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The effect of erbium concentration on spectroscopic properties of Er:CaF2 crystals was investigated. Two highest absorption cross-sections (σabs) at 967 nm were achieved in the 4at.% and 8at.% Er:CaF2 samples with the value of 0.22 × 10−20 cm2 and 0.23 × 10−20 cm2, respectively. And the 4at.% and 8at.% Er:CaF2 samples also had the highest emission cross-section (σem) at 2727 nm with the value about 0.67 × 10−20 cm2. Lifetime of 4I13/2 decreased faster than that of 4I11/2 with the increase of erbium concentration. Under laser diode (LD) pumping, the continuous-wave (CW) laser operations around 2.79 μm were demonstrated in the 4at.%, 8at.% and 11at.% Er:CaF2 samples. And the 4at.% Er:CaF2 sample had the best laser performance with a maximum output power of 0.282 W and a slop efficiency of 13.9%.
© 2016 Optical Society of America
1. Introduction
For various potential applications, such as medical surgery and remote atmosphere sensing, mid-infrared lasers at the wavelength region of 2.7 to 3 μm have attracted great attention [1–4]. The reason is that this region is overlapped by strong absorption bands of both water and CO2 [5]. Moreover, Lasers around 3 μm are suitable pumping sources for infrared optical parametric oscillation (OPO) [6]. Erbium doped laser materials can obtain emission from 2.58 to 2.94 μm [2,6,7], owning to the transition from 4I11/2 to 4I13/2 of Er3+. Researches about erbium doped laser materials concentrate mainly on oxide hosts (crystals and ceramics) like YAG [2], GGG [8], GSGG [6], YSGG [1], Y2O3 [3], CGA [5] and fluoride hosts (crystals, ceramics and fibers) like CaF2 [9,10], SrF2 [11], LiYF4 [12], ZBLAN [13]. However, the transition of Er3+ that generates emission around 2.7 μm is considered to be self-saturation, for the lifetime of 4I13/2 (lower laser level) is much longer than that of 4I11/2 (upper laser level) [6]. To deal with this unfavorable population bottleneck and depopulate the 4I13/2 state efficiently, two major solutions are proposed, either high erbium doping concentration or codoping sensitive ions like Pr3+, Nd3+, Ho3+ and so on [5,14,15]. Another important factor to improve the performance of the 2.7 μm emission is to choose host matrices with low phonon energy. Because the energy gap between 4I11/2 and 4I13/2 is narrow (about 3600 cm−1), such low phonon energy hosts reduce the nonradiative transition probability, which benefits the lifetime of laser levels [9]. CaF2 crystal is a famous laser host material with a low phonon energy of 322 cm−1, which is much lower than that of may oxide matrix such as YAG (700 cm−1 [16]), Y2O3 (591 cm−1 [3]). Recently, many breakthroughs had been made based on rare earth doped CaF2 crystals and ceramics. Nd,Y:CaF2 had achieved the shortest mode-locked pulse (103 fs) among the Nd-doped crystal lasers [17]. Yb:CaF2 thin-disk CW laser with an output power of 250 W was demonstrated [18]. 2.7 μm tunable laser with a tuning range of 118nm was demonstrated in a 5at.% Er:CaF2 ceramic under laser diode pumping [10]. Moreover, rare earth ions are easily clustered in CaF2 crystals [19] with adverse effects on 1.5 μm laser but benefits for 2.7 to 3 μm laser.
In this paper, we report the effect of erbium concentration on spectroscopic properties of Er:CaF2 crystals grown by tradition Bridgman method. Under 974 nm LD end-pumping, 2.79 μm CW laser was demonstrated in 4at.%, 8at.% and 11at.% Er:CaF2 samples. And the 4at.% Er:CaF2 sample had the best laser performance with a maximum output power 282 mW and a slop efficient of 13.9%.
2. Experiments
Erbium singly doped calcium fluoride crystals with the formula of (ErxCa1-x)F2+x (x = 0.2at.%, 1at.%, 3at.%, 4at.%, 8at.%, 11at.%, 13at.%) were grown by traditional Bridgman method. High purity (>99.995%) ErF3 and CaF2 fine powders were used as raw materials. The raw materials were mixed with additional deoxidant and sealed in the platinum crucibles during the process of growth. The crystal samples were cut and then polished into a size of 10 × 10 × 1.3 mm3 for spectral measurements. The actual concentrations of erbium in the grown crystals were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) method. Room temperature absorption spectra were measured by a Jasco V-570 UV/VIS/NIR spectro-photometer. Emission spectra were employed on a FLSP920 time-resolved fluorescence spectrometer. The 980 nm LD was used as the exciting source. The NIR and mid-infrared emission were measured by InAs photodetector. Luminescence decay curves were measured with μF900 microsecond lamp. All the measurements were carried out at room temperature.
3. Results and discussion
Figure 1 illustrated the absorption coefficient of Er:CaF2 crystals with different erbium concentration. The main 13 absorption bands were found to be centered at around 356, 364, 378, 406, 440, 449, 486, 522, 539, 652, 799, 980 and 1515 nm, which correspond to the transitions from ground state 4I15/2 to excited states 2G7/2, 2K15/2, 4G9/2, 4G11/2, 2H9/2, 4F3/2, 4F5/2, 4F7/2, 2H11/2, 4S3/2, 4F9/2, 4I9/2, 4I11/2 and 4I13/2, respectively. The absorption coefficient increased gradually with the increase of doping concentration. The full widths at half-maximum of the absorption peaks at 967 nm of these samples were similar, which were about 22 nm. The absorption cross-section at 799, 967, 1515 nm were calculated and shown in Fig. 2. The absorption cross-section variation tendency was shown in the insert of Fig. 2. Two highest absorption cross-section at 967 nm were achieved in the 4at.% and 8at.% Er:CaF2 samples with the value of 0.22 × 10−20 cm2 and 0.23 × 10−20 cm2.
Fig. 1 Absorption coefficient of (ErxCa1-x)F2 (x = 0.2at.%, 1at.%, 3at.%, 4at.%, 8at.%, 11at.%, 13at.%) at room temperature.
Fig. 2 The absorption cross-section at 799 nm (a), 1515 nm (b) and 967 nm (c). And variation tendency of absorption cross-section (red curve) and absorption coefficient (blue curve) in the insert and (d).
According to the Fuchtbauer-Ladenburg theory [20] and emission spectrum the emission cross-sections of seven samples were calculated and illustrated in Fig. 3 with the variation tendency was shown in the insert and Fig. 3d as well.
Where λ is the wavelength, Arad is the spontaneous transition probability, I(λ) is the emission spectrum, c is light speed, and n is the refractive index.
It could be seen that the 2.7 μm emission band was quiet broad ranging from 2500 to 2860 nm, which was beneficial for ultra-short pulse laser operation. The peak values were similar and varied from 0.37 × 10−20 cm2 to 0.67 × 10−20 cm2. The highest emission cross-section at 2727 nm was also obtained in the 4at.% and 8at.% Er:CaF2 samples with the value of 0.67 × 10−20 cm2.
The decay curves of luminescence for four analyzed samples recorded upon direct excitation of 4I11/2 at room temperature were presented in Fig. 4. As can be seen that lifetime of both 4I11/2 and 4I13/2 decreased with the increasing of erbium doping concentration. It was worth noting that lifetime of 4I13/2 decreased faster than that of 4I11/2 with the increase of erbium doping concentration, which was favorable for overcoming the “bottleneck” effect [16]. The lifetime difference between 4I11/2 and 4I13/2 reduced from 5.44 ms (4at.% Er:CaF2 sample) to 3.87 ms (13at.% Er:CaF2 sample). However, lifetime inversion was not achieved even when the erbium doping concentration was as high as 13at.%. C. Labbe et al [9] achieved lifetime inversion in a 10.34at.% Er:CaF2 crystal with the values of 4.7 ms and 2.4 ms for 4I11/2 and 4I13/2. The contradiction might be caused by the strong re-absorption effect of 4I13/2,for the samples had a thickness of 1.3 mm.
Fig. 4 Decay curves of luminescence for both 4I13/2 (a), 4I11/2 (b) of (ErxCa1-x)F2 (x = 4at.%, 8at.%, 11at.%, 13at.%) at room temperature.
The absorption cross-section of Er3+ between two excited levels 4I13/2 and 4I11/2 can be derived from the calculated emission cross-section through the McCumber method [20]:
where h is Planck’s constant, KB is the Boltzmann constant, T is the temperature, Ezl is the separation between the lowest Stark level of the lower and upper manifolds, and Zu and Zl are partition functions. The values of Zu and Zl are calculated to be 2.16 and 1.99, using the method proposed by Miniscale and Quimby [21]. The calculated absorption cross-section of the 4at.% erbium doped sample was presented in Fig. 5 along with the emission cross-section.
As the absorption and emission cross-section were calculated, the gain cross-section σG(λ) could be estimated by the following equation:
Where population inversion P is assigned to the concentration ratio of Er3+ in the 4I11/2 and 4I13/2 levels. The gain cross-sections of the 4at.% Er:CaF2 sample with the P various from 0 to 1 were estimated and illustrated in Fig. 6. Obviously, when P≥0.5, the positive gain cross-section was achieved.Preliminary, CW laser operations were carried out by inserting an uncoated Er:CaF2 sample inside a plane-concave laser resonator, with water cooling at 10°C, the setup was shown in Fig. 7. M1 was a flat mirror; M2 was a concave mirror having radius of 50 mm with output transmission of 1% at 2.7-2.95 μm. The resonator length was 44 mm. The laser samples were in dimensions of 3 × 3 × 10 mm3 (4at.% Er:CaF2), 3 × 3 × 6 mm3 (8at.% Er:CaF2) and 3 × 3 × 5 mm3 (11at.% Er:CaF2). Laser operations were demonstrated around 2.79 μm with a LD pumping at 974 nm.
It turned out that the 4at.% Er:CaF2 sample had the best laser performance with a maximum output power of 0.282 W versus incident power of 2.222 W and a slop efficiency of 13.9%, as shown in Fig. 8. It could be seen that the output power was not saturated. Better laser performance can be expected with anti-reflection coated samples, and by using an optimized laser resonator.
Fig. 8 Output power versus absorbed pump power for CW laser operation of 4at.%, 8at.%, 11 at.% Er:CaF2 samples.
The 8at.% Er:CaF2 sample had a slop efficiency of 12.2%, which was quiet similar with the 4at.% Er:CaF2 sample at the lower pumping power region. However, As a consequence of a higher doping concentration, its thermal conductivity and crystal quality were weakened. So its output power ended up in 0.112 W versus incident power of 1.129 W. The 11at.% Er:CaF2 sample had the same problem.
6. Conclusion
In summary, the spectroscopic properties of (ErxCa1-x)F2+x (x = 0.2at.%, 1at.%, 3at.%, 4at.%, 8at.%, 11at.%, 13at.%) crystals were investigated. Two highest σabs at 967 nm were achieved in the 4at.% and 8at.% Er:CaF2 samples with the value of 0.22 × 10−20 cm2 and 0.23 × 10−20 cm2, respectively. These two samples also had the highest σem at 2727 nm with the value about 0.67 × 10−20 cm2. Erbium concentration had a greater effect on the lifetime of 4I13/2 than that of 4I11/2. Under LD pumping, the CW laser operation around 2.79 μm were demonstrated in the 4at.%, 8at.% and 11at.% Er:CaF2 samples. And the 4at.% Er:CaF2 sample had the best laser performance with a maximum output power of 0.282 W and a slop efficiency of 13.9%. These results suggested that the best erbium doping concentration in CaF2 crystal for 2.79 μm laser could be near 4at.%.
Acknowledgments
The authors acknowledge support from the National Natural Science Foundation of China (Nos. 61422511, 51432007, 91222112 and 61575088).
References and links
1. A. Hiigele, G. Horbe, H. Lubatschowski, H. Welling, and W. Ertmer, “2.70 μm CrEr:YSGG laser with high output energy and FTIR-Q-switch,” J. Lumin. 125, 90–94 (1996).
2. C. Ziolek, H. Ernst, G. F. Will, H. Lubatschowski, H. Welling, and W. Ertmer, “High-repetition-rate, high-average-power, diode-pumped 2.94-microm Er:YAG laser,” Opt. Lett. 26(9), 599–601 (2001). [CrossRef] [PubMed]
3. T. Sanamyan, M. Kanskar, Y. Xiao, D. Kedlaya, and M. Dubinskii, “High power diode-pumped 2.7-μm Er3+:Y2O3 laser with nearly quantum defect-limited efficiency,” Opt. Express 19(5), A1082–A1087 (2011). [CrossRef] [PubMed]
4. D. Faucher, M. Bernier, G. Androz, N. Caron, and R. Vallée, “20 W passively cooled single-mode all-fiber laser at 2.8 μm,” Opt. Lett. 36(7), 1104–1106 (2011). [CrossRef] [PubMed]
5. Z. Zhu, J. Li, Z. You, Y. Wang, S. Lv, E. Ma, J. Xu, H. Wang, and C. Tu, “Benefit of Pr3+ ions to the spectral properties of Pr3+/Er3+:CaGdAlO4 crystal for a 2.7 μm laser,” Opt. Lett. 37(23), 4838–4840 (2012). [CrossRef] [PubMed]
6. J. Chen, D. Sun, J. Luo, H. Zhang, R. Dou, J. Xiao, Q. Zhang, and S. Yin, “Spectroscopic properties and diode end-pumped 2.79 μm laser performance of Er,Pr:GYSGG crystal,” Opt. Express 21(20), 23425–23432 (2013). [CrossRef] [PubMed]
7. A. Godard, “Infrared (2–12 µm) solid-state laser sources: a review,” C. R. Phys. 8(10), 1100–1128 (2007). [CrossRef]
8. B. J. Dinerman and P. F. Moulton, “3-µm cw laser operations in erbium-doped YSGG, GGG, and YAG,” Opt. Lett. 19(15), 1143–1145 (1994). [CrossRef] [PubMed]
9. C. Labbe, J. L. Doualan, P. Camy, R. Moncorge, and M. Thuau, “The 2.8 µm laser properties of Er3+ doped CaF2 crystals,” Opt. Commun. 209(1-3), 193–199 (2002). [CrossRef]
10. J. Šulc, M. Němec, R. Svejkar, H. Jelínková, M. E. Doroshenko, P. P. Fedorov, and V. V. Osiko, “Diode-pumped Er:CaF2 ceramic 2.7 μm tunable laser,” Opt. Lett. 38(17), 3406–3409 (2013). [CrossRef] [PubMed]
11. T. T. Basiev, Yu. V. Orlovskii, M. V. Polyachenkova, P. P. Fedorov, S. V. Kuznetsov, V. A. Konyushkin, V. V. Osiko, O. K. Alimov, and A. Yu. Dergachev, “Continuously tunable cw lasing near 2.75 μm in diode-pumped Er3+: SrF2 and Er3+: CaF2 crystals,” Quantum Electron. 36(7), 591–594 (2006). [CrossRef]
12. T. Jensen, A. Diening, G. Huber, and B. H. Chai, “Investigation of diode-pumped 2.8-microm Er:LiYF4 lasers with various doping levels,” Opt. Lett. 21(8), 585–587 (1996). [CrossRef] [PubMed]
13. S. Tokita, M. Hirokane, M. Murakami, S. Shimizu, M. Hashida, and S. Sakabe, “Stable 10 W Er:ZBLAN fiber laser operating at 2.71-2.88 μm,” Opt. Lett. 35(23), 3943–3945 (2010). [CrossRef] [PubMed]
14. H. Lin, D. Chen, Y. Yu, A. Yang, and Y. Wang, “Enhanced mid-infrared emissions of Er3+ at 2.7 μm via Nd3+ sensitization in chalcohalide glass,” Opt. Lett. 36(10), 1815–1817 (2011). [CrossRef] [PubMed]
15. F. Huang, X. Li, X. Liu, J. Zhang, L. Hu, and D. Chen, “Sensitizing effect of Ho3+ on the Er3+: 2.7 μm-emission in fluoride glass,” Opt. Mater. 36(5), 921–925 (2014). [CrossRef]
16. L. Fornasiero, E. Mix, V. Peters, K. Petermann, and G. Huber, “New Oxide Crystals for Solid State Lasers,” Cryst. Res. Technol. 34(2), 255–260 (1999). [CrossRef]
17. Z. P. Qin, G. Q. Xie, J. Ma, W. Y. Ge, P. Yuan, L. J. Qian, L. B. Su, D. P. Jiang, F. K. Ma, Q. Zhang, Y. X. Cao, and J. Xu, “Generation of 103 fs mode-locked pulses by a gain linewidth-variable Nd,Y:CaF2 disordered crystal,” Opt. Lett. 39(7), 1737–1739 (2014). [CrossRef] [PubMed]
18. K. S. Wentsch, B. Weichelt, S. Günster, F. Druon, P. Georges, M. Abdou Ahmed, and T. Graf, “Yb:CaF2 thin-disk laser,” Opt. Express 22(2), 1524–1532 (2014). [CrossRef] [PubMed]
19. C. R. Catlow, A. V. Chadwick, G. N. Greaves, and L. M. Moroney, “Direct observations of the dopant environment in fluorites using EXAFS,” Nature 312(13), 601–604 (1984). [CrossRef]
20. T. Schweizer, D. W. Hewak, B. N. Samson, and D. N. Payne, “Spectroscopic data of the 1.8-, 2.9-, and 4.3-microm transitions in dysprosium-doped gallium lanthanum sulfide glass,” Opt. Lett. 21(19), 1594–1596 (1996). [CrossRef] [PubMed]
21. W. J. Miniscalco and R. S. Quimby, “General procedure for the analysis of Er3+ cross sections,” Opt. Lett. 16(4), 258–260 (1991). [CrossRef] [PubMed]
