We report a high-efficiency Nd:YVO4 laser operating at 1342 nm pumped by an all-solid-state Q-switched Ti:Sapphire laser at 879 nm. A plano-concave cavity was optimized to obtain high efficiency and good beam quality. Output power for two Nd:YVO4 crystals with 1.0- and 3.0-at.% Nd3+ doping under 879-nm pumping was measured respectively. Comparative results obtained by traditional pumping at 808 nm into the highly absorbing 4 F 5/2 level were presented, showing that the slope efficiency of the 1.0-at.% Nd:YVO4 laser under 879-nm pumping was 10.5% higher than that of 808-nm pumping. In a 4-mm-thick, 1.0-at.% Nd:YVO4 crystal, a high slope efficiency of 64% was achieved under 879- nm pumping, with an optical-to-optical conversion efficiency of 41.3%.
©2008 Optical Society of America
High efficiency and compact lasers operating in the infrared spectral regions around 1.3 μm have attracted much attention for their important applications in spectroscopy, medicine, optical fibers, laser display and scientific research [1, 2]. The source operating near 1.3 μm is used in silica optical fibers  to coincide with the transmission window. Especially the 1342-nm emission is widely used for frequency conversion into the orange-yellow light (593 nm) by sum-frequency mixing with 1064 nm , into the red light (671 nm) by second harmonic generation  and into the blue light (447 nm) by third harmonic generation . Lasers based on the 1.3 μm can be basically achieved from 4 F 3/2→4 I 13/2 transition of Nd3+ doped materials and the corresponding emission lines are R 2→X 1 (1319 nm), R 2→X 3 (1338 nm) and R 1→X 4 (1356 nm) in Nd:YAG, R 2→X 2 (1342 nm) in Nd:YVO4. Traditionally, the Nd3+ ions are pumped into the highly absorbing 4 F 5/2 level by laser diode around 808 nm. In 1999, a continuous-wave(cw) 1342-nm Nd:YVO4 laser pumped by laser diode at 806 nm was achieved, with a slope efficiency in absorbed pump power of 28.1% . And then other diode-pumped 1.34-μm Nd:YVO4 lasers at 808 nm with slope efficiencies of around 40% were obtained [8–10]. An efficient diode-pumped 1342-nm Nd:YVO4 cw laser with slope efficiency of 42% and optical-to-optical conversion efficiency of 33% was demonstrated in 2003 . Recently, more efficient 1.3-μm lasers have been demonstrated, with slope efficiencies of 43.5% and 46%, respectively [12, 13]. However, the 808-nm pumping would introduce a large quantum defect of around 40% between the pump and lasing wavelength when operating at 1342 nm, which resulted in low efficiency and high heat deposition. Moreover, the output power and slope efficiency of Nd3+ doped laser operating at 1342 nm were also limited by the small stimulated-emission cross-section and the low quantum efficiency under 808-nm pumping.
A more efficient pumping method was presented recently, which was to pump the Nd3+ ions directly into the 4 F 3/2 upper lasing level. As a result, the slope efficiency and threshold can be increased and decreased respectively with lower heat generation while a ~20% decrease in quantum defect than traditional pumping was obtained. However, only several works about 1.34-μm emission by direct pumping were reported. In 2005, N. Pavel et al. reported a 885-nm pumped Nd:YAG laser operating at 1.34 μm with slope efficiency of 45% . In the same year, a 1.3-μm cw Nd:GdVO4 laser pumped by a 879-nm Ti:Sapphire laser was reported, with a slope efficiency of 60.7% .
In this paper, we report a high-efficiency 1342-nm Nd:YVO4 laser directly pumped by an all-solid-state Q-switched Ti:Sapphire laser at 879 nm. By optimizing the cavity, a high slope efficiency of 64% was obtained in a 4-mm-thick, 1.0-at.% Nd:YVO4 crystal, with a maximum output power of 0.743 W at the incident pump power of 1.8 W, leading to an optical-to-optical conversion efficiency of 41.3%. To the best of our knowledge, this is the highest reported slope efficiency and the optical-to-optical conversion efficiency of Nd:YVO4 laser operating at 1342 nm. The advantage of 879-nm pumping in increasing the slope efficiency and decreasing the threshold in absorbed pump power was also demonstrated by a comparative investigation on 808-nm pumping.
2. Experimental setup
The experimental setup of the 1342 nm Nd:YVO4 laser is shown schematically in Fig. 1. An all-solid-state Q-switched Ti:Sapphire laser with repetition rate of 6.8 kHz and tunable range from 700 nm to 950 nm was used as the pump source. The maximum output power (measured with a laser powermeter: Molectron EPM1000) is 2.7 W at 808 nm and 2.1 W at 879 nm, respectively, with full width at half maximum of ~2 nm and pulse duration of 38.4 ns. The spectrum of the 879 nm pump light is shown in Fig. 2. In our experiment, F was a focus lens with the focal length of 150 mm, which was used to enhance the density of pump power and obtain better volume matching between the pump and oscillating beam. The pump beam of 808 nm or 879 nm was focused into the Nd:YVO4 crystal with a waist spot radius of around 190 μm. When the pump power at 879 nm was 1.8 W, the thermal-lens focus length of the Nd:YVO4 crystal was 250 mm measured by a method which used the transform circle theory of resonator proposed in Ref. .
In experiment, a plano-concave cavity was carefully designed. The plane-reflector mirror M1 was high reflectivity (R>99%) coated at the lasing wavelength of 1342 nm and high transmission (T>99%) coated at the pump wavelengths of 808 nm and 879 nm. A planoconcave mirror M2 with a curvature-radius of 200 mm was employed, with a transmission of 8% at 1342 nm and high transmission (T>70%) at 1064 nm to suppress the strong parasitical oscillation at this transition. Two pieces of Nd:YVO4 crystals with the same dimensions of 3mm×3mm×4mm and different doping concentrations of 1.0- and 3.0-at.%, respectively, were a-cut to obtain the high-gain π transition. The Nd:YVO4 crystal, both surfaces were antireflection (AR) coated at 1342 nm and high transmission (T>99%) coated at 808 nm and 879 nm, was wrapped in indium foil and clamped in a copper holder while the water temperature was kept at 15◻. The absorption efficiencies for 1.0- and 3.0-at.% Nd:YVO4 crystals at 879 nm are shown in Fig. 3, while the pump power of 808 nm was totally absorbed for both crystals. The maximum pump power for 879-nm and 808-nm pumping was 1.8W and 2.2W after the focus lens, respectively.
Through theoretical calculating, a cavity length of 180 mm and a curvature-radius of 200 mm of M2 were chosen. The Nd:YVO4 crystal was placed closely to the M1. The U-shape curve of the fundamental mode spot radius in the 1.0-at.% Nd:YVO4 crystal as a function of the thermal-lens focus length is shown in Fig. 4. Over the entire pumping range, it is observed that the TEM00 mode spot radius in the Nd:YVO4 crystal was about 200 μm, which matched to the pump beam waist spot radius of 190 μm well and results in a larger beam overlap efficiency. The Nd:YVO4 laser still in the stability zone when operated at the maximum pump power, while the thermal-lens focus length of the Nd:YVO4 crystal was 250 mm.
3. Results and discussion
The output power at 1342 nm for the 1.0- and 3.0-at.% Nd:YVO4 crystals under 808-nm and 879-nm pumping was measured with a laser powermeter (Molectron EPM1000), respectively. The results are plotted in Fig. 5 and Fig. 6 as functions of the absorbed pump power. The slope efficiencies are obtained by fitting the result curves.
As shown in Fig. 5, for the 1.0-at.% Nd:YVO4 crystal, the best performances were obtained under 879-nm pumping. The slope efficiency in absorbed pump power was as high as 64%, which neared the quantum efficiency, to the best of our knowledge, this is the highest reported slope efficiency of Nd:YVO4 laser operating at 1342 nm. The threshold in absorbed pump power was 0.38 W, with a maximum output power of 0.743 W for the absorbed pump power of 1.5 W while the incident pump power was 1.8 W, leading to an optical-to-optical conversion efficiency of 41.3%. When pumped at 808 nm, the slope efficiency decreased to 53.5% and the threshold increased to 0.44 W, with a maximum output power of 0.76 W for the absorbed pump power of 2.12 W.
The output results obtained with the 3.0-at.% Nd:YVO4 crystal were shown in Fig. 6. Decreases of the slope efficiency and increases of the threshold were obtained as the doping concentration increases. Under 808-nm pumping, the slope efficiency was 48.6% and the threshold was 0.5 W. When pumped at 879 nm, the slope efficiency increased to 57.5% and threshold decreased to 0.45 W.
Contrast with the 808-nm pumping, direct pumping at 879 nm can increase the slope efficiency by 10.5% and reduce the threshold by ~12%. This is resulted from the larger quantum efficiency (ηqe=λp/λe, where subscripts p and e represent the pump and lasing beam) of ~8.8% under 879-nm pumping than that of 808-nm pumping. Besides, the optimization of plano-concave cavity is also an important contributing factor. Moreover, the expected less heat generation induced by a 20% smaller quantum defect ratio in 879-nm pumping than in 808-nm pumping also improve the laser parameters.
In experiment, a deterioration of slope efficiency and threshold in absorbed pump power with increasing doping concentration of Nd3+ was observed under the same pumping wavelengths. This mainly results from the decrease of emission quantum efficiency which depends on the doping concentration of Nd3+ [17, 18]. Additionally, the influence of nonuniform distribution of Nd3+ in crystal on laser parameters would be more severe in highly doped crystals.
The beam quality of the 1342 nm emission measured by a Laser Beam Diagnostics (Spiricon, M2-200) is shown in Fig. 7, a small M2 factor of 1.12 was achieved.
The experimental results shown above demonstrate that the direct pumping at 879 nm contributes significantly to increasing slope efficiency and decreasing threshold in absorbed pump power in solid-state lasers. Higher slope efficiency and good beam quality indicate that less heat is generated under direct pumping. Due to the higher quantum efficiency, an allsolid-state Nd:YVO4 laser operating at 1342 nm with high efficiency, high beam quality can be achieved by direct pumping under the condition of optimizing the cavity.
In summary, we present a high slope efficiency direct-pumped Nd:YVO4 laser operating at 1342 nm pumped by an all-solid-state Q-switched tunable Ti:Sapphire laser at 879 nm. A high slope efficiency of 64% was achieved in a 4-mm-thick, 1.0-at.% Nd:YVO4 crystal, with an optical-to-optical conversion efficiency of 41.3%. By contrasting with the output performances of 808-nm pumping, for the 1.0-at.% Nd:YVO4, the slope efficiency increased by 10.5% and the threshold in absorbed pump power decreased by ~12% under 879-nm pumping, due to the larger quantum efficiency between the pump and lasing wavelength.
This project was supported by the National Natural Science Foundation of China (Grant Nos. 60637010 and 60671036), the National Basic Research Program of China (Grant No. 2007CB310403) and Tianjin Applied Fundamental Research Project (Grant No. 07JCZDJC05900)
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