Characteristics of diode-wing-pumped highly efficient Tm:LuAG lasers running both in continuous wave (CW) and electro-optical Q-switching regimes have been investigated. Using a simple plane-plane cavity, a maximum CW output power of 8.5 W has been achieved with a corresponding slope efficiency of 44.5% by “wing pumping” at 790 nm. With a V-shaped cavity, a diode-wing-pumped MgO:LiNbO3 crystal based electro-optically Q-switched Tm:LuAG laser at 2022.9 nm delivered a maximum pulse energy of 10.8 mJ and a minimum pulse width of 52 ns at a corresponding repetition rate of 100 Hz. To the best of our knowledge, the achieved CW output power and Q-switched pulse energy have both set records for all-solid-state Tm:LuAG lasers, which well reveals an efficient way to generate high-power and high-energy lasers at 2 μm wavelength.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
2 μm eye-safe lasers have wide applications in the fields such as remote sensing, medicine and LIDAR . Particularly, such waveband lasers with high pulse energy and narrow pulse width are increasingly attracting attention, since they can be used to pump the optical parametric oscillators emitting 3-5 μm mid-infrared and 8-12 μm far-infrared radiations. Generally, active Q-switching technique, such as acoustic-optical (AO) and electro-optical (EO) Q-switching, provides an effective way to obtain large pulse energy with nanosecond durations. However, limited by the low single-pass extinction ratio, AO Q-switching usually generates relatively long pulse durations. In comparison with AO modulation, EO Q-switching based on Pockels cell possesses faster switching speed and higher extinction ratio, which could generate much shorter pulse duration with higher pulse energy. Concerning on the EO modulators suitable for 2 μm lasers, the crystals including LGS, RTP and LiNbO3 (LN) have been successfully applied. In 1981, a flash pumped LN electro-optically Q-switched Ho:YAG with a pulse energy of 80 mJ was reported at 77 K . In 2012, L. Wang et al. reported a lamp pumped LGS electro-optically Q-switched Cr,Tm,Ho:YAG laser with a pulse energy of 520 mJ and a pulse width of 35 ns . In 2013, a 550 mJ cryogenic Ho:YLF laser using RTP as EO Q-switch was demonstrated under pumping by a 100 W Tm:fiber laser, which emitted a pulse width of 14 ns . However, there are only few reports on the diode-pumped all-solid-state electro-optically Q-switched 2 μm lasers at room temperature. In 2016, a diode-pumped Tm:YAG slab laser by using RTP crystal based EO modulator delivered a pulse energy of 7.5 mJ with a pulse width of 58 ns . As a traditional EO crystal, LN crystal possesses advantages of low half-wave voltage, broad transmission spectrum (0.4-5 μm), non-hygroscopic and low cost. Using the LN crystal, our group reported a diode-pumped Q-switched Tm,Ho:YAP laser with a pulse width of 107 ns and a pulse energy of 1.65 mJ at a repetition rate of 200 Hz . Obviously, the obtained pulse energy scaling from the diode-pumped all-solid-state EO Q-switching 2 μm laser is still below 10 mJ.
The available pulse energy is not only limited by the damage threshold of EO modulators, but the energy storage ability and physical properties of the laser crystal. LuAG is developed by substituting the Y3+ ions with Lu3+ ions in YAG. Due to the similar ionic radii of Tm3+ ions and Lu3+ ions, a high dopant concentration is allowed in LuAG . And a long upper-level lifetime (10 ms) guarantees a strong energy storage ability for Tm:LuAG . Besides, the emission wavelength (2.023 μm) is closer to the atmospheric transmission window in comparison with Tm:YAG (2.016 μm) , which is helpful for avoiding the optical damage caused by strong water absorption peaks around 2 μm. So all-solid-state Tm:LuAG laser has great potential for generating high-energy Q-switched pulses. However, suffering from thermal effects, the maximum output power ever obtained from the diode-pumped Tm:LuAG lasers was only 4.91 W reported by Wu et al. in 2008 . Limited by the low output power, the achieved Q-switched pulse energy was also relatively low. Utilizing AO Q-switching techniques, the maximum pulse energy of 3.7 mJ was achieved with a pulse width of 204.1 ns in 2013 . To further narrow the pulse duration, efforts were subsequently paid on EO Q-switching techniques. In 2015, our group realized a diode-pumped EO Q-switched Tm:LuAG laser based on LN crystal, from which 2.51 mJ pulses with width of 88 ns were generated at a repetition rate of 50 Hz . In 2016, Y. Zhang et al. reported a 3 ns cavity-dumped Q-switched Tm:LuAG laser using a RTP pockels cell, where a maximum average output power of 1.22 W was obtained at a repetition rate of 100 kHz, only corresponding to a pulse energy of 12.2 μJ . Moreover, many passive Q-switching techniques based on various saturable absorbers such as Bi-doped GaAs, WS2, and Cr:ZnSe have been utilized to realize pulsed 2 μm Tm:LuAG lasers [13–15], but the output pulse energy were mainly in microjoules, much lower than those obtained by active Q-switching methods. Therefore, the potential of Tm:LuAG crystal in generating high power output and high energy pulses needs to be further investigated. In order to further improve the pulse energy, an effective way by using the weak absorption peak of Tm3+ as the pump wavelength, which is called “wing pumping” has been widely used in 1 μm and 2 μm lasers [16,17]. By using this method, more than 50 W average output power was obtained from a CW Tm:YAG laser early in 1996 . In this way, a long path length in the crystal is allowed to provide good heat removal and reduce thermal loading.
In this work, by optimally selecting pump sources, designing the cavity configuration, and choosing the operation regime of MgO:LN-based EO modulator, the ability of Tm:LuAG in generating high power output and high pulse energy has been experimentally demonstrated. In a simple two-mirror linear cavity, the maximum CW output power of 8.5 W was achieved with a slope efficiency of 44.5%. And the active Q-switching Tm:LuAG laser was realized with a maximum pulsed energy of 10.8 mJ and a minimum pulse width of 52 ns at a repetition rate of 100 Hz, using a LN-based EO modulator working with quarter-wave voltage on Q-switched operation. To the best of our knowledge, both the CW output power and Q-switched pulse energy are the maximum values ever achieved from the all-solid-state Tm:LuAG lasers.
2. Experimental results and discussions
2.1 Highly efficient CW operation
A compact plane-plane cavity with a physical length of 2 cm was employed to investigate the diode-wing-pumped Tm:LuAG laser efficiency, as shown in Fig. 1. The Tm:LuAG crystal with a dopant concentration of 6 at.% and dimensions of 4 × 4 × 8 mm3 was wrapped in an indium foil and mounted in a copper heat sink with water cooled at 12°C, since low cooling temperature was found to be helpful for enhancing the laser efficiency of Tm:LuAG . Both surfaces of the crystal were anti-reflection (AR) coated for the pump and laser wavelengths. The input coupling mirror (M1) was AR coated at 750-850 nm and high reflection (HR) coated at 1800-2100 nm. The output coupling mirrors (M2) had different transmissions of 3%, 9% and 20% at 1850-2150 nm. In this CW laser operation, the pump source was a fiber-coupled diode laser with a core diameter of 200 μm and a numerical aperture (NA) of 0.22. To realizing wing-pumping, the emission wavelength of LD was switched to 790 nm by adjusting the cooling temperature to 16°C. The pump light was imaged into Tm:LuAG crystal by a 1:1 optical coupling system, which was composed of two plano-convex lens with a total focal length of 50 mm. A PM100D power meter with a S314C power head (Thorlabs Inc., USA) was used to measure the average output power.
To realize high lasing efficiency, here “wing pumping” at 790 nm with a low absorbance was employed for the highly-doped Tm:LuAG crystal instead of its absorption peaks in a broad spectral range from 782 nm to 788 nm . Under wing-pumping, the absorption characteristics of the Tm:LuAG crystal were firstly measured and depicted in Fig. 2(a) without lasing. The absorbed pump power increased almost linearly with the incident pump power with a slope efficiency of 81.5%. However, from Fig. 2(a) it can be seen that the absorbance slowly declined from 87% to 82% with the incident pump power increased not exceeding 20.8 W, corresponding to an absorbed pump power of 17.1 W. After that, saturation absorption phenomenon happened and the absorbance dropped sharply down to 69.5% at the maximum incident pump power of 29 W, which was attributed to the severe bleaching of the ground state population of the laser-active ions under high pump intensity, especially for a quasi-three-level of Tm:LuAG crystal.
The CW Tm:LuAG laser output characteristics with different OCs are shown in Fig. 2(b). Similar to the results demonstrated in , a maximum CW output power of 8.5 W was achieved under the absorbed pump power of 20.1 W when a low output coupling rate of 3% was employed, corresponding to a threshold absorbed pump power of 1.27 W and a slope efficiency of 44.5%. However, roll-over phenomenon still appeared when the absorbed pump power was increased above 19 W, due to the accumulated thermal effect happening in the crystal under high pump power. With the other two OC transmissions, the maximum output powers declined to be 5.9 W (T = 9%) and 5 W (T = 20%), corresponding to slope efficiencies of 37.2% and 31%, respectively. Here we attribute the relatively low optimal transmission rate of 3% to the low gain coefficient of the Tm:LuAG crystal induced by the small stimulated emission cross-section of 1.38 × 10−21 cm2 at 2023 nm . For comparisons, the output performance of the ever obtained CW Tm:LuAG lasers is summarized in Table 1. As can be seen, the output power of 8.5 W is the highest CW output power ever obtained from diode-pumped all-solid-state Tm:LuAG lasers, to the best of our knowledge.
2.2 EO Q-switching operation
In Q-switching regime, a V-shaped cavity was designed with a large spot diameter and a high coupling rate to reduce the power density in the cavity. Limited by the size of MgO:LN-based EO modulator, a 29 cm-long V-shaped cavity configuration was employed with arm lengths of 13 cm between M1-M2 and 16 cm between M2-M3, as shown in Fig. 3. The concave mirror M2 with curvature radius of R = 200 mm and the plane mirror M1 were both HR coated at 1800-2100 nm and AR coated at 750-850 nm. To increase the output power and avoid the high power intensity induced damage to the intracavity elements, the flat output coupler M3 with a large transmission rate of 20% was employed. To increase the gain mode volume in the laser crystal, the pump spot diameter was enlarged to 400 μm by using a 1:2 optical coupling system. As for EO Q-switching, there are three operation modes including quarter-wave voltage on Q-switching regime, quarter-wave voltage off Q-switching regime and half-wave voltage regime . Among them, the quarter-wave voltage on Q-switching operation was adopted, which is helpful for prolonging the lifetime of EO modulator. A 1 mm-thick uncoated YAG plate was placed at Brewster angle (61°) in the cavity and acted as polarizer. In combination with a 1/4 wave plate (Thorlabs Inc., @2020nm), the light was blocked effectively. The 5 mol % MgO:LN crystal with a dimension of 9 × 9 × 25 mm3 was applied with a quarter-wave voltage of 3 kV along the x axis by a Pockels cell driver (QBU-BT-5020, OEM Tech, The Republic of Belarus). The practical quarter-wave voltage employed on the LN crystal was higher than the theoretical value of 2.5 kV due to the smaller EO coefficient γ22 under alternating current (AC) driving than that of direct current (DC) driving [11, 22]. The extinction ratio for the employed LN crystal based EO modulator was measured to be 324:1. A 0.5 mm-thick uncoated YAG etalon was inserted to prevent the Q-switched mode-locking phenomenon induced by the long cavity. The temporal pulse shapes were detected by fast InGaAs photodetector (EOT, ET-5000, USA) and recorded in a digital oscilloscope (1 GHZ band width, Tektronix DPO 7102, USA).
With the experimental setup shown in Fig. 3, the output characteristics of the EO Q-switched Tm:LuAG laser were firstly investigated under the maximum incident pump power of 11.7 W, which are demonstrated in Fig. 4. Since the LN crystal faces the piezoelectric ring effect at frequencies above 1 kHz , the modulation frequency of the EO modulator was switched from 100 Hz to 500 Hz. With the augment of the modulation frequency, the average output power increased slowly to be around 1.25 W and the peak power decreased from 208 kW to 26 kW, however, the output pulse energy decreased from 10.8 mJ to 2.56 mJ and the pulse width increased from 52 ns to 90 ns.
Under the modulation frequency of 100 Hz, the output power performance of both CW operation incorporating static EO modulator and EO Q-switching regime were recorded and depicted in Fig. 5(a). With the absorbed pump powers augmented from 3.31 W to 9.54 W, the CW average output power increased almost linearly up to 2.04 W with a corresponding slope efficiency of 27.6%. Under EO Q-switching regime, a maximum output power of 1.08 W was obtained at the repetition rate of 100 Hz, corresponding to a slope efficiency of 19.3%. As shown in Fig. 5(b), the pulse width decreased from 90 ns to 52 ns and the pulse energy increased from 0.5 to 10.8 mJ within the pump range. Under the maximum output pulse energy, the corresponding intracavity power intensity on MgO:LN crystal was found to be more than 200 MW/cm2. To prevent the MgO:LN crystal from being damaged, the pump power was not further increased. The temporal pulse train and pulse shape were recorded and shown in Fig. 6, and the pulse-to-pulse amplitude fluctuation was measured to be about 5%. The output spectrum was recorded by a laser spectrometer (APE WaveScan, APE Inc.). As shown in Fig. 7, the output center wavelength from the CW laser using a V-shaped cavity was located at 2022.8 nm with a full width at half-maximum (FWHM) of 2.1 nm. However, the emission wavelength from the EO Q-switched laser was found to be located at 2022.9 nm with a narrow FWHM of 0.15 nm, which provides a great potential to pump ZGP crystal based Mid-infrared optical parametric oscillators as a substitute of the Tm:YAP laser (1.99 μm) due to the higher transmittance in ZGP [24, 25].
As for the actively Q-switched Tm:LuAG lasers, Table 2 summarizes the ever reported output characteristics. It can be seen that our work has generated the highest pulse energy from the diode-pumped Q-switched Tm:LuAG laser. Furthermore, larger output power and higher pulse energy are highly expected by using double-end pumping to lower the thermal load of the crystal as well as enlarging the pump and laser spot sizes inside the resonant cavity to reduce the damage to the intracavity optical elements.
In conclusion, characteristics of diode-wing-pumped Tm:LuAG laser running both in CW and EO Q-switching regimes have been investigated. Under CW operation, a maximum output power of 8.5 W with a corresponding slope efficiency of 44.5% has been achieved under a maximum absorbed pump power of 20.1 W. By using the MgO:LiNbO3 crystal based EO modulator, diode-wing-pumped actively Q-switched Tm:LuAG laser at 2022.9 nm was realized, which delivered a maximum pulse energy of 10.8 mJ and a minimum pulse width of 52 ns at a corresponding repetition rate of 100 Hz. To the best of our knowledge, the achieved CW output power and Q-switched pulse energy have both set records for diode-pumped all-solid-state Tm:LuAG lasers, which well reveals an efficient way to generate high-power and high-energy lasers at 2 μm.
National Natural Science Foundation of China (NSFC) (61475088, 61378022, 61605100, 61422511 and 61775119); Key Research and Development Program of Shandong Province (2018GGX101006), Young Scholars Program of Shandong University (2015WLJH38); Open Research Fund of the State Key Laboratory of Pulsed Power Laser Technology, Electronic Engineering Institute, Hefei, China (SLK2016KF01).
References and links
1. C. T. Wu, Y. L. Ju, Z. G. Wang, Y. F. Li, H. Y. Ma, and Y. Z. Wang, “Lasing characteristics of a CW Tm:LuAG laser with a set of double cavity,” Laser Phys. Lett. 5(7), 510–513 (2008). [CrossRef]
2. N. P. Barnes and D. J. Gettemy, “Pulsed Ho:YAG oscillator and amplifier,” IEEE J. Quantum Electron. 17(7), 1303–1308 (1981). [CrossRef]
5. L. Jin, P. Liu, H. T. Huang, X. Liu, and D. Y. Shen, “Short pulse diode-pumped Tm:YAG slab laser electro-optically Q-switched by RbTiOPO4 crystal,” Opt. Mater. 60, 350–354 (2016). [CrossRef]
6. C. Liu, S. Zhao, Y. Li, K. Yang, M. Li, G. Li, D. Li, T. Li, W. Qiao, T. Feng, X. Chen, L. Zheng, L. Su, and J. Xu, “Stable kilo-hertz electro-optically Q-switched Tm,Ho:YAP laser at room temperature,” Opt. Laser Technol. 81, 189–193 (2016). [CrossRef]
7. T. L. Feng, K. J. Yang, S. Z. Zhao, J. Zhao, W. C. Qiao, T. Li, L. H. Zheng, J. Xu, Q. G. Wang, X. D. Xu, L. Bi Su, and Y. G. Wang, “Efficient CW Dual-Wavelength and Passively Q-Switched Tm:LuAG Lasers,” IEEE Photonic. Tech. Lett. 27(1), 7–10 (2015). [CrossRef]
9. C. T. Wu, Y. L. Ju, Y. F. Li, Z. G. Wang, and Y. Z. Wang, “Diode-pumped Tm:LuAG laser at room temperature,” Chin. Opt. Lett. 6(6), 415–416 (2008). [CrossRef]
10. C. T. Wu, Y. L. Ju, and Y. Z. Wang, “A diode-end-pumped long-pulse-width acoustic-optical Q-switched Tm:LuAG laser at room temperature,” Laser Phys. 23(10), 105810 (2013). [CrossRef]
11. C. Liu, K. Yang, S. Zhao, Y. Li, G. Li, D. Li, T. Li, W. Qiao, T. Feng, and X. Chen, “88 ns multi-millijoule LiNbO3 electro-optically Q-switched Tm:LuAG laser,” Opt. Commun. 355, 167–171 (2015). [CrossRef]
13. L. Wu, D. Li, S. Zhao, K. Yang, X. Li, R. Wang, and J. Liu, “Passive Q-switching with GaAs or Bi-doped GaAs saturable absorber in Tm:LuAG laser operating at 2μm wavelength,” Opt. Express 23(12), 15469–15476 (2015). [CrossRef] [PubMed]
14. C. Luan, K. Yang, J. Zhao, S. Zhao, L. Song, T. Li, H. Chu, J. Qiao, C. Wang, Z. Li, S. Jiang, B. Man, and L. Zheng, “WS2 as a saturable absorber for Q-switched 2 micron lasers,” Opt. Lett. 41(16), 3783–3786 (2016). [CrossRef] [PubMed]
15. Z. Y. Zhou, X. X. Huang, X. F. Guan, J. L. Lan, B. Xu, H. Y. Xu, Z. P. Cai, P. Liu, D. Y. Yan, X. D. Xu, J. Zhang, M. Lei, and J. Xu, “Continuous-wave and passively Q-switched Tm3+-doped LuAG ceramic lasers,” Opt. Mater. Express 7(9), 3441–3447 (2017). [CrossRef]
16. Q. L. Ma, H. D. Mo, and J. Zhao, “High-energy high-efficiency Nd:YLF laser end-pump by 808 nm diode,” Opt. Commun. 413, 220–223 (2018). [CrossRef]
17. Y. Y. Li, W. D. Chen, H. F. Lin, D. Ke, G. Zhang, S. Q. Zhu, and Z. Q. Chen, “Comparative investigation of diode-wing-pumped Tm:Y3Al5O12 laser between composite and non-composite crystal,” Opt. Laser Technol. 63, 132–136 (2014). [CrossRef]
18. R. J. Beach, S. B. Sutton, E. C. Honea, J. A. Skidmore, and M. A. Emanuel, “High-power 2-μm diode-pumped Tm:YAG laser,” Proc. SPIE 2698, 168–175 (1996). [CrossRef]
19. H. Kalaycioglu, A. Sennaroglu, A. Kurt, and G. Ozen, “Spectroscopic analysis of Tm3+:LuAG,” J. Phys. Condens. Matter 19(3), 036208 (2007). [CrossRef]
20. Y. F. Li, Y. Z. Wang, and Y. L. Ju, “Comparative Study of LD-Pumped Tm:YAG and Tm:LuAG Lasers,” Laser Phys. 18(6), 722–724 (2008). [CrossRef]
21. J. P. Salvestrini, M. Abarkan, and M. D. Fontana, “Comparative study of nonlinear optical crystals for electro-optic Q-switching of laser resonators,” Opt. Mater. 26(4), 449–458 (2004). [CrossRef]
22. W. Koechner, Solid-Sate Laser Engineering (Springer, 2005), Chap. 8.
23. M. Roth, M. Tseitlin, and N. Angert, “Oxide Crystals for Electro-Optic Q-Switching of Lasers,” Glass Phys. Chem. 31(1), 86–95 (2005). [CrossRef]
24. L. A. Pomeranz, P. A. Ketteridge, P. A. Budni, K. M. Ezzo, D. M. Rines, and E. P. Chicklis, “Tm:YAlO3 Laser Pumped ZGP Mid-IR Source,” in Advanced Solid-State Photonics, J. Zayhowski, ed., Vol. 83 of OSA Trends in Optics and Photonics (Optical Society of America, 2003), paper 142.
25. Y. J. Yang, Y. J. Zhang, Q. T. Gu, H. J. Zhang, and X. T. Tao, “Growth and annealing characterization of ZnGeP2 crystal,” J. Cryst. Growth 318(1), 721–724 (2011). [CrossRef]
26. C. T. Wu, F. Chen, and Y. L. Ju, “A Diode-Pumped Actively Q-Switched and Injection-Seeded Tm:LuAG Laser at Room Temperature,” J. Russ. Laser Res. 35(4), 347–354 (2014). [CrossRef]
27. F. Chen, C. T. Wu, Y. L. Ju, B. Q. Yao, and Y. Z. Wang, “Diode-pumped Q-switched Tm:LuAG ring laser operation at room temperature,” Laser Phys. 22(2), 371–374 (2012). [CrossRef]