Power scaling of end-pumped Nd:GdVO4 laser was realized by direct pumping, grown-together composite crystal and dual-end-pumping. A maximum CW output power of 46.0W with M2<1.1 was achieved, corresponding to a slope efficiency of 71.1% to absorbed pump power. In A-O Q-switch operation, peak power of 304.1kW, 58.6kW and 23.8kW, pulse width of 7.2ns, 11.3ns and 16.2ns were obtained at the repetition rates of 10kHz, 50kHz and 100kHz, respectively.
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Neodymium-doped vanadate, such as Nd:GdVO4, has now been recognized as a promising laser medium for diode-laser (LD) pumping due to its excellent physical and laser properties and also thought to be the favored gain medium when short pulses and high repetition rates are desired, owing to its high gain and limited upper-state lifetime. What is worth mentioning, Nd:GdVO4 has unexpectedly high thermal conductivity along the <1 1 0> direction , which is comparable to that of YAG. At present, many researchers have also indicated that Nd:GdVO4 is suitable for generation of diode-pumped solid-state lasers (DPSSLs) with high repetition rates [2–5]. The maximum repetition rate of Nd:GdVO4 laser had been up to 1.7MHz with the pulse width of about 34ns using acousto-optical Q-switch .
Based on the excellent thermal properties of Nd:GdVO4 crystal, people began to explore its potential to develop high-power lasers with good beam quality. Over 50W of multimode and 40W of predominantly TEM00 mode continuous-wave (CW) output from a diode-pumped bounce geometry Nd:GdVO4 oscillator was achieved. With master oscillator power amplifier (MOPA) configuration, 100W multimode and 104W TEM00 mode output was obtained . So far as we know, it’s the maximum CW TEM00 output power of Nd:GdVO4 laser. In thin disk configuration, the highest CW output power from Nd:GdVO4 disk lasers using one double pump pass was achieved to 18.1W . Although bounce geometry with MOPA configuration and thin disk configuration are effective methods for DPSSLs to obtain high output power, they have disadvantages of complex structure and design of pump system. In comparison, DPSSLs with end-pumped configuration have advantages of compactness, good beam quality, high energy conversion efficiency, simple design, and so on. What minor defect in something otherwise perfect is that achieving high output power with good beam quality is rather difficult, which is attributed to severe thermal effects. LD end-pumped Nd:GdVO4 laser had been researched more widely since it was induced. However, the power scaling of end-pumped Nd:GdVO4 laser was progressed slowly. LD end-pumped Nd:GdVO4 laser with CW output power exceed 10W was firstly reported by Liu . They achieved a 14.3W output power with conversion efficiency of 55% and M2<1.8. In 2004, J. Kong et al  reported a diode-end-pumped high-power CW Nd:GdVO4 laser pumped by 808nm, which produced maximum output power of 19.8W with beam quality M2~2.26 and a slope efficiency of 58.5%. Unfortunately, limited by the severe thermal effect under high pump power, the potential of the Nd:GdVO4 as a high-power laser crystal was not fully exploited. Until to 2009, the output power of Nd:GdVO4 laser was further advanced by means of double-end-pump configuration, and a maximum CW output power of 36W and M2=1.77 was obtained by Z. Zhao .
Theoretical and experimental results have demonstrated that power scaling of TEM00 operation in end-pumped lasers has been mainly limited by the severe thermal effects under high pumped power , such as temperature-dependent index change, temperature-dependent stress-induced birefringence, end-surface deformation and sometimes crystal destruction. Some methods was induced to reduce the thermal effects in order to scale the power, such as direct pumping [13,14], composite crystal [15–17] and dual-end-pumping [12,18], which were proved to be very effective and available means. Direct pumping has the potential to be the most efficient pumping scheme for a four-level laser because it reduces the losses induced by the Stokes factor to a minimum and eliminates quantum efficiency loss. The pumping of the Nd:GdVO4 crystal at 879nm instead of 808nm leads to the reduction of quantum defect ratio from 0.24 to 0.17 in the case of 4F3/2→4I11/2 emission, which reduces significantly the thermal loading by about 28% at 1.06μm. Consequently, higher incident pump power injection can be accomplished by direct pumping. Composite crystals, combining doped and undoped components, were adopted to improve the thermal lens of laser crystals, which was attributed to the undoped end of composite crystal acting as an effective heat diffuser. High-power grown-together GdVO4/Nd:GdVO4 composite crystal laser with single-end-pumped configuration pumped by 879nm was reported by our research group [19,20]. The maximum output power of about 20W with M2=1.9 was achieved. In addition, dual-end-pumping, as a method to reduce the thermal effects of crystals, distributes the total incident pump power to dual or multiple crystals and relaxes thermal effects of each crystal.
In this letter, power scaling of end-pumped Nd:GdVO4 laser was realized by direct pumping, grown-together composite GdVO4/Nd:GdVO4 crystal and dual-end-pumping. A maximum CW output power of 46.0W with the beam quality factor M2<1.1 was obtained. In A-O Q-switch operation, the peak power of 304.1kW, 58.6kW and 23.8kW, pulse width of 7.2ns, 11.3ns and 16.2ns were obtained at the repetition rates of 10kHz, 50kHz and 100kHz, respectively. The experimental results proved the feasibility of power scaling of Nd:GdVO4 laser with end-pumping configuration by means of direct pumping and grown-together composite crystal.
2. Experimental setup
Figure 1 shows the schematic diagram of a laser cavity utilized in our experiment. The 879nm pumping source employed in our experiments was a commercially available high-power fiber-coupled LD (NL-LDM-120-879, made by nLIGHT Inc.). The fiber has a N.A. of 0.22 and a core diameter of 400μm. The pump light of LD was separated into two beams of approximately equal intensity by beam splitter M1. M2, M3 and M4 were coated 45° high reflection (HR) at 879nm. Two beams were imaged spots of 533μm in diameter into the crystal rod C1 and C2 through two focusing achromatic lenses L2 and L3, respectively. The grown-together composite GdVO4/Nd:GdVO4 crystals were bought from Beijing Ke-Gang Electro-Optics Company in China. The total dimensions of C1 and C2 are both 3mm × 3mm × 10mm, and the undoped caps are 2mm long. The doped rods have a Nd3+ ion concentration of 0.3at.%. Two facets of C1 and C2 were coated antireflection (AR) at both 879nm and 1063nm. Both C1 and C2 were wrapped with indium foil and mounted in copper heat-sinks with microchannel structure, which had been proved to have more capability of thermal dissipation . A 200mm long laser cavity was constructed by two plano-plano mirrors M6 and M7. M6 was coated AR at 879nm and HR at 1063nm. M7, served as the output coupler, had four different transmissions at 1063nm of 20%, 30%, 35% and 40%, respectively. M5 was coated 45° high transmission (HT) at 879nm and 45° HR at 1063nm. The A-O Q-switch (39041-50DSFPS, made by Gooch and Housego Inc.) had AR coating at 1063nm on both sides and had a centre frequency of 41MHz and a radio-frequency power of 50W.
3. Simulation and analysis
Prior to the research on the power scaling of Nd:GdVO4 laser, the temperature distributions of laser rod along the optical axis were simulated according to different experimental conditions by LASCAD software, which is a laser cavity analysis and design software and combines all simulation tools necessary to model the 3-D nonlinear interaction of thermal and optical fields in DPSSL. The parameters used in simulation are as follows. The total incident pump power was set to 86.0W, the pump light was focused a 533µm spot in the center of laser rod.
Figure 2 shows the simulation results of temperature distributions. After mutual comparisons, we found that the temperature peak value was reduced from 330.7K to 318.7K when 808nm traditional pumping was replaced by 879nm direct pumping. Consequently, the decreased ratio of temperature peak value was reached to about 23.4%. It’s implied that 879nm direct pumping is more suitable to be employed in high pump laser. Furthermore, compared curve ① and ②, two main advantages can be found when composite crystal was adopted. One is the temperature peak value is decreased, another is the maximum temperature appears in the crystal interior rather than the surface of crystal, it is beneficial to limit the surface fracture of the crystal. Figure 3 and Fig. 4 show the 3-D temperature distributions. Compared Fig. 3 with Fig. 4, the temperature distributes the whole crystal rod more homogeneously in grown-together composite GdVO4/Nd:GdVO4 rod, which allows the reduction of the temperature and stress gradient.
From the comparison ④ with ① in Fig. 2, we can observe a significant reduction of temperature peak value owning to the adoption of dual-end-pumping, whose essential key is to distribute the total incident pump power to two or more crystals in order to split the thermal load on multiple crystals, therefore, the thermal effects of each crystal can be relaxed even though the total incident pump is very high. From the all comparisons above, it’s concluded that introduction of direct pumping to high power lasers is beneficial to realize higher incident pump power intensity. In order to relax the severe thermal effects of laser crystal end-pumped by high pump power, grown-together composite crystal and dual-end-pumping are effective means, they eliminate the surface expansion of the crystal and reduce the thermal load of the crystal, respectively. Following this rule, we had a try to scale the power of end-pumped Nd:GdVO4 laser to higher output power with good beam quality.
4. Experimental results and discussions
High power end-pumped CW Nd:GdVO4 laser was obtained by removing the A-O Q-switch from the laser resonator under direct pumping into the emitting level. The total incident pump power was about 86.0W. Figure 5 shows the CW output power as a function of the absorbed pump power. As seen in Fig. 5, the CW output power increased approximately linearly with the increase of the absorbed pump power. The maximum output powers were achieved to 46.0W, 44.3W, 44.0W and 40.8W at the absorbed pump power of about 70.8W with the transmissions of 35%, 40%, 30% and 20%, the corresponding slope efficiencies to absorption pump power were 71.1%, 70.6%, 61.2% and 66.4%, respectively. The 2-D and 3-D intensity distributions of laser beam at the maximum CW output power of 46.0W were measured by a laser beam analyzer (LBA-712PC-D, Spiricon Inc.) and shown in Fig. 7 and Fig. 8 . The M2 at the maximum CW output power of 46.0W was measured less than 1.1 by a beam propagation analyzer (M2-101, Spiricon Inc.). To the best of our knowledge, 46.0W is the highest output power with near quantum-limit beam quality factor from an end-pumped Nd:GdVO4 laser. The achievement of excellent laser properties is not only attributed to discreet laser design and good mode match, but also to the depression of direct pumping, composite crystal and dual-end-pumping to thermal effects of crystal rod pumped by high pump power.
In the process of experiments, we did not find the saturation of output power and the fracture of the crystal rod, which implies that the CW output power of Nd:GdVO4 laser will be further scaled to higher power level by increasing the incident pump power before the fracture limit of the crystal is reached. Unfortunately, we didn’t increasing the incident pump power further due to the limitation of 879nm diode laser. In order to explore the potential of direct pumping, composite crystal and dual-end-pumping in power scaling of Nd:GdVO4 laser, we simulated the CW output power as a function of pump power in single-end-pumped configuration by use of LASCAD software. The following parameters are used. The thermal conductivities along c-axial and a-axial were chosen to be 10.5W/m·K and 8.6W/m·K , respectively. The incident pump power was set to 80.0W, the cavity length was about 90mm, and the pump light was focused to a spot of about 533μm in diameter. The simulated result is shown in Fig. 6 . It can be seen that the maximum CW output power is reached to about 45.1W. The slope efficiencies in absorbed pump power and incident pump power were about uu71.8% and 58.9%, respectively. Compared Fig. 5 with Fig. 6, we found that the simulated results are in good agreement with the experimental results. In other words, it’s forecasted that over 90W output power with TEM00 mode and high efficiency can be obtained by adopting dual-end-pumping configuration. Therefore, the experimental and simulated results proved that direct pumping, grown-together composite crystal and dual-end-pumping have the potential in further power scaling of Nd:GdVO4 laser.
Efficient pulse operations of GdVO4/Nd:GdVO4 crystal with high repetition rates were also presented with the A-O Q-switch inserted into the resonator. Fig. 9 shows the average output power and the pulse width as a function of absorbed pump power for GdVO4/Nd:GdVO4 laser with the repetition rate of 100kHz. The average output power increases almost linearly and the pulse width decays exponentially with the increase of the absorbed pump power. The shortest pulse width of 16.2ns was obtained at the maximum absorbed pump power at the repetition rate of 100kHz. The peak power and pulse width as a function of repetition rate are shown in Fig. 10 . The peak power of 304.1kW, 58.6kW and 23.8kW, pulse width of 7.2ns, 11.3ns and 16.2ns were achieved at the repetition rates of 10kHz, 50kHz and 100kHz, respectively. Excellent pulse characteristics also indicated Nd:GdVO4 crystal is very suitable to be employed in high repetition rates lasers.
In conclusion, highly efficient CW and pulsed Nd:GdVO4 lasers scaling up to high power with near quantum-limit beam quality were accomplished. The CW output power was scaled to 46.0W with the beam quality M2<1.1 by 879nm direct pumping, grown-together GdVO4/Nd:GdVO4 crystal and dual-end-pumping, corresponding to a slope efficiency of 71.1% to absorbed pump power. Effective A-O Q-switched operations with high repetition rates were also achieved. The peak power of 304.1kW, 58.6kW and 23.8kW, pulse width of 7.2ns, 11.3ns and 16.2ns were obtained at the repetition rates of 10kHz, 50kHz and 100kHz, respectively. The achievement of power scaling of end-pumped Nd:GdVO4 laser implies the attractive potential of direct pumping, grown-together composite and dual-end-pumping to high power lasers.
This work was supported in part by the National Natural Science Foundation of China (No.60978016).
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