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1887 nm lasing in Tm3+-doped TeO2-BaF2-Y2O3 glass microstructured fibers
Optical Materials 66 (2017) 640e643
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
1887 nm lasing in Tm3þ-doped TeO2-BaF2-Y2O3 glass microstructured fibers Shunbin Wang, Chuanfei Yao, Zhixu Jia*, Guanshi Qin, Weiping Qin State Key Laboratory on Integrated Opto-Electronics, College of Electronic Science & Engineering, Jilin University, Changchun 130012, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 December 2016 Accepted 13 March 2017 Available online 21 March 2017
In this paper, we demonstrate ~2 mm lasing in Tm3þ-doped fluorotellurite microstructured fibers. The Tm3þ-doped fibers are based on TeO2-BaF2-Y2O3 glasses and fabricated by using a rod-in-tube method. Under the pump of a 1570 nm Er3þ-doped fiber laser, lasing at 1887 nm is obtained in a ~42.5 cm long Tm3þ-doped fiber with a threshold pump power of 94 mW. As the pump power increases to 780 mW, the obtained maximum unsaturated power reaches up to ~408 mW with a slop efficiency of ~58.1%. This result indicates that the Tm3þ-doped fluorotellurite fibers are promising gain media for ~2 mm fiber lasers. © 2017 Elsevier B.V. All rights reserved.
Keywords: Fluorotellurite glass Microstructured fiber Tm3þ-doped Fiber laser
1. Introduction Tm3þ-doped fiber lasers have been widely investigated for various applications in atmosphere and wind sensing, military, eyesafe laser radar and medical surgery [1e7]. Up to now, the reported output power of ~2 mm Tm3þ-doped silica fiber lasers has exceeded 1000 W [8]. ~2 mm lasing in Tm3þ-doped fibers is ascribed to the transition of 3F4 / 3H6; lower phonon energy of the matrix may lead to better “two-for-one” Tm-Tm self-quenching (3H4 þ 3H6 / 23F4), which can improve the slope efficiency and decrease the threshold of relative lasers [9]. For this purpose, glasses with low phonon energies (e.g. germanate glasses, tellurite glasses, and fluoride glasses) have been investigated for ~2 mm Tm3þ-doped fiber lasers [10e12]. In 2006, Jianfeng Wu et al. reported efficient ~2 mm lasing in a 4 cm long highly Tm3þ-doped germanate fiber pumped by an 800 nm laser. The obtained slope efficiency was 58% and the quantum efficiency was 179% [10]. In 2015, Xin Wen et al. reported an all-fiber laser at 1.95 mm in a barium gallo-germanate glass single-mode fiber with a maximum output power of 140 mW and a slope efficiency of 7.6% under the excitation of a 1568 nm fiber laser [11]. In 2010, Kefeng Li et al. reported a watt level ~2 mm CW fiber laser from a 40 cm long highly Tm3þ-doped tungsten tellurite glass double cladding fiber pumped by a 800 nm laser diode and the relative slope efficiency was 20%
* Corresponding author. E-mail address: [email protected] (Z. Jia). http://dx.doi.org/10.1016/j.optmat.2017.03.019 0925-3467/© 2017 Elsevier B.V. All rights reserved.
[12]. In 2008, M. Eichhorn and S. D. Jackson investigated ~2 mm lasing in Tm3þ-doped fluoride fibers, a slope efficiency of 49% and a maximum optical-to-optical efficiency of 45% at an incident pump power of 25 W were obtained [13]. Despite these progresses in this field, it is still necessary to explore new fiber materials with low phonon energy and low OH contents for further improving the performances of Tm3þ-doped fiber lasers. Recently, spectroscopic properties of Tm3þ-doped fluorotellurite glasses have been extensively studied [14e16]. In comparison to other oxide glasses, fluorotellurite glasses have relative low phonon energy and wide transmission window [17]. Moreover, introducing F ions into the tellurite glass system can significantly decrease the OH content, which is beneficial for realizing highly efficient emissions from rare earth ions doped matrixes [18]. In 2011, Gebavi et al. obtained enhanced 1.8 mm emission intensity and increased quantum efficiency in Tm3þ-doped fluorotellurite glasses compared to that in tellurite glasses [15]. One year later, Gebavi et al. demonstrated that the 3F4 manifold lifetime could be increased dramatically by introducing the Finto tellurite glasses [16]. These results indicate that fluorotellurite glasses are promising gain media for high-performance Tm3þ-doped fiber lasers. Very recently, we developed a fluorotellurite microstructured fiber (FTMF) based on TeO2-BaF2-Y2O3 (TBY) glasses [19]. However, lasing performances in Tm3þ-doped TBY glass fibers have not yet been investigated. In this work, we fabricated Tm3þ-doped FTMFs based on the TBY glasses by using a rod-in-tube method. Lasing at 1887 nm was achieved from a 42.5 ± 0.1 cm long Tm3þ-doped FTMF pumped by a
S. Wang et al. / Optical Materials 66 (2017) 640e643
1570 nm Er3þ-doped fiber laser. The maximum unsaturated output power was 408 ± 0.5 mW, and the corresponding slope efficiency was 58.1 ± 0.1%. The influence of the fiber length on lasing performances were also investigated. 2. Experiments 2.1. Preparation of TBY glass microstructured fibers The Tm3þ-doped FTMFs were fabricated by using a rod-in-tube method. The composition of core material and the cladding material were 70TeO2-10BaF2-9.5Y2O3-0.5Tm2O3 (TBYT) and 65TeO225BaF2-10Y2O3, respectively. The detailed procedure was similar to that described in Ref. [19]. Firstly, a glass rod was made by a modified suction method [20]. The glass rod consisted of a solid core (TBYT glass, n ¼ 1.798 ± 0.001 at 2 mm) surrounded by the TBY glass (n ¼ 1.773 ± 0.001 at 2 mm). Secondly, the glass rod was drawn and elongated to a thinner rod. Thirdly, the thinner rod, stacked with six capillary glass tubes, was inserted into a glass tube and elongated into a cane with a diameter of 2.7 ± 0.1 mm. Finally, the cane was inserted into another glass tube and drawn into Tm3þ-doped FTMFs. In the fiber-drawing process, dry nitrogen gas was pumped into the holes of the cane. By changing the pressure of dry nitrogen gas, FTMFs with varied diameter ratios of holey region to core could be obtained. All of the glass tubes and the capillary tubes were made from TBY glasses. Fig. 1 showed the cross-section of the Tm3þ-doped FTMF and the calculated confinement losses of the LP01, LP11, and LP12 modes in it by using the full-vectorial finite-difference method. The core was surrounded by a “thin cladding” and three air-hole layers. The core diameter of the Tm3þ-doped FTMFs was 7 ± 0.05 mm. The corresponding confinement losses at 2 mm of the LP01, LP11, and LP12 modes were 1.8 ± 0.2 1010 dB/m, 8 ± 0.2 108 dB/m and 9 ± 0.2 107 dB/m, respectively. This indicated that the Tm3þdoped FTMFs were few mode fibers. The background loss of an undoped FTMFs was measured to be 3.6 ± 0.1 dB/m at 1560 nm by using a cutback method. 2.2. Characterization of TBY glass microstructured fibers Fig. 2 showed the experimental setup for measuring the laser performances from the Tm3þ-doped FTMFs. A continuous wave
641
(CW) Er3þ-doped silica fiber laser operating at 1570 nm was used as the pump source. An isolator right behind the pump source was used to protect from any harmful feedbacks. Tm3þ-doped FTMFs with different lengths were used as the gain media. The pump light was launched into the Tm3þ-doped FTMFs by a 40 0.47 NA aspheric lens. The output end of the EDTMF was mechanically spliced with a silica fiber cable with large effective mode field (~3140 mm2) and a large NA of 0.45 by using a butt-joint method. The output signal was monitored by an optical spectrum analyzer (OSA) with a measurement range of 1200e2400 nm (Yokogawa AQ6375) and a power meter. 3. Results and discussions 3.1. Properties of TBYT glasses Fig. 3 shows the transmission spectrum of the TBYT glass with a thickness of 2 ± 0.01 mm. The glass has a wide transmission window of 370 nme6360 nm. The absorption bands in the spectral range of 450e2000 nm are due to the transitions from the ground level 3H6 to highly excited levels (including 1G4, 3F2,3, 3H4, 3H5 and 3 F4) of Tm3þ in the TBYT glass and the absorption in the range of 2750 nme3900 nm is ascribed to the residual OH in the glass. In our experiments, the OH content is dramatically reduced by introducing the F ions, which would be beneficial for obtaining highly efficient infrared emission from rare-earth ions [18]. The OH contents in glasses can be expressed by the absorption coefficient aOH [21].
1 L
aOH ¼ ln
T0 T
(1)
where, L is the thickness of TBYT glass, T0 and T are the incident and transmitted intensities, respectively. The absorption coefficient aOH was calculated to be 0.08 ± 0.01 cm1. Based on the transmission spectrum of TBYT glass, the absorption cross-section of Tm3þ in it could be calculated by using Beer-Lambert equation [22].
sabs ðlÞ ¼
2:303 log½I0 ðlÞ=IðlÞ Nl
(2)
where, I0(l) and I(l) were the incident and the transmitted optical intensity, N is the concentration of Tm3þ in the glass sample, and l is the thickness of TBYT glass. The stimulated emission cross-section of Tm3þ in the TBYT glass could be calculated by using McCumber theory [23].
sem ðlÞ ¼ sabs ðlÞ
Zl hc 1 1 exp kT lZL l Zu
(3)
where, sabs(l) is the absorption coefficient with respective to the wavelength, Zl and Zu are the partition functions of the lower and the upper energy states, respectively. h is the planck's constant, c is the velocity of light, k is the Boltzmann constant, T is the room temperature, and lZL is the zero line energy, respectively. Fig. 4 shows the calculated absorption (solid line) and stimulated emission (dashed line) cross-section spectra of Tm3þ in the TBYT glass. The integrated emission cross section is 1.99 ± 0.15 1018 cm2, such a large value is beneficial for obtaining highly efficient lasers [24]. 3.2. Performances of Tm3þ-doped FTMF lasers Fig. 1. Calculated confinement losses of the LP01, LP11, and LP12 modes in the Tm3þdoped fluorotellurite microstructured fiber. Inset: The cross-section of Tm3þ-doped fluorotellurite microstructured fiber.
Since the Fresnel reflection (~9% at 2 mm) existed on the end of FTMFs, a laser cavity could be formed by the Tm3þ-doped FTMF
642
S. Wang et al. / Optical Materials 66 (2017) 640e643
Fig. 2. Schematic diagram of the experimental setup for ~2 mm fiber laser.
Fig. 3. Transmission spectrum of TBYT glass.
Fig. 5. Output power of ~2 mm laser as a function of the launched pump power for the 42 cm Tm3þ-doped FTMF. Inset: output laser spectrum for a pump power of 779 mW.
efficiency was relatively high among the Tm3þ-doped fiber lasers based on multicomponent glasses [10e12]. This might be due to the reduced OH content and lower phonon energies through the addition of fluoride components into the glass matrix. The inset of Fig. 5 shows the output laser spectrum for a pump power of 779 mW. Fig. 6 showed the dependence of the slope efficiency and threshold on the fiber length of the Tm3þ-doped FTMF. With increasing the fiber length from 18 ± 0.1 to 42.5 ± 0.1 cm, the slope efficient increased from 6.3 ± 0.1% to 58.1 ± 0.1%, the lasing threshold firstly decreased from 112 mW to 61 mW (25 cm) and then increased to 94 mW. With further increasing the fiber length
Fig. 4. Absorption and emission cross-sections corresponding to the Tm3þ: 3H6 4 3F4 in the TBYT glass.
with two straight facets cleaved finely perpendicular to the fiber axis only. Therefore, no additional mirrors were necessary for obtaining lasing at 2 mm. Because of the same reflectance on the two fiber facets, lasing with the same power would output from both sides of short length Tm3þ-doped FTMFs. By increasing the launched pump power to 94 mW, we obtained lasing at 1887 nm from a 42.5 ± 0.1 cm long Tm3þ-doped FTMF. Fig. 5 showed the output laser power as a function of pump power for the 42.5 ± 0.1 cm long Tm3þ-doped FTMF. With further increasing the launched pump power to 779 mW, the unsaturated output power of the 1887 nm laser increased monotonically to 408 ± 0.5 mW, corresponding to a slope efficiency of 58.1 ± 0.1%. Such a slope
Fig. 6. Slope efficiency and threshold of ~2 mm fiber laser as a function of Tm3þ-doped FTMF length.
S. Wang et al. / Optical Materials 66 (2017) 640e643
from 42.5 ± 0.1 to 45 ± 0.1 cm, the slope efficiency decreased from 58.1 ± 0.1% to 50.6 ± 0.1%, and the lasing threshold increased to 97 mW. These results indicated that the optimized fiber length for achieving highly efficient lasers is about 42.5 ± 0.1 cm. 4. Conclusion In conclusion, Tm3þ-doped FTMFs were prepared based on the TBY glasses by using the rod-in-tube method. By using a 1570 nm Er3þ-doped fiber laser as the pump source, lasing at 1887 nm was realized in a ~42.5 cm long Tm3þ-doped FTMF for a threshold pump power of 94 mW and a slope efficiency of ~58.1%. Our results showed that Tm3þ-doped FTMFs could be used to construct highly efficient fiber lasers operating at 2 mm. Acknowledgements This work was supported by the NSFC (Grant Nos. 61378004, 61527823, 61605058, 60908001, 61077033, 60908031, 11274139, and 11474132), the Opened Fund of the State Key Laboratory on Integrated Optoelectronics, Tsinghua National Laboratory for Information Science, and Technology (TNList) Cross-discipline Foundation. References [1] R.C. Stoneman, L. Esterowitz, Opt. Lett. 15 (9) (1990) 486e488. [2] J. Yang, Y. Tang, J. Xu, Phot. Res. 1 (1) (2003) 52e57.
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[3] B. Guo, Y. Wang, C. Peng, H.L. Zhang, G.P. Luo, H.Q. Le, C. Gmach, D.L. Sivco, M.L. Peabody, A.Y. Cho, Opt. Express 12 (1) (2004) 208e219. [4] J.P. Cariou, B. Augere, M. Valla, C. R. Phys. 7 (2) (2006) 213e223. [5] J. Geng, Q. Wang, J. Wang, S. Jiang, K. Hsu, Opt. Lett. 32 (4) (2007) 355e357. [6] Y. Wang, J. Yang, C. Huang, Y. Luo, S. Wang, Y. Tang, J. Xu, Opt. Express 23 (3) (2015) 2991e2998. [7] P. Zhang, T. Wang, W. Ma, K. Dong, H. Jiang, Appl. Opt. 54 (15) (2015) 4667e4670. [8] Gavin P. Frith, David G. Lancaster, in: Proceedings of SPIE, The International Society for Optical Engineering, 2006. [9] B.M. Walsh, N.P. Barnes, Appl. Phys. B 78 (2004) 325e333. [10] J. Wu, S. Jiang, T. Luo, J. Geng, N. Peyghambarian, Norman P. Barnes, IEEE Photonics Technol. Lett. 18 (2006) 234e236. [11] X. Wen, G. Tang, J. Wang, X. Chen, Q. Qian, Z. Yang, Opt. Express 23 (6) (2015) 7722e7731. [12] K. Li, G. Zhang, L. Hu, Opt. Lett. 35 (24) (2010) 4136e4138. [13] M. Eichhorn, S.D. Jackson, Appl. Phys. B 90 (2008) 35e41. [14] Y. Cheng, Z. Wu, X. Hu, T. Wu, W. Zhou, J. Rare Earth 32 (12) (2014) 1154e1161. [15] H. Gebavi, D. Milanese, R. Balda, M. Ivanda, F. Auzel, J. Lousteau, J. Fernandez, M. Ferraris, Opt. Mater. 33 (2011) 428e437. [16] H. Gebavi, S. Taccheo, R. Balda, J.M. Fernandez, D. Milanese, F. Auzel, J. Non Cryst. Solids 358 (2012) 1497e1500. [17] M.D. O'Donnell, C.A. Miller, D. Furniss, V.K. Tikhomirov, A.B. Seddon, J. Non Cryst. Solids 331 (2003) 48e57. [18] Y. Guo, M. Li, L. Hu, J. Zhang, J. Am. Ceram. Soc. 116 (2012) 5571e5576. [19] C. Yao, C. He, Z. Jia, S. Wang, G. Qin, Y. Ohishi, W. Qin, Opt. Lett. 40 (20) (2015) 4695e4698. [20] N.K. Goel, G. Pickrell, R. Stolen, Opt. Fiber Technol. 20 (2014) 325e327. [21] R. Wang, X. Meng, F. Yin, Y. Feng, G. Qin, W. Qin, Opt. Mater. Express 21 (4) (2013) 4560e4566. [22] H. Abitan, H. Bohr, P. Buchhave, Appl. Opt. 47 (29) (2008) 5354e5357. [23] D.E. McCumber, Phys. Rev. 134 (2A) (1964) A299eA306. [24] J.A. Caird, L.G. DeShazer, J. Nella, IEEE J. Quant. Electron. 11 (11) (1975) 874e881.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
1887 nm lasing in Tm3þ-doped TeO2-BaF2-Y2O3 glass microstructured fibers Shunbin Wang, Chuanfei Yao, Zhixu Jia*, Guanshi Qin, Weiping Qin State Key Laboratory on Integrated Opto-Electronics, College of Electronic Science & Engineering, Jilin University, Changchun 130012, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 December 2016 Accepted 13 March 2017 Available online 21 March 2017
In this paper, we demonstrate ~2 mm lasing in Tm3þ-doped fluorotellurite microstructured fibers. The Tm3þ-doped fibers are based on TeO2-BaF2-Y2O3 glasses and fabricated by using a rod-in-tube method. Under the pump of a 1570 nm Er3þ-doped fiber laser, lasing at 1887 nm is obtained in a ~42.5 cm long Tm3þ-doped fiber with a threshold pump power of 94 mW. As the pump power increases to 780 mW, the obtained maximum unsaturated power reaches up to ~408 mW with a slop efficiency of ~58.1%. This result indicates that the Tm3þ-doped fluorotellurite fibers are promising gain media for ~2 mm fiber lasers. © 2017 Elsevier B.V. All rights reserved.
Keywords: Fluorotellurite glass Microstructured fiber Tm3þ-doped Fiber laser
1. Introduction Tm3þ-doped fiber lasers have been widely investigated for various applications in atmosphere and wind sensing, military, eyesafe laser radar and medical surgery [1e7]. Up to now, the reported output power of ~2 mm Tm3þ-doped silica fiber lasers has exceeded 1000 W [8]. ~2 mm lasing in Tm3þ-doped fibers is ascribed to the transition of 3F4 / 3H6; lower phonon energy of the matrix may lead to better “two-for-one” Tm-Tm self-quenching (3H4 þ 3H6 / 23F4), which can improve the slope efficiency and decrease the threshold of relative lasers [9]. For this purpose, glasses with low phonon energies (e.g. germanate glasses, tellurite glasses, and fluoride glasses) have been investigated for ~2 mm Tm3þ-doped fiber lasers [10e12]. In 2006, Jianfeng Wu et al. reported efficient ~2 mm lasing in a 4 cm long highly Tm3þ-doped germanate fiber pumped by an 800 nm laser. The obtained slope efficiency was 58% and the quantum efficiency was 179% [10]. In 2015, Xin Wen et al. reported an all-fiber laser at 1.95 mm in a barium gallo-germanate glass single-mode fiber with a maximum output power of 140 mW and a slope efficiency of 7.6% under the excitation of a 1568 nm fiber laser [11]. In 2010, Kefeng Li et al. reported a watt level ~2 mm CW fiber laser from a 40 cm long highly Tm3þ-doped tungsten tellurite glass double cladding fiber pumped by a 800 nm laser diode and the relative slope efficiency was 20%
* Corresponding author. E-mail address: [email protected] (Z. Jia). http://dx.doi.org/10.1016/j.optmat.2017.03.019 0925-3467/© 2017 Elsevier B.V. All rights reserved.
[12]. In 2008, M. Eichhorn and S. D. Jackson investigated ~2 mm lasing in Tm3þ-doped fluoride fibers, a slope efficiency of 49% and a maximum optical-to-optical efficiency of 45% at an incident pump power of 25 W were obtained [13]. Despite these progresses in this field, it is still necessary to explore new fiber materials with low phonon energy and low OH contents for further improving the performances of Tm3þ-doped fiber lasers. Recently, spectroscopic properties of Tm3þ-doped fluorotellurite glasses have been extensively studied [14e16]. In comparison to other oxide glasses, fluorotellurite glasses have relative low phonon energy and wide transmission window [17]. Moreover, introducing F ions into the tellurite glass system can significantly decrease the OH content, which is beneficial for realizing highly efficient emissions from rare earth ions doped matrixes [18]. In 2011, Gebavi et al. obtained enhanced 1.8 mm emission intensity and increased quantum efficiency in Tm3þ-doped fluorotellurite glasses compared to that in tellurite glasses [15]. One year later, Gebavi et al. demonstrated that the 3F4 manifold lifetime could be increased dramatically by introducing the Finto tellurite glasses [16]. These results indicate that fluorotellurite glasses are promising gain media for high-performance Tm3þ-doped fiber lasers. Very recently, we developed a fluorotellurite microstructured fiber (FTMF) based on TeO2-BaF2-Y2O3 (TBY) glasses [19]. However, lasing performances in Tm3þ-doped TBY glass fibers have not yet been investigated. In this work, we fabricated Tm3þ-doped FTMFs based on the TBY glasses by using a rod-in-tube method. Lasing at 1887 nm was achieved from a 42.5 ± 0.1 cm long Tm3þ-doped FTMF pumped by a
S. Wang et al. / Optical Materials 66 (2017) 640e643
1570 nm Er3þ-doped fiber laser. The maximum unsaturated output power was 408 ± 0.5 mW, and the corresponding slope efficiency was 58.1 ± 0.1%. The influence of the fiber length on lasing performances were also investigated. 2. Experiments 2.1. Preparation of TBY glass microstructured fibers The Tm3þ-doped FTMFs were fabricated by using a rod-in-tube method. The composition of core material and the cladding material were 70TeO2-10BaF2-9.5Y2O3-0.5Tm2O3 (TBYT) and 65TeO225BaF2-10Y2O3, respectively. The detailed procedure was similar to that described in Ref. [19]. Firstly, a glass rod was made by a modified suction method [20]. The glass rod consisted of a solid core (TBYT glass, n ¼ 1.798 ± 0.001 at 2 mm) surrounded by the TBY glass (n ¼ 1.773 ± 0.001 at 2 mm). Secondly, the glass rod was drawn and elongated to a thinner rod. Thirdly, the thinner rod, stacked with six capillary glass tubes, was inserted into a glass tube and elongated into a cane with a diameter of 2.7 ± 0.1 mm. Finally, the cane was inserted into another glass tube and drawn into Tm3þ-doped FTMFs. In the fiber-drawing process, dry nitrogen gas was pumped into the holes of the cane. By changing the pressure of dry nitrogen gas, FTMFs with varied diameter ratios of holey region to core could be obtained. All of the glass tubes and the capillary tubes were made from TBY glasses. Fig. 1 showed the cross-section of the Tm3þ-doped FTMF and the calculated confinement losses of the LP01, LP11, and LP12 modes in it by using the full-vectorial finite-difference method. The core was surrounded by a “thin cladding” and three air-hole layers. The core diameter of the Tm3þ-doped FTMFs was 7 ± 0.05 mm. The corresponding confinement losses at 2 mm of the LP01, LP11, and LP12 modes were 1.8 ± 0.2 1010 dB/m, 8 ± 0.2 108 dB/m and 9 ± 0.2 107 dB/m, respectively. This indicated that the Tm3þdoped FTMFs were few mode fibers. The background loss of an undoped FTMFs was measured to be 3.6 ± 0.1 dB/m at 1560 nm by using a cutback method. 2.2. Characterization of TBY glass microstructured fibers Fig. 2 showed the experimental setup for measuring the laser performances from the Tm3þ-doped FTMFs. A continuous wave
641
(CW) Er3þ-doped silica fiber laser operating at 1570 nm was used as the pump source. An isolator right behind the pump source was used to protect from any harmful feedbacks. Tm3þ-doped FTMFs with different lengths were used as the gain media. The pump light was launched into the Tm3þ-doped FTMFs by a 40 0.47 NA aspheric lens. The output end of the EDTMF was mechanically spliced with a silica fiber cable with large effective mode field (~3140 mm2) and a large NA of 0.45 by using a butt-joint method. The output signal was monitored by an optical spectrum analyzer (OSA) with a measurement range of 1200e2400 nm (Yokogawa AQ6375) and a power meter. 3. Results and discussions 3.1. Properties of TBYT glasses Fig. 3 shows the transmission spectrum of the TBYT glass with a thickness of 2 ± 0.01 mm. The glass has a wide transmission window of 370 nme6360 nm. The absorption bands in the spectral range of 450e2000 nm are due to the transitions from the ground level 3H6 to highly excited levels (including 1G4, 3F2,3, 3H4, 3H5 and 3 F4) of Tm3þ in the TBYT glass and the absorption in the range of 2750 nme3900 nm is ascribed to the residual OH in the glass. In our experiments, the OH content is dramatically reduced by introducing the F ions, which would be beneficial for obtaining highly efficient infrared emission from rare-earth ions [18]. The OH contents in glasses can be expressed by the absorption coefficient aOH [21].
1 L
aOH ¼ ln
T0 T
(1)
where, L is the thickness of TBYT glass, T0 and T are the incident and transmitted intensities, respectively. The absorption coefficient aOH was calculated to be 0.08 ± 0.01 cm1. Based on the transmission spectrum of TBYT glass, the absorption cross-section of Tm3þ in it could be calculated by using Beer-Lambert equation [22].
sabs ðlÞ ¼
2:303 log½I0 ðlÞ=IðlÞ Nl
(2)
where, I0(l) and I(l) were the incident and the transmitted optical intensity, N is the concentration of Tm3þ in the glass sample, and l is the thickness of TBYT glass. The stimulated emission cross-section of Tm3þ in the TBYT glass could be calculated by using McCumber theory [23].
sem ðlÞ ¼ sabs ðlÞ
Zl hc 1 1 exp kT lZL l Zu
(3)
where, sabs(l) is the absorption coefficient with respective to the wavelength, Zl and Zu are the partition functions of the lower and the upper energy states, respectively. h is the planck's constant, c is the velocity of light, k is the Boltzmann constant, T is the room temperature, and lZL is the zero line energy, respectively. Fig. 4 shows the calculated absorption (solid line) and stimulated emission (dashed line) cross-section spectra of Tm3þ in the TBYT glass. The integrated emission cross section is 1.99 ± 0.15 1018 cm2, such a large value is beneficial for obtaining highly efficient lasers [24]. 3.2. Performances of Tm3þ-doped FTMF lasers Fig. 1. Calculated confinement losses of the LP01, LP11, and LP12 modes in the Tm3þdoped fluorotellurite microstructured fiber. Inset: The cross-section of Tm3þ-doped fluorotellurite microstructured fiber.
Since the Fresnel reflection (~9% at 2 mm) existed on the end of FTMFs, a laser cavity could be formed by the Tm3þ-doped FTMF
642
S. Wang et al. / Optical Materials 66 (2017) 640e643
Fig. 2. Schematic diagram of the experimental setup for ~2 mm fiber laser.
Fig. 3. Transmission spectrum of TBYT glass.
Fig. 5. Output power of ~2 mm laser as a function of the launched pump power for the 42 cm Tm3þ-doped FTMF. Inset: output laser spectrum for a pump power of 779 mW.
efficiency was relatively high among the Tm3þ-doped fiber lasers based on multicomponent glasses [10e12]. This might be due to the reduced OH content and lower phonon energies through the addition of fluoride components into the glass matrix. The inset of Fig. 5 shows the output laser spectrum for a pump power of 779 mW. Fig. 6 showed the dependence of the slope efficiency and threshold on the fiber length of the Tm3þ-doped FTMF. With increasing the fiber length from 18 ± 0.1 to 42.5 ± 0.1 cm, the slope efficient increased from 6.3 ± 0.1% to 58.1 ± 0.1%, the lasing threshold firstly decreased from 112 mW to 61 mW (25 cm) and then increased to 94 mW. With further increasing the fiber length
Fig. 4. Absorption and emission cross-sections corresponding to the Tm3þ: 3H6 4 3F4 in the TBYT glass.
with two straight facets cleaved finely perpendicular to the fiber axis only. Therefore, no additional mirrors were necessary for obtaining lasing at 2 mm. Because of the same reflectance on the two fiber facets, lasing with the same power would output from both sides of short length Tm3þ-doped FTMFs. By increasing the launched pump power to 94 mW, we obtained lasing at 1887 nm from a 42.5 ± 0.1 cm long Tm3þ-doped FTMF. Fig. 5 showed the output laser power as a function of pump power for the 42.5 ± 0.1 cm long Tm3þ-doped FTMF. With further increasing the launched pump power to 779 mW, the unsaturated output power of the 1887 nm laser increased monotonically to 408 ± 0.5 mW, corresponding to a slope efficiency of 58.1 ± 0.1%. Such a slope
Fig. 6. Slope efficiency and threshold of ~2 mm fiber laser as a function of Tm3þ-doped FTMF length.
S. Wang et al. / Optical Materials 66 (2017) 640e643
from 42.5 ± 0.1 to 45 ± 0.1 cm, the slope efficiency decreased from 58.1 ± 0.1% to 50.6 ± 0.1%, and the lasing threshold increased to 97 mW. These results indicated that the optimized fiber length for achieving highly efficient lasers is about 42.5 ± 0.1 cm. 4. Conclusion In conclusion, Tm3þ-doped FTMFs were prepared based on the TBY glasses by using the rod-in-tube method. By using a 1570 nm Er3þ-doped fiber laser as the pump source, lasing at 1887 nm was realized in a ~42.5 cm long Tm3þ-doped FTMF for a threshold pump power of 94 mW and a slope efficiency of ~58.1%. Our results showed that Tm3þ-doped FTMFs could be used to construct highly efficient fiber lasers operating at 2 mm. Acknowledgements This work was supported by the NSFC (Grant Nos. 61378004, 61527823, 61605058, 60908001, 61077033, 60908031, 11274139, and 11474132), the Opened Fund of the State Key Laboratory on Integrated Optoelectronics, Tsinghua National Laboratory for Information Science, and Technology (TNList) Cross-discipline Foundation. References [1] R.C. Stoneman, L. Esterowitz, Opt. Lett. 15 (9) (1990) 486e488. [2] J. Yang, Y. Tang, J. Xu, Phot. Res. 1 (1) (2003) 52e57.
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