the cross-sections for the transition of 4F3/2→4I11/2 at 1076 and 1080 nm are larger than that of Nd:Y2O3 and Nd:Sc2O3. Because large emission cross-section facilitates the increase in output power, we believe that Nd:Lu2O3 should have a promising potential in the high power laser applications. All results calculated by J-O theory indicate that the Nd:Lu2O3 crystal is a promising laser material for high power laser sources.
3.2 laser performance
Detailed laser performance of Nd:Lu2O3 is shown in Fig. 4 . The threshold (Pthr) was measured to be 0.22 W, and the output power of 2.81 W was achieved under an absorbed pump power of 16.5 W with a slope efficiency of 17.3% and an optical-optical efficiency of 17%. To our knowledge, this is the highest output power with this crystal as the laser material, although the output power has not reached its highest level. With the threshold, emission cross-section and experimental fluorescence lifetime shown above, the round-trip loss inside the cavity mainly induced by the laser crystal was calculated to be about 2% based on the model for the longitudinally pumped lasers , which indicated that the crystal possesses good quality. Compared with the previous papers , we believe that this excellent result is generated by the good crystal quality. This laser is also much higher than the previous result achieved by a Nd:Lu2O3 ceramic (10 mW with a slope efficiency 5.4%) . It can be found that the output power has not saturated and it can be expected that a further rise of the output power can be achieved if the incident power increases.
Laser spectra measured by a spectral analyzer is shown in Fig. 5 , and its wavelength is found to be dual-wavelength centered at 1076 and 1080 nm. Because of the emission cross-section at 1076 nm and 1080 nm are almost identical, laser oscillation for such two wavelengths can be obtained simultaneously. Recently, multiple wavelengths lasing have been of great interest for many applications such as medical instrumentation, spectral analysis, optical frequency up-conversion, and THz frequency generation, ect. and dual-wavelength laser has potential applications for new-wavelength laser by sum-frequency, coherent terahertz radiation by difference frequency and ultrahigh repetition rate pulse by optical beating.
In conclusion, the spectra of the Nd:Lu2O3 crystal were investigated. The absorption cross-section at 806 nm was calculated to be 1.97×10−20 cm2, and the emission cross-sectionat 1076 and 1080 nm were calculated to be 8.52×10−20 and 8.49×10−20 cm2, respectively. Laser performance was demonstrated at 1076 and 1080 nm, and the maximum output power of 2.81 W was achieved under a slope efficiency of 17.3%. We also proposed that by optimization, the laser performance should be much better. All the results shows that Nd:Lu2O3 is an excellent material for laser applications.
This work was supported by the National Natural Science Foundation of China (Grant Nos. 51025210, 51032004 and 51021062), the National Basic Research Program of China (Grant No. 2010CB630702), the National High Technology Research and Development Program (“863”Program) of China (No.2009AA03Z436) and Innovation Foundation of Shandong University (2010TS090).
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