A high-efficiency 1342 nm Nd:YVO4 laser in-band pumped at 914 nm is demonstrated for the first time. Using an all-solid-state Nd:YVO4 laser operating at 914 nm as pump source, 0.86 W output was obtained with 1.82 W absorbed pump power. Corresponding slope efficiency of 65.4% was the highest of Nd:YVO4 lasers operating at 1342 nm to the best of our knowledge. Effects of crystal’s doping concentration and temperature on laser power and conversion efficiency were also investigated.
© 2011 OSA
High efficiency and compact lasers operating in the 1.3 µm infrared spectral regions have attracted much attention for their important applications in spectroscopy, medicine, optical fibers and scientific research [1,2]. It also can be used to generate red and blue lasers by nonlinear optical frequency conversion and as the pumping source for special lasers like Co:MgF2. Lasers around 1.3 μm can be basically obtained from 4F3/2→4I13/2 transition of Nd3+ and the corresponding emission lines are R2→X1 (1319 nm), R2→X3 (1338 nm) and R1→X4 (1356 nm) in Nd:YAG; R2→X2 (1342 nm) in Nd:YVO4. Traditionally, the Nd3+ ions are pumped from ground state into the highly absorbing level (4I9/2→4F5/2) by laser diode (LD) around 808 nm. In 1999, a continuous-wave (cw) 1342 nm Nd:YVO4 laser pumped by laser diode at 806 nm was demonstrated, with a slope efficiency of 28.1% versus absorbed pump power . Then many 1.3 µm Nd:YVO4 lasers pumped by 808 nm diode with slope efficiencies of around 40% were reported [4–7]. For 1.3 μm Nd:YAG lasers, slope efficiencies of higher than 40% were demonstrated [8,9]. However, due to long emitting wavelength, the large quantum defect has blocked the improvement of 1.3 µm laser efficiency. Great heat accompanied limited stable operation of solid-state lasers with high power and good beam quality. To solve this problem, N. Pavel et al. tried to use pump sources with longer wavelength to pump Nd3+ from ground state directly to upper lasing level (4I9/2→4F3/2) without a relaxation process (4F5/2→4F3/2). This kind of in-band pumping scheme could diminish the quantum defect effectively. In 2005, they reported a 1.34 µm Nd:YAG laser pump by 885 nm laser diode, high slope efficiency of 45% was achieved . In the same year, a 1.3 μm cw Nd:GdVO4 laser pumped by a 879 nm Ti:Sapphire laser was reported by Saikawa et al., with a slope efficiency of 60.7% . In 2008, X. Ding et al. reported a 1342 nm Nd:YVO4 laser pumped at 879 nm by a Q-switched Ti:Sapphire laser. Rather high slope efficiency of 64% was obtained with a 1.0-at.% Nd:YVO4 crystal . It is still possible to go further in this way since 880 nm is not the longest wavelength of the absorption band. Indeed, starting from the highest sublevel of the ground state manifold (Z5), Nd:YVO4 still presents absorption at 914 nm that can be used for pumping. 914 nm pumped Nd:YVO4 lasers emitting at 1064 nm have exhibited extremely high efficiency . In this letter, we report on a highly efficient 1342 nm Nd:YVO4 laser pumped at 914 nm by a laser diode (LD) end pumped Nd:YVO4 laser, for what we believe to be the first time. As two methods of promoting the pump absorption in this pump scheme, effects of heating the crystal and high doping concentration on the laser performance are experimentally studied.
2. Experiment arrangement
The experimental configuration is depicted in Fig. 1 . To investigate the influences of crystal temperature and doping concentration on 914 nm in-band pumped 1342 nm lasers accurately, a pump source with good beam quality and narrow spectral line-width is necessary. So we made a 914 nm Nd:YVO4 laser to pump the 1342 nm laser instead of a 914 nm laser diode. The pump source of the 914 nm Nd:YVO4 laser was an 808 nm fiber-coupled laser diode array with a fiber core diameter of 400 μm and a numerical aperture (NA) of 0.22. A multi-lens coupler reimaged the pump beam into a Nd:YVO4 crystal with a ratio of 1:1. The 3 × 3 × 5 mm3 crystal was 0.15-at.% doped and a-cut. Its entrance face was coated for highly reflective (HR, R>99.5%) at 914 nm and highly transmissive (HT, T>95%) at 808 nm. The other face has antireflection (AR, R<1%) coating at 914 nm. The crystal warped in indium foil was mounted in a cooper heat sink which cooled by refrigerant water at 10°C. A concave mirror with 150 mm radius of curvature and 5% transmittance at 914 nm was used as output coupler. The cavity length of 914 nm laser was 15 mm. It could provide 7.6 W maximum output power under 26 W incident LD pump power, with spectral line-width of 1 nm. The M2 factor of 914 nm laser output was measured to be 1.4 at the maximum output power. The power is sufficient and the beam quality is very suitable for our purpose of investigating the effects of crystal temperature and doping concentration on the performance of 1342 nm Nd:YVO4 lasers under 914 nm in-band pumping.
F was a focus lens with 40 mm focal length. It was used to control the 914 pump beam radius to realize better volume matching between pump and oscillating beam in the second Nd:YVO4 crystal utilized as 1342 nm laser gain medium. Two a-cut Nd:YVO4 crystals- 1.0-at.% doped, 3 × 3 × 8 mm3 and 3.0-at.% doped, 3 × 3 × 4 mm3 – were chosen in the experiment to investigate the influence of doping concentration on laser performance. Because the main drawback of 914 nm in-band pumping is the poor pump absorption and the absorption coefficient is nearly proportional to doping concentration, we chose the two samples with relatively high doping concentration. Besides, longer crystal could also increase the pump absorption effectively and thus improve the overall efficiency of 914 nm in-band pumped lasers. Thereby the 8-mm crystal length was chosen for the 1.0-at.% doped sample. The Nd:YVO4 crystals, both surfaces were antireflective (AR) coated at 1342 nm and highly transmissive (HT) coated at 914 nm, were wrapped in indium foil and clamped in a copper holder. The temperature of holder could be controlled by circulating water so that we can investigate the influence of temperature. M1 was a plane mirror highly reflective (R>99.5%) coated at the lasing wavelength of 1342 nm. Plane output coupler M2 was coated for 5% transmittance at 1342 nm. It was also coated HT at 1064 nm (T>70%) to prevent that from resonating and HT at 914 nm (T>99.5%) to make the measurements of absorption more accurate. The 914 nm pump polarized along c axis of the Nd:YVO4 crystal (π polarization) for higher absorption . Because F and M1 were not specially coated for HT at the pump wavelength, only 5.9 W of pump power could incident into the Nd:YVO4 crystal.
3. Results and discussion
We first measured the pump absorption in the two crystals. The absorption coefficients of π polarized 914 nm pump in the 1.0-at.% and 3.0-at.% doped Nd:YVO4 samples under the temperature of 20°C were 0.46 cm−1 and 1.80 cm−1. When the crystals were heated to 50°C, the absorption coefficients increased to 0.53 cm−1 and 1.96 cm−1, respectively. Heating crystals promoted the absorption slightly, as in reference .
When 5.9 W pump power incident into the 1.0-at.% doped, 3 × 3 × 8 mm3 sample, the theoretical thermal focal length was ~80 cm when pump radius was ~170 µm according to the following formula carried out by Innocenzi et al. 
Figure 2(a) shows 1342 nm laser output power versus absorbed 914 nm pump power with this crystal at different temperatures. When incident pump power was 5.9 W, the output power was 0.86 W with 1.82 W absorbed pump power at the crystal temperature of 20°C. High slope efficiency of 65.4%, which is close to the quantum efficiency of 68.1%, was achieved. To the best of our knowledge, this is the highest slope efficiency of 1342 nm Nd:YVO4 lasers ever reported. Such high slope efficiency indicates that the 1342 nm laser operation was not obviously harmed by the excited state absorption (ESA) centered at 1339.5 nm . When the temperature rose to 50°C, the output power under the fixed incident pump power of 5.9 W increased due to better pump absorption, as shown in Fig. 2(b). 0.94 W output power was obtained with 2.04 W absorbed pump power. The optical efficiency with respect to incident pump rose from 14.6% to 15.9% and slope efficiency rose from 20.2% to 22.0%. This exhibited that lasers in-band pumped at 914 nm have the potential of operating without cooling, because heat generation accompanied with laser operation could help increase the pump absorption thus promoting the output power and overall efficiency. However, the slope efficiency versus absorption fell slightly from 65.4% to 63.4%. We attribute this to the decrease of stimulated emission cross section induced by temperature rising. Many authors have reported that the stimulated emission cross section of the 4F3/2→4I11/2 transition (1064 nm) would decrease when temperature increased [16,17]. Considering the 4F3/2 state lifetime of Nd:YVO4 crystal showed very little variation in this temperature range [16,17], we can speculate that the decrease of stimulated emission cross section also occurred with 4F3/2→4I13/2 transition thus degenerated the laser efficiency. Meanwhile, the laser threshold also increased from 0.49 W to 0.54 W. For lasers operating at 1342 nm, 914 nm in-band pumping could reduce ~20% of thermal load by its lower pump photon energy compared with 808 nm pumping. This is not as effective as the 42% reduction with 1064 nm lasers. However, for 1342 nm lasers with relatively small stimulated emission cross section, thermal load induced stimulated emission cross section variation would affect the laser performance more obviously.
When it turned to the 3 × 3 × 4 mm3, 3.0-at.% doped sample, the theoretical thermal focal length was shortened to ~35 cm due to stronger pump absorption. Corresponding oscillating beam radius of ~185 µm in the Nd:YVO4 crystal still matched pump radius well, so we did not change the cavity arrangement. Figure 3 presents 1342 nm laser output power versus absorbed and incident pump power. The maximum output power of 1.09 W and 0.94 W were obtained with 3.03 W and 3.21 W absorbed pump power at the temperature of 20°C and 50°C, respectively, when the incident pump power was also 5.9 W. Laser thresholds were 0.48 W and 0.55 W absorbed pump power. Comparing with the former sample, higher doping concentration improved absorption greatly, but the output power did not show a corresponding increase due to short upper-laser-level lifetime induced by high doping concentration. High doping concentration led to significant cross relaxation. Meanwhile, higher absorption accompanied with high doping concentration also brought stronger Auger upconversion under the same incident pump power [18–20]. They both reduced the inversion population density. As a result, the upper-laser-level lifetime can be as short as ~50 μs in the 3.0-at.% doped Nd:YVO4 crystal . The slope efficiencies versus absorption were only 43.1% and 35.5%, much lower than those of the 1.0-at.% doped sample. Temperature rising from 20°C to 50°C induced 7.6% slope efficiency degeneration. This is also more serious than the 2% degeneration occurred with the 1.0-at.% doped sample. Moreover, when using the 3.0-at.% doped sample, the output power at the higher temperature was even lower (Fig. 3(b)) under the same incident pump power, though higher temperature led to better absorption. This is opposite to the 1.0-at.% crystal. It is can be seen more clearly in Fig. 4 , the output power showed a near linear decrease as temperature rising instead of the increase with the 1.0-at.% sample. This means, for highly doped Nd:YVO4 crystal, heating induced stimulated emission cross section decrease would harm more significantly on the laser performance. Therefore using highly doped crystals cannot be a good method to improve the absorption of 914 nm pump and meanwhile pursuing high efficiency.
When using the 1.0-at.% doped crystal under the temperature of 20°C and 5.9 W incident pump, the M2 factors of 1342 nm laser output were 1.15 and 1.10 in parallel and perpendicular directions, respectively. The beam profile is shown in Fig. 5 . In fact, the beam quality varied little when temperature and crystal changed. It must be mentioned that, though high efficiency versus absorbed pump power was achieved with the 1.0-at.% doped Nd:YVO4 crystal, the optical efficiency versus incident pump was still limited to lower than 20% by the poor absorption of no more than 35%. To solve this problem, longer crystal and pump feedback should be more proper. The absorption coefficient of 0.46 cm−1 (1.0-at.%, 20°C), with which we achieved the highest slope efficiency, indicates that more than 75% of pump can be absorbed when using a 15 mm long crystal with double pass pumping scheme. Thus an optical efficiency comparable with traditional 808 nm pumping can be expected. Then 914 nm in-band pumping would be more practical and attractive for its high efficiency and low heat generation.
In conclusion, we have demonstrated a 1342 nm Nd:YVO4 laser which we believe to be the first one in-band pumped at 914 nm. When using a 1.0-at.% doped sample at the temperature of 20°C, the slope efficiency of 65.4% with respect to absorbed pump power was obtained. This is the highest for a Nd:YVO4 laser operating at 1342 nm to our knowledge. We found that heating the crystal, which is considered as a method to improve the absorption of 914 nm pumping, will decrease the laser slope efficiency versus absorbed pump power. The reason might be the decrease of stimulated emission cross section of 4F3/2→4I13/2 transition induced by temperature rising, just like the decrease occurred in 4F3/2→4I11/2 transition. When using highly doped crystal whose upper-laser-level lifetime is short, the consequences of heating induced stimulated emission cross section decrease on laser performance would go more serious. Using longer gain medium with proper doping concentration and pump feedback instead of highly doped crystals should be the most promising method to ensure the absorption and high efficiency, which could bring 914 nm in-band pumped Nd:YVO4 practical.
This work was supported by the National Natural Science Foundation of China (Grant Nos. 60978021 & 10804055), the National Basic Research Program (973 Program: 2007CB310403) and Program for New Century Excellent Talents in University (NCET).
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