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Short pulse diode-pumped Tm:YAG slab laser electro-optically Q-switched by RbTiOPO4 crystal
Optical Materials 60 (2016) 350e354
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Short pulse diode-pumped Tm:YAG slab laser electro-optically Qswitched by RbTiOPO4 crystal Lin Jin, Pian Liu, Haitao Huang, Xuan Liu, Deyuan Shen* Jiangsu Key Laboratory of Advanced Laser Materials and Devices, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou, Jiangsu 221116, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 July 2016 Received in revised form 3 August 2016 Accepted 14 August 2016
In this paper, a laser diode end pumped RbTiOPO4 (RTP) electro-optically Q-switched Tm:YAG slab laser was demonstrated. Stable Q-switched pulse with the shortest pulse width of 58 ns and an average output power of 7.5 W were realized at the repetition rate of 1 KHz, corresponding to the slope efficiency of 21.7%. © 2016 Elsevier B.V. All rights reserved.
Keywords: Tm:YAG Slab laser End-pumped Electro-optically Q-switched
1. Introduction Pulsed lasers in 2 mm range have high importance for applications in LIDAR, medical operation, atmospheric monitoring, materials processing and so on [1e3]. Moreover, using high-energy short pulse -duration 2 mm laser to pump ZnGeP2 optical parametric oscillator to generate mid-infrared laser around 3e12 mm wavelength band has been increasingly investigated [4e6]. To obtain high peak power, high pulse energy and high repetition rates 2 mm lasers, the most widely used method is Q-switching by storing energy in laser gain medium during the high-loss periods and releasing high energy accumulated by population inversion during the low-loss periods. The acousto-optic (AO) Q-switching is frequently applied in 2 mm laser system. But the pulse duration of AO Q-switched laser is typically longer than 100 ns, which is limited by insufficient modulation depth (~80%) [7e9]. By contrast, an electro-optically (EO) Q-switching plays an important role in generating shorter pulse durations and achieving high efficiency due to its fast switching speed, high stability and controllability. The electro-optical crystals such as LiNbO3 (LN), langasites (LGS), rubidium titanyl phosphate (RTP) are favorable crystals for EO Q-switching. Early in 1981, LN crystal based EO Q-
* Corresponding author. E-mail address: [email protected] (D. Shen). http://dx.doi.org/10.1016/j.optmat.2016.08.011 0925-3467/© 2016 Elsevier B.V. All rights reserved.
switch was used in a flash pumped Ho:YAG laser, generating a pulse energy of 80 mJ and 30 ns pulse width at repetition rate of 5 Hz [10]. Recently, a pulse energy of 2.51 mJ with pulse duration of 88 ns at 50 Hz was generated in LN EO Q-switched Tm:LuAG laser [11]. LN crystal has the advantages on low half-wave voltage, broad transmission spectrum (0.4e5 mm), and disadvantages of it are the limited repetition rates of several kilohertz and piezoelectric ring effect which reduces the efficiency to a certain extent. Utilizing LGS crystal as the EO Q-switch, Li Wang et al. demonstrated a flash lamp pumped 2.09 mm Cr, Tm, Ho:YAG laser, 520 mJ pulse energy with 3 Hz repetition rate was achieved [12]. LGS has a high damage threshold of 950 MW/cm2 (1064 nm, 10 ms), but high half-wave voltage has to be applied on LGS crystal because of its low electro-optic coefficient (2.68 pm/V). Compared with LN and LGS crystal, RTP crystal possesses both of their advantages. RTP and KTP are isomorphism, the EO efficiencies can optimized by small cationic reorientation [13]. It also has large electro-optic coefficients (rc1 ¼ 30.2 pm/V, rc2 ¼ 23.6 pm/V), higher electrical resistivity (>1010 Um) and can operate at high repetition rates more than 200 KHz [7]. Tm3þ-doped YAG gain materials have upper-state lifetime of exceeding 10 ms [14,15], which is not relevant to the RTP's good performance at high repetition rates and makes itself become one of the high pulse energy materials. As a result of the crossrelaxation process (3H4 þ 3H6 / 2 3F4) in Tm3þ ions, Tm:YAG provides a high quantum efficiency [16,17]. RTP Q-switched
L. Jin et al. / Optical Materials 60 (2016) 350e354
Tm:YAG laser was first reported in 2002, the Tm:YAG rod was pumped by two fiber-coupled laser diodes, the maximum pulse energy of 2.4 mJ with a pulse width of 57 ns at repetition rate of 20 Hz was achieved but the slope efficiency was only 3% [18]. The highest pulse energy 4 mJ at 100 Hz was obtained in 2008 with the same equipment system [5]. However, both of the EO Q-switched Tm:YAG lasers operated at low repetition rates with hundreds of milliwatts output. Laser sources with higher output average power and higher repetition rates output are desired in many fields, such as materials processing, precise handling, and detecting. In our work, a laser diode end pumped RTP based EO Qswitched Tm:YAG slab laser was investigated. The highest output average power of 7.5 W at 1 KHz was obtained at 2 mm wavelength. The maximum pulse energy of 7.5 mJ with pulse width of 58 ns was obtained under the incident pump power of 43.8 W, corresponding to the slope efficiency of 21.7%.
351
was placed between the polarizer and the output coupler. RTP crystal is an optically biaxial crystal with the orthorhombic symmetry point group mm2, and the natural birefringence cannot be disregarded in Pockels cell configurations [8]. The double-crystal (two RTP optical axises perpendicular to each other) was designed to compensate for the natural birefringence caused by temperature variations. The dimension of each RTP crystal was 3 mm 3 mm 10 mm (Quantum Technology, Inc.) and the aperture of laser design was kept to be ~1 mm to minimize
2. Experiment set up The LD end pumped RTP electro-optically Q-switched Tm:YAG slab laser is shown in Fig. 1. In this experiment, the pump source was a laser diode stack consisted of five bars, the dimension of the diode arrays was about 12 mm along the slow axis (x direction) and 12 mm along the fast axis (y direction). The emission wavelength of the pump source had little fluctuations with water-cooled temperature changing, when the cooling water was set at 15 C, the emission wavelength was centered at around 785 nm which was coincidence with absorption wavelength of Tm:YAG gain medium. The resonator was constructed by two plano-concave mirrors M1 and M2, M1 with a radius of curvature of 300 mm was coated for high transmission at 760e810 nm and high reflection at 1879e2170 nm, the output coupler M2 had a radius of curvature of 100 mm and had a high reflection at 785 nm with the transmission of 10% at 1930e2100 nm. The pump light coupling system was composed of two cylindrical lens with the focal length of f1 ¼ 100 mm along the fast axis and f2 ¼ 60 mm along the slow axis. The focused pump spot had a dimension of 0.68 mm (x) 0.6 mm (y). The dimension of Tm:YAG was 1 mm 10 mm 15 mm with 3.5 at.% doping concentration. Both of the 1 mm 10 mm faces were coated for high transmission at 785 nm and 1890e2170 nm. The pump beams were incident on the left (1 mm 10 mm) face and about 94.7% of the pump beams can be absorbed. The two large area surfaces (10 mm 15 mm) were tightly sandwiched between the micro-channel water-cooled cooper heat sink and the cooling water was maintained at 15 C. The improved thermal module was a micro-channel heat sink with a channel width of 500 mm and had stress compensation mechanism. Two undoped YAG plates at Brewster's angle were used to generate linear polarized lasing output. The RTP Q-switched crystal
Fig. 2. Output power as a function of incident pump power in cw and Q-switched mode.
Fig. 3. Pulse energy and pulse width versus pulse repetition rate at the incident pump power of 43.8 W.
Fig. 1. Schematic of LD end pumped EO Q-switched Tm:YAG slab laser.
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damaging the crystal sides The dimension of pump beam size was designed less than 1 mm2 to avoid the damage of components. The entire cavity length was optimized to be 121 mm. A Sunfire-3PN100KHz-24 Pockels cell driver was used to drive the RTP Q-
switch (quarter-wave voltage, 1.05 kV). In the experiments, the output power was measured by a F150A-BB-26 power meter (Ophir Photonics). A fast InGaAs photodiode (DET10D/M) was used to detected Q-switched laser pulse and an oscilloscope (Keysight DSOS 104A) was used to record the pulse signal.
3. Experiment results and discussion
Fig. 4. Pulse width versus the incident pump power at 1 KHz.
Fig. 2 shows the output power with respect to incident pump power in continuous wave and Q-switched mode. In continuous wave operation, no components were placed in resonator cavity and the output power was also measured at 121 mm cavity length. With the increase of incident pump power, the output power takes a linear increase, at the incident pump power of 43.8 W, the maximum output power of 13.2 W was obtained, corresponding to the slope efficiency of 38.7%. In the pulse operation, a quarter-wave electrical voltage was applied on the RTP Q-switch crystal under the continuous pumping light, the resonator was at a low Q-state, and the linear polarized pulse laser was generated when the quarter-wave electrical voltage was removed. At the repetition rates of 10 KHz, 8 KHz, 5 KHz, 3 KHz, 1 KHz, the average output power of 8.14 W, 7.95 W, 7.86 W, 7.6 W, 7.5 W were obtained at the incident pump power of 43.8 W, respectively. The slope efficiency was 21.7% at the repetition rate of 1 KHz. With further increasing
Fig. 5. Typical pulse train and pulse waveform of 58 ns at 1 KHz under the incident pump power of 43.8 W.
L. Jin et al. / Optical Materials 60 (2016) 350e354
353
Fig. 6. (a) Output spectrum and (b) Beam quality of EO Q-switched Tm:YAG laser, insert, the profile of the laser beam.
the pump power in Q-switched mode, the output power decreased because of the resonator saturation as a result of the thermal lens effect, and no photo destruction was observed in the experiment due to the unreached damage threshold. The decreasing of the average output power and slope efficiency at Q-switched mode mainly contributed to the depolarization loss of thermal induced birefringence in Tm:YAG and RTP crystal and insertion loss of the components. Pulse energy and pulse width versus pulse repetition rate at the incident pump power of 43.8 W are summarized in Fig. 3. With the repetition rates changing from 10 KHz to 1 KHz, the pulse width reduced from 163 ns to 58 ns. At the repetition rate of 1 KHz, the maximum pulse energy of 7.5 mJ and the minimum pulse width of 58 ns were obtained at the maximum pump power, corresponding to the maximum peak power of ~125 KW. Fig. 4 shows the pulse width versus the incident pump power under the repetition rate of 1 KHz. With the incident pump power increased, pulse width decreased from 431 ns to 58 ns The shortest pulse width of 58 ns was obtained under the incident pump power of 43.8 W at 1 KHz. The temporal waveform of the 58 ns and typical pulse train at the repetition rate of 1 KHz under the incident pump power of 43.8 W was shown in Fig. 5. The output spectrum of RTP based EO Q-switched laser was centered at 2014.5 nm with a bandwidth of 0.3 nm and the laser spectrum was measured by an AQ6375 optical spectrum analyzer (shown in Fig. 6 (a)). Fig. 6 (b) shows the beam quality factors measured at 1 KHz under the maximum incident pump power and the insert shows the 3D profile of the laser beam near the focus. A slightly poor of the beam quality was because of the low overlapping degree between pump beam spot and TEM00 mode size. The beam quality factors of 7.5 W output at 1 KHz were measured as M2x ¼ 2.24 and M2y ¼ 1.63 along slow axis and fast axis respectively. Higher output average power should be achievable with optimized aperture of RTP crystal and improved mode matching in the future work.
4. Conclusion In conclusion, an RTP EO Q-switched Tm:YAG slab laser directly pumped by a 785 nm laser diode was demonstrated experimentally. An average output power of 7.5 W with 58 ns pulse width at 1 kHz was obtained, corresponding to the slope efficiency of 21.7%. The maximum pulse energy of 7.5 mJ and the peak power of 125 KW were achieved. The beam quality factors were M2x ¼ 2.24 and M2y ¼ 1.63. The results indicate that the combination of Tm:YAG
and RTP is a promising method for generating high energy pulsed laser with short pulse durations at 2 mm. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant Nos. U1430111, 61308047 and 11274144), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the Key Project of Scientific Research and Innovation of Jiangsu Normal University (Grant No. 2015YZD013). References [1] S. Ishii, K. Mizutani, H. Fukuoka, T. Ishikawa, B. Philippe, H. Iwai, T. Aoki, T. Itabe, A. Sato, K. Asai, Coherent 2 mm differential absorption and wind lidar with conductively cooled laser and two-axis scanning device, Appl. Opt. 49 (10) (2010) 1809e1817. [2] K. Scholle, P. Fuhrberg, P. Koopmann, S. Lamrini, 2 mm Laser Sources and Their Possible Applications, INTECH Open Access Publisher, 2010. [3] Z. Wang, C. Song, Y. Li, Y. Ju, Y. Wang, CW and pulsed operation of a diodeend-pumped Tm: GdVO4 laser at room temperature, Laser Phys. Lett. 6 (2) (2008) 105. [4] P. Budni, L. Pomeranz, M. Lemons, C. Miller, J. Mosto, E. Chicklis, Efficient midinfrared laser using 1.9-mm-pumped Ho: YAG and ZnGeP 2 optical parametric oscillators, JOSA B 17 (5) (2000) 723e728. [5] M. Eichhorn, A. Hirth, Electro-optically Q-switched Tm: YAG laser pumped ZGP optical-parametric oscillator, in: Conference on Lasers and Electro-Optics, Optical Society of America, 2008. CTuII3. [6] A.F. Nieuwenhuis, C.J. Lee, P.J. van der Slot, P. Groß, K.-J. Boller, Mid-infrared ZGP optical parametric oscillator directly pumped by a lamp-pumped, Qswitched Cr, Tm, Ho: YAG laser, in: Lasers and Applications in Science and Engineering, International Society for Optics and Photonics, 2007, pp. 645518e645519. [7] M. Roth, M. Tseitlin, N. Angert, Oxide crystals for electro-optic Q-switching of lasers, Glass Phys. Chem. 31 (1) (2005) 86e95. [8] C. Gao, Z. Lin, R. Wang, Y. Zhang, M. Gao, Y. Zheng, Experimental investigation of pulse diodes side-pumped 2 mm Tm: YAG lasers, Laser Phys. 21 (1) (2011) 70e73. [9] M. Yumoto, N. Saito, Y. Urata, S. Wada, 128 mJ/Pulse, laser-diode-pumped, Qswitched Tm: YAG laser, IEEE J. Sel. Top. Quantum Electron. 21 (1) (2015) 364e368. [10] N. Barnes, D. Gettemy, Pulsed Ho: YAG oscillator and amplifier, IEEE J. Quantum Electron. 17 (7) (1981) 1303e1308. [11] C. Liu, K. Yang, S. Zhao, Y. Li, G. Li, D. Li, T. Li, W. Qiao, T. Feng, X. Chen, 88ns multi-millijoule LiNbO 3 electro-optically Q-switched Tm: LuAG laser, Opt. Commun. 355 (2015) 167e171. [12] L. Wang, X. Cai, J. Yang, X. Wu, H. Jiang, J. Wang, 520 mJ langasite electrooptically Q-switched Cr, Tm, Ho: YAG laser, Opt. Lett. 37 (11) (2012) 1986e1988. [13] A.H. Reshak, I.V. Kityk, S. Auluck, Investigation of the linear and nonlinear optical susceptibilities of KTiOPO4 single crystals: theory and experiment, J. Phys. Chem. B 114 (50) (2010) 16705e16712. [14] M. Eichhorn, Quasi-three-level solid-state lasers in the near and mid infrared based on trivalent rare earth ions, Appl. Phys. B 93 (2e3) (2008) 269e316. €ll, R.K. Mohan, Long-time-storage mechanism for [15] N. Ohlsson, M. Nilsson, S. Kro
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Tm: YAG in a magnetic field, Opt. Lett. 28 (6) (2003) 450e452. [16] W. Gao, J. Ma, G. Xie, J. Zhang, D. Luo, H. Yang, D. Tang, P. Yuan, L. Qian, Highly efficient 2 mm Tm: YAG ceramic laser, Opt. Lett. 37 (6) (2012) 1076e1078. [17] Y. Zou, Z. Wei, Q. Wang, M. Zhan, D. Li, Z. Zhang, J. Zhang, D. Tang, High-efficiency diode-pumped Tm: YAG ceramic laser, Opt. Mater. 35 (4) (2013)
804e806. [18] S. Goldring, E. Lebiush, R. Lavi, RTP Q-switched 2-micron Tm: YAG laser, in: High-power Lasers and Applications, International Society for Optics and Photonics, 2002, pp. 13e16.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Short pulse diode-pumped Tm:YAG slab laser electro-optically Qswitched by RbTiOPO4 crystal Lin Jin, Pian Liu, Haitao Huang, Xuan Liu, Deyuan Shen* Jiangsu Key Laboratory of Advanced Laser Materials and Devices, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou, Jiangsu 221116, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 July 2016 Received in revised form 3 August 2016 Accepted 14 August 2016
In this paper, a laser diode end pumped RbTiOPO4 (RTP) electro-optically Q-switched Tm:YAG slab laser was demonstrated. Stable Q-switched pulse with the shortest pulse width of 58 ns and an average output power of 7.5 W were realized at the repetition rate of 1 KHz, corresponding to the slope efficiency of 21.7%. © 2016 Elsevier B.V. All rights reserved.
Keywords: Tm:YAG Slab laser End-pumped Electro-optically Q-switched
1. Introduction Pulsed lasers in 2 mm range have high importance for applications in LIDAR, medical operation, atmospheric monitoring, materials processing and so on [1e3]. Moreover, using high-energy short pulse -duration 2 mm laser to pump ZnGeP2 optical parametric oscillator to generate mid-infrared laser around 3e12 mm wavelength band has been increasingly investigated [4e6]. To obtain high peak power, high pulse energy and high repetition rates 2 mm lasers, the most widely used method is Q-switching by storing energy in laser gain medium during the high-loss periods and releasing high energy accumulated by population inversion during the low-loss periods. The acousto-optic (AO) Q-switching is frequently applied in 2 mm laser system. But the pulse duration of AO Q-switched laser is typically longer than 100 ns, which is limited by insufficient modulation depth (~80%) [7e9]. By contrast, an electro-optically (EO) Q-switching plays an important role in generating shorter pulse durations and achieving high efficiency due to its fast switching speed, high stability and controllability. The electro-optical crystals such as LiNbO3 (LN), langasites (LGS), rubidium titanyl phosphate (RTP) are favorable crystals for EO Q-switching. Early in 1981, LN crystal based EO Q-
* Corresponding author. E-mail address: [email protected] (D. Shen). http://dx.doi.org/10.1016/j.optmat.2016.08.011 0925-3467/© 2016 Elsevier B.V. All rights reserved.
switch was used in a flash pumped Ho:YAG laser, generating a pulse energy of 80 mJ and 30 ns pulse width at repetition rate of 5 Hz [10]. Recently, a pulse energy of 2.51 mJ with pulse duration of 88 ns at 50 Hz was generated in LN EO Q-switched Tm:LuAG laser [11]. LN crystal has the advantages on low half-wave voltage, broad transmission spectrum (0.4e5 mm), and disadvantages of it are the limited repetition rates of several kilohertz and piezoelectric ring effect which reduces the efficiency to a certain extent. Utilizing LGS crystal as the EO Q-switch, Li Wang et al. demonstrated a flash lamp pumped 2.09 mm Cr, Tm, Ho:YAG laser, 520 mJ pulse energy with 3 Hz repetition rate was achieved [12]. LGS has a high damage threshold of 950 MW/cm2 (1064 nm, 10 ms), but high half-wave voltage has to be applied on LGS crystal because of its low electro-optic coefficient (2.68 pm/V). Compared with LN and LGS crystal, RTP crystal possesses both of their advantages. RTP and KTP are isomorphism, the EO efficiencies can optimized by small cationic reorientation [13]. It also has large electro-optic coefficients (rc1 ¼ 30.2 pm/V, rc2 ¼ 23.6 pm/V), higher electrical resistivity (>1010 Um) and can operate at high repetition rates more than 200 KHz [7]. Tm3þ-doped YAG gain materials have upper-state lifetime of exceeding 10 ms [14,15], which is not relevant to the RTP's good performance at high repetition rates and makes itself become one of the high pulse energy materials. As a result of the crossrelaxation process (3H4 þ 3H6 / 2 3F4) in Tm3þ ions, Tm:YAG provides a high quantum efficiency [16,17]. RTP Q-switched
L. Jin et al. / Optical Materials 60 (2016) 350e354
Tm:YAG laser was first reported in 2002, the Tm:YAG rod was pumped by two fiber-coupled laser diodes, the maximum pulse energy of 2.4 mJ with a pulse width of 57 ns at repetition rate of 20 Hz was achieved but the slope efficiency was only 3% [18]. The highest pulse energy 4 mJ at 100 Hz was obtained in 2008 with the same equipment system [5]. However, both of the EO Q-switched Tm:YAG lasers operated at low repetition rates with hundreds of milliwatts output. Laser sources with higher output average power and higher repetition rates output are desired in many fields, such as materials processing, precise handling, and detecting. In our work, a laser diode end pumped RTP based EO Qswitched Tm:YAG slab laser was investigated. The highest output average power of 7.5 W at 1 KHz was obtained at 2 mm wavelength. The maximum pulse energy of 7.5 mJ with pulse width of 58 ns was obtained under the incident pump power of 43.8 W, corresponding to the slope efficiency of 21.7%.
351
was placed between the polarizer and the output coupler. RTP crystal is an optically biaxial crystal with the orthorhombic symmetry point group mm2, and the natural birefringence cannot be disregarded in Pockels cell configurations [8]. The double-crystal (two RTP optical axises perpendicular to each other) was designed to compensate for the natural birefringence caused by temperature variations. The dimension of each RTP crystal was 3 mm 3 mm 10 mm (Quantum Technology, Inc.) and the aperture of laser design was kept to be ~1 mm to minimize
2. Experiment set up The LD end pumped RTP electro-optically Q-switched Tm:YAG slab laser is shown in Fig. 1. In this experiment, the pump source was a laser diode stack consisted of five bars, the dimension of the diode arrays was about 12 mm along the slow axis (x direction) and 12 mm along the fast axis (y direction). The emission wavelength of the pump source had little fluctuations with water-cooled temperature changing, when the cooling water was set at 15 C, the emission wavelength was centered at around 785 nm which was coincidence with absorption wavelength of Tm:YAG gain medium. The resonator was constructed by two plano-concave mirrors M1 and M2, M1 with a radius of curvature of 300 mm was coated for high transmission at 760e810 nm and high reflection at 1879e2170 nm, the output coupler M2 had a radius of curvature of 100 mm and had a high reflection at 785 nm with the transmission of 10% at 1930e2100 nm. The pump light coupling system was composed of two cylindrical lens with the focal length of f1 ¼ 100 mm along the fast axis and f2 ¼ 60 mm along the slow axis. The focused pump spot had a dimension of 0.68 mm (x) 0.6 mm (y). The dimension of Tm:YAG was 1 mm 10 mm 15 mm with 3.5 at.% doping concentration. Both of the 1 mm 10 mm faces were coated for high transmission at 785 nm and 1890e2170 nm. The pump beams were incident on the left (1 mm 10 mm) face and about 94.7% of the pump beams can be absorbed. The two large area surfaces (10 mm 15 mm) were tightly sandwiched between the micro-channel water-cooled cooper heat sink and the cooling water was maintained at 15 C. The improved thermal module was a micro-channel heat sink with a channel width of 500 mm and had stress compensation mechanism. Two undoped YAG plates at Brewster's angle were used to generate linear polarized lasing output. The RTP Q-switched crystal
Fig. 2. Output power as a function of incident pump power in cw and Q-switched mode.
Fig. 3. Pulse energy and pulse width versus pulse repetition rate at the incident pump power of 43.8 W.
Fig. 1. Schematic of LD end pumped EO Q-switched Tm:YAG slab laser.
352
L. Jin et al. / Optical Materials 60 (2016) 350e354
damaging the crystal sides The dimension of pump beam size was designed less than 1 mm2 to avoid the damage of components. The entire cavity length was optimized to be 121 mm. A Sunfire-3PN100KHz-24 Pockels cell driver was used to drive the RTP Q-
switch (quarter-wave voltage, 1.05 kV). In the experiments, the output power was measured by a F150A-BB-26 power meter (Ophir Photonics). A fast InGaAs photodiode (DET10D/M) was used to detected Q-switched laser pulse and an oscilloscope (Keysight DSOS 104A) was used to record the pulse signal.
3. Experiment results and discussion
Fig. 4. Pulse width versus the incident pump power at 1 KHz.
Fig. 2 shows the output power with respect to incident pump power in continuous wave and Q-switched mode. In continuous wave operation, no components were placed in resonator cavity and the output power was also measured at 121 mm cavity length. With the increase of incident pump power, the output power takes a linear increase, at the incident pump power of 43.8 W, the maximum output power of 13.2 W was obtained, corresponding to the slope efficiency of 38.7%. In the pulse operation, a quarter-wave electrical voltage was applied on the RTP Q-switch crystal under the continuous pumping light, the resonator was at a low Q-state, and the linear polarized pulse laser was generated when the quarter-wave electrical voltage was removed. At the repetition rates of 10 KHz, 8 KHz, 5 KHz, 3 KHz, 1 KHz, the average output power of 8.14 W, 7.95 W, 7.86 W, 7.6 W, 7.5 W were obtained at the incident pump power of 43.8 W, respectively. The slope efficiency was 21.7% at the repetition rate of 1 KHz. With further increasing
Fig. 5. Typical pulse train and pulse waveform of 58 ns at 1 KHz under the incident pump power of 43.8 W.
L. Jin et al. / Optical Materials 60 (2016) 350e354
353
Fig. 6. (a) Output spectrum and (b) Beam quality of EO Q-switched Tm:YAG laser, insert, the profile of the laser beam.
the pump power in Q-switched mode, the output power decreased because of the resonator saturation as a result of the thermal lens effect, and no photo destruction was observed in the experiment due to the unreached damage threshold. The decreasing of the average output power and slope efficiency at Q-switched mode mainly contributed to the depolarization loss of thermal induced birefringence in Tm:YAG and RTP crystal and insertion loss of the components. Pulse energy and pulse width versus pulse repetition rate at the incident pump power of 43.8 W are summarized in Fig. 3. With the repetition rates changing from 10 KHz to 1 KHz, the pulse width reduced from 163 ns to 58 ns. At the repetition rate of 1 KHz, the maximum pulse energy of 7.5 mJ and the minimum pulse width of 58 ns were obtained at the maximum pump power, corresponding to the maximum peak power of ~125 KW. Fig. 4 shows the pulse width versus the incident pump power under the repetition rate of 1 KHz. With the incident pump power increased, pulse width decreased from 431 ns to 58 ns The shortest pulse width of 58 ns was obtained under the incident pump power of 43.8 W at 1 KHz. The temporal waveform of the 58 ns and typical pulse train at the repetition rate of 1 KHz under the incident pump power of 43.8 W was shown in Fig. 5. The output spectrum of RTP based EO Q-switched laser was centered at 2014.5 nm with a bandwidth of 0.3 nm and the laser spectrum was measured by an AQ6375 optical spectrum analyzer (shown in Fig. 6 (a)). Fig. 6 (b) shows the beam quality factors measured at 1 KHz under the maximum incident pump power and the insert shows the 3D profile of the laser beam near the focus. A slightly poor of the beam quality was because of the low overlapping degree between pump beam spot and TEM00 mode size. The beam quality factors of 7.5 W output at 1 KHz were measured as M2x ¼ 2.24 and M2y ¼ 1.63 along slow axis and fast axis respectively. Higher output average power should be achievable with optimized aperture of RTP crystal and improved mode matching in the future work.
4. Conclusion In conclusion, an RTP EO Q-switched Tm:YAG slab laser directly pumped by a 785 nm laser diode was demonstrated experimentally. An average output power of 7.5 W with 58 ns pulse width at 1 kHz was obtained, corresponding to the slope efficiency of 21.7%. The maximum pulse energy of 7.5 mJ and the peak power of 125 KW were achieved. The beam quality factors were M2x ¼ 2.24 and M2y ¼ 1.63. The results indicate that the combination of Tm:YAG
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