- Email: [email protected]
Growth and laser performance of Tm:SrGdGa3O7 crystal
Optical Materials xxx (xxxx) xxx
Contents lists available at ScienceDirect
Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Growth and laser performance of Tm:SrGdGa3O7 crystal Qi Yang a, Xulei Lun a, Yuanyuan Zhang b, *, Xuping Wang b, ** a b
International School for Optoelectronic Engineering, Qilu University of Technology (Shandong Academy of Sciences), China Advanced Materials Institute, Qilu University of Technology (Shandong Academy of Sciences), China
A R T I C L E I N F O
A B S T R A C T
Keywords: Diode-pumped Q-switched Tunable laser Tm-doped crystal
The TmSrGdGa3O7 crystal was grown by the Czochralski (CZ) method. Continuous-wave, Q-switched, and tunable laser performance are demonstrated based on Tm:SrGdGa3O7 crystal for the first time. The maximum continuous-wave output power was 0.49 W under the absorbed pump power of 8.6 W. Using a commercial WS2 as the saturable absorber, the passively Q-switched laser was successfully achieved. The maximum output power of Q-switched reached up to 0.13 W with the repetition rate of 21.4 kHz and the pulse width of 1.1 μs. A tunable laser was obtained with the whole tuning range of 60.8 nm from 1907.0 to 1967.8 nm.
1. Introduction In recent years, solid-state lasers based on thulium-doped crystals have been widely studied for efficient emitting at 1.9 μm via the 3 F4→3H6 transition. Thanks to the combination of specific features, the laser around 1.9 μm has potential applications such as medical surgery, LIDAR, remote sensing, material processing, and generation of mid-IR light sources [1,2]. The absorption bands of Tm-doped crystals are around 790 nm, which are suitable for commercially available high-power InGaAs laser diode, leading to compact and efficient laser sources. Up to now, laser actions at 1.9 μm have been demonstrated in many Tm3þ-doped crystals, such as the garnets Tm:YAG [3,4], Tm: LuAG [5], the aluminates Tm:YAP [6,7], the vanadates Tm: YVO4 [8], Tm:GdVO4 [9], the fluorides Tm:LiYF4 [10], Tm: LiLuF4 [11], Tm: BaY2F8 [12], Tm,Y:CaF2 [13,14], the silicate Tm:SSO [15], as well as tungstates Tm:KLu(WO4)2 [16], Tm:KGd(WO4)2 [17], etc. Although numerous works have already been carried out on the Tm3þ-doped laser, the development of new materials is still a longstanding goal. Disordered crystals have drawn much attention, benefitting from their broad absorption and emission spectra. SrGdGa3O7 (SGGM) crys tal, as a member of disordered crystals, belongs to the large ABC3O7 – La, Gd; and C– – Ga, Al. SrGdGa3O7 grouping, where A ¼ Ca, Sr, Ba; B– crystal is composed of layered GaO54 -tetrahedron, and between layers, Sr2þ and Gd3þ ions are randomly distributed at the corresponding lattice points in a 1:1 ratio. The disordered structure is caused by the differ ences of valence, particle radius and crystallization property of Sr2þ and Gd3þ ions. Doped activation ions replace Gd3þ ions, and different ion
activation centers are formed in matrix crystals, resulting in inhomo geneous broadening of spectra including absorption spectra and emis sion spectra. Wide absorption spectrum is good for diode pumping, and wide emission spectrum is beneficial to generate ultrashort pulse and tunable laser output. Spontaneous mode-locked laser at 1 μm based on Nd:SGGM has been realized [18]. W. Ryba-Romanowski has been re ported the spectroscopic of Tm:SGGM crystal [19]. Very recently, Gao et al. have reported a free-running Tm3þ/Ho3þ: SrLaGa3O7 laser at 2.04 μm. However, the duty cycle of the quasi-CW pump laser was 2%, the maximum average output power was only 123 mW [20]. In this paper, we present continuous wave (CW), Q-switched, and tunable laser performance with the disordered Tm3þ:SrGdGa3O7 (Tm: SGGM) crystal grown by the Czochralski method. To the best of our knowledge, it is the first time to report on the laser performance of Tm:SGGM crystal. 2. Crystal growth and characterizations Tm:SGGM crystal was grown by the Czochralski method using the intermediate-frequency furnace. The raw materials including 99.99% pure Tm2O3, SrCO3, Gd2O3, and Ga2O3 powders. The starting compo nents were dried, and then weighted according to the chemical formula SrGd0.95Tm0.05Ga3O7. Taking the volatilization of Ga2O3 into account during the growth process, the mass of Ga2O3 exceeded the 1% of the total mass. The starting materials were grinded, and then mixed for 24 h. The mixtures were put into a platinum crucible, and then sintered at 1100 � C for 10 h. The calcined powders were grinded and mixed for 24 h again. The presintering components were pressed into tablets and
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Zhang), [email protected] (X. Wang). https://doi.org/10.1016/j.optmat.2019.109482 Received 28 May 2019; Received in revised form 22 October 2019; Accepted 23 October 2019 0925-3467/© 2019 Published by Elsevier B.V.
Please cite this article as: Qi Yang, Optical Materials, https://doi.org/10.1016/j.optmat.2019.109482
Q. Yang et al.
Optical Materials xxx (xxxx) xxx
corresponding to 3 F4 →3 H6 was investigated to be 4.2 ms as shown in Fig. 2. According to the equation η ¼ ττradf , the quantum efficiency of upper 3F4 laser level η was 72%.
3. Laser experimental setup and results The experimental setup of CW and passive Q-switched laser was shown in Fig. 3 (a). The pump source was a fiber coupled laser diode with an emitting wavelength of 793 nm, which held a fiber core diam eter of 400 μm and numerical aperture of 0.22. The pumping light was focused into the laser gain medium by a focal lens with a beam diameter of about 320 μm. The laser gain medium was 3 � 3 � 8 mm3 c-cut Tm: SGGM. The crystal was wrapped by an indium foil and mounted in an aluminum block cooled by circulating water with a temperature of 17 � C. Concave mirror M1 with a radius of 50 mm was employed as the input mirror, which was AR coated from 750 to 850 nm (reflectivity <1%); HR coated from 1850 to 2150 nm (reflectivity >99%). Concave mirrors M2 with three different transmission of 1%, 2% and 5% from 1850 to 2150 nm were employed as output couplers (OC), which had a radius of 100 mm. The length of the whole cavity was about 20 mm. For the Q-switched laser operation, the commercial few-layer WS2 sample (sixcarbon Tech. Shenzhen) was employed as a saturable absorber (SA). The WS2 SA was prepared on a sapphire substrate by the CVD method. The modulation depth of SA was measured to be 10.0% at 1940 nm. Fig. 4 shows the CW output power with three different output coupler. Under the absorbed pump power of 8.6 W, the maximum output power of 0.31 W, 0.43 W and 0.49 W were obtained by the output cou plers (OCs) with transmissions of 1%, 2%, and 5%. The corresponding optical efficiency were 3.6%, 5.0%, and 5.7%, respectively. The optical efficiencies were not high, owing to the severe thermal effect. According to the experimental results in CW operation, output coupler of 5% transmission was employed in Q-switched operation. By using an optical
Fig. 1. The grown crystal.
Fig. 2. Fluorescence decay curves for the 3F4 → 3H6.
sintered at 1100 � C for 10 h to synthesize polycrystalline compounds. The crystal was grown using an iridium crucible in an atmosphere of N2þO2 (2% by volume), because of the high melting point of 1600 � C. To prevent dislocation, a <001>-oriented Tm:SGGM crystal was intro duced into the melt to obtain a thin neck growth. During the equal diameter growth, the pulling rate was 0.8 mm/h with a rotation rate of 15 rpm. After growth was completed, the crystal was cooled to room temperature at a rate of 30 K/h. The dimensions of the grown crystal was about Φ25 � 40 mm3 (Fig. 1). The concentration of Tm3þ was deter mined to be 3.05 at% using the X-ray fluorescence analysis method. The average Tm3þ density N0 was determined to be 1.76 � 1020/cm3 with a segregation coefficient of 0.61. W. Ryba-Romanowski et al. have re ported the crystal field splitting of the 1D2, 1G4 and 3H6 states by measuring the low temperature excited spectra and fluorescence spectra. Six energy levels compose the 3F4 state [19]. According to the method of Ref. [19,21], The radiative lifetime of 3F4 energy level τrad was calculated to be 5.8 ms. The fluorescence decay lifetime τf
Fig. 4. CW output power of three output couplers with different transmissions.
Fig. 3. (a) The experimental setup of the WS2 based passively Q-switched laser; (b) Wavelength-tuning with BF. 2
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2.98 W. Fig. 6 shows the average output power characteristic of Qswitched laser. The maximum Q-switched pulse average power of 0.13 W was obtained under the absorbed pump power of 8.6 W, with corresponding optical conversion efficiency of 1.5%. The laser spectra of Q-switched pulse was also shown in Fig. 5, the central wavelength and FWHM of Q-switched pulse were 1919.4 nm and 2.80 nm, respectively. The center wavelength of Q-switched laser was found to blue-shifted, which might be the Q-switched process leading to depopulation of the ground-state level and as a result, the reabsorption effect was reduced. Fig. 7 shows the pulse train and the temporal pulse profile of the Qswitched Tm:SGGM laser at the maximum output power. The shortest pulse width of 1.1 μs was achieved with the repetition rate of 21.4 kHz. The corresponding maximum single pulse energy and highest peak power were calculated to be about 6.1 μJ and 5.5 W. The experimental setup of tunable laser was shown in Fig. 3 (b). The linear cavity was employed, plane mirror M1 was used as an input mirror, concave mirror M2 with curvature radius of 100 mm was employed as OC, whose transmission was 2% from 1850 nm to 2150 nm. The length of the whole cavity was about 10 cm. A quartz BF was inserted into the cavity at Brewster’s angle to research the wavelength tunable performance of the Tm:SGGM crystal. A whole tuning range of 60.8 nm from 1907.0 to 1967.8 nm was attained by adjusting the BF angle, as depicted in Fig. 8. The smooth and broad tuning curve indicates that the crystal can be used as a gain medium for mode-locked lasers and has the potential to generate sub-100 fs pulses. Compared with other Tm-doped laser materials, the disordered Tm:
Fig. 5. Optical spectrum of the CW and Q-switched operation.
Fig. 6. Q-switched output power versus absorbed pump power with the OC of transmission 5%.
spectrum analyzer, the output spectrum was measured shown in Fig. 5. The central wavelength was 1928.3 nm and the full-width at halfmaximum (FWHM) was 3.17 nm. After inserting the WS2 saturable absorber into the cavity, the Qswitched pulse laser appeared as the absorbed pump power surpassed
Fig. 8. Laser-tuning scatter of Tm: SGGM crystal based on a quartz BF.
Fig. 7. (a) The typical pulse train at the maximum output power. (b) The single pulse width at the maximum output power. 3
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SGGM crystal perform wider spectrum range. We will improve the quality of crystal to increase the laser efficiency in the next step of the research. The tuning range will be largely expanded when the laser threshold is reduced.
[2] J.J. Zayhowski, J. Harrison, C. Dill III, J. Ochoa, Tm:YVO4 microchip laser, Appl. Opt. 34 (1995) 435–437. [3] C. Li, J. Song, D.Y. Shen, N.S. Kim, K.-i. Ueda, Y.J. Huo, S.F. He, Y.H. Cao, Diodepumped high-efficiency Tm:YAG lasers, Opt. Express 4 (1999) 12–18. [4] S. Zhang, X. Wang, W. Kong, Q. Yang, J. Xu, B. Jiang, Y. Pan, Efficient Q-switched Tm:YAG ceramic slab laser pumped by a 792 nm fiber laser, Opt. Commun. 286 (2013) 288–290. [5] C.T. Wu, Y.L. Ju, Z.G. Wang, Y.F. Li, H.Y. Ma, Y.Z. Wang, Lasing characteristics of a CW Tm: LuAG laser with a set of double cavity, Laser Phys. Lett. 5 (2008) 510–513. [6] H. Kalaycioglu, A. Sennaroglu, Low-threshold continuous-wave Tm3þ :YAlO3 laser, Opt. Commun. 281 (2008) 4071–4074. [7] J. Liu, Y.G. Wang, Z.S. Qu, X.W. Fan, 2 μm passive Q-switched mode-locked Tm3þ: YAP laser with single-walled carbon nanotube absorber, Opt. Laser. Technol. 44 (2012) 960–962. [8] J.J. Zayhowski, J. Harrison, C. Dill III, J. Ochoa, Tm:YVO4 microchip laser, Appl. Opt. 34 (1995) 435–437. [9] Y. Urata, S. Wada, 808-nm diode-pumped continuous wave Tm:GdVO4 laser at room temperature, Appl. Opt. 44 (2005) 3087–3092. [10] M. Schellhorn, S. Ngcobo, C. Bollig, High-power diode pumped Tm:YLF slab laser, Appl. Phys. B 94 (2009) 195–198. [11] N. Coluccelli, G. Galzerano, P. Laporta, F. Cornacchia, D. Parisi, M. Tonelli, Tmdoped LiLuF4 crystal for efficient laser action in the wavelength range from 1.82 to 2.06 μm, Opt. Lett. 32 (2007) 2040–2042. [12] F. Cornacchia, D. Parisi, C. Bernardini, A. Toncelli, M. Tonelli, Efficient, diodepumped Tm3þ :BaY2F8 vibronic laser, Opt. Express 12 (2004) 1982–1989. [13] C. Zhang, J. Liu, X.W. Fan, Q.Q. Peng, X.H. Guo, D.P. Jiang, X.B. Qian, L.B. Su, Compact passive Q-switching of a diode-pumped Tm,Y:CaF2 laser near 2 μm, Opt. Laser. Technol. 103 (2018) 89–92. [14] X. Liu, K. Yang, S. Zhao, T. Li, C. Luan, X. Guo, B. Zhao, L. Zheng, L. Su, J. Xu, J. Bian, Growth and lasing performance of a Tm,Y:CaF2 crystal, Opt. Lett. 42 (2017) 2567–2570. [15] J. Liu, Y.Q. Li, L.H. Zheng, L.B. Su, J. Xu, Y.G. Wang, Passive Q-switched mode locking of a diode-pumped Tm:SSO laser near 2 μm, Laser Phys. Lett. 10 (2013) 105812. [16] X. Mateos, V. Petrov, J. Liu, M.C. Pujol, U. Griebner, M. Aguil� o, F. Diaz, M. Galan, G. Viera, Efficient 2-μm continuous-wave laser oscillation of Tm3þ:KLu(WO4)2, IEEE J. Quantum Electron. 42 (2006) 1008–1050. [17] V. Petrov, F. Guell, J. Massons, J. Gavalda, R.M. Sole, M. Aguilo, F. Diaz, U. Griebner, Efficient tunable laser operation of Tm:KGd(WO4)2 in the continuouswave regime at room temperature, IEEE J. Quantum Electron. 40 (2004) 1244–1251. [18] Y. Chen, H. Liang, J. Tung, K. Su, Y. Zhang, H. Zhang, H. Yu, J. Wang, Spontaneous subpicosecond pulse formation with pulse repetition rate of 80 GHz in a diodepumped Nd:SrGdGa3O7 disordered crystal laser, Opt. Lett. 37 (2012) 461–463. [19] W. Ryba-Romanowski, S. Gołab, I. Sok� olska, G. Dominiak-Dzik, J. Zawadzka, M. Berkowski, J. Fink-Finowicki, M. Baba, Spectroscopic characterization of a Tm3 þ :SrGdGa3O7 crystal, Appl. Phys. B 68 (1999) 199–205. [20] S. Gao, S. Xu, C. Tu, Investigations on the spectroscopic properties and laser performance of Tm3þ/Ho3þ-SrLaGa3O7 crystal, Opt. Laser. Technol. 111 (2019) 775–780. [21] W. Ryba-Romanowski, M. Berkowski, B. Viana, P. Aschehoug, Relaxation dynamics of excited states of Tm3þ in SrGdGa3O7 crystals activated with Tm3þ and Tm3þþ Tb3þ, Appl. Phys. B 64 (1997) 525–529.
4. Conclusion In conclusion, we demonstrated diode-pumped CW, Q-switched, tunable laser performance with the Tm: SGGM crystal. The maximum CW output power was 0.49 W with the optical conversion of 5.7%. The maximum Q-switched output power was 0.13 W with the pulse width of 1.1 μs and the repetition rate of 21.4 kHz. A continuous tuning range of 60.8 nm (from 1907.0 to 1967.8 nm) was achieved. Benefitting from the large tuning range, the CW mode-locked laser around 1.9 μm may be achieved in the future. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors acknowledge the financial support partly from Youth Doctoral Cooperative Program of Qilu University of Technology (Shandong Academy of Sciences) (2017BSHZ018), the Natural Science Foundation of Shandong Province of China (ZR2017LF024), the Na tional Science Foundation of China (Grant no. 51672164 and 51302158), National College Students Innovation and Entrepreneurship Training Program (201810431044). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.optmat.2019.109482. References [1] E.C. Honea, R.J. Beach, S.B. Sutton, J.A. Speth, S.C. Mitchell, J.A. Skidmore, M. A. Emanuel, S.A. Payne, 115-W Tm:YAG diode-pumped solid-state laser, IEEE J. Quantum Electron. 33 (1997) 1592–1600.
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Contents lists available at ScienceDirect
Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Growth and laser performance of Tm:SrGdGa3O7 crystal Qi Yang a, Xulei Lun a, Yuanyuan Zhang b, *, Xuping Wang b, ** a b
International School for Optoelectronic Engineering, Qilu University of Technology (Shandong Academy of Sciences), China Advanced Materials Institute, Qilu University of Technology (Shandong Academy of Sciences), China
A R T I C L E I N F O
A B S T R A C T
Keywords: Diode-pumped Q-switched Tunable laser Tm-doped crystal
The TmSrGdGa3O7 crystal was grown by the Czochralski (CZ) method. Continuous-wave, Q-switched, and tunable laser performance are demonstrated based on Tm:SrGdGa3O7 crystal for the first time. The maximum continuous-wave output power was 0.49 W under the absorbed pump power of 8.6 W. Using a commercial WS2 as the saturable absorber, the passively Q-switched laser was successfully achieved. The maximum output power of Q-switched reached up to 0.13 W with the repetition rate of 21.4 kHz and the pulse width of 1.1 μs. A tunable laser was obtained with the whole tuning range of 60.8 nm from 1907.0 to 1967.8 nm.
1. Introduction In recent years, solid-state lasers based on thulium-doped crystals have been widely studied for efficient emitting at 1.9 μm via the 3 F4→3H6 transition. Thanks to the combination of specific features, the laser around 1.9 μm has potential applications such as medical surgery, LIDAR, remote sensing, material processing, and generation of mid-IR light sources [1,2]. The absorption bands of Tm-doped crystals are around 790 nm, which are suitable for commercially available high-power InGaAs laser diode, leading to compact and efficient laser sources. Up to now, laser actions at 1.9 μm have been demonstrated in many Tm3þ-doped crystals, such as the garnets Tm:YAG [3,4], Tm: LuAG [5], the aluminates Tm:YAP [6,7], the vanadates Tm: YVO4 [8], Tm:GdVO4 [9], the fluorides Tm:LiYF4 [10], Tm: LiLuF4 [11], Tm: BaY2F8 [12], Tm,Y:CaF2 [13,14], the silicate Tm:SSO [15], as well as tungstates Tm:KLu(WO4)2 [16], Tm:KGd(WO4)2 [17], etc. Although numerous works have already been carried out on the Tm3þ-doped laser, the development of new materials is still a longstanding goal. Disordered crystals have drawn much attention, benefitting from their broad absorption and emission spectra. SrGdGa3O7 (SGGM) crys tal, as a member of disordered crystals, belongs to the large ABC3O7 – La, Gd; and C– – Ga, Al. SrGdGa3O7 grouping, where A ¼ Ca, Sr, Ba; B– crystal is composed of layered GaO54 -tetrahedron, and between layers, Sr2þ and Gd3þ ions are randomly distributed at the corresponding lattice points in a 1:1 ratio. The disordered structure is caused by the differ ences of valence, particle radius and crystallization property of Sr2þ and Gd3þ ions. Doped activation ions replace Gd3þ ions, and different ion
activation centers are formed in matrix crystals, resulting in inhomo geneous broadening of spectra including absorption spectra and emis sion spectra. Wide absorption spectrum is good for diode pumping, and wide emission spectrum is beneficial to generate ultrashort pulse and tunable laser output. Spontaneous mode-locked laser at 1 μm based on Nd:SGGM has been realized [18]. W. Ryba-Romanowski has been re ported the spectroscopic of Tm:SGGM crystal [19]. Very recently, Gao et al. have reported a free-running Tm3þ/Ho3þ: SrLaGa3O7 laser at 2.04 μm. However, the duty cycle of the quasi-CW pump laser was 2%, the maximum average output power was only 123 mW [20]. In this paper, we present continuous wave (CW), Q-switched, and tunable laser performance with the disordered Tm3þ:SrGdGa3O7 (Tm: SGGM) crystal grown by the Czochralski method. To the best of our knowledge, it is the first time to report on the laser performance of Tm:SGGM crystal. 2. Crystal growth and characterizations Tm:SGGM crystal was grown by the Czochralski method using the intermediate-frequency furnace. The raw materials including 99.99% pure Tm2O3, SrCO3, Gd2O3, and Ga2O3 powders. The starting compo nents were dried, and then weighted according to the chemical formula SrGd0.95Tm0.05Ga3O7. Taking the volatilization of Ga2O3 into account during the growth process, the mass of Ga2O3 exceeded the 1% of the total mass. The starting materials were grinded, and then mixed for 24 h. The mixtures were put into a platinum crucible, and then sintered at 1100 � C for 10 h. The calcined powders were grinded and mixed for 24 h again. The presintering components were pressed into tablets and
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Zhang), [email protected] (X. Wang). https://doi.org/10.1016/j.optmat.2019.109482 Received 28 May 2019; Received in revised form 22 October 2019; Accepted 23 October 2019 0925-3467/© 2019 Published by Elsevier B.V.
Please cite this article as: Qi Yang, Optical Materials, https://doi.org/10.1016/j.optmat.2019.109482
Q. Yang et al.
Optical Materials xxx (xxxx) xxx
corresponding to 3 F4 →3 H6 was investigated to be 4.2 ms as shown in Fig. 2. According to the equation η ¼ ττradf , the quantum efficiency of upper 3F4 laser level η was 72%.
3. Laser experimental setup and results The experimental setup of CW and passive Q-switched laser was shown in Fig. 3 (a). The pump source was a fiber coupled laser diode with an emitting wavelength of 793 nm, which held a fiber core diam eter of 400 μm and numerical aperture of 0.22. The pumping light was focused into the laser gain medium by a focal lens with a beam diameter of about 320 μm. The laser gain medium was 3 � 3 � 8 mm3 c-cut Tm: SGGM. The crystal was wrapped by an indium foil and mounted in an aluminum block cooled by circulating water with a temperature of 17 � C. Concave mirror M1 with a radius of 50 mm was employed as the input mirror, which was AR coated from 750 to 850 nm (reflectivity <1%); HR coated from 1850 to 2150 nm (reflectivity >99%). Concave mirrors M2 with three different transmission of 1%, 2% and 5% from 1850 to 2150 nm were employed as output couplers (OC), which had a radius of 100 mm. The length of the whole cavity was about 20 mm. For the Q-switched laser operation, the commercial few-layer WS2 sample (sixcarbon Tech. Shenzhen) was employed as a saturable absorber (SA). The WS2 SA was prepared on a sapphire substrate by the CVD method. The modulation depth of SA was measured to be 10.0% at 1940 nm. Fig. 4 shows the CW output power with three different output coupler. Under the absorbed pump power of 8.6 W, the maximum output power of 0.31 W, 0.43 W and 0.49 W were obtained by the output cou plers (OCs) with transmissions of 1%, 2%, and 5%. The corresponding optical efficiency were 3.6%, 5.0%, and 5.7%, respectively. The optical efficiencies were not high, owing to the severe thermal effect. According to the experimental results in CW operation, output coupler of 5% transmission was employed in Q-switched operation. By using an optical
Fig. 1. The grown crystal.
Fig. 2. Fluorescence decay curves for the 3F4 → 3H6.
sintered at 1100 � C for 10 h to synthesize polycrystalline compounds. The crystal was grown using an iridium crucible in an atmosphere of N2þO2 (2% by volume), because of the high melting point of 1600 � C. To prevent dislocation, a <001>-oriented Tm:SGGM crystal was intro duced into the melt to obtain a thin neck growth. During the equal diameter growth, the pulling rate was 0.8 mm/h with a rotation rate of 15 rpm. After growth was completed, the crystal was cooled to room temperature at a rate of 30 K/h. The dimensions of the grown crystal was about Φ25 � 40 mm3 (Fig. 1). The concentration of Tm3þ was deter mined to be 3.05 at% using the X-ray fluorescence analysis method. The average Tm3þ density N0 was determined to be 1.76 � 1020/cm3 with a segregation coefficient of 0.61. W. Ryba-Romanowski et al. have re ported the crystal field splitting of the 1D2, 1G4 and 3H6 states by measuring the low temperature excited spectra and fluorescence spectra. Six energy levels compose the 3F4 state [19]. According to the method of Ref. [19,21], The radiative lifetime of 3F4 energy level τrad was calculated to be 5.8 ms. The fluorescence decay lifetime τf
Fig. 4. CW output power of three output couplers with different transmissions.
Fig. 3. (a) The experimental setup of the WS2 based passively Q-switched laser; (b) Wavelength-tuning with BF. 2
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2.98 W. Fig. 6 shows the average output power characteristic of Qswitched laser. The maximum Q-switched pulse average power of 0.13 W was obtained under the absorbed pump power of 8.6 W, with corresponding optical conversion efficiency of 1.5%. The laser spectra of Q-switched pulse was also shown in Fig. 5, the central wavelength and FWHM of Q-switched pulse were 1919.4 nm and 2.80 nm, respectively. The center wavelength of Q-switched laser was found to blue-shifted, which might be the Q-switched process leading to depopulation of the ground-state level and as a result, the reabsorption effect was reduced. Fig. 7 shows the pulse train and the temporal pulse profile of the Qswitched Tm:SGGM laser at the maximum output power. The shortest pulse width of 1.1 μs was achieved with the repetition rate of 21.4 kHz. The corresponding maximum single pulse energy and highest peak power were calculated to be about 6.1 μJ and 5.5 W. The experimental setup of tunable laser was shown in Fig. 3 (b). The linear cavity was employed, plane mirror M1 was used as an input mirror, concave mirror M2 with curvature radius of 100 mm was employed as OC, whose transmission was 2% from 1850 nm to 2150 nm. The length of the whole cavity was about 10 cm. A quartz BF was inserted into the cavity at Brewster’s angle to research the wavelength tunable performance of the Tm:SGGM crystal. A whole tuning range of 60.8 nm from 1907.0 to 1967.8 nm was attained by adjusting the BF angle, as depicted in Fig. 8. The smooth and broad tuning curve indicates that the crystal can be used as a gain medium for mode-locked lasers and has the potential to generate sub-100 fs pulses. Compared with other Tm-doped laser materials, the disordered Tm:
Fig. 5. Optical spectrum of the CW and Q-switched operation.
Fig. 6. Q-switched output power versus absorbed pump power with the OC of transmission 5%.
spectrum analyzer, the output spectrum was measured shown in Fig. 5. The central wavelength was 1928.3 nm and the full-width at halfmaximum (FWHM) was 3.17 nm. After inserting the WS2 saturable absorber into the cavity, the Qswitched pulse laser appeared as the absorbed pump power surpassed
Fig. 8. Laser-tuning scatter of Tm: SGGM crystal based on a quartz BF.
Fig. 7. (a) The typical pulse train at the maximum output power. (b) The single pulse width at the maximum output power. 3
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Optical Materials xxx (xxxx) xxx
SGGM crystal perform wider spectrum range. We will improve the quality of crystal to increase the laser efficiency in the next step of the research. The tuning range will be largely expanded when the laser threshold is reduced.
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4. Conclusion In conclusion, we demonstrated diode-pumped CW, Q-switched, tunable laser performance with the Tm: SGGM crystal. The maximum CW output power was 0.49 W with the optical conversion of 5.7%. The maximum Q-switched output power was 0.13 W with the pulse width of 1.1 μs and the repetition rate of 21.4 kHz. A continuous tuning range of 60.8 nm (from 1907.0 to 1967.8 nm) was achieved. Benefitting from the large tuning range, the CW mode-locked laser around 1.9 μm may be achieved in the future. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors acknowledge the financial support partly from Youth Doctoral Cooperative Program of Qilu University of Technology (Shandong Academy of Sciences) (2017BSHZ018), the Natural Science Foundation of Shandong Province of China (ZR2017LF024), the Na tional Science Foundation of China (Grant no. 51672164 and 51302158), National College Students Innovation and Entrepreneurship Training Program (201810431044). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.optmat.2019.109482. References [1] E.C. Honea, R.J. Beach, S.B. Sutton, J.A. Speth, S.C. Mitchell, J.A. Skidmore, M. A. Emanuel, S.A. Payne, 115-W Tm:YAG diode-pumped solid-state laser, IEEE J. Quantum Electron. 33 (1997) 1592–1600.
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