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We detail the comparison on laser performance of Nd:GdVO4 and Nd:YVO4 lasers at high repetition rates operated at 1.06µm under direct diode pumping of the upper laser level. The results reveal that Nd:GdVO4 and Nd:YVO4 are efficient laser crystals for solid-state lasers under direct pumping of the emitting level. However, Nd:YVO4 crystal, compared with Nd:GdVO4 crystal, is a more favorable gain medium when higher repetition rates and shorter pulse width are desired, owning to its larger stimulated emission cross-section.
©2009 Optical Society of America
1. Introduction
High repetition rates Q-switched solid-state lasers with short pulse width have a variety of applications such as remote sensing, ranging, micro-machining, marking and so on [1–3]. They can be realized by active Q-switching, such as acousto-optically (A-O) Q-switching, which takes the advantages of low modulation voltage, low insertion losses, high repetition rate and short pulse width. Short pulse width is benefit in obtaining high peak power for high repetition rates Q-switched lasers. Both Nd:GVO4 and Nd:YVO4, which are widely researched and used for their excellent physical and optical properties [4–6], are the favorable gain medium when short width and high repetition rates are desired, owing to its high gain and limited upper-state lifetime. A large stimulated emission cross section and effective absorption coefficient can provide higher gain which enhances Q-switching at high repetition rates. Meanwhile, their modest upper-state lifetime leads to the faster building up of the pulses to achieve short pulse width. Therefore, both Nd:GdVO4 and Nd:YVO4 crystals were regarded as excellent laser medium for Q-switching operation at high repetition rates with short pulse width [7,8].
Many researches on solid-state lasers with high repetition rates have been reported by making use of Nd:GdVO4 and Nd:YVO4 crystals [9–14]. For Nd:GdVO4 lasers, Li et al reported a 100kHz Nd:GdVO4 laser under 879nm diode-laser pumping, an average output power of 12.1W and a pulse width of 20.3ns were obtained in A-O Q-switched operation [9]. A. Minassian reported a diode-pumped TEM00 Nd:GdVO4 MOPA system and obtained an average power of 101W and a pulse width of about 20ns from 100~600kHz [11]. To the best of our knowledge, it’s the highest pulse repetition rate making use of A-O Q-switching Nd:GdVO4 laser. For Nd:YVO4 lasers, J. H. García-López et al reported a high power Nd:YVO4 slab laser with repetition rate up to 500kHz, which had a pulse width of 15ns and a average power of 15.9W at 200kHz [13]. Liu et al demonstrated a 850kHz A-O Q-switching diode-pumped MOPA Nd:YVO4 laser with average power of 183W and a pulse width of 72ns [14]. X. Yan reported a 2.2MHz A-O Q-switching Nd:YVO4 laser with a pulse width of 31ns. It is the highest pulse repetition rate ever reported based on A-O Q-switching Nd:YVO4 laser [15]. It’s forecasted that short pulse width solid-state lasers with higher and higher repetition rates will be desired and favored. Choosing a favorable crystal as the laser medium is benefit to shorten the pulse width and improve the performance of lasers with high repetition rates. Although some paper have shown that both Nd:GdVO4 and Nd:YVO4 are promising crystals to operate at high repetition rates, comparatively little research has been carried out on the comparison of their pulse performance, especially under direct diode pumping of the emitting level.
A major limitation in the scaling of a solid-state laser to high power is the quantum defect between the pump and the laser emission wavelengths, which has a major contribution to the heat generation in the laser material. The reduction of the quantum defect is an important issue in diminution of heat, and for Nd3+ laser materials this can be accomplished by direct pumping of the emitting level 4F3/2 [16,17]. Maik Frede et al reported an end-pumped Nd:YAG laser with direct pumping into the upper laser level [18], the maximum output power was 250W with an optical-optical efficiency of 57%. Y. Sato realized a near quantum-defect slope efficiency in Nd:YVO4 laser under direct diode pumping [19], 80% and 75% slope efficiency were obtain under Ti:sapphire and LD pumping at 880nm, respectively. An efficient A-O Q-switched Nd:GdVO4 laser under 879-nm pumping was also reported, a maximum average output power of over 4W was obtained at 100 kHz [7].
In this paper, we do in detail some comparative studies on the performance of A-O Q-switching Nd:GdVO4 and Nd:YVO4 lasers at high repetition rates operated at 1.06µm under direct diode pumping of the emitting level. The experimental results demonstrate that both Nd:GdVO4 and Nd:YVO4 are efficient and promising laser crystals for diode pumped solid-state lasers under direct pumping of the upper laser level. When higher repetition rates and shorter pulse width are desired, Nd:YVO4 shows superior laser pulse performance to Nd:GdVO4, which indicates that Nd:YVO4 crystal is a more favorable gain medium than Nd:GdVO4 crystal, owning to its larger stimulated emission cross-section.
2. Theoretical analysis
Nd:GdVO4 and Nd:YVO4 are isomorph and have the same crystal structure. Table 1 shows the thermal and laser properties of Nd:GdVO4 and Nd:YVO4 at room temperature. It’s noted that there are some controversies in the literatures surrounding the relative thermal conductivities of Nd:GdVO4 and Nd:YVO4 [20–22]. Here we just cited the data obtained by Yoichi Sato et al. They has measured the thermal conductivity by quasi-one-dimensional flash method and proved that there is no remarkable difference on thermal properties of YVO4 and GdVO4. However, the stimulated emission cross-section is greatly different between Nd:GdVO4 and Nd:YVO4. That of Nd:YVO4 is two more times greater than that of Nd:GdVO4. In the pulse operation, σem·τ is an important parameter for lasers with high repetition rates. σem is the stimulated emission cross-section at 1.06µm and τ is the upper-state lifetime. At the same repetition rates, the higher product of σem·τ, the higher gain of each pulse. The σem·τ of Nd:YVO4 is two more times greater than that of Nd:GdVO4. According to the simulation theoretically, we find that Nd:YVO4 laser can obtain shorter pulse width than Nd:GdVO4 laser operated at high repetition rates, owning to larger emission cross-section related to the higher single-pulse gain.
Table 1. Thermal and laser properties of Nd:GdVO4 and Nd:YVO4 at 25°C
From the theory of the continuously pumped and repetitively Q-switched system pulse width Δtp can be calculated by the following formulas [23]:
Where c is the velocity of light, L’ is the optical length of the cavity, T is the transmissivity of the output coupler, and L is the other loss of the cavity. ni, nf and nt are the initial population inversion density, the final population inversion density, and the population inversion density at threshold, respectively.We calculated theoretically Δtp for various repetition rates at the absorbed pump power of 20W for Nd:GdVO4 and Nd:YVO4 lasers, respectively. Figure 1 shows the theoretical pulse width as a function of repetition rate for Nd:GdVO4 and Nd:YVO4 lasers. As shown in Fig. 1, at relative lower repetition rates, such as lower than 30kHz, the difference of the pulse width between Nd:GdVO4 and Nd:YVO4 lasers is not obvious. As the repetition rate increases, the pulse width of Nd:YVO4 laser is gradually shorter than that of Nd:GdVO4 laser. The pulse width as a function of absorbed pump power at the repetition rate of 100kHz is also simulated for Nd:GdVO4 and Nd:YVO4 lasers, respectively. As shown in Fig. 2 , it is easy to observe the difference of pulse width between two lasers. As shown above, it’s forecasted that the pulse performance of Nd:YVO4 laser will be superior to that of Nd:GdVO4 laser at very high repetition rates according to the theoretical simulation.
3. Experimental setup
The experimental setup of A-O Q-switching operation is shown schematically in Fig. 3 . The laser crystals used in our experiments were the Nd:GdVO4 and Nd:YVO4, which were grown at Beijing Ke-Gang Electro-optics company in China by the Czochralski technique and had a comparable crystal quality. Nd:GdVO4 and Nd:YVO4 crystals were both polished and antireflection-coated at both the pump and laser wavelength on two facets of each crystal. The crystal was wrapped with indium foil and mounted in a copper heat-sink cooled by flowing-water at 18°C, with good thermal contact between crystal and heat-sink. The a-cut cuboid Nd:GdVO4 and Nd:YVO4 laser crystals employed had a same Nd3+ ion concentration of 0.5at.% and had a same dimensions of 4mm × 4mm × 8mm. The crystal was placed in a about 100mm long flat-flat resonator with an output coupler with a 35% transmissivity. The mirror M1 coated with antireflection at 879nm and high reflection at 1064nm and served as the front cavity mirror. A 879nm pumping source used in our experiments was a commercially available high-power fiber-coupled diode-laser (NL-LDM-120-879, made by nLIGHT Inc.), which had a top-hat intensity distribution, the FWHM of pumping radiation was less than 3nm. The pump light of diode-laser was imaged a spot of about 533µm into the crystal through two aplanatic lenses. The A-O Q-switch (39041-50DSFPS, made by Gooch and Housego Inc.) had antireflection-coating at 1064nm on both facets and the power of the radio-frequency driver was 50W at 41MHz.
4. Experimental results and discussion
Experiments for Nd:GdVO4 and Nd:YVO4 were carried out under the same conditions, respectively. End-pumped very highly efficient continuous-wave (CW) lasers system for Nd:GdVO4 and Nd:YVO4 were obtained under direct diode pumping of the emitting level by removing the A-O Q-switch from the laser resonator. The Nd:GdVO4 and Nd:YVO4 crystals used in our experiment did not absorb efficiently the pump radiation. The amount of absorbed pump power was determined by monitoring the transmitted pump power behind the laser crystal. The absorption efficiency to pump radiation was about 55.0% for Nd:GdVO4 crystal and 58.8% for Nd:YVO4 crystal, respectively. Figure 4 shows the CW output power as a function of the incident pump power. The maximum multi-mode CW output powers were 22.2W and 23.5W for Nd:GdVO4 and Nd:YVO4 lasers, respectively. The beam quality factors were measured with a beam propagation analyzer (M2-101, made by Spiricon Inc.). The M2 factors at the maximum CW output power were measured as M2 x = 2.27, M2 y = 2.30 for Nd:GdVO4 laser and M2 x = 2.28, M2 y = 2.32 for Nd:YVO4 laser, respectively. For Nd:GdVO4 laser, the maximum optical-to-optical efficiency and slope efficiency in the range of linear output with respect to incident pump power are 38.1% and 60.7%, respectively. For Nd:YVO4 laser, they are 40.5% and 66.5%. The CW and average output power as a function of absorbed pump power were also measured. As shown in Fig. 5 , the optical-optical efficiency of CW output power to absorbed pump power was about 69% and the slope efficiency achieved about 75% for both Nd:GdVO4 and Nd:YVO4 lasers. The differences in laser performance of Nd:GdVO4 and Nd:YVO4 crystals would be likely attributed to the different absorption coefficient to pump light. It’s concluded that the CW laser performance of Nd:YVO4 crystal is slightly superior to that of Nd:GdVO4 crystal under direct diode pumping of the emitting level. However, both Nd:GdVO4 and Nd:YVO4 are efficient laser crystal for diode pumped solid-state lasers under direct pumping of the emitting level.
Stable Q-switched mode operations for Nd:GdVO4 and Nd:YVO4 were accomplished with the A-O Q-switcher inserted into the resonator. In our experiments, the highest repetition rate was only up to 100kHz due to the restriction of A-O Q-switch. At the repetition rate of 100kHz, the comparison of average output power as a function of absorbed pump power is also shown in Fig. 5. More than 20W average output power were obtained and the slope efficiencies of average output power were nearly equal and above 67.6% for both Nd:GdVO4 and Nd:YVO4 lasers. Meanwhile, the output ratios of Q-switching to free running at 100 kHz were higher than 91% for both. It’s illuminated that Nd:GdVO4 and Nd:YVO4 were excellent laser crystals for Q-switching operation. Although the stimulated emission cross-sections of two crystals are different, it has had little influence on their CW and average output powers. We can see clearly that there is almost no remarkable difference on CW and average output power between Nd:GdVO4 and Nd:YVO4 lasers. The optical-optical efficiencies and the slope efficiencies to absorbed pump power for Nd:GdVO4 and Nd:YVO4 lasers are nearly equal.
At the absorbed pump power of about 33W, we studied the pulse width as a function of the repetition rate. The pulse width was detected by a high-speed silicon photo-detector (DET210, Thorlabs) and shown by a digitizing oscillograph (TDS3032B,Tektronix). Fig. 6 shows the comparative results of Nd:GdVO4 and Nd:YVO4 lasers. As seen in Fig. 6, the pulse width keeps on lengthening linearly as the repetition rate increases. It’s explained that the gain for each pulse is reduced when the repetition rate is increased, leading to increased pulse width. There is a remarkable difference between the Nd:GdVO4 and the Nd:YVO4 lasers. The pulse width of Nd:YVO4 laser is obviously shorter than that of Nd:GdVO4 laser at the same repetition rate from 30kHz to 100kHz. The minimum pulse widths at the repetition rate of 100kHz were 15.6ns for Nd:GdVO4 laser and 12.1ns for Nd:YVO4 laser, respectively. Figure 7 Shows the temporal single pulse profile of Nd:GdVO4 and Nd:YVO4 lasers at the repetition rate of 100kHz.The experimental pulse width as a function of the repetition rate is difference from the theoretical simulation. It can be resulted from the assumption that both the inversion population density and the photon density remain uniform across the transverse section of the laser crystal. This assumption limits the accuracy in the computation of pulse width [26]. However, it has no influence on the comparison of Nd:GdVO4 and the Nd:YVO4 lasers.
To further demonstrate the difference of the pulse performance of Nd:GdVO4 laser and Nd:YVO4 laser with high repetition rates, we studied the pulse width as a function of absorbed pump power at the repetition rate of 100kHz. The experimental results are shown in Fig. 8 . The pulse width decays approximately exponentially as the absorbed pump power increases for either Nd:GdVO4 laser or Nd:YVO4 laser. But at the same absorbed pump power, Nd:YVO4 laser can obtained shorter pulse width than Nd:GdVO4 laser. As the theory expected, we also observed the difference on pulse width of Nd:GdVO4 and Nd:YVO4 lasers with high repetition rates in our experiments. The results reveal that Nd:YVO4 crystal has more capability to obtain shorter pulse width and higher peak power at high repetition rates than Nd:GdVO4 crystal, even the repetition rate is much higher than 100kHz.
5. Conclusion
We compare the performance of A-O Q-switching Nd:GdVO4 and Nd:YVO4 lasers at the repetition rate of 100kHz operated at 1.06µm under direct diode pumping of the upper laser level. There is no remarkable difference on property of power-output for both Nd:GdVO4 and Nd:YVO4 lasers. Both of them are proved to be very efficient and promising laser crystals for diode pumped solid-state lasers under direct pumping of the emitting level. But when operated with high repetition rates, Nd:YVO4 laser can obtain shorter pulse width than Nd:GdVO4 laser. It’s concluded that Nd:YVO4 is a more favorable gain medium when higher repetition rates and shorter pulse width are desired, owning to its larger stimulated emission cross-section related to higher single-pulse gain. We believe that an efficient laser system with short pulse width at much higher repetition rates will be further realized by Nd:YVO4 crystal, direct pump scheme and A-O Q-switch.
Acknowledgments
This work was supported by program of excellent team in Harbin Institute of Technology.
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