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We report on a high repetition rate, high-power picosecond Nd:LuVO4 regenerative amplifier. Period doubling caused energy instability was eliminated at megahertz-level repetition rate with the modified seeding source. A multi-pass cell was used to improve the seed pulse energy to achieve complete suppression of the onset of bifurcation. At a maximum repetition rate of 1.43 MHz, the system produced 7.0-ps-long pulses with an average output power of 25.1 W, corresponding to a pulse energy of 17.6 μJ. At 100 kHz, the pulse energy increased to 205 μJ with an average power of 20.5 W. Moreover, the injected pulses with pulse duration of 5.1 ps broadened to 8.9 ps because of gain narrowing in the amplifier.
© 2016 Optical Society of America
1. Introduction
High-energy ultrafast pulses have gained significant interest due to their wide applications in biophysics, chemical spectroscopy, nonlinear optics and microfabrication [1–3]. Meanwhile, a higher possible repetition rate is also required in order to speed up the operation processes and save production costs. An average power up to 1.1 kW at 20 MHz, corresponding to a pulse energy of 55 μJ with 615 fs pulses has been demonstrated by the Innoslab Yb:YAG concept [4]. Thin-disk oscillator has been proven to be able to generate power as high as 242 W with pulse duration of 1.07 ps at 3.03 MHz, while the corresponding pulse energy was measured to be 80 μJ [5].
Pulses at repetition rates of up to a few megahertz with substantially higher energy are preferred for many applications. These pulse trains with pulse energy in microjoule range at MHz-level repetition rates could be achieved conveniently by the solid-state, diode pumped regenerative amplifiers (RAs). They are commonly used to amplify ultrafast pulses from laser oscillators by many orders of magnitude with a high efficiency [6, 7]. The highest output power reported to date for RA systems is as much as 160 W operating at 800 kHz with pulse duration of 750 fs by employing a Yb:YAG thin-disk active element [8]. The average output power is up to 28 W for a bulk Yb:CALGO regenerative amplifier at 500 kHz with a pulse width of 217 fs [9]. The repetition rates continuously tunable from 50 kHz up to 1.7 MHz were demonstrated by a cryogenically cooled Ti:sapphire RA laser regime, which produced sub-60-fs pulses with the pulse energy greater than 20 μJ and 3.5 μJ at 50 kHz and 1 MHz, respectively [10].
Meanwhile, detailed investigations have proved that picosecond pulses are well suited for materials micromachining with high precision in the area of industry and biomedicine [11–13]. Powerful picosecond RA sources with high repetition rates have drawn extensive attention based on Nd-doped gain materials [14, 15] such as Nd:YVO4 and Nd:GdVO4. These systems could maintain moderate compactness since they do not require any pulse stretching and compression. Those devices have added the complexity of overall system and reduced its efficiency. A RA system based on composite Nd:YVO4 crystal operating at 1342 nm produced output pulses of 13 ps duration at 300 kHz with the power of 11 W [16]. An average power up to 47.7 W at 100-816 kHz was generated in a solid-state Nd:YVO4 RA with pulse widths of 400-1000 ps at 1064 nm [17]. A Nd:GdVO4 RA has been designed which achieves 6.8 ps pulses with a pulse energy of 65 μJ at 200 kHz [18].
The neodymium-doped lutetium vanadate (Nd:LuVO4), as a new member of the vanadate family, has attractive advantageous properties, which makes it a promising candidate for high-power picosecond laser systems. The Nd-doped laser candidates, such as Nd:YVO4, Nd:GdVO4, Nd:LuVO4, have their own unique advantages in some respects. Among this three crystals, Nd:YVO4 has been widely applied, Nd:GdVO4 has the highest thermal conductivity and Nd:LuVO4 has the largest emission cross section, a relatively higher damage threshold [19,20]. In this Letter, we report a compact diode-pumped, high-power RA laser system based on Nd:LuVO4 crystal with picosecond pulses for the first time to the best of our knowledge. The operating repetition rate was up to 1.43 MHz, which was the highest rate in the picosecond RA regime.
However, the amplifier pulse train in RAs may fluctuate periodically between two or more output energies, when in particular operating at repetition rates close to or higher than the inverse lifetime of gain medium [21]. The RA is driven into a bifurcation regime, and some complex behavior may appear, including period doubling and energy instability [22–24]. In our experiments, a Herriott-style multi-pass cell (MPC) was first used to improve the pulse energy of seed laser to suppress complex pulse energy dynamics [26]. At a maximum repetition rate of 1.43 MHz, an average output power of 25.1 W was obtained, corresponding to a pulse energy of 17.6 μJ. The maximum pulse energy of 205 μJ was achieved at 100 kHz with an average power of 20.5 W. Compared with absorbed pump power of crystals, the overall efficiency of RA system was higher than 45%. Due to moderate gain narrowing, the injected pulses with pulse duration of 5.1 ps broadened to 8.9 ps during amplification. Moreover, pulse width was reduced from 8.9 ps at 100 kHz to 7.0 ps at 1.43 MHz because of a lower gain at higher repetition rates. The high-power, high repetition rate picosecond RA system with good beam qualities could be used as an excellent source for high-precision material processing with high efficiency.
2. Theoretical analysis
A theoretical simulation of the dynamics for high repetition rates Nd:LuVO4 RA was performed, and a fourth-order Runge-Kutta method was used to integrate the equations and calculate the output pulse energy [21–23, 25]. The parameters used for the simulation are shown in Table 1.
Table 1. Key parameter values for simulation
The RA system was operating at a repetition rate of 100 kHz and the cavity length was 2.2 m, corresponding to a cavity round-trip time of about 14.7 ns. The simulated bifurcation diagrams are illustrated in Fig. 1 for four seed pulse energies varied from 3 to 300 nJ. The horizontal axis was the gating time, which represented the amplified time of the seed laser in RA cavity, and the amplified pulse energies were recorded on the vertical scale. For a seed energy of 3 nJ, the amplification occurred at a gating time of 130 ns followed by a linear increase in the single-pulse energy regime. A period doubling of the amplified pulse energy appeared at a gating time between 130 and 330 ns. The system turned into the single-pulse energy regime again when the gating time was larger than 330 ns. When the seed energy was increased to 30 nJ, the range of gating time for period doubling was narrow, varied from 130 to 260 ns. Finally, the overall system operated in the single-pulse energy regime when the seed energy was changed to 300 nJ.
Fig. 1 Simulated bifurcation diagrams with the seed pulse energies of 3, 30, 100 and 300 nJ at 100 kHz, respectively.
It can be seen that an increase of the seed pulse energy could avoid bifurcation of the output pulse energy. Moreover, when the seed energy is high enough, the system could suppress period doubling and energy instability, and operate in the single-pulse energy regime at the high repetition rates. It is feasible that a preamplifier is utilized after seed oscillator to obtain the required seed level [21], while the complexity of overall RA system is increased. In our experiments, the seed pulses were provided by a homemade Nd:LuVO4 oscillator, and a MPC was first used to reduce the pulse repetition rate and improve the output pulse energy of the seed laser [26]. The MPC was inserted to obtain a total cavity length of approximately 13 m, corresponding to a repetition rate of 11.5 MHz. An output power of 5 W was achieved as the seed laser, corresponding to a pulse energy of 430 nJ, which was high enough to maintain RA system operating in the single-pulse energy regime.
3. Experimental setup
The experimental setup of high-power Nd:LuVO4 RA is shown in Fig. 2. It consisted of a seed oscillator, an optical isolator and a regenerative amplifier. A 0.5 at.-% doped Nd:LuVO4 crystal with the dimensions of 4 mm × 4 mm × 8 mm was pumped by a fiber-coupled diode laser in the seed laser system. A maximum average output power of 7 W was obtained at a repetition rate of 11.5 MHz with pulse duration of 5.1 ps (sech2-fit). The seed pulses passed a half-wave plate (HWP1) and a lens (F1), reflected by a thin-film polarizer (TFP1) and then went through HWP2, Faraday rotator (FR) and a TFP2. The seed power of 5 W was injected into the RA system by adjusting the HWP1. The optical isolator was composed of the FR, HWP2 and TFP1, which separated the amplified pulses in RA cavity from the injected pulses produced by the seed oscillator. Mode matching of beam divergence and waist location between the oscillator and RA was achieved by the lens F1 with a focal length of f = 750 mm.
Inside the RA cavity, a dual Nd:LuVO4 crystal configuration was used, allowing the thermal load to spread out to improve the extraction efficiency. The crystals were doped with 0.5 at.-% Nd with the sizes of 4 mm × 4 mm × 8 mm. Moreover, two surfaces of crystals were antireflection-coated at 880 nm and around 1 μm to decrease insertion loss. For heat dissipation, the crystals were wrapped with indium foil and mounted in a water-cooled copper heat sink, and the cooling water temperature was maintained at 15 °C. The crystals were pumped by two fiber-coupled laser diodes (LD1 and LD2) operating at 880nm, which had a core diameter of 400 μm, numerical aperture of 0.22, and a maximum output power of 26.5 W. The pump light was collimated and re-focused into the gain materials by a spherical lens.
M3 and M5 were concave mirrors with radii of curvature of 600 and 500 mm, respectively (Fig. 2). The continuous-wave (CW) operation was accomplished by M6, which was used as an output coupler with a transmission of 11.7%. Then, the mirror M6 was removed, the TFP2 and a Pockels cell (PC) were inserted to accomplish the RA cavity. A combination of a TFP, a quarter-wave plate (QWP) and a RTP PC with an aperture of 4 mm was used for injection of the seed pulses into the RA cavity. The switching frequency of the PC varied from 100 kHz to 1.5 MHz, and the RA cavity length was adapted to the rise and fall times (less than 5 ns) of the PC driver. The leaked laser from one mirror in seed oscillator was placed onto a photodiode, whose signal was used to synchronize a digital delay generator (DG 645, Stanford Research Systems). The repetition rate of seed laser (11.5 MHz) was divided by an integer and the synchronized signal was used to trigger the PC driver (such as 1.43 MHz, 1.27 MHz, 1.04 MHz…). Higher repetition rates (more than 1.5 MHz) were possible in principle but were not supported by the PC driver.
4. Experimental results and discussions
The CW operation of dual-Nd:LuVO4 crystals laser was first realized with mirror M6 inserted into the cavity, which was used as an output coupler with a transmission of 11.7%. In this configuration, the curve of average output power versus absorbed power is illustrated in Fig. 3. A maximum output power of 32 W was achieved at an absorbed pump power of 44 W and the corresponding slope efficiency was measured to be 75.2%.
In the RA regime, the mirror M6 was removed, the RTP PC and QWP were used to separate the input and output gates of the amplifier. The average power of seed laser injected into the RA system was adjusted by the HWP1. The output signals of RA system recorded by a fast photodiode at the rate of 100 kHz with the seed powers of 500 mW and 5 W are depicted in Fig. 4(a) and 4(b), respectively. It can be seen that period doubling of amplified pulses appeared when the average power of seed laser was 500 mW, corresponding to an injected pulse energy of 43 nJ. When the seed pulse energy was increased to 430 nJ (5 W), the output pulse train maintained stable and the onset of bifurcation was completely suppressed. Moreover, the complex pulse energy dynamics did not appear and the RA system operated in the single-pulse energy regime at all repetition rates because of a high seed energy.
Fig. 4 Oscilloscope traces of output pulse train at 100 kHz with the seed powers of: (a) 500 mW and (b) 5 W.
Pulses with the fixed power of 5 W at a repetition rate of 11.5 MHz were injected into the RA cavity. The repetition rate in the RA regime was reduced from 1.43 MHz to 100 kHz and the number of round trips was adjusted for each rate to yield the highest amplified pulse energy. The average output power and corresponding pulse energy are plotted as a function of repetition rate in Fig. 5. A maximum pulse energy of 205 μJ was achieved at the rate of 100 kHz and the average output power was measured to be 20.5 W at 16 round trips. At the highest rate of 1.43 MHz, an output power of 25.1 W was obtained, corresponding to a pulse energy of 17.6 μJ. The number of round trips was slightly decreased to approximately 13 to achieve the highest output power at 1.43 MHz. The measured amplitude standard deviation was below 1.5% for all repetition rates. A comparison of the extracted power and absorbed pump power indicated that the overall efficiency of the amplifier was higher than 45%.
By fitting of a sech2-shaped profile, the measured pulse width versus repetition rate curve is illustrated in Fig. 6(a). The measured duration was 5.1 ps for the injected pulses, but it turned to be 8.9 ps and 7.0 ps at 100 kHz and 1.43 MHz, respectively. The broadening of the amplified pulses can be attributed to gain narrowing in the RA regime [18, 27]. It can be seen that with an increasing repetition rate, the pulse width reduced from 8.9 ps at 100 kHz to 7.0 ps at 1.43 MHz. This behavior was caused by the lower gain in the higher repetition rate, which resulting in less gain narrowing and thus in a reduction of pulse broadening [18]. The measured autocorrelation trace at 1.43 MHz is depicted in Fig. 6(b), assuming the pulse had a sech2-shaped temporal intensity profile.
Fig. 6 (a) Output pulse duration (sech2-fit) versus repetition rate; (b) measured autocorrelation trace at 1.43 MHz, assuming a sech2-shaped pulse.
Figure 7 illustrated the M2 factor of the amplified pulse at a repetition rate of 100 kHz. The result was close to the diffraction limit, giving a beam quality of M2x = 1.21 and M2y = 1.17 in the two transverse directions. The measured M2 factors were all around 1.2 at other repetition rates, indicating that the RA output had a good beam quality.
5. Conclusion
In conclusion, we have demonstrated a high power, picosecond Nd:LuVO4 regenerative amplifier for the first time to the best of our knowledge. The repetition rate varied from 100 kHz to 1.43 MHz, which was the highest rate in the picosecond RA regime. A MPC was first used to improve the seed energy to suppress the complex pulse energy dynamics or even bifurcations. The maximum pulse energy of 205 μJ was achieved at 100 kHz, and a maximum average power of 25.1 W was obtained at 1.43 MHz, corresponding to a pulse energy of 17.6 μJ. Due to moderate gain narrowing, the duration of laser pulses broadened from 5.1 ps to 8.9 ps during amplification. Moreover, pulse width was reduced from 8.9 ps at 100 kHz to 7.0 ps at 1.43 MHz because of a low gain at the high repetition rates. The compact, stable and high-power laser amplifier system with good beam qualities could be applied for high efficiency material processing.
Acknowledgments
The authors acknowledge the support of the National Natural Science Foundation of China (Grant No. 11504394, 61521093 and 61378030).
References and links
1. U. Keller, “Recent developments in compact ultrafast lasers,” Nature 424(6950), 831–838 (2003). [CrossRef] [PubMed]
2. C. Y. Chien and M. C. Gupta, “Pulse width effect in ultrafast laser processing of materials,” Appl. Phys., A Mater. Sci. Process. 81(6), 1257–1263 (2005). [CrossRef]
3. A. Tuennermann, S. Nolte, and J. Limpert, “Femtosecond vs. picosecond laser material processing,” Laser Tech. J. 7(1), 34–38 (2010). [CrossRef]
4. P. Russbueldt, T. Mans, J. Weitenberg, H. D. Hoffmann, and R. Poprawe, “Compact diode-pumped 1.1 kW Yb:YAG Innoslab femtosecond amplifier,” Opt. Lett. 35(24), 4169–4171 (2010). [CrossRef] [PubMed]
5. C. J. Saraceno, F. Emaury, C. Schriber, M. Hoffmann, M. Golling, T. Südmeyer, and U. Keller, “Ultrafast thin-disk laser with 80 μJ pulse energy and 242 W of average power,” Opt. Lett. 39(1), 9–12 (2014). [CrossRef] [PubMed]
6. J. Li, P. Gao, L. Zheng, L. Su, J. Xu, and X. Liang, “Diode-pumped Yb:SSO chirped pulse amplifier with 1 ps pulse duration,” Chin. Opt. Lett. 13(1), 011403 (2015). [CrossRef]
7. E. Caracciolo, M. Kemnitzer, A. Guandalini, F. Pirzio, A. Agnesi, and J. Aus der Au, “High pulse energy multiwatt Yb:CaAlGdO4 and Yb:CaF2 regenerative amplifiers,” Opt. Express 22(17), 19912–19918 (2014). [CrossRef] [PubMed]
8. R. Fleischhaker, R. Gebs, A. Budnicki, M. Wolf, J. Kleinbauer, and D. H. Sutter, “Compact gigawatt-class subpicosecond Yb:YAG thin-disk regenerative chirped-pulse amplifier with high average power at up to 800 kHz,” in CLEO Europe (2013), paper CFIE-4.1.
9. E. Caracciolo, M. Kemnitzer, A. Guandalini, F. Pirzio, J. Aus der Au, and A. Agnesi, “28-W, 217 fs solid-state Yb:CAlGdO4 regenerative amplifiers,” Opt. Lett. 38(20), 4131–4133 (2013). [CrossRef] [PubMed]
10. X. Zhang, E. Schneider, G. Taft, H. Kaptyen, M. Murnane, and S. Backus, “Multi-microjoule, MHz repetition rate Ti:sapphire ultrafast regenerative amplifier system,” Opt. Express 20(7), 7015–7021 (2012). [CrossRef] [PubMed]
11. J. Kleinbauer, R. Knappe, and R. Wallenstein, “A powerful diode-pumped laser source for micro-machining with ps pulses in the infrared, the visible and the ultraviolet,” Appl. Phys. B 80(3), 315–320 (2005). [CrossRef]
12. C. Gerhard, F. Druon, P. Blandin, M. Hanna, F. Balembois, P. Georges, and F. Falcoz, “Efficient versatile-repetition-rate picosecond source for material processing applications,” Appl. Opt. 47(7), 967–974 (2008). [CrossRef] [PubMed]
13. A. Ancona, F. Röser, K. Rademaker, J. Limpert, S. Nolte, and A. Tünnermann, “High speed laser drilling of metals using a high repetition rate, high average power ultrafast fiber CPA system,” Opt. Express 16(12), 8958–8968 (2008). [CrossRef] [PubMed]
14. J. Guo, J. Li, P. Gao, L. Su, J. Xu, and X. Liang, “Laser performance and thermal lens of diode-pumped Nd, Y-codoped:SrF2 single crystals,” Chin. Opt. Lett. 12(12), 121403 (2014). [CrossRef]
15. C. Y. Cho, T. L. Huang, S. M. Wen, Y. J. Huang, K. F. Huang, and Y. F. Chen, “Nd:YLF laser at cryogenic temperature with orthogonally polarized simultaneous emission at 1047 nm and 1053 nm,” Opt. Express 22(21), 25318–25323 (2014). [CrossRef] [PubMed]
16. A. M. Rodin, M. Grishin, and A. Michailovas, “Picosecond laser with 11 W output power at 1342 nm based on composite multiple doping level Nd:YVO4 crystal,” Opt. Laser Technol. 76, 46–52 (2016). [CrossRef]
17. F. Harth, T. Ulm, M. Lührmann, R. Knappe, A. Klehr, T. Hoffmann, G. Erbert, and J. A. L’huillier, “High power laser pulses with voltage controlled durations of 400 - 1000 ps,” Opt. Express 20(7), 7002–7007 (2012). [CrossRef] [PubMed]
18. J. Kleinbauer, R. Knappe, and R. Wallenstein, “13-W picosecond Nd:GdVO4 regenerative amplifier with 200-kHz repetition rate,” Appl. Phys. B 81(2–3), 163–166 (2005). [CrossRef]
19. H. Yu, J. Liu, H. Zhang, A. A. Kaminskii, Z. Wang, and J. Wang, “Advances in vanadate laser crystals at a lasing wavelength of 1 micrometer,” Laser Photonics Rev. 8(6), 847–864 (2014). [CrossRef]
20. Y. P. Huang, C. Y. Cho, Y. J. Huang, and Y. F. Chen, “Orthogonally polarized dual-wavelength Nd:LuVO4 laser at 1086 nm and 1089 nm,” Opt. Express 20(5), 5644–5651 (2012). [CrossRef] [PubMed]
21. M. Grishin, V. Gulbinas, and A. Michailovas, “Bifurcation suppression for stability improvement in Nd:YVO4 regenerative amplifier,” Opt. Express 17(18), 15700–15708 (2009). [CrossRef] [PubMed]
22. J. Dörring, A. Killi, U. Morgner, A. Lang, M. Lederer, and D. Kopf, “Period doubling and deterministic chaos in continuously pumped regenerative amplifiers,” Opt. Express 12(8), 1759–1768 (2004). [CrossRef] [PubMed]
23. M. Grishin, V. Gulbinas, and A. Michailovas, “Dynamics of high repetition rate regenerative amplifiers,” Opt. Express 15(15), 9434–9443 (2007). [CrossRef] [PubMed]
24. P. Kroetz, A. Ruehl, G. Chatterjee, A.-L. Calendron, K. Murari, H. Cankaya, P. Li, F. X. Kärtner, I. Hartl, and R. J. Dwayne Miller, “Overcoming bifurcation instability in high-repetition-rate Ho:YLF regenerative amplifiers,” Opt. Lett. 40(23), 5427–5430 (2015). [CrossRef] [PubMed]
25. H. J. Bakker, P. C. M. Planken, and H. G. Muller, “Numerical calculation of optical frequency-conversion processes: a new approach,” J. Opt. Soc. Am. B 6(9), 1665–1672 (1989). [CrossRef]
26. A. Sennaroglu and J. Fujimoto, “Design criteria for Herriott-type multi-pass cavities for ultrashort pulse lasers,” Opt. Express 11(9), 1106–1113 (2003). [CrossRef] [PubMed]
27. A. E. Siegman, Lasers (University Science Books, 1986).
